bufmgr.c

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/*-------------------------------------------------------------------------
 *
 * bufmgr.c
 *    buffer manager interface routines
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/buffer/bufmgr.c
 *
 *-------------------------------------------------------------------------
 */
/*
 * Principal entry points:
 *
 * ReadBuffer() -- find or create a buffer holding the requested page,
 *      and pin it so that no one can destroy it while this process
 *      is using it.
 *
 * StartReadBuffer() -- as above, with separate wait step
 * StartReadBuffers() -- multiple block version
 * WaitReadBuffers() -- second step of above
 *
 * ReleaseBuffer() -- unpin a buffer
 *
 * MarkBufferDirty() -- mark a pinned buffer's contents as "dirty".
 *      The disk write is delayed until buffer replacement or checkpoint.
 *
 * See also these files:
 *      freelist.c -- chooses victim for buffer replacement
 *      buf_table.c -- manages the buffer lookup table
 */
#include "postgres.h"
 
#include <sys/file.h>
#include <unistd.h>
 
#include "access/tableam.h"
#include "access/xloginsert.h"
#include "access/xlogutils.h"
#ifdef USE_ASSERT_CHECKING
#include "catalog/pg_tablespace_d.h"
#endif
#include "catalog/storage.h"
#include "catalog/storage_xlog.h"
#include "executor/instrument.h"
#include "lib/binaryheap.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "pgstat.h"
#include "postmaster/bgwriter.h"
#include "storage/aio.h"
#include "storage/buf_internals.h"
#include "storage/bufmgr.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/proc.h"
#include "storage/read_stream.h"
#include "storage/smgr.h"
#include "storage/standby.h"
#include "utils/memdebug.h"
#include "utils/ps_status.h"
#include "utils/rel.h"
#include "utils/resowner.h"
#include "utils/timestamp.h"
 
 
/* Note: these two macros only work on shared buffers, not local ones! */
#define BufHdrGetBlock(bufHdr)  ((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
#define BufferGetLSN(bufHdr)    (PageGetLSN(BufHdrGetBlock(bufHdr)))
 
/* Note: this macro only works on local buffers, not shared ones! */
#define LocalBufHdrGetBlock(bufHdr) \
    LocalBufferBlockPointers[-((bufHdr)->buf_id + 2)]
 
/* Bits in SyncOneBuffer's return value */
#define BUF_WRITTEN             0x01
#define BUF_REUSABLE            0x02
 
#define RELS_BSEARCH_THRESHOLD      20
 
/*
 * This is the size (in the number of blocks) above which we scan the
 * entire buffer pool to remove the buffers for all the pages of relation
 * being dropped. For the relations with size below this threshold, we find
 * the buffers by doing lookups in BufMapping table.
 */
#define BUF_DROP_FULL_SCAN_THRESHOLD        (uint64) (NBuffers / 32)
 
typedef struct PrivateRefCountEntry
{
    Buffer      buffer;
    int32       refcount;
} PrivateRefCountEntry;
 
/* 64 bytes, about the size of a cache line on common systems */
#define REFCOUNT_ARRAY_ENTRIES 8
 
/*
 * Status of buffers to checkpoint for a particular tablespace, used
 * internally in BufferSync.
 */
typedef struct CkptTsStatus
{
    /* oid of the tablespace */
    Oid         tsId;
 
    /*
     * Checkpoint progress for this tablespace. To make progress comparable
     * between tablespaces the progress is, for each tablespace, measured as a
     * number between 0 and the total number of to-be-checkpointed pages. Each
     * page checkpointed in this tablespace increments this space's progress
     * by progress_slice.
     */
    float8      progress;
    float8      progress_slice;
 
    /* number of to-be checkpointed pages in this tablespace */
    int         num_to_scan;
    /* already processed pages in this tablespace */
    int         num_scanned;
 
    /* current offset in CkptBufferIds for this tablespace */
    int         index;
} CkptTsStatus;
 
/*
 * Type for array used to sort SMgrRelations
 *
 * FlushRelationsAllBuffers shares the same comparator function with
 * DropRelationsAllBuffers. Pointer to this struct and RelFileLocator must be
 * compatible.
 */
typedef struct SMgrSortArray
{
    RelFileLocator rlocator;    /* This must be the first member */
    SMgrRelation srel;
} SMgrSortArray;
 
/* GUC variables */
bool        zero_damaged_pages = false;
int         bgwriter_lru_maxpages = 100;
double      bgwriter_lru_multiplier = 2.0;
bool        track_io_timing = false;
 
/*
 * How many buffers PrefetchBuffer callers should try to stay ahead of their
 * ReadBuffer calls by.  Zero means "never prefetch".  This value is only used
 * for buffers not belonging to tablespaces that have their
 * effective_io_concurrency parameter set.
 */
int         effective_io_concurrency = DEFAULT_EFFECTIVE_IO_CONCURRENCY;
 
/*
 * Like effective_io_concurrency, but used by maintenance code paths that might
 * benefit from a higher setting because they work on behalf of many sessions.
 * Overridden by the tablespace setting of the same name.
 */
int         maintenance_io_concurrency = DEFAULT_MAINTENANCE_IO_CONCURRENCY;
 
/*
 * Limit on how many blocks should be handled in single I/O operations.
 * StartReadBuffers() callers should respect it, as should other operations
 * that call smgr APIs directly.  It is computed as the minimum of underlying
 * GUCs io_combine_limit_guc and io_max_combine_limit.
 */
int         io_combine_limit = DEFAULT_IO_COMBINE_LIMIT;
int         io_combine_limit_guc = DEFAULT_IO_COMBINE_LIMIT;
int         io_max_combine_limit = DEFAULT_IO_COMBINE_LIMIT;
 
/*
 * GUC variables about triggering kernel writeback for buffers written; OS
 * dependent defaults are set via the GUC mechanism.
 */
int         checkpoint_flush_after = DEFAULT_CHECKPOINT_FLUSH_AFTER;
int         bgwriter_flush_after = DEFAULT_BGWRITER_FLUSH_AFTER;
int         backend_flush_after = DEFAULT_BACKEND_FLUSH_AFTER;
 
/* local state for LockBufferForCleanup */
static BufferDesc *PinCountWaitBuf = NULL;
 
/*
 * Backend-Private refcount management:
 *
 * Each buffer also has a private refcount that keeps track of the number of
 * times the buffer is pinned in the current process.  This is so that the
 * shared refcount needs to be modified only once if a buffer is pinned more
 * than once by an individual backend.  It's also used to check that no buffers
 * are still pinned at the end of transactions and when exiting.
 *
 *
 * To avoid - as we used to - requiring an array with NBuffers entries to keep
 * track of local buffers, we use a small sequentially searched array
 * (PrivateRefCountArray) and an overflow hash table (PrivateRefCountHash) to
 * keep track of backend local pins.
 *
 * Until no more than REFCOUNT_ARRAY_ENTRIES buffers are pinned at once, all
 * refcounts are kept track of in the array; after that, new array entries
 * displace old ones into the hash table. That way a frequently used entry
 * can't get "stuck" in the hashtable while infrequent ones clog the array.
 *
 * Note that in most scenarios the number of pinned buffers will not exceed
 * REFCOUNT_ARRAY_ENTRIES.
 *
 *
 * To enter a buffer into the refcount tracking mechanism first reserve a free
 * entry using ReservePrivateRefCountEntry() and then later, if necessary,
 * fill it with NewPrivateRefCountEntry(). That split lets us avoid doing
 * memory allocations in NewPrivateRefCountEntry() which can be important
 * because in some scenarios it's called with a spinlock held...
 */
static struct PrivateRefCountEntry PrivateRefCountArray[REFCOUNT_ARRAY_ENTRIES];
static HTAB *PrivateRefCountHash = NULL;
static int32 PrivateRefCountOverflowed = 0;
static uint32 PrivateRefCountClock = 0;
static PrivateRefCountEntry *ReservedRefCountEntry = NULL;
 
static uint32 MaxProportionalPins;
 
static void ReservePrivateRefCountEntry(void);
static PrivateRefCountEntry *NewPrivateRefCountEntry(Buffer buffer);
static PrivateRefCountEntry *GetPrivateRefCountEntry(Buffer buffer, bool do_move);
static inline int32 GetPrivateRefCount(Buffer buffer);
static void ForgetPrivateRefCountEntry(PrivateRefCountEntry *ref);
 
/* ResourceOwner callbacks to hold in-progress I/Os and buffer pins */
static void ResOwnerReleaseBufferIO(Datum res);
static char *ResOwnerPrintBufferIO(Datum res);
static void ResOwnerReleaseBufferPin(Datum res);
static char *ResOwnerPrintBufferPin(Datum res);
 
const ResourceOwnerDesc buffer_io_resowner_desc =
{
    .name = "buffer io",
    .release_phase = RESOURCE_RELEASE_BEFORE_LOCKS,
    .release_priority = RELEASE_PRIO_BUFFER_IOS,
    .ReleaseResource = ResOwnerReleaseBufferIO,
    .DebugPrint = ResOwnerPrintBufferIO
};
 
const ResourceOwnerDesc buffer_pin_resowner_desc =
{
    .name = "buffer pin",
    .release_phase = RESOURCE_RELEASE_BEFORE_LOCKS,
    .release_priority = RELEASE_PRIO_BUFFER_PINS,
    .ReleaseResource = ResOwnerReleaseBufferPin,
    .DebugPrint = ResOwnerPrintBufferPin
};
 
/*
 * Ensure that the PrivateRefCountArray has sufficient space to store one more
 * entry. This has to be called before using NewPrivateRefCountEntry() to fill
 * a new entry - but it's perfectly fine to not use a reserved entry.
 */
static void
ReservePrivateRefCountEntry(void)
{
    /* Already reserved (or freed), nothing to do */
    if (ReservedRefCountEntry != NULL)
        return;
 
    /*
     * First search for a free entry the array, that'll be sufficient in the
     * majority of cases.
     */
    {
        int         i;
 
        for (i = 0; i < REFCOUNT_ARRAY_ENTRIES; i++)
        {
            PrivateRefCountEntry *res;
 
            res = &PrivateRefCountArray[i];
 
            if (res->buffer == InvalidBuffer)
            {
                ReservedRefCountEntry = res;
                return;
            }
        }
    }
 
    /*
     * No luck. All array entries are full. Move one array entry into the hash
     * table.
     */
    {
        /*
         * Move entry from the current clock position in the array into the
         * hashtable. Use that slot.
         */
        PrivateRefCountEntry *hashent;
        bool        found;
 
        /* select victim slot */
        ReservedRefCountEntry =
            &PrivateRefCountArray[PrivateRefCountClock++ % REFCOUNT_ARRAY_ENTRIES];
 
        /* Better be used, otherwise we shouldn't get here. */
        Assert(ReservedRefCountEntry->buffer != InvalidBuffer);
 
        /* enter victim array entry into hashtable */
        hashent = hash_search(PrivateRefCountHash,
                              &(ReservedRefCountEntry->buffer),
                              HASH_ENTER,
                              &found);
        Assert(!found);
        hashent->refcount = ReservedRefCountEntry->refcount;
 
        /* clear the now free array slot */
        ReservedRefCountEntry->buffer = InvalidBuffer;
        ReservedRefCountEntry->refcount = 0;
 
        PrivateRefCountOverflowed++;
    }
}
 
/*
 * Fill a previously reserved refcount entry.
 */
static PrivateRefCountEntry *
NewPrivateRefCountEntry(Buffer buffer)
{
    PrivateRefCountEntry *res;
 
    /* only allowed to be called when a reservation has been made */
    Assert(ReservedRefCountEntry != NULL);
 
    /* use up the reserved entry */
    res = ReservedRefCountEntry;
    ReservedRefCountEntry = NULL;
 
    /* and fill it */
    res->buffer = buffer;
    res->refcount = 0;
 
    return res;
}
 
/*
 * Return the PrivateRefCount entry for the passed buffer.
 *
 * Returns NULL if a buffer doesn't have a refcount entry. Otherwise, if
 * do_move is true, and the entry resides in the hashtable the entry is
 * optimized for frequent access by moving it to the array.
 */
static PrivateRefCountEntry *
GetPrivateRefCountEntry(Buffer buffer, bool do_move)
{
    PrivateRefCountEntry *res;
    int         i;
 
    Assert(BufferIsValid(buffer));
    Assert(!BufferIsLocal(buffer));
 
    /*
     * First search for references in the array, that'll be sufficient in the
     * majority of cases.
     */
    for (i = 0; i < REFCOUNT_ARRAY_ENTRIES; i++)
    {
        res = &PrivateRefCountArray[i];
 
        if (res->buffer == buffer)
            return res;
    }
 
    /*
     * By here we know that the buffer, if already pinned, isn't residing in
     * the array.
     *
     * Only look up the buffer in the hashtable if we've previously overflowed
     * into it.
     */
    if (PrivateRefCountOverflowed == 0)
        return NULL;
 
    res = hash_search(PrivateRefCountHash, &buffer, HASH_FIND, NULL);
 
    if (res == NULL)
        return NULL;
    else if (!do_move)
    {
        /* caller doesn't want us to move the hash entry into the array */
        return res;
    }
    else
    {
        /* move buffer from hashtable into the free array slot */
        bool        found;
        PrivateRefCountEntry *free;
 
        /* Ensure there's a free array slot */
        ReservePrivateRefCountEntry();
 
        /* Use up the reserved slot */
        Assert(ReservedRefCountEntry != NULL);
        free = ReservedRefCountEntry;
        ReservedRefCountEntry = NULL;
        Assert(free->buffer == InvalidBuffer);
 
        /* and fill it */
        free->buffer = buffer;
        free->refcount = res->refcount;
 
        /* delete from hashtable */
        hash_search(PrivateRefCountHash, &buffer, HASH_REMOVE, &found);
        Assert(found);
        Assert(PrivateRefCountOverflowed > 0);
        PrivateRefCountOverflowed--;
 
        return free;
    }
}
 
/*
 * Returns how many times the passed buffer is pinned by this backend.
 *
 * Only works for shared memory buffers!
 */
static inline int32
GetPrivateRefCount(Buffer buffer)
{
    PrivateRefCountEntry *ref;
 
    Assert(BufferIsValid(buffer));
    Assert(!BufferIsLocal(buffer));
 
    /*
     * Not moving the entry - that's ok for the current users, but we might
     * want to change this one day.
     */
    ref = GetPrivateRefCountEntry(buffer, false);
 
    if (ref == NULL)
        return 0;
    return ref->refcount;
}
 
/*
 * Release resources used to track the reference count of a buffer which we no
 * longer have pinned and don't want to pin again immediately.
 */
static void
ForgetPrivateRefCountEntry(PrivateRefCountEntry *ref)
{
    Assert(ref->refcount == 0);
 
    if (ref >= &PrivateRefCountArray[0] &&
        ref < &PrivateRefCountArray[REFCOUNT_ARRAY_ENTRIES])
    {
        ref->buffer = InvalidBuffer;
 
        /*
         * Mark the just used entry as reserved - in many scenarios that
         * allows us to avoid ever having to search the array/hash for free
         * entries.
         */
        ReservedRefCountEntry = ref;
    }
    else
    {
        bool        found;
        Buffer      buffer = ref->buffer;
 
        hash_search(PrivateRefCountHash, &buffer, HASH_REMOVE, &found);
        Assert(found);
        Assert(PrivateRefCountOverflowed > 0);
        PrivateRefCountOverflowed--;
    }
}
 
/*
 * BufferIsPinned
 *      True iff the buffer is pinned (also checks for valid buffer number).
 *
 *      NOTE: what we check here is that *this* backend holds a pin on
 *      the buffer.  We do not care whether some other backend does.
 */
#define BufferIsPinned(bufnum) \
( \
    !BufferIsValid(bufnum) ? \
        false \
    : \
        BufferIsLocal(bufnum) ? \
            (LocalRefCount[-(bufnum) - 1] > 0) \
        : \
    (GetPrivateRefCount(bufnum) > 0) \
)
 
 
static Buffer ReadBuffer_common(Relation rel,
                                SMgrRelation smgr, char smgr_persistence,
                                ForkNumber forkNum, BlockNumber blockNum,
                                ReadBufferMode mode, BufferAccessStrategy strategy);
static BlockNumber ExtendBufferedRelCommon(BufferManagerRelation bmr,
                                           ForkNumber fork,
                                           BufferAccessStrategy strategy,
                                           uint32 flags,
                                           uint32 extend_by,
                                           BlockNumber extend_upto,
                                           Buffer *buffers,
                                           uint32 *extended_by);
static BlockNumber ExtendBufferedRelShared(BufferManagerRelation bmr,
                                           ForkNumber fork,
                                           BufferAccessStrategy strategy,
                                           uint32 flags,
                                           uint32 extend_by,
                                           BlockNumber extend_upto,
                                           Buffer *buffers,
                                           uint32 *extended_by);
static bool PinBuffer(BufferDesc *buf, BufferAccessStrategy strategy);
static void PinBuffer_Locked(BufferDesc *buf);
static void UnpinBuffer(BufferDesc *buf);
static void UnpinBufferNoOwner(BufferDesc *buf);
static void BufferSync(int flags);
static uint32 WaitBufHdrUnlocked(BufferDesc *buf);
static int  SyncOneBuffer(int buf_id, bool skip_recently_used,
                          WritebackContext *wb_context);
static void WaitIO(BufferDesc *buf);
static void AbortBufferIO(Buffer buffer);
static void shared_buffer_write_error_callback(void *arg);
static void local_buffer_write_error_callback(void *arg);
static inline BufferDesc *BufferAlloc(SMgrRelation smgr,
                                      char relpersistence,
                                      ForkNumber forkNum,
                                      BlockNumber blockNum,
                                      BufferAccessStrategy strategy,
                                      bool *foundPtr, IOContext io_context);
static bool AsyncReadBuffers(ReadBuffersOperation *operation, int *nblocks_progress);
static void CheckReadBuffersOperation(ReadBuffersOperation *operation, bool is_complete);
static Buffer GetVictimBuffer(BufferAccessStrategy strategy, IOContext io_context);
static void FlushBuffer(BufferDesc *buf, SMgrRelation reln,
                        IOObject io_object, IOContext io_context);
static void FindAndDropRelationBuffers(RelFileLocator rlocator,
                                       ForkNumber forkNum,
                                       BlockNumber nForkBlock,
                                       BlockNumber firstDelBlock);
static void RelationCopyStorageUsingBuffer(RelFileLocator srclocator,
                                           RelFileLocator dstlocator,
                                           ForkNumber forkNum, bool permanent);
static void AtProcExit_Buffers(int code, Datum arg);
static void CheckForBufferLeaks(void);
#ifdef USE_ASSERT_CHECKING
static void AssertNotCatalogBufferLock(LWLock *lock, LWLockMode mode,
                                       void *unused_context);
#endif
static int  rlocator_comparator(const void *p1, const void *p2);
static inline int buffertag_comparator(const BufferTag *ba, const BufferTag *bb);
static inline int ckpt_buforder_comparator(const CkptSortItem *a, const CkptSortItem *b);
static int  ts_ckpt_progress_comparator(Datum a, Datum b, void *arg);
 
 
/*
 * Implementation of PrefetchBuffer() for shared buffers.
 */
PrefetchBufferResult
PrefetchSharedBuffer(SMgrRelation smgr_reln,
                     ForkNumber forkNum,
                     BlockNumber blockNum)
{
    PrefetchBufferResult result = {InvalidBuffer, false};
    BufferTag   newTag;         /* identity of requested block */
    uint32      newHash;        /* hash value for newTag */
    LWLock     *newPartitionLock;   /* buffer partition lock for it */
    int         buf_id;
 
    Assert(BlockNumberIsValid(blockNum));
 
    /* create a tag so we can lookup the buffer */
    InitBufferTag(&newTag, &smgr_reln->smgr_rlocator.locator,
                  forkNum, blockNum);
 
    /* determine its hash code and partition lock ID */
    newHash = BufTableHashCode(&newTag);
    newPartitionLock = BufMappingPartitionLock(newHash);
 
    /* see if the block is in the buffer pool already */
    LWLockAcquire(newPartitionLock, LW_SHARED);
    buf_id = BufTableLookup(&newTag, newHash);
    LWLockRelease(newPartitionLock);
 
    /* If not in buffers, initiate prefetch */
    if (buf_id < 0)
    {
#ifdef USE_PREFETCH
        /*
         * Try to initiate an asynchronous read.  This returns false in
         * recovery if the relation file doesn't exist.
         */
        if ((io_direct_flags & IO_DIRECT_DATA) == 0 &&
            smgrprefetch(smgr_reln, forkNum, blockNum, 1))
        {
            result.initiated_io = true;
        }
#endif                          /* USE_PREFETCH */
    }
    else
    {
        /*
         * Report the buffer it was in at that time.  The caller may be able
         * to avoid a buffer table lookup, but it's not pinned and it must be
         * rechecked!
         */
        result.recent_buffer = buf_id + 1;
    }
 
    /*
     * If the block *is* in buffers, we do nothing.  This is not really ideal:
     * the block might be just about to be evicted, which would be stupid
     * since we know we are going to need it soon.  But the only easy answer
     * is to bump the usage_count, which does not seem like a great solution:
     * when the caller does ultimately touch the block, usage_count would get
     * bumped again, resulting in too much favoritism for blocks that are
     * involved in a prefetch sequence. A real fix would involve some
     * additional per-buffer state, and it's not clear that there's enough of
     * a problem to justify that.
     */
 
    return result;
}
 
/*
 * PrefetchBuffer -- initiate asynchronous read of a block of a relation
 *
 * This is named by analogy to ReadBuffer but doesn't actually allocate a
 * buffer.  Instead it tries to ensure that a future ReadBuffer for the given
 * block will not be delayed by the I/O.  Prefetching is optional.
 *
 * There are three possible outcomes:
 *
 * 1.  If the block is already cached, the result includes a valid buffer that
 * could be used by the caller to avoid the need for a later buffer lookup, but
 * it's not pinned, so the caller must recheck it.
 *
 * 2.  If the kernel has been asked to initiate I/O, the initiated_io member is
 * true.  Currently there is no way to know if the data was already cached by
 * the kernel and therefore didn't really initiate I/O, and no way to know when
 * the I/O completes other than using synchronous ReadBuffer().
 *
 * 3.  Otherwise, the buffer wasn't already cached by PostgreSQL, and
 * USE_PREFETCH is not defined (this build doesn't support prefetching due to
 * lack of a kernel facility), direct I/O is enabled, or the underlying
 * relation file wasn't found and we are in recovery.  (If the relation file
 * wasn't found and we are not in recovery, an error is raised).
 */
PrefetchBufferResult
PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
{
    Assert(RelationIsValid(reln));
    Assert(BlockNumberIsValid(blockNum));
 
    if (RelationUsesLocalBuffers(reln))
    {
        /* see comments in ReadBufferExtended */
        if (RELATION_IS_OTHER_TEMP(reln))
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("cannot access temporary tables of other sessions")));
 
        /* pass it off to localbuf.c */
        return PrefetchLocalBuffer(RelationGetSmgr(reln), forkNum, blockNum);
    }
    else
    {
        /* pass it to the shared buffer version */
        return PrefetchSharedBuffer(RelationGetSmgr(reln), forkNum, blockNum);
    }
}
 
/*
 * ReadRecentBuffer -- try to pin a block in a recently observed buffer
 *
 * Compared to ReadBuffer(), this avoids a buffer mapping lookup when it's
 * successful.  Return true if the buffer is valid and still has the expected
 * tag.  In that case, the buffer is pinned and the usage count is bumped.
 */
bool
ReadRecentBuffer(RelFileLocator rlocator, ForkNumber forkNum, BlockNumber blockNum,
                 Buffer recent_buffer)
{
    BufferDesc *bufHdr;
    BufferTag   tag;
    uint32      buf_state;
    bool        have_private_ref;
 
    Assert(BufferIsValid(recent_buffer));
 
    ResourceOwnerEnlarge(CurrentResourceOwner);
    ReservePrivateRefCountEntry();
    InitBufferTag(&tag, &rlocator, forkNum, blockNum);
 
    if (BufferIsLocal(recent_buffer))
    {
        int         b = -recent_buffer - 1;
 
        bufHdr = GetLocalBufferDescriptor(b);
        buf_state = pg_atomic_read_u32(&bufHdr->state);
 
        /* Is it still valid and holding the right tag? */
        if ((buf_state & BM_VALID) && BufferTagsEqual(&tag, &bufHdr->tag))
        {
            PinLocalBuffer(bufHdr, true);
 
            pgBufferUsage.local_blks_hit++;
 
            return true;
        }
    }
    else
    {
        bufHdr = GetBufferDescriptor(recent_buffer - 1);
        have_private_ref = GetPrivateRefCount(recent_buffer) > 0;
 
        /*
         * Do we already have this buffer pinned with a private reference?  If
         * so, it must be valid and it is safe to check the tag without
         * locking.  If not, we have to lock the header first and then check.
         */
        if (have_private_ref)
            buf_state = pg_atomic_read_u32(&bufHdr->state);
        else
            buf_state = LockBufHdr(bufHdr);
 
        if ((buf_state & BM_VALID) && BufferTagsEqual(&tag, &bufHdr->tag))
        {
            /*
             * It's now safe to pin the buffer.  We can't pin first and ask
             * questions later, because it might confuse code paths like
             * InvalidateBuffer() if we pinned a random non-matching buffer.
             */
            if (have_private_ref)
                PinBuffer(bufHdr, NULL);    /* bump pin count */
            else
                PinBuffer_Locked(bufHdr);   /* pin for first time */
 
            pgBufferUsage.shared_blks_hit++;
 
            return true;
        }
 
        /* If we locked the header above, now unlock. */
        if (!have_private_ref)
            UnlockBufHdr(bufHdr, buf_state);
    }
 
    return false;
}
 
/*
 * ReadBuffer -- a shorthand for ReadBufferExtended, for reading from main
 *      fork with RBM_NORMAL mode and default strategy.
 */
Buffer
ReadBuffer(Relation reln, BlockNumber blockNum)
{
    return ReadBufferExtended(reln, MAIN_FORKNUM, blockNum, RBM_NORMAL, NULL);
}
 
/*
 * ReadBufferExtended -- returns a buffer containing the requested
 *      block of the requested relation.  If the blknum
 *      requested is P_NEW, extend the relation file and
 *      allocate a new block.  (Caller is responsible for
 *      ensuring that only one backend tries to extend a
 *      relation at the same time!)
 *
 * Returns: the buffer number for the buffer containing
 *      the block read.  The returned buffer has been pinned.
 *      Does not return on error --- elog's instead.
 *
 * Assume when this function is called, that reln has been opened already.
 *
 * In RBM_NORMAL mode, the page is read from disk, and the page header is
 * validated.  An error is thrown if the page header is not valid.  (But
 * note that an all-zero page is considered "valid"; see
 * PageIsVerified().)
 *
 * RBM_ZERO_ON_ERROR is like the normal mode, but if the page header is not
 * valid, the page is zeroed instead of throwing an error. This is intended
 * for non-critical data, where the caller is prepared to repair errors.
 *
 * In RBM_ZERO_AND_LOCK mode, if the page isn't in buffer cache already, it's
 * filled with zeros instead of reading it from disk.  Useful when the caller
 * is going to fill the page from scratch, since this saves I/O and avoids
 * unnecessary failure if the page-on-disk has corrupt page headers.
 * The page is returned locked to ensure that the caller has a chance to
 * initialize the page before it's made visible to others.
 * Caution: do not use this mode to read a page that is beyond the relation's
 * current physical EOF; that is likely to cause problems in md.c when
 * the page is modified and written out. P_NEW is OK, though.
 *
 * RBM_ZERO_AND_CLEANUP_LOCK is the same as RBM_ZERO_AND_LOCK, but acquires
 * a cleanup-strength lock on the page.
 *
 * RBM_NORMAL_NO_LOG mode is treated the same as RBM_NORMAL here.
 *
 * If strategy is not NULL, a nondefault buffer access strategy is used.
 * See buffer/README for details.
 */
inline Buffer
ReadBufferExtended(Relation reln, ForkNumber forkNum, BlockNumber blockNum,
                   ReadBufferMode mode, BufferAccessStrategy strategy)
{
    Buffer      buf;
 
    /*
     * Reject attempts to read non-local temporary relations; we would be
     * likely to get wrong data since we have no visibility into the owning
     * session's local buffers.
     */
    if (RELATION_IS_OTHER_TEMP(reln))
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("cannot access temporary tables of other sessions")));
 
    /*
     * Read the buffer, and update pgstat counters to reflect a cache hit or
     * miss.
     */
    buf = ReadBuffer_common(reln, RelationGetSmgr(reln), 0,
                            forkNum, blockNum, mode, strategy);
 
    return buf;
}
 
 
/*
 * ReadBufferWithoutRelcache -- like ReadBufferExtended, but doesn't require
 *      a relcache entry for the relation.
 *
 * Pass permanent = true for a RELPERSISTENCE_PERMANENT relation, and
 * permanent = false for a RELPERSISTENCE_UNLOGGED relation. This function
 * cannot be used for temporary relations (and making that work might be
 * difficult, unless we only want to read temporary relations for our own
 * ProcNumber).
 */
Buffer
ReadBufferWithoutRelcache(RelFileLocator rlocator, ForkNumber forkNum,
                          BlockNumber blockNum, ReadBufferMode mode,
                          BufferAccessStrategy strategy, bool permanent)
{
    SMgrRelation smgr = smgropen(rlocator, INVALID_PROC_NUMBER);
 
    return ReadBuffer_common(NULL, smgr,
                             permanent ? RELPERSISTENCE_PERMANENT : RELPERSISTENCE_UNLOGGED,
                             forkNum, blockNum,
                             mode, strategy);
}
 
/*
 * Convenience wrapper around ExtendBufferedRelBy() extending by one block.
 */
Buffer
ExtendBufferedRel(BufferManagerRelation bmr,
                  ForkNumber forkNum,
                  BufferAccessStrategy strategy,
                  uint32 flags)
{
    Buffer      buf;
    uint32      extend_by = 1;
 
    ExtendBufferedRelBy(bmr, forkNum, strategy, flags, extend_by,
                        &buf, &extend_by);
 
    return buf;
}
 
/*
 * Extend relation by multiple blocks.
 *
 * Tries to extend the relation by extend_by blocks. Depending on the
 * availability of resources the relation may end up being extended by a
 * smaller number of pages (unless an error is thrown, always by at least one
 * page). *extended_by is updated to the number of pages the relation has been
 * extended to.
 *
 * buffers needs to be an array that is at least extend_by long. Upon
 * completion, the first extend_by array elements will point to a pinned
 * buffer.
 *
 * If EB_LOCK_FIRST is part of flags, the first returned buffer is
 * locked. This is useful for callers that want a buffer that is guaranteed to
 * be empty.
 */
BlockNumber
ExtendBufferedRelBy(BufferManagerRelation bmr,
                    ForkNumber fork,
                    BufferAccessStrategy strategy,
                    uint32 flags,
                    uint32 extend_by,
                    Buffer *buffers,
                    uint32 *extended_by)
{
    Assert((bmr.rel != NULL) != (bmr.smgr != NULL));
    Assert(bmr.smgr == NULL || bmr.relpersistence != 0);
    Assert(extend_by > 0);
 
    if (bmr.smgr == NULL)
    {
        bmr.smgr = RelationGetSmgr(bmr.rel);
        bmr.relpersistence = bmr.rel->rd_rel->relpersistence;
    }
 
    return ExtendBufferedRelCommon(bmr, fork, strategy, flags,
                                   extend_by, InvalidBlockNumber,
                                   buffers, extended_by);
}
 
/*
 * Extend the relation so it is at least extend_to blocks large, return buffer
 * (extend_to - 1).
 *
 * This is useful for callers that want to write a specific page, regardless
 * of the current size of the relation (e.g. useful for visibilitymap and for
 * crash recovery).
 */
Buffer
ExtendBufferedRelTo(BufferManagerRelation bmr,
                    ForkNumber fork,
                    BufferAccessStrategy strategy,
                    uint32 flags,
                    BlockNumber extend_to,
                    ReadBufferMode mode)
{
    BlockNumber current_size;
    uint32      extended_by = 0;
    Buffer      buffer = InvalidBuffer;
    Buffer      buffers[64];
 
    Assert((bmr.rel != NULL) != (bmr.smgr != NULL));
    Assert(bmr.smgr == NULL || bmr.relpersistence != 0);
    Assert(extend_to != InvalidBlockNumber && extend_to > 0);
 
    if (bmr.smgr == NULL)
    {
        bmr.smgr = RelationGetSmgr(bmr.rel);
        bmr.relpersistence = bmr.rel->rd_rel->relpersistence;
    }
 
    /*
     * If desired, create the file if it doesn't exist.  If
     * smgr_cached_nblocks[fork] is positive then it must exist, no need for
     * an smgrexists call.
     */
    if ((flags & EB_CREATE_FORK_IF_NEEDED) &&
        (bmr.smgr->smgr_cached_nblocks[fork] == 0 ||
         bmr.smgr->smgr_cached_nblocks[fork] == InvalidBlockNumber) &&
        !smgrexists(bmr.smgr, fork))
    {
        LockRelationForExtension(bmr.rel, ExclusiveLock);
 
        /* recheck, fork might have been created concurrently */
        if (!smgrexists(bmr.smgr, fork))
            smgrcreate(bmr.smgr, fork, flags & EB_PERFORMING_RECOVERY);
 
        UnlockRelationForExtension(bmr.rel, ExclusiveLock);
    }
 
    /*
     * If requested, invalidate size cache, so that smgrnblocks asks the
     * kernel.
     */
    if (flags & EB_CLEAR_SIZE_CACHE)
        bmr.smgr->smgr_cached_nblocks[fork] = InvalidBlockNumber;
 
    /*
     * Estimate how many pages we'll need to extend by. This avoids acquiring
     * unnecessarily many victim buffers.
     */
    current_size = smgrnblocks(bmr.smgr, fork);
 
    /*
     * Since no-one else can be looking at the page contents yet, there is no
     * difference between an exclusive lock and a cleanup-strength lock. Note
     * that we pass the original mode to ReadBuffer_common() below, when
     * falling back to reading the buffer to a concurrent relation extension.
     */
    if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
        flags |= EB_LOCK_TARGET;
 
    while (current_size < extend_to)
    {
        uint32      num_pages = lengthof(buffers);
        BlockNumber first_block;
 
        if ((uint64) current_size + num_pages > extend_to)
            num_pages = extend_to - current_size;
 
        first_block = ExtendBufferedRelCommon(bmr, fork, strategy, flags,
                                              num_pages, extend_to,
                                              buffers, &extended_by);
 
        current_size = first_block + extended_by;
        Assert(num_pages != 0 || current_size >= extend_to);
 
        for (uint32 i = 0; i < extended_by; i++)
        {
            if (first_block + i != extend_to - 1)
                ReleaseBuffer(buffers[i]);
            else
                buffer = buffers[i];
        }
    }
 
    /*
     * It's possible that another backend concurrently extended the relation.
     * In that case read the buffer.
     *
     * XXX: Should we control this via a flag?
     */
    if (buffer == InvalidBuffer)
    {
        Assert(extended_by == 0);
        buffer = ReadBuffer_common(bmr.rel, bmr.smgr, bmr.relpersistence,
                                   fork, extend_to - 1, mode, strategy);
    }
 
    return buffer;
}
 
/*
 * Lock and optionally zero a buffer, as part of the implementation of
 * RBM_ZERO_AND_LOCK or RBM_ZERO_AND_CLEANUP_LOCK.  The buffer must be already
 * pinned.  If the buffer is not already valid, it is zeroed and made valid.
 */
static void
ZeroAndLockBuffer(Buffer buffer, ReadBufferMode mode, bool already_valid)
{
    BufferDesc *bufHdr;
    bool        need_to_zero;
    bool        isLocalBuf = BufferIsLocal(buffer);
 
    Assert(mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK);
 
    if (already_valid)
    {
        /*
         * If the caller already knew the buffer was valid, we can skip some
         * header interaction.  The caller just wants to lock the buffer.
         */
        need_to_zero = false;
    }
    else if (isLocalBuf)
    {
        /* Simple case for non-shared buffers. */
        bufHdr = GetLocalBufferDescriptor(-buffer - 1);
        need_to_zero = StartLocalBufferIO(bufHdr, true, false);
    }
    else
    {
        /*
         * Take BM_IO_IN_PROGRESS, or discover that BM_VALID has been set
         * concurrently.  Even though we aren't doing I/O, that ensures that
         * we don't zero a page that someone else has pinned.  An exclusive
         * content lock wouldn't be enough, because readers are allowed to
         * drop the content lock after determining that a tuple is visible
         * (see buffer access rules in README).
         */
        bufHdr = GetBufferDescriptor(buffer - 1);
        need_to_zero = StartBufferIO(bufHdr, true, false);
    }
 
    if (need_to_zero)
    {
        memset(BufferGetPage(buffer), 0, BLCKSZ);
 
        /*
         * Grab the buffer content lock before marking the page as valid, to
         * make sure that no other backend sees the zeroed page before the
         * caller has had a chance to initialize it.
         *
         * Since no-one else can be looking at the page contents yet, there is
         * no difference between an exclusive lock and a cleanup-strength
         * lock. (Note that we cannot use LockBuffer() or
         * LockBufferForCleanup() here, because they assert that the buffer is
         * already valid.)
         */
        if (!isLocalBuf)
            LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_EXCLUSIVE);
 
        /* Set BM_VALID, terminate IO, and wake up any waiters */
        if (isLocalBuf)
            TerminateLocalBufferIO(bufHdr, false, BM_VALID, false);
        else
            TerminateBufferIO(bufHdr, false, BM_VALID, true, false);
    }
    else if (!isLocalBuf)
    {
        /*
         * The buffer is valid, so we can't zero it.  The caller still expects
         * the page to be locked on return.
         */
        if (mode == RBM_ZERO_AND_LOCK)
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
        else
            LockBufferForCleanup(buffer);
    }
}
 
/*
 * Pin a buffer for a given block.  *foundPtr is set to true if the block was
 * already present, or false if more work is required to either read it in or
 * zero it.
 */
static pg_attribute_always_inline Buffer
PinBufferForBlock(Relation rel,
                  SMgrRelation smgr,
                  char persistence,
                  ForkNumber forkNum,
                  BlockNumber blockNum,
                  BufferAccessStrategy strategy,
                  bool *foundPtr)
{
    BufferDesc *bufHdr;
    IOContext   io_context;
    IOObject    io_object;
 
    Assert(blockNum != P_NEW);
 
    /* Persistence should be set before */
    Assert((persistence == RELPERSISTENCE_TEMP ||
            persistence == RELPERSISTENCE_PERMANENT ||
            persistence == RELPERSISTENCE_UNLOGGED));
 
    if (persistence == RELPERSISTENCE_TEMP)
    {
        io_context = IOCONTEXT_NORMAL;
        io_object = IOOBJECT_TEMP_RELATION;
    }
    else
    {
        io_context = IOContextForStrategy(strategy);
        io_object = IOOBJECT_RELATION;
    }
 
    TRACE_POSTGRESQL_BUFFER_READ_START(forkNum, blockNum,
                                       smgr->smgr_rlocator.locator.spcOid,
                                       smgr->smgr_rlocator.locator.dbOid,
                                       smgr->smgr_rlocator.locator.relNumber,
                                       smgr->smgr_rlocator.backend);
 
    if (persistence == RELPERSISTENCE_TEMP)
    {
        bufHdr = LocalBufferAlloc(smgr, forkNum, blockNum, foundPtr);
        if (*foundPtr)
            pgBufferUsage.local_blks_hit++;
    }
    else
    {
        bufHdr = BufferAlloc(smgr, persistence, forkNum, blockNum,
                             strategy, foundPtr, io_context);
        if (*foundPtr)
            pgBufferUsage.shared_blks_hit++;
    }
    if (rel)
    {
        /*
         * While pgBufferUsage's "read" counter isn't bumped unless we reach
         * WaitReadBuffers() (so, not for hits, and not for buffers that are
         * zeroed instead), the per-relation stats always count them.
         */
        pgstat_count_buffer_read(rel);
        if (*foundPtr)
            pgstat_count_buffer_hit(rel);
    }
    if (*foundPtr)
    {
        pgstat_count_io_op(io_object, io_context, IOOP_HIT, 1, 0);
        if (VacuumCostActive)
            VacuumCostBalance += VacuumCostPageHit;
 
        TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
                                          smgr->smgr_rlocator.locator.spcOid,
                                          smgr->smgr_rlocator.locator.dbOid,
                                          smgr->smgr_rlocator.locator.relNumber,
                                          smgr->smgr_rlocator.backend,
                                          true);
    }
 
    return BufferDescriptorGetBuffer(bufHdr);
}
 
/*
 * ReadBuffer_common -- common logic for all ReadBuffer variants
 *
 * smgr is required, rel is optional unless using P_NEW.
 */
static pg_attribute_always_inline Buffer
ReadBuffer_common(Relation rel, SMgrRelation smgr, char smgr_persistence,
                  ForkNumber forkNum,
                  BlockNumber blockNum, ReadBufferMode mode,
                  BufferAccessStrategy strategy)
{
    ReadBuffersOperation operation;
    Buffer      buffer;
    int         flags;
    char        persistence;
 
    /*
     * Backward compatibility path, most code should use ExtendBufferedRel()
     * instead, as acquiring the extension lock inside ExtendBufferedRel()
     * scales a lot better.
     */
    if (unlikely(blockNum == P_NEW))
    {
        uint32      flags = EB_SKIP_EXTENSION_LOCK;
 
        /*
         * Since no-one else can be looking at the page contents yet, there is
         * no difference between an exclusive lock and a cleanup-strength
         * lock.
         */
        if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
            flags |= EB_LOCK_FIRST;
 
        return ExtendBufferedRel(BMR_REL(rel), forkNum, strategy, flags);
    }
 
    if (rel)
        persistence = rel->rd_rel->relpersistence;
    else
        persistence = smgr_persistence;
 
    if (unlikely(mode == RBM_ZERO_AND_CLEANUP_LOCK ||
                 mode == RBM_ZERO_AND_LOCK))
    {
        bool        found;
 
        buffer = PinBufferForBlock(rel, smgr, persistence,
                                   forkNum, blockNum, strategy, &found);
        ZeroAndLockBuffer(buffer, mode, found);
        return buffer;
    }
 
    /*
     * Signal that we are going to immediately wait. If we're immediately
     * waiting, there is no benefit in actually executing the IO
     * asynchronously, it would just add dispatch overhead.
     */
    flags = READ_BUFFERS_SYNCHRONOUSLY;
    if (mode == RBM_ZERO_ON_ERROR)
        flags |= READ_BUFFERS_ZERO_ON_ERROR;
    operation.smgr = smgr;
    operation.rel = rel;
    operation.persistence = persistence;
    operation.forknum = forkNum;
    operation.strategy = strategy;
    if (StartReadBuffer(&operation,
                        &buffer,
                        blockNum,
                        flags))
        WaitReadBuffers(&operation);
 
    return buffer;
}
 
static pg_attribute_always_inline bool
StartReadBuffersImpl(ReadBuffersOperation *operation,
                     Buffer *buffers,
                     BlockNumber blockNum,
                     int *nblocks,
                     int flags,
                     bool allow_forwarding)
{
    int         actual_nblocks = *nblocks;
    int         maxcombine = 0;
    bool        did_start_io;
 
    Assert(*nblocks == 1 || allow_forwarding);
    Assert(*nblocks > 0);
    Assert(*nblocks <= MAX_IO_COMBINE_LIMIT);
 
    for (int i = 0; i < actual_nblocks; ++i)
    {
        bool        found;
 
        if (allow_forwarding && buffers[i] != InvalidBuffer)
        {
            BufferDesc *bufHdr;
 
            /*
             * This is a buffer that was pinned by an earlier call to
             * StartReadBuffers(), but couldn't be handled in one operation at
             * that time.  The operation was split, and the caller has passed
             * an already pinned buffer back to us to handle the rest of the
             * operation.  It must continue at the expected block number.
             */
            Assert(BufferGetBlockNumber(buffers[i]) == blockNum + i);
 
            /*
             * It might be an already valid buffer (a hit) that followed the
             * final contiguous block of an earlier I/O (a miss) marking the
             * end of it, or a buffer that some other backend has since made
             * valid by performing the I/O for us, in which case we can handle
             * it as a hit now.  It is safe to check for a BM_VALID flag with
             * a relaxed load, because we got a fresh view of it while pinning
             * it in the previous call.
             *
             * On the other hand if we don't see BM_VALID yet, it must be an
             * I/O that was split by the previous call and we need to try to
             * start a new I/O from this block.  We're also racing against any
             * other backend that might start the I/O or even manage to mark
             * it BM_VALID after this check, but StartBufferIO() will handle
             * those cases.
             */
            if (BufferIsLocal(buffers[i]))
                bufHdr = GetLocalBufferDescriptor(-buffers[i] - 1);
            else
                bufHdr = GetBufferDescriptor(buffers[i] - 1);
            Assert(pg_atomic_read_u32(&bufHdr->state) & BM_TAG_VALID);
            found = pg_atomic_read_u32(&bufHdr->state) & BM_VALID;
        }
        else
        {
            buffers[i] = PinBufferForBlock(operation->rel,
                                           operation->smgr,
                                           operation->persistence,
                                           operation->forknum,
                                           blockNum + i,
                                           operation->strategy,
                                           &found);
        }
 
        if (found)
        {
            /*
             * We have a hit.  If it's the first block in the requested range,
             * we can return it immediately and report that WaitReadBuffers()
             * does not need to be called.  If the initial value of *nblocks
             * was larger, the caller will have to call again for the rest.
             */
            if (i == 0)
            {
                *nblocks = 1;
 
#ifdef USE_ASSERT_CHECKING
 
                /*
                 * Initialize enough of ReadBuffersOperation to make
                 * CheckReadBuffersOperation() work. Outside of assertions
                 * that's not necessary when no IO is issued.
                 */
                operation->buffers = buffers;
                operation->blocknum = blockNum;
                operation->nblocks = 1;
                operation->nblocks_done = 1;
                CheckReadBuffersOperation(operation, true);
#endif
                return false;
            }
 
            /*
             * Otherwise we already have an I/O to perform, but this block
             * can't be included as it is already valid.  Split the I/O here.
             * There may or may not be more blocks requiring I/O after this
             * one, we haven't checked, but they can't be contiguous with this
             * one in the way.  We'll leave this buffer pinned, forwarding it
             * to the next call, avoiding the need to unpin it here and re-pin
             * it in the next call.
             */
            actual_nblocks = i;
            break;
        }
        else
        {
            /*
             * Check how many blocks we can cover with the same IO. The smgr
             * implementation might e.g. be limited due to a segment boundary.
             */
            if (i == 0 && actual_nblocks > 1)
            {
                maxcombine = smgrmaxcombine(operation->smgr,
                                            operation->forknum,
                                            blockNum);
                if (unlikely(maxcombine < actual_nblocks))
                {
                    elog(DEBUG2, "limiting nblocks at %u from %u to %u",
                         blockNum, actual_nblocks, maxcombine);
                    actual_nblocks = maxcombine;
                }
            }
        }
    }
    *nblocks = actual_nblocks;
 
    /* Populate information needed for I/O. */
    operation->buffers = buffers;
    operation->blocknum = blockNum;
    operation->flags = flags;
    operation->nblocks = actual_nblocks;
    operation->nblocks_done = 0;
    pgaio_wref_clear(&operation->io_wref);
 
    /*
     * When using AIO, start the IO in the background. If not, issue prefetch
     * requests if desired by the caller.
     *
     * The reason we have a dedicated path for IOMETHOD_SYNC here is to
     * de-risk the introduction of AIO somewhat. It's a large architectural
     * change, with lots of chances for unanticipated performance effects.
     *
     * Use of IOMETHOD_SYNC already leads to not actually performing IO
     * asynchronously, but without the check here we'd execute IO earlier than
     * we used to. Eventually this IOMETHOD_SYNC specific path should go away.
     */
    if (io_method != IOMETHOD_SYNC)
    {
        /*
         * Try to start IO asynchronously. It's possible that no IO needs to
         * be started, if another backend already performed the IO.
         *
         * Note that if an IO is started, it might not cover the entire
         * requested range, e.g. because an intermediary block has been read
         * in by another backend.  In that case any "trailing" buffers we
         * already pinned above will be "forwarded" by read_stream.c to the
         * next call to StartReadBuffers().
         *
         * This is signalled to the caller by decrementing *nblocks *and*
         * reducing operation->nblocks. The latter is done here, but not below
         * WaitReadBuffers(), as in WaitReadBuffers() we can't "shorten" the
         * overall read size anymore, we need to retry until done in its
         * entirety or until failed.
         */
        did_start_io = AsyncReadBuffers(operation, nblocks);
 
        operation->nblocks = *nblocks;
    }
    else
    {
        operation->flags |= READ_BUFFERS_SYNCHRONOUSLY;
 
        if (flags & READ_BUFFERS_ISSUE_ADVICE)
        {
            /*
             * In theory we should only do this if PinBufferForBlock() had to
             * allocate new buffers above.  That way, if two calls to
             * StartReadBuffers() were made for the same blocks before
             * WaitReadBuffers(), only the first would issue the advice.
             * That'd be a better simulation of true asynchronous I/O, which
             * would only start the I/O once, but isn't done here for
             * simplicity.
             */
            smgrprefetch(operation->smgr,
                         operation->forknum,
                         blockNum,
                         actual_nblocks);
        }
 
        /*
         * Indicate that WaitReadBuffers() should be called. WaitReadBuffers()
         * will initiate the necessary IO.
         */
        did_start_io = true;
    }
 
    CheckReadBuffersOperation(operation, !did_start_io);
 
    return did_start_io;
}
 
/*
 * Begin reading a range of blocks beginning at blockNum and extending for
 * *nblocks.  *nblocks and the buffers array are in/out parameters.  On entry,
 * the buffers elements covered by *nblocks must hold either InvalidBuffer or
 * buffers forwarded by an earlier call to StartReadBuffers() that was split
 * and is now being continued.  On return, *nblocks holds the number of blocks
 * accepted by this operation.  If it is less than the original number then
 * this operation has been split, but buffer elements up to the original
 * requested size may hold forwarded buffers to be used for a continuing
 * operation.  The caller must either start a new I/O beginning at the block
 * immediately following the blocks accepted by this call and pass those
 * buffers back in, or release them if it chooses not to.  It shouldn't make
 * any other use of or assumptions about forwarded buffers.
 *
 * If false is returned, no I/O is necessary and the buffers covered by
 * *nblocks on exit are valid and ready to be accessed.  If true is returned,
 * an I/O has been started, and WaitReadBuffers() must be called with the same
 * operation object before the buffers covered by *nblocks on exit can be
 * accessed.  Along with the operation object, the caller-supplied array of
 * buffers must remain valid until WaitReadBuffers() is called, and any
 * forwarded buffers must also be preserved for a continuing call unless
 * they are explicitly released.
 *
 * Currently the I/O is only started with optional operating system advice if
 * requested by the caller with READ_BUFFERS_ISSUE_ADVICE, and the real I/O
 * happens synchronously in WaitReadBuffers().  In future work, true I/O could
 * be initiated here.
 */
bool
StartReadBuffers(ReadBuffersOperation *operation,
                 Buffer *buffers,
                 BlockNumber blockNum,
                 int *nblocks,
                 int flags)
{
    return StartReadBuffersImpl(operation, buffers, blockNum, nblocks, flags,
                                true /* expect forwarded buffers */ );
}
 
/*
 * Single block version of the StartReadBuffers().  This might save a few
 * instructions when called from another translation unit, because it is
 * specialized for nblocks == 1.
 *
 * This version does not support "forwarded" buffers: they cannot be created
 * by reading only one block and *buffer is ignored on entry.
 */
bool
StartReadBuffer(ReadBuffersOperation *operation,
                Buffer *buffer,
                BlockNumber blocknum,
                int flags)
{
    int         nblocks = 1;
    bool        result;
 
    result = StartReadBuffersImpl(operation, buffer, blocknum, &nblocks, flags,
                                  false /* single block, no forwarding */ );
    Assert(nblocks == 1);       /* single block can't be short */
 
    return result;
}
 
/*
 * Perform sanity checks on the ReadBuffersOperation.
 */
static void
CheckReadBuffersOperation(ReadBuffersOperation *operation, bool is_complete)
{
#ifdef USE_ASSERT_CHECKING
    Assert(operation->nblocks_done <= operation->nblocks);
    Assert(!is_complete || operation->nblocks == operation->nblocks_done);
 
    for (int i = 0; i < operation->nblocks; i++)
    {
        Buffer      buffer = operation->buffers[i];
        BufferDesc *buf_hdr = BufferIsLocal(buffer) ?
            GetLocalBufferDescriptor(-buffer - 1) :
            GetBufferDescriptor(buffer - 1);
 
        Assert(BufferGetBlockNumber(buffer) == operation->blocknum + i);
        Assert(pg_atomic_read_u32(&buf_hdr->state) & BM_TAG_VALID);
 
        if (i < operation->nblocks_done)
            Assert(pg_atomic_read_u32(&buf_hdr->state) & BM_VALID);
    }
#endif
}
 
/* helper for ReadBuffersCanStartIO(), to avoid repetition */
static inline bool
ReadBuffersCanStartIOOnce(Buffer buffer, bool nowait)
{
    if (BufferIsLocal(buffer))
        return StartLocalBufferIO(GetLocalBufferDescriptor(-buffer - 1),
                                  true, nowait);
    else
        return StartBufferIO(GetBufferDescriptor(buffer - 1), true, nowait);
}
 
/*
 * Helper for AsyncReadBuffers that tries to get the buffer ready for IO.
 */
static inline bool
ReadBuffersCanStartIO(Buffer buffer, bool nowait)
{
    /*
     * If this backend currently has staged IO, we need to submit the pending
     * IO before waiting for the right to issue IO, to avoid the potential for
     * deadlocks (and, more commonly, unnecessary delays for other backends).
     */
    if (!nowait && pgaio_have_staged())
    {
        if (ReadBuffersCanStartIOOnce(buffer, true))
            return true;
 
        /*
         * Unfortunately StartBufferIO() returning false doesn't allow to
         * distinguish between the buffer already being valid and IO already
         * being in progress. Since IO already being in progress is quite
         * rare, this approach seems fine.
         */
        pgaio_submit_staged();
    }
 
    return ReadBuffersCanStartIOOnce(buffer, nowait);
}
 
/*
 * Helper for WaitReadBuffers() that processes the results of a readv
 * operation, raising an error if necessary.
 */
static void
ProcessReadBuffersResult(ReadBuffersOperation *operation)
{
    PgAioReturn *aio_ret = &operation->io_return;
    PgAioResultStatus rs = aio_ret->result.status;
    int         newly_read_blocks = 0;
 
    Assert(pgaio_wref_valid(&operation->io_wref));
    Assert(aio_ret->result.status != PGAIO_RS_UNKNOWN);
 
    /*
     * SMGR reports the number of blocks successfully read as the result of
     * the IO operation. Thus we can simply add that to ->nblocks_done.
     */
 
    if (likely(rs != PGAIO_RS_ERROR))
        newly_read_blocks = aio_ret->result.result;
 
    if (rs == PGAIO_RS_ERROR || rs == PGAIO_RS_WARNING)
        pgaio_result_report(aio_ret->result, &aio_ret->target_data,
                            rs == PGAIO_RS_ERROR ? ERROR : WARNING);
    else if (aio_ret->result.status == PGAIO_RS_PARTIAL)
    {
        /*
         * We'll retry, so we just emit a debug message to the server log (or
         * not even that in prod scenarios).
         */
        pgaio_result_report(aio_ret->result, &aio_ret->target_data, DEBUG1);
        elog(DEBUG3, "partial read, will retry");
    }
 
    Assert(newly_read_blocks > 0);
    Assert(newly_read_blocks <= MAX_IO_COMBINE_LIMIT);
 
    operation->nblocks_done += newly_read_blocks;
 
    Assert(operation->nblocks_done <= operation->nblocks);
}
 
void
WaitReadBuffers(ReadBuffersOperation *operation)
{
    PgAioReturn *aio_ret = &operation->io_return;
    IOContext   io_context;
    IOObject    io_object;
 
    if (operation->persistence == RELPERSISTENCE_TEMP)
    {
        io_context = IOCONTEXT_NORMAL;
        io_object = IOOBJECT_TEMP_RELATION;
    }
    else
    {
        io_context = IOContextForStrategy(operation->strategy);
        io_object = IOOBJECT_RELATION;
    }
 
    /*
     * If we get here without an IO operation having been issued, the
     * io_method == IOMETHOD_SYNC path must have been used. Otherwise the
     * caller should not have called WaitReadBuffers().
     *
     * In the case of IOMETHOD_SYNC, we start - as we used to before the
     * introducing of AIO - the IO in WaitReadBuffers(). This is done as part
     * of the retry logic below, no extra code is required.
     *
     * This path is expected to eventually go away.
     */
    if (!pgaio_wref_valid(&operation->io_wref) && io_method != IOMETHOD_SYNC)
        elog(ERROR, "waiting for read operation that didn't read");
 
    /*
     * To handle partial reads, and IOMETHOD_SYNC, we re-issue IO until we're
     * done. We may need multiple retries, not just because we could get
     * multiple partial reads, but also because some of the remaining
     * to-be-read buffers may have been read in by other backends, limiting
     * the IO size.
     */
    while (true)
    {
        int         ignored_nblocks_progress;
 
        CheckReadBuffersOperation(operation, false);
 
        /*
         * If there is an IO associated with the operation, we may need to
         * wait for it.
         */
        if (pgaio_wref_valid(&operation->io_wref))
        {
            /*
             * Track the time spent waiting for the IO to complete. As
             * tracking a wait even if we don't actually need to wait
             *
             * a) is not cheap, due to the timestamping overhead
             *
             * b) reports some time as waiting, even if we never waited
             *
             * we first check if we already know the IO is complete.
             */
            if (aio_ret->result.status == PGAIO_RS_UNKNOWN &&
                !pgaio_wref_check_done(&operation->io_wref))
            {
                instr_time  io_start = pgstat_prepare_io_time(track_io_timing);
 
                pgaio_wref_wait(&operation->io_wref);
 
                /*
                 * The IO operation itself was already counted earlier, in
                 * AsyncReadBuffers(), this just accounts for the wait time.
                 */
                pgstat_count_io_op_time(io_object, io_context, IOOP_READ,
                                        io_start, 0, 0);
            }
            else
            {
                Assert(pgaio_wref_check_done(&operation->io_wref));
            }
 
            /*
             * We now are sure the IO completed. Check the results. This
             * includes reporting on errors if there were any.
             */
            ProcessReadBuffersResult(operation);
        }
 
        /*
         * Most of the time, the one IO we already started, will read in
         * everything.  But we need to deal with partial reads and buffers not
         * needing IO anymore.
         */
        if (operation->nblocks_done == operation->nblocks)
            break;
 
        CHECK_FOR_INTERRUPTS();
 
        /*
         * This may only complete the IO partially, either because some
         * buffers were already valid, or because of a partial read.
         *
         * NB: In contrast to after the AsyncReadBuffers() call in
         * StartReadBuffers(), we do *not* reduce
         * ReadBuffersOperation->nblocks here, callers expect the full
         * operation to be completed at this point (as more operations may
         * have been queued).
         */
        AsyncReadBuffers(operation, &ignored_nblocks_progress);
    }
 
    CheckReadBuffersOperation(operation, true);
 
    /* NB: READ_DONE tracepoint was already executed in completion callback */
}
 
/*
 * Initiate IO for the ReadBuffersOperation
 *
 * This function only starts a single IO at a time. The size of the IO may be
 * limited to below the to-be-read blocks, if one of the buffers has
 * concurrently been read in. If the first to-be-read buffer is already valid,
 * no IO will be issued.
 *
 * To support retries after partial reads, the first operation->nblocks_done
 * buffers are skipped.
 *
 * On return *nblocks_progress is updated to reflect the number of buffers
 * affected by the call. If the first buffer is valid, *nblocks_progress is
 * set to 1 and operation->nblocks_done is incremented.
 *
 * Returns true if IO was initiated, false if no IO was necessary.
 */
static bool
AsyncReadBuffers(ReadBuffersOperation *operation, int *nblocks_progress)
{
    Buffer     *buffers = &operation->buffers[0];
    int         flags = operation->flags;
    BlockNumber blocknum = operation->blocknum;
    ForkNumber  forknum = operation->forknum;
    char        persistence = operation->persistence;
    int16       nblocks_done = operation->nblocks_done;
    Buffer     *io_buffers = &operation->buffers[nblocks_done];
    int         io_buffers_len = 0;
    PgAioHandle *ioh;
    uint32      ioh_flags = 0;
    void       *io_pages[MAX_IO_COMBINE_LIMIT];
    IOContext   io_context;
    IOObject    io_object;
    bool        did_start_io;
 
    /*
     * When this IO is executed synchronously, either because the caller will
     * immediately block waiting for the IO or because IOMETHOD_SYNC is used,
     * the AIO subsystem needs to know.
     */
    if (flags & READ_BUFFERS_SYNCHRONOUSLY)
        ioh_flags |= PGAIO_HF_SYNCHRONOUS;
 
    if (persistence == RELPERSISTENCE_TEMP)
    {
        io_context = IOCONTEXT_NORMAL;
        io_object = IOOBJECT_TEMP_RELATION;
        ioh_flags |= PGAIO_HF_REFERENCES_LOCAL;
    }
    else
    {
        io_context = IOContextForStrategy(operation->strategy);
        io_object = IOOBJECT_RELATION;
    }
 
    /*
     * If zero_damaged_pages is enabled, add the READ_BUFFERS_ZERO_ON_ERROR
     * flag. The reason for that is that, hopefully, zero_damaged_pages isn't
     * set globally, but on a per-session basis. The completion callback,
     * which may be run in other processes, e.g. in IO workers, may have a
     * different value of the zero_damaged_pages GUC.
     *
     * XXX: We probably should eventually use a different flag for
     * zero_damaged_pages, so we can report different log levels / error codes
     * for zero_damaged_pages and ZERO_ON_ERROR.
     */
    if (zero_damaged_pages)
        flags |= READ_BUFFERS_ZERO_ON_ERROR;
 
    /*
     * For the same reason as with zero_damaged_pages we need to use this
     * backend's ignore_checksum_failure value.
     */
    if (ignore_checksum_failure)
        flags |= READ_BUFFERS_IGNORE_CHECKSUM_FAILURES;
 
 
    /*
     * To be allowed to report stats in the local completion callback we need
     * to prepare to report stats now. This ensures we can safely report the
     * checksum failure even in a critical section.
     */
    pgstat_prepare_report_checksum_failure(operation->smgr->smgr_rlocator.locator.dbOid);
 
    /*
     * Get IO handle before ReadBuffersCanStartIO(), as pgaio_io_acquire()
     * might block, which we don't want after setting IO_IN_PROGRESS.
     *
     * If we need to wait for IO before we can get a handle, submit
     * already-staged IO first, so that other backends don't need to wait.
     * There wouldn't be a deadlock risk, as pgaio_io_acquire() just needs to
     * wait for already submitted IO, which doesn't require additional locks,
     * but it could still cause undesirable waits.
     *
     * A secondary benefit is that this would allow us to measure the time in
     * pgaio_io_acquire() without causing undue timer overhead in the common,
     * non-blocking, case.  However, currently the pgstats infrastructure
     * doesn't really allow that, as it a) asserts that an operation can't
     * have time without operations b) doesn't have an API to report
     * "accumulated" time.
     */
    ioh = pgaio_io_acquire_nb(CurrentResourceOwner, &operation->io_return);
    if (unlikely(!ioh))
    {
        pgaio_submit_staged();
 
        ioh = pgaio_io_acquire(CurrentResourceOwner, &operation->io_return);
    }
 
    /*
     * Check if we can start IO on the first to-be-read buffer.
     *
     * If an I/O is already in progress in another backend, we want to wait
     * for the outcome: either done, or something went wrong and we will
     * retry.
     */
    if (!ReadBuffersCanStartIO(buffers[nblocks_done], false))
    {
        /*
         * Someone else has already completed this block, we're done.
         *
         * When IO is necessary, ->nblocks_done is updated in
         * ProcessReadBuffersResult(), but that is not called if no IO is
         * necessary. Thus update here.
         */
        operation->nblocks_done += 1;
        *nblocks_progress = 1;
 
        pgaio_io_release(ioh);
        pgaio_wref_clear(&operation->io_wref);
        did_start_io = false;
 
        /*
         * Report and track this as a 'hit' for this backend, even though it
         * must have started out as a miss in PinBufferForBlock(). The other
         * backend will track this as a 'read'.
         */
        TRACE_POSTGRESQL_BUFFER_READ_DONE(forknum, blocknum + operation->nblocks_done,
                                          operation->smgr->smgr_rlocator.locator.spcOid,
                                          operation->smgr->smgr_rlocator.locator.dbOid,
                                          operation->smgr->smgr_rlocator.locator.relNumber,
                                          operation->smgr->smgr_rlocator.backend,
                                          true);
 
        if (persistence == RELPERSISTENCE_TEMP)
            pgBufferUsage.local_blks_hit += 1;
        else
            pgBufferUsage.shared_blks_hit += 1;
 
        if (operation->rel)
            pgstat_count_buffer_hit(operation->rel);
 
        pgstat_count_io_op(io_object, io_context, IOOP_HIT, 1, 0);
 
        if (VacuumCostActive)
            VacuumCostBalance += VacuumCostPageHit;
    }
    else
    {
        instr_time  io_start;
 
        /* We found a buffer that we need to read in. */
        Assert(io_buffers[0] == buffers[nblocks_done]);
        io_pages[0] = BufferGetBlock(buffers[nblocks_done]);
        io_buffers_len = 1;
 
        /*
         * How many neighboring-on-disk blocks can we scatter-read into other
         * buffers at the same time?  In this case we don't wait if we see an
         * I/O already in progress.  We already set BM_IO_IN_PROGRESS for the
         * head block, so we should get on with that I/O as soon as possible.
         */
        for (int i = nblocks_done + 1; i < operation->nblocks; i++)
        {
            if (!ReadBuffersCanStartIO(buffers[i], true))
                break;
            /* Must be consecutive block numbers. */
            Assert(BufferGetBlockNumber(buffers[i - 1]) ==
                   BufferGetBlockNumber(buffers[i]) - 1);
            Assert(io_buffers[io_buffers_len] == buffers[i]);
 
            io_pages[io_buffers_len++] = BufferGetBlock(buffers[i]);
        }
 
        /* get a reference to wait for in WaitReadBuffers() */
        pgaio_io_get_wref(ioh, &operation->io_wref);
 
        /* provide the list of buffers to the completion callbacks */
        pgaio_io_set_handle_data_32(ioh, (uint32 *) io_buffers, io_buffers_len);
 
        pgaio_io_register_callbacks(ioh,
                                    persistence == RELPERSISTENCE_TEMP ?
                                    PGAIO_HCB_LOCAL_BUFFER_READV :
                                    PGAIO_HCB_SHARED_BUFFER_READV,
                                    flags);
 
        pgaio_io_set_flag(ioh, ioh_flags);
 
        /* ---
         * Even though we're trying to issue IO asynchronously, track the time
         * in smgrstartreadv():
         * - if io_method == IOMETHOD_SYNC, we will always perform the IO
         *   immediately
         * - the io method might not support the IO (e.g. worker IO for a temp
         *   table)
         * ---
         */
        io_start = pgstat_prepare_io_time(track_io_timing);
        smgrstartreadv(ioh, operation->smgr, forknum,
                       blocknum + nblocks_done,
                       io_pages, io_buffers_len);
        pgstat_count_io_op_time(io_object, io_context, IOOP_READ,
                                io_start, 1, io_buffers_len * BLCKSZ);
 
        if (persistence == RELPERSISTENCE_TEMP)
            pgBufferUsage.local_blks_read += io_buffers_len;
        else
            pgBufferUsage.shared_blks_read += io_buffers_len;
 
        /*
         * Track vacuum cost when issuing IO, not after waiting for it.
         * Otherwise we could end up issuing a lot of IO in a short timespan,
         * despite a low cost limit.
         */
        if (VacuumCostActive)
            VacuumCostBalance += VacuumCostPageMiss * io_buffers_len;
 
        *nblocks_progress = io_buffers_len;
        did_start_io = true;
    }
 
    return did_start_io;
}
 
/*
 * BufferAlloc -- subroutine for PinBufferForBlock.  Handles lookup of a shared
 *      buffer.  If no buffer exists already, selects a replacement victim and
 *      evicts the old page, but does NOT read in new page.
 *
 * "strategy" can be a buffer replacement strategy object, or NULL for
 * the default strategy.  The selected buffer's usage_count is advanced when
 * using the default strategy, but otherwise possibly not (see PinBuffer).
 *
 * The returned buffer is pinned and is already marked as holding the
 * desired page.  If it already did have the desired page, *foundPtr is
 * set true.  Otherwise, *foundPtr is set false.
 *
 * io_context is passed as an output parameter to avoid calling
 * IOContextForStrategy() when there is a shared buffers hit and no IO
 * statistics need be captured.
 *
 * No locks are held either at entry or exit.
 */
static pg_attribute_always_inline BufferDesc *
BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
            BlockNumber blockNum,
            BufferAccessStrategy strategy,
            bool *foundPtr, IOContext io_context)
{
    BufferTag   newTag;         /* identity of requested block */
    uint32      newHash;        /* hash value for newTag */
    LWLock     *newPartitionLock;   /* buffer partition lock for it */
    int         existing_buf_id;
    Buffer      victim_buffer;
    BufferDesc *victim_buf_hdr;
    uint32      victim_buf_state;
 
    /* Make sure we will have room to remember the buffer pin */
    ResourceOwnerEnlarge(CurrentResourceOwner);
    ReservePrivateRefCountEntry();
 
    /* create a tag so we can lookup the buffer */
    InitBufferTag(&newTag, &smgr->smgr_rlocator.locator, forkNum, blockNum);
 
    /* determine its hash code and partition lock ID */
    newHash = BufTableHashCode(&newTag);
    newPartitionLock = BufMappingPartitionLock(newHash);
 
    /* see if the block is in the buffer pool already */
    LWLockAcquire(newPartitionLock, LW_SHARED);
    existing_buf_id = BufTableLookup(&newTag, newHash);
    if (existing_buf_id >= 0)
    {
        BufferDesc *buf;
        bool        valid;
 
        /*
         * Found it.  Now, pin the buffer so no one can steal it from the
         * buffer pool, and check to see if the correct data has been loaded
         * into the buffer.
         */
        buf = GetBufferDescriptor(existing_buf_id);
 
        valid = PinBuffer(buf, strategy);
 
        /* Can release the mapping lock as soon as we've pinned it */
        LWLockRelease(newPartitionLock);
 
        *foundPtr = true;
 
        if (!valid)
        {
            /*
             * We can only get here if (a) someone else is still reading in
             * the page, (b) a previous read attempt failed, or (c) someone
             * called StartReadBuffers() but not yet WaitReadBuffers().
             */
            *foundPtr = false;
        }
 
        return buf;
    }
 
    /*
     * Didn't find it in the buffer pool.  We'll have to initialize a new
     * buffer.  Remember to unlock the mapping lock while doing the work.
     */
    LWLockRelease(newPartitionLock);
 
    /*
     * Acquire a victim buffer. Somebody else might try to do the same, we
     * don't hold any conflicting locks. If so we'll have to undo our work
     * later.
     */
    victim_buffer = GetVictimBuffer(strategy, io_context);
    victim_buf_hdr = GetBufferDescriptor(victim_buffer - 1);
 
    /*
     * Try to make a hashtable entry for the buffer under its new tag. If
     * somebody else inserted another buffer for the tag, we'll release the
     * victim buffer we acquired and use the already inserted one.
     */
    LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
    existing_buf_id = BufTableInsert(&newTag, newHash, victim_buf_hdr->buf_id);
    if (existing_buf_id >= 0)
    {
        BufferDesc *existing_buf_hdr;
        bool        valid;
 
        /*
         * Got a collision. Someone has already done what we were about to do.
         * We'll just handle this as if it were found in the buffer pool in
         * the first place.  First, give up the buffer we were planning to
         * use.
         *
         * We could do this after releasing the partition lock, but then we'd
         * have to call ResourceOwnerEnlarge() & ReservePrivateRefCountEntry()
         * before acquiring the lock, for the rare case of such a collision.
         */
        UnpinBuffer(victim_buf_hdr);
 
        /*
         * The victim buffer we acquired previously is clean and unused, let
         * it be found again quickly
         */
        StrategyFreeBuffer(victim_buf_hdr);
 
        /* remaining code should match code at top of routine */
 
        existing_buf_hdr = GetBufferDescriptor(existing_buf_id);
 
        valid = PinBuffer(existing_buf_hdr, strategy);
 
        /* Can release the mapping lock as soon as we've pinned it */
        LWLockRelease(newPartitionLock);
 
        *foundPtr = true;
 
        if (!valid)
        {
            /*
             * We can only get here if (a) someone else is still reading in
             * the page, (b) a previous read attempt failed, or (c) someone
             * called StartReadBuffers() but not yet WaitReadBuffers().
             */
            *foundPtr = false;
        }
 
        return existing_buf_hdr;
    }
 
    /*
     * Need to lock the buffer header too in order to change its tag.
     */
    victim_buf_state = LockBufHdr(victim_buf_hdr);
 
    /* some sanity checks while we hold the buffer header lock */
    Assert(BUF_STATE_GET_REFCOUNT(victim_buf_state) == 1);
    Assert(!(victim_buf_state & (BM_TAG_VALID | BM_VALID | BM_DIRTY | BM_IO_IN_PROGRESS)));
 
    victim_buf_hdr->tag = newTag;
 
    /*
     * Make sure BM_PERMANENT is set for buffers that must be written at every
     * checkpoint.  Unlogged buffers only need to be written at shutdown
     * checkpoints, except for their "init" forks, which need to be treated
     * just like permanent relations.
     */
    victim_buf_state |= BM_TAG_VALID | BUF_USAGECOUNT_ONE;
    if (relpersistence == RELPERSISTENCE_PERMANENT || forkNum == INIT_FORKNUM)
        victim_buf_state |= BM_PERMANENT;
 
    UnlockBufHdr(victim_buf_hdr, victim_buf_state);
 
    LWLockRelease(newPartitionLock);
 
    /*
     * Buffer contents are currently invalid.
     */
    *foundPtr = false;
 
    return victim_buf_hdr;
}
 
/*
 * InvalidateBuffer -- mark a shared buffer invalid and return it to the
 * freelist.
 *
 * The buffer header spinlock must be held at entry.  We drop it before
 * returning.  (This is sane because the caller must have locked the
 * buffer in order to be sure it should be dropped.)
 *
 * This is used only in contexts such as dropping a relation.  We assume
 * that no other backend could possibly be interested in using the page,
 * so the only reason the buffer might be pinned is if someone else is
 * trying to write it out.  We have to let them finish before we can
 * reclaim the buffer.
 *
 * The buffer could get reclaimed by someone else while we are waiting
 * to acquire the necessary locks; if so, don't mess it up.
 */
static void
InvalidateBuffer(BufferDesc *buf)
{
    BufferTag   oldTag;
    uint32      oldHash;        /* hash value for oldTag */
    LWLock     *oldPartitionLock;   /* buffer partition lock for it */
    uint32      oldFlags;
    uint32      buf_state;
 
    /* Save the original buffer tag before dropping the spinlock */
    oldTag = buf->tag;
 
    buf_state = pg_atomic_read_u32(&buf->state);
    Assert(buf_state & BM_LOCKED);
    UnlockBufHdr(buf, buf_state);
 
    /*
     * Need to compute the old tag's hashcode and partition lock ID. XXX is it
     * worth storing the hashcode in BufferDesc so we need not recompute it
     * here?  Probably not.
     */
    oldHash = BufTableHashCode(&oldTag);
    oldPartitionLock = BufMappingPartitionLock(oldHash);
 
retry:
 
    /*
     * Acquire exclusive mapping lock in preparation for changing the buffer's
     * association.
     */
    LWLockAcquire(oldPartitionLock, LW_EXCLUSIVE);
 
    /* Re-lock the buffer header */
    buf_state = LockBufHdr(buf);
 
    /* If it's changed while we were waiting for lock, do nothing */
    if (!BufferTagsEqual(&buf->tag, &oldTag))
    {
        UnlockBufHdr(buf, buf_state);
        LWLockRelease(oldPartitionLock);
        return;
    }
 
    /*
     * We assume the reason for it to be pinned is that either we were
     * asynchronously reading the page in before erroring out or someone else
     * is flushing the page out.  Wait for the IO to finish.  (This could be
     * an infinite loop if the refcount is messed up... it would be nice to
     * time out after awhile, but there seems no way to be sure how many loops
     * may be needed.  Note that if the other guy has pinned the buffer but
     * not yet done StartBufferIO, WaitIO will fall through and we'll
     * effectively be busy-looping here.)
     */
    if (BUF_STATE_GET_REFCOUNT(buf_state) != 0)
    {
        UnlockBufHdr(buf, buf_state);
        LWLockRelease(oldPartitionLock);
        /* safety check: should definitely not be our *own* pin */
        if (GetPrivateRefCount(BufferDescriptorGetBuffer(buf)) > 0)
            elog(ERROR, "buffer is pinned in InvalidateBuffer");
        WaitIO(buf);
        goto retry;
    }
 
    /*
     * Clear out the buffer's tag and flags.  We must do this to ensure that
     * linear scans of the buffer array don't think the buffer is valid.
     */
    oldFlags = buf_state & BUF_FLAG_MASK;
    ClearBufferTag(&buf->tag);
    buf_state &= ~(BUF_FLAG_MASK | BUF_USAGECOUNT_MASK);
    UnlockBufHdr(buf, buf_state);
 
    /*
     * Remove the buffer from the lookup hashtable, if it was in there.
     */
    if (oldFlags & BM_TAG_VALID)
        BufTableDelete(&oldTag, oldHash);
 
    /*
     * Done with mapping lock.
     */
    LWLockRelease(oldPartitionLock);
 
    /*
     * Insert the buffer at the head of the list of free buffers.
     */
    StrategyFreeBuffer(buf);
}
 
/*
 * Helper routine for GetVictimBuffer()
 *
 * Needs to be called on a buffer with a valid tag, pinned, but without the
 * buffer header spinlock held.
 *
 * Returns true if the buffer can be reused, in which case the buffer is only
 * pinned by this backend and marked as invalid, false otherwise.
 */
static bool
InvalidateVictimBuffer(BufferDesc *buf_hdr)
{
    uint32      buf_state;
    uint32      hash;
    LWLock     *partition_lock;
    BufferTag   tag;
 
    Assert(GetPrivateRefCount(BufferDescriptorGetBuffer(buf_hdr)) == 1);
 
    /* have buffer pinned, so it's safe to read tag without lock */
    tag = buf_hdr->tag;
 
    hash = BufTableHashCode(&tag);
    partition_lock = BufMappingPartitionLock(hash);
 
    LWLockAcquire(partition_lock, LW_EXCLUSIVE);
 
    /* lock the buffer header */
    buf_state = LockBufHdr(buf_hdr);
 
    /*
     * We have the buffer pinned nobody else should have been able to unset
     * this concurrently.
     */
    Assert(buf_state & BM_TAG_VALID);
    Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
    Assert(BufferTagsEqual(&buf_hdr->tag, &tag));
 
    /*
     * If somebody else pinned the buffer since, or even worse, dirtied it,
     * give up on this buffer: It's clearly in use.
     */
    if (BUF_STATE_GET_REFCOUNT(buf_state) != 1 || (buf_state & BM_DIRTY))
    {
        Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
 
        UnlockBufHdr(buf_hdr, buf_state);
        LWLockRelease(partition_lock);
 
        return false;
    }
 
    /*
     * Clear out the buffer's tag and flags and usagecount.  This is not
     * strictly required, as BM_TAG_VALID/BM_VALID needs to be checked before
     * doing anything with the buffer. But currently it's beneficial, as the
     * cheaper pre-check for several linear scans of shared buffers use the
     * tag (see e.g. FlushDatabaseBuffers()).
     */
    ClearBufferTag(&buf_hdr->tag);
    buf_state &= ~(BUF_FLAG_MASK | BUF_USAGECOUNT_MASK);
    UnlockBufHdr(buf_hdr, buf_state);
 
    Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
 
    /* finally delete buffer from the buffer mapping table */
    BufTableDelete(&tag, hash);
 
    LWLockRelease(partition_lock);
 
    Assert(!(buf_state & (BM_DIRTY | BM_VALID | BM_TAG_VALID)));
    Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
    Assert(BUF_STATE_GET_REFCOUNT(pg_atomic_read_u32(&buf_hdr->state)) > 0);
 
    return true;
}
 
static Buffer
GetVictimBuffer(BufferAccessStrategy strategy, IOContext io_context)
{
    BufferDesc *buf_hdr;
    Buffer      buf;
    uint32      buf_state;
    bool        from_ring;
 
    /*
     * Ensure, while the spinlock's not yet held, that there's a free refcount
     * entry, and a resource owner slot for the pin.
     */
    ReservePrivateRefCountEntry();
    ResourceOwnerEnlarge(CurrentResourceOwner);
 
    /* we return here if a prospective victim buffer gets used concurrently */
again:
 
    /*
     * Select a victim buffer.  The buffer is returned with its header
     * spinlock still held!
     */
    buf_hdr = StrategyGetBuffer(strategy, &buf_state, &from_ring);
    buf = BufferDescriptorGetBuffer(buf_hdr);
 
    Assert(BUF_STATE_GET_REFCOUNT(buf_state) == 0);
 
    /* Pin the buffer and then release the buffer spinlock */
    PinBuffer_Locked(buf_hdr);
 
    /*
     * We shouldn't have any other pins for this buffer.
     */
    CheckBufferIsPinnedOnce(buf);
 
    /*
     * If the buffer was dirty, try to write it out.  There is a race
     * condition here, in that someone might dirty it after we released the
     * buffer header lock above, or even while we are writing it out (since
     * our share-lock won't prevent hint-bit updates).  We will recheck the
     * dirty bit after re-locking the buffer header.
     */
    if (buf_state & BM_DIRTY)
    {
        LWLock     *content_lock;
 
        Assert(buf_state & BM_TAG_VALID);
        Assert(buf_state & BM_VALID);
 
        /*
         * We need a share-lock on the buffer contents to write it out (else
         * we might write invalid data, eg because someone else is compacting
         * the page contents while we write).  We must use a conditional lock
         * acquisition here to avoid deadlock.  Even though the buffer was not
         * pinned (and therefore surely not locked) when StrategyGetBuffer
         * returned it, someone else could have pinned and exclusive-locked it
         * by the time we get here. If we try to get the lock unconditionally,
         * we'd block waiting for them; if they later block waiting for us,
         * deadlock ensues. (This has been observed to happen when two
         * backends are both trying to split btree index pages, and the second
         * one just happens to be trying to split the page the first one got
         * from StrategyGetBuffer.)
         */
        content_lock = BufferDescriptorGetContentLock(buf_hdr);
        if (!LWLockConditionalAcquire(content_lock, LW_SHARED))
        {
            /*
             * Someone else has locked the buffer, so give it up and loop back
             * to get another one.
             */
            UnpinBuffer(buf_hdr);
            goto again;
        }
 
        /*
         * If using a nondefault strategy, and writing the buffer would
         * require a WAL flush, let the strategy decide whether to go ahead
         * and write/reuse the buffer or to choose another victim.  We need a
         * lock to inspect the page LSN, so this can't be done inside
         * StrategyGetBuffer.
         */
        if (strategy != NULL)
        {
            XLogRecPtr  lsn;
 
            /* Read the LSN while holding buffer header lock */
            buf_state = LockBufHdr(buf_hdr);
            lsn = BufferGetLSN(buf_hdr);
            UnlockBufHdr(buf_hdr, buf_state);
 
            if (XLogNeedsFlush(lsn)
                && StrategyRejectBuffer(strategy, buf_hdr, from_ring))
            {
                LWLockRelease(content_lock);
                UnpinBuffer(buf_hdr);
                goto again;
            }
        }
 
        /* OK, do the I/O */
        FlushBuffer(buf_hdr, NULL, IOOBJECT_RELATION, io_context);
        LWLockRelease(content_lock);
 
        ScheduleBufferTagForWriteback(&BackendWritebackContext, io_context,
                                      &buf_hdr->tag);
    }
 
 
    if (buf_state & BM_VALID)
    {
        /*
         * When a BufferAccessStrategy is in use, blocks evicted from shared
         * buffers are counted as IOOP_EVICT in the corresponding context
         * (e.g. IOCONTEXT_BULKWRITE). Shared buffers are evicted by a
         * strategy in two cases: 1) while initially claiming buffers for the
         * strategy ring 2) to replace an existing strategy ring buffer
         * because it is pinned or in use and cannot be reused.
         *
         * Blocks evicted from buffers already in the strategy ring are
         * counted as IOOP_REUSE in the corresponding strategy context.
         *
         * At this point, we can accurately count evictions and reuses,
         * because we have successfully claimed the valid buffer. Previously,
         * we may have been forced to release the buffer due to concurrent
         * pinners or erroring out.
         */
        pgstat_count_io_op(IOOBJECT_RELATION, io_context,
                           from_ring ? IOOP_REUSE : IOOP_EVICT, 1, 0);
    }
 
    /*
     * If the buffer has an entry in the buffer mapping table, delete it. This
     * can fail because another backend could have pinned or dirtied the
     * buffer.
     */
    if ((buf_state & BM_TAG_VALID) && !InvalidateVictimBuffer(buf_hdr))
    {
        UnpinBuffer(buf_hdr);
        goto again;
    }
 
    /* a final set of sanity checks */
#ifdef USE_ASSERT_CHECKING
    buf_state = pg_atomic_read_u32(&buf_hdr->state);
 
    Assert(BUF_STATE_GET_REFCOUNT(buf_state) == 1);
    Assert(!(buf_state & (BM_TAG_VALID | BM_VALID | BM_DIRTY)));
 
    CheckBufferIsPinnedOnce(buf);
#endif
 
    return buf;
}
 
/*
 * Return the maximum number of buffers that a backend should try to pin once,
 * to avoid exceeding its fair share.  This is the highest value that
 * GetAdditionalPinLimit() could ever return.  Note that it may be zero on a
 * system with a very small buffer pool relative to max_connections.
 */
uint32
GetPinLimit(void)
{
    return MaxProportionalPins;
}
 
/*
 * Return the maximum number of additional buffers that this backend should
 * pin if it wants to stay under the per-backend limit, considering the number
 * of buffers it has already pinned.  Unlike LimitAdditionalPins(), the limit
 * return by this function can be zero.
 */
uint32
GetAdditionalPinLimit(void)
{
    uint32      estimated_pins_held;
 
    /*
     * We get the number of "overflowed" pins for free, but don't know the
     * number of pins in PrivateRefCountArray.  The cost of calculating that
     * exactly doesn't seem worth it, so just assume the max.
     */
    estimated_pins_held = PrivateRefCountOverflowed + REFCOUNT_ARRAY_ENTRIES;
 
    /* Is this backend already holding more than its fair share? */
    if (estimated_pins_held > MaxProportionalPins)
        return 0;
 
    return MaxProportionalPins - estimated_pins_held;
}
 
/*
 * Limit the number of pins a batch operation may additionally acquire, to
 * avoid running out of pinnable buffers.
 *
 * One additional pin is always allowed, on the assumption that the operation
 * requires at least one to make progress.
 */
void
LimitAdditionalPins(uint32 *additional_pins)
{
    uint32      limit;
 
    if (*additional_pins <= 1)
        return;
 
    limit = GetAdditionalPinLimit();
    limit = Max(limit, 1);
    if (limit < *additional_pins)
        *additional_pins = limit;
}
 
/*
 * Logic shared between ExtendBufferedRelBy(), ExtendBufferedRelTo(). Just to
 * avoid duplicating the tracing and relpersistence related logic.
 */
static BlockNumber
ExtendBufferedRelCommon(BufferManagerRelation bmr,
                        ForkNumber fork,
                        BufferAccessStrategy strategy,
                        uint32 flags,
                        uint32 extend_by,
                        BlockNumber extend_upto,
                        Buffer *buffers,
                        uint32 *extended_by)
{
    BlockNumber first_block;
 
    TRACE_POSTGRESQL_BUFFER_EXTEND_START(fork,
                                         bmr.smgr->smgr_rlocator.locator.spcOid,
                                         bmr.smgr->smgr_rlocator.locator.dbOid,
                                         bmr.smgr->smgr_rlocator.locator.relNumber,
                                         bmr.smgr->smgr_rlocator.backend,
                                         extend_by);
 
    if (bmr.relpersistence == RELPERSISTENCE_TEMP)
        first_block = ExtendBufferedRelLocal(bmr, fork, flags,
                                             extend_by, extend_upto,
                                             buffers, &extend_by);
    else
        first_block = ExtendBufferedRelShared(bmr, fork, strategy, flags,
                                              extend_by, extend_upto,
                                              buffers, &extend_by);
    *extended_by = extend_by;
 
    TRACE_POSTGRESQL_BUFFER_EXTEND_DONE(fork,
                                        bmr.smgr->smgr_rlocator.locator.spcOid,
                                        bmr.smgr->smgr_rlocator.locator.dbOid,
                                        bmr.smgr->smgr_rlocator.locator.relNumber,
                                        bmr.smgr->smgr_rlocator.backend,
                                        *extended_by,
                                        first_block);
 
    return first_block;
}
 
/*
 * Implementation of ExtendBufferedRelBy() and ExtendBufferedRelTo() for
 * shared buffers.
 */
static BlockNumber
ExtendBufferedRelShared(BufferManagerRelation bmr,
                        ForkNumber fork,
                        BufferAccessStrategy strategy,
                        uint32 flags,
                        uint32 extend_by,
                        BlockNumber extend_upto,
                        Buffer *buffers,
                        uint32 *extended_by)
{
    BlockNumber first_block;
    IOContext   io_context = IOContextForStrategy(strategy);
    instr_time  io_start;
 
    LimitAdditionalPins(&extend_by);
 
    /*
     * Acquire victim buffers for extension without holding extension lock.
     * Writing out victim buffers is the most expensive part of extending the
     * relation, particularly when doing so requires WAL flushes. Zeroing out
     * the buffers is also quite expensive, so do that before holding the
     * extension lock as well.
     *
     * These pages are pinned by us and not valid. While we hold the pin they
     * can't be acquired as victim buffers by another backend.
     */
    for (uint32 i = 0; i < extend_by; i++)
    {
        Block       buf_block;
 
        buffers[i] = GetVictimBuffer(strategy, io_context);
        buf_block = BufHdrGetBlock(GetBufferDescriptor(buffers[i] - 1));
 
        /* new buffers are zero-filled */
        MemSet(buf_block, 0, BLCKSZ);
    }
 
    /*
     * Lock relation against concurrent extensions, unless requested not to.
     *
     * We use the same extension lock for all forks. That's unnecessarily
     * restrictive, but currently extensions for forks don't happen often
     * enough to make it worth locking more granularly.
     *
     * Note that another backend might have extended the relation by the time
     * we get the lock.
     */
    if (!(flags & EB_SKIP_EXTENSION_LOCK))
        LockRelationForExtension(bmr.rel, ExclusiveLock);
 
    /*
     * If requested, invalidate size cache, so that smgrnblocks asks the
     * kernel.
     */
    if (flags & EB_CLEAR_SIZE_CACHE)
        bmr.smgr->smgr_cached_nblocks[fork] = InvalidBlockNumber;
 
    first_block = smgrnblocks(bmr.smgr, fork);
 
    /*
     * Now that we have the accurate relation size, check if the caller wants
     * us to extend to only up to a specific size. If there were concurrent
     * extensions, we might have acquired too many buffers and need to release
     * them.
     */
    if (extend_upto != InvalidBlockNumber)
    {
        uint32      orig_extend_by = extend_by;
 
        if (first_block > extend_upto)
            extend_by = 0;
        else if ((uint64) first_block + extend_by > extend_upto)
            extend_by = extend_upto - first_block;
 
        for (uint32 i = extend_by; i < orig_extend_by; i++)
        {
            BufferDesc *buf_hdr = GetBufferDescriptor(buffers[i] - 1);
 
            /*
             * The victim buffer we acquired previously is clean and unused,
             * let it be found again quickly
             */
            StrategyFreeBuffer(buf_hdr);
            UnpinBuffer(buf_hdr);
        }
 
        if (extend_by == 0)
        {
            if (!(flags & EB_SKIP_EXTENSION_LOCK))
                UnlockRelationForExtension(bmr.rel, ExclusiveLock);
            *extended_by = extend_by;
            return first_block;
        }
    }
 
    /* Fail if relation is already at maximum possible length */
    if ((uint64) first_block + extend_by >= MaxBlockNumber)
        ereport(ERROR,
                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                 errmsg("cannot extend relation %s beyond %u blocks",
                        relpath(bmr.smgr->smgr_rlocator, fork).str,
                        MaxBlockNumber)));
 
    /*
     * Insert buffers into buffer table, mark as IO_IN_PROGRESS.
     *
     * This needs to happen before we extend the relation, because as soon as
     * we do, other backends can start to read in those pages.
     */
    for (uint32 i = 0; i < extend_by; i++)
    {
        Buffer      victim_buf = buffers[i];
        BufferDesc *victim_buf_hdr = GetBufferDescriptor(victim_buf - 1);
        BufferTag   tag;
        uint32      hash;
        LWLock     *partition_lock;
        int         existing_id;
 
        /* in case we need to pin an existing buffer below */
        ResourceOwnerEnlarge(CurrentResourceOwner);
        ReservePrivateRefCountEntry();
 
        InitBufferTag(&tag, &bmr.smgr->smgr_rlocator.locator, fork, first_block + i);
        hash = BufTableHashCode(&tag);
        partition_lock = BufMappingPartitionLock(hash);
 
        LWLockAcquire(partition_lock, LW_EXCLUSIVE);
 
        existing_id = BufTableInsert(&tag, hash, victim_buf_hdr->buf_id);
 
        /*
         * We get here only in the corner case where we are trying to extend
         * the relation but we found a pre-existing buffer. This can happen
         * because a prior attempt at extending the relation failed, and
         * because mdread doesn't complain about reads beyond EOF (when
         * zero_damaged_pages is ON) and so a previous attempt to read a block
         * beyond EOF could have left a "valid" zero-filled buffer.
         * Unfortunately, we have also seen this case occurring because of
         * buggy Linux kernels that sometimes return an lseek(SEEK_END) result
         * that doesn't account for a recent write. In that situation, the
         * pre-existing buffer would contain valid data that we don't want to
         * overwrite.  Since the legitimate cases should always have left a
         * zero-filled buffer, complain if not PageIsNew.
         */
        if (existing_id >= 0)
        {
            BufferDesc *existing_hdr = GetBufferDescriptor(existing_id);
            Block       buf_block;
            bool        valid;
 
            /*
             * Pin the existing buffer before releasing the partition lock,
             * preventing it from being evicted.
             */
            valid = PinBuffer(existing_hdr, strategy);
 
            LWLockRelease(partition_lock);
 
            /*
             * The victim buffer we acquired previously is clean and unused,
             * let it be found again quickly
             */
            StrategyFreeBuffer(victim_buf_hdr);
            UnpinBuffer(victim_buf_hdr);
 
            buffers[i] = BufferDescriptorGetBuffer(existing_hdr);
            buf_block = BufHdrGetBlock(existing_hdr);
 
            if (valid && !PageIsNew((Page) buf_block))
                ereport(ERROR,
                        (errmsg("unexpected data beyond EOF in block %u of relation %s",
                                existing_hdr->tag.blockNum,
                                relpath(bmr.smgr->smgr_rlocator, fork).str),
                         errhint("This has been seen to occur with buggy kernels; consider updating your system.")));
 
            /*
             * We *must* do smgr[zero]extend before succeeding, else the page
             * will not be reserved by the kernel, and the next P_NEW call
             * will decide to return the same page.  Clear the BM_VALID bit,
             * do StartBufferIO() and proceed.
             *
             * Loop to handle the very small possibility that someone re-sets
             * BM_VALID between our clearing it and StartBufferIO inspecting
             * it.
             */
            do
            {
                uint32      buf_state = LockBufHdr(existing_hdr);
 
                buf_state &= ~BM_VALID;
                UnlockBufHdr(existing_hdr, buf_state);
            } while (!StartBufferIO(existing_hdr, true, false));
        }
        else
        {
            uint32      buf_state;
 
            buf_state = LockBufHdr(victim_buf_hdr);
 
            /* some sanity checks while we hold the buffer header lock */
            Assert(!(buf_state & (BM_VALID | BM_TAG_VALID | BM_DIRTY | BM_JUST_DIRTIED)));
            Assert(BUF_STATE_GET_REFCOUNT(buf_state) == 1);
 
            victim_buf_hdr->tag = tag;
 
            buf_state |= BM_TAG_VALID | BUF_USAGECOUNT_ONE;
            if (bmr.relpersistence == RELPERSISTENCE_PERMANENT || fork == INIT_FORKNUM)
                buf_state |= BM_PERMANENT;
 
            UnlockBufHdr(victim_buf_hdr, buf_state);
 
            LWLockRelease(partition_lock);
 
            /* XXX: could combine the locked operations in it with the above */
            StartBufferIO(victim_buf_hdr, true, false);
        }
    }
 
    io_start = pgstat_prepare_io_time(track_io_timing);
 
    /*
     * Note: if smgrzeroextend fails, we will end up with buffers that are
     * allocated but not marked BM_VALID.  The next relation extension will
     * still select the same block number (because the relation didn't get any
     * longer on disk) and so future attempts to extend the relation will find
     * the same buffers (if they have not been recycled) but come right back
     * here to try smgrzeroextend again.
     *
     * We don't need to set checksum for all-zero pages.
     */
    smgrzeroextend(bmr.smgr, fork, first_block, extend_by, false);
 
    /*
     * Release the file-extension lock; it's now OK for someone else to extend
     * the relation some more.
     *
     * We remove IO_IN_PROGRESS after this, as waking up waiting backends can
     * take noticeable time.
     */
    if (!(flags & EB_SKIP_EXTENSION_LOCK))
        UnlockRelationForExtension(bmr.rel, ExclusiveLock);
 
    pgstat_count_io_op_time(IOOBJECT_RELATION, io_context, IOOP_EXTEND,
                            io_start, 1, extend_by * BLCKSZ);
 
    /* Set BM_VALID, terminate IO, and wake up any waiters */
    for (uint32 i = 0; i < extend_by; i++)
    {
        Buffer      buf = buffers[i];
        BufferDesc *buf_hdr = GetBufferDescriptor(buf - 1);
        bool        lock = false;
 
        if (flags & EB_LOCK_FIRST && i == 0)
            lock = true;
        else if (flags & EB_LOCK_TARGET)
        {
            Assert(extend_upto != InvalidBlockNumber);
            if (first_block + i + 1 == extend_upto)
                lock = true;
        }
 
        if (lock)
            LWLockAcquire(BufferDescriptorGetContentLock(buf_hdr), LW_EXCLUSIVE);
 
        TerminateBufferIO(buf_hdr, false, BM_VALID, true, false);
    }
 
    pgBufferUsage.shared_blks_written += extend_by;
 
    *extended_by = extend_by;
 
    return first_block;
}
 
/*
 * BufferIsExclusiveLocked
 *
 *      Checks if buffer is exclusive-locked.
 *
 * Buffer must be pinned.
 */
bool
BufferIsExclusiveLocked(Buffer buffer)
{
    BufferDesc *bufHdr;
 
    Assert(BufferIsPinned(buffer));
 
    if (BufferIsLocal(buffer))
    {
        /* Content locks are not maintained for local buffers. */
        return true;
    }
    else
    {
        bufHdr = GetBufferDescriptor(buffer - 1);
        return LWLockHeldByMeInMode(BufferDescriptorGetContentLock(bufHdr),
                                    LW_EXCLUSIVE);
    }
}
 
/*
 * BufferIsDirty
 *
 *      Checks if buffer is already dirty.
 *
 * Buffer must be pinned and exclusive-locked.  (Without an exclusive lock,
 * the result may be stale before it's returned.)
 */
bool
BufferIsDirty(Buffer buffer)
{
    BufferDesc *bufHdr;
 
    Assert(BufferIsPinned(buffer));
 
    if (BufferIsLocal(buffer))
    {
        int         bufid = -buffer - 1;
 
        bufHdr = GetLocalBufferDescriptor(bufid);
        /* Content locks are not maintained for local buffers. */
    }
    else
    {
        bufHdr = GetBufferDescriptor(buffer - 1);
        Assert(LWLockHeldByMeInMode(BufferDescriptorGetContentLock(bufHdr),
                                    LW_EXCLUSIVE));
    }
 
    return pg_atomic_read_u32(&bufHdr->state) & BM_DIRTY;
}
 
/*
 * MarkBufferDirty
 *
 *      Marks buffer contents as dirty (actual write happens later).
 *
 * Buffer must be pinned and exclusive-locked.  (If caller does not hold
 * exclusive lock, then somebody could be in process of writing the buffer,
 * leading to risk of bad data written to disk.)
 */
void
MarkBufferDirty(Buffer buffer)
{
    BufferDesc *bufHdr;
    uint32      buf_state;
    uint32      old_buf_state;
 
    if (!BufferIsValid(buffer))
        elog(ERROR, "bad buffer ID: %d", buffer);
 
    if (BufferIsLocal(buffer))
    {
        MarkLocalBufferDirty(buffer);
        return;
    }
 
    bufHdr = GetBufferDescriptor(buffer - 1);
 
    Assert(BufferIsPinned(buffer));
    Assert(LWLockHeldByMeInMode(BufferDescriptorGetContentLock(bufHdr),
                                LW_EXCLUSIVE));
 
    old_buf_state = pg_atomic_read_u32(&bufHdr->state);
    for (;;)
    {
        if (old_buf_state & BM_LOCKED)
            old_buf_state = WaitBufHdrUnlocked(bufHdr);
 
        buf_state = old_buf_state;
 
        Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
        buf_state |= BM_DIRTY | BM_JUST_DIRTIED;
 
        if (pg_atomic_compare_exchange_u32(&bufHdr->state, &old_buf_state,
                                           buf_state))
            break;
    }
 
    /*
     * If the buffer was not dirty already, do vacuum accounting.
     */
    if (!(old_buf_state & BM_DIRTY))
    {
        pgBufferUsage.shared_blks_dirtied++;
        if (VacuumCostActive)
            VacuumCostBalance += VacuumCostPageDirty;
    }
}
 
/*
 * ReleaseAndReadBuffer -- combine ReleaseBuffer() and ReadBuffer()
 *
 * Formerly, this saved one cycle of acquiring/releasing the BufMgrLock
 * compared to calling the two routines separately.  Now it's mainly just
 * a convenience function.  However, if the passed buffer is valid and
 * already contains the desired block, we just return it as-is; and that
 * does save considerable work compared to a full release and reacquire.
 *
 * Note: it is OK to pass buffer == InvalidBuffer, indicating that no old
 * buffer actually needs to be released.  This case is the same as ReadBuffer,
 * but can save some tests in the caller.
 */
Buffer
ReleaseAndReadBuffer(Buffer buffer,
                     Relation relation,
                     BlockNumber blockNum)
{
    ForkNumber  forkNum = MAIN_FORKNUM;
    BufferDesc *bufHdr;
 
    if (BufferIsValid(buffer))
    {
        Assert(BufferIsPinned(buffer));
        if (BufferIsLocal(buffer))
        {
            bufHdr = GetLocalBufferDescriptor(-buffer - 1);
            if (bufHdr->tag.blockNum == blockNum &&
                BufTagMatchesRelFileLocator(&bufHdr->tag, &relation->rd_locator) &&
                BufTagGetForkNum(&bufHdr->tag) == forkNum)
                return buffer;
            UnpinLocalBuffer(buffer);
        }
        else
        {
            bufHdr = GetBufferDescriptor(buffer - 1);
            /* we have pin, so it's ok to examine tag without spinlock */
            if (bufHdr->tag.blockNum == blockNum &&
                BufTagMatchesRelFileLocator(&bufHdr->tag, &relation->rd_locator) &&
                BufTagGetForkNum(&bufHdr->tag) == forkNum)
                return buffer;
            UnpinBuffer(bufHdr);
        }
    }
 
    return ReadBuffer(relation, blockNum);
}
 
/*
 * PinBuffer -- make buffer unavailable for replacement.
 *
 * For the default access strategy, the buffer's usage_count is incremented
 * when we first pin it; for other strategies we just make sure the usage_count
 * isn't zero.  (The idea of the latter is that we don't want synchronized
 * heap scans to inflate the count, but we need it to not be zero to discourage
 * other backends from stealing buffers from our ring.  As long as we cycle
 * through the ring faster than the global clock-sweep cycles, buffers in
 * our ring won't be chosen as victims for replacement by other backends.)
 *
 * This should be applied only to shared buffers, never local ones.
 *
 * Since buffers are pinned/unpinned very frequently, pin buffers without
 * taking the buffer header lock; instead update the state variable in loop of
 * CAS operations. Hopefully it's just a single CAS.
 *
 * Note that ResourceOwnerEnlarge() and ReservePrivateRefCountEntry()
 * must have been done already.
 *
 * Returns true if buffer is BM_VALID, else false.  This provision allows
 * some callers to avoid an extra spinlock cycle.
 */
static bool
PinBuffer(BufferDesc *buf, BufferAccessStrategy strategy)
{
    Buffer      b = BufferDescriptorGetBuffer(buf);
    bool        result;
    PrivateRefCountEntry *ref;
 
    Assert(!BufferIsLocal(b));
    Assert(ReservedRefCountEntry != NULL);
 
    ref = GetPrivateRefCountEntry(b, true);
 
    if (ref == NULL)
    {
        uint32      buf_state;
        uint32      old_buf_state;
 
        ref = NewPrivateRefCountEntry(b);
 
        old_buf_state = pg_atomic_read_u32(&buf->state);
        for (;;)
        {
            if (old_buf_state & BM_LOCKED)
                old_buf_state = WaitBufHdrUnlocked(buf);
 
            buf_state = old_buf_state;
 
            /* increase refcount */
            buf_state += BUF_REFCOUNT_ONE;
 
            if (strategy == NULL)
            {
                /* Default case: increase usagecount unless already max. */
                if (BUF_STATE_GET_USAGECOUNT(buf_state) < BM_MAX_USAGE_COUNT)
                    buf_state += BUF_USAGECOUNT_ONE;
            }
            else
            {
                /*
                 * Ring buffers shouldn't evict others from pool.  Thus we
                 * don't make usagecount more than 1.
                 */
                if (BUF_STATE_GET_USAGECOUNT(buf_state) == 0)
                    buf_state += BUF_USAGECOUNT_ONE;
            }
 
            if (pg_atomic_compare_exchange_u32(&buf->state, &old_buf_state,
                                               buf_state))
            {
                result = (buf_state & BM_VALID) != 0;
 
                /*
                 * Assume that we acquired a buffer pin for the purposes of
                 * Valgrind buffer client checks (even in !result case) to
                 * keep things simple.  Buffers that are unsafe to access are
                 * not generally guaranteed to be marked undefined or
                 * non-accessible in any case.
                 */
                VALGRIND_MAKE_MEM_DEFINED(BufHdrGetBlock(buf), BLCKSZ);
                break;
            }
        }
    }
    else
    {
        /*
         * If we previously pinned the buffer, it is likely to be valid, but
         * it may not be if StartReadBuffers() was called and
         * WaitReadBuffers() hasn't been called yet.  We'll check by loading
         * the flags without locking.  This is racy, but it's OK to return
         * false spuriously: when WaitReadBuffers() calls StartBufferIO(),
         * it'll see that it's now valid.
         *
         * Note: We deliberately avoid a Valgrind client request here.
         * Individual access methods can optionally superimpose buffer page
         * client requests on top of our client requests to enforce that
         * buffers are only accessed while locked (and pinned).  It's possible
         * that the buffer page is legitimately non-accessible here.  We
         * cannot meddle with that.
         */
        result = (pg_atomic_read_u32(&buf->state) & BM_VALID) != 0;
    }
 
    ref->refcount++;
    Assert(ref->refcount > 0);
    ResourceOwnerRememberBuffer(CurrentResourceOwner, b);
    return result;
}
 
/*
 * PinBuffer_Locked -- as above, but caller already locked the buffer header.
 * The spinlock is released before return.
 *
 * As this function is called with the spinlock held, the caller has to
 * previously call ReservePrivateRefCountEntry() and
 * ResourceOwnerEnlarge(CurrentResourceOwner);
 *
 * Currently, no callers of this function want to modify the buffer's
 * usage_count at all, so there's no need for a strategy parameter.
 * Also we don't bother with a BM_VALID test (the caller could check that for
 * itself).
 *
 * Also all callers only ever use this function when it's known that the
 * buffer can't have a preexisting pin by this backend. That allows us to skip
 * searching the private refcount array & hash, which is a boon, because the
 * spinlock is still held.
 *
 * Note: use of this routine is frequently mandatory, not just an optimization
 * to save a spin lock/unlock cycle, because we need to pin a buffer before
 * its state can change under us.
 */
static void
PinBuffer_Locked(BufferDesc *buf)
{
    Buffer      b;
    PrivateRefCountEntry *ref;
    uint32      buf_state;
 
    /*
     * As explained, We don't expect any preexisting pins. That allows us to
     * manipulate the PrivateRefCount after releasing the spinlock
     */
    Assert(GetPrivateRefCountEntry(BufferDescriptorGetBuffer(buf), false) == NULL);
 
    /*
     * Buffer can't have a preexisting pin, so mark its page as defined to
     * Valgrind (this is similar to the PinBuffer() case where the backend
     * doesn't already have a buffer pin)
     */
    VALGRIND_MAKE_MEM_DEFINED(BufHdrGetBlock(buf), BLCKSZ);
 
    /*
     * Since we hold the buffer spinlock, we can update the buffer state and
     * release the lock in one operation.
     */
    buf_state = pg_atomic_read_u32(&buf->state);
    Assert(buf_state & BM_LOCKED);
    buf_state += BUF_REFCOUNT_ONE;
    UnlockBufHdr(buf, buf_state);
 
    b = BufferDescriptorGetBuffer(buf);
 
    ref = NewPrivateRefCountEntry(b);
    ref->refcount++;
 
    ResourceOwnerRememberBuffer(CurrentResourceOwner, b);
}
 
/*
 * Support for waking up another backend that is waiting for the cleanup lock
 * to be released using BM_PIN_COUNT_WAITER.
 *
 * See LockBufferForCleanup().
 *
 * Expected to be called just after releasing a buffer pin (in a BufferDesc,
 * not just reducing the backend-local pincount for the buffer).
 */
static void
WakePinCountWaiter(BufferDesc *buf)
{
    /*
     * Acquire the buffer header lock, re-check that there's a waiter. Another
     * backend could have unpinned this buffer, and already woken up the
     * waiter.
     *
     * There's no danger of the buffer being replaced after we unpinned it
     * above, as it's pinned by the waiter. The waiter removes
     * BM_PIN_COUNT_WAITER if it stops waiting for a reason other than this
     * backend waking it up.
     */
    uint32      buf_state = LockBufHdr(buf);
 
    if ((buf_state & BM_PIN_COUNT_WAITER) &&
        BUF_STATE_GET_REFCOUNT(buf_state) == 1)
    {
        /* we just released the last pin other than the waiter's */
        int         wait_backend_pgprocno = buf->wait_backend_pgprocno;
 
        buf_state &= ~BM_PIN_COUNT_WAITER;
        UnlockBufHdr(buf, buf_state);
        ProcSendSignal(wait_backend_pgprocno);
    }
    else
        UnlockBufHdr(buf, buf_state);
}
 
/*
 * UnpinBuffer -- make buffer available for replacement.
 *
 * This should be applied only to shared buffers, never local ones.  This
 * always adjusts CurrentResourceOwner.
 */
static void
UnpinBuffer(BufferDesc *buf)
{
    Buffer      b = BufferDescriptorGetBuffer(buf);
 
    ResourceOwnerForgetBuffer(CurrentResourceOwner, b);
    UnpinBufferNoOwner(buf);
}
 
static void
UnpinBufferNoOwner(BufferDesc *buf)
{
    PrivateRefCountEntry *ref;
    Buffer      b = BufferDescriptorGetBuffer(buf);
 
    Assert(!BufferIsLocal(b));
 
    /* not moving as we're likely deleting it soon anyway */
    ref = GetPrivateRefCountEntry(b, false);
    Assert(ref != NULL);
    Assert(ref->refcount > 0);
    ref->refcount--;
    if (ref->refcount == 0)
    {
        uint32      buf_state;
        uint32      old_buf_state;
 
        /*
         * Mark buffer non-accessible to Valgrind.
         *
         * Note that the buffer may have already been marked non-accessible
         * within access method code that enforces that buffers are only
         * accessed while a buffer lock is held.
         */
        VALGRIND_MAKE_MEM_NOACCESS(BufHdrGetBlock(buf), BLCKSZ);
 
        /* I'd better not still hold the buffer content lock */
        Assert(!LWLockHeldByMe(BufferDescriptorGetContentLock(buf)));
 
        /*
         * Decrement the shared reference count.
         *
         * Since buffer spinlock holder can update status using just write,
         * it's not safe to use atomic decrement here; thus use a CAS loop.
         */
        old_buf_state = pg_atomic_read_u32(&buf->state);
        for (;;)
        {
            if (old_buf_state & BM_LOCKED)
                old_buf_state = WaitBufHdrUnlocked(buf);
 
            buf_state = old_buf_state;
 
            buf_state -= BUF_REFCOUNT_ONE;
 
            if (pg_atomic_compare_exchange_u32(&buf->state, &old_buf_state,
                                               buf_state))
                break;
        }
 
        /* Support LockBufferForCleanup() */
        if (buf_state & BM_PIN_COUNT_WAITER)
            WakePinCountWaiter(buf);
 
        ForgetPrivateRefCountEntry(ref);
    }
}
 
#define ST_SORT sort_checkpoint_bufferids
#define ST_ELEMENT_TYPE CkptSortItem
#define ST_COMPARE(a, b) ckpt_buforder_comparator(a, b)
#define ST_SCOPE static
#define ST_DEFINE
#include "lib/sort_template.h"
 
/*
 * BufferSync -- Write out all dirty buffers in the pool.
 *
 * This is called at checkpoint time to write out all dirty shared buffers.
 * The checkpoint request flags should be passed in.  If CHECKPOINT_IMMEDIATE
 * is set, we disable delays between writes; if CHECKPOINT_IS_SHUTDOWN,
 * CHECKPOINT_END_OF_RECOVERY or CHECKPOINT_FLUSH_ALL is set, we write even
 * unlogged buffers, which are otherwise skipped.  The remaining flags
 * currently have no effect here.
 */
static void
BufferSync(int flags)
{
    uint32      buf_state;
    int         buf_id;
    int         num_to_scan;
    int         num_spaces;
    int         num_processed;
    int         num_written;
    CkptTsStatus *per_ts_stat = NULL;
    Oid         last_tsid;
    binaryheap *ts_heap;
    int         i;
    int         mask = BM_DIRTY;
    WritebackContext wb_context;
 
    /*
     * Unless this is a shutdown checkpoint or we have been explicitly told,
     * we write only permanent, dirty buffers.  But at shutdown or end of
     * recovery, we write all dirty buffers.
     */
    if (!((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY |
                    CHECKPOINT_FLUSH_ALL))))
        mask |= BM_PERMANENT;
 
    /*
     * Loop over all buffers, and mark the ones that need to be written with
     * BM_CHECKPOINT_NEEDED.  Count them as we go (num_to_scan), so that we
     * can estimate how much work needs to be done.
     *
     * This allows us to write only those pages that were dirty when the
     * checkpoint began, and not those that get dirtied while it proceeds.
     * Whenever a page with BM_CHECKPOINT_NEEDED is written out, either by us
     * later in this function, or by normal backends or the bgwriter cleaning
     * scan, the flag is cleared.  Any buffer dirtied after this point won't
     * have the flag set.
     *
     * Note that if we fail to write some buffer, we may leave buffers with
     * BM_CHECKPOINT_NEEDED still set.  This is OK since any such buffer would
     * certainly need to be written for the next checkpoint attempt, too.
     */
    num_to_scan = 0;
    for (buf_id = 0; buf_id < NBuffers; buf_id++)
    {
        BufferDesc *bufHdr = GetBufferDescriptor(buf_id);
 
        /*
         * Header spinlock is enough to examine BM_DIRTY, see comment in
         * SyncOneBuffer.
         */
        buf_state = LockBufHdr(bufHdr);
 
        if ((buf_state & mask) == mask)
        {
            CkptSortItem *item;
 
            buf_state |= BM_CHECKPOINT_NEEDED;
 
            item = &CkptBufferIds[num_to_scan++];
            item->buf_id = buf_id;
            item->tsId = bufHdr->tag.spcOid;
            item->relNumber = BufTagGetRelNumber(&bufHdr->tag);
            item->forkNum = BufTagGetForkNum(&bufHdr->tag);
            item->blockNum = bufHdr->tag.blockNum;
        }
 
        UnlockBufHdr(bufHdr, buf_state);
 
        /* Check for barrier events in case NBuffers is large. */
        if (ProcSignalBarrierPending)
            ProcessProcSignalBarrier();
    }
 
    if (num_to_scan == 0)
        return;                 /* nothing to do */
 
    WritebackContextInit(&wb_context, &checkpoint_flush_after);
 
    TRACE_POSTGRESQL_BUFFER_SYNC_START(NBuffers, num_to_scan);
 
    /*
     * Sort buffers that need to be written to reduce the likelihood of random
     * IO. The sorting is also important for the implementation of balancing
     * writes between tablespaces. Without balancing writes we'd potentially
     * end up writing to the tablespaces one-by-one; possibly overloading the
     * underlying system.
     */
    sort_checkpoint_bufferids(CkptBufferIds, num_to_scan);
 
    num_spaces = 0;
 
    /*
     * Allocate progress status for each tablespace with buffers that need to
     * be flushed. This requires the to-be-flushed array to be sorted.
     */
    last_tsid = InvalidOid;
    for (i = 0; i < num_to_scan; i++)
    {
        CkptTsStatus *s;
        Oid         cur_tsid;
 
        cur_tsid = CkptBufferIds[i].tsId;
 
        /*
         * Grow array of per-tablespace status structs, every time a new
         * tablespace is found.
         */
        if (last_tsid == InvalidOid || last_tsid != cur_tsid)
        {
            Size        sz;
 
            num_spaces++;
 
            /*
             * Not worth adding grow-by-power-of-2 logic here - even with a
             * few hundred tablespaces this should be fine.
             */
            sz = sizeof(CkptTsStatus) * num_spaces;
 
            if (per_ts_stat == NULL)
                per_ts_stat = (CkptTsStatus *) palloc(sz);
            else
                per_ts_stat = (CkptTsStatus *) repalloc(per_ts_stat, sz);
 
            s = &per_ts_stat[num_spaces - 1];
            memset(s, 0, sizeof(*s));
            s->tsId = cur_tsid;
 
            /*
             * The first buffer in this tablespace. As CkptBufferIds is sorted
             * by tablespace all (s->num_to_scan) buffers in this tablespace
             * will follow afterwards.
             */
            s->index = i;
 
            /*
             * progress_slice will be determined once we know how many buffers
             * are in each tablespace, i.e. after this loop.
             */
 
            last_tsid = cur_tsid;
        }
        else
        {
            s = &per_ts_stat[num_spaces - 1];
        }
 
        s->num_to_scan++;
 
        /* Check for barrier events. */
        if (ProcSignalBarrierPending)
            ProcessProcSignalBarrier();
    }
 
    Assert(num_spaces > 0);
 
    /*
     * Build a min-heap over the write-progress in the individual tablespaces,
     * and compute how large a portion of the total progress a single
     * processed buffer is.
     */
    ts_heap = binaryheap_allocate(num_spaces,
                                  ts_ckpt_progress_comparator,
                                  NULL);
 
    for (i = 0; i < num_spaces; i++)
    {
        CkptTsStatus *ts_stat = &per_ts_stat[i];
 
        ts_stat->progress_slice = (float8) num_to_scan / ts_stat->num_to_scan;
 
        binaryheap_add_unordered(ts_heap, PointerGetDatum(ts_stat));
    }
 
    binaryheap_build(ts_heap);
 
    /*
     * Iterate through to-be-checkpointed buffers and write the ones (still)
     * marked with BM_CHECKPOINT_NEEDED. The writes are balanced between
     * tablespaces; otherwise the sorting would lead to only one tablespace
     * receiving writes at a time, making inefficient use of the hardware.
     */
    num_processed = 0;
    num_written = 0;
    while (!binaryheap_empty(ts_heap))
    {
        BufferDesc *bufHdr = NULL;
        CkptTsStatus *ts_stat = (CkptTsStatus *)
            DatumGetPointer(binaryheap_first(ts_heap));
 
        buf_id = CkptBufferIds[ts_stat->index].buf_id;
        Assert(buf_id != -1);
 
        bufHdr = GetBufferDescriptor(buf_id);
 
        num_processed++;
 
        /*
         * We don't need to acquire the lock here, because we're only looking
         * at a single bit. It's possible that someone else writes the buffer
         * and clears the flag right after we check, but that doesn't matter
         * since SyncOneBuffer will then do nothing.  However, there is a
         * further race condition: it's conceivable that between the time we
         * examine the bit here and the time SyncOneBuffer acquires the lock,
         * someone else not only wrote the buffer but replaced it with another
         * page and dirtied it.  In that improbable case, SyncOneBuffer will
         * write the buffer though we didn't need to.  It doesn't seem worth
         * guarding against this, though.
         */
        if (pg_atomic_read_u32(&bufHdr->state) & BM_CHECKPOINT_NEEDED)
        {
            if (SyncOneBuffer(buf_id, false, &wb_context) & BUF_WRITTEN)
            {
                TRACE_POSTGRESQL_BUFFER_SYNC_WRITTEN(buf_id);
                PendingCheckpointerStats.buffers_written++;
                num_written++;
            }
        }
 
        /*
         * Measure progress independent of actually having to flush the buffer
         * - otherwise writing become unbalanced.
         */
        ts_stat->progress += ts_stat->progress_slice;
        ts_stat->num_scanned++;
        ts_stat->index++;
 
        /* Have all the buffers from the tablespace been processed? */
        if (ts_stat->num_scanned == ts_stat->num_to_scan)
        {
            binaryheap_remove_first(ts_heap);
        }
        else
        {
            /* update heap with the new progress */
            binaryheap_replace_first(ts_heap, PointerGetDatum(ts_stat));
        }
 
        /*
         * Sleep to throttle our I/O rate.
         *
         * (This will check for barrier events even if it doesn't sleep.)
         */
        CheckpointWriteDelay(flags, (double) num_processed / num_to_scan);
    }
 
    /*
     * Issue all pending flushes. Only checkpointer calls BufferSync(), so
     * IOContext will always be IOCONTEXT_NORMAL.
     */
    IssuePendingWritebacks(&wb_context, IOCONTEXT_NORMAL);
 
    pfree(per_ts_stat);
    per_ts_stat = NULL;
    binaryheap_free(ts_heap);
 
    /*
     * Update checkpoint statistics. As noted above, this doesn't include
     * buffers written by other backends or bgwriter scan.
     */
    CheckpointStats.ckpt_bufs_written += num_written;
 
    TRACE_POSTGRESQL_BUFFER_SYNC_DONE(NBuffers, num_written, num_to_scan);
}
 
/*
 * BgBufferSync -- Write out some dirty buffers in the pool.
 *
 * This is called periodically by the background writer process.
 *
 * Returns true if it's appropriate for the bgwriter process to go into
 * low-power hibernation mode.  (This happens if the strategy clock sweep
 * has been "lapped" and no buffer allocations have occurred recently,
 * or if the bgwriter has been effectively disabled by setting
 * bgwriter_lru_maxpages to 0.)
 */
bool
BgBufferSync(WritebackContext *wb_context)
{
    /* info obtained from freelist.c */
    int         strategy_buf_id;
    uint32      strategy_passes;
    uint32      recent_alloc;
 
    /*
     * Information saved between calls so we can determine the strategy
     * point's advance rate and avoid scanning already-cleaned buffers.
     */
    static bool saved_info_valid = false;
    static int  prev_strategy_buf_id;
    static uint32 prev_strategy_passes;
    static int  next_to_clean;
    static uint32 next_passes;
 
    /* Moving averages of allocation rate and clean-buffer density */
    static float smoothed_alloc = 0;
    static float smoothed_density = 10.0;
 
    /* Potentially these could be tunables, but for now, not */
    float       smoothing_samples = 16;
    float       scan_whole_pool_milliseconds = 120000.0;
 
    /* Used to compute how far we scan ahead */
    long        strategy_delta;
    int         bufs_to_lap;
    int         bufs_ahead;
    float       scans_per_alloc;
    int         reusable_buffers_est;
    int         upcoming_alloc_est;
    int         min_scan_buffers;
 
    /* Variables for the scanning loop proper */
    int         num_to_scan;
    int         num_written;
    int         reusable_buffers;
 
    /* Variables for final smoothed_density update */
    long        new_strategy_delta;
    uint32      new_recent_alloc;
 
    /*
     * Find out where the freelist clock sweep currently is, and how many
     * buffer allocations have happened since our last call.
     */
    strategy_buf_id = StrategySyncStart(&strategy_passes, &recent_alloc);
 
    /* Report buffer alloc counts to pgstat */
    PendingBgWriterStats.buf_alloc += recent_alloc;
 
    /*
     * If we're not running the LRU scan, just stop after doing the stats
     * stuff.  We mark the saved state invalid so that we can recover sanely
     * if LRU scan is turned back on later.
     */
    if (bgwriter_lru_maxpages <= 0)
    {
        saved_info_valid = false;
        return true;
    }
 
    /*
     * Compute strategy_delta = how many buffers have been scanned by the
     * clock sweep since last time.  If first time through, assume none. Then
     * see if we are still ahead of the clock sweep, and if so, how many
     * buffers we could scan before we'd catch up with it and "lap" it. Note:
     * weird-looking coding of xxx_passes comparisons are to avoid bogus
     * behavior when the passes counts wrap around.
     */
    if (saved_info_valid)
    {
        int32       passes_delta = strategy_passes - prev_strategy_passes;
 
        strategy_delta = strategy_buf_id - prev_strategy_buf_id;
        strategy_delta += (long) passes_delta * NBuffers;
 
        Assert(strategy_delta >= 0);
 
        if ((int32) (next_passes - strategy_passes) > 0)
        {
            /* we're one pass ahead of the strategy point */
            bufs_to_lap = strategy_buf_id - next_to_clean;
#ifdef BGW_DEBUG
            elog(DEBUG2, "bgwriter ahead: bgw %u-%u strategy %u-%u delta=%ld lap=%d",
                 next_passes, next_to_clean,
                 strategy_passes, strategy_buf_id,
                 strategy_delta, bufs_to_lap);
#endif
        }
        else if (next_passes == strategy_passes &&
                 next_to_clean >= strategy_buf_id)
        {
            /* on same pass, but ahead or at least not behind */
            bufs_to_lap = NBuffers - (next_to_clean - strategy_buf_id);
#ifdef BGW_DEBUG
            elog(DEBUG2, "bgwriter ahead: bgw %u-%u strategy %u-%u delta=%ld lap=%d",
                 next_passes, next_to_clean,
                 strategy_passes, strategy_buf_id,
                 strategy_delta, bufs_to_lap);
#endif
        }
        else
        {
            /*
             * We're behind, so skip forward to the strategy point and start
             * cleaning from there.
             */
#ifdef BGW_DEBUG
            elog(DEBUG2, "bgwriter behind: bgw %u-%u strategy %u-%u delta=%ld",
                 next_passes, next_to_clean,
                 strategy_passes, strategy_buf_id,
                 strategy_delta);
#endif
            next_to_clean = strategy_buf_id;
            next_passes = strategy_passes;
            bufs_to_lap = NBuffers;
        }
    }
    else
    {
        /*
         * Initializing at startup or after LRU scanning had been off. Always
         * start at the strategy point.
         */
#ifdef BGW_DEBUG
        elog(DEBUG2, "bgwriter initializing: strategy %u-%u",
             strategy_passes, strategy_buf_id);
#endif
        strategy_delta = 0;
        next_to_clean = strategy_buf_id;
        next_passes = strategy_passes;
        bufs_to_lap = NBuffers;
    }
 
    /* Update saved info for next time */
    prev_strategy_buf_id = strategy_buf_id;
    prev_strategy_passes = strategy_passes;
    saved_info_valid = true;
 
    /*
     * Compute how many buffers had to be scanned for each new allocation, ie,
     * 1/density of reusable buffers, and track a moving average of that.
     *
     * If the strategy point didn't move, we don't update the density estimate
     */
    if (strategy_delta > 0 && recent_alloc > 0)
    {
        scans_per_alloc = (float) strategy_delta / (float) recent_alloc;
        smoothed_density += (scans_per_alloc - smoothed_density) /
            smoothing_samples;
    }
 
    /*
     * Estimate how many reusable buffers there are between the current
     * strategy point and where we've scanned ahead to, based on the smoothed
     * density estimate.
     */
    bufs_ahead = NBuffers - bufs_to_lap;
    reusable_buffers_est = (float) bufs_ahead / smoothed_density;
 
    /*
     * Track a moving average of recent buffer allocations.  Here, rather than
     * a true average we want a fast-attack, slow-decline behavior: we
     * immediately follow any increase.
     */
    if (smoothed_alloc <= (float) recent_alloc)
        smoothed_alloc = recent_alloc;
    else
        smoothed_alloc += ((float) recent_alloc - smoothed_alloc) /
            smoothing_samples;
 
    /* Scale the estimate by a GUC to allow more aggressive tuning. */
    upcoming_alloc_est = (int) (smoothed_alloc * bgwriter_lru_multiplier);
 
    /*
     * If recent_alloc remains at zero for many cycles, smoothed_alloc will
     * eventually underflow to zero, and the underflows produce annoying
     * kernel warnings on some platforms.  Once upcoming_alloc_est has gone to
     * zero, there's no point in tracking smaller and smaller values of
     * smoothed_alloc, so just reset it to exactly zero to avoid this
     * syndrome.  It will pop back up as soon as recent_alloc increases.
     */
    if (upcoming_alloc_est == 0)
        smoothed_alloc = 0;
 
    /*
     * Even in cases where there's been little or no buffer allocation
     * activity, we want to make a small amount of progress through the buffer
     * cache so that as many reusable buffers as possible are clean after an
     * idle period.
     *
     * (scan_whole_pool_milliseconds / BgWriterDelay) computes how many times
     * the BGW will be called during the scan_whole_pool time; slice the
     * buffer pool into that many sections.
     */
    min_scan_buffers = (int) (NBuffers / (scan_whole_pool_milliseconds / BgWriterDelay));
 
    if (upcoming_alloc_est < (min_scan_buffers + reusable_buffers_est))
    {
#ifdef BGW_DEBUG
        elog(DEBUG2, "bgwriter: alloc_est=%d too small, using min=%d + reusable_est=%d",
             upcoming_alloc_est, min_scan_buffers, reusable_buffers_est);
#endif
        upcoming_alloc_est = min_scan_buffers + reusable_buffers_est;
    }
 
    /*
     * Now write out dirty reusable buffers, working forward from the
     * next_to_clean point, until we have lapped the strategy scan, or cleaned
     * enough buffers to match our estimate of the next cycle's allocation
     * requirements, or hit the bgwriter_lru_maxpages limit.
     */
 
    num_to_scan = bufs_to_lap;
    num_written = 0;
    reusable_buffers = reusable_buffers_est;
 
    /* Execute the LRU scan */
    while (num_to_scan > 0 && reusable_buffers < upcoming_alloc_est)
    {
        int         sync_state = SyncOneBuffer(next_to_clean, true,
                                               wb_context);
 
        if (++next_to_clean >= NBuffers)
        {
            next_to_clean = 0;
            next_passes++;
        }
        num_to_scan--;
 
        if (sync_state & BUF_WRITTEN)
        {
            reusable_buffers++;
            if (++num_written >= bgwriter_lru_maxpages)
            {
                PendingBgWriterStats.maxwritten_clean++;
                break;
            }
        }
        else if (sync_state & BUF_REUSABLE)
            reusable_buffers++;
    }
 
    PendingBgWriterStats.buf_written_clean += num_written;
 
#ifdef BGW_DEBUG
    elog(DEBUG1, "bgwriter: recent_alloc=%u smoothed=%.2f delta=%ld ahead=%d density=%.2f reusable_est=%d upcoming_est=%d scanned=%d wrote=%d reusable=%d",
         recent_alloc, smoothed_alloc, strategy_delta, bufs_ahead,
         smoothed_density, reusable_buffers_est, upcoming_alloc_est,
         bufs_to_lap - num_to_scan,
         num_written,
         reusable_buffers - reusable_buffers_est);
#endif
 
    /*
     * Consider the above scan as being like a new allocation scan.
     * Characterize its density and update the smoothed one based on it. This
     * effectively halves the moving average period in cases where both the
     * strategy and the background writer are doing some useful scanning,
     * which is helpful because a long memory isn't as desirable on the
     * density estimates.
     */
    new_strategy_delta = bufs_to_lap - num_to_scan;
    new_recent_alloc = reusable_buffers - reusable_buffers_est;
    if (new_strategy_delta > 0 && new_recent_alloc > 0)
    {
        scans_per_alloc = (float) new_strategy_delta / (float) new_recent_alloc;
        smoothed_density += (scans_per_alloc - smoothed_density) /
            smoothing_samples;
 
#ifdef BGW_DEBUG
        elog(DEBUG2, "bgwriter: cleaner density alloc=%u scan=%ld density=%.2f new smoothed=%.2f",
             new_recent_alloc, new_strategy_delta,
             scans_per_alloc, smoothed_density);
#endif
    }
 
    /* Return true if OK to hibernate */
    return (bufs_to_lap == 0 && recent_alloc == 0);
}
 
/*
 * SyncOneBuffer -- process a single buffer during syncing.
 *
 * If skip_recently_used is true, we don't write currently-pinned buffers, nor
 * buffers marked recently used, as these are not replacement candidates.
 *
 * Returns a bitmask containing the following flag bits:
 *  BUF_WRITTEN: we wrote the buffer.
 *  BUF_REUSABLE: buffer is available for replacement, ie, it has
 *      pin count 0 and usage count 0.
 *
 * (BUF_WRITTEN could be set in error if FlushBuffer finds the buffer clean
 * after locking it, but we don't care all that much.)
 */
static int
SyncOneBuffer(int buf_id, bool skip_recently_used, WritebackContext *wb_context)
{
    BufferDesc *bufHdr = GetBufferDescriptor(buf_id);
    int         result = 0;
    uint32      buf_state;
    BufferTag   tag;
 
    /* Make sure we can handle the pin */
    ReservePrivateRefCountEntry();
    ResourceOwnerEnlarge(CurrentResourceOwner);
 
    /*
     * Check whether buffer needs writing.
     *
     * We can make this check without taking the buffer content lock so long
     * as we mark pages dirty in access methods *before* logging changes with
     * XLogInsert(): if someone marks the buffer dirty just after our check we
     * don't worry because our checkpoint.redo points before log record for
     * upcoming changes and so we are not required to write such dirty buffer.
     */
    buf_state = LockBufHdr(bufHdr);
 
    if (BUF_STATE_GET_REFCOUNT(buf_state) == 0 &&
        BUF_STATE_GET_USAGECOUNT(buf_state) == 0)
    {
        result |= BUF_REUSABLE;
    }
    else if (skip_recently_used)
    {
        /* Caller told us not to write recently-used buffers */
        UnlockBufHdr(bufHdr, buf_state);
        return result;
    }
 
    if (!(buf_state & BM_VALID) || !(buf_state & BM_DIRTY))
    {
        /* It's clean, so nothing to do */
        UnlockBufHdr(bufHdr, buf_state);
        return result;
    }
 
    /*
     * Pin it, share-lock it, write it.  (FlushBuffer will do nothing if the
     * buffer is clean by the time we've locked it.)
     */
    PinBuffer_Locked(bufHdr);
    LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
 
    FlushBuffer(bufHdr, NULL, IOOBJECT_RELATION, IOCONTEXT_NORMAL);
 
    LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
 
    tag = bufHdr->tag;
 
    UnpinBuffer(bufHdr);
 
    /*
     * SyncOneBuffer() is only called by checkpointer and bgwriter, so
     * IOContext will always be IOCONTEXT_NORMAL.
     */
    ScheduleBufferTagForWriteback(wb_context, IOCONTEXT_NORMAL, &tag);
 
    return result | BUF_WRITTEN;
}
 
/*
 *      AtEOXact_Buffers - clean up at end of transaction.
 *
 *      As of PostgreSQL 8.0, buffer pins should get released by the
 *      ResourceOwner mechanism.  This routine is just a debugging
 *      cross-check that no pins remain.
 */
void
AtEOXact_Buffers(bool isCommit)
{
    CheckForBufferLeaks();
 
    AtEOXact_LocalBuffers(isCommit);
 
    Assert(PrivateRefCountOverflowed == 0);
}
 
/*
 * Initialize access to shared buffer pool
 *
 * This is called during backend startup (whether standalone or under the
 * postmaster).  It sets up for this backend's access to the already-existing
 * buffer pool.
 */
void
InitBufferManagerAccess(void)
{
    HASHCTL     hash_ctl;
 
    /*
     * An advisory limit on the number of pins each backend should hold, based
     * on shared_buffers and the maximum number of connections possible.
     * That's very pessimistic, but outside toy-sized shared_buffers it should
     * allow plenty of pins.  LimitAdditionalPins() and
     * GetAdditionalPinLimit() can be used to check the remaining balance.
     */
    MaxProportionalPins = NBuffers / (MaxBackends + NUM_AUXILIARY_PROCS);
 
    memset(&PrivateRefCountArray, 0, sizeof(PrivateRefCountArray));
 
    hash_ctl.keysize = sizeof(int32);
    hash_ctl.entrysize = sizeof(PrivateRefCountEntry);
 
    PrivateRefCountHash = hash_create("PrivateRefCount", 100, &hash_ctl,
                                      HASH_ELEM | HASH_BLOBS);
 
    /*
     * AtProcExit_Buffers needs LWLock access, and thereby has to be called at
     * the corresponding phase of backend shutdown.
     */
    Assert(MyProc != NULL);
    on_shmem_exit(AtProcExit_Buffers, 0);
}
 
/*
 * During backend exit, ensure that we released all shared-buffer locks and
 * assert that we have no remaining pins.
 */
static void
AtProcExit_Buffers(int code, Datum arg)
{
    UnlockBuffers();
 
    CheckForBufferLeaks();
 
    /* localbuf.c needs a chance too */
    AtProcExit_LocalBuffers();
}
 
/*
 *      CheckForBufferLeaks - ensure this backend holds no buffer pins
 *
 *      As of PostgreSQL 8.0, buffer pins should get released by the
 *      ResourceOwner mechanism.  This routine is just a debugging
 *      cross-check that no pins remain.
 */
static void
CheckForBufferLeaks(void)
{
#ifdef USE_ASSERT_CHECKING
    int         RefCountErrors = 0;
    PrivateRefCountEntry *res;
    int         i;
    char       *s;
 
    /* check the array */
    for (i = 0; i < REFCOUNT_ARRAY_ENTRIES; i++)
    {
        res = &PrivateRefCountArray[i];
 
        if (res->buffer != InvalidBuffer)
        {
            s = DebugPrintBufferRefcount(res->buffer);
            elog(WARNING, "buffer refcount leak: %s", s);
            pfree(s);
 
            RefCountErrors++;
        }
    }
 
    /* if necessary search the hash */
    if (PrivateRefCountOverflowed)
    {
        HASH_SEQ_STATUS hstat;
 
        hash_seq_init(&hstat, PrivateRefCountHash);
        while ((res = (PrivateRefCountEntry *) hash_seq_search(&hstat)) != NULL)
        {
            s = DebugPrintBufferRefcount(res->buffer);
            elog(WARNING, "buffer refcount leak: %s", s);
            pfree(s);
            RefCountErrors++;
        }
    }
 
    Assert(RefCountErrors == 0);
#endif
}
 
#ifdef USE_ASSERT_CHECKING
/*
 * Check for exclusive-locked catalog buffers.  This is the core of
 * AssertCouldGetRelation().
 *
 * A backend would self-deadlock on LWLocks if the catalog scan read the
 * exclusive-locked buffer.  The main threat is exclusive-locked buffers of
 * catalogs used in relcache, because a catcache search on any catalog may
 * build that catalog's relcache entry.  We don't have an inventory of
 * catalogs relcache uses, so just check buffers of most catalogs.
 *
 * It's better to minimize waits while holding an exclusive buffer lock, so it
 * would be nice to broaden this check not to be catalog-specific.  However,
 * bttextcmp() accesses pg_collation, and non-core opclasses might similarly
 * read tables.  That is deadlock-free as long as there's no loop in the
 * dependency graph: modifying table A may cause an opclass to read table B,
 * but it must not cause a read of table A.
 */
void
AssertBufferLocksPermitCatalogRead(void)
{
    ForEachLWLockHeldByMe(AssertNotCatalogBufferLock, NULL);
}
 
static void
AssertNotCatalogBufferLock(LWLock *lock, LWLockMode mode,
                           void *unused_context)
{
    BufferDesc *bufHdr;
    BufferTag   tag;
    Oid         relid;
 
    if (mode != LW_EXCLUSIVE)
        return;
 
    if (!((BufferDescPadded *) lock > BufferDescriptors &&
          (BufferDescPadded *) lock < BufferDescriptors + NBuffers))
        return;                 /* not a buffer lock */
 
    bufHdr = (BufferDesc *)
        ((char *) lock - offsetof(BufferDesc, content_lock));
    tag = bufHdr->tag;
 
    /*
     * This relNumber==relid assumption holds until a catalog experiences
     * VACUUM FULL or similar.  After a command like that, relNumber will be
     * in the normal (non-catalog) range, and we lose the ability to detect
     * hazardous access to that catalog.  Calling RelidByRelfilenumber() would
     * close that gap, but RelidByRelfilenumber() might then deadlock with a
     * held lock.
     */
    relid = tag.relNumber;
 
    if (IsCatalogTextUniqueIndexOid(relid)) /* see comments at the callee */
        return;
 
    Assert(!IsCatalogRelationOid(relid));
}
#endif
 
 
/*
 * Helper routine to issue warnings when a buffer is unexpectedly pinned
 */
char *
DebugPrintBufferRefcount(Buffer buffer)
{
    BufferDesc *buf;
    int32       loccount;
    char       *result;
    ProcNumber  backend;
    uint32      buf_state;
 
    Assert(BufferIsValid(buffer));
    if (BufferIsLocal(buffer))
    {
        buf = GetLocalBufferDescriptor(-buffer - 1);
        loccount = LocalRefCount[-buffer - 1];
        backend = MyProcNumber;
    }
    else
    {
        buf = GetBufferDescriptor(buffer - 1);
        loccount = GetPrivateRefCount(buffer);
        backend = INVALID_PROC_NUMBER;
    }
 
    /* theoretically we should lock the bufhdr here */
    buf_state = pg_atomic_read_u32(&buf->state);
 
    result = psprintf("[%03d] (rel=%s, blockNum=%u, flags=0x%x, refcount=%u %d)",
                      buffer,
                      relpathbackend(BufTagGetRelFileLocator(&buf->tag), backend,
                                     BufTagGetForkNum(&buf->tag)).str,
                      buf->tag.blockNum, buf_state & BUF_FLAG_MASK,
                      BUF_STATE_GET_REFCOUNT(buf_state), loccount);
    return result;
}
 
/*
 * CheckPointBuffers
 *
 * Flush all dirty blocks in buffer pool to disk at checkpoint time.
 *
 * Note: temporary relations do not participate in checkpoints, so they don't
 * need to be flushed.
 */
void
CheckPointBuffers(int flags)
{
    BufferSync(flags);
}
 
/*
 * BufferGetBlockNumber
 *      Returns the block number associated with a buffer.
 *
 * Note:
 *      Assumes that the buffer is valid and pinned, else the
 *      value may be obsolete immediately...
 */
BlockNumber
BufferGetBlockNumber(Buffer buffer)
{
    BufferDesc *bufHdr;
 
    Assert(BufferIsPinned(buffer));
 
    if (BufferIsLocal(buffer))
        bufHdr = GetLocalBufferDescriptor(-buffer - 1);
    else
        bufHdr = GetBufferDescriptor(buffer - 1);
 
    /* pinned, so OK to read tag without spinlock */
    return bufHdr->tag.blockNum;
}
 
/*
 * BufferGetTag
 *      Returns the relfilelocator, fork number and block number associated with
 *      a buffer.
 */
void
BufferGetTag(Buffer buffer, RelFileLocator *rlocator, ForkNumber *forknum,
             BlockNumber *blknum)
{
    BufferDesc *bufHdr;
 
    /* Do the same checks as BufferGetBlockNumber. */
    Assert(BufferIsPinned(buffer));
 
    if (BufferIsLocal(buffer))
        bufHdr = GetLocalBufferDescriptor(-buffer - 1);
    else
        bufHdr = GetBufferDescriptor(buffer - 1);
 
    /* pinned, so OK to read tag without spinlock */
    *rlocator = BufTagGetRelFileLocator(&bufHdr->tag);
    *forknum = BufTagGetForkNum(&bufHdr->tag);
    *blknum = bufHdr->tag.blockNum;
}
 
/*
 * FlushBuffer
 *      Physically write out a shared buffer.
 *
 * NOTE: this actually just passes the buffer contents to the kernel; the
 * real write to disk won't happen until the kernel feels like it.  This
 * is okay from our point of view since we can redo the changes from WAL.
 * However, we will need to force the changes to disk via fsync before
 * we can checkpoint WAL.
 *
 * The caller must hold a pin on the buffer and have share-locked the
 * buffer contents.  (Note: a share-lock does not prevent updates of
 * hint bits in the buffer, so the page could change while the write
 * is in progress, but we assume that that will not invalidate the data
 * written.)
 *
 * If the caller has an smgr reference for the buffer's relation, pass it
 * as the second parameter.  If not, pass NULL.
 */
static void
FlushBuffer(BufferDesc *buf, SMgrRelation reln, IOObject io_object,
            IOContext io_context)
{
    XLogRecPtr  recptr;
    ErrorContextCallback errcallback;
    instr_time  io_start;
    Block       bufBlock;
    char       *bufToWrite;
    uint32      buf_state;
 
    /*
     * Try to start an I/O operation.  If StartBufferIO returns false, then
     * someone else flushed the buffer before we could, so we need not do
     * anything.
     */
    if (!StartBufferIO(buf, false, false))
        return;
 
    /* Setup error traceback support for ereport() */
    errcallback.callback = shared_buffer_write_error_callback;
    errcallback.arg = buf;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* Find smgr relation for buffer */
    if (reln == NULL)
        reln = smgropen(BufTagGetRelFileLocator(&buf->tag), INVALID_PROC_NUMBER);
 
    TRACE_POSTGRESQL_BUFFER_FLUSH_START(BufTagGetForkNum(&buf->tag),
                                        buf->tag.blockNum,
                                        reln->smgr_rlocator.locator.spcOid,
                                        reln->smgr_rlocator.locator.dbOid,
                                        reln->smgr_rlocator.locator.relNumber);
 
    buf_state = LockBufHdr(buf);
 
    /*
     * Run PageGetLSN while holding header lock, since we don't have the
     * buffer locked exclusively in all cases.
     */
    recptr = BufferGetLSN(buf);
 
    /* To check if block content changes while flushing. - vadim 01/17/97 */
    buf_state &= ~BM_JUST_DIRTIED;
    UnlockBufHdr(buf, buf_state);
 
    /*
     * Force XLOG flush up to buffer's LSN.  This implements the basic WAL
     * rule that log updates must hit disk before any of the data-file changes
     * they describe do.
     *
     * However, this rule does not apply to unlogged relations, which will be
     * lost after a crash anyway.  Most unlogged relation pages do not bear
     * LSNs since we never emit WAL records for them, and therefore flushing
     * up through the buffer LSN would be useless, but harmless.  However,
     * GiST indexes use LSNs internally to track page-splits, and therefore
     * unlogged GiST pages bear "fake" LSNs generated by
     * GetFakeLSNForUnloggedRel.  It is unlikely but possible that the fake
     * LSN counter could advance past the WAL insertion point; and if it did
     * happen, attempting to flush WAL through that location would fail, with
     * disastrous system-wide consequences.  To make sure that can't happen,
     * skip the flush if the buffer isn't permanent.
     */
    if (buf_state & BM_PERMANENT)
        XLogFlush(recptr);
 
    /*
     * Now it's safe to write the buffer to disk. Note that no one else should
     * have been able to write it, while we were busy with log flushing,
     * because we got the exclusive right to perform I/O by setting the
     * BM_IO_IN_PROGRESS bit.
     */
    bufBlock = BufHdrGetBlock(buf);
 
    /*
     * Update page checksum if desired.  Since we have only shared lock on the
     * buffer, other processes might be updating hint bits in it, so we must
     * copy the page to private storage if we do checksumming.
     */
    bufToWrite = PageSetChecksumCopy((Page) bufBlock, buf->tag.blockNum);
 
    io_start = pgstat_prepare_io_time(track_io_timing);
 
    /*
     * bufToWrite is either the shared buffer or a copy, as appropriate.
     */
    smgrwrite(reln,
              BufTagGetForkNum(&buf->tag),
              buf->tag.blockNum,
              bufToWrite,
              false);
 
    /*
     * When a strategy is in use, only flushes of dirty buffers already in the
     * strategy ring are counted as strategy writes (IOCONTEXT
     * [BULKREAD|BULKWRITE|VACUUM] IOOP_WRITE) for the purpose of IO
     * statistics tracking.
     *
     * If a shared buffer initially added to the ring must be flushed before
     * being used, this is counted as an IOCONTEXT_NORMAL IOOP_WRITE.
     *
     * If a shared buffer which was added to the ring later because the
     * current strategy buffer is pinned or in use or because all strategy
     * buffers were dirty and rejected (for BAS_BULKREAD operations only)
     * requires flushing, this is counted as an IOCONTEXT_NORMAL IOOP_WRITE
     * (from_ring will be false).
     *
     * When a strategy is not in use, the write can only be a "regular" write
     * of a dirty shared buffer (IOCONTEXT_NORMAL IOOP_WRITE).
     */
    pgstat_count_io_op_time(IOOBJECT_RELATION, io_context,
                            IOOP_WRITE, io_start, 1, BLCKSZ);
 
    pgBufferUsage.shared_blks_written++;
 
    /*
     * Mark the buffer as clean (unless BM_JUST_DIRTIED has become set) and
     * end the BM_IO_IN_PROGRESS state.
     */
    TerminateBufferIO(buf, true, 0, true, false);
 
    TRACE_POSTGRESQL_BUFFER_FLUSH_DONE(BufTagGetForkNum(&buf->tag),
                                       buf->tag.blockNum,
                                       reln->smgr_rlocator.locator.spcOid,
                                       reln->smgr_rlocator.locator.dbOid,
                                       reln->smgr_rlocator.locator.relNumber);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
/*
 * RelationGetNumberOfBlocksInFork
 *      Determines the current number of pages in the specified relation fork.
 *
 * Note that the accuracy of the result will depend on the details of the
 * relation's storage. For builtin AMs it'll be accurate, but for external AMs
 * it might not be.
 */
BlockNumber
RelationGetNumberOfBlocksInFork(Relation relation, ForkNumber forkNum)
{
    if (RELKIND_HAS_TABLE_AM(relation->rd_rel->relkind))
    {
        /*
         * Not every table AM uses BLCKSZ wide fixed size blocks. Therefore
         * tableam returns the size in bytes - but for the purpose of this
         * routine, we want the number of blocks. Therefore divide, rounding
         * up.
         */
        uint64      szbytes;
 
        szbytes = table_relation_size(relation, forkNum);
 
        return (szbytes + (BLCKSZ - 1)) / BLCKSZ;
    }
    else if (RELKIND_HAS_STORAGE(relation->rd_rel->relkind))
    {
        return smgrnblocks(RelationGetSmgr(relation), forkNum);
    }
    else
        Assert(false);
 
    return 0;                   /* keep compiler quiet */
}
 
/*
 * BufferIsPermanent
 *      Determines whether a buffer will potentially still be around after
 *      a crash.  Caller must hold a buffer pin.
 */
bool
BufferIsPermanent(Buffer buffer)
{
    BufferDesc *bufHdr;
 
    /* Local buffers are used only for temp relations. */
    if (BufferIsLocal(buffer))
        return false;
 
    /* Make sure we've got a real buffer, and that we hold a pin on it. */
    Assert(BufferIsValid(buffer));
    Assert(BufferIsPinned(buffer));
 
    /*
     * BM_PERMANENT can't be changed while we hold a pin on the buffer, so we
     * need not bother with the buffer header spinlock.  Even if someone else
     * changes the buffer header state while we're doing this, the state is
     * changed atomically, so we'll read the old value or the new value, but
     * not random garbage.
     */
    bufHdr = GetBufferDescriptor(buffer - 1);
    return (pg_atomic_read_u32(&bufHdr->state) & BM_PERMANENT) != 0;
}
 
/*
 * BufferGetLSNAtomic
 *      Retrieves the LSN of the buffer atomically using a buffer header lock.
 *      This is necessary for some callers who may not have an exclusive lock
 *      on the buffer.
 */
XLogRecPtr
BufferGetLSNAtomic(Buffer buffer)
{
    char       *page = BufferGetPage(buffer);
    BufferDesc *bufHdr;
    XLogRecPtr  lsn;
    uint32      buf_state;
 
    /*
     * If we don't need locking for correctness, fastpath out.
     */
    if (!XLogHintBitIsNeeded() || BufferIsLocal(buffer))
        return PageGetLSN(page);
 
    /* Make sure we've got a real buffer, and that we hold a pin on it. */
    Assert(BufferIsValid(buffer));
    Assert(BufferIsPinned(buffer));
 
    bufHdr = GetBufferDescriptor(buffer - 1);
    buf_state = LockBufHdr(bufHdr);
    lsn = PageGetLSN(page);
    UnlockBufHdr(bufHdr, buf_state);
 
    return lsn;
}
 
/* ---------------------------------------------------------------------
 *      DropRelationBuffers
 *
 *      This function removes from the buffer pool all the pages of the
 *      specified relation forks that have block numbers >= firstDelBlock.
 *      (In particular, with firstDelBlock = 0, all pages are removed.)
 *      Dirty pages are simply dropped, without bothering to write them
 *      out first.  Therefore, this is NOT rollback-able, and so should be
 *      used only with extreme caution!
 *
 *      Currently, this is called only from smgr.c when the underlying file
 *      is about to be deleted or truncated (firstDelBlock is needed for
 *      the truncation case).  The data in the affected pages would therefore
 *      be deleted momentarily anyway, and there is no point in writing it.
 *      It is the responsibility of higher-level code to ensure that the
 *      deletion or truncation does not lose any data that could be needed
 *      later.  It is also the responsibility of higher-level code to ensure
 *      that no other process could be trying to load more pages of the
 *      relation into buffers.
 * --------------------------------------------------------------------
 */
void
DropRelationBuffers(SMgrRelation smgr_reln, ForkNumber *forkNum,
                    int nforks, BlockNumber *firstDelBlock)
{
    int         i;
    int         j;
    RelFileLocatorBackend rlocator;
    BlockNumber nForkBlock[MAX_FORKNUM];
    uint64      nBlocksToInvalidate = 0;
 
    rlocator = smgr_reln->smgr_rlocator;
 
    /* If it's a local relation, it's localbuf.c's problem. */
    if (RelFileLocatorBackendIsTemp(rlocator))
    {
        if (rlocator.backend == MyProcNumber)
        {
            for (j = 0; j < nforks; j++)
                DropRelationLocalBuffers(rlocator.locator, forkNum[j],
                                         firstDelBlock[j]);
        }
        return;
    }
 
    /*
     * To remove all the pages of the specified relation forks from the buffer
     * pool, we need to scan the entire buffer pool but we can optimize it by
     * finding the buffers from BufMapping table provided we know the exact
     * size of each fork of the relation. The exact size is required to ensure
     * that we don't leave any buffer for the relation being dropped as
     * otherwise the background writer or checkpointer can lead to a PANIC
     * error while flushing buffers corresponding to files that don't exist.
     *
     * To know the exact size, we rely on the size cached for each fork by us
     * during recovery which limits the optimization to recovery and on
     * standbys but we can easily extend it once we have shared cache for
     * relation size.
     *
     * In recovery, we cache the value returned by the first lseek(SEEK_END)
     * and the future writes keeps the cached value up-to-date. See
     * smgrextend. It is possible that the value of the first lseek is smaller
     * than the actual number of existing blocks in the file due to buggy
     * Linux kernels that might not have accounted for the recent write. But
     * that should be fine because there must not be any buffers after that
     * file size.
     */
    for (i = 0; i < nforks; i++)
    {
        /* Get the number of blocks for a relation's fork */
        nForkBlock[i] = smgrnblocks_cached(smgr_reln, forkNum[i]);
 
        if (nForkBlock[i] == InvalidBlockNumber)
        {
            nBlocksToInvalidate = InvalidBlockNumber;
            break;
        }
 
        /* calculate the number of blocks to be invalidated */
        nBlocksToInvalidate += (nForkBlock[i] - firstDelBlock[i]);
    }
 
    /*
     * We apply the optimization iff the total number of blocks to invalidate
     * is below the BUF_DROP_FULL_SCAN_THRESHOLD.
     */
    if (BlockNumberIsValid(nBlocksToInvalidate) &&
        nBlocksToInvalidate < BUF_DROP_FULL_SCAN_THRESHOLD)
    {
        for (j = 0; j < nforks; j++)
            FindAndDropRelationBuffers(rlocator.locator, forkNum[j],
                                       nForkBlock[j], firstDelBlock[j]);
        return;
    }
 
    for (i = 0; i < NBuffers; i++)
    {
        BufferDesc *bufHdr = GetBufferDescriptor(i);
        uint32      buf_state;
 
        /*
         * We can make this a tad faster by prechecking the buffer tag before
         * we attempt to lock the buffer; this saves a lot of lock
         * acquisitions in typical cases.  It should be safe because the
         * caller must have AccessExclusiveLock on the relation, or some other
         * reason to be certain that no one is loading new pages of the rel
         * into the buffer pool.  (Otherwise we might well miss such pages
         * entirely.)  Therefore, while the tag might be changing while we
         * look at it, it can't be changing *to* a value we care about, only
         * *away* from such a value.  So false negatives are impossible, and
         * false positives are safe because we'll recheck after getting the
         * buffer lock.
         *
         * We could check forkNum and blockNum as well as the rlocator, but
         * the incremental win from doing so seems small.
         */
        if (!BufTagMatchesRelFileLocator(&bufHdr->tag, &rlocator.locator))
            continue;
 
        buf_state = LockBufHdr(bufHdr);
 
        for (j = 0; j < nforks; j++)
        {
            if (BufTagMatchesRelFileLocator(&bufHdr->tag, &rlocator.locator) &&
                BufTagGetForkNum(&bufHdr->tag) == forkNum[j] &&
                bufHdr->tag.blockNum >= firstDelBlock[j])
            {
                InvalidateBuffer(bufHdr);   /* releases spinlock */
                break;
            }
        }
        if (j >= nforks)
            UnlockBufHdr(bufHdr, buf_state);
    }
}
 
/* ---------------------------------------------------------------------
 *      DropRelationsAllBuffers
 *
 *      This function removes from the buffer pool all the pages of all
 *      forks of the specified relations.  It's equivalent to calling
 *      DropRelationBuffers once per fork per relation with firstDelBlock = 0.
 *      --------------------------------------------------------------------
 */
void
DropRelationsAllBuffers(SMgrRelation *smgr_reln, int nlocators)
{
    int         i;
    int         n = 0;
    SMgrRelation *rels;
    BlockNumber (*block)[MAX_FORKNUM + 1];
    uint64      nBlocksToInvalidate = 0;
    RelFileLocator *locators;
    bool        cached = true;
    bool        use_bsearch;
 
    if (nlocators == 0)
        return;
 
    rels = palloc(sizeof(SMgrRelation) * nlocators);    /* non-local relations */
 
    /* If it's a local relation, it's localbuf.c's problem. */
    for (i = 0; i < nlocators; i++)
    {
        if (RelFileLocatorBackendIsTemp(smgr_reln[i]->smgr_rlocator))
        {
            if (smgr_reln[i]->smgr_rlocator.backend == MyProcNumber)
                DropRelationAllLocalBuffers(smgr_reln[i]->smgr_rlocator.locator);
        }
        else
            rels[n++] = smgr_reln[i];
    }
 
    /*
     * If there are no non-local relations, then we're done. Release the
     * memory and return.
     */
    if (n == 0)
    {
        pfree(rels);
        return;
    }
 
    /*
     * This is used to remember the number of blocks for all the relations
     * forks.
     */
    block = (BlockNumber (*)[MAX_FORKNUM + 1])
        palloc(sizeof(BlockNumber) * n * (MAX_FORKNUM + 1));
 
    /*
     * We can avoid scanning the entire buffer pool if we know the exact size
     * of each of the given relation forks. See DropRelationBuffers.
     */
    for (i = 0; i < n && cached; i++)
    {
        for (int j = 0; j <= MAX_FORKNUM; j++)
        {
            /* Get the number of blocks for a relation's fork. */
            block[i][j] = smgrnblocks_cached(rels[i], j);
 
            /* We need to only consider the relation forks that exists. */
            if (block[i][j] == InvalidBlockNumber)
            {
                if (!smgrexists(rels[i], j))
                    continue;
                cached = false;
                break;
            }
 
            /* calculate the total number of blocks to be invalidated */
            nBlocksToInvalidate += block[i][j];
        }
    }
 
    /*
     * We apply the optimization iff the total number of blocks to invalidate
     * is below the BUF_DROP_FULL_SCAN_THRESHOLD.
     */
    if (cached && nBlocksToInvalidate < BUF_DROP_FULL_SCAN_THRESHOLD)
    {
        for (i = 0; i < n; i++)
        {
            for (int j = 0; j <= MAX_FORKNUM; j++)
            {
                /* ignore relation forks that doesn't exist */
                if (!BlockNumberIsValid(block[i][j]))
                    continue;
 
                /* drop all the buffers for a particular relation fork */
                FindAndDropRelationBuffers(rels[i]->smgr_rlocator.locator,
                                           j, block[i][j], 0);
            }
        }
 
        pfree(block);
        pfree(rels);
        return;
    }
 
    pfree(block);
    locators = palloc(sizeof(RelFileLocator) * n);  /* non-local relations */
    for (i = 0; i < n; i++)
        locators[i] = rels[i]->smgr_rlocator.locator;
 
    /*
     * For low number of relations to drop just use a simple walk through, to
     * save the bsearch overhead. The threshold to use is rather a guess than
     * an exactly determined value, as it depends on many factors (CPU and RAM
     * speeds, amount of shared buffers etc.).
     */
    use_bsearch = n > RELS_BSEARCH_THRESHOLD;
 
    /* sort the list of rlocators if necessary */
    if (use_bsearch)
        qsort(locators, n, sizeof(RelFileLocator), rlocator_comparator);
 
    for (i = 0; i < NBuffers; i++)
    {
        RelFileLocator *rlocator = NULL;
        BufferDesc *bufHdr = GetBufferDescriptor(i);
        uint32      buf_state;
 
        /*
         * As in DropRelationBuffers, an unlocked precheck should be safe and
         * saves some cycles.
         */
 
        if (!use_bsearch)
        {
            int         j;
 
            for (j = 0; j < n; j++)
            {
                if (BufTagMatchesRelFileLocator(&bufHdr->tag, &locators[j]))
                {
                    rlocator = &locators[j];
                    break;
                }
            }
        }
        else
        {
            RelFileLocator locator;
 
            locator = BufTagGetRelFileLocator(&bufHdr->tag);
            rlocator = bsearch(&locator,
                               locators, n, sizeof(RelFileLocator),
                               rlocator_comparator);
        }
 
        /* buffer doesn't belong to any of the given relfilelocators; skip it */
        if (rlocator == NULL)
            continue;
 
        buf_state = LockBufHdr(bufHdr);
        if (BufTagMatchesRelFileLocator(&bufHdr->tag, rlocator))
            InvalidateBuffer(bufHdr);   /* releases spinlock */
        else
            UnlockBufHdr(bufHdr, buf_state);
    }
 
    pfree(locators);
    pfree(rels);
}
 
/* ---------------------------------------------------------------------
 *      FindAndDropRelationBuffers
 *
 *      This function performs look up in BufMapping table and removes from the
 *      buffer pool all the pages of the specified relation fork that has block
 *      number >= firstDelBlock. (In particular, with firstDelBlock = 0, all
 *      pages are removed.)
 * --------------------------------------------------------------------
 */
static void
FindAndDropRelationBuffers(RelFileLocator rlocator, ForkNumber forkNum,
                           BlockNumber nForkBlock,
                           BlockNumber firstDelBlock)
{
    BlockNumber curBlock;
 
    for (curBlock = firstDelBlock; curBlock < nForkBlock; curBlock++)
    {
        uint32      bufHash;    /* hash value for tag */
        BufferTag   bufTag;     /* identity of requested block */
        LWLock     *bufPartitionLock;   /* buffer partition lock for it */
        int         buf_id;
        BufferDesc *bufHdr;
        uint32      buf_state;
 
        /* create a tag so we can lookup the buffer */
        InitBufferTag(&bufTag, &rlocator, forkNum, curBlock);
 
        /* determine its hash code and partition lock ID */
        bufHash = BufTableHashCode(&bufTag);
        bufPartitionLock = BufMappingPartitionLock(bufHash);
 
        /* Check that it is in the buffer pool. If not, do nothing. */
        LWLockAcquire(bufPartitionLock, LW_SHARED);
        buf_id = BufTableLookup(&bufTag, bufHash);
        LWLockRelease(bufPartitionLock);
 
        if (buf_id < 0)
            continue;
 
        bufHdr = GetBufferDescriptor(buf_id);
 
        /*
         * We need to lock the buffer header and recheck if the buffer is
         * still associated with the same block because the buffer could be
         * evicted by some other backend loading blocks for a different
         * relation after we release lock on the BufMapping table.
         */
        buf_state = LockBufHdr(bufHdr);
 
        if (BufTagMatchesRelFileLocator(&bufHdr->tag, &rlocator) &&
            BufTagGetForkNum(&bufHdr->tag) == forkNum &&
            bufHdr->tag.blockNum >= firstDelBlock)
            InvalidateBuffer(bufHdr);   /* releases spinlock */
        else
            UnlockBufHdr(bufHdr, buf_state);
    }
}
 
/* ---------------------------------------------------------------------
 *      DropDatabaseBuffers
 *
 *      This function removes all the buffers in the buffer cache for a
 *      particular database.  Dirty pages are simply dropped, without
 *      bothering to write them out first.  This is used when we destroy a
 *      database, to avoid trying to flush data to disk when the directory
 *      tree no longer exists.  Implementation is pretty similar to
 *      DropRelationBuffers() which is for destroying just one relation.
 * --------------------------------------------------------------------
 */
void
DropDatabaseBuffers(Oid dbid)
{
    int         i;
 
    /*
     * We needn't consider local buffers, since by assumption the target
     * database isn't our own.
     */
 
    for (i = 0; i < NBuffers; i++)
    {
        BufferDesc *bufHdr = GetBufferDescriptor(i);
        uint32      buf_state;
 
        /*
         * As in DropRelationBuffers, an unlocked precheck should be safe and
         * saves some cycles.
         */
        if (bufHdr->tag.dbOid != dbid)
            continue;
 
        buf_state = LockBufHdr(bufHdr);
        if (bufHdr->tag.dbOid == dbid)
            InvalidateBuffer(bufHdr);   /* releases spinlock */
        else
            UnlockBufHdr(bufHdr, buf_state);
    }
}
 
/* ---------------------------------------------------------------------
 *      FlushRelationBuffers
 *
 *      This function writes all dirty pages of a relation out to disk
 *      (or more accurately, out to kernel disk buffers), ensuring that the
 *      kernel has an up-to-date view of the relation.
 *
 *      Generally, the caller should be holding AccessExclusiveLock on the
 *      target relation to ensure that no other backend is busy dirtying
 *      more blocks of the relation; the effects can't be expected to last
 *      after the lock is released.
 *
 *      XXX currently it sequentially searches the buffer pool, should be
 *      changed to more clever ways of searching.  This routine is not
 *      used in any performance-critical code paths, so it's not worth
 *      adding additional overhead to normal paths to make it go faster.
 * --------------------------------------------------------------------
 */
void
FlushRelationBuffers(Relation rel)
{
    int         i;
    BufferDesc *bufHdr;
    SMgrRelation srel = RelationGetSmgr(rel);
 
    if (RelationUsesLocalBuffers(rel))
    {
        for (i = 0; i < NLocBuffer; i++)
        {
            uint32      buf_state;
 
            bufHdr = GetLocalBufferDescriptor(i);
            if (BufTagMatchesRelFileLocator(&bufHdr->tag, &rel->rd_locator) &&
                ((buf_state = pg_atomic_read_u32(&bufHdr->state)) &
                 (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
            {
                ErrorContextCallback errcallback;
 
                /* Setup error traceback support for ereport() */
                errcallback.callback = local_buffer_write_error_callback;
                errcallback.arg = bufHdr;
                errcallback.previous = error_context_stack;
                error_context_stack = &errcallback;
 
                /* Make sure we can handle the pin */
                ReservePrivateRefCountEntry();
                ResourceOwnerEnlarge(CurrentResourceOwner);
 
                /*
                 * Pin/unpin mostly to make valgrind work, but it also seems
                 * like the right thing to do.
                 */
                PinLocalBuffer(bufHdr, false);
 
 
                FlushLocalBuffer(bufHdr, srel);
 
                UnpinLocalBuffer(BufferDescriptorGetBuffer(bufHdr));
 
                /* Pop the error context stack */
                error_context_stack = errcallback.previous;
            }
        }
 
        return;
    }
 
    for (i = 0; i < NBuffers; i++)
    {
        uint32      buf_state;
 
        bufHdr = GetBufferDescriptor(i);
 
        /*
         * As in DropRelationBuffers, an unlocked precheck should be safe and
         * saves some cycles.
         */
        if (!BufTagMatchesRelFileLocator(&bufHdr->tag, &rel->rd_locator))
            continue;
 
        /* Make sure we can handle the pin */
        ReservePrivateRefCountEntry();
        ResourceOwnerEnlarge(CurrentResourceOwner);
 
        buf_state = LockBufHdr(bufHdr);
        if (BufTagMatchesRelFileLocator(&bufHdr->tag, &rel->rd_locator) &&
            (buf_state & (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
        {
            PinBuffer_Locked(bufHdr);
            LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
            FlushBuffer(bufHdr, srel, IOOBJECT_RELATION, IOCONTEXT_NORMAL);
            LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
            UnpinBuffer(bufHdr);
        }
        else
            UnlockBufHdr(bufHdr, buf_state);
    }
}
 
/* ---------------------------------------------------------------------
 *      FlushRelationsAllBuffers
 *
 *      This function flushes out of the buffer pool all the pages of all
 *      forks of the specified smgr relations.  It's equivalent to calling
 *      FlushRelationBuffers once per relation.  The relations are assumed not
 *      to use local buffers.
 * --------------------------------------------------------------------
 */
void
FlushRelationsAllBuffers(SMgrRelation *smgrs, int nrels)
{
    int         i;
    SMgrSortArray *srels;
    bool        use_bsearch;
 
    if (nrels == 0)
        return;
 
    /* fill-in array for qsort */
    srels = palloc(sizeof(SMgrSortArray) * nrels);
 
    for (i = 0; i < nrels; i++)
    {
        Assert(!RelFileLocatorBackendIsTemp(smgrs[i]->smgr_rlocator));
 
        srels[i].rlocator = smgrs[i]->smgr_rlocator.locator;
        srels[i].srel = smgrs[i];
    }
 
    /*
     * Save the bsearch overhead for low number of relations to sync. See
     * DropRelationsAllBuffers for details.
     */
    use_bsearch = nrels > RELS_BSEARCH_THRESHOLD;
 
    /* sort the list of SMgrRelations if necessary */
    if (use_bsearch)
        qsort(srels, nrels, sizeof(SMgrSortArray), rlocator_comparator);
 
    for (i = 0; i < NBuffers; i++)
    {
        SMgrSortArray *srelent = NULL;
        BufferDesc *bufHdr = GetBufferDescriptor(i);
        uint32      buf_state;
 
        /*
         * As in DropRelationBuffers, an unlocked precheck should be safe and
         * saves some cycles.
         */
 
        if (!use_bsearch)
        {
            int         j;
 
            for (j = 0; j < nrels; j++)
            {
                if (BufTagMatchesRelFileLocator(&bufHdr->tag, &srels[j].rlocator))
                {
                    srelent = &srels[j];
                    break;
                }
            }
        }
        else
        {
            RelFileLocator rlocator;
 
            rlocator = BufTagGetRelFileLocator(&bufHdr->tag);
            srelent = bsearch(&rlocator,
                              srels, nrels, sizeof(SMgrSortArray),
                              rlocator_comparator);
        }
 
        /* buffer doesn't belong to any of the given relfilelocators; skip it */
        if (srelent == NULL)
            continue;
 
        /* Make sure we can handle the pin */
        ReservePrivateRefCountEntry();
        ResourceOwnerEnlarge(CurrentResourceOwner);
 
        buf_state = LockBufHdr(bufHdr);
        if (BufTagMatchesRelFileLocator(&bufHdr->tag, &srelent->rlocator) &&
            (buf_state & (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
        {
            PinBuffer_Locked(bufHdr);
            LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
            FlushBuffer(bufHdr, srelent->srel, IOOBJECT_RELATION, IOCONTEXT_NORMAL);
            LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
            UnpinBuffer(bufHdr);
        }
        else
            UnlockBufHdr(bufHdr, buf_state);
    }
 
    pfree(srels);
}
 
/* ---------------------------------------------------------------------
 *      RelationCopyStorageUsingBuffer
 *
 *      Copy fork's data using bufmgr.  Same as RelationCopyStorage but instead
 *      of using smgrread and smgrextend this will copy using bufmgr APIs.
 *
 *      Refer comments atop CreateAndCopyRelationData() for details about
 *      'permanent' parameter.
 * --------------------------------------------------------------------
 */
static void
RelationCopyStorageUsingBuffer(RelFileLocator srclocator,
                               RelFileLocator dstlocator,
                               ForkNumber forkNum, bool permanent)
{
    Buffer      srcBuf;
    Buffer      dstBuf;
    Page        srcPage;
    Page        dstPage;
    bool        use_wal;
    BlockNumber nblocks;
    BlockNumber blkno;
    PGIOAlignedBlock buf;
    BufferAccessStrategy bstrategy_src;
    BufferAccessStrategy bstrategy_dst;
    BlockRangeReadStreamPrivate p;
    ReadStream *src_stream;
    SMgrRelation src_smgr;
 
    /*
     * In general, we want to write WAL whenever wal_level > 'minimal', but we
     * can skip it when copying any fork of an unlogged relation other than
     * the init fork.
     */
    use_wal = XLogIsNeeded() && (permanent || forkNum == INIT_FORKNUM);
 
    /* Get number of blocks in the source relation. */
    nblocks = smgrnblocks(smgropen(srclocator, INVALID_PROC_NUMBER),
                          forkNum);
 
    /* Nothing to copy; just return. */
    if (nblocks == 0)
        return;
 
    /*
     * Bulk extend the destination relation of the same size as the source
     * relation before starting to copy block by block.
     */
    memset(buf.data, 0, BLCKSZ);
    smgrextend(smgropen(dstlocator, INVALID_PROC_NUMBER), forkNum, nblocks - 1,
               buf.data, true);
 
    /* This is a bulk operation, so use buffer access strategies. */
    bstrategy_src = GetAccessStrategy(BAS_BULKREAD);
    bstrategy_dst = GetAccessStrategy(BAS_BULKWRITE);
 
    /* Initialize streaming read */
    p.current_blocknum = 0;
    p.last_exclusive = nblocks;
    src_smgr = smgropen(srclocator, INVALID_PROC_NUMBER);
 
    /*
     * It is safe to use batchmode as block_range_read_stream_cb takes no
     * locks.
     */
    src_stream = read_stream_begin_smgr_relation(READ_STREAM_FULL |
                                                 READ_STREAM_USE_BATCHING,
                                                 bstrategy_src,
                                                 src_smgr,
                                                 permanent ? RELPERSISTENCE_PERMANENT : RELPERSISTENCE_UNLOGGED,
                                                 forkNum,
                                                 block_range_read_stream_cb,
                                                 &p,
                                                 0);
 
    /* Iterate over each block of the source relation file. */
    for (blkno = 0; blkno < nblocks; blkno++)
    {
        CHECK_FOR_INTERRUPTS();
 
        /* Read block from source relation. */
        srcBuf = read_stream_next_buffer(src_stream, NULL);
        LockBuffer(srcBuf, BUFFER_LOCK_SHARE);
        srcPage = BufferGetPage(srcBuf);
 
        dstBuf = ReadBufferWithoutRelcache(dstlocator, forkNum,
                                           BufferGetBlockNumber(srcBuf),
                                           RBM_ZERO_AND_LOCK, bstrategy_dst,
                                           permanent);
        dstPage = BufferGetPage(dstBuf);
 
        START_CRIT_SECTION();
 
        /* Copy page data from the source to the destination. */
        memcpy(dstPage, srcPage, BLCKSZ);
        MarkBufferDirty(dstBuf);
 
        /* WAL-log the copied page. */
        if (use_wal)
            log_newpage_buffer(dstBuf, true);
 
        END_CRIT_SECTION();
 
        UnlockReleaseBuffer(dstBuf);
        UnlockReleaseBuffer(srcBuf);
    }
    Assert(read_stream_next_buffer(src_stream, NULL) == InvalidBuffer);
    read_stream_end(src_stream);
 
    FreeAccessStrategy(bstrategy_src);
    FreeAccessStrategy(bstrategy_dst);
}
 
/* ---------------------------------------------------------------------
 *      CreateAndCopyRelationData
 *
 *      Create destination relation storage and copy all forks from the
 *      source relation to the destination.
 *
 *      Pass permanent as true for permanent relations and false for
 *      unlogged relations.  Currently this API is not supported for
 *      temporary relations.
 * --------------------------------------------------------------------
 */
void
CreateAndCopyRelationData(RelFileLocator src_rlocator,
                          RelFileLocator dst_rlocator, bool permanent)
{
    char        relpersistence;
    SMgrRelation src_rel;
    SMgrRelation dst_rel;
 
    /* Set the relpersistence. */
    relpersistence = permanent ?
        RELPERSISTENCE_PERMANENT : RELPERSISTENCE_UNLOGGED;
 
    src_rel = smgropen(src_rlocator, INVALID_PROC_NUMBER);
    dst_rel = smgropen(dst_rlocator, INVALID_PROC_NUMBER);
 
    /*
     * Create and copy all forks of the relation.  During create database we
     * have a separate cleanup mechanism which deletes complete database
     * directory.  Therefore, each individual relation doesn't need to be
     * registered for cleanup.
     */
    RelationCreateStorage(dst_rlocator, relpersistence, false);
 
    /* copy main fork. */
    RelationCopyStorageUsingBuffer(src_rlocator, dst_rlocator, MAIN_FORKNUM,
                                   permanent);
 
    /* copy those extra forks that exist */
    for (ForkNumber forkNum = MAIN_FORKNUM + 1;
         forkNum <= MAX_FORKNUM; forkNum++)
    {
        if (smgrexists(src_rel, forkNum))
        {
            smgrcreate(dst_rel, forkNum, false);
 
            /*
             * WAL log creation if the relation is persistent, or this is the
             * init fork of an unlogged relation.
             */
            if (permanent || forkNum == INIT_FORKNUM)
                log_smgrcreate(&dst_rlocator, forkNum);
 
            /* Copy a fork's data, block by block. */
            RelationCopyStorageUsingBuffer(src_rlocator, dst_rlocator, forkNum,
                                           permanent);
        }
    }
}
 
/* ---------------------------------------------------------------------
 *      FlushDatabaseBuffers
 *
 *      This function writes all dirty pages of a database out to disk
 *      (or more accurately, out to kernel disk buffers), ensuring that the
 *      kernel has an up-to-date view of the database.
 *
 *      Generally, the caller should be holding an appropriate lock to ensure
 *      no other backend is active in the target database; otherwise more
 *      pages could get dirtied.
 *
 *      Note we don't worry about flushing any pages of temporary relations.
 *      It's assumed these wouldn't be interesting.
 * --------------------------------------------------------------------
 */
void
FlushDatabaseBuffers(Oid dbid)
{
    int         i;
    BufferDesc *bufHdr;
 
    for (i = 0; i < NBuffers; i++)
    {
        uint32      buf_state;
 
        bufHdr = GetBufferDescriptor(i);
 
        /*
         * As in DropRelationBuffers, an unlocked precheck should be safe and
         * saves some cycles.
         */
        if (bufHdr->tag.dbOid != dbid)
            continue;
 
        /* Make sure we can handle the pin */
        ReservePrivateRefCountEntry();
        ResourceOwnerEnlarge(CurrentResourceOwner);
 
        buf_state = LockBufHdr(bufHdr);
        if (bufHdr->tag.dbOid == dbid &&
            (buf_state & (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
        {
            PinBuffer_Locked(bufHdr);
            LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
            FlushBuffer(bufHdr, NULL, IOOBJECT_RELATION, IOCONTEXT_NORMAL);
            LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
            UnpinBuffer(bufHdr);
        }
        else
            UnlockBufHdr(bufHdr, buf_state);
    }
}
 
/*
 * Flush a previously, shared or exclusively, locked and pinned buffer to the
 * OS.
 */
void
FlushOneBuffer(Buffer buffer)
{
    BufferDesc *bufHdr;
 
    /* currently not needed, but no fundamental reason not to support */
    Assert(!BufferIsLocal(buffer));
 
    Assert(BufferIsPinned(buffer));
 
    bufHdr = GetBufferDescriptor(buffer - 1);
 
    Assert(LWLockHeldByMe(BufferDescriptorGetContentLock(bufHdr)));
 
    FlushBuffer(bufHdr, NULL, IOOBJECT_RELATION, IOCONTEXT_NORMAL);
}
 
/*
 * ReleaseBuffer -- release the pin on a buffer
 */
void
ReleaseBuffer(Buffer buffer)
{
    if (!BufferIsValid(buffer))
        elog(ERROR, "bad buffer ID: %d", buffer);
 
    if (BufferIsLocal(buffer))
        UnpinLocalBuffer(buffer);
    else
        UnpinBuffer(GetBufferDescriptor(buffer - 1));
}
 
/*
 * UnlockReleaseBuffer -- release the content lock and pin on a buffer
 *
 * This is just a shorthand for a common combination.
 */
void
UnlockReleaseBuffer(Buffer buffer)
{
    LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
    ReleaseBuffer(buffer);
}
 
/*
 * IncrBufferRefCount
 *      Increment the pin count on a buffer that we have *already* pinned
 *      at least once.
 *
 *      This function cannot be used on a buffer we do not have pinned,
 *      because it doesn't change the shared buffer state.
 */
void
IncrBufferRefCount(Buffer buffer)
{
    Assert(BufferIsPinned(buffer));
    ResourceOwnerEnlarge(CurrentResourceOwner);
    if (BufferIsLocal(buffer))
        LocalRefCount[-buffer - 1]++;
    else
    {
        PrivateRefCountEntry *ref;
 
        ref = GetPrivateRefCountEntry(buffer, true);
        Assert(ref != NULL);
        ref->refcount++;
    }
    ResourceOwnerRememberBuffer(CurrentResourceOwner, buffer);
}
 
/*
 * MarkBufferDirtyHint
 *
 *  Mark a buffer dirty for non-critical changes.
 *
 * This is essentially the same as MarkBufferDirty, except:
 *
 * 1. The caller does not write WAL; so if checksums are enabled, we may need
 *    to write an XLOG_FPI_FOR_HINT WAL record to protect against torn pages.
 * 2. The caller might have only share-lock instead of exclusive-lock on the
 *    buffer's content lock.
 * 3. This function does not guarantee that the buffer is always marked dirty
 *    (due to a race condition), so it cannot be used for important changes.
 */
void
MarkBufferDirtyHint(Buffer buffer, bool buffer_std)
{
    BufferDesc *bufHdr;
    Page        page = BufferGetPage(buffer);
 
    if (!BufferIsValid(buffer))
        elog(ERROR, "bad buffer ID: %d", buffer);
 
    if (BufferIsLocal(buffer))
    {
        MarkLocalBufferDirty(buffer);
        return;
    }
 
    bufHdr = GetBufferDescriptor(buffer - 1);
 
    Assert(GetPrivateRefCount(buffer) > 0);
    /* here, either share or exclusive lock is OK */
    Assert(LWLockHeldByMe(BufferDescriptorGetContentLock(bufHdr)));
 
    /*
     * This routine might get called many times on the same page, if we are
     * making the first scan after commit of an xact that added/deleted many
     * tuples. So, be as quick as we can if the buffer is already dirty.  We
     * do this by not acquiring spinlock if it looks like the status bits are
     * already set.  Since we make this test unlocked, there's a chance we
     * might fail to notice that the flags have just been cleared, and failed
     * to reset them, due to memory-ordering issues.  But since this function
     * is only intended to be used in cases where failing to write out the
     * data would be harmless anyway, it doesn't really matter.
     */
    if ((pg_atomic_read_u32(&bufHdr->state) & (BM_DIRTY | BM_JUST_DIRTIED)) !=
        (BM_DIRTY | BM_JUST_DIRTIED))
    {
        XLogRecPtr  lsn = InvalidXLogRecPtr;
        bool        dirtied = false;
        bool        delayChkptFlags = false;
        uint32      buf_state;
 
        /*
         * If we need to protect hint bit updates from torn writes, WAL-log a
         * full page image of the page. This full page image is only necessary
         * if the hint bit update is the first change to the page since the
         * last checkpoint.
         *
         * We don't check full_page_writes here because that logic is included
         * when we call XLogInsert() since the value changes dynamically.
         */
        if (XLogHintBitIsNeeded() &&
            (pg_atomic_read_u32(&bufHdr->state) & BM_PERMANENT))
        {
            /*
             * If we must not write WAL, due to a relfilelocator-specific
             * condition or being in recovery, don't dirty the page.  We can
             * set the hint, just not dirty the page as a result so the hint
             * is lost when we evict the page or shutdown.
             *
             * See src/backend/storage/page/README for longer discussion.
             */
            if (RecoveryInProgress() ||
                RelFileLocatorSkippingWAL(BufTagGetRelFileLocator(&bufHdr->tag)))
                return;
 
            /*
             * If the block is already dirty because we either made a change
             * or set a hint already, then we don't need to write a full page
             * image.  Note that aggressive cleaning of blocks dirtied by hint
             * bit setting would increase the call rate. Bulk setting of hint
             * bits would reduce the call rate...
             *
             * We must issue the WAL record before we mark the buffer dirty.
             * Otherwise we might write the page before we write the WAL. That
             * causes a race condition, since a checkpoint might occur between
             * writing the WAL record and marking the buffer dirty. We solve
             * that with a kluge, but one that is already in use during
             * transaction commit to prevent race conditions. Basically, we
             * simply prevent the checkpoint WAL record from being written
             * until we have marked the buffer dirty. We don't start the
             * checkpoint flush until we have marked dirty, so our checkpoint
             * must flush the change to disk successfully or the checkpoint
             * never gets written, so crash recovery will fix.
             *
             * It's possible we may enter here without an xid, so it is
             * essential that CreateCheckPoint waits for virtual transactions
             * rather than full transactionids.
             */
            Assert((MyProc->delayChkptFlags & DELAY_CHKPT_START) == 0);
            MyProc->delayChkptFlags |= DELAY_CHKPT_START;
            delayChkptFlags = true;
            lsn = XLogSaveBufferForHint(buffer, buffer_std);
        }
 
        buf_state = LockBufHdr(bufHdr);
 
        Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
 
        if (!(buf_state & BM_DIRTY))
        {
            dirtied = true;     /* Means "will be dirtied by this action" */
 
            /*
             * Set the page LSN if we wrote a backup block. We aren't supposed
             * to set this when only holding a share lock but as long as we
             * serialise it somehow we're OK. We choose to set LSN while
             * holding the buffer header lock, which causes any reader of an
             * LSN who holds only a share lock to also obtain a buffer header
             * lock before using PageGetLSN(), which is enforced in
             * BufferGetLSNAtomic().
             *
             * If checksums are enabled, you might think we should reset the
             * checksum here. That will happen when the page is written
             * sometime later in this checkpoint cycle.
             */
            if (!XLogRecPtrIsInvalid(lsn))
                PageSetLSN(page, lsn);
        }
 
        buf_state |= BM_DIRTY | BM_JUST_DIRTIED;
        UnlockBufHdr(bufHdr, buf_state);
 
        if (delayChkptFlags)
            MyProc->delayChkptFlags &= ~DELAY_CHKPT_START;
 
        if (dirtied)
        {
            pgBufferUsage.shared_blks_dirtied++;
            if (VacuumCostActive)
                VacuumCostBalance += VacuumCostPageDirty;
        }
    }
}
 
/*
 * Release buffer content locks for shared buffers.
 *
 * Used to clean up after errors.
 *
 * Currently, we can expect that lwlock.c's LWLockReleaseAll() took care
 * of releasing buffer content locks per se; the only thing we need to deal
 * with here is clearing any PIN_COUNT request that was in progress.
 */
void
UnlockBuffers(void)
{
    BufferDesc *buf = PinCountWaitBuf;
 
    if (buf)
    {
        uint32      buf_state;
 
        buf_state = LockBufHdr(buf);
 
        /*
         * Don't complain if flag bit not set; it could have been reset but we
         * got a cancel/die interrupt before getting the signal.
         */
        if ((buf_state & BM_PIN_COUNT_WAITER) != 0 &&
            buf->wait_backend_pgprocno == MyProcNumber)
            buf_state &= ~BM_PIN_COUNT_WAITER;
 
        UnlockBufHdr(buf, buf_state);
 
        PinCountWaitBuf = NULL;
    }
}
 
/*
 * Acquire or release the content_lock for the buffer.
 */
void
LockBuffer(Buffer buffer, int mode)
{
    BufferDesc *buf;
 
    Assert(BufferIsPinned(buffer));
    if (BufferIsLocal(buffer))
        return;                 /* local buffers need no lock */
 
    buf = GetBufferDescriptor(buffer - 1);
 
    if (mode == BUFFER_LOCK_UNLOCK)
        LWLockRelease(BufferDescriptorGetContentLock(buf));
    else if (mode == BUFFER_LOCK_SHARE)
        LWLockAcquire(BufferDescriptorGetContentLock(buf), LW_SHARED);
    else if (mode == BUFFER_LOCK_EXCLUSIVE)
        LWLockAcquire(BufferDescriptorGetContentLock(buf), LW_EXCLUSIVE);
    else
        elog(ERROR, "unrecognized buffer lock mode: %d", mode);
}
 
/*
 * Acquire the content_lock for the buffer, but only if we don't have to wait.
 *
 * This assumes the caller wants BUFFER_LOCK_EXCLUSIVE mode.
 */
bool
ConditionalLockBuffer(Buffer buffer)
{
    BufferDesc *buf;
 
    Assert(BufferIsPinned(buffer));
    if (BufferIsLocal(buffer))
        return true;            /* act as though we got it */
 
    buf = GetBufferDescriptor(buffer - 1);
 
    return LWLockConditionalAcquire(BufferDescriptorGetContentLock(buf),
                                    LW_EXCLUSIVE);
}
 
/*
 * Verify that this backend is pinning the buffer exactly once.
 *
 * NOTE: Like in BufferIsPinned(), what we check here is that *this* backend
 * holds a pin on the buffer.  We do not care whether some other backend does.
 */
void
CheckBufferIsPinnedOnce(Buffer buffer)
{
    if (BufferIsLocal(buffer))
    {
        if (LocalRefCount[-buffer - 1] != 1)
            elog(ERROR, "incorrect local pin count: %d",
                 LocalRefCount[-buffer - 1]);
    }
    else
    {
        if (GetPrivateRefCount(buffer) != 1)
            elog(ERROR, "incorrect local pin count: %d",
                 GetPrivateRefCount(buffer));
    }
}
 
/*
 * LockBufferForCleanup - lock a buffer in preparation for deleting items
 *
 * Items may be deleted from a disk page only when the caller (a) holds an
 * exclusive lock on the buffer and (b) has observed that no other backend
 * holds a pin on the buffer.  If there is a pin, then the other backend
 * might have a pointer into the buffer (for example, a heapscan reference
 * to an item --- see README for more details).  It's OK if a pin is added
 * after the cleanup starts, however; the newly-arrived backend will be
 * unable to look at the page until we release the exclusive lock.
 *
 * To implement this protocol, a would-be deleter must pin the buffer and
 * then call LockBufferForCleanup().  LockBufferForCleanup() is similar to
 * LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE), except that it loops until
 * it has successfully observed pin count = 1.
 */
void
LockBufferForCleanup(Buffer buffer)
{
    BufferDesc *bufHdr;
    TimestampTz waitStart = 0;
    bool        waiting = false;
    bool        logged_recovery_conflict = false;
 
    Assert(BufferIsPinned(buffer));
    Assert(PinCountWaitBuf == NULL);
 
    CheckBufferIsPinnedOnce(buffer);
 
    /*
     * We do not yet need to be worried about in-progress AIOs holding a pin,
     * as we, so far, only support doing reads via AIO and this function can
     * only be called once the buffer is valid (i.e. no read can be in
     * flight).
     */
 
    /* Nobody else to wait for */
    if (BufferIsLocal(buffer))
        return;
 
    bufHdr = GetBufferDescriptor(buffer - 1);
 
    for (;;)
    {
        uint32      buf_state;
 
        /* Try to acquire lock */
        LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
        buf_state = LockBufHdr(bufHdr);
 
        Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
        if (BUF_STATE_GET_REFCOUNT(buf_state) == 1)
        {
            /* Successfully acquired exclusive lock with pincount 1 */
            UnlockBufHdr(bufHdr, buf_state);
 
            /*
             * Emit the log message if recovery conflict on buffer pin was
             * resolved but the startup process waited longer than
             * deadlock_timeout for it.
             */
            if (logged_recovery_conflict)
                LogRecoveryConflict(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN,
                                    waitStart, GetCurrentTimestamp(),
                                    NULL, false);
 
            if (waiting)
            {
                /* reset ps display to remove the suffix if we added one */
                set_ps_display_remove_suffix();
                waiting = false;
            }
            return;
        }
        /* Failed, so mark myself as waiting for pincount 1 */
        if (buf_state & BM_PIN_COUNT_WAITER)
        {
            UnlockBufHdr(bufHdr, buf_state);
            LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
            elog(ERROR, "multiple backends attempting to wait for pincount 1");
        }
        bufHdr->wait_backend_pgprocno = MyProcNumber;
        PinCountWaitBuf = bufHdr;
        buf_state |= BM_PIN_COUNT_WAITER;
        UnlockBufHdr(bufHdr, buf_state);
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
 
        /* Wait to be signaled by UnpinBuffer() */
        if (InHotStandby)
        {
            if (!waiting)
            {
                /* adjust the process title to indicate that it's waiting */
                set_ps_display_suffix("waiting");
                waiting = true;
            }
 
            /*
             * Emit the log message if the startup process is waiting longer
             * than deadlock_timeout for recovery conflict on buffer pin.
             *
             * Skip this if first time through because the startup process has
             * not started waiting yet in this case. So, the wait start
             * timestamp is set after this logic.
             */
            if (waitStart != 0 && !logged_recovery_conflict)
            {
                TimestampTz now = GetCurrentTimestamp();
 
                if (TimestampDifferenceExceeds(waitStart, now,
                                               DeadlockTimeout))
                {
                    LogRecoveryConflict(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN,
                                        waitStart, now, NULL, true);
                    logged_recovery_conflict = true;
                }
            }
 
            /*
             * Set the wait start timestamp if logging is enabled and first
             * time through.
             */
            if (log_recovery_conflict_waits && waitStart == 0)
                waitStart = GetCurrentTimestamp();
 
            /* Publish the bufid that Startup process waits on */
            SetStartupBufferPinWaitBufId(buffer - 1);
            /* Set alarm and then wait to be signaled by UnpinBuffer() */
            ResolveRecoveryConflictWithBufferPin();
            /* Reset the published bufid */
            SetStartupBufferPinWaitBufId(-1);
        }
        else
            ProcWaitForSignal(WAIT_EVENT_BUFFER_PIN);
 
        /*
         * Remove flag marking us as waiter. Normally this will not be set
         * anymore, but ProcWaitForSignal() can return for other signals as
         * well.  We take care to only reset the flag if we're the waiter, as
         * theoretically another backend could have started waiting. That's
         * impossible with the current usages due to table level locking, but
         * better be safe.
         */
        buf_state = LockBufHdr(bufHdr);
        if ((buf_state & BM_PIN_COUNT_WAITER) != 0 &&
            bufHdr->wait_backend_pgprocno == MyProcNumber)
            buf_state &= ~BM_PIN_COUNT_WAITER;
        UnlockBufHdr(bufHdr, buf_state);
 
        PinCountWaitBuf = NULL;
        /* Loop back and try again */
    }
}
 
/*
 * Check called from ProcessRecoveryConflictInterrupts() when Startup process
 * requests cancellation of all pin holders that are blocking it.
 */
bool
HoldingBufferPinThatDelaysRecovery(void)
{
    int         bufid = GetStartupBufferPinWaitBufId();
 
    /*
     * If we get woken slowly then it's possible that the Startup process was
     * already woken by other backends before we got here. Also possible that
     * we get here by multiple interrupts or interrupts at inappropriate
     * times, so make sure we do nothing if the bufid is not set.
     */
    if (bufid < 0)
        return false;
 
    if (GetPrivateRefCount(bufid + 1) > 0)
        return true;
 
    return false;
}
 
/*
 * ConditionalLockBufferForCleanup - as above, but don't wait to get the lock
 *
 * We won't loop, but just check once to see if the pin count is OK.  If
 * not, return false with no lock held.
 */
bool
ConditionalLockBufferForCleanup(Buffer buffer)
{
    BufferDesc *bufHdr;
    uint32      buf_state,
                refcount;
 
    Assert(BufferIsValid(buffer));
 
    /* see AIO related comment in LockBufferForCleanup() */
 
    if (BufferIsLocal(buffer))
    {
        refcount = LocalRefCount[-buffer - 1];
        /* There should be exactly one pin */
        Assert(refcount > 0);
        if (refcount != 1)
            return false;
        /* Nobody else to wait for */
        return true;
    }
 
    /* There should be exactly one local pin */
    refcount = GetPrivateRefCount(buffer);
    Assert(refcount);
    if (refcount != 1)
        return false;
 
    /* Try to acquire lock */
    if (!ConditionalLockBuffer(buffer))
        return false;
 
    bufHdr = GetBufferDescriptor(buffer - 1);
    buf_state = LockBufHdr(bufHdr);
    refcount = BUF_STATE_GET_REFCOUNT(buf_state);
 
    Assert(refcount > 0);
    if (refcount == 1)
    {
        /* Successfully acquired exclusive lock with pincount 1 */
        UnlockBufHdr(bufHdr, buf_state);
        return true;
    }
 
    /* Failed, so release the lock */
    UnlockBufHdr(bufHdr, buf_state);
    LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
    return false;
}
 
/*
 * IsBufferCleanupOK - as above, but we already have the lock
 *
 * Check whether it's OK to perform cleanup on a buffer we've already
 * locked.  If we observe that the pin count is 1, our exclusive lock
 * happens to be a cleanup lock, and we can proceed with anything that
 * would have been allowable had we sought a cleanup lock originally.
 */
bool
IsBufferCleanupOK(Buffer buffer)
{
    BufferDesc *bufHdr;
    uint32      buf_state;
 
    Assert(BufferIsValid(buffer));
 
    /* see AIO related comment in LockBufferForCleanup() */
 
    if (BufferIsLocal(buffer))
    {
        /* There should be exactly one pin */
        if (LocalRefCount[-buffer - 1] != 1)
            return false;
        /* Nobody else to wait for */
        return true;
    }
 
    /* There should be exactly one local pin */
    if (GetPrivateRefCount(buffer) != 1)
        return false;
 
    bufHdr = GetBufferDescriptor(buffer - 1);
 
    /* caller must hold exclusive lock on buffer */
    Assert(LWLockHeldByMeInMode(BufferDescriptorGetContentLock(bufHdr),
                                LW_EXCLUSIVE));
 
    buf_state = LockBufHdr(bufHdr);
 
    Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
    if (BUF_STATE_GET_REFCOUNT(buf_state) == 1)
    {
        /* pincount is OK. */
        UnlockBufHdr(bufHdr, buf_state);
        return true;
    }
 
    UnlockBufHdr(bufHdr, buf_state);
    return false;
}
 
 
/*
 *  Functions for buffer I/O handling
 *
 *  Also note that these are used only for shared buffers, not local ones.
 */
 
/*
 * WaitIO -- Block until the IO_IN_PROGRESS flag on 'buf' is cleared.
 */
static void
WaitIO(BufferDesc *buf)
{
    ConditionVariable *cv = BufferDescriptorGetIOCV(buf);
 
    ConditionVariablePrepareToSleep(cv);
    for (;;)
    {
        uint32      buf_state;
        PgAioWaitRef iow;
 
        /*
         * It may not be necessary to acquire the spinlock to check the flag
         * here, but since this test is essential for correctness, we'd better
         * play it safe.
         */
        buf_state = LockBufHdr(buf);
 
        /*
         * Copy the wait reference while holding the spinlock. This protects
         * against a concurrent TerminateBufferIO() in another backend from
         * clearing the wref while it's being read.
         */
        iow = buf->io_wref;
        UnlockBufHdr(buf, buf_state);
 
        /* no IO in progress, we don't need to wait */
        if (!(buf_state & BM_IO_IN_PROGRESS))
            break;
 
        /*
         * The buffer has asynchronous IO in progress, wait for it to
         * complete.
         */
        if (pgaio_wref_valid(&iow))
        {
            pgaio_wref_wait(&iow);
 
            /*
             * The AIO subsystem internally uses condition variables and thus
             * might remove this backend from the BufferDesc's CV. While that
             * wouldn't cause a correctness issue (the first CV sleep just
             * immediately returns if not already registered), it seems worth
             * avoiding unnecessary loop iterations, given that we take care
             * to do so at the start of the function.
             */
            ConditionVariablePrepareToSleep(cv);
            continue;
        }
 
        /* wait on BufferDesc->cv, e.g. for concurrent synchronous IO */
        ConditionVariableSleep(cv, WAIT_EVENT_BUFFER_IO);
    }
    ConditionVariableCancelSleep();
}
 
/*
 * StartBufferIO: begin I/O on this buffer
 *  (Assumptions)
 *  My process is executing no IO on this buffer
 *  The buffer is Pinned
 *
 * In some scenarios multiple backends could attempt the same I/O operation
 * concurrently.  If someone else has already started I/O on this buffer then
 * we will wait for completion of the IO using WaitIO().
 *
 * Input operations are only attempted on buffers that are not BM_VALID,
 * and output operations only on buffers that are BM_VALID and BM_DIRTY,
 * so we can always tell if the work is already done.
 *
 * Returns true if we successfully marked the buffer as I/O busy,
 * false if someone else already did the work.
 *
 * If nowait is true, then we don't wait for an I/O to be finished by another
 * backend.  In that case, false indicates either that the I/O was already
 * finished, or is still in progress.  This is useful for callers that want to
 * find out if they can perform the I/O as part of a larger operation, without
 * waiting for the answer or distinguishing the reasons why not.
 */
bool
StartBufferIO(BufferDesc *buf, bool forInput, bool nowait)
{
    uint32      buf_state;
 
    ResourceOwnerEnlarge(CurrentResourceOwner);
 
    for (;;)
    {
        buf_state = LockBufHdr(buf);
 
        if (!(buf_state & BM_IO_IN_PROGRESS))
            break;
        UnlockBufHdr(buf, buf_state);
        if (nowait)
            return false;
        WaitIO(buf);
    }
 
    /* Once we get here, there is definitely no I/O active on this buffer */
 
    /* Check if someone else already did the I/O */
    if (forInput ? (buf_state & BM_VALID) : !(buf_state & BM_DIRTY))
    {
        UnlockBufHdr(buf, buf_state);
        return false;
    }
 
    buf_state |= BM_IO_IN_PROGRESS;
    UnlockBufHdr(buf, buf_state);
 
    ResourceOwnerRememberBufferIO(CurrentResourceOwner,
                                  BufferDescriptorGetBuffer(buf));
 
    return true;
}
 
/*
 * TerminateBufferIO: release a buffer we were doing I/O on
 *  (Assumptions)
 *  My process is executing IO for the buffer
 *  BM_IO_IN_PROGRESS bit is set for the buffer
 *  The buffer is Pinned
 *
 * If clear_dirty is true and BM_JUST_DIRTIED is not set, we clear the
 * buffer's BM_DIRTY flag.  This is appropriate when terminating a
 * successful write.  The check on BM_JUST_DIRTIED is necessary to avoid
 * marking the buffer clean if it was re-dirtied while we were writing.
 *
 * set_flag_bits gets ORed into the buffer's flags.  It must include
 * BM_IO_ERROR in a failure case.  For successful completion it could
 * be 0, or BM_VALID if we just finished reading in the page.
 *
 * If forget_owner is true, we release the buffer I/O from the current
 * resource owner. (forget_owner=false is used when the resource owner itself
 * is being released)
 */
void
TerminateBufferIO(BufferDesc *buf, bool clear_dirty, uint32 set_flag_bits,
                  bool forget_owner, bool release_aio)
{
    uint32      buf_state;
 
    buf_state = LockBufHdr(buf);
 
    Assert(buf_state & BM_IO_IN_PROGRESS);
    buf_state &= ~BM_IO_IN_PROGRESS;
 
    /* Clear earlier errors, if this IO failed, it'll be marked again */
    buf_state &= ~BM_IO_ERROR;
 
    if (clear_dirty && !(buf_state & BM_JUST_DIRTIED))
        buf_state &= ~(BM_DIRTY | BM_CHECKPOINT_NEEDED);
 
    if (release_aio)
    {
        /* release ownership by the AIO subsystem */
        Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
        buf_state -= BUF_REFCOUNT_ONE;
        pgaio_wref_clear(&buf->io_wref);
    }
 
    buf_state |= set_flag_bits;
    UnlockBufHdr(buf, buf_state);
 
    if (forget_owner)
        ResourceOwnerForgetBufferIO(CurrentResourceOwner,
                                    BufferDescriptorGetBuffer(buf));
 
    ConditionVariableBroadcast(BufferDescriptorGetIOCV(buf));
 
    /*
     * Support LockBufferForCleanup()
     *
     * We may have just released the last pin other than the waiter's. In most
     * cases, this backend holds another pin on the buffer. But, if, for
     * example, this backend is completing an IO issued by another backend, it
     * may be time to wake the waiter.
     */
    if (release_aio && (buf_state & BM_PIN_COUNT_WAITER))
        WakePinCountWaiter(buf);
}
 
/*
 * AbortBufferIO: Clean up active buffer I/O after an error.
 *
 *  All LWLocks we might have held have been released,
 *  but we haven't yet released buffer pins, so the buffer is still pinned.
 *
 *  If I/O was in progress, we always set BM_IO_ERROR, even though it's
 *  possible the error condition wasn't related to the I/O.
 *
 *  Note: this does not remove the buffer I/O from the resource owner.
 *  That's correct when we're releasing the whole resource owner, but
 *  beware if you use this in other contexts.
 */
static void
AbortBufferIO(Buffer buffer)
{
    BufferDesc *buf_hdr = GetBufferDescriptor(buffer - 1);
    uint32      buf_state;
 
    buf_state = LockBufHdr(buf_hdr);
    Assert(buf_state & (BM_IO_IN_PROGRESS | BM_TAG_VALID));
 
    if (!(buf_state & BM_VALID))
    {
        Assert(!(buf_state & BM_DIRTY));
        UnlockBufHdr(buf_hdr, buf_state);
    }
    else
    {
        Assert(buf_state & BM_DIRTY);
        UnlockBufHdr(buf_hdr, buf_state);
 
        /* Issue notice if this is not the first failure... */
        if (buf_state & BM_IO_ERROR)
        {
            /* Buffer is pinned, so we can read tag without spinlock */
            ereport(WARNING,
                    (errcode(ERRCODE_IO_ERROR),
                     errmsg("could not write block %u of %s",
                            buf_hdr->tag.blockNum,
                            relpathperm(BufTagGetRelFileLocator(&buf_hdr->tag),
                                        BufTagGetForkNum(&buf_hdr->tag)).str),
                     errdetail("Multiple failures --- write error might be permanent.")));
        }
    }
 
    TerminateBufferIO(buf_hdr, false, BM_IO_ERROR, false, false);
}
 
/*
 * Error context callback for errors occurring during shared buffer writes.
 */
static void
shared_buffer_write_error_callback(void *arg)
{
    BufferDesc *bufHdr = (BufferDesc *) arg;
 
    /* Buffer is pinned, so we can read the tag without locking the spinlock */
    if (bufHdr != NULL)
        errcontext("writing block %u of relation %s",
                   bufHdr->tag.blockNum,
                   relpathperm(BufTagGetRelFileLocator(&bufHdr->tag),
                               BufTagGetForkNum(&bufHdr->tag)).str);
}
 
/*
 * Error context callback for errors occurring during local buffer writes.
 */
static void
local_buffer_write_error_callback(void *arg)
{
    BufferDesc *bufHdr = (BufferDesc *) arg;
 
    if (bufHdr != NULL)
        errcontext("writing block %u of relation %s",
                   bufHdr->tag.blockNum,
                   relpathbackend(BufTagGetRelFileLocator(&bufHdr->tag),
                                  MyProcNumber,
                                  BufTagGetForkNum(&bufHdr->tag)).str);
}
 
/*
 * RelFileLocator qsort/bsearch comparator; see RelFileLocatorEquals.
 */
static int
rlocator_comparator(const void *p1, const void *p2)
{
    RelFileLocator n1 = *(const RelFileLocator *) p1;
    RelFileLocator n2 = *(const RelFileLocator *) p2;
 
    if (n1.relNumber < n2.relNumber)
        return -1;
    else if (n1.relNumber > n2.relNumber)
        return 1;
 
    if (n1.dbOid < n2.dbOid)
        return -1;
    else if (n1.dbOid > n2.dbOid)
        return 1;
 
    if (n1.spcOid < n2.spcOid)
        return -1;
    else if (n1.spcOid > n2.spcOid)
        return 1;
    else
        return 0;
}
 
/*
 * Lock buffer header - set BM_LOCKED in buffer state.
 */
uint32
LockBufHdr(BufferDesc *desc)
{
    SpinDelayStatus delayStatus;
    uint32      old_buf_state;
 
    Assert(!BufferIsLocal(BufferDescriptorGetBuffer(desc)));
 
    init_local_spin_delay(&delayStatus);
 
    while (true)
    {
        /* set BM_LOCKED flag */
        old_buf_state = pg_atomic_fetch_or_u32(&desc->state, BM_LOCKED);
        /* if it wasn't set before we're OK */
        if (!(old_buf_state & BM_LOCKED))
            break;
        perform_spin_delay(&delayStatus);
    }
    finish_spin_delay(&delayStatus);
    return old_buf_state | BM_LOCKED;
}
 
/*
 * Wait until the BM_LOCKED flag isn't set anymore and return the buffer's
 * state at that point.
 *
 * Obviously the buffer could be locked by the time the value is returned, so
 * this is primarily useful in CAS style loops.
 */
static uint32
WaitBufHdrUnlocked(BufferDesc *buf)
{
    SpinDelayStatus delayStatus;
    uint32      buf_state;
 
    init_local_spin_delay(&delayStatus);
 
    buf_state = pg_atomic_read_u32(&buf->state);
 
    while (buf_state & BM_LOCKED)
    {
        perform_spin_delay(&delayStatus);
        buf_state = pg_atomic_read_u32(&buf->state);
    }
 
    finish_spin_delay(&delayStatus);
 
    return buf_state;
}
 
/*
 * BufferTag comparator.
 */
static inline int
buffertag_comparator(const BufferTag *ba, const BufferTag *bb)
{
    int         ret;
    RelFileLocator rlocatora;
    RelFileLocator rlocatorb;
 
    rlocatora = BufTagGetRelFileLocator(ba);
    rlocatorb = BufTagGetRelFileLocator(bb);
 
    ret = rlocator_comparator(&rlocatora, &rlocatorb);
 
    if (ret != 0)
        return ret;
 
    if (BufTagGetForkNum(ba) < BufTagGetForkNum(bb))
        return -1;
    if (BufTagGetForkNum(ba) > BufTagGetForkNum(bb))
        return 1;
 
    if (ba->blockNum < bb->blockNum)
        return -1;
    if (ba->blockNum > bb->blockNum)
        return 1;
 
    return 0;
}
 
/*
 * Comparator determining the writeout order in a checkpoint.
 *
 * It is important that tablespaces are compared first, the logic balancing
 * writes between tablespaces relies on it.
 */
static inline int
ckpt_buforder_comparator(const CkptSortItem *a, const CkptSortItem *b)
{
    /* compare tablespace */
    if (a->tsId < b->tsId)
        return -1;
    else if (a->tsId > b->tsId)
        return 1;
    /* compare relation */
    if (a->relNumber < b->relNumber)
        return -1;
    else if (a->relNumber > b->relNumber)
        return 1;
    /* compare fork */
    else if (a->forkNum < b->forkNum)
        return -1;
    else if (a->forkNum > b->forkNum)
        return 1;
    /* compare block number */
    else if (a->blockNum < b->blockNum)
        return -1;
    else if (a->blockNum > b->blockNum)
        return 1;
    /* equal page IDs are unlikely, but not impossible */
    return 0;
}
 
/*
 * Comparator for a Min-Heap over the per-tablespace checkpoint completion
 * progress.
 */
static int
ts_ckpt_progress_comparator(Datum a, Datum b, void *arg)
{
    CkptTsStatus *sa = (CkptTsStatus *) a;
    CkptTsStatus *sb = (CkptTsStatus *) b;
 
    /* we want a min-heap, so return 1 for the a < b */
    if (sa->progress < sb->progress)
        return 1;
    else if (sa->progress == sb->progress)
        return 0;
    else
        return -1;
}
 
/*
 * Initialize a writeback context, discarding potential previous state.
 *
 * *max_pending is a pointer instead of an immediate value, so the coalesce
 * limits can easily changed by the GUC mechanism, and so calling code does
 * not have to check the current configuration. A value of 0 means that no
 * writeback control will be performed.
 */
void
WritebackContextInit(WritebackContext *context, int *max_pending)
{
    Assert(*max_pending <= WRITEBACK_MAX_PENDING_FLUSHES);
 
    context->max_pending = max_pending;
    context->nr_pending = 0;
}
 
/*
 * Add buffer to list of pending writeback requests.
 */
void
ScheduleBufferTagForWriteback(WritebackContext *wb_context, IOContext io_context,
                              BufferTag *tag)
{
    PendingWriteback *pending;
 
    /*
     * As pg_flush_data() doesn't do anything with fsync disabled, there's no
     * point in tracking in that case.
     */
    if (io_direct_flags & IO_DIRECT_DATA ||
        !enableFsync)
        return;
 
    /*
     * Add buffer to the pending writeback array, unless writeback control is
     * disabled.
     */
    if (*wb_context->max_pending > 0)
    {
        Assert(*wb_context->max_pending <= WRITEBACK_MAX_PENDING_FLUSHES);
 
        pending = &wb_context->pending_writebacks[wb_context->nr_pending++];
 
        pending->tag = *tag;
    }
 
    /*
     * Perform pending flushes if the writeback limit is exceeded. This
     * includes the case where previously an item has been added, but control
     * is now disabled.
     */
    if (wb_context->nr_pending >= *wb_context->max_pending)
        IssuePendingWritebacks(wb_context, io_context);
}
 
#define ST_SORT sort_pending_writebacks
#define ST_ELEMENT_TYPE PendingWriteback
#define ST_COMPARE(a, b) buffertag_comparator(&a->tag, &b->tag)
#define ST_SCOPE static
#define ST_DEFINE
#include "lib/sort_template.h"
 
/*
 * Issue all pending writeback requests, previously scheduled with
 * ScheduleBufferTagForWriteback, to the OS.
 *
 * Because this is only used to improve the OSs IO scheduling we try to never
 * error out - it's just a hint.
 */
void
IssuePendingWritebacks(WritebackContext *wb_context, IOContext io_context)
{
    instr_time  io_start;
    int         i;
 
    if (wb_context->nr_pending == 0)
        return;
 
    /*
     * Executing the writes in-order can make them a lot faster, and allows to
     * merge writeback requests to consecutive blocks into larger writebacks.
     */
    sort_pending_writebacks(wb_context->pending_writebacks,
                            wb_context->nr_pending);
 
    io_start = pgstat_prepare_io_time(track_io_timing);
 
    /*
     * Coalesce neighbouring writes, but nothing else. For that we iterate
     * through the, now sorted, array of pending flushes, and look forward to
     * find all neighbouring (or identical) writes.
     */
    for (i = 0; i < wb_context->nr_pending; i++)
    {
        PendingWriteback *cur;
        PendingWriteback *next;
        SMgrRelation reln;
        int         ahead;
        BufferTag   tag;
        RelFileLocator currlocator;
        Size        nblocks = 1;
 
        cur = &wb_context->pending_writebacks[i];
        tag = cur->tag;
        currlocator = BufTagGetRelFileLocator(&tag);
 
        /*
         * Peek ahead, into following writeback requests, to see if they can
         * be combined with the current one.
         */
        for (ahead = 0; i + ahead + 1 < wb_context->nr_pending; ahead++)
        {
 
            next = &wb_context->pending_writebacks[i + ahead + 1];
 
            /* different file, stop */
            if (!RelFileLocatorEquals(currlocator,
                                      BufTagGetRelFileLocator(&next->tag)) ||
                BufTagGetForkNum(&cur->tag) != BufTagGetForkNum(&next->tag))
                break;
 
            /* ok, block queued twice, skip */
            if (cur->tag.blockNum == next->tag.blockNum)
                continue;
 
            /* only merge consecutive writes */
            if (cur->tag.blockNum + 1 != next->tag.blockNum)
                break;
 
            nblocks++;
            cur = next;
        }
 
        i += ahead;
 
        /* and finally tell the kernel to write the data to storage */
        reln = smgropen(currlocator, INVALID_PROC_NUMBER);
        smgrwriteback(reln, BufTagGetForkNum(&tag), tag.blockNum, nblocks);
    }
 
    /*
     * Assume that writeback requests are only issued for buffers containing
     * blocks of permanent relations.
     */
    pgstat_count_io_op_time(IOOBJECT_RELATION, io_context,
                            IOOP_WRITEBACK, io_start, wb_context->nr_pending, 0);
 
    wb_context->nr_pending = 0;
}
 
/* ResourceOwner callbacks */
 
static void
ResOwnerReleaseBufferIO(Datum res)
{
    Buffer      buffer = DatumGetInt32(res);
 
    AbortBufferIO(buffer);
}
 
static char *
ResOwnerPrintBufferIO(Datum res)
{
    Buffer      buffer = DatumGetInt32(res);
 
    return psprintf("lost track of buffer IO on buffer %d", buffer);
}
 
static void
ResOwnerReleaseBufferPin(Datum res)
{
    Buffer      buffer = DatumGetInt32(res);
 
    /* Like ReleaseBuffer, but don't call ResourceOwnerForgetBuffer */
    if (!BufferIsValid(buffer))
        elog(ERROR, "bad buffer ID: %d", buffer);
 
    if (BufferIsLocal(buffer))
        UnpinLocalBufferNoOwner(buffer);
    else
        UnpinBufferNoOwner(GetBufferDescriptor(buffer - 1));
}
 
static char *
ResOwnerPrintBufferPin(Datum res)
{
    return DebugPrintBufferRefcount(DatumGetInt32(res));
}
 
/*
 * Helper function to evict unpinned buffer whose buffer header lock is
 * already acquired.
 */
static bool
EvictUnpinnedBufferInternal(BufferDesc *desc, bool *buffer_flushed)
{
    uint32      buf_state;
    bool        result;
 
    *buffer_flushed = false;
 
    buf_state = pg_atomic_read_u32(&(desc->state));
    Assert(buf_state & BM_LOCKED);
 
    if ((buf_state & BM_VALID) == 0)
    {
        UnlockBufHdr(desc, buf_state);
        return false;
    }
 
    /* Check that it's not pinned already. */
    if (BUF_STATE_GET_REFCOUNT(buf_state) > 0)
    {
        UnlockBufHdr(desc, buf_state);
        return false;
    }
 
    PinBuffer_Locked(desc);     /* releases spinlock */
 
    /* If it was dirty, try to clean it once. */
    if (buf_state & BM_DIRTY)
    {
        LWLockAcquire(BufferDescriptorGetContentLock(desc), LW_SHARED);
        FlushBuffer(desc, NULL, IOOBJECT_RELATION, IOCONTEXT_NORMAL);
        *buffer_flushed = true;
        LWLockRelease(BufferDescriptorGetContentLock(desc));
    }
 
    /* This will return false if it becomes dirty or someone else pins it. */
    result = InvalidateVictimBuffer(desc);
 
    UnpinBuffer(desc);
 
    return result;
}
 
/*
 * Try to evict the current block in a shared buffer.
 *
 * This function is intended for testing/development use only!
 *
 * To succeed, the buffer must not be pinned on entry, so if the caller had a
 * particular block in mind, it might already have been replaced by some other
 * block by the time this function runs.  It's also unpinned on return, so the
 * buffer might be occupied again by the time control is returned, potentially
 * even by the same block.  This inherent raciness without other interlocking
 * makes the function unsuitable for non-testing usage.
 *
 * *buffer_flushed is set to true if the buffer was dirty and has been
 * flushed, false otherwise.  However, *buffer_flushed=true does not
 * necessarily mean that we flushed the buffer, it could have been flushed by
 * someone else.
 *
 * Returns true if the buffer was valid and it has now been made invalid.
 * Returns false if it wasn't valid, if it couldn't be evicted due to a pin,
 * or if the buffer becomes dirty again while we're trying to write it out.
 */
bool
EvictUnpinnedBuffer(Buffer buf, bool *buffer_flushed)
{
    BufferDesc *desc;
 
    Assert(BufferIsValid(buf) && !BufferIsLocal(buf));
 
    /* Make sure we can pin the buffer. */
    ResourceOwnerEnlarge(CurrentResourceOwner);
    ReservePrivateRefCountEntry();
 
    desc = GetBufferDescriptor(buf - 1);
    LockBufHdr(desc);
 
    return EvictUnpinnedBufferInternal(desc, buffer_flushed);
}
 
/*
 * Try to evict all the shared buffers.
 *
 * This function is intended for testing/development use only! See
 * EvictUnpinnedBuffer().
 *
 * The buffers_* parameters are mandatory and indicate the total count of
 * buffers that:
 * - buffers_evicted - were evicted
 * - buffers_flushed - were flushed
 * - buffers_skipped - could not be evicted
 */
void
EvictAllUnpinnedBuffers(int32 *buffers_evicted, int32 *buffers_flushed,
                        int32 *buffers_skipped)
{
    *buffers_evicted = 0;
    *buffers_skipped = 0;
    *buffers_flushed = 0;
 
    for (int buf = 1; buf <= NBuffers; buf++)
    {
        BufferDesc *desc = GetBufferDescriptor(buf - 1);
        uint32      buf_state;
        bool        buffer_flushed;
 
        buf_state = pg_atomic_read_u32(&desc->state);
        if (!(buf_state & BM_VALID))
            continue;
 
        ResourceOwnerEnlarge(CurrentResourceOwner);
        ReservePrivateRefCountEntry();
 
        LockBufHdr(desc);
 
        if (EvictUnpinnedBufferInternal(desc, &buffer_flushed))
            (*buffers_evicted)++;
        else
            (*buffers_skipped)++;
 
        if (buffer_flushed)
            (*buffers_flushed)++;
    }
}
 
/*
 * Try to evict all the shared buffers containing provided relation's pages.
 *
 * This function is intended for testing/development use only! See
 * EvictUnpinnedBuffer().
 *
 * The caller must hold at least AccessShareLock on the relation to prevent
 * the relation from being dropped.
 *
 * The buffers_* parameters are mandatory and indicate the total count of
 * buffers that:
 * - buffers_evicted - were evicted
 * - buffers_flushed - were flushed
 * - buffers_skipped - could not be evicted
 */
void
EvictRelUnpinnedBuffers(Relation rel, int32 *buffers_evicted,
                        int32 *buffers_flushed, int32 *buffers_skipped)
{
    Assert(!RelationUsesLocalBuffers(rel));
 
    *buffers_skipped = 0;
    *buffers_evicted = 0;
    *buffers_flushed = 0;
 
    for (int buf = 1; buf <= NBuffers; buf++)
    {
        BufferDesc *desc = GetBufferDescriptor(buf - 1);
        uint32      buf_state = pg_atomic_read_u32(&(desc->state));
        bool        buffer_flushed;
 
        /* An unlocked precheck should be safe and saves some cycles. */
        if ((buf_state & BM_VALID) == 0 ||
            !BufTagMatchesRelFileLocator(&desc->tag, &rel->rd_locator))
            continue;
 
        /* Make sure we can pin the buffer. */
        ResourceOwnerEnlarge(CurrentResourceOwner);
        ReservePrivateRefCountEntry();
 
        buf_state = LockBufHdr(desc);
 
        /* recheck, could have changed without the lock */
        if ((buf_state & BM_VALID) == 0 ||
            !BufTagMatchesRelFileLocator(&desc->tag, &rel->rd_locator))
        {
            UnlockBufHdr(desc, buf_state);
            continue;
        }
 
        if (EvictUnpinnedBufferInternal(desc, &buffer_flushed))
            (*buffers_evicted)++;
        else
            (*buffers_skipped)++;
 
        if (buffer_flushed)
            (*buffers_flushed)++;
    }
}
 
/*
 * Generic implementation of the AIO handle staging callback for readv/writev
 * on local/shared buffers.
 *
 * Each readv/writev can target multiple buffers. The buffers have already
 * been registered with the IO handle.
 *
 * To make the IO ready for execution ("staging"), we need to ensure that the
 * targeted buffers are in an appropriate state while the IO is ongoing. For
 * that the AIO subsystem needs to have its own buffer pin, otherwise an error
 * in this backend could lead to this backend's buffer pin being released as
 * part of error handling, which in turn could lead to the buffer being
 * replaced while IO is ongoing.
 */
static pg_attribute_always_inline void
buffer_stage_common(PgAioHandle *ioh, bool is_write, bool is_temp)
{
    uint64     *io_data;
    uint8       handle_data_len;
    PgAioWaitRef io_ref;
    BufferTag   first PG_USED_FOR_ASSERTS_ONLY = {0};
 
    io_data = pgaio_io_get_handle_data(ioh, &handle_data_len);
 
    pgaio_io_get_wref(ioh, &io_ref);
 
    /* iterate over all buffers affected by the vectored readv/writev */
    for (int i = 0; i < handle_data_len; i++)
    {
        Buffer      buffer = (Buffer) io_data[i];
        BufferDesc *buf_hdr = is_temp ?
            GetLocalBufferDescriptor(-buffer - 1)
            : GetBufferDescriptor(buffer - 1);
        uint32      buf_state;
 
        /*
         * Check that all the buffers are actually ones that could conceivably
         * be done in one IO, i.e. are sequential. This is the last
         * buffer-aware code before IO is actually executed and confusion
         * about which buffers are targeted by IO can be hard to debug, making
         * it worth doing extra-paranoid checks.
         */
        if (i == 0)
            first = buf_hdr->tag;
        else
        {
            Assert(buf_hdr->tag.relNumber == first.relNumber);
            Assert(buf_hdr->tag.blockNum == first.blockNum + i);
        }
 
        if (is_temp)
            buf_state = pg_atomic_read_u32(&buf_hdr->state);
        else
            buf_state = LockBufHdr(buf_hdr);
 
        /* verify the buffer is in the expected state */
        Assert(buf_state & BM_TAG_VALID);
        if (is_write)
        {
            Assert(buf_state & BM_VALID);
            Assert(buf_state & BM_DIRTY);
        }
        else
        {
            Assert(!(buf_state & BM_VALID));
            Assert(!(buf_state & BM_DIRTY));
        }
 
        /* temp buffers don't use BM_IO_IN_PROGRESS */
        if (!is_temp)
            Assert(buf_state & BM_IO_IN_PROGRESS);
 
        Assert(BUF_STATE_GET_REFCOUNT(buf_state) >= 1);
 
        /*
         * Reflect that the buffer is now owned by the AIO subsystem.
         *
         * For local buffers: This can't be done just via LocalRefCount, as
         * one might initially think, as this backend could error out while
         * AIO is still in progress, releasing all the pins by the backend
         * itself.
         *
         * This pin is released again in TerminateBufferIO().
         */
        buf_state += BUF_REFCOUNT_ONE;
        buf_hdr->io_wref = io_ref;
 
        if (is_temp)
            pg_atomic_unlocked_write_u32(&buf_hdr->state, buf_state);
        else
            UnlockBufHdr(buf_hdr, buf_state);
 
        /*
         * Ensure the content lock that prevents buffer modifications while
         * the buffer is being written out is not released early due to an
         * error.
         */
        if (is_write && !is_temp)
        {
            LWLock     *content_lock;
 
            content_lock = BufferDescriptorGetContentLock(buf_hdr);
 
            Assert(LWLockHeldByMe(content_lock));
 
            /*
             * Lock is now owned by AIO subsystem.
             */
            LWLockDisown(content_lock);
        }
 
        /*
         * Stop tracking this buffer via the resowner - the AIO system now
         * keeps track.
         */
        if (!is_temp)
            ResourceOwnerForgetBufferIO(CurrentResourceOwner, buffer);
    }
}
 
/*
 * Decode readv errors as encoded by buffer_readv_encode_error().
 */
static inline void
buffer_readv_decode_error(PgAioResult result,
                          bool *zeroed_any,
                          bool *ignored_any,
                          uint8 *zeroed_or_error_count,
                          uint8 *checkfail_count,
                          uint8 *first_off)
{
    uint32      rem_error = result.error_data;
 
    /* see static asserts in buffer_readv_encode_error */
#define READV_COUNT_BITS    7
#define READV_COUNT_MASK    ((1 << READV_COUNT_BITS) - 1)
 
    *zeroed_any = rem_error & 1;
    rem_error >>= 1;
 
    *ignored_any = rem_error & 1;
    rem_error >>= 1;
 
    *zeroed_or_error_count = rem_error & READV_COUNT_MASK;
    rem_error >>= READV_COUNT_BITS;
 
    *checkfail_count = rem_error & READV_COUNT_MASK;
    rem_error >>= READV_COUNT_BITS;
 
    *first_off = rem_error & READV_COUNT_MASK;
    rem_error >>= READV_COUNT_BITS;
}
 
/*
 * Helper to encode errors for buffer_readv_complete()
 *
 * Errors are encoded as follows:
 * - bit 0 indicates whether any page was zeroed (1) or not (0)
 * - bit 1 indicates whether any checksum failure was ignored (1) or not (0)
 * - next READV_COUNT_BITS bits indicate the number of errored or zeroed pages
 * - next READV_COUNT_BITS bits indicate the number of checksum failures
 * - next READV_COUNT_BITS bits indicate the first offset of the first page
 *   that was errored or zeroed or, if no errors/zeroes, the first ignored
 *   checksum
 */
static inline void
buffer_readv_encode_error(PgAioResult *result,
                          bool is_temp,
                          bool zeroed_any,
                          bool ignored_any,
                          uint8 error_count,
                          uint8 zeroed_count,
                          uint8 checkfail_count,
                          uint8 first_error_off,
                          uint8 first_zeroed_off,
                          uint8 first_ignored_off)
{
 
    uint8       shift = 0;
    uint8       zeroed_or_error_count =
        error_count > 0 ? error_count : zeroed_count;
    uint8       first_off;
 
    StaticAssertStmt(PG_IOV_MAX <= 1 << READV_COUNT_BITS,
                     "PG_IOV_MAX is bigger than reserved space for error data");
    StaticAssertStmt((1 + 1 + 3 * READV_COUNT_BITS) <= PGAIO_RESULT_ERROR_BITS,
                     "PGAIO_RESULT_ERROR_BITS is insufficient for buffer_readv");
 
    /*
     * We only have space to encode one offset - but luckily that's good
     * enough. If there is an error, the error is the interesting offset, same
     * with a zeroed buffer vs an ignored buffer.
     */
    if (error_count > 0)
        first_off = first_error_off;
    else if (zeroed_count > 0)
        first_off = first_zeroed_off;
    else
        first_off = first_ignored_off;
 
    Assert(!zeroed_any || error_count == 0);
 
    result->error_data = 0;
 
    result->error_data |= zeroed_any << shift;
    shift += 1;
 
    result->error_data |= ignored_any << shift;
    shift += 1;
 
    result->error_data |= ((uint32) zeroed_or_error_count) << shift;
    shift += READV_COUNT_BITS;
 
    result->error_data |= ((uint32) checkfail_count) << shift;
    shift += READV_COUNT_BITS;
 
    result->error_data |= ((uint32) first_off) << shift;
    shift += READV_COUNT_BITS;
 
    result->id = is_temp ? PGAIO_HCB_LOCAL_BUFFER_READV :
        PGAIO_HCB_SHARED_BUFFER_READV;
 
    if (error_count > 0)
        result->status = PGAIO_RS_ERROR;
    else
        result->status = PGAIO_RS_WARNING;
 
    /*
     * The encoding is complicated enough to warrant cross-checking it against
     * the decode function.
     */
#ifdef USE_ASSERT_CHECKING
    {
        bool        zeroed_any_2,
                    ignored_any_2;
        uint8       zeroed_or_error_count_2,
                    checkfail_count_2,
                    first_off_2;
 
        buffer_readv_decode_error(*result,
                                  &zeroed_any_2, &ignored_any_2,
                                  &zeroed_or_error_count_2,
                                  &checkfail_count_2,
                                  &first_off_2);
        Assert(zeroed_any == zeroed_any_2);
        Assert(ignored_any == ignored_any_2);
        Assert(zeroed_or_error_count == zeroed_or_error_count_2);
        Assert(checkfail_count == checkfail_count_2);
        Assert(first_off == first_off_2);
    }
#endif
 
#undef READV_COUNT_BITS
#undef READV_COUNT_MASK
}
 
/*
 * Helper for AIO readv completion callbacks, supporting both shared and temp
 * buffers. Gets called once for each buffer in a multi-page read.
 */
static pg_attribute_always_inline void
buffer_readv_complete_one(PgAioTargetData *td, uint8 buf_off, Buffer buffer,
                          uint8 flags, bool failed, bool is_temp,
                          bool *buffer_invalid,
                          bool *failed_checksum,
                          bool *ignored_checksum,
                          bool *zeroed_buffer)
{
    BufferDesc *buf_hdr = is_temp ?
        GetLocalBufferDescriptor(-buffer - 1)
        : GetBufferDescriptor(buffer - 1);
    BufferTag   tag = buf_hdr->tag;
    char       *bufdata = BufferGetBlock(buffer);
    uint32      set_flag_bits;
    int         piv_flags;
 
    /* check that the buffer is in the expected state for a read */
#ifdef USE_ASSERT_CHECKING
    {
        uint32      buf_state = pg_atomic_read_u32(&buf_hdr->state);
 
        Assert(buf_state & BM_TAG_VALID);
        Assert(!(buf_state & BM_VALID));
        /* temp buffers don't use BM_IO_IN_PROGRESS */
        if (!is_temp)
            Assert(buf_state & BM_IO_IN_PROGRESS);
        Assert(!(buf_state & BM_DIRTY));
    }
#endif
 
    *buffer_invalid = false;
    *failed_checksum = false;
    *ignored_checksum = false;
    *zeroed_buffer = false;
 
    /*
     * We ask PageIsVerified() to only log the message about checksum errors,
     * as the completion might be run in any backend (or IO workers). We will
     * report checksum errors in buffer_readv_report().
     */
    piv_flags = PIV_LOG_LOG;
 
    /* the local zero_damaged_pages may differ from the definer's */
    if (flags & READ_BUFFERS_IGNORE_CHECKSUM_FAILURES)
        piv_flags |= PIV_IGNORE_CHECKSUM_FAILURE;
 
    /* Check for garbage data. */
    if (!failed)
    {
        /*
         * If the buffer is not currently pinned by this backend, e.g. because
         * we're completing this IO after an error, the buffer data will have
         * been marked as inaccessible when the buffer was unpinned. The AIO
         * subsystem holds a pin, but that doesn't prevent the buffer from
         * having been marked as inaccessible. The completion might also be
         * executed in a different process.
         */
#ifdef USE_VALGRIND
        if (!BufferIsPinned(buffer))
            VALGRIND_MAKE_MEM_DEFINED(bufdata, BLCKSZ);
#endif
 
        if (!PageIsVerified((Page) bufdata, tag.blockNum, piv_flags,
                            failed_checksum))
        {
            if (flags & READ_BUFFERS_ZERO_ON_ERROR)
            {
                memset(bufdata, 0, BLCKSZ);
                *zeroed_buffer = true;
            }
            else
            {
                *buffer_invalid = true;
                /* mark buffer as having failed */
                failed = true;
            }
        }
        else if (*failed_checksum)
            *ignored_checksum = true;
 
        /* undo what we did above */
#ifdef USE_VALGRIND
        if (!BufferIsPinned(buffer))
            VALGRIND_MAKE_MEM_NOACCESS(bufdata, BLCKSZ);
#endif
 
        /*
         * Immediately log a message about the invalid page, but only to the
         * server log. The reason to do so immediately is that this may be
         * executed in a different backend than the one that originated the
         * request. The reason to do so immediately is that the originator
         * might not process the query result immediately (because it is busy
         * doing another part of query processing) or at all (e.g. if it was
         * cancelled or errored out due to another IO also failing). The
         * definer of the IO will emit an ERROR or WARNING when processing the
         * IO's results
         *
         * To avoid duplicating the code to emit these log messages, we reuse
         * buffer_readv_report().
         */
        if (*buffer_invalid || *failed_checksum || *zeroed_buffer)
        {
            PgAioResult result_one = {0};
 
            buffer_readv_encode_error(&result_one, is_temp,
                                      *zeroed_buffer,
                                      *ignored_checksum,
                                      *buffer_invalid,
                                      *zeroed_buffer ? 1 : 0,
                                      *failed_checksum ? 1 : 0,
                                      buf_off, buf_off, buf_off);
            pgaio_result_report(result_one, td, LOG_SERVER_ONLY);
        }
    }
 
    /* Terminate I/O and set BM_VALID. */
    set_flag_bits = failed ? BM_IO_ERROR : BM_VALID;
    if (is_temp)
        TerminateLocalBufferIO(buf_hdr, false, set_flag_bits, true);
    else
        TerminateBufferIO(buf_hdr, false, set_flag_bits, false, true);
 
    /*
     * Call the BUFFER_READ_DONE tracepoint in the callback, even though the
     * callback may not be executed in the same backend that called
     * BUFFER_READ_START. The alternative would be to defer calling the
     * tracepoint to a later point (e.g. the local completion callback for
     * shared buffer reads), which seems even less helpful.
     */
    TRACE_POSTGRESQL_BUFFER_READ_DONE(tag.forkNum,
                                      tag.blockNum,
                                      tag.spcOid,
                                      tag.dbOid,
                                      tag.relNumber,
                                      is_temp ? MyProcNumber : INVALID_PROC_NUMBER,
                                      false);
}
 
/*
 * Perform completion handling of a single AIO read. This read may cover
 * multiple blocks / buffers.
 *
 * Shared between shared and local buffers, to reduce code duplication.
 */
static pg_attribute_always_inline PgAioResult
buffer_readv_complete(PgAioHandle *ioh, PgAioResult prior_result,
                      uint8 cb_data, bool is_temp)
{
    PgAioResult result = prior_result;
    PgAioTargetData *td = pgaio_io_get_target_data(ioh);
    uint8       first_error_off = 0;
    uint8       first_zeroed_off = 0;
    uint8       first_ignored_off = 0;
    uint8       error_count = 0;
    uint8       zeroed_count = 0;
    uint8       ignored_count = 0;
    uint8       checkfail_count = 0;
    uint64     *io_data;
    uint8       handle_data_len;
 
    if (is_temp)
    {
        Assert(td->smgr.is_temp);
        Assert(pgaio_io_get_owner(ioh) == MyProcNumber);
    }
    else
        Assert(!td->smgr.is_temp);
 
    /*
     * Iterate over all the buffers affected by this IO and call the
     * per-buffer completion function for each buffer.
     */
    io_data = pgaio_io_get_handle_data(ioh, &handle_data_len);
    for (uint8 buf_off = 0; buf_off < handle_data_len; buf_off++)
    {
        Buffer      buf = io_data[buf_off];
        bool        failed;
        bool        failed_verification = false;
        bool        failed_checksum = false;
        bool        zeroed_buffer = false;
        bool        ignored_checksum = false;
 
        Assert(BufferIsValid(buf));
 
        /*
         * If the entire I/O failed on a lower-level, each buffer needs to be
         * marked as failed. In case of a partial read, the first few buffers
         * may be ok.
         */
        failed =
            prior_result.status == PGAIO_RS_ERROR
            || prior_result.result <= buf_off;
 
        buffer_readv_complete_one(td, buf_off, buf, cb_data, failed, is_temp,
                                  &failed_verification,
                                  &failed_checksum,
                                  &ignored_checksum,
                                  &zeroed_buffer);
 
        /*
         * Track information about the number of different kinds of error
         * conditions across all pages, as there can be multiple pages failing
         * verification as part of one IO.
         */
        if (failed_verification && !zeroed_buffer && error_count++ == 0)
            first_error_off = buf_off;
        if (zeroed_buffer && zeroed_count++ == 0)
            first_zeroed_off = buf_off;
        if (ignored_checksum && ignored_count++ == 0)
            first_ignored_off = buf_off;
        if (failed_checksum)
            checkfail_count++;
    }
 
    /*
     * If the smgr read succeeded [partially] and page verification failed for
     * some of the pages, adjust the IO's result state appropriately.
     */
    if (prior_result.status != PGAIO_RS_ERROR &&
        (error_count > 0 || ignored_count > 0 || zeroed_count > 0))
    {
        buffer_readv_encode_error(&result, is_temp,
                                  zeroed_count > 0, ignored_count > 0,
                                  error_count, zeroed_count, checkfail_count,
                                  first_error_off, first_zeroed_off,
                                  first_ignored_off);
        pgaio_result_report(result, td, DEBUG1);
    }
 
    /*
     * For shared relations this reporting is done in
     * shared_buffer_readv_complete_local().
     */
    if (is_temp && checkfail_count > 0)
        pgstat_report_checksum_failures_in_db(td->smgr.rlocator.dbOid,
                                              checkfail_count);
 
    return result;
}
 
/*
 * AIO error reporting callback for aio_shared_buffer_readv_cb and
 * aio_local_buffer_readv_cb.
 *
 * The error is encoded / decoded in buffer_readv_encode_error() /
 * buffer_readv_decode_error().
 */
static void
buffer_readv_report(PgAioResult result, const PgAioTargetData *td,
                    int elevel)
{
    int         nblocks = td->smgr.nblocks;
    BlockNumber first = td->smgr.blockNum;
    BlockNumber last = first + nblocks - 1;
    ProcNumber  errProc =
        td->smgr.is_temp ? MyProcNumber : INVALID_PROC_NUMBER;
    RelPathStr  rpath =
        relpathbackend(td->smgr.rlocator, errProc, td->smgr.forkNum);
    bool        zeroed_any,
                ignored_any;
    uint8       zeroed_or_error_count,
                checkfail_count,
                first_off;
    uint8       affected_count;
    const char *msg_one,
               *msg_mult,
               *det_mult,
               *hint_mult;
 
    buffer_readv_decode_error(result, &zeroed_any, &ignored_any,
                              &zeroed_or_error_count,
                              &checkfail_count,
                              &first_off);
 
    /*
     * Treat a read that had both zeroed buffers *and* ignored checksums as a
     * special case, it's too irregular to be emitted the same way as the
     * other cases.
     */
    if (zeroed_any && ignored_any)
    {
        Assert(zeroed_any && ignored_any);
        Assert(nblocks > 1);    /* same block can't be both zeroed and ignored */
        Assert(result.status != PGAIO_RS_ERROR);
        affected_count = zeroed_or_error_count;
 
        ereport(elevel,
                errcode(ERRCODE_DATA_CORRUPTED),
                errmsg("zeroing %u page(s) and ignoring %u checksum failure(s) among blocks %u..%u of relation %s",
                       affected_count, checkfail_count, first, last, rpath.str),
                affected_count > 1 ?
                errdetail("Block %u held first zeroed page.",
                          first + first_off) : 0,
                errhint("See server log for details about the other %u invalid block(s).",
                        affected_count + checkfail_count - 1));
        return;
    }
 
    /*
     * The other messages are highly repetitive. To avoid duplicating a long
     * and complicated ereport(), gather the translated format strings
     * separately and then do one common ereport.
     */
    if (result.status == PGAIO_RS_ERROR)
    {
        Assert(!zeroed_any);    /* can't have invalid pages when zeroing them */
        affected_count = zeroed_or_error_count;
        msg_one = _("invalid page in block %u of relation %s");
        msg_mult = _("%u invalid pages among blocks %u..%u of relation %s");
        det_mult = _("Block %u held first invalid page.");
        hint_mult = _("See server log for the other %u invalid block(s).");
    }
    else if (zeroed_any && !ignored_any)
    {
        affected_count = zeroed_or_error_count;
        msg_one = _("invalid page in block %u of relation %s; zeroing out page");
        msg_mult = _("zeroing out %u invalid pages among blocks %u..%u of relation %s");
        det_mult = _("Block %u held first zeroed page.");
        hint_mult = _("See server log for the other %u zeroed block(s).");
    }
    else if (!zeroed_any && ignored_any)
    {
        affected_count = checkfail_count;
        msg_one = _("ignoring checksum failure in block %u of relation %s");
        msg_mult = _("ignoring %u checksum failures among blocks %u..%u of relation %s");
        det_mult = _("Block %u held first ignored page.");
        hint_mult = _("See server log for the other %u ignored block(s).");
    }
    else
        pg_unreachable();
 
    ereport(elevel,
            errcode(ERRCODE_DATA_CORRUPTED),
            affected_count == 1 ?
            errmsg_internal(msg_one, first + first_off, rpath.str) :
            errmsg_internal(msg_mult, affected_count, first, last, rpath.str),
            affected_count > 1 ? errdetail_internal(det_mult, first + first_off) : 0,
            affected_count > 1 ? errhint_internal(hint_mult, affected_count - 1) : 0);
}
 
static void
shared_buffer_readv_stage(PgAioHandle *ioh, uint8 cb_data)
{
    buffer_stage_common(ioh, false, false);
}
 
static PgAioResult
shared_buffer_readv_complete(PgAioHandle *ioh, PgAioResult prior_result,
                             uint8 cb_data)
{
    return buffer_readv_complete(ioh, prior_result, cb_data, false);
}
 
/*
 * We need a backend-local completion callback for shared buffers, to be able
 * to report checksum errors correctly. Unfortunately that can only safely
 * happen if the reporting backend has previously called
 * pgstat_prepare_report_checksum_failure(), which we can only guarantee in
 * the backend that started the IO. Hence this callback.
 */
static PgAioResult
shared_buffer_readv_complete_local(PgAioHandle *ioh, PgAioResult prior_result,
                                   uint8 cb_data)
{
    bool        zeroed_any,
                ignored_any;
    uint8       zeroed_or_error_count,
                checkfail_count,
                first_off;
 
    if (prior_result.status == PGAIO_RS_OK)
        return prior_result;
 
    buffer_readv_decode_error(prior_result,
                              &zeroed_any,
                              &ignored_any,
                              &zeroed_or_error_count,
                              &checkfail_count,
                              &first_off);
 
    if (checkfail_count)
    {
        PgAioTargetData *td = pgaio_io_get_target_data(ioh);
 
        pgstat_report_checksum_failures_in_db(td->smgr.rlocator.dbOid,
                                              checkfail_count);
    }
 
    return prior_result;
}
 
static void
local_buffer_readv_stage(PgAioHandle *ioh, uint8 cb_data)
{
    buffer_stage_common(ioh, false, true);
}
 
static PgAioResult
local_buffer_readv_complete(PgAioHandle *ioh, PgAioResult prior_result,
                            uint8 cb_data)
{
    return buffer_readv_complete(ioh, prior_result, cb_data, true);
}
 
/* readv callback is passed READ_BUFFERS_* flags as callback data */
const PgAioHandleCallbacks aio_shared_buffer_readv_cb = {
    .stage = shared_buffer_readv_stage,
    .complete_shared = shared_buffer_readv_complete,
    /* need a local callback to report checksum failures */
    .complete_local = shared_buffer_readv_complete_local,
    .report = buffer_readv_report,
};
 
/* readv callback is passed READ_BUFFERS_* flags as callback data */
const PgAioHandleCallbacks aio_local_buffer_readv_cb = {
    .stage = local_buffer_readv_stage,
 
    /*
     * Note that this, in contrast to the shared_buffers case, uses
     * complete_local, as only the issuing backend has access to the required
     * datastructures. This is important in case the IO completion may be
     * consumed incidentally by another backend.
     */
    .complete_local = local_buffer_readv_complete,
    .report = buffer_readv_report,
};