bufmgr.c

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/*-------------------------------------------------------------------------
 *
 * bufmgr.c
 *    buffer manager interface routines
 *
 * Portions Copyright (c) 1996-2020, 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.
 *
 * 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/xlog.h"
#include "catalog/catalog.h"
#include "catalog/storage.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/buf_internals.h"
#include "storage/bufmgr.h"
#include "storage/ipc.h"
#include "storage/proc.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_private.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

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
 * DropRelFileNodesAllBuffers. Pointer to this struct and RelFileNode must be
 * compatible.
 */
typedef struct SMgrSortArray
{
    RelFileNode rnode;          /* 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 = 0;

/*
 * 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 = 0;

/*
 * GUC variables about triggering kernel writeback for buffers written; OS
 * dependent defaults are set via the GUC mechanism.
 */
int         checkpoint_flush_after = 0;
int         bgwriter_flush_after = 0;
int         backend_flush_after = 0;

/* local state for StartBufferIO and related functions */
static BufferDesc *InProgressBuf = NULL;
static bool IsForInput;

/* 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 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);

/*
 * 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,
                              (void *) &(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,
                      (void *) &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,
                    (void *) &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,
                    (void *) &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(SMgrRelation reln, char relpersistence,
                                ForkNumber forkNum, BlockNumber blockNum,
                                ReadBufferMode mode, BufferAccessStrategy strategy,
                                bool *hit);
static bool PinBuffer(BufferDesc *buf, BufferAccessStrategy strategy);
static void PinBuffer_Locked(BufferDesc *buf);
static void UnpinBuffer(BufferDesc *buf, bool fixOwner);
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 bool StartBufferIO(BufferDesc *buf, bool forInput);
static void TerminateBufferIO(BufferDesc *buf, bool clear_dirty,
                              uint32 set_flag_bits);
static void shared_buffer_write_error_callback(void *arg);
static void local_buffer_write_error_callback(void *arg);
static BufferDesc *BufferAlloc(SMgrRelation smgr,
                               char relpersistence,
                               ForkNumber forkNum,
                               BlockNumber blockNum,
                               BufferAccessStrategy strategy,
                               bool *foundPtr);
static void FlushBuffer(BufferDesc *buf, SMgrRelation reln);
static void AtProcExit_Buffers(int code, Datum arg);
static void CheckForBufferLeaks(void);
static int  rnode_comparator(const void *p1, const void *p2);
static int  buffertag_comparator(const void *p1, const void *p2);
static int  ckpt_buforder_comparator(const void *pa, const void *pb);
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 */
    INIT_BUFFERTAG(newTag, smgr_reln->smgr_rnode.node,
                   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 (smgrprefetch(smgr_reln, forkNum, blockNum))
            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 either
 * USE_PREFETCH is not defined (this build doesn't support prefetching due to
 * lack of a kernel facility), 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));

    /* Open it at the smgr level if not already done */
    RelationOpenSmgr(reln);

    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(reln->rd_smgr, forkNum, blockNum);
    }
    else
    {
        /* pass it to the shared buffer version */
        return PrefetchSharedBuffer(reln->rd_smgr, forkNum, blockNum);
    }
}


/*
 * 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
 * PageIsVerifiedExtended().)
 *
 * 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.
 */
Buffer
ReadBufferExtended(Relation reln, ForkNumber forkNum, BlockNumber blockNum,
                   ReadBufferMode mode, BufferAccessStrategy strategy)
{
    bool        hit;
    Buffer      buf;

    /* Open it at the smgr level if not already done */
    RelationOpenSmgr(reln);

    /*
     * 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.
     */
    pgstat_count_buffer_read(reln);
    buf = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
                            forkNum, blockNum, mode, strategy, &hit);
    if (hit)
        pgstat_count_buffer_hit(reln);
    return buf;
}


/*
 * ReadBufferWithoutRelcache -- like ReadBufferExtended, but doesn't require
 *      a relcache entry for the relation.
 *
 * NB: At present, this function may only be used on permanent relations, which
 * is OK, because we only use it during XLOG replay.  If in the future we
 * want to use it on temporary or unlogged relations, we could pass additional
 * parameters.
 */
Buffer
ReadBufferWithoutRelcache(RelFileNode rnode, ForkNumber forkNum,
                          BlockNumber blockNum, ReadBufferMode mode,
                          BufferAccessStrategy strategy)
{
    bool        hit;

    SMgrRelation smgr = smgropen(rnode, InvalidBackendId);

    Assert(InRecovery);

    return ReadBuffer_common(smgr, RELPERSISTENCE_PERMANENT, forkNum, blockNum,
                             mode, strategy, &hit);
}


/*
 * ReadBuffer_common -- common logic for all ReadBuffer variants
 *
 * *hit is set to true if the request was satisfied from shared buffer cache.
 */
static Buffer
ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
                  BlockNumber blockNum, ReadBufferMode mode,
                  BufferAccessStrategy strategy, bool *hit)
{
    BufferDesc *bufHdr;
    Block       bufBlock;
    bool        found;
    bool        isExtend;
    bool        isLocalBuf = SmgrIsTemp(smgr);

    *hit = false;

    /* Make sure we will have room to remember the buffer pin */
    ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

    isExtend = (blockNum == P_NEW);

    TRACE_POSTGRESQL_BUFFER_READ_START(forkNum, blockNum,
                                       smgr->smgr_rnode.node.spcNode,
                                       smgr->smgr_rnode.node.dbNode,
                                       smgr->smgr_rnode.node.relNode,
                                       smgr->smgr_rnode.backend,
                                       isExtend);

    /* Substitute proper block number if caller asked for P_NEW */
    if (isExtend)
        blockNum = smgrnblocks(smgr, forkNum);

    if (isLocalBuf)
    {
        bufHdr = LocalBufferAlloc(smgr, forkNum, blockNum, &found);
        if (found)
            pgBufferUsage.local_blks_hit++;
        else if (isExtend)
            pgBufferUsage.local_blks_written++;
        else if (mode == RBM_NORMAL || mode == RBM_NORMAL_NO_LOG ||
                 mode == RBM_ZERO_ON_ERROR)
            pgBufferUsage.local_blks_read++;
    }
    else
    {
        /*
         * lookup the buffer.  IO_IN_PROGRESS is set if the requested block is
         * not currently in memory.
         */
        bufHdr = BufferAlloc(smgr, relpersistence, forkNum, blockNum,
                             strategy, &found);
        if (found)
            pgBufferUsage.shared_blks_hit++;
        else if (isExtend)
            pgBufferUsage.shared_blks_written++;
        else if (mode == RBM_NORMAL || mode == RBM_NORMAL_NO_LOG ||
                 mode == RBM_ZERO_ON_ERROR)
            pgBufferUsage.shared_blks_read++;
    }

    /* At this point we do NOT hold any locks. */

    /* if it was already in the buffer pool, we're done */
    if (found)
    {
        if (!isExtend)
        {
            /* Just need to update stats before we exit */
            *hit = true;
            VacuumPageHit++;

            if (VacuumCostActive)
                VacuumCostBalance += VacuumCostPageHit;

            TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
                                              smgr->smgr_rnode.node.spcNode,
                                              smgr->smgr_rnode.node.dbNode,
                                              smgr->smgr_rnode.node.relNode,
                                              smgr->smgr_rnode.backend,
                                              isExtend,
                                              found);

            /*
             * In RBM_ZERO_AND_LOCK mode the caller expects the page to be
             * locked on return.
             */
            if (!isLocalBuf)
            {
                if (mode == RBM_ZERO_AND_LOCK)
                    LWLockAcquire(BufferDescriptorGetContentLock(bufHdr),
                                  LW_EXCLUSIVE);
                else if (mode == RBM_ZERO_AND_CLEANUP_LOCK)
                    LockBufferForCleanup(BufferDescriptorGetBuffer(bufHdr));
            }

            return BufferDescriptorGetBuffer(bufHdr);
        }

        /*
         * We get here only in the corner case where we are trying to extend
         * the relation but we found a pre-existing buffer marked BM_VALID.
         * This can happen 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 case should
         * always have left a zero-filled buffer, complain if not PageIsNew.
         */
        bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
        if (!PageIsNew((Page) bufBlock))
            ereport(ERROR,
                    (errmsg("unexpected data beyond EOF in block %u of relation %s",
                            blockNum, relpath(smgr->smgr_rnode, forkNum)),
                     errhint("This has been seen to occur with buggy kernels; consider updating your system.")));

        /*
         * We *must* do smgrextend 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 the StartBufferIO
         * call that BufferAlloc didn't, and proceed.
         */
        if (isLocalBuf)
        {
            /* Only need to adjust flags */
            uint32      buf_state = pg_atomic_read_u32(&bufHdr->state);

            Assert(buf_state & BM_VALID);
            buf_state &= ~BM_VALID;
            pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
        }
        else
        {
            /*
             * 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(bufHdr);

                Assert(buf_state & BM_VALID);
                buf_state &= ~BM_VALID;
                UnlockBufHdr(bufHdr, buf_state);
            } while (!StartBufferIO(bufHdr, true));
        }
    }

    /*
     * if we have gotten to this point, we have allocated a buffer for the
     * page but its contents are not yet valid.  IO_IN_PROGRESS is set for it,
     * if it's a shared buffer.
     *
     * Note: if smgrextend fails, we will end up with a buffer that is
     * allocated but not marked BM_VALID.  P_NEW 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 buffer (if
     * it's not been recycled) but come right back here to try smgrextend
     * again.
     */
    Assert(!(pg_atomic_read_u32(&bufHdr->state) & BM_VALID));   /* spinlock not needed */

    bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);

    if (isExtend)
    {
        /* new buffers are zero-filled */
        MemSet((char *) bufBlock, 0, BLCKSZ);
        /* don't set checksum for all-zero page */
        smgrextend(smgr, forkNum, blockNum, (char *) bufBlock, false);

        /*
         * NB: we're *not* doing a ScheduleBufferTagForWriteback here;
         * although we're essentially performing a write. At least on linux
         * doing so defeats the 'delayed allocation' mechanism, leading to
         * increased file fragmentation.
         */
    }
    else
    {
        /*
         * Read in the page, unless the caller intends to overwrite it and
         * just wants us to allocate a buffer.
         */
        if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
            MemSet((char *) bufBlock, 0, BLCKSZ);
        else
        {
            instr_time  io_start,
                        io_time;

            if (track_io_timing)
                INSTR_TIME_SET_CURRENT(io_start);

            smgrread(smgr, forkNum, blockNum, (char *) bufBlock);

            if (track_io_timing)
            {
                INSTR_TIME_SET_CURRENT(io_time);
                INSTR_TIME_SUBTRACT(io_time, io_start);
                pgstat_count_buffer_read_time(INSTR_TIME_GET_MICROSEC(io_time));
                INSTR_TIME_ADD(pgBufferUsage.blk_read_time, io_time);
            }

            /* check for garbage data */
            if (!PageIsVerifiedExtended((Page) bufBlock, blockNum,
                                        PIV_LOG_WARNING | PIV_REPORT_STAT))
            {
                if (mode == RBM_ZERO_ON_ERROR || zero_damaged_pages)
                {
                    ereport(WARNING,
                            (errcode(ERRCODE_DATA_CORRUPTED),
                             errmsg("invalid page in block %u of relation %s; zeroing out page",
                                    blockNum,
                                    relpath(smgr->smgr_rnode, forkNum))));
                    MemSet((char *) bufBlock, 0, BLCKSZ);
                }
                else
                    ereport(ERROR,
                            (errcode(ERRCODE_DATA_CORRUPTED),
                             errmsg("invalid page in block %u of relation %s",
                                    blockNum,
                                    relpath(smgr->smgr_rnode, forkNum))));
            }
        }
    }

    /*
     * In RBM_ZERO_AND_LOCK mode, 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 ((mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK) &&
        !isLocalBuf)
    {
        LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_EXCLUSIVE);
    }

    if (isLocalBuf)
    {
        /* Only need to adjust flags */
        uint32      buf_state = pg_atomic_read_u32(&bufHdr->state);

        buf_state |= BM_VALID;
        pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
    }
    else
    {
        /* Set BM_VALID, terminate IO, and wake up any waiters */
        TerminateBufferIO(bufHdr, false, BM_VALID);
    }

    VacuumPageMiss++;
    if (VacuumCostActive)
        VacuumCostBalance += VacuumCostPageMiss;

    TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
                                      smgr->smgr_rnode.node.spcNode,
                                      smgr->smgr_rnode.node.dbNode,
                                      smgr->smgr_rnode.node.relNode,
                                      smgr->smgr_rnode.backend,
                                      isExtend,
                                      found);

    return BufferDescriptorGetBuffer(bufHdr);
}

/*
 * BufferAlloc -- subroutine for ReadBuffer.  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 and the buffer is marked
 * as IO_IN_PROGRESS; ReadBuffer will now need to do I/O to fill it.
 *
 * *foundPtr is actually redundant with the buffer's BM_VALID flag, but
 * we keep it for simplicity in ReadBuffer.
 *
 * No locks are held either at entry or exit.
 */
static BufferDesc *
BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
            BlockNumber blockNum,
            BufferAccessStrategy strategy,
            bool *foundPtr)
{
    BufferTag   newTag;         /* identity of requested block */
    uint32      newHash;        /* hash value for newTag */
    LWLock     *newPartitionLock;   /* buffer partition lock for it */
    BufferTag   oldTag;         /* previous identity of selected buffer */
    uint32      oldHash;        /* hash value for oldTag */
    LWLock     *oldPartitionLock;   /* buffer partition lock for it */
    uint32      oldFlags;
    int         buf_id;
    BufferDesc *buf;
    bool        valid;
    uint32      buf_state;

    /* create a tag so we can lookup the buffer */
    INIT_BUFFERTAG(newTag, smgr->smgr_rnode.node, 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);
    if (buf_id >= 0)
    {
        /*
         * 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(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, or (b) a previous read attempt failed.  We have to
             * wait for any active read attempt to finish, and then set up our
             * own read attempt if the page is still not BM_VALID.
             * StartBufferIO does it all.
             */
            if (StartBufferIO(buf, true))
            {
                /*
                 * If we get here, previous attempts to read the buffer must
                 * have failed ... but we shall bravely try again.
                 */
                *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);

    /* Loop here in case we have to try another victim buffer */
    for (;;)
    {
        /*
         * Ensure, while the spinlock's not yet held, that there's a free
         * refcount entry.
         */
        ReservePrivateRefCountEntry();

        /*
         * Select a victim buffer.  The buffer is returned with its header
         * spinlock still held!
         */
        buf = StrategyGetBuffer(strategy, &buf_state);

        Assert(BUF_STATE_GET_REFCOUNT(buf_state) == 0);

        /* Must copy buffer flags while we still hold the spinlock */
        oldFlags = buf_state & BUF_FLAG_MASK;

        /* Pin the buffer and then release the buffer spinlock */
        PinBuffer_Locked(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 it
         * 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 (oldFlags & BM_DIRTY)
        {
            /*
             * 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.)
             */
            if (LWLockConditionalAcquire(BufferDescriptorGetContentLock(buf),
                                         LW_SHARED))
            {
                /*
                 * 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 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);
                    lsn = BufferGetLSN(buf);
                    UnlockBufHdr(buf, buf_state);

                    if (XLogNeedsFlush(lsn) &&
                        StrategyRejectBuffer(strategy, buf))
                    {
                        /* Drop lock/pin and loop around for another buffer */
                        LWLockRelease(BufferDescriptorGetContentLock(buf));
                        UnpinBuffer(buf, true);
                        continue;
                    }
                }

                /* OK, do the I/O */
                TRACE_POSTGRESQL_BUFFER_WRITE_DIRTY_START(forkNum, blockNum,
                                                          smgr->smgr_rnode.node.spcNode,
                                                          smgr->smgr_rnode.node.dbNode,
                                                          smgr->smgr_rnode.node.relNode);

                FlushBuffer(buf, NULL);
                LWLockRelease(BufferDescriptorGetContentLock(buf));

                ScheduleBufferTagForWriteback(&BackendWritebackContext,
                                              &buf->tag);

                TRACE_POSTGRESQL_BUFFER_WRITE_DIRTY_DONE(forkNum, blockNum,
                                                         smgr->smgr_rnode.node.spcNode,
                                                         smgr->smgr_rnode.node.dbNode,
                                                         smgr->smgr_rnode.node.relNode);
            }
            else
            {
                /*
                 * Someone else has locked the buffer, so give it up and loop
                 * back to get another one.
                 */
                UnpinBuffer(buf, true);
                continue;
            }
        }

        /*
         * To change the association of a valid buffer, we'll need to have
         * exclusive lock on both the old and new mapping partitions.
         */
        if (oldFlags & BM_TAG_VALID)
        {
            /*
             * 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.
             */
            oldTag = buf->tag;
            oldHash = BufTableHashCode(&oldTag);
            oldPartitionLock = BufMappingPartitionLock(oldHash);

            /*
             * Must lock the lower-numbered partition first to avoid
             * deadlocks.
             */
            if (oldPartitionLock < newPartitionLock)
            {
                LWLockAcquire(oldPartitionLock, LW_EXCLUSIVE);
                LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
            }
            else if (oldPartitionLock > newPartitionLock)
            {
                LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
                LWLockAcquire(oldPartitionLock, LW_EXCLUSIVE);
            }
            else
            {
                /* only one partition, only one lock */
                LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
            }
        }
        else
        {
            /* if it wasn't valid, we need only the new partition */
            LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
            /* remember we have no old-partition lock or tag */
            oldPartitionLock = NULL;
            /* keep the compiler quiet about uninitialized variables */
            oldHash = 0;
        }

        /*
         * Try to make a hashtable entry for the buffer under its new tag.
         * This could fail because while we were writing someone else
         * allocated another buffer for the same block we want to read in.
         * Note that we have not yet removed the hashtable entry for the old
         * tag.
         */
        buf_id = BufTableInsert(&newTag, newHash, buf->buf_id);

        if (buf_id >= 0)
        {
            /*
             * 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.
             */
            UnpinBuffer(buf, true);

            /* Can give up that buffer's mapping partition lock now */
            if (oldPartitionLock != NULL &&
                oldPartitionLock != newPartitionLock)
                LWLockRelease(oldPartitionLock);

            /* remaining code should match code at top of routine */

            buf = GetBufferDescriptor(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, or (b) a previous read attempt failed.  We
                 * have to wait for any active read attempt to finish, and
                 * then set up our own read attempt if the page is still not
                 * BM_VALID.  StartBufferIO does it all.
                 */
                if (StartBufferIO(buf, true))
                {
                    /*
                     * If we get here, previous attempts to read the buffer
                     * must have failed ... but we shall bravely try again.
                     */
                    *foundPtr = false;
                }
            }

            return buf;
        }

        /*
         * Need to lock the buffer header too in order to change its tag.
         */
        buf_state = LockBufHdr(buf);

        /*
         * Somebody could have pinned or re-dirtied the buffer while we were
         * doing the I/O and making the new hashtable entry.  If so, we can't
         * recycle this buffer; we must undo everything we've done and start
         * over with a new victim buffer.
         */
        oldFlags = buf_state & BUF_FLAG_MASK;
        if (BUF_STATE_GET_REFCOUNT(buf_state) == 1 && !(oldFlags & BM_DIRTY))
            break;

        UnlockBufHdr(buf, buf_state);
        BufTableDelete(&newTag, newHash);
        if (oldPartitionLock != NULL &&
            oldPartitionLock != newPartitionLock)
            LWLockRelease(oldPartitionLock);
        LWLockRelease(newPartitionLock);
        UnpinBuffer(buf, true);
    }

    /*
     * Okay, it's finally safe to rename the buffer.
     *
     * Clearing BM_VALID here is necessary, clearing the dirtybits is just
     * paranoia.  We also reset the usage_count since any recency of use of
     * the old content is no longer relevant.  (The usage_count starts out at
     * 1 so that the buffer can survive one clock-sweep pass.)
     *
     * 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.
     */
    buf->tag = newTag;
    buf_state &= ~(BM_VALID | BM_DIRTY | BM_JUST_DIRTIED |
                   BM_CHECKPOINT_NEEDED | BM_IO_ERROR | BM_PERMANENT |
                   BUF_USAGECOUNT_MASK);
    if (relpersistence == RELPERSISTENCE_PERMANENT || forkNum == INIT_FORKNUM)
        buf_state |= BM_TAG_VALID | BM_PERMANENT | BUF_USAGECOUNT_ONE;
    else
        buf_state |= BM_TAG_VALID | BUF_USAGECOUNT_ONE;

    UnlockBufHdr(buf, buf_state);

    if (oldPartitionLock != NULL)
    {
        BufTableDelete(&oldTag, oldHash);
        if (oldPartitionLock != newPartitionLock)
            LWLockRelease(oldPartitionLock);
    }

    LWLockRelease(newPartitionLock);

    /*
     * Buffer contents are currently invalid.  Try to get the io_in_progress
     * lock.  If StartBufferIO returns false, then someone else managed to
     * read it before we did, so there's nothing left for BufferAlloc() to do.
     */
    if (StartBufferIO(buf, true))
        *foundPtr = false;
    else
        *foundPtr = true;

    return buf;
}

/*
 * 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 (!BUFFERTAGS_EQUAL(buf->tag, oldTag))
    {
        UnlockBufHdr(buf, buf_state);
        LWLockRelease(oldPartitionLock);
        return;
    }

    /*
     * We assume the only reason for it to be pinned is that someone else is
     * flushing the page out.  Wait for them 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;
    CLEAR_BUFFERTAG(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);
}

/*
 * 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))
    {
        VacuumPageDirty++;
        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 &&
                RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
                bufHdr->tag.forkNum == forkNum)
                return buffer;
            ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
            LocalRefCount[-buffer - 1]--;
        }
        else
        {
            bufHdr = GetBufferDescriptor(buffer - 1);
            /* we have pin, so it's ok to examine tag without spinlock */
            if (bufHdr->tag.blockNum == blockNum &&
                RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
                bufHdr->tag.forkNum == forkNum)
                return buffer;
            UnpinBuffer(bufHdr, true);
        }
    }

    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 ResourceOwnerEnlargeBuffers 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;

    ref = GetPrivateRefCountEntry(b, true);

    if (ref == NULL)
    {
        uint32      buf_state;
        uint32      old_buf_state;

        ReservePrivateRefCountEntry();
        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 must surely be 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 = true;
    }

    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().
 *
 * 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);
}

/*
 * UnpinBuffer -- make buffer available for replacement.
 *
 * This should be applied only to shared buffers, never local ones.
 *
 * Most but not all callers want CurrentResourceOwner to be adjusted.
 * Those that don't should pass fixOwner = false.
 */
static void
UnpinBuffer(BufferDesc *buf, bool fixOwner)
{
    PrivateRefCountEntry *ref;
    Buffer      b = BufferDescriptorGetBuffer(buf);

    /* not moving as we're likely deleting it soon anyway */
    ref = GetPrivateRefCountEntry(b, false);
    Assert(ref != NULL);

    if (fixOwner)
        ResourceOwnerForgetBuffer(CurrentResourceOwner, b);

    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 any locks on the buffer */
        Assert(!LWLockHeldByMe(BufferDescriptorGetContentLock(buf)));
        Assert(!LWLockHeldByMe(BufferDescriptorGetIOLock(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)
        {
            /*
             * 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.
             */
            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_pid = buf->wait_backend_pid;

                buf_state &= ~BM_PIN_COUNT_WAITER;
                UnlockBufHdr(buf, buf_state);
                ProcSendSignal(wait_backend_pid);
            }
            else
                UnlockBufHdr(buf, buf_state);
        }
        ForgetPrivateRefCountEntry(ref);
    }
}

/*
 * 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;

    /* Make sure we can handle the pin inside SyncOneBuffer */
    ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

    /*
     * 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.rnode.spcNode;
            item->relNode = bufHdr->tag.rnode.relNode;
            item->forkNum = bufHdr->tag.forkNum;
            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.
     */
    qsort(CkptBufferIds, num_to_scan, sizeof(CkptSortItem),
          ckpt_buforder_comparator);

    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);
                BgWriterStats.m_buf_written_checkpoints++;
                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 */
    IssuePendingWritebacks(&wb_context);

    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 */
    BgWriterStats.m_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.
     */

    /* Make sure we can handle the pin inside SyncOneBuffer */
    ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

    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)
            {
                BgWriterStats.m_maxwritten_clean++;
                break;
            }
        }
        else if (sync_state & BUF_REUSABLE)
            reusable_buffers++;
    }

    BgWriterStats.m_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.)
 *
 * Note: caller must have done ResourceOwnerEnlargeBuffers.
 */
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;

    ReservePrivateRefCountEntry();

    /*
     * 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);

    LWLockRelease(BufferDescriptorGetContentLock(bufHdr));

    tag = bufHdr->tag;

    UnpinBuffer(bufHdr, true);

    ScheduleBufferTagForWriteback(wb_context, &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.
 *
 * NB: this is called before InitProcess(), so we do not have a PGPROC and
 * cannot do LWLockAcquire; hence we can't actually access stuff in
 * shared memory yet.  We are only initializing local data here.
 * (See also InitBufferPoolBackend)
 */
void
InitBufferPoolAccess(void)
{
    HASHCTL     hash_ctl;

    memset(&PrivateRefCountArray, 0, sizeof(PrivateRefCountArray));

    MemSet(&hash_ctl, 0, sizeof(hash_ctl));
    hash_ctl.keysize = sizeof(int32);
    hash_ctl.entrysize = sizeof(PrivateRefCountEntry);

    PrivateRefCountHash = hash_create("PrivateRefCount", 100, &hash_ctl,
                                      HASH_ELEM | HASH_BLOBS);
}

/*
 * InitBufferPoolBackend --- second-stage initialization of a new backend
 *
 * This is called after we have acquired a PGPROC and so can safely get
 * LWLocks.  We don't currently need to do anything at this stage ...
 * except register a shmem-exit callback.  AtProcExit_Buffers needs LWLock
 * access, and thereby has to be called at the corresponding phase of
 * backend shutdown.
 */
void
InitBufferPoolBackend(void)
{
    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)
{
    AbortBufferIO();
    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;

    /* check the array */
    for (i = 0; i < REFCOUNT_ARRAY_ENTRIES; i++)
    {
        res = &PrivateRefCountArray[i];

        if (res->buffer != InvalidBuffer)
        {
            PrintBufferLeakWarning(res->buffer);
            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)
        {
            PrintBufferLeakWarning(res->buffer);
            RefCountErrors++;
        }

    }

    Assert(RefCountErrors == 0);
#endif
}

/*
 * Helper routine to issue warnings when a buffer is unexpectedly pinned
 */
void
PrintBufferLeakWarning(Buffer buffer)
{
    BufferDesc *buf;
    int32       loccount;
    char       *path;
    BackendId   backend;
    uint32      buf_state;

    Assert(BufferIsValid(buffer));
    if (BufferIsLocal(buffer))
    {
        buf = GetLocalBufferDescriptor(-buffer - 1);
        loccount = LocalRefCount[-buffer - 1];
        backend = MyBackendId;
    }
    else
    {
        buf = GetBufferDescriptor(buffer - 1);
        loccount = GetPrivateRefCount(buffer);
        backend = InvalidBackendId;
    }

    /* theoretically we should lock the bufhdr here */
    path = relpathbackend(buf->tag.rnode, backend, buf->tag.forkNum);
    buf_state = pg_atomic_read_u32(&buf->state);
    elog(WARNING,
         "buffer refcount leak: [%03d] "
         "(rel=%s, blockNum=%u, flags=0x%x, refcount=%u %d)",
         buffer, path,
         buf->tag.blockNum, buf_state & BUF_FLAG_MASK,
         BUF_STATE_GET_REFCOUNT(buf_state), loccount);
    pfree(path);
}

/*
 * 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);
}


/*
 * Do whatever is needed to prepare for commit at the bufmgr and smgr levels
 */
void
BufmgrCommit(void)
{
    /* Nothing to do in bufmgr anymore... */
}

/*
 * 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 relfilenode, fork number and block number associated with
 *      a buffer.
 */
void
BufferGetTag(Buffer buffer, RelFileNode *rnode, 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 */
    *rnode = bufHdr->tag.rnode;
    *forknum = bufHdr->tag.forkNum;
    *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)
{
    XLogRecPtr  recptr;
    ErrorContextCallback errcallback;
    instr_time  io_start,
                io_time;
    Block       bufBlock;
    char       *bufToWrite;
    uint32      buf_state;

    /*
     * Acquire the buffer's io_in_progress lock.  If StartBufferIO returns
     * false, then someone else flushed the buffer before we could, so we need
     * not do anything.
     */
    if (!StartBufferIO(buf, false))
        return;

    /* Setup error traceback support for ereport() */
    errcallback.callback = shared_buffer_write_error_callback;
    errcallback.arg = (void *) buf;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;

    /* Find smgr relation for buffer */
    if (reln == NULL)
        reln = smgropen(buf->tag.rnode, InvalidBackendId);

    TRACE_POSTGRESQL_BUFFER_FLUSH_START(buf->tag.forkNum,
                                        buf->tag.blockNum,
                                        reln->smgr_rnode.node.spcNode,
                                        reln->smgr_rnode.node.dbNode,
                                        reln->smgr_rnode.node.relNode);

    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 buffer to disk. Note that no one else should
     * have been able to write it while we were busy with log flushing because
     * we have the io_in_progress lock.
     */
    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);

    if (track_io_timing)
        INSTR_TIME_SET_CURRENT(io_start);

    /*
     * bufToWrite is either the shared buffer or a copy, as appropriate.
     */
    smgrwrite(reln,
              buf->tag.forkNum,
              buf->tag.blockNum,
              bufToWrite,
              false);

    if (track_io_timing)
    {
        INSTR_TIME_SET_CURRENT(io_time);
        INSTR_TIME_SUBTRACT(io_time, io_start);
        pgstat_count_buffer_write_time(INSTR_TIME_GET_MICROSEC(io_time));
        INSTR_TIME_ADD(pgBufferUsage.blk_write_time, io_time);
    }

    pgBufferUsage.shared_blks_written++;

    /*
     * Mark the buffer as clean (unless BM_JUST_DIRTIED has become set) and
     * end the io_in_progress state.
     */
    TerminateBufferIO(buf, true, 0);

    TRACE_POSTGRESQL_BUFFER_FLUSH_DONE(buf->tag.forkNum,
                                       buf->tag.blockNum,
                                       reln->smgr_rnode.node.spcNode,
                                       reln->smgr_rnode.node.dbNode,
                                       reln->smgr_rnode.node.relNode);

    /* 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)
{
    switch (relation->rd_rel->relkind)
    {
        case RELKIND_SEQUENCE:
        case RELKIND_INDEX:
        case RELKIND_PARTITIONED_INDEX:
            /* Open it at the smgr level if not already done */
            RelationOpenSmgr(relation);

            return smgrnblocks(relation->rd_smgr, forkNum);

        case RELKIND_RELATION:
        case RELKIND_TOASTVALUE:
        case RELKIND_MATVIEW:
            {
                /*
                 * 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;
            }
        case RELKIND_VIEW:
        case RELKIND_COMPOSITE_TYPE:
        case RELKIND_FOREIGN_TABLE:
        case RELKIND_PARTITIONED_TABLE:
        default:
            Assert(false);
            break;
    }

    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)
{
    BufferDesc *bufHdr = GetBufferDescriptor(buffer - 1);
    char       *page = BufferGetPage(buffer);
    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));

    buf_state = LockBufHdr(bufHdr);
    lsn = PageGetLSN(page);
    UnlockBufHdr(bufHdr, buf_state);

    return lsn;
}

/* ---------------------------------------------------------------------
 *      DropRelFileNodeBuffers
 *
 *      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.
 *
 *      XXX currently it sequentially searches the buffer pool, should be
 *      changed to more clever ways of searching.  However, this routine
 *      is used only in code paths that aren't very performance-critical,
 *      and we shouldn't slow down the hot paths to make it faster ...
 * --------------------------------------------------------------------
 */
void
DropRelFileNodeBuffers(RelFileNodeBackend rnode, ForkNumber *forkNum,
                       int nforks, BlockNumber *firstDelBlock)
{
    int         i;
    int         j;

    /* If it's a local relation, it's localbuf.c's problem. */
    if (RelFileNodeBackendIsTemp(rnode))
    {
        if (rnode.backend == MyBackendId)
        {
            for (j = 0; j < nforks; j++)
                DropRelFileNodeLocalBuffers(rnode.node, forkNum[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 rnode, but the
         * incremental win from doing so seems small.
         */
        if (!RelFileNodeEquals(bufHdr->tag.rnode, rnode.node))
            continue;

        buf_state = LockBufHdr(bufHdr);

        for (j = 0; j < nforks; j++)
        {
            if (RelFileNodeEquals(bufHdr->tag.rnode, rnode.node) &&
                bufHdr->tag.forkNum == forkNum[j] &&
                bufHdr->tag.blockNum >= firstDelBlock[j])
            {
                InvalidateBuffer(bufHdr);   /* releases spinlock */
                break;
            }
        }
        if (j >= nforks)
            UnlockBufHdr(bufHdr, buf_state);
    }
}

/* ---------------------------------------------------------------------
 *      DropRelFileNodesAllBuffers
 *
 *      This function removes from the buffer pool all the pages of all
 *      forks of the specified relations.  It's equivalent to calling
 *      DropRelFileNodeBuffers once per fork per relation with
 *      firstDelBlock = 0.
 * --------------------------------------------------------------------
 */
void
DropRelFileNodesAllBuffers(RelFileNodeBackend *rnodes, int nnodes)
{
    int         i,
                n = 0;
    RelFileNode *nodes;
    bool        use_bsearch;

    if (nnodes == 0)
        return;

    nodes = palloc(sizeof(RelFileNode) * nnodes);   /* non-local relations */

    /* If it's a local relation, it's localbuf.c's problem. */
    for (i = 0; i < nnodes; i++)
    {
        if (RelFileNodeBackendIsTemp(rnodes[i]))
        {
            if (rnodes[i].backend == MyBackendId)
                DropRelFileNodeAllLocalBuffers(rnodes[i].node);
        }
        else
            nodes[n++] = rnodes[i].node;
    }

    /*
     * If there are no non-local relations, then we're done. Release the
     * memory and return.
     */
    if (n == 0)
    {
        pfree(nodes);
        return;
    }

    /*
     * 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 rnodes if necessary */
    if (use_bsearch)
        pg_qsort(nodes, n, sizeof(RelFileNode), rnode_comparator);

    for (i = 0; i < NBuffers; i++)
    {
        RelFileNode *rnode = NULL;
        BufferDesc *bufHdr = GetBufferDescriptor(i);
        uint32      buf_state;

        /*
         * As in DropRelFileNodeBuffers, an unlocked precheck should be safe
         * and saves some cycles.
         */

        if (!use_bsearch)
        {
            int         j;

            for (j = 0; j < n; j++)
            {
                if (RelFileNodeEquals(bufHdr->tag.rnode, nodes[j]))
                {
                    rnode = &nodes[j];
                    break;
                }
            }
        }
        else
        {
            rnode = bsearch((const void *) &(bufHdr->tag.rnode),
                            nodes, n, sizeof(RelFileNode),
                            rnode_comparator);
        }

        /* buffer doesn't belong to any of the given relfilenodes; skip it */
        if (rnode == NULL)
            continue;

        buf_state = LockBufHdr(bufHdr);
        if (RelFileNodeEquals(bufHdr->tag.rnode, (*rnode)))
            InvalidateBuffer(bufHdr);   /* releases spinlock */
        else
            UnlockBufHdr(bufHdr, buf_state);
    }

    pfree(nodes);
}

/* ---------------------------------------------------------------------
 *      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
 *      DropRelFileNodeBuffers() 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 DropRelFileNodeBuffers, an unlocked precheck should be safe
         * and saves some cycles.
         */
        if (bufHdr->tag.rnode.dbNode != dbid)
            continue;

        buf_state = LockBufHdr(bufHdr);
        if (bufHdr->tag.rnode.dbNode == dbid)
            InvalidateBuffer(bufHdr);   /* releases spinlock */
        else
            UnlockBufHdr(bufHdr, buf_state);
    }
}

/* -----------------------------------------------------------------
 *      PrintBufferDescs
 *
 *      this function prints all the buffer descriptors, for debugging
 *      use only.
 * -----------------------------------------------------------------
 */
#ifdef NOT_USED
void
PrintBufferDescs(void)
{
    int         i;

    for (i = 0; i < NBuffers; ++i)
    {
        BufferDesc *buf = GetBufferDescriptor(i);
        Buffer      b = BufferDescriptorGetBuffer(buf);

        /* theoretically we should lock the bufhdr here */
        elog(LOG,
             "[%02d] (freeNext=%d, rel=%s, "
             "blockNum=%u, flags=0x%x, refcount=%u %d)",
             i, buf->freeNext,
             relpathbackend(buf->tag.rnode, InvalidBackendId, buf->tag.forkNum),
             buf->tag.blockNum, buf->flags,
             buf->refcount, GetPrivateRefCount(b));
    }
}
#endif

#ifdef NOT_USED
void
PrintPinnedBufs(void)
{
    int         i;

    for (i = 0; i < NBuffers; ++i)
    {
        BufferDesc *buf = GetBufferDescriptor(i);
        Buffer      b = BufferDescriptorGetBuffer(buf);

        if (GetPrivateRefCount(b) > 0)
        {
            /* theoretically we should lock the bufhdr here */
            elog(LOG,
                 "[%02d] (freeNext=%d, rel=%s, "
                 "blockNum=%u, flags=0x%x, refcount=%u %d)",
                 i, buf->freeNext,
                 relpathperm(buf->tag.rnode, buf->tag.forkNum),
                 buf->tag.blockNum, buf->flags,
                 buf->refcount, GetPrivateRefCount(b));
        }
    }
}
#endif

/* ---------------------------------------------------------------------
 *      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;
 *      but see also DropRelFileNodeBuffers.
 * --------------------------------------------------------------------
 */
void
FlushRelationBuffers(Relation rel)
{
    int         i;
    BufferDesc *bufHdr;

    /* Open rel at the smgr level if not already done */
    RelationOpenSmgr(rel);

    if (RelationUsesLocalBuffers(rel))
    {
        for (i = 0; i < NLocBuffer; i++)
        {
            uint32      buf_state;

            bufHdr = GetLocalBufferDescriptor(i);
            if (RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node) &&
                ((buf_state = pg_atomic_read_u32(&bufHdr->state)) &
                 (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
            {
                ErrorContextCallback errcallback;
                Page        localpage;

                localpage = (char *) LocalBufHdrGetBlock(bufHdr);

                /* Setup error traceback support for ereport() */
                errcallback.callback = local_buffer_write_error_callback;
                errcallback.arg = (void *) bufHdr;
                errcallback.previous = error_context_stack;
                error_context_stack = &errcallback;

                PageSetChecksumInplace(localpage, bufHdr->tag.blockNum);

                smgrwrite(rel->rd_smgr,
                          bufHdr->tag.forkNum,
                          bufHdr->tag.blockNum,
                          localpage,
                          false);

                buf_state &= ~(BM_DIRTY | BM_JUST_DIRTIED);
                pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);

                /* Pop the error context stack */
                error_context_stack = errcallback.previous;
            }
        }

        return;
    }

    /* Make sure we can handle the pin inside the loop */
    ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

    for (i = 0; i < NBuffers; i++)
    {
        uint32      buf_state;

        bufHdr = GetBufferDescriptor(i);

        /*
         * As in DropRelFileNodeBuffers, an unlocked precheck should be safe
         * and saves some cycles.
         */
        if (!RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node))
            continue;

        ReservePrivateRefCountEntry();

        buf_state = LockBufHdr(bufHdr);
        if (RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node) &&
            (buf_state & (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
        {
            PinBuffer_Locked(bufHdr);
            LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
            FlushBuffer(bufHdr, rel->rd_smgr);
            LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
            UnpinBuffer(bufHdr, true);
        }
        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 fork 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(!RelFileNodeBackendIsTemp(smgrs[i]->smgr_rnode));

        srels[i].rnode = smgrs[i]->smgr_rnode.node;
        srels[i].srel = smgrs[i];
    }

    /*
     * Save the bsearch overhead for low number of relations to sync. See
     * DropRelFileNodesAllBuffers for details.
     */
    use_bsearch = nrels > RELS_BSEARCH_THRESHOLD;

    /* sort the list of SMgrRelations if necessary */
    if (use_bsearch)
        pg_qsort(srels, nrels, sizeof(SMgrSortArray), rnode_comparator);

    /* Make sure we can handle the pin inside the loop */
    ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

    for (i = 0; i < NBuffers; i++)
    {
        SMgrSortArray *srelent = NULL;
        BufferDesc *bufHdr = GetBufferDescriptor(i);
        uint32      buf_state;

        /*
         * As in DropRelFileNodeBuffers, an unlocked precheck should be safe
         * and saves some cycles.
         */

        if (!use_bsearch)
        {
            int         j;

            for (j = 0; j < nrels; j++)
            {
                if (RelFileNodeEquals(bufHdr->tag.rnode, srels[j].rnode))
                {
                    srelent = &srels[j];
                    break;
                }
            }

        }
        else
        {
            srelent = bsearch((const void *) &(bufHdr->tag.rnode),
                              srels, nrels, sizeof(SMgrSortArray),
                              rnode_comparator);
        }

        /* buffer doesn't belong to any of the given relfilenodes; skip it */
        if (srelent == NULL)
            continue;

        ReservePrivateRefCountEntry();

        buf_state = LockBufHdr(bufHdr);
        if (RelFileNodeEquals(bufHdr->tag.rnode, srelent->rnode) &&
            (buf_state & (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
        {
            PinBuffer_Locked(bufHdr);
            LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
            FlushBuffer(bufHdr, srelent->srel);
            LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
            UnpinBuffer(bufHdr, true);
        }
        else
            UnlockBufHdr(bufHdr, buf_state);
    }

    pfree(srels);
}

/* ---------------------------------------------------------------------
 *      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;

    /* Make sure we can handle the pin inside the loop */
    ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

    for (i = 0; i < NBuffers; i++)
    {
        uint32      buf_state;

        bufHdr = GetBufferDescriptor(i);

        /*
         * As in DropRelFileNodeBuffers, an unlocked precheck should be safe
         * and saves some cycles.
         */
        if (bufHdr->tag.rnode.dbNode != dbid)
            continue;

        ReservePrivateRefCountEntry();

        buf_state = LockBufHdr(bufHdr);
        if (bufHdr->tag.rnode.dbNode == dbid &&
            (buf_state & (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
        {
            PinBuffer_Locked(bufHdr);
            LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
            FlushBuffer(bufHdr, NULL);
            LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
            UnpinBuffer(bufHdr, true);
        }
        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);
}

/*
 * ReleaseBuffer -- release the pin on a buffer
 */
void
ReleaseBuffer(Buffer buffer)
{
    if (!BufferIsValid(buffer))
        elog(ERROR, "bad buffer ID: %d", buffer);

    if (BufferIsLocal(buffer))
    {
        ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);

        Assert(LocalRefCount[-buffer - 1] > 0);
        LocalRefCount[-buffer - 1]--;
        return;
    }

    UnpinBuffer(GetBufferDescriptor(buffer - 1), true);
}

/*
 * 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));
    ResourceOwnerEnlargeBuffers(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        delayChkpt = 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 relfilenode-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() ||
                RelFileNodeSkippingWAL(bufHdr->tag.rnode))
                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.
             */
            MyProc->delayChkpt = delayChkpt = 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 (delayChkpt)
            MyProc->delayChkpt = false;

        if (dirtied)
        {
            VacuumPageDirty++;
            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_pid == MyProcPid)
            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);
}

/*
 * 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;
    char       *new_status = NULL;

    Assert(BufferIsPinned(buffer));
    Assert(PinCountWaitBuf == NULL);

    if (BufferIsLocal(buffer))
    {
        /* There should be exactly one pin */
        if (LocalRefCount[-buffer - 1] != 1)
            elog(ERROR, "incorrect local pin count: %d",
                 LocalRefCount[-buffer - 1]);
        /* Nobody else to wait for */
        return;
    }

    /* There should be exactly one local pin */
    if (GetPrivateRefCount(buffer) != 1)
        elog(ERROR, "incorrect local pin count: %d",
             GetPrivateRefCount(buffer));

    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);

            /* Report change to non-waiting status */
            if (new_status)
            {
                set_ps_display(new_status);
                pfree(new_status);
            }
            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_pid = MyProcPid;
        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)
        {
            /* Report change to waiting status */
            if (update_process_title && new_status == NULL)
            {
                const char *old_status;
                int         len;

                old_status = get_ps_display(&len);
                new_status = (char *) palloc(len + 8 + 1);
                memcpy(new_status, old_status, len);
                strcpy(new_status + len, " waiting");
                set_ps_display(new_status);
                new_status[len] = '\0'; /* truncate off " waiting" */
            }

            /* 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(PG_WAIT_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_pid == MyProcPid)
            buf_state &= ~BM_PIN_COUNT_WAITER;
        UnlockBufHdr(bufHdr, buf_state);

        PinCountWaitBuf = NULL;
        /* Loop back and try again */
    }
}

/*
 * Check called from RecoveryConflictInterrupt handler 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));

    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));

    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
 *
 *  Note: We assume that nested buffer I/O never occurs.
 *  i.e at most one io_in_progress lock is held per proc.
 *
 *  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)
{
    /*
     * Changed to wait until there's no IO - Inoue 01/13/2000
     *
     * Note this is *necessary* because an error abort in the process doing
     * I/O could release the io_in_progress_lock prematurely. See
     * AbortBufferIO.
     */
    for (;;)
    {
        uint32      buf_state;

        /*
         * 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);
        UnlockBufHdr(buf, buf_state);

        if (!(buf_state & BM_IO_IN_PROGRESS))
            break;
        LWLockAcquire(BufferDescriptorGetIOLock(buf), LW_SHARED);
        LWLockRelease(BufferDescriptorGetIOLock(buf));
    }
}

/*
 * StartBufferIO: begin I/O on this buffer
 *  (Assumptions)
 *  My process is executing no IO
 *  The buffer is Pinned
 *
 * In some scenarios there are race conditions in which multiple backends
 * could attempt the same I/O operation concurrently.  If someone else
 * has already started I/O on this buffer then we will block on the
 * io_in_progress lock until he's done.
 *
 * 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.
 */
static bool
StartBufferIO(BufferDesc *buf, bool forInput)
{
    uint32      buf_state;

    Assert(!InProgressBuf);

    for (;;)
    {
        /*
         * Grab the io_in_progress lock so that other processes can wait for
         * me to finish the I/O.
         */
        LWLockAcquire(BufferDescriptorGetIOLock(buf), LW_EXCLUSIVE);

        buf_state = LockBufHdr(buf);

        if (!(buf_state & BM_IO_IN_PROGRESS))
            break;

        /*
         * The only way BM_IO_IN_PROGRESS could be set when the io_in_progress
         * lock isn't held is if the process doing the I/O is recovering from
         * an error (see AbortBufferIO).  If that's the case, we must wait for
         * him to get unwedged.
         */
        UnlockBufHdr(buf, buf_state);
        LWLockRelease(BufferDescriptorGetIOLock(buf));
        WaitIO(buf);
    }

    /* Once we get here, there is definitely no I/O active on this buffer */

    if (forInput ? (buf_state & BM_VALID) : !(buf_state & BM_DIRTY))
    {
        /* someone else already did the I/O */
        UnlockBufHdr(buf, buf_state);
        LWLockRelease(BufferDescriptorGetIOLock(buf));
        return false;
    }

    buf_state |= BM_IO_IN_PROGRESS;
    UnlockBufHdr(buf, buf_state);

    InProgressBuf = buf;
    IsForInput = forInput;

    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
 *  We hold the buffer's io_in_progress lock
 *  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.
 */
static void
TerminateBufferIO(BufferDesc *buf, bool clear_dirty, uint32 set_flag_bits)
{
    uint32      buf_state;

    Assert(buf == InProgressBuf);

    buf_state = LockBufHdr(buf);

    Assert(buf_state & BM_IO_IN_PROGRESS);

    buf_state &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR);
    if (clear_dirty && !(buf_state & BM_JUST_DIRTIED))
        buf_state &= ~(BM_DIRTY | BM_CHECKPOINT_NEEDED);

    buf_state |= set_flag_bits;
    UnlockBufHdr(buf, buf_state);

    InProgressBuf = NULL;

    LWLockRelease(BufferDescriptorGetIOLock(buf));
}

/*
 * AbortBufferIO: Clean up any 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.
 */
void
AbortBufferIO(void)
{
    BufferDesc *buf = InProgressBuf;

    if (buf)
    {
        uint32      buf_state;

        /*
         * Since LWLockReleaseAll has already been called, we're not holding
         * the buffer's io_in_progress_lock. We have to re-acquire it so that
         * we can use TerminateBufferIO. Anyone who's executing WaitIO on the
         * buffer will be in a busy spin until we succeed in doing this.
         */
        LWLockAcquire(BufferDescriptorGetIOLock(buf), LW_EXCLUSIVE);

        buf_state = LockBufHdr(buf);
        Assert(buf_state & BM_IO_IN_PROGRESS);
        if (IsForInput)
        {
            Assert(!(buf_state & BM_DIRTY));

            /* We'd better not think buffer is valid yet */
            Assert(!(buf_state & BM_VALID));
            UnlockBufHdr(buf, buf_state);
        }
        else
        {
            Assert(buf_state & BM_DIRTY);
            UnlockBufHdr(buf, 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 */
                char       *path;

                path = relpathperm(buf->tag.rnode, buf->tag.forkNum);
                ereport(WARNING,
                        (errcode(ERRCODE_IO_ERROR),
                         errmsg("could not write block %u of %s",
                                buf->tag.blockNum, path),
                         errdetail("Multiple failures --- write error might be permanent.")));
                pfree(path);
            }
        }
        TerminateBufferIO(buf, false, BM_IO_ERROR);
    }
}

/*
 * 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)
    {
        char       *path = relpathperm(bufHdr->tag.rnode, bufHdr->tag.forkNum);

        errcontext("writing block %u of relation %s",
                   bufHdr->tag.blockNum, path);
        pfree(path);
    }
}

/*
 * 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)
    {
        char       *path = relpathbackend(bufHdr->tag.rnode, MyBackendId,
                                          bufHdr->tag.forkNum);

        errcontext("writing block %u of relation %s",
                   bufHdr->tag.blockNum, path);
        pfree(path);
    }
}

/*
 * RelFileNode qsort/bsearch comparator; see RelFileNodeEquals.
 */
static int
rnode_comparator(const void *p1, const void *p2)
{
    RelFileNode n1 = *(const RelFileNode *) p1;
    RelFileNode n2 = *(const RelFileNode *) p2;

    if (n1.relNode < n2.relNode)
        return -1;
    else if (n1.relNode > n2.relNode)
        return 1;

    if (n1.dbNode < n2.dbNode)
        return -1;
    else if (n1.dbNode > n2.dbNode)
        return 1;

    if (n1.spcNode < n2.spcNode)
        return -1;
    else if (n1.spcNode > n2.spcNode)
        return 1;
    else
        return 0;
}

/*
 * Lock buffer header - set BM_LOCKED in buffer state.
 */
uint32
LockBufHdr(BufferDesc *desc)
{
    SpinDelayStatus delayStatus;
    uint32      old_buf_state;

    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 int
buffertag_comparator(const void *a, const void *b)
{
    const BufferTag *ba = (const BufferTag *) a;
    const BufferTag *bb = (const BufferTag *) b;
    int         ret;

    ret = rnode_comparator(&ba->rnode, &bb->rnode);

    if (ret != 0)
        return ret;

    if (ba->forkNum < bb->forkNum)
        return -1;
    if (ba->forkNum > bb->forkNum)
        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 int
ckpt_buforder_comparator(const void *pa, const void *pb)
{
    const CkptSortItem *a = (const CkptSortItem *) pa;
    const CkptSortItem *b = (const CkptSortItem *) pb;

    /* compare tablespace */
    if (a->tsId < b->tsId)
        return -1;
    else if (a->tsId > b->tsId)
        return 1;
    /* compare relation */
    if (a->relNode < b->relNode)
        return -1;
    else if (a->relNode > b->relNode)
        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 *context, BufferTag *tag)
{
    PendingWriteback *pending;

    /*
     * Add buffer to the pending writeback array, unless writeback control is
     * disabled.
     */
    if (*context->max_pending > 0)
    {
        Assert(*context->max_pending <= WRITEBACK_MAX_PENDING_FLUSHES);

        pending = &context->pending_writebacks[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 (context->nr_pending >= *context->max_pending)
        IssuePendingWritebacks(context);
}

/*
 * 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 *context)
{
    int         i;

    if (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.
     */
    qsort(&context->pending_writebacks, context->nr_pending,
          sizeof(PendingWriteback), buffertag_comparator);

    /*
     * 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 < context->nr_pending; i++)
    {
        PendingWriteback *cur;
        PendingWriteback *next;
        SMgrRelation reln;
        int         ahead;
        BufferTag   tag;
        Size        nblocks = 1;

        cur = &context->pending_writebacks[i];
        tag = cur->tag;

        /*
         * Peek ahead, into following writeback requests, to see if they can
         * be combined with the current one.
         */
        for (ahead = 0; i + ahead + 1 < context->nr_pending; ahead++)
        {
            next = &context->pending_writebacks[i + ahead + 1];

            /* different file, stop */
            if (!RelFileNodeEquals(cur->tag.rnode, next->tag.rnode) ||
                cur->tag.forkNum != next->tag.forkNum)
                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(tag.rnode, InvalidBackendId);
        smgrwriteback(reln, tag.forkNum, tag.blockNum, nblocks);
    }

    context->nr_pending = 0;
}


/*
 * Implement slower/larger portions of TestForOldSnapshot
 *
 * Smaller/faster portions are put inline, but the entire set of logic is too
 * big for that.
 */
void
TestForOldSnapshot_impl(Snapshot snapshot, Relation relation)
{
    if (RelationAllowsEarlyPruning(relation)
        && (snapshot)->whenTaken < GetOldSnapshotThresholdTimestamp())
        ereport(ERROR,
                (errcode(ERRCODE_SNAPSHOT_TOO_OLD),
                 errmsg("snapshot too old")));
}