localbuf.c

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/*-------------------------------------------------------------------------
 *
 * localbuf.c
 *    local buffer manager. Fast buffer manager for temporary tables,
 *    which never need to be WAL-logged or checkpointed, etc.
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994-5, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/buffer/localbuf.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/parallel.h"
#include "executor/instrument.h"
#include "pgstat.h"
#include "storage/buf_internals.h"
#include "storage/bufmgr.h"
#include "storage/fd.h"
#include "utils/guc_hooks.h"
#include "utils/memutils.h"
#include "utils/resowner.h"
 
 
/*#define LBDEBUG*/
 
/* entry for buffer lookup hashtable */
typedef struct
{
    BufferTag   key;            /* Tag of a disk page */
    int         id;             /* Associated local buffer's index */
} LocalBufferLookupEnt;
 
/* Note: this macro only works on local buffers, not shared ones! */
#define LocalBufHdrGetBlock(bufHdr) \
    LocalBufferBlockPointers[-((bufHdr)->buf_id + 2)]
 
int         NLocBuffer = 0;     /* until buffers are initialized */
 
BufferDesc *LocalBufferDescriptors = NULL;
Block      *LocalBufferBlockPointers = NULL;
int32      *LocalRefCount = NULL;
 
static int  nextFreeLocalBufId = 0;
 
static HTAB *LocalBufHash = NULL;
 
/* number of local buffers pinned at least once */
static int  NLocalPinnedBuffers = 0;
 
 
static void InitLocalBuffers(void);
static Block GetLocalBufferStorage(void);
static Buffer GetLocalVictimBuffer(void);
 
 
/*
 * PrefetchLocalBuffer -
 *    initiate asynchronous read of a block of a relation
 *
 * Do PrefetchBuffer's work for temporary relations.
 * No-op if prefetching isn't compiled in.
 */
PrefetchBufferResult
PrefetchLocalBuffer(SMgrRelation smgr, ForkNumber forkNum,
                    BlockNumber blockNum)
{
    PrefetchBufferResult result = {InvalidBuffer, false};
    BufferTag   newTag;         /* identity of requested block */
    LocalBufferLookupEnt *hresult;
 
    InitBufferTag(&newTag, &smgr->smgr_rlocator.locator, forkNum, blockNum);
 
    /* Initialize local buffers if first request in this session */
    if (LocalBufHash == NULL)
        InitLocalBuffers();
 
    /* See if the desired buffer already exists */
    hresult = (LocalBufferLookupEnt *)
        hash_search(LocalBufHash, &newTag, HASH_FIND, NULL);
 
    if (hresult)
    {
        /* Yes, so nothing to do */
        result.recent_buffer = -hresult->id - 1;
    }
    else
    {
#ifdef USE_PREFETCH
        /* Not in buffers, so initiate prefetch */
        if ((io_direct_flags & IO_DIRECT_DATA) == 0 &&
            smgrprefetch(smgr, forkNum, blockNum, 1))
        {
            result.initiated_io = true;
        }
#endif                          /* USE_PREFETCH */
    }
 
    return result;
}
 
 
/*
 * LocalBufferAlloc -
 *    Find or create a local buffer for the given page of the given relation.
 *
 * API is similar to bufmgr.c's BufferAlloc, except that we do not need to do
 * any locking since this is all local.  We support only default access
 * strategy (hence, usage_count is always advanced).
 */
BufferDesc *
LocalBufferAlloc(SMgrRelation smgr, ForkNumber forkNum, BlockNumber blockNum,
                 bool *foundPtr)
{
    BufferTag   newTag;         /* identity of requested block */
    LocalBufferLookupEnt *hresult;
    BufferDesc *bufHdr;
    Buffer      victim_buffer;
    int         bufid;
    bool        found;
 
    InitBufferTag(&newTag, &smgr->smgr_rlocator.locator, forkNum, blockNum);
 
    /* Initialize local buffers if first request in this session */
    if (LocalBufHash == NULL)
        InitLocalBuffers();
 
    ResourceOwnerEnlarge(CurrentResourceOwner);
 
    /* See if the desired buffer already exists */
    hresult = (LocalBufferLookupEnt *)
        hash_search(LocalBufHash, &newTag, HASH_FIND, NULL);
 
    if (hresult)
    {
        bufid = hresult->id;
        bufHdr = GetLocalBufferDescriptor(bufid);
        Assert(BufferTagsEqual(&bufHdr->tag, &newTag));
 
        *foundPtr = PinLocalBuffer(bufHdr, true);
    }
    else
    {
        uint32      buf_state;
 
        victim_buffer = GetLocalVictimBuffer();
        bufid = -victim_buffer - 1;
        bufHdr = GetLocalBufferDescriptor(bufid);
 
        hresult = (LocalBufferLookupEnt *)
            hash_search(LocalBufHash, &newTag, HASH_ENTER, &found);
        if (found)              /* shouldn't happen */
            elog(ERROR, "local buffer hash table corrupted");
        hresult->id = bufid;
 
        /*
         * it's all ours now.
         */
        bufHdr->tag = newTag;
 
        buf_state = pg_atomic_read_u32(&bufHdr->state);
        buf_state &= ~(BUF_FLAG_MASK | BUF_USAGECOUNT_MASK);
        buf_state |= BM_TAG_VALID | BUF_USAGECOUNT_ONE;
        pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
 
        *foundPtr = false;
    }
 
    return bufHdr;
}
 
static Buffer
GetLocalVictimBuffer(void)
{
    int         victim_bufid;
    int         trycounter;
    uint32      buf_state;
    BufferDesc *bufHdr;
 
    ResourceOwnerEnlarge(CurrentResourceOwner);
 
    /*
     * Need to get a new buffer.  We use a clock sweep algorithm (essentially
     * the same as what freelist.c does now...)
     */
    trycounter = NLocBuffer;
    for (;;)
    {
        victim_bufid = nextFreeLocalBufId;
 
        if (++nextFreeLocalBufId >= NLocBuffer)
            nextFreeLocalBufId = 0;
 
        bufHdr = GetLocalBufferDescriptor(victim_bufid);
 
        if (LocalRefCount[victim_bufid] == 0)
        {
            buf_state = pg_atomic_read_u32(&bufHdr->state);
 
            if (BUF_STATE_GET_USAGECOUNT(buf_state) > 0)
            {
                buf_state -= BUF_USAGECOUNT_ONE;
                pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
                trycounter = NLocBuffer;
            }
            else
            {
                /* Found a usable buffer */
                PinLocalBuffer(bufHdr, false);
                break;
            }
        }
        else if (--trycounter == 0)
            ereport(ERROR,
                    (errcode(ERRCODE_INSUFFICIENT_RESOURCES),
                     errmsg("no empty local buffer available")));
    }
 
    /*
     * lazy memory allocation: allocate space on first use of a buffer.
     */
    if (LocalBufHdrGetBlock(bufHdr) == NULL)
    {
        /* Set pointer for use by BufferGetBlock() macro */
        LocalBufHdrGetBlock(bufHdr) = GetLocalBufferStorage();
    }
 
    /*
     * this buffer is not referenced but it might still be dirty. if that's
     * the case, write it out before reusing it!
     */
    if (buf_state & BM_DIRTY)
    {
        instr_time  io_start;
        SMgrRelation oreln;
        Page        localpage = (char *) LocalBufHdrGetBlock(bufHdr);
 
        /* Find smgr relation for buffer */
        oreln = smgropen(BufTagGetRelFileLocator(&bufHdr->tag), MyProcNumber);
 
        PageSetChecksumInplace(localpage, bufHdr->tag.blockNum);
 
        io_start = pgstat_prepare_io_time(track_io_timing);
 
        /* And write... */
        smgrwrite(oreln,
                  BufTagGetForkNum(&bufHdr->tag),
                  bufHdr->tag.blockNum,
                  localpage,
                  false);
 
        /* Temporary table I/O does not use Buffer Access Strategies */
        pgstat_count_io_op_time(IOOBJECT_TEMP_RELATION, IOCONTEXT_NORMAL,
                                IOOP_WRITE, io_start, 1, BLCKSZ);
 
        /* Mark not-dirty now in case we error out below */
        buf_state &= ~BM_DIRTY;
        pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
 
        pgBufferUsage.local_blks_written++;
    }
 
    /*
     * Remove the victim buffer from the hashtable and mark as invalid.
     */
    if (buf_state & BM_TAG_VALID)
    {
        LocalBufferLookupEnt *hresult;
 
        hresult = (LocalBufferLookupEnt *)
            hash_search(LocalBufHash, &bufHdr->tag, HASH_REMOVE, NULL);
        if (!hresult)           /* shouldn't happen */
            elog(ERROR, "local buffer hash table corrupted");
        /* mark buffer invalid just in case hash insert fails */
        ClearBufferTag(&bufHdr->tag);
        buf_state &= ~(BUF_FLAG_MASK | BUF_USAGECOUNT_MASK);
        pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
 
        pgstat_count_io_op(IOOBJECT_TEMP_RELATION, IOCONTEXT_NORMAL, IOOP_EVICT, 1, 0);
    }
 
    return BufferDescriptorGetBuffer(bufHdr);
}
 
/* see LimitAdditionalPins() */
void
LimitAdditionalLocalPins(uint32 *additional_pins)
{
    uint32      max_pins;
 
    if (*additional_pins <= 1)
        return;
 
    /*
     * In contrast to LimitAdditionalPins() other backends don't play a role
     * here. We can allow up to NLocBuffer pins in total, but it might not be
     * initialized yet so read num_temp_buffers.
     */
    max_pins = (num_temp_buffers - NLocalPinnedBuffers);
 
    if (*additional_pins >= max_pins)
        *additional_pins = max_pins;
}
 
/*
 * Implementation of ExtendBufferedRelBy() and ExtendBufferedRelTo() for
 * temporary buffers.
 */
BlockNumber
ExtendBufferedRelLocal(BufferManagerRelation bmr,
                       ForkNumber fork,
                       uint32 flags,
                       uint32 extend_by,
                       BlockNumber extend_upto,
                       Buffer *buffers,
                       uint32 *extended_by)
{
    BlockNumber first_block;
    instr_time  io_start;
 
    /* Initialize local buffers if first request in this session */
    if (LocalBufHash == NULL)
        InitLocalBuffers();
 
    LimitAdditionalLocalPins(&extend_by);
 
    for (uint32 i = 0; i < extend_by; i++)
    {
        BufferDesc *buf_hdr;
        Block       buf_block;
 
        buffers[i] = GetLocalVictimBuffer();
        buf_hdr = GetLocalBufferDescriptor(-buffers[i] - 1);
        buf_block = LocalBufHdrGetBlock(buf_hdr);
 
        /* new buffers are zero-filled */
        MemSet((char *) buf_block, 0, BLCKSZ);
    }
 
    first_block = smgrnblocks(bmr.smgr, fork);
 
    if (extend_upto != InvalidBlockNumber)
    {
        /*
         * In contrast to shared relations, nothing could change the relation
         * size concurrently. Thus we shouldn't end up finding that we don't
         * need to do anything.
         */
        Assert(first_block <= extend_upto);
 
        Assert((uint64) first_block + extend_by <= extend_upto);
    }
 
    /* Fail if relation is already at maximum possible length */
    if ((uint64) first_block + extend_by >= MaxBlockNumber)
        ereport(ERROR,
                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                 errmsg("cannot extend relation %s beyond %u blocks",
                        relpath(bmr.smgr->smgr_rlocator, fork),
                        MaxBlockNumber)));
 
    for (uint32 i = 0; i < extend_by; i++)
    {
        int         victim_buf_id;
        BufferDesc *victim_buf_hdr;
        BufferTag   tag;
        LocalBufferLookupEnt *hresult;
        bool        found;
 
        victim_buf_id = -buffers[i] - 1;
        victim_buf_hdr = GetLocalBufferDescriptor(victim_buf_id);
 
        /* in case we need to pin an existing buffer below */
        ResourceOwnerEnlarge(CurrentResourceOwner);
 
        InitBufferTag(&tag, &bmr.smgr->smgr_rlocator.locator, fork, first_block + i);
 
        hresult = (LocalBufferLookupEnt *)
            hash_search(LocalBufHash, &tag, HASH_ENTER, &found);
        if (found)
        {
            BufferDesc *existing_hdr;
            uint32      buf_state;
 
            UnpinLocalBuffer(BufferDescriptorGetBuffer(victim_buf_hdr));
 
            existing_hdr = GetLocalBufferDescriptor(hresult->id);
            PinLocalBuffer(existing_hdr, false);
            buffers[i] = BufferDescriptorGetBuffer(existing_hdr);
 
            buf_state = pg_atomic_read_u32(&existing_hdr->state);
            Assert(buf_state & BM_TAG_VALID);
            Assert(!(buf_state & BM_DIRTY));
            buf_state &= ~BM_VALID;
            pg_atomic_unlocked_write_u32(&existing_hdr->state, buf_state);
        }
        else
        {
            uint32      buf_state = pg_atomic_read_u32(&victim_buf_hdr->state);
 
            Assert(!(buf_state & (BM_VALID | BM_TAG_VALID | BM_DIRTY | BM_JUST_DIRTIED)));
 
            victim_buf_hdr->tag = tag;
 
            buf_state |= BM_TAG_VALID | BUF_USAGECOUNT_ONE;
 
            pg_atomic_unlocked_write_u32(&victim_buf_hdr->state, buf_state);
 
            hresult->id = victim_buf_id;
        }
    }
 
    io_start = pgstat_prepare_io_time(track_io_timing);
 
    /* actually extend relation */
    smgrzeroextend(bmr.smgr, fork, first_block, extend_by, false);
 
    pgstat_count_io_op_time(IOOBJECT_TEMP_RELATION, IOCONTEXT_NORMAL, IOOP_EXTEND,
                            io_start, 1, extend_by * BLCKSZ);
 
    for (uint32 i = 0; i < extend_by; i++)
    {
        Buffer      buf = buffers[i];
        BufferDesc *buf_hdr;
        uint32      buf_state;
 
        buf_hdr = GetLocalBufferDescriptor(-buf - 1);
 
        buf_state = pg_atomic_read_u32(&buf_hdr->state);
        buf_state |= BM_VALID;
        pg_atomic_unlocked_write_u32(&buf_hdr->state, buf_state);
    }
 
    *extended_by = extend_by;
 
    pgBufferUsage.local_blks_written += extend_by;
 
    return first_block;
}
 
/*
 * MarkLocalBufferDirty -
 *    mark a local buffer dirty
 */
void
MarkLocalBufferDirty(Buffer buffer)
{
    int         bufid;
    BufferDesc *bufHdr;
    uint32      buf_state;
 
    Assert(BufferIsLocal(buffer));
 
#ifdef LBDEBUG
    fprintf(stderr, "LB DIRTY %d\n", buffer);
#endif
 
    bufid = -buffer - 1;
 
    Assert(LocalRefCount[bufid] > 0);
 
    bufHdr = GetLocalBufferDescriptor(bufid);
 
    buf_state = pg_atomic_read_u32(&bufHdr->state);
 
    if (!(buf_state & BM_DIRTY))
        pgBufferUsage.local_blks_dirtied++;
 
    buf_state |= BM_DIRTY;
 
    pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
}
 
/*
 * DropRelationLocalBuffers
 *      This function removes from the buffer pool all the pages of the
 *      specified relation 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!
 *
 *      See DropRelationBuffers in bufmgr.c for more notes.
 */
void
DropRelationLocalBuffers(RelFileLocator rlocator, ForkNumber forkNum,
                         BlockNumber firstDelBlock)
{
    int         i;
 
    for (i = 0; i < NLocBuffer; i++)
    {
        BufferDesc *bufHdr = GetLocalBufferDescriptor(i);
        LocalBufferLookupEnt *hresult;
        uint32      buf_state;
 
        buf_state = pg_atomic_read_u32(&bufHdr->state);
 
        if ((buf_state & BM_TAG_VALID) &&
            BufTagMatchesRelFileLocator(&bufHdr->tag, &rlocator) &&
            BufTagGetForkNum(&bufHdr->tag) == forkNum &&
            bufHdr->tag.blockNum >= firstDelBlock)
        {
            if (LocalRefCount[i] != 0)
                elog(ERROR, "block %u of %s is still referenced (local %u)",
                     bufHdr->tag.blockNum,
                     relpathbackend(BufTagGetRelFileLocator(&bufHdr->tag),
                                    MyProcNumber,
                                    BufTagGetForkNum(&bufHdr->tag)),
                     LocalRefCount[i]);
 
            /* Remove entry from hashtable */
            hresult = (LocalBufferLookupEnt *)
                hash_search(LocalBufHash, &bufHdr->tag, HASH_REMOVE, NULL);
            if (!hresult)       /* shouldn't happen */
                elog(ERROR, "local buffer hash table corrupted");
            /* Mark buffer invalid */
            ClearBufferTag(&bufHdr->tag);
            buf_state &= ~BUF_FLAG_MASK;
            buf_state &= ~BUF_USAGECOUNT_MASK;
            pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
        }
    }
}
 
/*
 * DropRelationAllLocalBuffers
 *      This function removes from the buffer pool all pages of all forks
 *      of the specified relation.
 *
 *      See DropRelationsAllBuffers in bufmgr.c for more notes.
 */
void
DropRelationAllLocalBuffers(RelFileLocator rlocator)
{
    int         i;
 
    for (i = 0; i < NLocBuffer; i++)
    {
        BufferDesc *bufHdr = GetLocalBufferDescriptor(i);
        LocalBufferLookupEnt *hresult;
        uint32      buf_state;
 
        buf_state = pg_atomic_read_u32(&bufHdr->state);
 
        if ((buf_state & BM_TAG_VALID) &&
            BufTagMatchesRelFileLocator(&bufHdr->tag, &rlocator))
        {
            if (LocalRefCount[i] != 0)
                elog(ERROR, "block %u of %s is still referenced (local %u)",
                     bufHdr->tag.blockNum,
                     relpathbackend(BufTagGetRelFileLocator(&bufHdr->tag),
                                    MyProcNumber,
                                    BufTagGetForkNum(&bufHdr->tag)),
                     LocalRefCount[i]);
            /* Remove entry from hashtable */
            hresult = (LocalBufferLookupEnt *)
                hash_search(LocalBufHash, &bufHdr->tag, HASH_REMOVE, NULL);
            if (!hresult)       /* shouldn't happen */
                elog(ERROR, "local buffer hash table corrupted");
            /* Mark buffer invalid */
            ClearBufferTag(&bufHdr->tag);
            buf_state &= ~BUF_FLAG_MASK;
            buf_state &= ~BUF_USAGECOUNT_MASK;
            pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
        }
    }
}
 
/*
 * InitLocalBuffers -
 *    init the local buffer cache. Since most queries (esp. multi-user ones)
 *    don't involve local buffers, we delay allocating actual memory for the
 *    buffers until we need them; just make the buffer headers here.
 */
static void
InitLocalBuffers(void)
{
    int         nbufs = num_temp_buffers;
    HASHCTL     info;
    int         i;
 
    /*
     * Parallel workers can't access data in temporary tables, because they
     * have no visibility into the local buffers of their leader.  This is a
     * convenient, low-cost place to provide a backstop check for that.  Note
     * that we don't wish to prevent a parallel worker from accessing catalog
     * metadata about a temp table, so checks at higher levels would be
     * inappropriate.
     */
    if (IsParallelWorker())
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TRANSACTION_STATE),
                 errmsg("cannot access temporary tables during a parallel operation")));
 
    /* Allocate and zero buffer headers and auxiliary arrays */
    LocalBufferDescriptors = (BufferDesc *) calloc(nbufs, sizeof(BufferDesc));
    LocalBufferBlockPointers = (Block *) calloc(nbufs, sizeof(Block));
    LocalRefCount = (int32 *) calloc(nbufs, sizeof(int32));
    if (!LocalBufferDescriptors || !LocalBufferBlockPointers || !LocalRefCount)
        ereport(FATAL,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of memory")));
 
    nextFreeLocalBufId = 0;
 
    /* initialize fields that need to start off nonzero */
    for (i = 0; i < nbufs; i++)
    {
        BufferDesc *buf = GetLocalBufferDescriptor(i);
 
        /*
         * negative to indicate local buffer. This is tricky: shared buffers
         * start with 0. We have to start with -2. (Note that the routine
         * BufferDescriptorGetBuffer adds 1 to buf_id so our first buffer id
         * is -1.)
         */
        buf->buf_id = -i - 2;
 
        /*
         * Intentionally do not initialize the buffer's atomic variable
         * (besides zeroing the underlying memory above). That way we get
         * errors on platforms without atomics, if somebody (re-)introduces
         * atomic operations for local buffers.
         */
    }
 
    /* Create the lookup hash table */
    info.keysize = sizeof(BufferTag);
    info.entrysize = sizeof(LocalBufferLookupEnt);
 
    LocalBufHash = hash_create("Local Buffer Lookup Table",
                               nbufs,
                               &info,
                               HASH_ELEM | HASH_BLOBS);
 
    if (!LocalBufHash)
        elog(ERROR, "could not initialize local buffer hash table");
 
    /* Initialization done, mark buffers allocated */
    NLocBuffer = nbufs;
}
 
/*
 * XXX: We could have a slightly more efficient version of PinLocalBuffer()
 * that does not support adjusting the usagecount - but so far it does not
 * seem worth the trouble.
 *
 * Note that ResourceOwnerEnlarge() must have been done already.
 */
bool
PinLocalBuffer(BufferDesc *buf_hdr, bool adjust_usagecount)
{
    uint32      buf_state;
    Buffer      buffer = BufferDescriptorGetBuffer(buf_hdr);
    int         bufid = -buffer - 1;
 
    buf_state = pg_atomic_read_u32(&buf_hdr->state);
 
    if (LocalRefCount[bufid] == 0)
    {
        NLocalPinnedBuffers++;
        if (adjust_usagecount &&
            BUF_STATE_GET_USAGECOUNT(buf_state) < BM_MAX_USAGE_COUNT)
        {
            buf_state += BUF_USAGECOUNT_ONE;
            pg_atomic_unlocked_write_u32(&buf_hdr->state, buf_state);
        }
    }
    LocalRefCount[bufid]++;
    ResourceOwnerRememberBuffer(CurrentResourceOwner,
                                BufferDescriptorGetBuffer(buf_hdr));
 
    return buf_state & BM_VALID;
}
 
void
UnpinLocalBuffer(Buffer buffer)
{
    UnpinLocalBufferNoOwner(buffer);
    ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
}
 
void
UnpinLocalBufferNoOwner(Buffer buffer)
{
    int         buffid = -buffer - 1;
 
    Assert(BufferIsLocal(buffer));
    Assert(LocalRefCount[buffid] > 0);
    Assert(NLocalPinnedBuffers > 0);
 
    if (--LocalRefCount[buffid] == 0)
        NLocalPinnedBuffers--;
}
 
/*
 * GUC check_hook for temp_buffers
 */
bool
check_temp_buffers(int *newval, void **extra, GucSource source)
{
    /*
     * Once local buffers have been initialized, it's too late to change this.
     * However, if this is only a test call, allow it.
     */
    if (source != PGC_S_TEST && NLocBuffer && NLocBuffer != *newval)
    {
        GUC_check_errdetail("\"temp_buffers\" cannot be changed after any temporary tables have been accessed in the session.");
        return false;
    }
    return true;
}
 
/*
 * GetLocalBufferStorage - allocate memory for a local buffer
 *
 * The idea of this function is to aggregate our requests for storage
 * so that the memory manager doesn't see a whole lot of relatively small
 * requests.  Since we'll never give back a local buffer once it's created
 * within a particular process, no point in burdening memmgr with separately
 * managed chunks.
 */
static Block
GetLocalBufferStorage(void)
{
    static char *cur_block = NULL;
    static int  next_buf_in_block = 0;
    static int  num_bufs_in_block = 0;
    static int  total_bufs_allocated = 0;
    static MemoryContext LocalBufferContext = NULL;
 
    char       *this_buf;
 
    Assert(total_bufs_allocated < NLocBuffer);
 
    if (next_buf_in_block >= num_bufs_in_block)
    {
        /* Need to make a new request to memmgr */
        int         num_bufs;
 
        /*
         * We allocate local buffers in a context of their own, so that the
         * space eaten for them is easily recognizable in MemoryContextStats
         * output.  Create the context on first use.
         */
        if (LocalBufferContext == NULL)
            LocalBufferContext =
                AllocSetContextCreate(TopMemoryContext,
                                      "LocalBufferContext",
                                      ALLOCSET_DEFAULT_SIZES);
 
        /* Start with a 16-buffer request; subsequent ones double each time */
        num_bufs = Max(num_bufs_in_block * 2, 16);
        /* But not more than what we need for all remaining local bufs */
        num_bufs = Min(num_bufs, NLocBuffer - total_bufs_allocated);
        /* And don't overflow MaxAllocSize, either */
        num_bufs = Min(num_bufs, MaxAllocSize / BLCKSZ);
 
        /* Buffers should be I/O aligned. */
        cur_block = (char *)
            TYPEALIGN(PG_IO_ALIGN_SIZE,
                      MemoryContextAlloc(LocalBufferContext,
                                         num_bufs * BLCKSZ + PG_IO_ALIGN_SIZE));
        next_buf_in_block = 0;
        num_bufs_in_block = num_bufs;
    }
 
    /* Allocate next buffer in current memory block */
    this_buf = cur_block + next_buf_in_block * BLCKSZ;
    next_buf_in_block++;
    total_bufs_allocated++;
 
    return (Block) this_buf;
}
 
/*
 * CheckForLocalBufferLeaks - ensure this backend holds no local buffer pins
 *
 * This is just like CheckForBufferLeaks(), but for local buffers.
 */
static void
CheckForLocalBufferLeaks(void)
{
#ifdef USE_ASSERT_CHECKING
    if (LocalRefCount)
    {
        int         RefCountErrors = 0;
        int         i;
 
        for (i = 0; i < NLocBuffer; i++)
        {
            if (LocalRefCount[i] != 0)
            {
                Buffer      b = -i - 1;
                char       *s;
 
                s = DebugPrintBufferRefcount(b);
                elog(WARNING, "local buffer refcount leak: %s", s);
                pfree(s);
 
                RefCountErrors++;
            }
        }
        Assert(RefCountErrors == 0);
    }
#endif
}
 
/*
 * AtEOXact_LocalBuffers - clean up at end of transaction.
 *
 * This is just like AtEOXact_Buffers, but for local buffers.
 */
void
AtEOXact_LocalBuffers(bool isCommit)
{
    CheckForLocalBufferLeaks();
}
 
/*
 * AtProcExit_LocalBuffers - ensure we have dropped pins during backend exit.
 *
 * This is just like AtProcExit_Buffers, but for local buffers.
 */
void
AtProcExit_LocalBuffers(void)
{
    /*
     * We shouldn't be holding any remaining pins; if we are, and assertions
     * aren't enabled, we'll fail later in DropRelationBuffers while trying to
     * drop the temp rels.
     */
    CheckForLocalBufferLeaks();
}