hio.c

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/*-------------------------------------------------------------------------
 *
 * hio.c
 *    POSTGRES heap access method input/output code.
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/access/heap/hio.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/heapam.h"
#include "access/hio.h"
#include "access/htup_details.h"
#include "access/visibilitymap.h"
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "storage/lmgr.h"
#include "storage/smgr.h"
 
 
/*
 * RelationPutHeapTuple - place tuple at specified page
 *
 * !!! EREPORT(ERROR) IS DISALLOWED HERE !!!  Must PANIC on failure!!!
 *
 * Note - caller must hold BUFFER_LOCK_EXCLUSIVE on the buffer.
 */
void
RelationPutHeapTuple(Relation relation,
                     Buffer buffer,
                     HeapTuple tuple,
                     bool token)
{
    Page        pageHeader;
    OffsetNumber offnum;
 
    /*
     * A tuple that's being inserted speculatively should already have its
     * token set.
     */
    Assert(!token || HeapTupleHeaderIsSpeculative(tuple->t_data));
 
    /*
     * Do not allow tuples with invalid combinations of hint bits to be placed
     * on a page.  This combination is detected as corruption by the
     * contrib/amcheck logic, so if you disable this assertion, make
     * corresponding changes there.
     */
    Assert(!((tuple->t_data->t_infomask & HEAP_XMAX_COMMITTED) &&
             (tuple->t_data->t_infomask & HEAP_XMAX_IS_MULTI)));
 
    /* Add the tuple to the page */
    pageHeader = BufferGetPage(buffer);
 
    offnum = PageAddItem(pageHeader, (Item) tuple->t_data,
                         tuple->t_len, InvalidOffsetNumber, false, true);
 
    if (offnum == InvalidOffsetNumber)
        elog(PANIC, "failed to add tuple to page");
 
    /* Update tuple->t_self to the actual position where it was stored */
    ItemPointerSet(&(tuple->t_self), BufferGetBlockNumber(buffer), offnum);
 
    /*
     * Insert the correct position into CTID of the stored tuple, too (unless
     * this is a speculative insertion, in which case the token is held in
     * CTID field instead)
     */
    if (!token)
    {
        ItemId      itemId = PageGetItemId(pageHeader, offnum);
        HeapTupleHeader item = (HeapTupleHeader) PageGetItem(pageHeader, itemId);
 
        item->t_ctid = tuple->t_self;
    }
}
 
/*
 * Read in a buffer in mode, using bulk-insert strategy if bistate isn't NULL.
 */
static Buffer
ReadBufferBI(Relation relation, BlockNumber targetBlock,
             ReadBufferMode mode, BulkInsertState bistate)
{
    Buffer      buffer;
 
    /* If not bulk-insert, exactly like ReadBuffer */
    if (!bistate)
        return ReadBufferExtended(relation, MAIN_FORKNUM, targetBlock,
                                  mode, NULL);
 
    /* If we have the desired block already pinned, re-pin and return it */
    if (bistate->current_buf != InvalidBuffer)
    {
        if (BufferGetBlockNumber(bistate->current_buf) == targetBlock)
        {
            /*
             * Currently the LOCK variants are only used for extending
             * relation, which should never reach this branch.
             */
            Assert(mode != RBM_ZERO_AND_LOCK &&
                   mode != RBM_ZERO_AND_CLEANUP_LOCK);
 
            IncrBufferRefCount(bistate->current_buf);
            return bistate->current_buf;
        }
        /* ... else drop the old buffer */
        ReleaseBuffer(bistate->current_buf);
        bistate->current_buf = InvalidBuffer;
    }
 
    /* Perform a read using the buffer strategy */
    buffer = ReadBufferExtended(relation, MAIN_FORKNUM, targetBlock,
                                mode, bistate->strategy);
 
    /* Save the selected block as target for future inserts */
    IncrBufferRefCount(buffer);
    bistate->current_buf = buffer;
 
    return buffer;
}
 
/*
 * For each heap page which is all-visible, acquire a pin on the appropriate
 * visibility map page, if we haven't already got one.
 *
 * buffer2 may be InvalidBuffer, if only one buffer is involved.  buffer1
 * must not be InvalidBuffer.  If both buffers are specified, block1 must
 * be less than block2.
 */
static void
GetVisibilityMapPins(Relation relation, Buffer buffer1, Buffer buffer2,
                     BlockNumber block1, BlockNumber block2,
                     Buffer *vmbuffer1, Buffer *vmbuffer2)
{
    bool        need_to_pin_buffer1;
    bool        need_to_pin_buffer2;
 
    Assert(BufferIsValid(buffer1));
    Assert(buffer2 == InvalidBuffer || block1 <= block2);
 
    while (1)
    {
        /* Figure out which pins we need but don't have. */
        need_to_pin_buffer1 = PageIsAllVisible(BufferGetPage(buffer1))
            && !visibilitymap_pin_ok(block1, *vmbuffer1);
        need_to_pin_buffer2 = buffer2 != InvalidBuffer
            && PageIsAllVisible(BufferGetPage(buffer2))
            && !visibilitymap_pin_ok(block2, *vmbuffer2);
        if (!need_to_pin_buffer1 && !need_to_pin_buffer2)
            return;
 
        /* We must unlock both buffers before doing any I/O. */
        LockBuffer(buffer1, BUFFER_LOCK_UNLOCK);
        if (buffer2 != InvalidBuffer && buffer2 != buffer1)
            LockBuffer(buffer2, BUFFER_LOCK_UNLOCK);
 
        /* Get pins. */
        if (need_to_pin_buffer1)
            visibilitymap_pin(relation, block1, vmbuffer1);
        if (need_to_pin_buffer2)
            visibilitymap_pin(relation, block2, vmbuffer2);
 
        /* Relock buffers. */
        LockBuffer(buffer1, BUFFER_LOCK_EXCLUSIVE);
        if (buffer2 != InvalidBuffer && buffer2 != buffer1)
            LockBuffer(buffer2, BUFFER_LOCK_EXCLUSIVE);
 
        /*
         * If there are two buffers involved and we pinned just one of them,
         * it's possible that the second one became all-visible while we were
         * busy pinning the first one.  If it looks like that's a possible
         * scenario, we'll need to make a second pass through this loop.
         */
        if (buffer2 == InvalidBuffer || buffer1 == buffer2
            || (need_to_pin_buffer1 && need_to_pin_buffer2))
            break;
    }
}
 
/*
 * Extend a relation by multiple blocks to avoid future contention on the
 * relation extension lock.  Our goal is to pre-extend the relation by an
 * amount which ramps up as the degree of contention ramps up, but limiting
 * the result to some sane overall value.
 */
static void
RelationAddExtraBlocks(Relation relation, BulkInsertState bistate)
{
    BlockNumber blockNum,
                firstBlock = InvalidBlockNumber;
    int         extraBlocks;
    int         lockWaiters;
 
    /* Use the length of the lock wait queue to judge how much to extend. */
    lockWaiters = RelationExtensionLockWaiterCount(relation);
    if (lockWaiters <= 0)
        return;
 
    /*
     * It might seem like multiplying the number of lock waiters by as much as
     * 20 is too aggressive, but benchmarking revealed that smaller numbers
     * were insufficient.  512 is just an arbitrary cap to prevent
     * pathological results.
     */
    extraBlocks = Min(512, lockWaiters * 20);
 
    do
    {
        Buffer      buffer;
        Page        page;
        Size        freespace;
 
        /*
         * Extend by one page.  This should generally match the main-line
         * extension code in RelationGetBufferForTuple, except that we hold
         * the relation extension lock throughout, and we don't immediately
         * initialize the page (see below).
         */
        buffer = ReadBufferBI(relation, P_NEW, RBM_ZERO_AND_LOCK, bistate);
        page = BufferGetPage(buffer);
 
        if (!PageIsNew(page))
            elog(ERROR, "page %u of relation \"%s\" should be empty but is not",
                 BufferGetBlockNumber(buffer),
                 RelationGetRelationName(relation));
 
        /*
         * Add the page to the FSM without initializing. If we were to
         * initialize here, the page would potentially get flushed out to disk
         * before we add any useful content. There's no guarantee that that'd
         * happen before a potential crash, so we need to deal with
         * uninitialized pages anyway, thus avoid the potential for
         * unnecessary writes.
         */
 
        /* we'll need this info below */
        blockNum = BufferGetBlockNumber(buffer);
        freespace = BufferGetPageSize(buffer) - SizeOfPageHeaderData;
 
        UnlockReleaseBuffer(buffer);
 
        /* Remember first block number thus added. */
        if (firstBlock == InvalidBlockNumber)
            firstBlock = blockNum;
 
        /*
         * Immediately update the bottom level of the FSM.  This has a good
         * chance of making this page visible to other concurrently inserting
         * backends, and we want that to happen without delay.
         */
        RecordPageWithFreeSpace(relation, blockNum, freespace);
    }
    while (--extraBlocks > 0);
 
    /*
     * Updating the upper levels of the free space map is too expensive to do
     * for every block, but it's worth doing once at the end to make sure that
     * subsequent insertion activity sees all of those nifty free pages we
     * just inserted.
     */
    FreeSpaceMapVacuumRange(relation, firstBlock, blockNum + 1);
}
 
/*
 * RelationGetBufferForTuple
 *
 *  Returns pinned and exclusive-locked buffer of a page in given relation
 *  with free space >= given len.
 *
 *  If otherBuffer is not InvalidBuffer, then it references a previously
 *  pinned buffer of another page in the same relation; on return, this
 *  buffer will also be exclusive-locked.  (This case is used by heap_update;
 *  the otherBuffer contains the tuple being updated.)
 *
 *  The reason for passing otherBuffer is that if two backends are doing
 *  concurrent heap_update operations, a deadlock could occur if they try
 *  to lock the same two buffers in opposite orders.  To ensure that this
 *  can't happen, we impose the rule that buffers of a relation must be
 *  locked in increasing page number order.  This is most conveniently done
 *  by having RelationGetBufferForTuple lock them both, with suitable care
 *  for ordering.
 *
 *  NOTE: it is unlikely, but not quite impossible, for otherBuffer to be the
 *  same buffer we select for insertion of the new tuple (this could only
 *  happen if space is freed in that page after heap_update finds there's not
 *  enough there).  In that case, the page will be pinned and locked only once.
 *
 *  We also handle the possibility that the all-visible flag will need to be
 *  cleared on one or both pages.  If so, pin on the associated visibility map
 *  page must be acquired before acquiring buffer lock(s), to avoid possibly
 *  doing I/O while holding buffer locks.  The pins are passed back to the
 *  caller using the input-output arguments vmbuffer and vmbuffer_other.
 *  Note that in some cases the caller might have already acquired such pins,
 *  which is indicated by these arguments not being InvalidBuffer on entry.
 *
 *  We normally use FSM to help us find free space.  However,
 *  if HEAP_INSERT_SKIP_FSM is specified, we just append a new empty page to
 *  the end of the relation if the tuple won't fit on the current target page.
 *  This can save some cycles when we know the relation is new and doesn't
 *  contain useful amounts of free space.
 *
 *  HEAP_INSERT_SKIP_FSM is also useful for non-WAL-logged additions to a
 *  relation, if the caller holds exclusive lock and is careful to invalidate
 *  relation's smgr_targblock before the first insertion --- that ensures that
 *  all insertions will occur into newly added pages and not be intermixed
 *  with tuples from other transactions.  That way, a crash can't risk losing
 *  any committed data of other transactions.  (See heap_insert's comments
 *  for additional constraints needed for safe usage of this behavior.)
 *
 *  The caller can also provide a BulkInsertState object to optimize many
 *  insertions into the same relation.  This keeps a pin on the current
 *  insertion target page (to save pin/unpin cycles) and also passes a
 *  BULKWRITE buffer selection strategy object to the buffer manager.
 *  Passing NULL for bistate selects the default behavior.
 *
 *  We don't fill existing pages further than the fillfactor, except for large
 *  tuples in nearly-empty pages.  This is OK since this routine is not
 *  consulted when updating a tuple and keeping it on the same page, which is
 *  the scenario fillfactor is meant to reserve space for.
 *
 *  ereport(ERROR) is allowed here, so this routine *must* be called
 *  before any (unlogged) changes are made in buffer pool.
 */
Buffer
RelationGetBufferForTuple(Relation relation, Size len,
                          Buffer otherBuffer, int options,
                          BulkInsertState bistate,
                          Buffer *vmbuffer, Buffer *vmbuffer_other)
{
    bool        use_fsm = !(options & HEAP_INSERT_SKIP_FSM);
    Buffer      buffer = InvalidBuffer;
    Page        page;
    Size        nearlyEmptyFreeSpace,
                pageFreeSpace = 0,
                saveFreeSpace = 0,
                targetFreeSpace = 0;
    BlockNumber targetBlock,
                otherBlock;
    bool        needLock;
 
    len = MAXALIGN(len);        /* be conservative */
 
    /* Bulk insert is not supported for updates, only inserts. */
    Assert(otherBuffer == InvalidBuffer || !bistate);
 
    /*
     * If we're gonna fail for oversize tuple, do it right away
     */
    if (len > MaxHeapTupleSize)
        ereport(ERROR,
                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                 errmsg("row is too big: size %zu, maximum size %zu",
                        len, MaxHeapTupleSize)));
 
    /* Compute desired extra freespace due to fillfactor option */
    saveFreeSpace = RelationGetTargetPageFreeSpace(relation,
                                                   HEAP_DEFAULT_FILLFACTOR);
 
    /*
     * Since pages without tuples can still have line pointers, we consider
     * pages "empty" when the unavailable space is slight.  This threshold is
     * somewhat arbitrary, but it should prevent most unnecessary relation
     * extensions while inserting large tuples into low-fillfactor tables.
     */
    nearlyEmptyFreeSpace = MaxHeapTupleSize -
        (MaxHeapTuplesPerPage / 8 * sizeof(ItemIdData));
    if (len + saveFreeSpace > nearlyEmptyFreeSpace)
        targetFreeSpace = Max(len, nearlyEmptyFreeSpace);
    else
        targetFreeSpace = len + saveFreeSpace;
 
    if (otherBuffer != InvalidBuffer)
        otherBlock = BufferGetBlockNumber(otherBuffer);
    else
        otherBlock = InvalidBlockNumber;    /* just to keep compiler quiet */
 
    /*
     * We first try to put the tuple on the same page we last inserted a tuple
     * on, as cached in the BulkInsertState or relcache entry.  If that
     * doesn't work, we ask the Free Space Map to locate a suitable page.
     * Since the FSM's info might be out of date, we have to be prepared to
     * loop around and retry multiple times. (To insure this isn't an infinite
     * loop, we must update the FSM with the correct amount of free space on
     * each page that proves not to be suitable.)  If the FSM has no record of
     * a page with enough free space, we give up and extend the relation.
     *
     * When use_fsm is false, we either put the tuple onto the existing target
     * page or extend the relation.
     */
    if (bistate && bistate->current_buf != InvalidBuffer)
        targetBlock = BufferGetBlockNumber(bistate->current_buf);
    else
        targetBlock = RelationGetTargetBlock(relation);
 
    if (targetBlock == InvalidBlockNumber && use_fsm)
    {
        /*
         * We have no cached target page, so ask the FSM for an initial
         * target.
         */
        targetBlock = GetPageWithFreeSpace(relation, targetFreeSpace);
    }
 
    /*
     * If the FSM knows nothing of the rel, try the last page before we give
     * up and extend.  This avoids one-tuple-per-page syndrome during
     * bootstrapping or in a recently-started system.
     */
    if (targetBlock == InvalidBlockNumber)
    {
        BlockNumber nblocks = RelationGetNumberOfBlocks(relation);
 
        if (nblocks > 0)
            targetBlock = nblocks - 1;
    }
 
loop:
    while (targetBlock != InvalidBlockNumber)
    {
        /*
         * Read and exclusive-lock the target block, as well as the other
         * block if one was given, taking suitable care with lock ordering and
         * the possibility they are the same block.
         *
         * If the page-level all-visible flag is set, caller will need to
         * clear both that and the corresponding visibility map bit.  However,
         * by the time we return, we'll have x-locked the buffer, and we don't
         * want to do any I/O while in that state.  So we check the bit here
         * before taking the lock, and pin the page if it appears necessary.
         * Checking without the lock creates a risk of getting the wrong
         * answer, so we'll have to recheck after acquiring the lock.
         */
        if (otherBuffer == InvalidBuffer)
        {
            /* easy case */
            buffer = ReadBufferBI(relation, targetBlock, RBM_NORMAL, bistate);
            if (PageIsAllVisible(BufferGetPage(buffer)))
                visibilitymap_pin(relation, targetBlock, vmbuffer);
 
            /*
             * If the page is empty, pin vmbuffer to set all_frozen bit later.
             */
            if ((options & HEAP_INSERT_FROZEN) &&
                (PageGetMaxOffsetNumber(BufferGetPage(buffer)) == 0))
                visibilitymap_pin(relation, targetBlock, vmbuffer);
 
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
        }
        else if (otherBlock == targetBlock)
        {
            /* also easy case */
            buffer = otherBuffer;
            if (PageIsAllVisible(BufferGetPage(buffer)))
                visibilitymap_pin(relation, targetBlock, vmbuffer);
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
        }
        else if (otherBlock < targetBlock)
        {
            /* lock other buffer first */
            buffer = ReadBuffer(relation, targetBlock);
            if (PageIsAllVisible(BufferGetPage(buffer)))
                visibilitymap_pin(relation, targetBlock, vmbuffer);
            LockBuffer(otherBuffer, BUFFER_LOCK_EXCLUSIVE);
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
        }
        else
        {
            /* lock target buffer first */
            buffer = ReadBuffer(relation, targetBlock);
            if (PageIsAllVisible(BufferGetPage(buffer)))
                visibilitymap_pin(relation, targetBlock, vmbuffer);
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
            LockBuffer(otherBuffer, BUFFER_LOCK_EXCLUSIVE);
        }
 
        /*
         * We now have the target page (and the other buffer, if any) pinned
         * and locked.  However, since our initial PageIsAllVisible checks
         * were performed before acquiring the lock, the results might now be
         * out of date, either for the selected victim buffer, or for the
         * other buffer passed by the caller.  In that case, we'll need to
         * give up our locks, go get the pin(s) we failed to get earlier, and
         * re-lock.  That's pretty painful, but hopefully shouldn't happen
         * often.
         *
         * Note that there's a small possibility that we didn't pin the page
         * above but still have the correct page pinned anyway, either because
         * we've already made a previous pass through this loop, or because
         * caller passed us the right page anyway.
         *
         * Note also that it's possible that by the time we get the pin and
         * retake the buffer locks, the visibility map bit will have been
         * cleared by some other backend anyway.  In that case, we'll have
         * done a bit of extra work for no gain, but there's no real harm
         * done.
         */
        if (otherBuffer == InvalidBuffer || targetBlock <= otherBlock)
            GetVisibilityMapPins(relation, buffer, otherBuffer,
                                 targetBlock, otherBlock, vmbuffer,
                                 vmbuffer_other);
        else
            GetVisibilityMapPins(relation, otherBuffer, buffer,
                                 otherBlock, targetBlock, vmbuffer_other,
                                 vmbuffer);
 
        /*
         * Now we can check to see if there's enough free space here. If so,
         * we're done.
         */
        page = BufferGetPage(buffer);
 
        /*
         * If necessary initialize page, it'll be used soon.  We could avoid
         * dirtying the buffer here, and rely on the caller to do so whenever
         * it puts a tuple onto the page, but there seems not much benefit in
         * doing so.
         */
        if (PageIsNew(page))
        {
            PageInit(page, BufferGetPageSize(buffer), 0);
            MarkBufferDirty(buffer);
        }
 
        pageFreeSpace = PageGetHeapFreeSpace(page);
        if (targetFreeSpace <= pageFreeSpace)
        {
            /* use this page as future insert target, too */
            RelationSetTargetBlock(relation, targetBlock);
            return buffer;
        }
 
        /*
         * Not enough space, so we must give up our page locks and pin (if
         * any) and prepare to look elsewhere.  We don't care which order we
         * unlock the two buffers in, so this can be slightly simpler than the
         * code above.
         */
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
        if (otherBuffer == InvalidBuffer)
            ReleaseBuffer(buffer);
        else if (otherBlock != targetBlock)
        {
            LockBuffer(otherBuffer, BUFFER_LOCK_UNLOCK);
            ReleaseBuffer(buffer);
        }
 
        /* Without FSM, always fall out of the loop and extend */
        if (!use_fsm)
            break;
 
        /*
         * Update FSM as to condition of this page, and ask for another page
         * to try.
         */
        targetBlock = RecordAndGetPageWithFreeSpace(relation,
                                                    targetBlock,
                                                    pageFreeSpace,
                                                    targetFreeSpace);
    }
 
    /*
     * Have to extend the relation.
     *
     * We have to use a lock to ensure no one else is extending the rel at the
     * same time, else we will both try to initialize the same new page.  We
     * can skip locking for new or temp relations, however, since no one else
     * could be accessing them.
     */
    needLock = !RELATION_IS_LOCAL(relation);
 
    /*
     * If we need the lock but are not able to acquire it immediately, we'll
     * consider extending the relation by multiple blocks at a time to manage
     * contention on the relation extension lock.  However, this only makes
     * sense if we're using the FSM; otherwise, there's no point.
     */
    if (needLock)
    {
        if (!use_fsm)
            LockRelationForExtension(relation, ExclusiveLock);
        else if (!ConditionalLockRelationForExtension(relation, ExclusiveLock))
        {
            /* Couldn't get the lock immediately; wait for it. */
            LockRelationForExtension(relation, ExclusiveLock);
 
            /*
             * Check if some other backend has extended a block for us while
             * we were waiting on the lock.
             */
            targetBlock = GetPageWithFreeSpace(relation, targetFreeSpace);
 
            /*
             * If some other waiter has already extended the relation, we
             * don't need to do so; just use the existing freespace.
             */
            if (targetBlock != InvalidBlockNumber)
            {
                UnlockRelationForExtension(relation, ExclusiveLock);
                goto loop;
            }
 
            /* Time to bulk-extend. */
            RelationAddExtraBlocks(relation, bistate);
        }
    }
 
    /*
     * In addition to whatever extension we performed above, we always add at
     * least one block to satisfy our own request.
     *
     * XXX This does an lseek - rather expensive - but at the moment it is the
     * only way to accurately determine how many blocks are in a relation.  Is
     * it worth keeping an accurate file length in shared memory someplace,
     * rather than relying on the kernel to do it for us?
     */
    buffer = ReadBufferBI(relation, P_NEW, RBM_ZERO_AND_LOCK, bistate);
 
    /*
     * We need to initialize the empty new page.  Double-check that it really
     * is empty (this should never happen, but if it does we don't want to
     * risk wiping out valid data).
     */
    page = BufferGetPage(buffer);
 
    if (!PageIsNew(page))
        elog(ERROR, "page %u of relation \"%s\" should be empty but is not",
             BufferGetBlockNumber(buffer),
             RelationGetRelationName(relation));
 
    PageInit(page, BufferGetPageSize(buffer), 0);
    MarkBufferDirty(buffer);
 
    /*
     * The page is empty, pin vmbuffer to set all_frozen bit.
     */
    if (options & HEAP_INSERT_FROZEN)
    {
        Assert(PageGetMaxOffsetNumber(BufferGetPage(buffer)) == 0);
        visibilitymap_pin(relation, BufferGetBlockNumber(buffer), vmbuffer);
    }
 
    /*
     * Release the file-extension lock; it's now OK for someone else to extend
     * the relation some more.
     */
    if (needLock)
        UnlockRelationForExtension(relation, ExclusiveLock);
 
    /*
     * Lock the other buffer. It's guaranteed to be of a lower page number
     * than the new page. To conform with the deadlock prevent rules, we ought
     * to lock otherBuffer first, but that would give other backends a chance
     * to put tuples on our page. To reduce the likelihood of that, attempt to
     * lock the other buffer conditionally, that's very likely to work.
     * Otherwise we need to lock buffers in the correct order, and retry if
     * the space has been used in the mean time.
     *
     * Alternatively, we could acquire the lock on otherBuffer before
     * extending the relation, but that'd require holding the lock while
     * performing IO, which seems worse than an unlikely retry.
     */
    if (otherBuffer != InvalidBuffer)
    {
        Assert(otherBuffer != buffer);
        targetBlock = BufferGetBlockNumber(buffer);
        Assert(targetBlock > otherBlock);
 
        if (unlikely(!ConditionalLockBuffer(otherBuffer)))
        {
            LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
            LockBuffer(otherBuffer, BUFFER_LOCK_EXCLUSIVE);
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
 
            /*
             * Because the buffers were unlocked for a while, it's possible,
             * although unlikely, that an all-visible flag became set or that
             * somebody used up the available space in the new page.  We can
             * use GetVisibilityMapPins to deal with the first case.  In the
             * second case, just retry from start.
             */
            GetVisibilityMapPins(relation, otherBuffer, buffer,
                                 otherBlock, targetBlock, vmbuffer_other,
                                 vmbuffer);
 
            if (len > PageGetHeapFreeSpace(page))
            {
                LockBuffer(otherBuffer, BUFFER_LOCK_UNLOCK);
                UnlockReleaseBuffer(buffer);
 
                goto loop;
            }
        }
    }
 
    if (len > PageGetHeapFreeSpace(page))
    {
        /* We should not get here given the test at the top */
        elog(PANIC, "tuple is too big: size %zu", len);
    }
 
    /*
     * Remember the new page as our target for future insertions.
     *
     * XXX should we enter the new page into the free space map immediately,
     * or just keep it for this backend's exclusive use in the short run
     * (until VACUUM sees it)?  Seems to depend on whether you expect the
     * current backend to make more insertions or not, which is probably a
     * good bet most of the time.  So for now, don't add it to FSM yet.
     */
    RelationSetTargetBlock(relation, BufferGetBlockNumber(buffer));
 
    return buffer;
}