hio.c

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/*-------------------------------------------------------------------------
 *
 * hio.c
 *    POSTGRES heap access method input/output code.
 *
 * Portions Copyright (c) 1996-2025, 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"
 
 
/*
 * 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.
 *
 * To avoid complexity in the callers, either buffer1 or buffer2 may be
 * InvalidBuffer if only one buffer is involved. For the same reason, block2
 * may be smaller than block1.
 *
 * Returns whether buffer locks were temporarily released.
 */
static bool
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;
    bool        released_locks = false;
 
    /*
     * Swap buffers around to handle case of a single block/buffer, and to
     * handle if lock ordering rules require to lock block2 first.
     */
    if (!BufferIsValid(buffer1) ||
        (BufferIsValid(buffer2) && block1 > block2))
    {
        Buffer      tmpbuf = buffer1;
        Buffer     *tmpvmbuf = vmbuffer1;
        BlockNumber tmpblock = block1;
 
        buffer1 = buffer2;
        vmbuffer1 = vmbuffer2;
        block1 = block2;
 
        buffer2 = tmpbuf;
        vmbuffer2 = tmpvmbuf;
        block2 = tmpblock;
    }
 
    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)
            break;
 
        /* We must unlock both buffers before doing any I/O. */
        released_locks = true;
        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;
    }
 
    return released_locks;
}
 
/*
 * Extend the relation. By multiple pages, if beneficial.
 *
 * If the caller needs multiple pages (num_pages > 1), we always try to extend
 * by at least that much.
 *
 * If there is contention on the extension lock, we don't just extend "for
 * ourselves", but we try to help others. We can do so by adding empty pages
 * into the FSM. Typically there is no contention when we can't use the FSM.
 *
 * We do have to limit the number of pages to extend by to some value, as the
 * buffers for all the extended pages need to, temporarily, be pinned. For now
 * we define MAX_BUFFERS_TO_EXTEND_BY to be 64 buffers, it's hard to see
 * benefits with higher numbers. This partially is because copyfrom.c's
 * MAX_BUFFERED_TUPLES / MAX_BUFFERED_BYTES prevents larger multi_inserts.
 *
 * Returns a buffer for a newly extended block. If possible, the buffer is
 * returned exclusively locked. *did_unlock is set to true if the lock had to
 * be released, false otherwise.
 *
 *
 * XXX: It would likely be beneficial for some workloads to extend more
 * aggressively, e.g. using a heuristic based on the relation size.
 */
static Buffer
RelationAddBlocks(Relation relation, BulkInsertState bistate,
                  int num_pages, bool use_fsm, bool *did_unlock)
{
#define MAX_BUFFERS_TO_EXTEND_BY 64
    Buffer      victim_buffers[MAX_BUFFERS_TO_EXTEND_BY];
    BlockNumber first_block = InvalidBlockNumber;
    BlockNumber last_block = InvalidBlockNumber;
    uint32      extend_by_pages;
    uint32      not_in_fsm_pages;
    Buffer      buffer;
    Page        page;
 
    /*
     * Determine by how many pages to try to extend by.
     */
    if (bistate == NULL && !use_fsm)
    {
        /*
         * If we have neither bistate, nor can use the FSM, we can't bulk
         * extend - there'd be no way to find the additional pages.
         */
        extend_by_pages = 1;
    }
    else
    {
        uint32      waitcount;
 
        /*
         * Try to extend at least by the number of pages the caller needs. We
         * can remember the additional pages (either via FSM or bistate).
         */
        extend_by_pages = num_pages;
 
        if (!RELATION_IS_LOCAL(relation))
            waitcount = RelationExtensionLockWaiterCount(relation);
        else
            waitcount = 0;
 
        /*
         * Multiply the number of pages to extend by the number of waiters. Do
         * this even if we're not using the FSM, as it still relieves
         * contention, by deferring the next time this backend needs to
         * extend. In that case the extended pages will be found via
         * bistate->next_free.
         */
        extend_by_pages += extend_by_pages * waitcount;
 
        /* ---
         * If we previously extended using the same bistate, it's very likely
         * we'll extend some more. Try to extend by as many pages as
         * before. This can be important for performance for several reasons,
         * including:
         *
         * - It prevents mdzeroextend() switching between extending the
         *   relation in different ways, which is inefficient for some
         *   filesystems.
         *
         * - Contention is often intermittent. Even if we currently don't see
         *   other waiters (see above), extending by larger amounts can
         *   prevent future contention.
         * ---
         */
        if (bistate)
            extend_by_pages = Max(extend_by_pages, bistate->already_extended_by);
 
        /*
         * Can't extend by more than MAX_BUFFERS_TO_EXTEND_BY, we need to pin
         * them all concurrently.
         */
        extend_by_pages = Min(extend_by_pages, MAX_BUFFERS_TO_EXTEND_BY);
    }
 
    /*
     * How many of the extended pages should be entered into the FSM?
     *
     * If we have a bistate, only enter pages that we don't need ourselves
     * into the FSM.  Otherwise every other backend will immediately try to
     * use the pages this backend needs for itself, causing unnecessary
     * contention.  If we don't have a bistate, we can't avoid the FSM.
     *
     * Never enter the page returned into the FSM, we'll immediately use it.
     */
    if (num_pages > 1 && bistate == NULL)
        not_in_fsm_pages = 1;
    else
        not_in_fsm_pages = num_pages;
 
    /* prepare to put another buffer into the bistate */
    if (bistate && bistate->current_buf != InvalidBuffer)
    {
        ReleaseBuffer(bistate->current_buf);
        bistate->current_buf = InvalidBuffer;
    }
 
    /*
     * Extend the relation. We ask for the first returned page to be locked,
     * so that we are sure that nobody has inserted into the page
     * concurrently.
     *
     * With the current MAX_BUFFERS_TO_EXTEND_BY there's no danger of
     * [auto]vacuum trying to truncate later pages as REL_TRUNCATE_MINIMUM is
     * way larger.
     */
    first_block = ExtendBufferedRelBy(BMR_REL(relation), MAIN_FORKNUM,
                                      bistate ? bistate->strategy : NULL,
                                      EB_LOCK_FIRST,
                                      extend_by_pages,
                                      victim_buffers,
                                      &extend_by_pages);
    buffer = victim_buffers[0]; /* the buffer the function will return */
    last_block = first_block + (extend_by_pages - 1);
    Assert(first_block == BufferGetBlockNumber(buffer));
 
    /*
     * Relation is now extended. Initialize the page. We do this here, before
     * potentially releasing the lock on the page, because it allows us to
     * double check that the page contents are 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",
             first_block,
             RelationGetRelationName(relation));
 
    PageInit(page, BufferGetPageSize(buffer), 0);
    MarkBufferDirty(buffer);
 
    /*
     * If we decided to put pages into the FSM, release the buffer lock (but
     * not pin), we don't want to do IO while holding a buffer lock. This will
     * necessitate a bit more extensive checking in our caller.
     */
    if (use_fsm && not_in_fsm_pages < extend_by_pages)
    {
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
        *did_unlock = true;
    }
    else
        *did_unlock = false;
 
    /*
     * Relation is now extended. Release pins on all buffers, except for the
     * first (which we'll return).  If we decided to put pages into the FSM,
     * we can do that as part of the same loop.
     */
    for (uint32 i = 1; i < extend_by_pages; i++)
    {
        BlockNumber curBlock = first_block + i;
 
        Assert(curBlock == BufferGetBlockNumber(victim_buffers[i]));
        Assert(BlockNumberIsValid(curBlock));
 
        ReleaseBuffer(victim_buffers[i]);
 
        if (use_fsm && i >= not_in_fsm_pages)
        {
            Size        freespace = BufferGetPageSize(victim_buffers[i]) -
                SizeOfPageHeaderData;
 
            RecordPageWithFreeSpace(relation, curBlock, freespace);
        }
    }
 
    if (use_fsm && not_in_fsm_pages < extend_by_pages)
    {
        BlockNumber first_fsm_block = first_block + not_in_fsm_pages;
 
        FreeSpaceMapVacuumRange(relation, first_fsm_block, last_block);
    }
 
    if (bistate)
    {
        /*
         * Remember the additional pages we extended by, so we later can use
         * them without looking into the FSM.
         */
        if (extend_by_pages > 1)
        {
            bistate->next_free = first_block + 1;
            bistate->last_free = last_block;
        }
        else
        {
            bistate->next_free = InvalidBlockNumber;
            bistate->last_free = InvalidBlockNumber;
        }
 
        /* maintain bistate->current_buf */
        IncrBufferRefCount(buffer);
        bistate->current_buf = buffer;
        bistate->already_extended_by += extend_by_pages;
    }
 
    return buffer;
#undef MAX_BUFFERS_TO_EXTEND_BY
}
 
/*
 * RelationGetBufferForTuple
 *
 *  Returns pinned and exclusive-locked buffer of a page in given relation
 *  with free space >= given len.
 *
 *  If num_pages is > 1, we will try to extend the relation by at least that
 *  many pages when we decide to extend the relation. This is more efficient
 *  for callers that know they will need multiple pages
 *  (e.g. heap_multi_insert()).
 *
 *  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,
                          int num_pages)
{
    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        unlockedTargetBuffer;
    bool        recheckVmPins;
 
    len = MAXALIGN(len);        /* be conservative */
 
    /* if the caller doesn't know by how many pages to extend, extend by 1 */
    if (num_pages <= 0)
        num_pages = 1;
 
    /* 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 ensure 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.
         */
        GetVisibilityMapPins(relation, buffer, otherBuffer,
                             targetBlock, otherBlock, vmbuffer,
                             vmbuffer_other);
 
        /*
         * 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);
        }
 
        /* Is there an ongoing bulk extension? */
        if (bistate && bistate->next_free != InvalidBlockNumber)
        {
            Assert(bistate->next_free <= bistate->last_free);
 
            /*
             * We bulk extended the relation before, and there are still some
             * unused pages from that extension, so we don't need to look in
             * the FSM for a new page. But do record the free space from the
             * last page, somebody might insert narrower tuples later.
             */
            if (use_fsm)
                RecordPageWithFreeSpace(relation, targetBlock, pageFreeSpace);
 
            targetBlock = bistate->next_free;
            if (bistate->next_free >= bistate->last_free)
            {
                bistate->next_free = InvalidBlockNumber;
                bistate->last_free = InvalidBlockNumber;
            }
            else
                bistate->next_free++;
        }
        else if (!use_fsm)
        {
            /* Without FSM, always fall out of the loop and extend */
            break;
        }
        else
        {
            /*
             * 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 */
    buffer = RelationAddBlocks(relation, bistate, num_pages, use_fsm,
                               &unlockedTargetBuffer);
 
    targetBlock = BufferGetBlockNumber(buffer);
    page = BufferGetPage(buffer);
 
    /*
     * The page is empty, pin vmbuffer to set all_frozen bit. We don't want to
     * do IO while the buffer is locked, so we unlock the page first if IO is
     * needed (necessitating checks below).
     */
    if (options & HEAP_INSERT_FROZEN)
    {
        Assert(PageGetMaxOffsetNumber(page) == 0);
 
        if (!visibilitymap_pin_ok(targetBlock, *vmbuffer))
        {
            if (!unlockedTargetBuffer)
                LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
            unlockedTargetBuffer = true;
            visibilitymap_pin(relation, targetBlock, vmbuffer);
        }
    }
 
    /*
     * Reacquire locks if necessary.
     *
     * If the target buffer was unlocked above, or is unlocked while
     * reacquiring the lock on otherBuffer below, it's unlikely, but possible,
     * that another backend used space on this page. We check for that below,
     * and retry if necessary.
     */
    recheckVmPins = false;
    if (unlockedTargetBuffer)
    {
        /* released lock on target buffer above */
        if (otherBuffer != InvalidBuffer)
            LockBuffer(otherBuffer, BUFFER_LOCK_EXCLUSIVE);
        LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
        recheckVmPins = true;
    }
    else if (otherBuffer != InvalidBuffer)
    {
        /*
         * We did not release the target buffer, and otherBuffer is valid,
         * need to 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.
         *
         * 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.
         */
        Assert(otherBuffer != buffer);
        Assert(targetBlock > otherBlock);
 
        if (unlikely(!ConditionalLockBuffer(otherBuffer)))
        {
            unlockedTargetBuffer = true;
            LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
            LockBuffer(otherBuffer, BUFFER_LOCK_EXCLUSIVE);
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
        }
        recheckVmPins = true;
    }
 
    /*
     * If one of the buffers was unlocked (always the case if otherBuffer is
     * valid), it's possible, although unlikely, that an all-visible flag
     * became set.  We can use GetVisibilityMapPins to deal with that. It's
     * possible that GetVisibilityMapPins() might need to temporarily release
     * buffer locks, in which case we'll need to check if there's still enough
     * space on the page below.
     */
    if (recheckVmPins)
    {
        if (GetVisibilityMapPins(relation, otherBuffer, buffer,
                                 otherBlock, targetBlock, vmbuffer_other,
                                 vmbuffer))
            unlockedTargetBuffer = true;
    }
 
    /*
     * If the target buffer was temporarily unlocked since the relation
     * extension, it's possible, although unlikely, that all the space on the
     * page was already used. If so, we just retry from the start.  If we
     * didn't unlock, something has gone wrong if there's not enough space -
     * the test at the top should have prevented reaching this case.
     */
    pageFreeSpace = PageGetHeapFreeSpace(page);
    if (len > pageFreeSpace)
    {
        if (unlockedTargetBuffer)
        {
            if (otherBuffer != InvalidBuffer)
                LockBuffer(otherBuffer, BUFFER_LOCK_UNLOCK);
            UnlockReleaseBuffer(buffer);
 
            goto loop;
        }
        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, targetBlock);
 
    return buffer;
}