heapam.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * heapam.c
 *    heap access method code
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/access/heap/heapam.c
 *
 *
 * INTERFACE ROUTINES
 *      relation_open   - open any relation by relation OID
 *      relation_openrv - open any relation specified by a RangeVar
 *      relation_close  - close any relation
 *      heap_open       - open a heap relation by relation OID
 *      heap_openrv     - open a heap relation specified by a RangeVar
 *      heap_close      - (now just a macro for relation_close)
 *      heap_beginscan  - begin relation scan
 *      heap_rescan     - restart a relation scan
 *      heap_endscan    - end relation scan
 *      heap_getnext    - retrieve next tuple in scan
 *      heap_fetch      - retrieve tuple with given tid
 *      heap_insert     - insert tuple into a relation
 *      heap_multi_insert - insert multiple tuples into a relation
 *      heap_delete     - delete a tuple from a relation
 *      heap_update     - replace a tuple in a relation with another tuple
 *      heap_sync       - sync heap, for when no WAL has been written
 *
 * NOTES
 *    This file contains the heap_ routines which implement
 *    the POSTGRES heap access method used for all POSTGRES
 *    relations.
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/bufmask.h"
#include "access/heapam.h"
#include "access/heapam_xlog.h"
#include "access/hio.h"
#include "access/multixact.h"
#include "access/parallel.h"
#include "access/relscan.h"
#include "access/sysattr.h"
#include "access/transam.h"
#include "access/tuptoaster.h"
#include "access/valid.h"
#include "access/visibilitymap.h"
#include "access/xact.h"
#include "access/xlog.h"
#include "access/xloginsert.h"
#include "access/xlogutils.h"
#include "catalog/catalog.h"
#include "catalog/namespace.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "port/atomics.h"
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "storage/lmgr.h"
#include "storage/predicate.h"
#include "storage/procarray.h"
#include "storage/smgr.h"
#include "storage/spin.h"
#include "storage/standby.h"
#include "utils/datum.h"
#include "utils/inval.h"
#include "utils/lsyscache.h"
#include "utils/relcache.h"
#include "utils/snapmgr.h"
#include "utils/syscache.h"
#include "utils/tqual.h"


/* GUC variable */
bool        synchronize_seqscans = true;


static HeapScanDesc heap_beginscan_internal(Relation relation,
                        Snapshot snapshot,
                        int nkeys, ScanKey key,
                        ParallelHeapScanDesc parallel_scan,
                        bool allow_strat,
                        bool allow_sync,
                        bool allow_pagemode,
                        bool is_bitmapscan,
                        bool is_samplescan,
                        bool temp_snap);
static void heap_parallelscan_startblock_init(HeapScanDesc scan);
static BlockNumber heap_parallelscan_nextpage(HeapScanDesc scan);
static HeapTuple heap_prepare_insert(Relation relation, HeapTuple tup,
                    TransactionId xid, CommandId cid, int options);
static XLogRecPtr log_heap_update(Relation reln, Buffer oldbuf,
                Buffer newbuf, HeapTuple oldtup,
                HeapTuple newtup, HeapTuple old_key_tup,
                bool all_visible_cleared, bool new_all_visible_cleared);
static Bitmapset *HeapDetermineModifiedColumns(Relation relation,
                             Bitmapset *interesting_cols,
                             HeapTuple oldtup, HeapTuple newtup);
static bool heap_acquire_tuplock(Relation relation, ItemPointer tid,
                     LockTupleMode mode, LockWaitPolicy wait_policy,
                     bool *have_tuple_lock);
static void compute_new_xmax_infomask(TransactionId xmax, uint16 old_infomask,
                          uint16 old_infomask2, TransactionId add_to_xmax,
                          LockTupleMode mode, bool is_update,
                          TransactionId *result_xmax, uint16 *result_infomask,
                          uint16 *result_infomask2);
static HTSU_Result heap_lock_updated_tuple(Relation rel, HeapTuple tuple,
                        ItemPointer ctid, TransactionId xid,
                        LockTupleMode mode);
static void GetMultiXactIdHintBits(MultiXactId multi, uint16 *new_infomask,
                       uint16 *new_infomask2);
static TransactionId MultiXactIdGetUpdateXid(TransactionId xmax,
                        uint16 t_infomask);
static bool DoesMultiXactIdConflict(MultiXactId multi, uint16 infomask,
                        LockTupleMode lockmode);
static void MultiXactIdWait(MultiXactId multi, MultiXactStatus status, uint16 infomask,
                Relation rel, ItemPointer ctid, XLTW_Oper oper,
                int *remaining);
static bool ConditionalMultiXactIdWait(MultiXactId multi, MultiXactStatus status,
                           uint16 infomask, Relation rel, int *remaining);
static XLogRecPtr log_heap_new_cid(Relation relation, HeapTuple tup);
static HeapTuple ExtractReplicaIdentity(Relation rel, HeapTuple tup, bool key_modified,
                       bool *copy);


/*
 * Each tuple lock mode has a corresponding heavyweight lock, and one or two
 * corresponding MultiXactStatuses (one to merely lock tuples, another one to
 * update them).  This table (and the macros below) helps us determine the
 * heavyweight lock mode and MultiXactStatus values to use for any particular
 * tuple lock strength.
 *
 * Don't look at lockstatus/updstatus directly!  Use get_mxact_status_for_lock
 * instead.
 */
static const struct
{
    LOCKMODE    hwlock;
    int         lockstatus;
    int         updstatus;
}

            tupleLockExtraInfo[MaxLockTupleMode + 1] =
{
    {                           /* LockTupleKeyShare */
        AccessShareLock,
        MultiXactStatusForKeyShare,
        -1                      /* KeyShare does not allow updating tuples */
    },
    {                           /* LockTupleShare */
        RowShareLock,
        MultiXactStatusForShare,
        -1                      /* Share does not allow updating tuples */
    },
    {                           /* LockTupleNoKeyExclusive */
        ExclusiveLock,
        MultiXactStatusForNoKeyUpdate,
        MultiXactStatusNoKeyUpdate
    },
    {                           /* LockTupleExclusive */
        AccessExclusiveLock,
        MultiXactStatusForUpdate,
        MultiXactStatusUpdate
    }
};

/* Get the LOCKMODE for a given MultiXactStatus */
#define LOCKMODE_from_mxstatus(status) \
            (tupleLockExtraInfo[TUPLOCK_from_mxstatus((status))].hwlock)

/*
 * Acquire heavyweight locks on tuples, using a LockTupleMode strength value.
 * This is more readable than having every caller translate it to lock.h's
 * LOCKMODE.
 */
#define LockTupleTuplock(rel, tup, mode) \
    LockTuple((rel), (tup), tupleLockExtraInfo[mode].hwlock)
#define UnlockTupleTuplock(rel, tup, mode) \
    UnlockTuple((rel), (tup), tupleLockExtraInfo[mode].hwlock)
#define ConditionalLockTupleTuplock(rel, tup, mode) \
    ConditionalLockTuple((rel), (tup), tupleLockExtraInfo[mode].hwlock)

/*
 * This table maps tuple lock strength values for each particular
 * MultiXactStatus value.
 */
static const int MultiXactStatusLock[MaxMultiXactStatus + 1] =
{
    LockTupleKeyShare,          /* ForKeyShare */
    LockTupleShare,             /* ForShare */
    LockTupleNoKeyExclusive,    /* ForNoKeyUpdate */
    LockTupleExclusive,         /* ForUpdate */
    LockTupleNoKeyExclusive,    /* NoKeyUpdate */
    LockTupleExclusive          /* Update */
};

/* Get the LockTupleMode for a given MultiXactStatus */
#define TUPLOCK_from_mxstatus(status) \
            (MultiXactStatusLock[(status)])

/* ----------------------------------------------------------------
 *                       heap support routines
 * ----------------------------------------------------------------
 */

/* ----------------
 *      initscan - scan code common to heap_beginscan and heap_rescan
 * ----------------
 */
static void
initscan(HeapScanDesc scan, ScanKey key, bool keep_startblock)
{
    bool        allow_strat;
    bool        allow_sync;

    /*
     * Determine the number of blocks we have to scan.
     *
     * It is sufficient to do this once at scan start, since any tuples added
     * while the scan is in progress will be invisible to my snapshot anyway.
     * (That is not true when using a non-MVCC snapshot.  However, we couldn't
     * guarantee to return tuples added after scan start anyway, since they
     * might go into pages we already scanned.  To guarantee consistent
     * results for a non-MVCC snapshot, the caller must hold some higher-level
     * lock that ensures the interesting tuple(s) won't change.)
     */
    if (scan->rs_parallel != NULL)
        scan->rs_nblocks = scan->rs_parallel->phs_nblocks;
    else
        scan->rs_nblocks = RelationGetNumberOfBlocks(scan->rs_rd);

    /*
     * If the table is large relative to NBuffers, use a bulk-read access
     * strategy and enable synchronized scanning (see syncscan.c).  Although
     * the thresholds for these features could be different, we make them the
     * same so that there are only two behaviors to tune rather than four.
     * (However, some callers need to be able to disable one or both of these
     * behaviors, independently of the size of the table; also there is a GUC
     * variable that can disable synchronized scanning.)
     *
     * Note that heap_parallelscan_initialize has a very similar test; if you
     * change this, consider changing that one, too.
     */
    if (!RelationUsesLocalBuffers(scan->rs_rd) &&
        scan->rs_nblocks > NBuffers / 4)
    {
        allow_strat = scan->rs_allow_strat;
        allow_sync = scan->rs_allow_sync;
    }
    else
        allow_strat = allow_sync = false;

    if (allow_strat)
    {
        /* During a rescan, keep the previous strategy object. */
        if (scan->rs_strategy == NULL)
            scan->rs_strategy = GetAccessStrategy(BAS_BULKREAD);
    }
    else
    {
        if (scan->rs_strategy != NULL)
            FreeAccessStrategy(scan->rs_strategy);
        scan->rs_strategy = NULL;
    }

    if (scan->rs_parallel != NULL)
    {
        /* For parallel scan, believe whatever ParallelHeapScanDesc says. */
        scan->rs_syncscan = scan->rs_parallel->phs_syncscan;
    }
    else if (keep_startblock)
    {
        /*
         * When rescanning, we want to keep the previous startblock setting,
         * so that rewinding a cursor doesn't generate surprising results.
         * Reset the active syncscan setting, though.
         */
        scan->rs_syncscan = (allow_sync && synchronize_seqscans);
    }
    else if (allow_sync && synchronize_seqscans)
    {
        scan->rs_syncscan = true;
        scan->rs_startblock = ss_get_location(scan->rs_rd, scan->rs_nblocks);
    }
    else
    {
        scan->rs_syncscan = false;
        scan->rs_startblock = 0;
    }

    scan->rs_numblocks = InvalidBlockNumber;
    scan->rs_inited = false;
    scan->rs_ctup.t_data = NULL;
    ItemPointerSetInvalid(&scan->rs_ctup.t_self);
    scan->rs_cbuf = InvalidBuffer;
    scan->rs_cblock = InvalidBlockNumber;

    /* page-at-a-time fields are always invalid when not rs_inited */

    /*
     * copy the scan key, if appropriate
     */
    if (key != NULL)
        memcpy(scan->rs_key, key, scan->rs_nkeys * sizeof(ScanKeyData));

    /*
     * Currently, we don't have a stats counter for bitmap heap scans (but the
     * underlying bitmap index scans will be counted) or sample scans (we only
     * update stats for tuple fetches there)
     */
    if (!scan->rs_bitmapscan && !scan->rs_samplescan)
        pgstat_count_heap_scan(scan->rs_rd);
}

/*
 * heap_setscanlimits - restrict range of a heapscan
 *
 * startBlk is the page to start at
 * numBlks is number of pages to scan (InvalidBlockNumber means "all")
 */
void
heap_setscanlimits(HeapScanDesc scan, BlockNumber startBlk, BlockNumber numBlks)
{
    Assert(!scan->rs_inited);   /* else too late to change */
    Assert(!scan->rs_syncscan); /* else rs_startblock is significant */

    /* Check startBlk is valid (but allow case of zero blocks...) */
    Assert(startBlk == 0 || startBlk < scan->rs_nblocks);

    scan->rs_startblock = startBlk;
    scan->rs_numblocks = numBlks;
}

/*
 * heapgetpage - subroutine for heapgettup()
 *
 * This routine reads and pins the specified page of the relation.
 * In page-at-a-time mode it performs additional work, namely determining
 * which tuples on the page are visible.
 */
void
heapgetpage(HeapScanDesc scan, BlockNumber page)
{
    Buffer      buffer;
    Snapshot    snapshot;
    Page        dp;
    int         lines;
    int         ntup;
    OffsetNumber lineoff;
    ItemId      lpp;
    bool        all_visible;

    Assert(page < scan->rs_nblocks);

    /* release previous scan buffer, if any */
    if (BufferIsValid(scan->rs_cbuf))
    {
        ReleaseBuffer(scan->rs_cbuf);
        scan->rs_cbuf = InvalidBuffer;
    }

    /*
     * Be sure to check for interrupts at least once per page.  Checks at
     * higher code levels won't be able to stop a seqscan that encounters many
     * pages' worth of consecutive dead tuples.
     */
    CHECK_FOR_INTERRUPTS();

    /* read page using selected strategy */
    scan->rs_cbuf = ReadBufferExtended(scan->rs_rd, MAIN_FORKNUM, page,
                                       RBM_NORMAL, scan->rs_strategy);
    scan->rs_cblock = page;

    if (!scan->rs_pageatatime)
        return;

    buffer = scan->rs_cbuf;
    snapshot = scan->rs_snapshot;

    /*
     * Prune and repair fragmentation for the whole page, if possible.
     */
    heap_page_prune_opt(scan->rs_rd, buffer);

    /*
     * We must hold share lock on the buffer content while examining tuple
     * visibility.  Afterwards, however, the tuples we have found to be
     * visible are guaranteed good as long as we hold the buffer pin.
     */
    LockBuffer(buffer, BUFFER_LOCK_SHARE);

    dp = BufferGetPage(buffer);
    TestForOldSnapshot(snapshot, scan->rs_rd, dp);
    lines = PageGetMaxOffsetNumber(dp);
    ntup = 0;

    /*
     * If the all-visible flag indicates that all tuples on the page are
     * visible to everyone, we can skip the per-tuple visibility tests.
     *
     * Note: In hot standby, a tuple that's already visible to all
     * transactions in the master might still be invisible to a read-only
     * transaction in the standby. We partly handle this problem by tracking
     * the minimum xmin of visible tuples as the cut-off XID while marking a
     * page all-visible on master and WAL log that along with the visibility
     * map SET operation. In hot standby, we wait for (or abort) all
     * transactions that can potentially may not see one or more tuples on the
     * page. That's how index-only scans work fine in hot standby. A crucial
     * difference between index-only scans and heap scans is that the
     * index-only scan completely relies on the visibility map where as heap
     * scan looks at the page-level PD_ALL_VISIBLE flag. We are not sure if
     * the page-level flag can be trusted in the same way, because it might
     * get propagated somehow without being explicitly WAL-logged, e.g. via a
     * full page write. Until we can prove that beyond doubt, let's check each
     * tuple for visibility the hard way.
     */
    all_visible = PageIsAllVisible(dp) && !snapshot->takenDuringRecovery;

    for (lineoff = FirstOffsetNumber, lpp = PageGetItemId(dp, lineoff);
         lineoff <= lines;
         lineoff++, lpp++)
    {
        if (ItemIdIsNormal(lpp))
        {
            HeapTupleData loctup;
            bool        valid;

            loctup.t_tableOid = RelationGetRelid(scan->rs_rd);
            loctup.t_data = (HeapTupleHeader) PageGetItem((Page) dp, lpp);
            loctup.t_len = ItemIdGetLength(lpp);
            ItemPointerSet(&(loctup.t_self), page, lineoff);

            if (all_visible)
                valid = true;
            else
                valid = HeapTupleSatisfiesVisibility(&loctup, snapshot, buffer);

            CheckForSerializableConflictOut(valid, scan->rs_rd, &loctup,
                                            buffer, snapshot);

            if (valid)
                scan->rs_vistuples[ntup++] = lineoff;
        }
    }

    LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

    Assert(ntup <= MaxHeapTuplesPerPage);
    scan->rs_ntuples = ntup;
}

/* ----------------
 *      heapgettup - fetch next heap tuple
 *
 *      Initialize the scan if not already done; then advance to the next
 *      tuple as indicated by "dir"; return the next tuple in scan->rs_ctup,
 *      or set scan->rs_ctup.t_data = NULL if no more tuples.
 *
 * dir == NoMovementScanDirection means "re-fetch the tuple indicated
 * by scan->rs_ctup".
 *
 * Note: the reason nkeys/key are passed separately, even though they are
 * kept in the scan descriptor, is that the caller may not want us to check
 * the scankeys.
 *
 * Note: when we fall off the end of the scan in either direction, we
 * reset rs_inited.  This means that a further request with the same
 * scan direction will restart the scan, which is a bit odd, but a
 * request with the opposite scan direction will start a fresh scan
 * in the proper direction.  The latter is required behavior for cursors,
 * while the former case is generally undefined behavior in Postgres
 * so we don't care too much.
 * ----------------
 */
static void
heapgettup(HeapScanDesc scan,
           ScanDirection dir,
           int nkeys,
           ScanKey key)
{
    HeapTuple   tuple = &(scan->rs_ctup);
    Snapshot    snapshot = scan->rs_snapshot;
    bool        backward = ScanDirectionIsBackward(dir);
    BlockNumber page;
    bool        finished;
    Page        dp;
    int         lines;
    OffsetNumber lineoff;
    int         linesleft;
    ItemId      lpp;

    /*
     * calculate next starting lineoff, given scan direction
     */
    if (ScanDirectionIsForward(dir))
    {
        if (!scan->rs_inited)
        {
            /*
             * return null immediately if relation is empty
             */
            if (scan->rs_nblocks == 0 || scan->rs_numblocks == 0)
            {
                Assert(!BufferIsValid(scan->rs_cbuf));
                tuple->t_data = NULL;
                return;
            }
            if (scan->rs_parallel != NULL)
            {
                heap_parallelscan_startblock_init(scan);

                page = heap_parallelscan_nextpage(scan);

                /* Other processes might have already finished the scan. */
                if (page == InvalidBlockNumber)
                {
                    Assert(!BufferIsValid(scan->rs_cbuf));
                    tuple->t_data = NULL;
                    return;
                }
            }
            else
                page = scan->rs_startblock; /* first page */
            heapgetpage(scan, page);
            lineoff = FirstOffsetNumber;    /* first offnum */
            scan->rs_inited = true;
        }
        else
        {
            /* continue from previously returned page/tuple */
            page = scan->rs_cblock; /* current page */
            lineoff =           /* next offnum */
                OffsetNumberNext(ItemPointerGetOffsetNumber(&(tuple->t_self)));
        }

        LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);

        dp = BufferGetPage(scan->rs_cbuf);
        TestForOldSnapshot(snapshot, scan->rs_rd, dp);
        lines = PageGetMaxOffsetNumber(dp);
        /* page and lineoff now reference the physically next tid */

        linesleft = lines - lineoff + 1;
    }
    else if (backward)
    {
        /* backward parallel scan not supported */
        Assert(scan->rs_parallel == NULL);

        if (!scan->rs_inited)
        {
            /*
             * return null immediately if relation is empty
             */
            if (scan->rs_nblocks == 0 || scan->rs_numblocks == 0)
            {
                Assert(!BufferIsValid(scan->rs_cbuf));
                tuple->t_data = NULL;
                return;
            }

            /*
             * Disable reporting to syncscan logic in a backwards scan; it's
             * not very likely anyone else is doing the same thing at the same
             * time, and much more likely that we'll just bollix things for
             * forward scanners.
             */
            scan->rs_syncscan = false;
            /* start from last page of the scan */
            if (scan->rs_startblock > 0)
                page = scan->rs_startblock - 1;
            else
                page = scan->rs_nblocks - 1;
            heapgetpage(scan, page);
        }
        else
        {
            /* continue from previously returned page/tuple */
            page = scan->rs_cblock; /* current page */
        }

        LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);

        dp = BufferGetPage(scan->rs_cbuf);
        TestForOldSnapshot(snapshot, scan->rs_rd, dp);
        lines = PageGetMaxOffsetNumber(dp);

        if (!scan->rs_inited)
        {
            lineoff = lines;    /* final offnum */
            scan->rs_inited = true;
        }
        else
        {
            lineoff =           /* previous offnum */
                OffsetNumberPrev(ItemPointerGetOffsetNumber(&(tuple->t_self)));
        }
        /* page and lineoff now reference the physically previous tid */

        linesleft = lineoff;
    }
    else
    {
        /*
         * ``no movement'' scan direction: refetch prior tuple
         */
        if (!scan->rs_inited)
        {
            Assert(!BufferIsValid(scan->rs_cbuf));
            tuple->t_data = NULL;
            return;
        }

        page = ItemPointerGetBlockNumber(&(tuple->t_self));
        if (page != scan->rs_cblock)
            heapgetpage(scan, page);

        /* Since the tuple was previously fetched, needn't lock page here */
        dp = BufferGetPage(scan->rs_cbuf);
        TestForOldSnapshot(snapshot, scan->rs_rd, dp);
        lineoff = ItemPointerGetOffsetNumber(&(tuple->t_self));
        lpp = PageGetItemId(dp, lineoff);
        Assert(ItemIdIsNormal(lpp));

        tuple->t_data = (HeapTupleHeader) PageGetItem((Page) dp, lpp);
        tuple->t_len = ItemIdGetLength(lpp);

        return;
    }

    /*
     * advance the scan until we find a qualifying tuple or run out of stuff
     * to scan
     */
    lpp = PageGetItemId(dp, lineoff);
    for (;;)
    {
        while (linesleft > 0)
        {
            if (ItemIdIsNormal(lpp))
            {
                bool        valid;

                tuple->t_data = (HeapTupleHeader) PageGetItem((Page) dp, lpp);
                tuple->t_len = ItemIdGetLength(lpp);
                ItemPointerSet(&(tuple->t_self), page, lineoff);

                /*
                 * if current tuple qualifies, return it.
                 */
                valid = HeapTupleSatisfiesVisibility(tuple,
                                                     snapshot,
                                                     scan->rs_cbuf);

                CheckForSerializableConflictOut(valid, scan->rs_rd, tuple,
                                                scan->rs_cbuf, snapshot);

                if (valid && key != NULL)
                    HeapKeyTest(tuple, RelationGetDescr(scan->rs_rd),
                                nkeys, key, valid);

                if (valid)
                {
                    LockBuffer(scan->rs_cbuf, BUFFER_LOCK_UNLOCK);
                    return;
                }
            }

            /*
             * otherwise move to the next item on the page
             */
            --linesleft;
            if (backward)
            {
                --lpp;          /* move back in this page's ItemId array */
                --lineoff;
            }
            else
            {
                ++lpp;          /* move forward in this page's ItemId array */
                ++lineoff;
            }
        }

        /*
         * if we get here, it means we've exhausted the items on this page and
         * it's time to move to the next.
         */
        LockBuffer(scan->rs_cbuf, BUFFER_LOCK_UNLOCK);

        /*
         * advance to next/prior page and detect end of scan
         */
        if (backward)
        {
            finished = (page == scan->rs_startblock) ||
                (scan->rs_numblocks != InvalidBlockNumber ? --scan->rs_numblocks == 0 : false);
            if (page == 0)
                page = scan->rs_nblocks;
            page--;
        }
        else if (scan->rs_parallel != NULL)
        {
            page = heap_parallelscan_nextpage(scan);
            finished = (page == InvalidBlockNumber);
        }
        else
        {
            page++;
            if (page >= scan->rs_nblocks)
                page = 0;
            finished = (page == scan->rs_startblock) ||
                (scan->rs_numblocks != InvalidBlockNumber ? --scan->rs_numblocks == 0 : false);

            /*
             * Report our new scan position for synchronization purposes. We
             * don't do that when moving backwards, however. That would just
             * mess up any other forward-moving scanners.
             *
             * Note: we do this before checking for end of scan so that the
             * final state of the position hint is back at the start of the
             * rel.  That's not strictly necessary, but otherwise when you run
             * the same query multiple times the starting position would shift
             * a little bit backwards on every invocation, which is confusing.
             * We don't guarantee any specific ordering in general, though.
             */
            if (scan->rs_syncscan)
                ss_report_location(scan->rs_rd, page);
        }

        /*
         * return NULL if we've exhausted all the pages
         */
        if (finished)
        {
            if (BufferIsValid(scan->rs_cbuf))
                ReleaseBuffer(scan->rs_cbuf);
            scan->rs_cbuf = InvalidBuffer;
            scan->rs_cblock = InvalidBlockNumber;
            tuple->t_data = NULL;
            scan->rs_inited = false;
            return;
        }

        heapgetpage(scan, page);

        LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);

        dp = BufferGetPage(scan->rs_cbuf);
        TestForOldSnapshot(snapshot, scan->rs_rd, dp);
        lines = PageGetMaxOffsetNumber((Page) dp);
        linesleft = lines;
        if (backward)
        {
            lineoff = lines;
            lpp = PageGetItemId(dp, lines);
        }
        else
        {
            lineoff = FirstOffsetNumber;
            lpp = PageGetItemId(dp, FirstOffsetNumber);
        }
    }
}

/* ----------------
 *      heapgettup_pagemode - fetch next heap tuple in page-at-a-time mode
 *
 *      Same API as heapgettup, but used in page-at-a-time mode
 *
 * The internal logic is much the same as heapgettup's too, but there are some
 * differences: we do not take the buffer content lock (that only needs to
 * happen inside heapgetpage), and we iterate through just the tuples listed
 * in rs_vistuples[] rather than all tuples on the page.  Notice that
 * lineindex is 0-based, where the corresponding loop variable lineoff in
 * heapgettup is 1-based.
 * ----------------
 */
static void
heapgettup_pagemode(HeapScanDesc scan,
                    ScanDirection dir,
                    int nkeys,
                    ScanKey key)
{
    HeapTuple   tuple = &(scan->rs_ctup);
    bool        backward = ScanDirectionIsBackward(dir);
    BlockNumber page;
    bool        finished;
    Page        dp;
    int         lines;
    int         lineindex;
    OffsetNumber lineoff;
    int         linesleft;
    ItemId      lpp;

    /*
     * calculate next starting lineindex, given scan direction
     */
    if (ScanDirectionIsForward(dir))
    {
        if (!scan->rs_inited)
        {
            /*
             * return null immediately if relation is empty
             */
            if (scan->rs_nblocks == 0 || scan->rs_numblocks == 0)
            {
                Assert(!BufferIsValid(scan->rs_cbuf));
                tuple->t_data = NULL;
                return;
            }
            if (scan->rs_parallel != NULL)
            {
                heap_parallelscan_startblock_init(scan);

                page = heap_parallelscan_nextpage(scan);

                /* Other processes might have already finished the scan. */
                if (page == InvalidBlockNumber)
                {
                    Assert(!BufferIsValid(scan->rs_cbuf));
                    tuple->t_data = NULL;
                    return;
                }
            }
            else
                page = scan->rs_startblock; /* first page */
            heapgetpage(scan, page);
            lineindex = 0;
            scan->rs_inited = true;
        }
        else
        {
            /* continue from previously returned page/tuple */
            page = scan->rs_cblock; /* current page */
            lineindex = scan->rs_cindex + 1;
        }

        dp = BufferGetPage(scan->rs_cbuf);
        TestForOldSnapshot(scan->rs_snapshot, scan->rs_rd, dp);
        lines = scan->rs_ntuples;
        /* page and lineindex now reference the next visible tid */

        linesleft = lines - lineindex;
    }
    else if (backward)
    {
        /* backward parallel scan not supported */
        Assert(scan->rs_parallel == NULL);

        if (!scan->rs_inited)
        {
            /*
             * return null immediately if relation is empty
             */
            if (scan->rs_nblocks == 0 || scan->rs_numblocks == 0)
            {
                Assert(!BufferIsValid(scan->rs_cbuf));
                tuple->t_data = NULL;
                return;
            }

            /*
             * Disable reporting to syncscan logic in a backwards scan; it's
             * not very likely anyone else is doing the same thing at the same
             * time, and much more likely that we'll just bollix things for
             * forward scanners.
             */
            scan->rs_syncscan = false;
            /* start from last page of the scan */
            if (scan->rs_startblock > 0)
                page = scan->rs_startblock - 1;
            else
                page = scan->rs_nblocks - 1;
            heapgetpage(scan, page);
        }
        else
        {
            /* continue from previously returned page/tuple */
            page = scan->rs_cblock; /* current page */
        }

        dp = BufferGetPage(scan->rs_cbuf);
        TestForOldSnapshot(scan->rs_snapshot, scan->rs_rd, dp);
        lines = scan->rs_ntuples;

        if (!scan->rs_inited)
        {
            lineindex = lines - 1;
            scan->rs_inited = true;
        }
        else
        {
            lineindex = scan->rs_cindex - 1;
        }
        /* page and lineindex now reference the previous visible tid */

        linesleft = lineindex + 1;
    }
    else
    {
        /*
         * ``no movement'' scan direction: refetch prior tuple
         */
        if (!scan->rs_inited)
        {
            Assert(!BufferIsValid(scan->rs_cbuf));
            tuple->t_data = NULL;
            return;
        }

        page = ItemPointerGetBlockNumber(&(tuple->t_self));
        if (page != scan->rs_cblock)
            heapgetpage(scan, page);

        /* Since the tuple was previously fetched, needn't lock page here */
        dp = BufferGetPage(scan->rs_cbuf);
        TestForOldSnapshot(scan->rs_snapshot, scan->rs_rd, dp);
        lineoff = ItemPointerGetOffsetNumber(&(tuple->t_self));
        lpp = PageGetItemId(dp, lineoff);
        Assert(ItemIdIsNormal(lpp));

        tuple->t_data = (HeapTupleHeader) PageGetItem((Page) dp, lpp);
        tuple->t_len = ItemIdGetLength(lpp);

        /* check that rs_cindex is in sync */
        Assert(scan->rs_cindex < scan->rs_ntuples);
        Assert(lineoff == scan->rs_vistuples[scan->rs_cindex]);

        return;
    }

    /*
     * advance the scan until we find a qualifying tuple or run out of stuff
     * to scan
     */
    for (;;)
    {
        while (linesleft > 0)
        {
            lineoff = scan->rs_vistuples[lineindex];
            lpp = PageGetItemId(dp, lineoff);
            Assert(ItemIdIsNormal(lpp));

            tuple->t_data = (HeapTupleHeader) PageGetItem((Page) dp, lpp);
            tuple->t_len = ItemIdGetLength(lpp);
            ItemPointerSet(&(tuple->t_self), page, lineoff);

            /*
             * if current tuple qualifies, return it.
             */
            if (key != NULL)
            {
                bool        valid;

                HeapKeyTest(tuple, RelationGetDescr(scan->rs_rd),
                            nkeys, key, valid);
                if (valid)
                {
                    scan->rs_cindex = lineindex;
                    return;
                }
            }
            else
            {
                scan->rs_cindex = lineindex;
                return;
            }

            /*
             * otherwise move to the next item on the page
             */
            --linesleft;
            if (backward)
                --lineindex;
            else
                ++lineindex;
        }

        /*
         * if we get here, it means we've exhausted the items on this page and
         * it's time to move to the next.
         */
        if (backward)
        {
            finished = (page == scan->rs_startblock) ||
                (scan->rs_numblocks != InvalidBlockNumber ? --scan->rs_numblocks == 0 : false);
            if (page == 0)
                page = scan->rs_nblocks;
            page--;
        }
        else if (scan->rs_parallel != NULL)
        {
            page = heap_parallelscan_nextpage(scan);
            finished = (page == InvalidBlockNumber);
        }
        else
        {
            page++;
            if (page >= scan->rs_nblocks)
                page = 0;
            finished = (page == scan->rs_startblock) ||
                (scan->rs_numblocks != InvalidBlockNumber ? --scan->rs_numblocks == 0 : false);

            /*
             * Report our new scan position for synchronization purposes. We
             * don't do that when moving backwards, however. That would just
             * mess up any other forward-moving scanners.
             *
             * Note: we do this before checking for end of scan so that the
             * final state of the position hint is back at the start of the
             * rel.  That's not strictly necessary, but otherwise when you run
             * the same query multiple times the starting position would shift
             * a little bit backwards on every invocation, which is confusing.
             * We don't guarantee any specific ordering in general, though.
             */
            if (scan->rs_syncscan)
                ss_report_location(scan->rs_rd, page);
        }

        /*
         * return NULL if we've exhausted all the pages
         */
        if (finished)
        {
            if (BufferIsValid(scan->rs_cbuf))
                ReleaseBuffer(scan->rs_cbuf);
            scan->rs_cbuf = InvalidBuffer;
            scan->rs_cblock = InvalidBlockNumber;
            tuple->t_data = NULL;
            scan->rs_inited = false;
            return;
        }

        heapgetpage(scan, page);

        dp = BufferGetPage(scan->rs_cbuf);
        TestForOldSnapshot(scan->rs_snapshot, scan->rs_rd, dp);
        lines = scan->rs_ntuples;
        linesleft = lines;
        if (backward)
            lineindex = lines - 1;
        else
            lineindex = 0;
    }
}


#if defined(DISABLE_COMPLEX_MACRO)
/*
 * This is formatted so oddly so that the correspondence to the macro
 * definition in access/htup_details.h is maintained.
 */
Datum
fastgetattr(HeapTuple tup, int attnum, TupleDesc tupleDesc,
            bool *isnull)
{
    return (
            (attnum) > 0 ?
            (
             (*(isnull) = false),
             HeapTupleNoNulls(tup) ?
             (
              TupleDescAttr((tupleDesc), (attnum) - 1)->attcacheoff >= 0 ?
              (
               fetchatt(TupleDescAttr((tupleDesc), (attnum) - 1),
                        (char *) (tup)->t_data + (tup)->t_data->t_hoff +
                        TupleDescAttr((tupleDesc), (attnum) - 1)->attcacheoff)
               )
              :
              nocachegetattr((tup), (attnum), (tupleDesc))
              )
             :
             (
              att_isnull((attnum) - 1, (tup)->t_data->t_bits) ?
              (
               (*(isnull) = true),
               (Datum) NULL
               )
              :
              (
               nocachegetattr((tup), (attnum), (tupleDesc))
               )
              )
             )
            :
            (
             (Datum) NULL
             )
        );
}
#endif                          /* defined(DISABLE_COMPLEX_MACRO) */


/* ----------------------------------------------------------------
 *                   heap access method interface
 * ----------------------------------------------------------------
 */

/* ----------------
 *      relation_open - open any relation by relation OID
 *
 *      If lockmode is not "NoLock", the specified kind of lock is
 *      obtained on the relation.  (Generally, NoLock should only be
 *      used if the caller knows it has some appropriate lock on the
 *      relation already.)
 *
 *      An error is raised if the relation does not exist.
 *
 *      NB: a "relation" is anything with a pg_class entry.  The caller is
 *      expected to check whether the relkind is something it can handle.
 * ----------------
 */
Relation
relation_open(Oid relationId, LOCKMODE lockmode)
{
    Relation    r;

    Assert(lockmode >= NoLock && lockmode < MAX_LOCKMODES);

    /* Get the lock before trying to open the relcache entry */
    if (lockmode != NoLock)
        LockRelationOid(relationId, lockmode);

    /* The relcache does all the real work... */
    r = RelationIdGetRelation(relationId);

    if (!RelationIsValid(r))
        elog(ERROR, "could not open relation with OID %u", relationId);

    /* Make note that we've accessed a temporary relation */
    if (RelationUsesLocalBuffers(r))
        MyXactFlags |= XACT_FLAGS_ACCESSEDTEMPREL;

    pgstat_initstats(r);

    return r;
}

/* ----------------
 *      try_relation_open - open any relation by relation OID
 *
 *      Same as relation_open, except return NULL instead of failing
 *      if the relation does not exist.
 * ----------------
 */
Relation
try_relation_open(Oid relationId, LOCKMODE lockmode)
{
    Relation    r;

    Assert(lockmode >= NoLock && lockmode < MAX_LOCKMODES);

    /* Get the lock first */
    if (lockmode != NoLock)
        LockRelationOid(relationId, lockmode);

    /*
     * Now that we have the lock, probe to see if the relation really exists
     * or not.
     */
    if (!SearchSysCacheExists1(RELOID, ObjectIdGetDatum(relationId)))
    {
        /* Release useless lock */
        if (lockmode != NoLock)
            UnlockRelationOid(relationId, lockmode);

        return NULL;
    }

    /* Should be safe to do a relcache load */
    r = RelationIdGetRelation(relationId);

    if (!RelationIsValid(r))
        elog(ERROR, "could not open relation with OID %u", relationId);

    /* Make note that we've accessed a temporary relation */
    if (RelationUsesLocalBuffers(r))
        MyXactFlags |= XACT_FLAGS_ACCESSEDTEMPREL;

    pgstat_initstats(r);

    return r;
}

/* ----------------
 *      relation_openrv - open any relation specified by a RangeVar
 *
 *      Same as relation_open, but the relation is specified by a RangeVar.
 * ----------------
 */
Relation
relation_openrv(const RangeVar *relation, LOCKMODE lockmode)
{
    Oid         relOid;

    /*
     * Check for shared-cache-inval messages before trying to open the
     * relation.  This is needed even if we already hold a lock on the
     * relation, because GRANT/REVOKE are executed without taking any lock on
     * the target relation, and we want to be sure we see current ACL
     * information.  We can skip this if asked for NoLock, on the assumption
     * that such a call is not the first one in the current command, and so we
     * should be reasonably up-to-date already.  (XXX this all could stand to
     * be redesigned, but for the moment we'll keep doing this like it's been
     * done historically.)
     */
    if (lockmode != NoLock)
        AcceptInvalidationMessages();

    /* Look up and lock the appropriate relation using namespace search */
    relOid = RangeVarGetRelid(relation, lockmode, false);

    /* Let relation_open do the rest */
    return relation_open(relOid, NoLock);
}

/* ----------------
 *      relation_openrv_extended - open any relation specified by a RangeVar
 *
 *      Same as relation_openrv, but with an additional missing_ok argument
 *      allowing a NULL return rather than an error if the relation is not
 *      found.  (Note that some other causes, such as permissions problems,
 *      will still result in an ereport.)
 * ----------------
 */
Relation
relation_openrv_extended(const RangeVar *relation, LOCKMODE lockmode,
                         bool missing_ok)
{
    Oid         relOid;

    /*
     * Check for shared-cache-inval messages before trying to open the
     * relation.  See comments in relation_openrv().
     */
    if (lockmode != NoLock)
        AcceptInvalidationMessages();

    /* Look up and lock the appropriate relation using namespace search */
    relOid = RangeVarGetRelid(relation, lockmode, missing_ok);

    /* Return NULL on not-found */
    if (!OidIsValid(relOid))
        return NULL;

    /* Let relation_open do the rest */
    return relation_open(relOid, NoLock);
}

/* ----------------
 *      relation_close - close any relation
 *
 *      If lockmode is not "NoLock", we then release the specified lock.
 *
 *      Note that it is often sensible to hold a lock beyond relation_close;
 *      in that case, the lock is released automatically at xact end.
 * ----------------
 */
void
relation_close(Relation relation, LOCKMODE lockmode)
{
    LockRelId   relid = relation->rd_lockInfo.lockRelId;

    Assert(lockmode >= NoLock && lockmode < MAX_LOCKMODES);

    /* The relcache does the real work... */
    RelationClose(relation);

    if (lockmode != NoLock)
        UnlockRelationId(&relid, lockmode);
}


/* ----------------
 *      heap_open - open a heap relation by relation OID
 *
 *      This is essentially relation_open plus check that the relation
 *      is not an index nor a composite type.  (The caller should also
 *      check that it's not a view or foreign table before assuming it has
 *      storage.)
 * ----------------
 */
Relation
heap_open(Oid relationId, LOCKMODE lockmode)
{
    Relation    r;

    r = relation_open(relationId, lockmode);

    if (r->rd_rel->relkind == RELKIND_INDEX)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("\"%s\" is an index",
                        RelationGetRelationName(r))));
    else if (r->rd_rel->relkind == RELKIND_COMPOSITE_TYPE)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("\"%s\" is a composite type",
                        RelationGetRelationName(r))));

    return r;
}

/* ----------------
 *      heap_openrv - open a heap relation specified
 *      by a RangeVar node
 *
 *      As above, but relation is specified by a RangeVar.
 * ----------------
 */
Relation
heap_openrv(const RangeVar *relation, LOCKMODE lockmode)
{
    Relation    r;

    r = relation_openrv(relation, lockmode);

    if (r->rd_rel->relkind == RELKIND_INDEX)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("\"%s\" is an index",
                        RelationGetRelationName(r))));
    else if (r->rd_rel->relkind == RELKIND_COMPOSITE_TYPE)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("\"%s\" is a composite type",
                        RelationGetRelationName(r))));

    return r;
}

/* ----------------
 *      heap_openrv_extended - open a heap relation specified
 *      by a RangeVar node
 *
 *      As above, but optionally return NULL instead of failing for
 *      relation-not-found.
 * ----------------
 */
Relation
heap_openrv_extended(const RangeVar *relation, LOCKMODE lockmode,
                     bool missing_ok)
{
    Relation    r;

    r = relation_openrv_extended(relation, lockmode, missing_ok);

    if (r)
    {
        if (r->rd_rel->relkind == RELKIND_INDEX)
            ereport(ERROR,
                    (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                     errmsg("\"%s\" is an index",
                            RelationGetRelationName(r))));
        else if (r->rd_rel->relkind == RELKIND_COMPOSITE_TYPE)
            ereport(ERROR,
                    (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                     errmsg("\"%s\" is a composite type",
                            RelationGetRelationName(r))));
    }

    return r;
}


/* ----------------
 *      heap_beginscan  - begin relation scan
 *
 * heap_beginscan is the "standard" case.
 *
 * heap_beginscan_catalog differs in setting up its own temporary snapshot.
 *
 * heap_beginscan_strat offers an extended API that lets the caller control
 * whether a nondefault buffer access strategy can be used, and whether
 * syncscan can be chosen (possibly resulting in the scan not starting from
 * block zero).  Both of these default to TRUE with plain heap_beginscan.
 *
 * heap_beginscan_bm is an alternative entry point for setting up a
 * HeapScanDesc for a bitmap heap scan.  Although that scan technology is
 * really quite unlike a standard seqscan, there is just enough commonality
 * to make it worth using the same data structure.
 *
 * heap_beginscan_sampling is an alternative entry point for setting up a
 * HeapScanDesc for a TABLESAMPLE scan.  As with bitmap scans, it's worth
 * using the same data structure although the behavior is rather different.
 * In addition to the options offered by heap_beginscan_strat, this call
 * also allows control of whether page-mode visibility checking is used.
 * ----------------
 */
HeapScanDesc
heap_beginscan(Relation relation, Snapshot snapshot,
               int nkeys, ScanKey key)
{
    return heap_beginscan_internal(relation, snapshot, nkeys, key, NULL,
                                   true, true, true, false, false, false);
}

HeapScanDesc
heap_beginscan_catalog(Relation relation, int nkeys, ScanKey key)
{
    Oid         relid = RelationGetRelid(relation);
    Snapshot    snapshot = RegisterSnapshot(GetCatalogSnapshot(relid));

    return heap_beginscan_internal(relation, snapshot, nkeys, key, NULL,
                                   true, true, true, false, false, true);
}

HeapScanDesc
heap_beginscan_strat(Relation relation, Snapshot snapshot,
                     int nkeys, ScanKey key,
                     bool allow_strat, bool allow_sync)
{
    return heap_beginscan_internal(relation, snapshot, nkeys, key, NULL,
                                   allow_strat, allow_sync, true,
                                   false, false, false);
}

HeapScanDesc
heap_beginscan_bm(Relation relation, Snapshot snapshot,
                  int nkeys, ScanKey key)
{
    return heap_beginscan_internal(relation, snapshot, nkeys, key, NULL,
                                   false, false, true, true, false, false);
}

HeapScanDesc
heap_beginscan_sampling(Relation relation, Snapshot snapshot,
                        int nkeys, ScanKey key,
                        bool allow_strat, bool allow_sync, bool allow_pagemode)
{
    return heap_beginscan_internal(relation, snapshot, nkeys, key, NULL,
                                   allow_strat, allow_sync, allow_pagemode,
                                   false, true, false);
}

static HeapScanDesc
heap_beginscan_internal(Relation relation, Snapshot snapshot,
                        int nkeys, ScanKey key,
                        ParallelHeapScanDesc parallel_scan,
                        bool allow_strat,
                        bool allow_sync,
                        bool allow_pagemode,
                        bool is_bitmapscan,
                        bool is_samplescan,
                        bool temp_snap)
{
    HeapScanDesc scan;

    /*
     * increment relation ref count while scanning relation
     *
     * This is just to make really sure the relcache entry won't go away while
     * the scan has a pointer to it.  Caller should be holding the rel open
     * anyway, so this is redundant in all normal scenarios...
     */
    RelationIncrementReferenceCount(relation);

    /*
     * allocate and initialize scan descriptor
     */
    scan = (HeapScanDesc) palloc(sizeof(HeapScanDescData));

    scan->rs_rd = relation;
    scan->rs_snapshot = snapshot;
    scan->rs_nkeys = nkeys;
    scan->rs_bitmapscan = is_bitmapscan;
    scan->rs_samplescan = is_samplescan;
    scan->rs_strategy = NULL;   /* set in initscan */
    scan->rs_allow_strat = allow_strat;
    scan->rs_allow_sync = allow_sync;
    scan->rs_temp_snap = temp_snap;
    scan->rs_parallel = parallel_scan;

    /*
     * we can use page-at-a-time mode if it's an MVCC-safe snapshot
     */
    scan->rs_pageatatime = allow_pagemode && IsMVCCSnapshot(snapshot);

    /*
     * For a seqscan in a serializable transaction, acquire a predicate lock
     * on the entire relation. This is required not only to lock all the
     * matching tuples, but also to conflict with new insertions into the
     * table. In an indexscan, we take page locks on the index pages covering
     * the range specified in the scan qual, but in a heap scan there is
     * nothing more fine-grained to lock. A bitmap scan is a different story,
     * there we have already scanned the index and locked the index pages
     * covering the predicate. But in that case we still have to lock any
     * matching heap tuples.
     */
    if (!is_bitmapscan)
        PredicateLockRelation(relation, snapshot);

    /* we only need to set this up once */
    scan->rs_ctup.t_tableOid = RelationGetRelid(relation);

    /*
     * we do this here instead of in initscan() because heap_rescan also calls
     * initscan() and we don't want to allocate memory again
     */
    if (nkeys > 0)
        scan->rs_key = (ScanKey) palloc(sizeof(ScanKeyData) * nkeys);
    else
        scan->rs_key = NULL;

    initscan(scan, key, false);

    return scan;
}

/* ----------------
 *      heap_rescan     - restart a relation scan
 * ----------------
 */
void
heap_rescan(HeapScanDesc scan,
            ScanKey key)
{
    /*
     * unpin scan buffers
     */
    if (BufferIsValid(scan->rs_cbuf))
        ReleaseBuffer(scan->rs_cbuf);

    /*
     * reinitialize scan descriptor
     */
    initscan(scan, key, true);
}

/* ----------------
 *      heap_rescan_set_params  - restart a relation scan after changing params
 *
 * This call allows changing the buffer strategy, syncscan, and pagemode
 * options before starting a fresh scan.  Note that although the actual use
 * of syncscan might change (effectively, enabling or disabling reporting),
 * the previously selected startblock will be kept.
 * ----------------
 */
void
heap_rescan_set_params(HeapScanDesc scan, ScanKey key,
                       bool allow_strat, bool allow_sync, bool allow_pagemode)
{
    /* adjust parameters */
    scan->rs_allow_strat = allow_strat;
    scan->rs_allow_sync = allow_sync;
    scan->rs_pageatatime = allow_pagemode && IsMVCCSnapshot(scan->rs_snapshot);
    /* ... and rescan */
    heap_rescan(scan, key);
}

/* ----------------
 *      heap_endscan    - end relation scan
 *
 *      See how to integrate with index scans.
 *      Check handling if reldesc caching.
 * ----------------
 */
void
heap_endscan(HeapScanDesc scan)
{
    /* Note: no locking manipulations needed */

    /*
     * unpin scan buffers
     */
    if (BufferIsValid(scan->rs_cbuf))
        ReleaseBuffer(scan->rs_cbuf);

    /*
     * decrement relation reference count and free scan descriptor storage
     */
    RelationDecrementReferenceCount(scan->rs_rd);

    if (scan->rs_key)
        pfree(scan->rs_key);

    if (scan->rs_strategy != NULL)
        FreeAccessStrategy(scan->rs_strategy);

    if (scan->rs_temp_snap)
        UnregisterSnapshot(scan->rs_snapshot);

    pfree(scan);
}

/* ----------------
 *      heap_parallelscan_estimate - estimate storage for ParallelHeapScanDesc
 *
 *      Sadly, this doesn't reduce to a constant, because the size required
 *      to serialize the snapshot can vary.
 * ----------------
 */
Size
heap_parallelscan_estimate(Snapshot snapshot)
{
    return add_size(offsetof(ParallelHeapScanDescData, phs_snapshot_data),
                    EstimateSnapshotSpace(snapshot));
}

/* ----------------
 *      heap_parallelscan_initialize - initialize ParallelHeapScanDesc
 *
 *      Must allow as many bytes of shared memory as returned by
 *      heap_parallelscan_estimate.  Call this just once in the leader
 *      process; then, individual workers attach via heap_beginscan_parallel.
 * ----------------
 */
void
heap_parallelscan_initialize(ParallelHeapScanDesc target, Relation relation,
                             Snapshot snapshot)
{
    target->phs_relid = RelationGetRelid(relation);
    target->phs_nblocks = RelationGetNumberOfBlocks(relation);
    /* compare phs_syncscan initialization to similar logic in initscan */
    target->phs_syncscan = synchronize_seqscans &&
        !RelationUsesLocalBuffers(relation) &&
        target->phs_nblocks > NBuffers / 4;
    SpinLockInit(&target->phs_mutex);
    target->phs_startblock = InvalidBlockNumber;
    pg_atomic_init_u64(&target->phs_nallocated, 0);
    SerializeSnapshot(snapshot, target->phs_snapshot_data);
}

/* ----------------
 *      heap_parallelscan_reinitialize - reset a parallel scan
 *
 *      Call this in the leader process.  Caller is responsible for
 *      making sure that all workers have finished the scan beforehand.
 * ----------------
 */
void
heap_parallelscan_reinitialize(ParallelHeapScanDesc parallel_scan)
{
    pg_atomic_write_u64(&parallel_scan->phs_nallocated, 0);
}

/* ----------------
 *      heap_beginscan_parallel - join a parallel scan
 *
 *      Caller must hold a suitable lock on the correct relation.
 * ----------------
 */
HeapScanDesc
heap_beginscan_parallel(Relation relation, ParallelHeapScanDesc parallel_scan)
{
    Snapshot    snapshot;

    Assert(RelationGetRelid(relation) == parallel_scan->phs_relid);
    snapshot = RestoreSnapshot(parallel_scan->phs_snapshot_data);
    RegisterSnapshot(snapshot);

    return heap_beginscan_internal(relation, snapshot, 0, NULL, parallel_scan,
                                   true, true, true, false, false, true);
}

/* ----------------
 *      heap_parallelscan_startblock_init - find and set the scan's startblock
 *
 *      Determine where the parallel seq scan should start.  This function may
 *      be called many times, once by each parallel worker.  We must be careful
 *      only to set the startblock once.
 * ----------------
 */
static void
heap_parallelscan_startblock_init(HeapScanDesc scan)
{
    BlockNumber sync_startpage = InvalidBlockNumber;
    ParallelHeapScanDesc parallel_scan;

    Assert(scan->rs_parallel);
    parallel_scan = scan->rs_parallel;

retry:
    /* Grab the spinlock. */
    SpinLockAcquire(&parallel_scan->phs_mutex);

    /*
     * If the scan's startblock has not yet been initialized, we must do so
     * now.  If this is not a synchronized scan, we just start at block 0, but
     * if it is a synchronized scan, we must get the starting position from
     * the synchronized scan machinery.  We can't hold the spinlock while
     * doing that, though, so release the spinlock, get the information we
     * need, and retry.  If nobody else has initialized the scan in the
     * meantime, we'll fill in the value we fetched on the second time
     * through.
     */
    if (parallel_scan->phs_startblock == InvalidBlockNumber)
    {
        if (!parallel_scan->phs_syncscan)
            parallel_scan->phs_startblock = 0;
        else if (sync_startpage != InvalidBlockNumber)
            parallel_scan->phs_startblock = sync_startpage;
        else
        {
            SpinLockRelease(&parallel_scan->phs_mutex);
            sync_startpage = ss_get_location(scan->rs_rd, scan->rs_nblocks);
            goto retry;
        }
    }
    SpinLockRelease(&parallel_scan->phs_mutex);
}

/* ----------------
 *      heap_parallelscan_nextpage - get the next page to scan
 *
 *      Get the next page to scan.  Even if there are no pages left to scan,
 *      another backend could have grabbed a page to scan and not yet finished
 *      looking at it, so it doesn't follow that the scan is done when the
 *      first backend gets an InvalidBlockNumber return.
 * ----------------
 */
static BlockNumber
heap_parallelscan_nextpage(HeapScanDesc scan)
{
    BlockNumber page;
    ParallelHeapScanDesc parallel_scan;
    uint64      nallocated;

    Assert(scan->rs_parallel);
    parallel_scan = scan->rs_parallel;

    /*
     * phs_nallocated tracks how many pages have been allocated to workers
     * already.  When phs_nallocated >= rs_nblocks, all blocks have been
     * allocated.
     *
     * Because we use an atomic fetch-and-add to fetch the current value, the
     * phs_nallocated counter will exceed rs_nblocks, because workers will
     * still increment the value, when they try to allocate the next block but
     * all blocks have been allocated already. The counter must be 64 bits
     * wide because of that, to avoid wrapping around when rs_nblocks is close
     * to 2^32.
     *
     * The actual page to return is calculated by adding the counter to the
     * starting block number, modulo nblocks.
     */
    nallocated = pg_atomic_fetch_add_u64(&parallel_scan->phs_nallocated, 1);
    if (nallocated >= scan->rs_nblocks)
        page = InvalidBlockNumber;  /* all blocks have been allocated */
    else
        page = (nallocated + parallel_scan->phs_startblock) % scan->rs_nblocks;

    /*
     * Report scan location.  Normally, we report the current page number.
     * When we reach the end of the scan, though, we report the starting page,
     * not the ending page, just so the starting positions for later scans
     * doesn't slew backwards.  We only report the position at the end of the
     * scan once, though: subsequent callers will report nothing.
     */
    if (scan->rs_syncscan)
    {
        if (page != InvalidBlockNumber)
            ss_report_location(scan->rs_rd, page);
        else if (nallocated == scan->rs_nblocks)
            ss_report_location(scan->rs_rd, parallel_scan->phs_startblock);
    }

    return page;
}

/* ----------------
 *      heap_update_snapshot
 *
 *      Update snapshot info in heap scan descriptor.
 * ----------------
 */
void
heap_update_snapshot(HeapScanDesc scan, Snapshot snapshot)
{
    Assert(IsMVCCSnapshot(snapshot));

    RegisterSnapshot(snapshot);
    scan->rs_snapshot = snapshot;
    scan->rs_temp_snap = true;
}

/* ----------------
 *      heap_getnext    - retrieve next tuple in scan
 *
 *      Fix to work with index relations.
 *      We don't return the buffer anymore, but you can get it from the
 *      returned HeapTuple.
 * ----------------
 */

#ifdef HEAPDEBUGALL
#define HEAPDEBUG_1 \
    elog(DEBUG2, "heap_getnext([%s,nkeys=%d],dir=%d) called", \
         RelationGetRelationName(scan->rs_rd), scan->rs_nkeys, (int) direction)
#define HEAPDEBUG_2 \
    elog(DEBUG2, "heap_getnext returning EOS")
#define HEAPDEBUG_3 \
    elog(DEBUG2, "heap_getnext returning tuple")
#else
#define HEAPDEBUG_1
#define HEAPDEBUG_2
#define HEAPDEBUG_3
#endif                          /* !defined(HEAPDEBUGALL) */


HeapTuple
heap_getnext(HeapScanDesc scan, ScanDirection direction)
{
    /* Note: no locking manipulations needed */

    HEAPDEBUG_1;                /* heap_getnext( info ) */

    if (scan->rs_pageatatime)
        heapgettup_pagemode(scan, direction,
                            scan->rs_nkeys, scan->rs_key);
    else
        heapgettup(scan, direction, scan->rs_nkeys, scan->rs_key);

    if (scan->rs_ctup.t_data == NULL)
    {
        HEAPDEBUG_2;            /* heap_getnext returning EOS */
        return NULL;
    }

    /*
     * if we get here it means we have a new current scan tuple, so point to
     * the proper return buffer and return the tuple.
     */
    HEAPDEBUG_3;                /* heap_getnext returning tuple */

    pgstat_count_heap_getnext(scan->rs_rd);

    return &(scan->rs_ctup);
}

/*
 *  heap_fetch      - retrieve tuple with given tid
 *
 * On entry, tuple->t_self is the TID to fetch.  We pin the buffer holding
 * the tuple, fill in the remaining fields of *tuple, and check the tuple
 * against the specified snapshot.
 *
 * If successful (tuple found and passes snapshot time qual), then *userbuf
 * is set to the buffer holding the tuple and TRUE is returned.  The caller
 * must unpin the buffer when done with the tuple.
 *
 * If the tuple is not found (ie, item number references a deleted slot),
 * then tuple->t_data is set to NULL and FALSE is returned.
 *
 * If the tuple is found but fails the time qual check, then FALSE is returned
 * but tuple->t_data is left pointing to the tuple.
 *
 * keep_buf determines what is done with the buffer in the FALSE-result cases.
 * When the caller specifies keep_buf = true, we retain the pin on the buffer
 * and return it in *userbuf (so the caller must eventually unpin it); when
 * keep_buf = false, the pin is released and *userbuf is set to InvalidBuffer.
 *
 * stats_relation is the relation to charge the heap_fetch operation against
 * for statistical purposes.  (This could be the heap rel itself, an
 * associated index, or NULL to not count the fetch at all.)
 *
 * heap_fetch does not follow HOT chains: only the exact TID requested will
 * be fetched.
 *
 * It is somewhat inconsistent that we ereport() on invalid block number but
 * return false on invalid item number.  There are a couple of reasons though.
 * One is that the caller can relatively easily check the block number for
 * validity, but cannot check the item number without reading the page
 * himself.  Another is that when we are following a t_ctid link, we can be
 * reasonably confident that the page number is valid (since VACUUM shouldn't
 * truncate off the destination page without having killed the referencing
 * tuple first), but the item number might well not be good.
 */
bool
heap_fetch(Relation relation,
           Snapshot snapshot,
           HeapTuple tuple,
           Buffer *userbuf,
           bool keep_buf,
           Relation stats_relation)
{
    ItemPointer tid = &(tuple->t_self);
    ItemId      lp;
    Buffer      buffer;
    Page        page;
    OffsetNumber offnum;
    bool        valid;

    /*
     * Fetch and pin the appropriate page of the relation.
     */
    buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(tid));

    /*
     * Need share lock on buffer to examine tuple commit status.
     */
    LockBuffer(buffer, BUFFER_LOCK_SHARE);
    page = BufferGetPage(buffer);
    TestForOldSnapshot(snapshot, relation, page);

    /*
     * We'd better check for out-of-range offnum in case of VACUUM since the
     * TID was obtained.
     */
    offnum = ItemPointerGetOffsetNumber(tid);
    if (offnum < FirstOffsetNumber || offnum > PageGetMaxOffsetNumber(page))
    {
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
        if (keep_buf)
            *userbuf = buffer;
        else
        {
            ReleaseBuffer(buffer);
            *userbuf = InvalidBuffer;
        }
        tuple->t_data = NULL;
        return false;
    }

    /*
     * get the item line pointer corresponding to the requested tid
     */
    lp = PageGetItemId(page, offnum);

    /*
     * Must check for deleted tuple.
     */
    if (!ItemIdIsNormal(lp))
    {
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
        if (keep_buf)
            *userbuf = buffer;
        else
        {
            ReleaseBuffer(buffer);
            *userbuf = InvalidBuffer;
        }
        tuple->t_data = NULL;
        return false;
    }

    /*
     * fill in *tuple fields
     */
    tuple->t_data = (HeapTupleHeader) PageGetItem(page, lp);
    tuple->t_len = ItemIdGetLength(lp);
    tuple->t_tableOid = RelationGetRelid(relation);

    /*
     * check time qualification of tuple, then release lock
     */
    valid = HeapTupleSatisfiesVisibility(tuple, snapshot, buffer);

    if (valid)
        PredicateLockTuple(relation, tuple, snapshot);

    CheckForSerializableConflictOut(valid, relation, tuple, buffer, snapshot);

    LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

    if (valid)
    {
        /*
         * All checks passed, so return the tuple as valid. Caller is now
         * responsible for releasing the buffer.
         */
        *userbuf = buffer;

        /* Count the successful fetch against appropriate rel, if any */
        if (stats_relation != NULL)
            pgstat_count_heap_fetch(stats_relation);

        return true;
    }

    /* Tuple failed time qual, but maybe caller wants to see it anyway. */
    if (keep_buf)
        *userbuf = buffer;
    else
    {
        ReleaseBuffer(buffer);
        *userbuf = InvalidBuffer;
    }

    return false;
}

/*
 *  heap_hot_search_buffer  - search HOT chain for tuple satisfying snapshot
 *
 * On entry, *tid is the TID of a tuple (either a simple tuple, or the root
 * of a HOT chain), and buffer is the buffer holding this tuple.  We search
 * for the first chain member satisfying the given snapshot.  If one is
 * found, we update *tid to reference that tuple's offset number, and
 * return TRUE.  If no match, return FALSE without modifying *tid.
 *
 * heapTuple is a caller-supplied buffer.  When a match is found, we return
 * the tuple here, in addition to updating *tid.  If no match is found, the
 * contents of this buffer on return are undefined.
 *
 * If all_dead is not NULL, we check non-visible tuples to see if they are
 * globally dead; *all_dead is set TRUE if all members of the HOT chain
 * are vacuumable, FALSE if not.
 *
 * Unlike heap_fetch, the caller must already have pin and (at least) share
 * lock on the buffer; it is still pinned/locked at exit.  Also unlike
 * heap_fetch, we do not report any pgstats count; caller may do so if wanted.
 */
bool
heap_hot_search_buffer(ItemPointer tid, Relation relation, Buffer buffer,
                       Snapshot snapshot, HeapTuple heapTuple,
                       bool *all_dead, bool first_call)
{
    Page        dp = (Page) BufferGetPage(buffer);
    TransactionId prev_xmax = InvalidTransactionId;
    OffsetNumber offnum;
    bool        at_chain_start;
    bool        valid;
    bool        skip;

    /* If this is not the first call, previous call returned a (live!) tuple */
    if (all_dead)
        *all_dead = first_call;

    Assert(TransactionIdIsValid(RecentGlobalXmin));

    Assert(ItemPointerGetBlockNumber(tid) == BufferGetBlockNumber(buffer));
    offnum = ItemPointerGetOffsetNumber(tid);
    at_chain_start = first_call;
    skip = !first_call;

    heapTuple->t_self = *tid;

    /* Scan through possible multiple members of HOT-chain */
    for (;;)
    {
        ItemId      lp;

        /* check for bogus TID */
        if (offnum < FirstOffsetNumber || offnum > PageGetMaxOffsetNumber(dp))
            break;

        lp = PageGetItemId(dp, offnum);

        /* check for unused, dead, or redirected items */
        if (!ItemIdIsNormal(lp))
        {
            /* We should only see a redirect at start of chain */
            if (ItemIdIsRedirected(lp) && at_chain_start)
            {
                /* Follow the redirect */
                offnum = ItemIdGetRedirect(lp);
                at_chain_start = false;
                continue;
            }
            /* else must be end of chain */
            break;
        }

        heapTuple->t_data = (HeapTupleHeader) PageGetItem(dp, lp);
        heapTuple->t_len = ItemIdGetLength(lp);
        heapTuple->t_tableOid = RelationGetRelid(relation);
        ItemPointerSetOffsetNumber(&heapTuple->t_self, offnum);

        /*
         * Shouldn't see a HEAP_ONLY tuple at chain start.
         */
        if (at_chain_start && HeapTupleIsHeapOnly(heapTuple))
            break;

        /*
         * The xmin should match the previous xmax value, else chain is
         * broken.
         */
        if (TransactionIdIsValid(prev_xmax) &&
            !TransactionIdEquals(prev_xmax,
                                 HeapTupleHeaderGetXmin(heapTuple->t_data)))
            break;

        /*
         * When first_call is true (and thus, skip is initially false) we'll
         * return the first tuple we find.  But on later passes, heapTuple
         * will initially be pointing to the tuple we returned last time.
         * Returning it again would be incorrect (and would loop forever), so
         * we skip it and return the next match we find.
         */
        if (!skip)
        {
            /*
             * For the benefit of logical decoding, have t_self point at the
             * element of the HOT chain we're currently investigating instead
             * of the root tuple of the HOT chain. This is important because
             * the *Satisfies routine for historical mvcc snapshots needs the
             * correct tid to decide about the visibility in some cases.
             */
            ItemPointerSet(&(heapTuple->t_self), BufferGetBlockNumber(buffer), offnum);

            /* If it's visible per the snapshot, we must return it */
            valid = HeapTupleSatisfiesVisibility(heapTuple, snapshot, buffer);
            CheckForSerializableConflictOut(valid, relation, heapTuple,
                                            buffer, snapshot);
            /* reset to original, non-redirected, tid */
            heapTuple->t_self = *tid;

            if (valid)
            {
                ItemPointerSetOffsetNumber(tid, offnum);
                PredicateLockTuple(relation, heapTuple, snapshot);
                if (all_dead)
                    *all_dead = false;
                return true;
            }
        }
        skip = false;

        /*
         * If we can't see it, maybe no one else can either.  At caller
         * request, check whether all chain members are dead to all
         * transactions.
         *
         * Note: if you change the criterion here for what is "dead", fix the
         * planner's get_actual_variable_range() function to match.
         */
        if (all_dead && *all_dead &&
            !HeapTupleIsSurelyDead(heapTuple, RecentGlobalXmin))
            *all_dead = false;

        /*
         * Check to see if HOT chain continues past this tuple; if so fetch
         * the next offnum and loop around.
         */
        if (HeapTupleIsHotUpdated(heapTuple))
        {
            Assert(ItemPointerGetBlockNumber(&heapTuple->t_data->t_ctid) ==
                   ItemPointerGetBlockNumber(tid));
            offnum = ItemPointerGetOffsetNumber(&heapTuple->t_data->t_ctid);
            at_chain_start = false;
            prev_xmax = HeapTupleHeaderGetUpdateXid(heapTuple->t_data);
        }
        else
            break;              /* end of chain */
    }

    return false;
}

/*
 *  heap_hot_search     - search HOT chain for tuple satisfying snapshot
 *
 * This has the same API as heap_hot_search_buffer, except that the caller
 * does not provide the buffer containing the page, rather we access it
 * locally.
 */
bool
heap_hot_search(ItemPointer tid, Relation relation, Snapshot snapshot,
                bool *all_dead)
{
    bool        result;
    Buffer      buffer;
    HeapTupleData heapTuple;

    buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(tid));
    LockBuffer(buffer, BUFFER_LOCK_SHARE);
    result = heap_hot_search_buffer(tid, relation, buffer, snapshot,
                                    &heapTuple, all_dead, true);
    LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
    ReleaseBuffer(buffer);
    return result;
}

/*
 *  heap_get_latest_tid -  get the latest tid of a specified tuple
 *
 * Actually, this gets the latest version that is visible according to
 * the passed snapshot.  You can pass SnapshotDirty to get the very latest,
 * possibly uncommitted version.
 *
 * *tid is both an input and an output parameter: it is updated to
 * show the latest version of the row.  Note that it will not be changed
 * if no version of the row passes the snapshot test.
 */
void
heap_get_latest_tid(Relation relation,
                    Snapshot snapshot,
                    ItemPointer tid)
{
    BlockNumber blk;
    ItemPointerData ctid;
    TransactionId priorXmax;

    /* this is to avoid Assert failures on bad input */
    if (!ItemPointerIsValid(tid))
        return;

    /*
     * Since this can be called with user-supplied TID, don't trust the input
     * too much.  (RelationGetNumberOfBlocks is an expensive check, so we
     * don't check t_ctid links again this way.  Note that it would not do to
     * call it just once and save the result, either.)
     */
    blk = ItemPointerGetBlockNumber(tid);
    if (blk >= RelationGetNumberOfBlocks(relation))
        elog(ERROR, "block number %u is out of range for relation \"%s\"",
             blk, RelationGetRelationName(relation));

    /*
     * Loop to chase down t_ctid links.  At top of loop, ctid is the tuple we
     * need to examine, and *tid is the TID we will return if ctid turns out
     * to be bogus.
     *
     * Note that we will loop until we reach the end of the t_ctid chain.
     * Depending on the snapshot passed, there might be at most one visible
     * version of the row, but we don't try to optimize for that.
     */
    ctid = *tid;
    priorXmax = InvalidTransactionId;   /* cannot check first XMIN */
    for (;;)
    {
        Buffer      buffer;
        Page        page;
        OffsetNumber offnum;
        ItemId      lp;
        HeapTupleData tp;
        bool        valid;

        /*
         * Read, pin, and lock the page.
         */
        buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(&ctid));
        LockBuffer(buffer, BUFFER_LOCK_SHARE);
        page = BufferGetPage(buffer);
        TestForOldSnapshot(snapshot, relation, page);

        /*
         * Check for bogus item number.  This is not treated as an error
         * condition because it can happen while following a t_ctid link. We
         * just assume that the prior tid is OK and return it unchanged.
         */
        offnum = ItemPointerGetOffsetNumber(&ctid);
        if (offnum < FirstOffsetNumber || offnum > PageGetMaxOffsetNumber(page))
        {
            UnlockReleaseBuffer(buffer);
            break;
        }
        lp = PageGetItemId(page, offnum);
        if (!ItemIdIsNormal(lp))
        {
            UnlockReleaseBuffer(buffer);
            break;
        }

        /* OK to access the tuple */
        tp.t_self = ctid;
        tp.t_data = (HeapTupleHeader) PageGetItem(page, lp);
        tp.t_len = ItemIdGetLength(lp);
        tp.t_tableOid = RelationGetRelid(relation);

        /*
         * After following a t_ctid link, we might arrive at an unrelated
         * tuple.  Check for XMIN match.
         */
        if (TransactionIdIsValid(priorXmax) &&
            !TransactionIdEquals(priorXmax, HeapTupleHeaderGetXmin(tp.t_data)))
        {
            UnlockReleaseBuffer(buffer);
            break;
        }

        /*
         * Check time qualification of tuple; if visible, set it as the new
         * result candidate.
         */
        valid = HeapTupleSatisfiesVisibility(&tp, snapshot, buffer);
        CheckForSerializableConflictOut(valid, relation, &tp, buffer, snapshot);
        if (valid)
            *tid = ctid;

        /*
         * If there's a valid t_ctid link, follow it, else we're done.
         */
        if ((tp.t_data->t_infomask & HEAP_XMAX_INVALID) ||
            HeapTupleHeaderIsOnlyLocked(tp.t_data) ||
            ItemPointerEquals(&tp.t_self, &tp.t_data->t_ctid))
        {
            UnlockReleaseBuffer(buffer);
            break;
        }

        ctid = tp.t_data->t_ctid;
        priorXmax = HeapTupleHeaderGetUpdateXid(tp.t_data);
        UnlockReleaseBuffer(buffer);
    }                           /* end of loop */
}


/*
 * UpdateXmaxHintBits - update tuple hint bits after xmax transaction ends
 *
 * This is called after we have waited for the XMAX transaction to terminate.
 * If the transaction aborted, we guarantee the XMAX_INVALID hint bit will
 * be set on exit.  If the transaction committed, we set the XMAX_COMMITTED
 * hint bit if possible --- but beware that that may not yet be possible,
 * if the transaction committed asynchronously.
 *
 * Note that if the transaction was a locker only, we set HEAP_XMAX_INVALID
 * even if it commits.
 *
 * Hence callers should look only at XMAX_INVALID.
 *
 * Note this is not allowed for tuples whose xmax is a multixact.
 */
static void
UpdateXmaxHintBits(HeapTupleHeader tuple, Buffer buffer, TransactionId xid)
{
    Assert(TransactionIdEquals(HeapTupleHeaderGetRawXmax(tuple), xid));
    Assert(!(tuple->t_infomask & HEAP_XMAX_IS_MULTI));

    if (!(tuple->t_infomask & (HEAP_XMAX_COMMITTED | HEAP_XMAX_INVALID)))
    {
        if (!HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask) &&
            TransactionIdDidCommit(xid))
            HeapTupleSetHintBits(tuple, buffer, HEAP_XMAX_COMMITTED,
                                 xid);
        else
            HeapTupleSetHintBits(tuple, buffer, HEAP_XMAX_INVALID,
                                 InvalidTransactionId);
    }
}


/*
 * GetBulkInsertState - prepare status object for a bulk insert
 */
BulkInsertState
GetBulkInsertState(void)
{
    BulkInsertState bistate;

    bistate = (BulkInsertState) palloc(sizeof(BulkInsertStateData));
    bistate->strategy = GetAccessStrategy(BAS_BULKWRITE);
    bistate->current_buf = InvalidBuffer;
    return bistate;
}

/*
 * FreeBulkInsertState - clean up after finishing a bulk insert
 */
void
FreeBulkInsertState(BulkInsertState bistate)
{
    if (bistate->current_buf != InvalidBuffer)
        ReleaseBuffer(bistate->current_buf);
    FreeAccessStrategy(bistate->strategy);
    pfree(bistate);
}

/*
 * ReleaseBulkInsertStatePin - release a buffer currently held in bistate
 */
void
ReleaseBulkInsertStatePin(BulkInsertState bistate)
{
    if (bistate->current_buf != InvalidBuffer)
        ReleaseBuffer(bistate->current_buf);
    bistate->current_buf = InvalidBuffer;
}


/*
 *  heap_insert     - insert tuple into a heap
 *
 * The new tuple is stamped with current transaction ID and the specified
 * command ID.
 *
 * If the HEAP_INSERT_SKIP_WAL option is specified, the new tuple is not
 * logged in WAL, even for a non-temp relation.  Safe usage of this behavior
 * requires that we arrange that all new tuples go into new pages not
 * containing any tuples from other transactions, and that the relation gets
 * fsync'd before commit.  (See also heap_sync() comments)
 *
 * The HEAP_INSERT_SKIP_FSM option is passed directly to
 * RelationGetBufferForTuple, which see for more info.
 *
 * HEAP_INSERT_FROZEN should only be specified for inserts into
 * relfilenodes created during the current subtransaction and when
 * there are no prior snapshots or pre-existing portals open.
 * This causes rows to be frozen, which is an MVCC violation and
 * requires explicit options chosen by user.
 *
 * HEAP_INSERT_IS_SPECULATIVE is used on so-called "speculative insertions",
 * which can be backed out afterwards without aborting the whole transaction.
 * Other sessions can wait for the speculative insertion to be confirmed,
 * turning it into a regular tuple, or aborted, as if it never existed.
 * Speculatively inserted tuples behave as "value locks" of short duration,
 * used to implement INSERT .. ON CONFLICT.
 *
 * Note that most of these options will be applied when inserting into the
 * heap's TOAST table, too, if the tuple requires any out-of-line data.  Only
 * HEAP_INSERT_IS_SPECULATIVE is explicitly ignored, as the toast data does
 * not partake in speculative insertion.
 *
 * The BulkInsertState object (if any; bistate can be NULL for default
 * behavior) is also just passed through to RelationGetBufferForTuple.
 *
 * The return value is the OID assigned to the tuple (either here or by the
 * caller), or InvalidOid if no OID.  The header fields of *tup are updated
 * to match the stored tuple; in particular tup->t_self receives the actual
 * TID where the tuple was stored.  But note that any toasting of fields
 * within the tuple data is NOT reflected into *tup.
 */
Oid
heap_insert(Relation relation, HeapTuple tup, CommandId cid,
            int options, BulkInsertState bistate)
{
    TransactionId xid = GetCurrentTransactionId();
    HeapTuple   heaptup;
    Buffer      buffer;
    Buffer      vmbuffer = InvalidBuffer;
    bool        all_visible_cleared = false;

    /*
     * Fill in tuple header fields, assign an OID, and toast the tuple if
     * necessary.
     *
     * Note: below this point, heaptup is the data we actually intend to store
     * into the relation; tup is the caller's original untoasted data.
     */
    heaptup = heap_prepare_insert(relation, tup, xid, cid, options);

    /*
     * Find buffer to insert this tuple into.  If the page is all visible,
     * this will also pin the requisite visibility map page.
     */
    buffer = RelationGetBufferForTuple(relation, heaptup->t_len,
                                       InvalidBuffer, options, bistate,
                                       &vmbuffer, NULL);

    /*
     * We're about to do the actual insert -- but check for conflict first, to
     * avoid possibly having to roll back work we've just done.
     *
     * This is safe without a recheck as long as there is no possibility of
     * another process scanning the page between this check and the insert
     * being visible to the scan (i.e., an exclusive buffer content lock is
     * continuously held from this point until the tuple insert is visible).
     *
     * For a heap insert, we only need to check for table-level SSI locks. Our
     * new tuple can't possibly conflict with existing tuple locks, and heap
     * page locks are only consolidated versions of tuple locks; they do not
     * lock "gaps" as index page locks do.  So we don't need to specify a
     * buffer when making the call, which makes for a faster check.
     */
    CheckForSerializableConflictIn(relation, NULL, InvalidBuffer);

    /* NO EREPORT(ERROR) from here till changes are logged */
    START_CRIT_SECTION();

    RelationPutHeapTuple(relation, buffer, heaptup,
                         (options & HEAP_INSERT_SPECULATIVE) != 0);

    if (PageIsAllVisible(BufferGetPage(buffer)))
    {
        all_visible_cleared = true;
        PageClearAllVisible(BufferGetPage(buffer));
        visibilitymap_clear(relation,
                            ItemPointerGetBlockNumber(&(heaptup->t_self)),
                            vmbuffer, VISIBILITYMAP_VALID_BITS);
    }

    /*
     * XXX Should we set PageSetPrunable on this page ?
     *
     * The inserting transaction may eventually abort thus making this tuple
     * DEAD and hence available for pruning. Though we don't want to optimize
     * for aborts, if no other tuple in this page is UPDATEd/DELETEd, the
     * aborted tuple will never be pruned until next vacuum is triggered.
     *
     * If you do add PageSetPrunable here, add it in heap_xlog_insert too.
     */

    MarkBufferDirty(buffer);

    /* XLOG stuff */
    if (!(options & HEAP_INSERT_SKIP_WAL) && RelationNeedsWAL(relation))
    {
        xl_heap_insert xlrec;
        xl_heap_header xlhdr;
        XLogRecPtr  recptr;
        Page        page = BufferGetPage(buffer);
        uint8       info = XLOG_HEAP_INSERT;
        int         bufflags = 0;

        /*
         * If this is a catalog, we need to transmit combocids to properly
         * decode, so log that as well.
         */
        if (RelationIsAccessibleInLogicalDecoding(relation))
            log_heap_new_cid(relation, heaptup);

        /*
         * If this is the single and first tuple on page, we can reinit the
         * page instead of restoring the whole thing.  Set flag, and hide
         * buffer references from XLogInsert.
         */
        if (ItemPointerGetOffsetNumber(&(heaptup->t_self)) == FirstOffsetNumber &&
            PageGetMaxOffsetNumber(page) == FirstOffsetNumber)
        {
            info |= XLOG_HEAP_INIT_PAGE;
            bufflags |= REGBUF_WILL_INIT;
        }

        xlrec.offnum = ItemPointerGetOffsetNumber(&heaptup->t_self);
        xlrec.flags = 0;
        if (all_visible_cleared)
            xlrec.flags |= XLH_INSERT_ALL_VISIBLE_CLEARED;
        if (options & HEAP_INSERT_SPECULATIVE)
            xlrec.flags |= XLH_INSERT_IS_SPECULATIVE;
        Assert(ItemPointerGetBlockNumber(&heaptup->t_self) == BufferGetBlockNumber(buffer));

        /*
         * For logical decoding, we need the tuple even if we're doing a full
         * page write, so make sure it's included even if we take a full-page
         * image. (XXX We could alternatively store a pointer into the FPW).
         */
        if (RelationIsLogicallyLogged(relation))
        {
            xlrec.flags |= XLH_INSERT_CONTAINS_NEW_TUPLE;
            bufflags |= REGBUF_KEEP_DATA;
        }

        XLogBeginInsert();
        XLogRegisterData((char *) &xlrec, SizeOfHeapInsert);

        xlhdr.t_infomask2 = heaptup->t_data->t_infomask2;
        xlhdr.t_infomask = heaptup->t_data->t_infomask;
        xlhdr.t_hoff = heaptup->t_data->t_hoff;

        /*
         * note we mark xlhdr as belonging to buffer; if XLogInsert decides to
         * write the whole page to the xlog, we don't need to store
         * xl_heap_header in the xlog.
         */
        XLogRegisterBuffer(0, buffer, REGBUF_STANDARD | bufflags);
        XLogRegisterBufData(0, (char *) &xlhdr, SizeOfHeapHeader);
        /* PG73FORMAT: write bitmap [+ padding] [+ oid] + data */
        XLogRegisterBufData(0,
                            (char *) heaptup->t_data + SizeofHeapTupleHeader,
                            heaptup->t_len - SizeofHeapTupleHeader);

        /* filtering by origin on a row level is much more efficient */
        XLogSetRecordFlags(XLOG_INCLUDE_ORIGIN);

        recptr = XLogInsert(RM_HEAP_ID, info);

        PageSetLSN(page, recptr);
    }

    END_CRIT_SECTION();

    UnlockReleaseBuffer(buffer);
    if (vmbuffer != InvalidBuffer)
        ReleaseBuffer(vmbuffer);

    /*
     * If tuple is cachable, mark it for invalidation from the caches in case
     * we abort.  Note it is OK to do this after releasing the buffer, because
     * the heaptup data structure is all in local memory, not in the shared
     * buffer.
     */
    CacheInvalidateHeapTuple(relation, heaptup, NULL);

    /* Note: speculative insertions are counted too, even if aborted later */
    pgstat_count_heap_insert(relation, 1);

    /*
     * If heaptup is a private copy, release it.  Don't forget to copy t_self
     * back to the caller's image, too.
     */
    if (heaptup != tup)
    {
        tup->t_self = heaptup->t_self;
        heap_freetuple(heaptup);
    }

    return HeapTupleGetOid(tup);
}

/*
 * Subroutine for heap_insert(). Prepares a tuple for insertion. This sets the
 * tuple header fields, assigns an OID, and toasts the tuple if necessary.
 * Returns a toasted version of the tuple if it was toasted, or the original
 * tuple if not. Note that in any case, the header fields are also set in
 * the original tuple.
 */
static HeapTuple
heap_prepare_insert(Relation relation, HeapTuple tup, TransactionId xid,
                    CommandId cid, int options)
{
    /*
     * For now, parallel operations are required to be strictly read-only.
     * Unlike heap_update() and heap_delete(), an insert should never create a
     * combo CID, so it might be possible to relax this restriction, but not
     * without more thought and testing.
     */
    if (IsInParallelMode())
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TRANSACTION_STATE),
                 errmsg("cannot insert tuples during a parallel operation")));

    if (relation->rd_rel->relhasoids)
    {
#ifdef NOT_USED
        /* this is redundant with an Assert in HeapTupleSetOid */
        Assert(tup->t_data->t_infomask & HEAP_HASOID);
#endif

        /*
         * If the object id of this tuple has already been assigned, trust the
         * caller.  There are a couple of ways this can happen.  At initial db
         * creation, the backend program sets oids for tuples. When we define
         * an index, we set the oid.  Finally, in the future, we may allow
         * users to set their own object ids in order to support a persistent
         * object store (objects need to contain pointers to one another).
         */
        if (!OidIsValid(HeapTupleGetOid(tup)))
            HeapTupleSetOid(tup, GetNewOid(relation));
    }
    else
    {
        /* check there is not space for an OID */
        Assert(!(tup->t_data->t_infomask & HEAP_HASOID));
    }

    tup->t_data->t_infomask &= ~(HEAP_XACT_MASK);
    tup->t_data->t_infomask2 &= ~(HEAP2_XACT_MASK);
    tup->t_data->t_infomask |= HEAP_XMAX_INVALID;
    HeapTupleHeaderSetXmin(tup->t_data, xid);
    if (options & HEAP_INSERT_FROZEN)
        HeapTupleHeaderSetXminFrozen(tup->t_data);

    HeapTupleHeaderSetCmin(tup->t_data, cid);
    HeapTupleHeaderSetXmax(tup->t_data, 0); /* for cleanliness */
    tup->t_tableOid = RelationGetRelid(relation);

    /*
     * If the new tuple is too big for storage or contains already toasted
     * out-of-line attributes from some other relation, invoke the toaster.
     */
    if (relation->rd_rel->relkind != RELKIND_RELATION &&
        relation->rd_rel->relkind != RELKIND_MATVIEW)
    {
        /* toast table entries should never be recursively toasted */
        Assert(!HeapTupleHasExternal(tup));
        return tup;
    }
    else if (HeapTupleHasExternal(tup) || tup->t_len > TOAST_TUPLE_THRESHOLD)
        return toast_insert_or_update(relation, tup, NULL, options);
    else
        return tup;
}

/*
 *  heap_multi_insert   - insert multiple tuple into a heap
 *
 * This is like heap_insert(), but inserts multiple tuples in one operation.
 * That's faster than calling heap_insert() in a loop, because when multiple
 * tuples can be inserted on a single page, we can write just a single WAL
 * record covering all of them, and only need to lock/unlock the page once.
 *
 * Note: this leaks memory into the current memory context. You can create a
 * temporary context before calling this, if that's a problem.
 */
void
heap_multi_insert(Relation relation, HeapTuple *tuples, int ntuples,
                  CommandId cid, int options, BulkInsertState bistate)
{
    TransactionId xid = GetCurrentTransactionId();
    HeapTuple  *heaptuples;
    int         i;
    int         ndone;
    char       *scratch = NULL;
    Page        page;
    bool        needwal;
    Size        saveFreeSpace;
    bool        need_tuple_data = RelationIsLogicallyLogged(relation);
    bool        need_cids = RelationIsAccessibleInLogicalDecoding(relation);

    needwal = !(options & HEAP_INSERT_SKIP_WAL) && RelationNeedsWAL(relation);
    saveFreeSpace = RelationGetTargetPageFreeSpace(relation,
                                                   HEAP_DEFAULT_FILLFACTOR);

    /* Toast and set header data in all the tuples */
    heaptuples = palloc(ntuples * sizeof(HeapTuple));
    for (i = 0; i < ntuples; i++)
        heaptuples[i] = heap_prepare_insert(relation, tuples[i],
                                            xid, cid, options);

    /*
     * Allocate some memory to use for constructing the WAL record. Using
     * palloc() within a critical section is not safe, so we allocate this
     * beforehand.
     */
    if (needwal)
        scratch = palloc(BLCKSZ);

    /*
     * We're about to do the actual inserts -- but check for conflict first,
     * to minimize the possibility of having to roll back work we've just
     * done.
     *
     * A check here does not definitively prevent a serialization anomaly;
     * that check MUST be done at least past the point of acquiring an
     * exclusive buffer content lock on every buffer that will be affected,
     * and MAY be done after all inserts are reflected in the buffers and
     * those locks are released; otherwise there race condition.  Since
     * multiple buffers can be locked and unlocked in the loop below, and it
     * would not be feasible to identify and lock all of those buffers before
     * the loop, we must do a final check at the end.
     *
     * The check here could be omitted with no loss of correctness; it is
     * present strictly as an optimization.
     *
     * For heap inserts, we only need to check for table-level SSI locks. Our
     * new tuples can't possibly conflict with existing tuple locks, and heap
     * page locks are only consolidated versions of tuple locks; they do not
     * lock "gaps" as index page locks do.  So we don't need to specify a
     * buffer when making the call, which makes for a faster check.
     */
    CheckForSerializableConflictIn(relation, NULL, InvalidBuffer);

    ndone = 0;
    while (ndone < ntuples)
    {
        Buffer      buffer;
        Buffer      vmbuffer = InvalidBuffer;
        bool        all_visible_cleared = false;
        int         nthispage;

        CHECK_FOR_INTERRUPTS();

        /*
         * Find buffer where at least the next tuple will fit.  If the page is
         * all-visible, this will also pin the requisite visibility map page.
         */
        buffer = RelationGetBufferForTuple(relation, heaptuples[ndone]->t_len,
                                           InvalidBuffer, options, bistate,
                                           &vmbuffer, NULL);
        page = BufferGetPage(buffer);

        /* NO EREPORT(ERROR) from here till changes are logged */
        START_CRIT_SECTION();

        /*
         * RelationGetBufferForTuple has ensured that the first tuple fits.
         * Put that on the page, and then as many other tuples as fit.
         */
        RelationPutHeapTuple(relation, buffer, heaptuples[ndone], false);
        for (nthispage = 1; ndone + nthispage < ntuples; nthispage++)
        {
            HeapTuple   heaptup = heaptuples[ndone + nthispage];

            if (PageGetHeapFreeSpace(page) < MAXALIGN(heaptup->t_len) + saveFreeSpace)
                break;

            RelationPutHeapTuple(relation, buffer, heaptup, false);

            /*
             * We don't use heap_multi_insert for catalog tuples yet, but
             * better be prepared...
             */
            if (needwal && need_cids)
                log_heap_new_cid(relation, heaptup);
        }

        if (PageIsAllVisible(page))
        {
            all_visible_cleared = true;
            PageClearAllVisible(page);
            visibilitymap_clear(relation,
                                BufferGetBlockNumber(buffer),
                                vmbuffer, VISIBILITYMAP_VALID_BITS);
        }

        /*
         * XXX Should we set PageSetPrunable on this page ? See heap_insert()
         */

        MarkBufferDirty(buffer);

        /* XLOG stuff */
        if (needwal)
        {
            XLogRecPtr  recptr;
            xl_heap_multi_insert *xlrec;
            uint8       info = XLOG_HEAP2_MULTI_INSERT;
            char       *tupledata;
            int         totaldatalen;
            char       *scratchptr = scratch;
            bool        init;
            int         bufflags = 0;

            /*
             * If the page was previously empty, we can reinit the page
             * instead of restoring the whole thing.
             */
            init = (ItemPointerGetOffsetNumber(&(heaptuples[ndone]->t_self)) == FirstOffsetNumber &&
                    PageGetMaxOffsetNumber(page) == FirstOffsetNumber + nthispage - 1);

            /* allocate xl_heap_multi_insert struct from the scratch area */
            xlrec = (xl_heap_multi_insert *) scratchptr;
            scratchptr += SizeOfHeapMultiInsert;

            /*
             * Allocate offsets array. Unless we're reinitializing the page,
             * in that case the tuples are stored in order starting at
             * FirstOffsetNumber and we don't need to store the offsets
             * explicitly.
             */
            if (!init)
                scratchptr += nthispage * sizeof(OffsetNumber);

            /* the rest of the scratch space is used for tuple data */
            tupledata = scratchptr;

            xlrec->flags = all_visible_cleared ? XLH_INSERT_ALL_VISIBLE_CLEARED : 0;
            xlrec->ntuples = nthispage;

            /*
             * Write out an xl_multi_insert_tuple and the tuple data itself
             * for each tuple.
             */
            for (i = 0; i < nthispage; i++)
            {
                HeapTuple   heaptup = heaptuples[ndone + i];
                xl_multi_insert_tuple *tuphdr;
                int         datalen;

                if (!init)
                    xlrec->offsets[i] = ItemPointerGetOffsetNumber(&heaptup->t_self);
                /* xl_multi_insert_tuple needs two-byte alignment. */
                tuphdr = (xl_multi_insert_tuple *) SHORTALIGN(scratchptr);
                scratchptr = ((char *) tuphdr) + SizeOfMultiInsertTuple;

                tuphdr->t_infomask2 = heaptup->t_data->t_infomask2;
                tuphdr->t_infomask = heaptup->t_data->t_infomask;
                tuphdr->t_hoff = heaptup->t_data->t_hoff;

                /* write bitmap [+ padding] [+ oid] + data */
                datalen = heaptup->t_len - SizeofHeapTupleHeader;
                memcpy(scratchptr,
                       (char *) heaptup->t_data + SizeofHeapTupleHeader,
                       datalen);
                tuphdr->datalen = datalen;
                scratchptr += datalen;
            }
            totaldatalen = scratchptr - tupledata;
            Assert((scratchptr - scratch) < BLCKSZ);

            if (need_tuple_data)
                xlrec->flags |= XLH_INSERT_CONTAINS_NEW_TUPLE;

            /*
             * Signal that this is the last xl_heap_multi_insert record
             * emitted by this call to heap_multi_insert(). Needed for logical
             * decoding so it knows when to cleanup temporary data.
             */
            if (ndone + nthispage == ntuples)
                xlrec->flags |= XLH_INSERT_LAST_IN_MULTI;

            if (init)
            {
                info |= XLOG_HEAP_INIT_PAGE;
                bufflags |= REGBUF_WILL_INIT;
            }

            /*
             * If we're doing logical decoding, include the new tuple data
             * even if we take a full-page image of the page.
             */
            if (need_tuple_data)
                bufflags |= REGBUF_KEEP_DATA;

            XLogBeginInsert();
            XLogRegisterData((char *) xlrec, tupledata - scratch);
            XLogRegisterBuffer(0, buffer, REGBUF_STANDARD | bufflags);

            XLogRegisterBufData(0, tupledata, totaldatalen);

            /* filtering by origin on a row level is much more efficient */
            XLogSetRecordFlags(XLOG_INCLUDE_ORIGIN);

            recptr = XLogInsert(RM_HEAP2_ID, info);

            PageSetLSN(page, recptr);
        }

        END_CRIT_SECTION();

        UnlockReleaseBuffer(buffer);
        if (vmbuffer != InvalidBuffer)
            ReleaseBuffer(vmbuffer);

        ndone += nthispage;
    }

    /*
     * We're done with the actual inserts.  Check for conflicts again, to
     * ensure that all rw-conflicts in to these inserts are detected.  Without
     * this final check, a sequential scan of the heap may have locked the
     * table after the "before" check, missing one opportunity to detect the
     * conflict, and then scanned the table before the new tuples were there,
     * missing the other chance to detect the conflict.
     *
     * For heap inserts, we only need to check for table-level SSI locks. Our
     * new tuples can't possibly conflict with existing tuple locks, and heap
     * page locks are only consolidated versions of tuple locks; they do not
     * lock "gaps" as index page locks do.  So we don't need to specify a
     * buffer when making the call.
     */
    CheckForSerializableConflictIn(relation, NULL, InvalidBuffer);

    /*
     * If tuples are cachable, mark them for invalidation from the caches in
     * case we abort.  Note it is OK to do this after releasing the buffer,
     * because the heaptuples data structure is all in local memory, not in
     * the shared buffer.
     */
    if (IsCatalogRelation(relation))
    {
        for (i = 0; i < ntuples; i++)
            CacheInvalidateHeapTuple(relation, heaptuples[i], NULL);
    }

    /*
     * Copy t_self fields back to the caller's original tuples. This does
     * nothing for untoasted tuples (tuples[i] == heaptuples[i)], but it's
     * probably faster to always copy than check.
     */
    for (i = 0; i < ntuples; i++)
        tuples[i]->t_self = heaptuples[i]->t_self;

    pgstat_count_heap_insert(relation, ntuples);
}

/*
 *  simple_heap_insert - insert a tuple
 *
 * Currently, this routine differs from heap_insert only in supplying
 * a default command ID and not allowing access to the speedup options.
 *
 * This should be used rather than using heap_insert directly in most places
 * where we are modifying system catalogs.
 */
Oid
simple_heap_insert(Relation relation, HeapTuple tup)
{
    return heap_insert(relation, tup, GetCurrentCommandId(true), 0, NULL);
}

/*
 * Given infomask/infomask2, compute the bits that must be saved in the
 * "infobits" field of xl_heap_delete, xl_heap_update, xl_heap_lock,
 * xl_heap_lock_updated WAL records.
 *
 * See fix_infomask_from_infobits.
 */
static uint8
compute_infobits(uint16 infomask, uint16 infomask2)
{
    return
        ((infomask & HEAP_XMAX_IS_MULTI) != 0 ? XLHL_XMAX_IS_MULTI : 0) |
        ((infomask & HEAP_XMAX_LOCK_ONLY) != 0 ? XLHL_XMAX_LOCK_ONLY : 0) |
        ((infomask & HEAP_XMAX_EXCL_LOCK) != 0 ? XLHL_XMAX_EXCL_LOCK : 0) |
    /* note we ignore HEAP_XMAX_SHR_LOCK here */
        ((infomask & HEAP_XMAX_KEYSHR_LOCK) != 0 ? XLHL_XMAX_KEYSHR_LOCK : 0) |
        ((infomask2 & HEAP_KEYS_UPDATED) != 0 ?
         XLHL_KEYS_UPDATED : 0);
}

/*
 * Given two versions of the same t_infomask for a tuple, compare them and
 * return whether the relevant status for a tuple Xmax has changed.  This is
 * used after a buffer lock has been released and reacquired: we want to ensure
 * that the tuple state continues to be the same it was when we previously
 * examined it.
 *
 * Note the Xmax field itself must be compared separately.
 */
static inline bool
xmax_infomask_changed(uint16 new_infomask, uint16 old_infomask)
{
    const uint16 interesting =
    HEAP_XMAX_IS_MULTI | HEAP_XMAX_LOCK_ONLY | HEAP_LOCK_MASK;

    if ((new_infomask & interesting) != (old_infomask & interesting))
        return true;

    return false;
}

/*
 *  heap_delete - delete a tuple
 *
 * NB: do not call this directly unless you are prepared to deal with
 * concurrent-update conditions.  Use simple_heap_delete instead.
 *
 *  relation - table to be modified (caller must hold suitable lock)
 *  tid - TID of tuple to be deleted
 *  cid - delete command ID (used for visibility test, and stored into
 *      cmax if successful)
 *  crosscheck - if not InvalidSnapshot, also check tuple against this
 *  wait - true if should wait for any conflicting update to commit/abort
 *  hufd - output parameter, filled in failure cases (see below)
 *
 * Normal, successful return value is HeapTupleMayBeUpdated, which
 * actually means we did delete it.  Failure return codes are
 * HeapTupleSelfUpdated, HeapTupleUpdated, or HeapTupleBeingUpdated
 * (the last only possible if wait == false).
 *
 * In the failure cases, the routine fills *hufd with the tuple's t_ctid,
 * t_xmax (resolving a possible MultiXact, if necessary), and t_cmax
 * (the last only for HeapTupleSelfUpdated, since we
 * cannot obtain cmax from a combocid generated by another transaction).
 * See comments for struct HeapUpdateFailureData for additional info.
 */
HTSU_Result
heap_delete(Relation relation, ItemPointer tid,
            CommandId cid, Snapshot crosscheck, bool wait,
            HeapUpdateFailureData *hufd)
{
    HTSU_Result result;
    TransactionId xid = GetCurrentTransactionId();
    ItemId      lp;
    HeapTupleData tp;
    Page        page;
    BlockNumber block;
    Buffer      buffer;
    Buffer      vmbuffer = InvalidBuffer;
    TransactionId new_xmax;
    uint16      new_infomask,
                new_infomask2;
    bool        have_tuple_lock = false;
    bool        iscombo;
    bool        all_visible_cleared = false;
    HeapTuple   old_key_tuple = NULL;   /* replica identity of the tuple */
    bool        old_key_copied = false;

    Assert(ItemPointerIsValid(tid));

    /*
     * Forbid this during a parallel operation, lest it allocate a combocid.
     * Other workers might need that combocid for visibility checks, and we
     * have no provision for broadcasting it to them.
     */
    if (IsInParallelMode())
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TRANSACTION_STATE),
                 errmsg("cannot delete tuples during a parallel operation")));

    block = ItemPointerGetBlockNumber(tid);
    buffer = ReadBuffer(relation, block);
    page = BufferGetPage(buffer);

    /*
     * Before locking the buffer, pin the visibility map page if it appears to
     * be necessary.  Since we haven't got the lock yet, someone else might be
     * in the middle of changing this, so we'll need to recheck after we have
     * the lock.
     */
    if (PageIsAllVisible(page))
        visibilitymap_pin(relation, block, &vmbuffer);

    LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

    /*
     * If we didn't pin the visibility map page and the page has become all
     * visible while we were busy locking the buffer, we'll have to unlock and
     * re-lock, to avoid holding the buffer lock across an I/O.  That's a bit
     * unfortunate, but hopefully shouldn't happen often.
     */
    if (vmbuffer == InvalidBuffer && PageIsAllVisible(page))
    {
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
        visibilitymap_pin(relation, block, &vmbuffer);
        LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
    }

    lp = PageGetItemId(page, ItemPointerGetOffsetNumber(tid));
    Assert(ItemIdIsNormal(lp));

    tp.t_tableOid = RelationGetRelid(relation);
    tp.t_data = (HeapTupleHeader) PageGetItem(page, lp);
    tp.t_len = ItemIdGetLength(lp);
    tp.t_self = *tid;

l1:
    result = HeapTupleSatisfiesUpdate(&tp, cid, buffer);

    if (result == HeapTupleInvisible)
    {
        UnlockReleaseBuffer(buffer);
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("attempted to delete invisible tuple")));
    }
    else if (result == HeapTupleBeingUpdated && wait)
    {
        TransactionId xwait;
        uint16      infomask;

        /* must copy state data before unlocking buffer */
        xwait = HeapTupleHeaderGetRawXmax(tp.t_data);
        infomask = tp.t_data->t_infomask;

        /*
         * Sleep until concurrent transaction ends -- except when there's a
         * single locker and it's our own transaction.  Note we don't care
         * which lock mode the locker has, because we need the strongest one.
         *
         * Before sleeping, we need to acquire tuple lock to establish our
         * priority for the tuple (see heap_lock_tuple).  LockTuple will
         * release us when we are next-in-line for the tuple.
         *
         * If we are forced to "start over" below, we keep the tuple lock;
         * this arranges that we stay at the head of the line while rechecking
         * tuple state.
         */
        if (infomask & HEAP_XMAX_IS_MULTI)
        {
            /* wait for multixact */
            if (DoesMultiXactIdConflict((MultiXactId) xwait, infomask,
                                        LockTupleExclusive))
            {
                LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

                /* acquire tuple lock, if necessary */
                heap_acquire_tuplock(relation, &(tp.t_self), LockTupleExclusive,
                                     LockWaitBlock, &have_tuple_lock);

                /* wait for multixact */
                MultiXactIdWait((MultiXactId) xwait, MultiXactStatusUpdate, infomask,
                                relation, &(tp.t_self), XLTW_Delete,
                                NULL);
                LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

                /*
                 * If xwait had just locked the tuple then some other xact
                 * could update this tuple before we get to this point.  Check
                 * for xmax change, and start over if so.
                 */
                if (xmax_infomask_changed(tp.t_data->t_infomask, infomask) ||
                    !TransactionIdEquals(HeapTupleHeaderGetRawXmax(tp.t_data),
                                         xwait))
                    goto l1;
            }

            /*
             * You might think the multixact is necessarily done here, but not
             * so: it could have surviving members, namely our own xact or
             * other subxacts of this backend.  It is legal for us to delete
             * the tuple in either case, however (the latter case is
             * essentially a situation of upgrading our former shared lock to
             * exclusive).  We don't bother changing the on-disk hint bits
             * since we are about to overwrite the xmax altogether.
             */
        }
        else if (!TransactionIdIsCurrentTransactionId(xwait))
        {
            /*
             * Wait for regular transaction to end; but first, acquire tuple
             * lock.
             */
            LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
            heap_acquire_tuplock(relation, &(tp.t_self), LockTupleExclusive,
                                 LockWaitBlock, &have_tuple_lock);
            XactLockTableWait(xwait, relation, &(tp.t_self), XLTW_Delete);
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

            /*
             * xwait is done, but if xwait had just locked the tuple then some
             * other xact could update this tuple before we get to this point.
             * Check for xmax change, and start over if so.
             */
            if (xmax_infomask_changed(tp.t_data->t_infomask, infomask) ||
                !TransactionIdEquals(HeapTupleHeaderGetRawXmax(tp.t_data),
                                     xwait))
                goto l1;

            /* Otherwise check if it committed or aborted */
            UpdateXmaxHintBits(tp.t_data, buffer, xwait);
        }

        /*
         * We may overwrite if previous xmax aborted, or if it committed but
         * only locked the tuple without updating it.
         */
        if ((tp.t_data->t_infomask & HEAP_XMAX_INVALID) ||
            HEAP_XMAX_IS_LOCKED_ONLY(tp.t_data->t_infomask) ||
            HeapTupleHeaderIsOnlyLocked(tp.t_data))
            result = HeapTupleMayBeUpdated;
        else
            result = HeapTupleUpdated;
    }

    if (crosscheck != InvalidSnapshot && result == HeapTupleMayBeUpdated)
    {
        /* Perform additional check for transaction-snapshot mode RI updates */
        if (!HeapTupleSatisfiesVisibility(&tp, crosscheck, buffer))
            result = HeapTupleUpdated;
    }

    if (result != HeapTupleMayBeUpdated)
    {
        Assert(result == HeapTupleSelfUpdated ||
               result == HeapTupleUpdated ||
               result == HeapTupleBeingUpdated);
        Assert(!(tp.t_data->t_infomask & HEAP_XMAX_INVALID));
        hufd->ctid = tp.t_data->t_ctid;
        hufd->xmax = HeapTupleHeaderGetUpdateXid(tp.t_data);
        if (result == HeapTupleSelfUpdated)
            hufd->cmax = HeapTupleHeaderGetCmax(tp.t_data);
        else
            hufd->cmax = InvalidCommandId;
        UnlockReleaseBuffer(buffer);
        if (have_tuple_lock)
            UnlockTupleTuplock(relation, &(tp.t_self), LockTupleExclusive);
        if (vmbuffer != InvalidBuffer)
            ReleaseBuffer(vmbuffer);
        return result;
    }

    /*
     * We're about to do the actual delete -- check for conflict first, to
     * avoid possibly having to roll back work we've just done.
     *
     * This is safe without a recheck as long as there is no possibility of
     * another process scanning the page between this check and the delete
     * being visible to the scan (i.e., an exclusive buffer content lock is
     * continuously held from this point until the tuple delete is visible).
     */
    CheckForSerializableConflictIn(relation, &tp, buffer);

    /* replace cid with a combo cid if necessary */
    HeapTupleHeaderAdjustCmax(tp.t_data, &cid, &iscombo);

    /*
     * Compute replica identity tuple before entering the critical section so
     * we don't PANIC upon a memory allocation failure.
     */
    old_key_tuple = ExtractReplicaIdentity(relation, &tp, true, &old_key_copied);

    /*
     * If this is the first possibly-multixact-able operation in the current
     * transaction, set my per-backend OldestMemberMXactId setting. We can be
     * certain that the transaction will never become a member of any older
     * MultiXactIds than that.  (We have to do this even if we end up just
     * using our own TransactionId below, since some other backend could
     * incorporate our XID into a MultiXact immediately afterwards.)
     */
    MultiXactIdSetOldestMember();

    compute_new_xmax_infomask(HeapTupleHeaderGetRawXmax(tp.t_data),
                              tp.t_data->t_infomask, tp.t_data->t_infomask2,
                              xid, LockTupleExclusive, true,
                              &new_xmax, &new_infomask, &new_infomask2);

    START_CRIT_SECTION();

    /*
     * If this transaction commits, the tuple will become DEAD sooner or
     * later.  Set flag that this page is a candidate for pruning once our xid
     * falls below the OldestXmin horizon.  If the transaction finally aborts,
     * the subsequent page pruning will be a no-op and the hint will be
     * cleared.
     */
    PageSetPrunable(page, xid);

    if (PageIsAllVisible(page))
    {
        all_visible_cleared = true;
        PageClearAllVisible(page);
        visibilitymap_clear(relation, BufferGetBlockNumber(buffer),
                            vmbuffer, VISIBILITYMAP_VALID_BITS);
    }

    /* store transaction information of xact deleting the tuple */
    tp.t_data->t_infomask &= ~(HEAP_XMAX_BITS | HEAP_MOVED);
    tp.t_data->t_infomask2 &= ~HEAP_KEYS_UPDATED;
    tp.t_data->t_infomask |= new_infomask;
    tp.t_data->t_infomask2 |= new_infomask2;
    HeapTupleHeaderClearHotUpdated(tp.t_data);
    HeapTupleHeaderSetXmax(tp.t_data, new_xmax);
    HeapTupleHeaderSetCmax(tp.t_data, cid, iscombo);
    /* Make sure there is no forward chain link in t_ctid */
    tp.t_data->t_ctid = tp.t_self;

    MarkBufferDirty(buffer);

    /*
     * XLOG stuff
     *
     * NB: heap_abort_speculative() uses the same xlog record and replay
     * routines.
     */
    if (RelationNeedsWAL(relation))
    {
        xl_heap_delete xlrec;
        XLogRecPtr  recptr;

        /* For logical decode we need combocids to properly decode the catalog */
        if (RelationIsAccessibleInLogicalDecoding(relation))
            log_heap_new_cid(relation, &tp);

        xlrec.flags = all_visible_cleared ? XLH_DELETE_ALL_VISIBLE_CLEARED : 0;
        xlrec.infobits_set = compute_infobits(tp.t_data->t_infomask,
                                              tp.t_data->t_infomask2);
        xlrec.offnum = ItemPointerGetOffsetNumber(&tp.t_self);
        xlrec.xmax = new_xmax;

        if (old_key_tuple != NULL)
        {
            if (relation->rd_rel->relreplident == REPLICA_IDENTITY_FULL)
                xlrec.flags |= XLH_DELETE_CONTAINS_OLD_TUPLE;
            else
                xlrec.flags |= XLH_DELETE_CONTAINS_OLD_KEY;
        }

        XLogBeginInsert();
        XLogRegisterData((char *) &xlrec, SizeOfHeapDelete);

        XLogRegisterBuffer(0, buffer, REGBUF_STANDARD);

        /*
         * Log replica identity of the deleted tuple if there is one
         */
        if (old_key_tuple != NULL)
        {
            xl_heap_header xlhdr;

            xlhdr.t_infomask2 = old_key_tuple->t_data->t_infomask2;
            xlhdr.t_infomask = old_key_tuple->t_data->t_infomask;
            xlhdr.t_hoff = old_key_tuple->t_data->t_hoff;

            XLogRegisterData((char *) &xlhdr, SizeOfHeapHeader);
            XLogRegisterData((char *) old_key_tuple->t_data
                             + SizeofHeapTupleHeader,
                             old_key_tuple->t_len
                             - SizeofHeapTupleHeader);
        }

        /* filtering by origin on a row level is much more efficient */
        XLogSetRecordFlags(XLOG_INCLUDE_ORIGIN);

        recptr = XLogInsert(RM_HEAP_ID, XLOG_HEAP_DELETE);

        PageSetLSN(page, recptr);
    }

    END_CRIT_SECTION();

    LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

    if (vmbuffer != InvalidBuffer)
        ReleaseBuffer(vmbuffer);

    /*
     * If the tuple has toasted out-of-line attributes, we need to delete
     * those items too.  We have to do this before releasing the buffer
     * because we need to look at the contents of the tuple, but it's OK to
     * release the content lock on the buffer first.
     */
    if (relation->rd_rel->relkind != RELKIND_RELATION &&
        relation->rd_rel->relkind != RELKIND_MATVIEW)
    {
        /* toast table entries should never be recursively toasted */
        Assert(!HeapTupleHasExternal(&tp));
    }
    else if (HeapTupleHasExternal(&tp))
        toast_delete(relation, &tp, false);

    /*
     * Mark tuple for invalidation from system caches at next command
     * boundary. We have to do this before releasing the buffer because we
     * need to look at the contents of the tuple.
     */
    CacheInvalidateHeapTuple(relation, &tp, NULL);

    /* Now we can release the buffer */
    ReleaseBuffer(buffer);

    /*
     * Release the lmgr tuple lock, if we had it.
     */
    if (have_tuple_lock)
        UnlockTupleTuplock(relation, &(tp.t_self), LockTupleExclusive);

    pgstat_count_heap_delete(relation);

    if (old_key_tuple != NULL && old_key_copied)
        heap_freetuple(old_key_tuple);

    return HeapTupleMayBeUpdated;
}

/*
 *  simple_heap_delete - delete a tuple
 *
 * This routine may be used to delete a tuple when concurrent updates of
 * the target tuple are not expected (for example, because we have a lock
 * on the relation associated with the tuple).  Any failure is reported
 * via ereport().
 */
void
simple_heap_delete(Relation relation, ItemPointer tid)
{
    HTSU_Result result;
    HeapUpdateFailureData hufd;

    result = heap_delete(relation, tid,
                         GetCurrentCommandId(true), InvalidSnapshot,
                         true /* wait for commit */ ,
                         &hufd);
    switch (result)
    {
        case HeapTupleSelfUpdated:
            /* Tuple was already updated in current command? */
            elog(ERROR, "tuple already updated by self");
            break;

        case HeapTupleMayBeUpdated:
            /* done successfully */
            break;

        case HeapTupleUpdated:
            elog(ERROR, "tuple concurrently updated");
            break;

        default:
            elog(ERROR, "unrecognized heap_delete status: %u", result);
            break;
    }
}

/*
 *  heap_update - replace a tuple
 *
 * NB: do not call this directly unless you are prepared to deal with
 * concurrent-update conditions.  Use simple_heap_update instead.
 *
 *  relation - table to be modified (caller must hold suitable lock)
 *  otid - TID of old tuple to be replaced
 *  newtup - newly constructed tuple data to store
 *  cid - update command ID (used for visibility test, and stored into
 *      cmax/cmin if successful)
 *  crosscheck - if not InvalidSnapshot, also check old tuple against this
 *  wait - true if should wait for any conflicting update to commit/abort
 *  hufd - output parameter, filled in failure cases (see below)
 *  lockmode - output parameter, filled with lock mode acquired on tuple
 *
 * Normal, successful return value is HeapTupleMayBeUpdated, which
 * actually means we *did* update it.  Failure return codes are
 * HeapTupleSelfUpdated, HeapTupleUpdated, or HeapTupleBeingUpdated
 * (the last only possible if wait == false).
 *
 * On success, the header fields of *newtup are updated to match the new
 * stored tuple; in particular, newtup->t_self is set to the TID where the
 * new tuple was inserted, and its HEAP_ONLY_TUPLE flag is set iff a HOT
 * update was done.  However, any TOAST changes in the new tuple's
 * data are not reflected into *newtup.
 *
 * In the failure cases, the routine fills *hufd with the tuple's t_ctid,
 * t_xmax (resolving a possible MultiXact, if necessary), and t_cmax
 * (the last only for HeapTupleSelfUpdated, since we
 * cannot obtain cmax from a combocid generated by another transaction).
 * See comments for struct HeapUpdateFailureData for additional info.
 */
HTSU_Result
heap_update(Relation relation, ItemPointer otid, HeapTuple newtup,
            CommandId cid, Snapshot crosscheck, bool wait,
            HeapUpdateFailureData *hufd, LockTupleMode *lockmode)
{
    HTSU_Result result;
    TransactionId xid = GetCurrentTransactionId();
    Bitmapset  *hot_attrs;
    Bitmapset  *key_attrs;
    Bitmapset  *id_attrs;
    Bitmapset  *interesting_attrs;
    Bitmapset  *modified_attrs;
    ItemId      lp;
    HeapTupleData oldtup;
    HeapTuple   heaptup;
    HeapTuple   old_key_tuple = NULL;
    bool        old_key_copied = false;
    Page        page;
    BlockNumber block;
    MultiXactStatus mxact_status;
    Buffer      buffer,
                newbuf,
                vmbuffer = InvalidBuffer,
                vmbuffer_new = InvalidBuffer;
    bool        need_toast;
    Size        newtupsize,
                pagefree;
    bool        have_tuple_lock = false;
    bool        iscombo;
    bool        use_hot_update = false;
    bool        hot_attrs_checked = false;
    bool        key_intact;
    bool        all_visible_cleared = false;
    bool        all_visible_cleared_new = false;
    bool        checked_lockers;
    bool        locker_remains;
    TransactionId xmax_new_tuple,
                xmax_old_tuple;
    uint16      infomask_old_tuple,
                infomask2_old_tuple,
                infomask_new_tuple,
                infomask2_new_tuple;

    Assert(ItemPointerIsValid(otid));

    /*
     * Forbid this during a parallel operation, lest it allocate a combocid.
     * Other workers might need that combocid for visibility checks, and we
     * have no provision for broadcasting it to them.
     */
    if (IsInParallelMode())
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TRANSACTION_STATE),
                 errmsg("cannot update tuples during a parallel operation")));

    /*
     * Fetch the list of attributes to be checked for various operations.
     *
     * For HOT considerations, this is wasted effort if we fail to update or
     * have to put the new tuple on a different page.  But we must compute the
     * list before obtaining buffer lock --- in the worst case, if we are
     * doing an update on one of the relevant system catalogs, we could
     * deadlock if we try to fetch the list later.  In any case, the relcache
     * caches the data so this is usually pretty cheap.
     *
     * We also need columns used by the replica identity and columns that are
     * considered the "key" of rows in the table.
     *
     * Note that we get copies of each bitmap, so we need not worry about
     * relcache flush happening midway through.
     */
    hot_attrs = RelationGetIndexAttrBitmap(relation, INDEX_ATTR_BITMAP_ALL);
    key_attrs = RelationGetIndexAttrBitmap(relation, INDEX_ATTR_BITMAP_KEY);
    id_attrs = RelationGetIndexAttrBitmap(relation,
                                          INDEX_ATTR_BITMAP_IDENTITY_KEY);


    block = ItemPointerGetBlockNumber(otid);
    buffer = ReadBuffer(relation, block);
    page = BufferGetPage(buffer);

    interesting_attrs = NULL;

    /*
     * If the page is already full, there is hardly any chance of doing a HOT
     * update on this page. It might be wasteful effort to look for index
     * column updates only to later reject HOT updates for lack of space in
     * the same page. So we be conservative and only fetch hot_attrs if the
     * page is not already full. Since we are already holding a pin on the
     * buffer, there is no chance that the buffer can get cleaned up
     * concurrently and even if that was possible, in the worst case we lose a
     * chance to do a HOT update.
     */
    if (!PageIsFull(page))
    {
        interesting_attrs = bms_add_members(interesting_attrs, hot_attrs);
        hot_attrs_checked = true;
    }
    interesting_attrs = bms_add_members(interesting_attrs, key_attrs);
    interesting_attrs = bms_add_members(interesting_attrs, id_attrs);

    /*
     * Before locking the buffer, pin the visibility map page if it appears to
     * be necessary.  Since we haven't got the lock yet, someone else might be
     * in the middle of changing this, so we'll need to recheck after we have
     * the lock.
     */
    if (PageIsAllVisible(page))
        visibilitymap_pin(relation, block, &vmbuffer);

    LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

    lp = PageGetItemId(page, ItemPointerGetOffsetNumber(otid));
    Assert(ItemIdIsNormal(lp));

    /*
     * Fill in enough data in oldtup for HeapDetermineModifiedColumns to work
     * properly.
     */
    oldtup.t_tableOid = RelationGetRelid(relation);
    oldtup.t_data = (HeapTupleHeader) PageGetItem(page, lp);
    oldtup.t_len = ItemIdGetLength(lp);
    oldtup.t_self = *otid;

    /* the new tuple is ready, except for this: */
    newtup->t_tableOid = RelationGetRelid(relation);

    /* Fill in OID for newtup */
    if (relation->rd_rel->relhasoids)
    {
#ifdef NOT_USED
        /* this is redundant with an Assert in HeapTupleSetOid */
        Assert(newtup->t_data->t_infomask & HEAP_HASOID);
#endif
        HeapTupleSetOid(newtup, HeapTupleGetOid(&oldtup));
    }
    else
    {
        /* check there is not space for an OID */
        Assert(!(newtup->t_data->t_infomask & HEAP_HASOID));
    }

    /* Determine columns modified by the update. */
    modified_attrs = HeapDetermineModifiedColumns(relation, interesting_attrs,
                                                  &oldtup, newtup);

    /*
     * If we're not updating any "key" column, we can grab a weaker lock type.
     * This allows for more concurrency when we are running simultaneously
     * with foreign key checks.
     *
     * Note that if a column gets detoasted while executing the update, but
     * the value ends up being the same, this test will fail and we will use
     * the stronger lock.  This is acceptable; the important case to optimize
     * is updates that don't manipulate key columns, not those that
     * serendipitiously arrive at the same key values.
     */
    if (!bms_overlap(modified_attrs, key_attrs))
    {
        *lockmode = LockTupleNoKeyExclusive;
        mxact_status = MultiXactStatusNoKeyUpdate;
        key_intact = true;

        /*
         * If this is the first possibly-multixact-able operation in the
         * current transaction, set my per-backend OldestMemberMXactId
         * setting. We can be certain that the transaction will never become a
         * member of any older MultiXactIds than that.  (We have to do this
         * even if we end up just using our own TransactionId below, since
         * some other backend could incorporate our XID into a MultiXact
         * immediately afterwards.)
         */
        MultiXactIdSetOldestMember();
    }
    else
    {
        *lockmode = LockTupleExclusive;
        mxact_status = MultiXactStatusUpdate;
        key_intact = false;
    }

    /*
     * Note: beyond this point, use oldtup not otid to refer to old tuple.
     * otid may very well point at newtup->t_self, which we will overwrite
     * with the new tuple's location, so there's great risk of confusion if we
     * use otid anymore.
     */

l2:
    checked_lockers = false;
    locker_remains = false;
    result = HeapTupleSatisfiesUpdate(&oldtup, cid, buffer);

    /* see below about the "no wait" case */
    Assert(result != HeapTupleBeingUpdated || wait);

    if (result == HeapTupleInvisible)
    {
        UnlockReleaseBuffer(buffer);
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("attempted to update invisible tuple")));
    }
    else if (result == HeapTupleBeingUpdated && wait)
    {
        TransactionId xwait;
        uint16      infomask;
        bool        can_continue = false;

        /*
         * XXX note that we don't consider the "no wait" case here.  This
         * isn't a problem currently because no caller uses that case, but it
         * should be fixed if such a caller is introduced.  It wasn't a
         * problem previously because this code would always wait, but now
         * that some tuple locks do not conflict with one of the lock modes we
         * use, it is possible that this case is interesting to handle
         * specially.
         *
         * This may cause failures with third-party code that calls
         * heap_update directly.
         */

        /* must copy state data before unlocking buffer */
        xwait = HeapTupleHeaderGetRawXmax(oldtup.t_data);
        infomask = oldtup.t_data->t_infomask;

        /*
         * Now we have to do something about the existing locker.  If it's a
         * multi, sleep on it; we might be awakened before it is completely
         * gone (or even not sleep at all in some cases); we need to preserve
         * it as locker, unless it is gone completely.
         *
         * If it's not a multi, we need to check for sleeping conditions
         * before actually going to sleep.  If the update doesn't conflict
         * with the locks, we just continue without sleeping (but making sure
         * it is preserved).
         *
         * Before sleeping, we need to acquire tuple lock to establish our
         * priority for the tuple (see heap_lock_tuple).  LockTuple will
         * release us when we are next-in-line for the tuple.  Note we must
         * not acquire the tuple lock until we're sure we're going to sleep;
         * otherwise we're open for race conditions with other transactions
         * holding the tuple lock which sleep on us.
         *
         * If we are forced to "start over" below, we keep the tuple lock;
         * this arranges that we stay at the head of the line while rechecking
         * tuple state.
         */
        if (infomask & HEAP_XMAX_IS_MULTI)
        {
            TransactionId update_xact;
            int         remain;

            if (DoesMultiXactIdConflict((MultiXactId) xwait, infomask,
                                        *lockmode))
            {
                LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

                /* acquire tuple lock, if necessary */
                heap_acquire_tuplock(relation, &(oldtup.t_self), *lockmode,
                                     LockWaitBlock, &have_tuple_lock);

                /* wait for multixact */
                MultiXactIdWait((MultiXactId) xwait, mxact_status, infomask,
                                relation, &oldtup.t_self, XLTW_Update,
                                &remain);
                checked_lockers = true;
                locker_remains = remain != 0;
                LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

                /*
                 * If xwait had just locked the tuple then some other xact
                 * could update this tuple before we get to this point.  Check
                 * for xmax change, and start over if so.
                 */
                if (xmax_infomask_changed(oldtup.t_data->t_infomask,
                                          infomask) ||
                    !TransactionIdEquals(HeapTupleHeaderGetRawXmax(oldtup.t_data),
                                         xwait))
                    goto l2;
            }

            /*
             * Note that the multixact may not be done by now.  It could have
             * surviving members; our own xact or other subxacts of this
             * backend, and also any other concurrent transaction that locked
             * the tuple with KeyShare if we only got TupleLockUpdate.  If
             * this is the case, we have to be careful to mark the updated
             * tuple with the surviving members in Xmax.
             *
             * Note that there could have been another update in the
             * MultiXact. In that case, we need to check whether it committed
             * or aborted. If it aborted we are safe to update it again;
             * otherwise there is an update conflict, and we have to return
             * HeapTupleUpdated below.
             *
             * In the LockTupleExclusive case, we still need to preserve the
             * surviving members: those would include the tuple locks we had
             * before this one, which are important to keep in case this
             * subxact aborts.
             */
            if (!HEAP_XMAX_IS_LOCKED_ONLY(oldtup.t_data->t_infomask))
                update_xact = HeapTupleGetUpdateXid(oldtup.t_data);
            else
                update_xact = InvalidTransactionId;

            /*
             * There was no UPDATE in the MultiXact; or it aborted. No
             * TransactionIdIsInProgress() call needed here, since we called
             * MultiXactIdWait() above.
             */
            if (!TransactionIdIsValid(update_xact) ||
                TransactionIdDidAbort(update_xact))
                can_continue = true;
        }
        else if (TransactionIdIsCurrentTransactionId(xwait))
        {
            /*
             * The only locker is ourselves; we can avoid grabbing the tuple
             * lock here, but must preserve our locking information.
             */
            checked_lockers = true;
            locker_remains = true;
            can_continue = true;
        }
        else if (HEAP_XMAX_IS_KEYSHR_LOCKED(infomask) && key_intact)
        {
            /*
             * If it's just a key-share locker, and we're not changing the key
             * columns, we don't need to wait for it to end; but we need to
             * preserve it as locker.
             */
            checked_lockers = true;
            locker_remains = true;
            can_continue = true;
        }
        else
        {
            /*
             * Wait for regular transaction to end; but first, acquire tuple
             * lock.
             */
            LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
            heap_acquire_tuplock(relation, &(oldtup.t_self), *lockmode,
                                 LockWaitBlock, &have_tuple_lock);
            XactLockTableWait(xwait, relation, &oldtup.t_self,
                              XLTW_Update);
            checked_lockers = true;
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

            /*
             * xwait is done, but if xwait had just locked the tuple then some
             * other xact could update this tuple before we get to this point.
             * Check for xmax change, and start over if so.
             */
            if (xmax_infomask_changed(oldtup.t_data->t_infomask, infomask) ||
                !TransactionIdEquals(xwait,
                                     HeapTupleHeaderGetRawXmax(oldtup.t_data)))
                goto l2;

            /* Otherwise check if it committed or aborted */
            UpdateXmaxHintBits(oldtup.t_data, buffer, xwait);
            if (oldtup.t_data->t_infomask & HEAP_XMAX_INVALID)
                can_continue = true;
        }

        result = can_continue ? HeapTupleMayBeUpdated : HeapTupleUpdated;
    }

    if (crosscheck != InvalidSnapshot && result == HeapTupleMayBeUpdated)
    {
        /* Perform additional check for transaction-snapshot mode RI updates */
        if (!HeapTupleSatisfiesVisibility(&oldtup, crosscheck, buffer))
            result = HeapTupleUpdated;
    }

    if (result != HeapTupleMayBeUpdated)
    {
        Assert(result == HeapTupleSelfUpdated ||
               result == HeapTupleUpdated ||
               result == HeapTupleBeingUpdated);
        Assert(!(oldtup.t_data->t_infomask & HEAP_XMAX_INVALID));
        hufd->ctid = oldtup.t_data->t_ctid;
        hufd->xmax = HeapTupleHeaderGetUpdateXid(oldtup.t_data);
        if (result == HeapTupleSelfUpdated)
            hufd->cmax = HeapTupleHeaderGetCmax(oldtup.t_data);
        else
            hufd->cmax = InvalidCommandId;
        UnlockReleaseBuffer(buffer);
        if (have_tuple_lock)
            UnlockTupleTuplock(relation, &(oldtup.t_self), *lockmode);
        if (vmbuffer != InvalidBuffer)
            ReleaseBuffer(vmbuffer);
        bms_free(hot_attrs);
        bms_free(key_attrs);
        bms_free(id_attrs);
        bms_free(modified_attrs);
        bms_free(interesting_attrs);
        return result;
    }

    /*
     * If we didn't pin the visibility map page and the page has become all
     * visible while we were busy locking the buffer, or during some
     * subsequent window during which we had it unlocked, we'll have to unlock
     * and re-lock, to avoid holding the buffer lock across an I/O.  That's a
     * bit unfortunate, especially since we'll now have to recheck whether the
     * tuple has been locked or updated under us, but hopefully it won't
     * happen very often.
     */
    if (vmbuffer == InvalidBuffer && PageIsAllVisible(page))
    {
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
        visibilitymap_pin(relation, block, &vmbuffer);
        LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
        goto l2;
    }

    /* Fill in transaction status data */

    /*
     * If the tuple we're updating is locked, we need to preserve the locking
     * info in the old tuple's Xmax.  Prepare a new Xmax value for this.
     */
    compute_new_xmax_infomask(HeapTupleHeaderGetRawXmax(oldtup.t_data),
                              oldtup.t_data->t_infomask,
                              oldtup.t_data->t_infomask2,
                              xid, *lockmode, true,
                              &xmax_old_tuple, &infomask_old_tuple,
                              &infomask2_old_tuple);

    /*
     * And also prepare an Xmax value for the new copy of the tuple.  If there
     * was no xmax previously, or there was one but all lockers are now gone,
     * then use InvalidXid; otherwise, get the xmax from the old tuple.  (In
     * rare cases that might also be InvalidXid and yet not have the
     * HEAP_XMAX_INVALID bit set; that's fine.)
     */
    if ((oldtup.t_data->t_infomask & HEAP_XMAX_INVALID) ||
        HEAP_LOCKED_UPGRADED(oldtup.t_data->t_infomask) ||
        (checked_lockers && !locker_remains))
        xmax_new_tuple = InvalidTransactionId;
    else
        xmax_new_tuple = HeapTupleHeaderGetRawXmax(oldtup.t_data);

    if (!TransactionIdIsValid(xmax_new_tuple))
    {
        infomask_new_tuple = HEAP_XMAX_INVALID;
        infomask2_new_tuple = 0;
    }
    else
    {
        /*
         * If we found a valid Xmax for the new tuple, then the infomask bits
         * to use on the new tuple depend on what was there on the old one.
         * Note that since we're doing an update, the only possibility is that
         * the lockers had FOR KEY SHARE lock.
         */
        if (oldtup.t_data->t_infomask & HEAP_XMAX_IS_MULTI)
        {
            GetMultiXactIdHintBits(xmax_new_tuple, &infomask_new_tuple,
                                   &infomask2_new_tuple);
        }
        else
        {
            infomask_new_tuple = HEAP_XMAX_KEYSHR_LOCK | HEAP_XMAX_LOCK_ONLY;
            infomask2_new_tuple = 0;
        }
    }

    /*
     * Prepare the new tuple with the appropriate initial values of Xmin and
     * Xmax, as well as initial infomask bits as computed above.
     */
    newtup->t_data->t_infomask &= ~(HEAP_XACT_MASK);
    newtup->t_data->t_infomask2 &= ~(HEAP2_XACT_MASK);
    HeapTupleHeaderSetXmin(newtup->t_data, xid);
    HeapTupleHeaderSetCmin(newtup->t_data, cid);
    newtup->t_data->t_infomask |= HEAP_UPDATED | infomask_new_tuple;
    newtup->t_data->t_infomask2 |= infomask2_new_tuple;
    HeapTupleHeaderSetXmax(newtup->t_data, xmax_new_tuple);

    /*
     * Replace cid with a combo cid if necessary.  Note that we already put
     * the plain cid into the new tuple.
     */
    HeapTupleHeaderAdjustCmax(oldtup.t_data, &cid, &iscombo);

    /*
     * If the toaster needs to be activated, OR if the new tuple will not fit
     * on the same page as the old, then we need to release the content lock
     * (but not the pin!) on the old tuple's buffer while we are off doing
     * TOAST and/or table-file-extension work.  We must mark the old tuple to
     * show that it's locked, else other processes may try to update it
     * themselves.
     *
     * We need to invoke the toaster if there are already any out-of-line
     * toasted values present, or if the new tuple is over-threshold.
     */
    if (relation->rd_rel->relkind != RELKIND_RELATION &&
        relation->rd_rel->relkind != RELKIND_MATVIEW)
    {
        /* toast table entries should never be recursively toasted */
        Assert(!HeapTupleHasExternal(&oldtup));
        Assert(!HeapTupleHasExternal(newtup));
        need_toast = false;
    }
    else
        need_toast = (HeapTupleHasExternal(&oldtup) ||
                      HeapTupleHasExternal(newtup) ||
                      newtup->t_len > TOAST_TUPLE_THRESHOLD);

    pagefree = PageGetHeapFreeSpace(page);

    newtupsize = MAXALIGN(newtup->t_len);

    if (need_toast || newtupsize > pagefree)
    {
        TransactionId xmax_lock_old_tuple;
        uint16      infomask_lock_old_tuple,
                    infomask2_lock_old_tuple;
        bool        cleared_all_frozen = false;

        /*
         * To prevent concurrent sessions from updating the tuple, we have to
         * temporarily mark it locked, while we release the page-level lock.
         *
         * To satisfy the rule that any xid potentially appearing in a buffer
         * written out to disk, we unfortunately have to WAL log this
         * temporary modification.  We can reuse xl_heap_lock for this
         * purpose.  If we crash/error before following through with the
         * actual update, xmax will be of an aborted transaction, allowing
         * other sessions to proceed.
         */

        /*
         * Compute xmax / infomask appropriate for locking the tuple. This has
         * to be done separately from the combo that's going to be used for
         * updating, because the potentially created multixact would otherwise
         * be wrong.
         */
        compute_new_xmax_infomask(HeapTupleHeaderGetRawXmax(oldtup.t_data),
                                  oldtup.t_data->t_infomask,
                                  oldtup.t_data->t_infomask2,
                                  xid, *lockmode, false,
                                  &xmax_lock_old_tuple, &infomask_lock_old_tuple,
                                  &infomask2_lock_old_tuple);

        Assert(HEAP_XMAX_IS_LOCKED_ONLY(infomask_lock_old_tuple));

        START_CRIT_SECTION();

        /* Clear obsolete visibility flags ... */
        oldtup.t_data->t_infomask &= ~(HEAP_XMAX_BITS | HEAP_MOVED);
        oldtup.t_data->t_infomask2 &= ~HEAP_KEYS_UPDATED;
        HeapTupleClearHotUpdated(&oldtup);
        /* ... and store info about transaction updating this tuple */
        Assert(TransactionIdIsValid(xmax_lock_old_tuple));
        HeapTupleHeaderSetXmax(oldtup.t_data, xmax_lock_old_tuple);
        oldtup.t_data->t_infomask |= infomask_lock_old_tuple;
        oldtup.t_data->t_infomask2 |= infomask2_lock_old_tuple;
        HeapTupleHeaderSetCmax(oldtup.t_data, cid, iscombo);

        /* temporarily make it look not-updated, but locked */
        oldtup.t_data->t_ctid = oldtup.t_self;

        /*
         * Clear all-frozen bit on visibility map if needed. We could
         * immediately reset ALL_VISIBLE, but given that the WAL logging
         * overhead would be unchanged, that doesn't seem necessarily
         * worthwhile.
         */
        if (PageIsAllVisible(BufferGetPage(buffer)) &&
            visibilitymap_clear(relation, block, vmbuffer,
                                VISIBILITYMAP_ALL_FROZEN))
            cleared_all_frozen = true;

        MarkBufferDirty(buffer);

        if (RelationNeedsWAL(relation))
        {
            xl_heap_lock xlrec;
            XLogRecPtr  recptr;

            XLogBeginInsert();
            XLogRegisterBuffer(0, buffer, REGBUF_STANDARD);

            xlrec.offnum = ItemPointerGetOffsetNumber(&oldtup.t_self);
            xlrec.locking_xid = xmax_lock_old_tuple;
            xlrec.infobits_set = compute_infobits(oldtup.t_data->t_infomask,
                                                  oldtup.t_data->t_infomask2);
            xlrec.flags =
                cleared_all_frozen ? XLH_LOCK_ALL_FROZEN_CLEARED : 0;
            XLogRegisterData((char *) &xlrec, SizeOfHeapLock);
            recptr = XLogInsert(RM_HEAP_ID, XLOG_HEAP_LOCK);
            PageSetLSN(page, recptr);
        }

        END_CRIT_SECTION();

        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

        /*
         * Let the toaster do its thing, if needed.
         *
         * Note: below this point, heaptup is the data we actually intend to
         * store into the relation; newtup is the caller's original untoasted
         * data.
         */
        if (need_toast)
        {
            /* Note we always use WAL and FSM during updates */
            heaptup = toast_insert_or_update(relation, newtup, &oldtup, 0);
            newtupsize = MAXALIGN(heaptup->t_len);
        }
        else
            heaptup = newtup;

        /*
         * Now, do we need a new page for the tuple, or not?  This is a bit
         * tricky since someone else could have added tuples to the page while
         * we weren't looking.  We have to recheck the available space after
         * reacquiring the buffer lock.  But don't bother to do that if the
         * former amount of free space is still not enough; it's unlikely
         * there's more free now than before.
         *
         * What's more, if we need to get a new page, we will need to acquire
         * buffer locks on both old and new pages.  To avoid deadlock against
         * some other backend trying to get the same two locks in the other
         * order, we must be consistent about the order we get the locks in.
         * We use the rule "lock the lower-numbered page of the relation
         * first".  To implement this, we must do RelationGetBufferForTuple
         * while not holding the lock on the old page, and we must rely on it
         * to get the locks on both pages in the correct order.
         */
        if (newtupsize > pagefree)
        {
            /* Assume there's no chance to put heaptup on same page. */
            newbuf = RelationGetBufferForTuple(relation, heaptup->t_len,
                                               buffer, 0, NULL,
                                               &vmbuffer_new, &vmbuffer);
        }
        else
        {
            /* Re-acquire the lock on the old tuple's page. */
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
            /* Re-check using the up-to-date free space */
            pagefree = PageGetHeapFreeSpace(page);
            if (newtupsize > pagefree)
            {
                /*
                 * Rats, it doesn't fit anymore.  We must now unlock and
                 * relock to avoid deadlock.  Fortunately, this path should
                 * seldom be taken.
                 */
                LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
                newbuf = RelationGetBufferForTuple(relation, heaptup->t_len,
                                                   buffer, 0, NULL,
                                                   &vmbuffer_new, &vmbuffer);
            }
            else
            {
                /* OK, it fits here, so we're done. */
                newbuf = buffer;
            }
        }
    }
    else
    {
        /* No TOAST work needed, and it'll fit on same page */
        newbuf = buffer;
        heaptup = newtup;
    }

    /*
     * We're about to do the actual update -- check for conflict first, to
     * avoid possibly having to roll back work we've just done.
     *
     * This is safe without a recheck as long as there is no possibility of
     * another process scanning the pages between this check and the update
     * being visible to the scan (i.e., exclusive buffer content lock(s) are
     * continuously held from this point until the tuple update is visible).
     *
     * For the new tuple the only check needed is at the relation level, but
     * since both tuples are in the same relation and the check for oldtup
     * will include checking the relation level, there is no benefit to a
     * separate check for the new tuple.
     */
    CheckForSerializableConflictIn(relation, &oldtup, buffer);

    /*
     * At this point newbuf and buffer are both pinned and locked, and newbuf
     * has enough space for the new tuple.  If they are the same buffer, only
     * one pin is held.
     */

    if (newbuf == buffer)
    {
        /*
         * Since the new tuple is going into the same page, we might be able
         * to do a HOT update.  Check if any of the index columns have been
         * changed. If the page was already full, we may have skipped checking
         * for index columns. If so, HOT update is possible.
         */
        if (hot_attrs_checked && !bms_overlap(modified_attrs, hot_attrs))
            use_hot_update = true;
    }
    else
    {
        /* Set a hint that the old page could use prune/defrag */
        PageSetFull(page);
    }

    /*
     * Compute replica identity tuple before entering the critical section so
     * we don't PANIC upon a memory allocation failure.
     * ExtractReplicaIdentity() will return NULL if nothing needs to be
     * logged.
     */
    old_key_tuple = ExtractReplicaIdentity(relation, &oldtup,
                                           bms_overlap(modified_attrs, id_attrs),
                                           &old_key_copied);

    /* NO EREPORT(ERROR) from here till changes are logged */
    START_CRIT_SECTION();

    /*
     * If this transaction commits, the old tuple will become DEAD sooner or
     * later.  Set flag that this page is a candidate for pruning once our xid
     * falls below the OldestXmin horizon.  If the transaction finally aborts,
     * the subsequent page pruning will be a no-op and the hint will be
     * cleared.
     *
     * XXX Should we set hint on newbuf as well?  If the transaction aborts,
     * there would be a prunable tuple in the newbuf; but for now we choose
     * not to optimize for aborts.  Note that heap_xlog_update must be kept in
     * sync if this decision changes.
     */
    PageSetPrunable(page, xid);

    if (use_hot_update)
    {
        /* Mark the old tuple as HOT-updated */
        HeapTupleSetHotUpdated(&oldtup);
        /* And mark the new tuple as heap-only */
        HeapTupleSetHeapOnly(heaptup);
        /* Mark the caller's copy too, in case different from heaptup */
        HeapTupleSetHeapOnly(newtup);
    }
    else
    {
        /* Make sure tuples are correctly marked as not-HOT */
        HeapTupleClearHotUpdated(&oldtup);
        HeapTupleClearHeapOnly(heaptup);
        HeapTupleClearHeapOnly(newtup);
    }

    RelationPutHeapTuple(relation, newbuf, heaptup, false); /* insert new tuple */


    /* Clear obsolete visibility flags, possibly set by ourselves above... */
    oldtup.t_data->t_infomask &= ~(HEAP_XMAX_BITS | HEAP_MOVED);
    oldtup.t_data->t_infomask2 &= ~HEAP_KEYS_UPDATED;
    /* ... and store info about transaction updating this tuple */
    Assert(TransactionIdIsValid(xmax_old_tuple));
    HeapTupleHeaderSetXmax(oldtup.t_data, xmax_old_tuple);
    oldtup.t_data->t_infomask |= infomask_old_tuple;
    oldtup.t_data->t_infomask2 |= infomask2_old_tuple;
    HeapTupleHeaderSetCmax(oldtup.t_data, cid, iscombo);

    /* record address of new tuple in t_ctid of old one */
    oldtup.t_data->t_ctid = heaptup->t_self;

    /* clear PD_ALL_VISIBLE flags, reset all visibilitymap bits */
    if (PageIsAllVisible(BufferGetPage(buffer)))
    {
        all_visible_cleared = true;
        PageClearAllVisible(BufferGetPage(buffer));
        visibilitymap_clear(relation, BufferGetBlockNumber(buffer),
                            vmbuffer, VISIBILITYMAP_VALID_BITS);
    }
    if (newbuf != buffer && PageIsAllVisible(BufferGetPage(newbuf)))
    {
        all_visible_cleared_new = true;
        PageClearAllVisible(BufferGetPage(newbuf));
        visibilitymap_clear(relation, BufferGetBlockNumber(newbuf),
                            vmbuffer_new, VISIBILITYMAP_VALID_BITS);
    }

    if (newbuf != buffer)
        MarkBufferDirty(newbuf);
    MarkBufferDirty(buffer);

    /* XLOG stuff */
    if (RelationNeedsWAL(relation))
    {
        XLogRecPtr  recptr;

        /*
         * For logical decoding we need combocids to properly decode the
         * catalog.
         */
        if (RelationIsAccessibleInLogicalDecoding(relation))
        {
            log_heap_new_cid(relation, &oldtup);
            log_heap_new_cid(relation, heaptup);
        }

        recptr = log_heap_update(relation, buffer,
                                 newbuf, &oldtup, heaptup,
                                 old_key_tuple,
                                 all_visible_cleared,
                                 all_visible_cleared_new);
        if (newbuf != buffer)
        {
            PageSetLSN(BufferGetPage(newbuf), recptr);
        }
        PageSetLSN(BufferGetPage(buffer), recptr);
    }

    END_CRIT_SECTION();

    if (newbuf != buffer)
        LockBuffer(newbuf, BUFFER_LOCK_UNLOCK);
    LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

    /*
     * Mark old tuple for invalidation from system caches at next command
     * boundary, and mark the new tuple for invalidation in case we abort. We
     * have to do this before releasing the buffer because oldtup is in the
     * buffer.  (heaptup is all in local memory, but it's necessary to process
     * both tuple versions in one call to inval.c so we can avoid redundant
     * sinval messages.)
     */
    CacheInvalidateHeapTuple(relation, &oldtup, heaptup);

    /* Now we can release the buffer(s) */
    if (newbuf != buffer)
        ReleaseBuffer(newbuf);
    ReleaseBuffer(buffer);
    if (BufferIsValid(vmbuffer_new))
        ReleaseBuffer(vmbuffer_new);
    if (BufferIsValid(vmbuffer))
        ReleaseBuffer(vmbuffer);

    /*
     * Release the lmgr tuple lock, if we had it.
     */
    if (have_tuple_lock)
        UnlockTupleTuplock(relation, &(oldtup.t_self), *lockmode);

    pgstat_count_heap_update(relation, use_hot_update);

    /*
     * If heaptup is a private copy, release it.  Don't forget to copy t_self
     * back to the caller's image, too.
     */
    if (heaptup != newtup)
    {
        newtup->t_self = heaptup->t_self;
        heap_freetuple(heaptup);
    }

    if (old_key_tuple != NULL && old_key_copied)
        heap_freetuple(old_key_tuple);

    bms_free(hot_attrs);
    bms_free(key_attrs);
    bms_free(id_attrs);
    bms_free(modified_attrs);
    bms_free(interesting_attrs);

    return HeapTupleMayBeUpdated;
}

/*
 * Check if the specified attribute's value is same in both given tuples.
 * Subroutine for HeapDetermineModifiedColumns.
 */
static bool
heap_tuple_attr_equals(TupleDesc tupdesc, int attrnum,
                       HeapTuple tup1, HeapTuple tup2)
{
    Datum       value1,
                value2;
    bool        isnull1,
                isnull2;
    Form_pg_attribute att;

    /*
     * If it's a whole-tuple reference, say "not equal".  It's not really
     * worth supporting this case, since it could only succeed after a no-op
     * update, which is hardly a case worth optimizing for.
     */
    if (attrnum == 0)
        return false;

    /*
     * Likewise, automatically say "not equal" for any system attribute other
     * than OID and tableOID; we cannot expect these to be consistent in a HOT
     * chain, or even to be set correctly yet in the new tuple.
     */
    if (attrnum < 0)
    {
        if (attrnum != ObjectIdAttributeNumber &&
            attrnum != TableOidAttributeNumber)
            return false;
    }

    /*
     * Extract the corresponding values.  XXX this is pretty inefficient if
     * there are many indexed columns.  Should HeapDetermineModifiedColumns do
     * a single heap_deform_tuple call on each tuple, instead?  But that
     * doesn't work for system columns ...
     */
    value1 = heap_getattr(tup1, attrnum, tupdesc, &isnull1);
    value2 = heap_getattr(tup2, attrnum, tupdesc, &isnull2);

    /*
     * If one value is NULL and other is not, then they are certainly not
     * equal
     */
    if (isnull1 != isnull2)
        return false;

    /*
     * If both are NULL, they can be considered equal.
     */
    if (isnull1)
        return true;

    /*
     * We do simple binary comparison of the two datums.  This may be overly
     * strict because there can be multiple binary representations for the
     * same logical value.  But we should be OK as long as there are no false
     * positives.  Using a type-specific equality operator is messy because
     * there could be multiple notions of equality in different operator
     * classes; furthermore, we cannot safely invoke user-defined functions
     * while holding exclusive buffer lock.
     */
    if (attrnum <= 0)
    {
        /* The only allowed system columns are OIDs, so do this */
        return (DatumGetObjectId(value1) == DatumGetObjectId(value2));
    }
    else
    {
        Assert(attrnum <= tupdesc->natts);
        att = TupleDescAttr(tupdesc, attrnum - 1);
        return datumIsEqual(value1, value2, att->attbyval, att->attlen);
    }
}

/*
 * Check which columns are being updated.
 *
 * Given an updated tuple, determine (and return into the output bitmapset),
 * from those listed as interesting, the set of columns that changed.
 *
 * The input bitmapset is destructively modified; that is OK since this is
 * invoked at most once in heap_update.
 */
static Bitmapset *
HeapDetermineModifiedColumns(Relation relation, Bitmapset *interesting_cols,
                             HeapTuple oldtup, HeapTuple newtup)
{
    int         attnum;
    Bitmapset  *modified = NULL;

    while ((attnum = bms_first_member(interesting_cols)) >= 0)
    {
        attnum += FirstLowInvalidHeapAttributeNumber;

        if (!heap_tuple_attr_equals(RelationGetDescr(relation),
                                    attnum, oldtup, newtup))
            modified = bms_add_member(modified,
                                      attnum - FirstLowInvalidHeapAttributeNumber);
    }

    return modified;
}

/*
 *  simple_heap_update - replace a tuple
 *
 * This routine may be used to update a tuple when concurrent updates of
 * the target tuple are not expected (for example, because we have a lock
 * on the relation associated with the tuple).  Any failure is reported
 * via ereport().
 */
void
simple_heap_update(Relation relation, ItemPointer otid, HeapTuple tup)
{
    HTSU_Result result;
    HeapUpdateFailureData hufd;
    LockTupleMode lockmode;

    result = heap_update(relation, otid, tup,
                         GetCurrentCommandId(true), InvalidSnapshot,
                         true /* wait for commit */ ,
                         &hufd, &lockmode);
    switch (result)
    {
        case HeapTupleSelfUpdated:
            /* Tuple was already updated in current command? */
            elog(ERROR, "tuple already updated by self");
            break;

        case HeapTupleMayBeUpdated:
            /* done successfully */
            break;

        case HeapTupleUpdated:
            elog(ERROR, "tuple concurrently updated");
            break;

        default:
            elog(ERROR, "unrecognized heap_update status: %u", result);
            break;
    }
}


/*
 * Return the MultiXactStatus corresponding to the given tuple lock mode.
 */
static MultiXactStatus
get_mxact_status_for_lock(LockTupleMode mode, bool is_update)
{
    int         retval;

    if (is_update)
        retval = tupleLockExtraInfo[mode].updstatus;
    else
        retval = tupleLockExtraInfo[mode].lockstatus;

    if (retval == -1)
        elog(ERROR, "invalid lock tuple mode %d/%s", mode,
             is_update ? "true" : "false");

    return (MultiXactStatus) retval;
}

/*
 *  heap_lock_tuple - lock a tuple in shared or exclusive mode
 *
 * Note that this acquires a buffer pin, which the caller must release.
 *
 * Input parameters:
 *  relation: relation containing tuple (caller must hold suitable lock)
 *  tuple->t_self: TID of tuple to lock (rest of struct need not be valid)
 *  cid: current command ID (used for visibility test, and stored into
 *      tuple's cmax if lock is successful)
 *  mode: indicates if shared or exclusive tuple lock is desired
 *  wait_policy: what to do if tuple lock is not available
 *  follow_updates: if true, follow the update chain to also lock descendant
 *      tuples.
 *
 * Output parameters:
 *  *tuple: all fields filled in
 *  *buffer: set to buffer holding tuple (pinned but not locked at exit)
 *  *hufd: filled in failure cases (see below)
 *
 * Function result may be:
 *  HeapTupleMayBeUpdated: lock was successfully acquired
 *  HeapTupleInvisible: lock failed because tuple was never visible to us
 *  HeapTupleSelfUpdated: lock failed because tuple updated by self
 *  HeapTupleUpdated: lock failed because tuple updated by other xact
 *  HeapTupleWouldBlock: lock couldn't be acquired and wait_policy is skip
 *
 * In the failure cases other than HeapTupleInvisible, the routine fills
 * *hufd with the tuple's t_ctid, t_xmax (resolving a possible MultiXact,
 * if necessary), and t_cmax (the last only for HeapTupleSelfUpdated,
 * since we cannot obtain cmax from a combocid generated by another
 * transaction).
 * See comments for struct HeapUpdateFailureData for additional info.
 *
 * See README.tuplock for a thorough explanation of this mechanism.
 */
HTSU_Result
heap_lock_tuple(Relation relation, HeapTuple tuple,
                CommandId cid, LockTupleMode mode, LockWaitPolicy wait_policy,
                bool follow_updates,
                Buffer *buffer, HeapUpdateFailureData *hufd)
{
    HTSU_Result result;
    ItemPointer tid = &(tuple->t_self);
    ItemId      lp;
    Page        page;
    Buffer      vmbuffer = InvalidBuffer;
    BlockNumber block;
    TransactionId xid,
                xmax;
    uint16      old_infomask,
                new_infomask,
                new_infomask2;
    bool        first_time = true;
    bool        have_tuple_lock = false;
    bool        cleared_all_frozen = false;

    *buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(tid));
    block = ItemPointerGetBlockNumber(tid);

    /*
     * Before locking the buffer, pin the visibility map page if it appears to
     * be necessary.  Since we haven't got the lock yet, someone else might be
     * in the middle of changing this, so we'll need to recheck after we have
     * the lock.
     */
    if (PageIsAllVisible(BufferGetPage(*buffer)))
        visibilitymap_pin(relation, block, &vmbuffer);

    LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);

    page = BufferGetPage(*buffer);
    lp = PageGetItemId(page, ItemPointerGetOffsetNumber(tid));
    Assert(ItemIdIsNormal(lp));

    tuple->t_data = (HeapTupleHeader) PageGetItem(page, lp);
    tuple->t_len = ItemIdGetLength(lp);
    tuple->t_tableOid = RelationGetRelid(relation);

l3:
    result = HeapTupleSatisfiesUpdate(tuple, cid, *buffer);

    if (result == HeapTupleInvisible)
    {
        /*
         * This is possible, but only when locking a tuple for ON CONFLICT
         * UPDATE.  We return this value here rather than throwing an error in
         * order to give that case the opportunity to throw a more specific
         * error.
         */
        result = HeapTupleInvisible;
        goto out_locked;
    }
    else if (result == HeapTupleBeingUpdated || result == HeapTupleUpdated)
    {
        TransactionId xwait;
        uint16      infomask;
        uint16      infomask2;
        bool        require_sleep;
        ItemPointerData t_ctid;

        /* must copy state data before unlocking buffer */
        xwait = HeapTupleHeaderGetRawXmax(tuple->t_data);
        infomask = tuple->t_data->t_infomask;
        infomask2 = tuple->t_data->t_infomask2;
        ItemPointerCopy(&tuple->t_data->t_ctid, &t_ctid);

        LockBuffer(*buffer, BUFFER_LOCK_UNLOCK);

        /*
         * If any subtransaction of the current top transaction already holds
         * a lock as strong as or stronger than what we're requesting, we
         * effectively hold the desired lock already.  We *must* succeed
         * without trying to take the tuple lock, else we will deadlock
         * against anyone wanting to acquire a stronger lock.
         *
         * Note we only do this the first time we loop on the HTSU result;
         * there is no point in testing in subsequent passes, because
         * evidently our own transaction cannot have acquired a new lock after
         * the first time we checked.
         */
        if (first_time)
        {
            first_time = false;

            if (infomask & HEAP_XMAX_IS_MULTI)
            {
                int         i;
                int         nmembers;
                MultiXactMember *members;

                /*
                 * We don't need to allow old multixacts here; if that had
                 * been the case, HeapTupleSatisfiesUpdate would have returned
                 * MayBeUpdated and we wouldn't be here.
                 */
                nmembers =
                    GetMultiXactIdMembers(xwait, &members, false,
                                          HEAP_XMAX_IS_LOCKED_ONLY(infomask));

                for (i = 0; i < nmembers; i++)
                {
                    /* only consider members of our own transaction */
                    if (!TransactionIdIsCurrentTransactionId(members[i].xid))
                        continue;

                    if (TUPLOCK_from_mxstatus(members[i].status) >= mode)
                    {
                        pfree(members);
                        result = HeapTupleMayBeUpdated;
                        goto out_unlocked;
                    }
                }

                if (members)
                    pfree(members);
            }
            else if (TransactionIdIsCurrentTransactionId(xwait))
            {
                switch (mode)
                {
                    case LockTupleKeyShare:
                        Assert(HEAP_XMAX_IS_KEYSHR_LOCKED(infomask) ||
                               HEAP_XMAX_IS_SHR_LOCKED(infomask) ||
                               HEAP_XMAX_IS_EXCL_LOCKED(infomask));
                        result = HeapTupleMayBeUpdated;
                        goto out_unlocked;
                    case LockTupleShare:
                        if (HEAP_XMAX_IS_SHR_LOCKED(infomask) ||
                            HEAP_XMAX_IS_EXCL_LOCKED(infomask))
                        {
                            result = HeapTupleMayBeUpdated;
                            goto out_unlocked;
                        }
                        break;
                    case LockTupleNoKeyExclusive:
                        if (HEAP_XMAX_IS_EXCL_LOCKED(infomask))
                        {
                            result = HeapTupleMayBeUpdated;
                            goto out_unlocked;
                        }
                        break;
                    case LockTupleExclusive:
                        if (HEAP_XMAX_IS_EXCL_LOCKED(infomask) &&
                            infomask2 & HEAP_KEYS_UPDATED)
                        {
                            result = HeapTupleMayBeUpdated;
                            goto out_unlocked;
                        }
                        break;
                }
            }
        }

        /*
         * Initially assume that we will have to wait for the locking
         * transaction(s) to finish.  We check various cases below in which
         * this can be turned off.
         */
        require_sleep = true;
        if (mode == LockTupleKeyShare)
        {
            /*
             * If we're requesting KeyShare, and there's no update present, we
             * don't need to wait.  Even if there is an update, we can still
             * continue if the key hasn't been modified.
             *
             * However, if there are updates, we need to walk the update chain
             * to mark future versions of the row as locked, too.  That way,
             * if somebody deletes that future version, we're protected
             * against the key going away.  This locking of future versions
             * could block momentarily, if a concurrent transaction is
             * deleting a key; or it could return a value to the effect that
             * the transaction deleting the key has already committed.  So we
             * do this before re-locking the buffer; otherwise this would be
             * prone to deadlocks.
             *
             * Note that the TID we're locking was grabbed before we unlocked
             * the buffer.  For it to change while we're not looking, the
             * other properties we're testing for below after re-locking the
             * buffer would also change, in which case we would restart this
             * loop above.
             */
            if (!(infomask2 & HEAP_KEYS_UPDATED))
            {
                bool        updated;

                updated = !HEAP_XMAX_IS_LOCKED_ONLY(infomask);

                /*
                 * If there are updates, follow the update chain; bail out if
                 * that cannot be done.
                 */
                if (follow_updates && updated)
                {
                    HTSU_Result res;

                    res = heap_lock_updated_tuple(relation, tuple, &t_ctid,
                                                  GetCurrentTransactionId(),
                                                  mode);
                    if (res != HeapTupleMayBeUpdated)
                    {
                        result = res;
                        /* recovery code expects to have buffer lock held */
                        LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
                        goto failed;
                    }
                }

                LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);

                /*
                 * Make sure it's still an appropriate lock, else start over.
                 * Also, if it wasn't updated before we released the lock, but
                 * is updated now, we start over too; the reason is that we
                 * now need to follow the update chain to lock the new
                 * versions.
                 */
                if (!HeapTupleHeaderIsOnlyLocked(tuple->t_data) &&
                    ((tuple->t_data->t_infomask2 & HEAP_KEYS_UPDATED) ||
                     !updated))
                    goto l3;

                /* Things look okay, so we can skip sleeping */
                require_sleep = false;

                /*
                 * Note we allow Xmax to change here; other updaters/lockers
                 * could have modified it before we grabbed the buffer lock.
                 * However, this is not a problem, because with the recheck we
                 * just did we ensure that they still don't conflict with the
                 * lock we want.
                 */
            }
        }
        else if (mode == LockTupleShare)
        {
            /*
             * If we're requesting Share, we can similarly avoid sleeping if
             * there's no update and no exclusive lock present.
             */
            if (HEAP_XMAX_IS_LOCKED_ONLY(infomask) &&
                !HEAP_XMAX_IS_EXCL_LOCKED(infomask))
            {
                LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);

                /*
                 * Make sure it's still an appropriate lock, else start over.
                 * See above about allowing xmax to change.
                 */
                if (!HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_data->t_infomask) ||
                    HEAP_XMAX_IS_EXCL_LOCKED(tuple->t_data->t_infomask))
                    goto l3;
                require_sleep = false;
            }
        }
        else if (mode == LockTupleNoKeyExclusive)
        {
            /*
             * If we're requesting NoKeyExclusive, we might also be able to
             * avoid sleeping; just ensure that there no conflicting lock
             * already acquired.
             */
            if (infomask & HEAP_XMAX_IS_MULTI)
            {
                if (!DoesMultiXactIdConflict((MultiXactId) xwait, infomask,
                                             mode))
                {
                    /*
                     * No conflict, but if the xmax changed under us in the
                     * meantime, start over.
                     */
                    LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
                    if (xmax_infomask_changed(tuple->t_data->t_infomask, infomask) ||
                        !TransactionIdEquals(HeapTupleHeaderGetRawXmax(tuple->t_data),
                                             xwait))
                        goto l3;

                    /* otherwise, we're good */
                    require_sleep = false;
                }
            }
            else if (HEAP_XMAX_IS_KEYSHR_LOCKED(infomask))
            {
                LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);

                /* if the xmax changed in the meantime, start over */
                if (xmax_infomask_changed(tuple->t_data->t_infomask, infomask) ||
                    !TransactionIdEquals(
                                         HeapTupleHeaderGetRawXmax(tuple->t_data),
                                         xwait))
                    goto l3;
                /* otherwise, we're good */
                require_sleep = false;
            }
        }

        /*
         * As a check independent from those above, we can also avoid sleeping
         * if the current transaction is the sole locker of the tuple.  Note
         * that the strength of the lock already held is irrelevant; this is
         * not about recording the lock in Xmax (which will be done regardless
         * of this optimization, below).  Also, note that the cases where we
         * hold a lock stronger than we are requesting are already handled
         * above by not doing anything.
         *
         * Note we only deal with the non-multixact case here; MultiXactIdWait
         * is well equipped to deal with this situation on its own.
         */
        if (require_sleep && !(infomask & HEAP_XMAX_IS_MULTI) &&
            TransactionIdIsCurrentTransactionId(xwait))
        {
            /* ... but if the xmax changed in the meantime, start over */
            LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
            if (xmax_infomask_changed(tuple->t_data->t_infomask, infomask) ||
                !TransactionIdEquals(HeapTupleHeaderGetRawXmax(tuple->t_data),
                                     xwait))
                goto l3;
            Assert(HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_data->t_infomask));
            require_sleep = false;
        }

        /*
         * Time to sleep on the other transaction/multixact, if necessary.
         *
         * If the other transaction is an update that's already committed,
         * then sleeping cannot possibly do any good: if we're required to
         * sleep, get out to raise an error instead.
         *
         * By here, we either have already acquired the buffer exclusive lock,
         * or we must wait for the locking transaction or multixact; so below
         * we ensure that we grab buffer lock after the sleep.
         */
        if (require_sleep && result == HeapTupleUpdated)
        {
            LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
            goto failed;
        }
        else if (require_sleep)
        {
            /*
             * Acquire tuple lock to establish our priority for the tuple, or
             * die trying.  LockTuple will release us when we are next-in-line
             * for the tuple.  We must do this even if we are share-locking.
             *
             * If we are forced to "start over" below, we keep the tuple lock;
             * this arranges that we stay at the head of the line while
             * rechecking tuple state.
             */
            if (!heap_acquire_tuplock(relation, tid, mode, wait_policy,
                                      &have_tuple_lock))
            {
                /*
                 * This can only happen if wait_policy is Skip and the lock
                 * couldn't be obtained.
                 */
                result = HeapTupleWouldBlock;
                /* recovery code expects to have buffer lock held */
                LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
                goto failed;
            }

            if (infomask & HEAP_XMAX_IS_MULTI)
            {
                MultiXactStatus status = get_mxact_status_for_lock(mode, false);

                /* We only ever lock tuples, never update them */
                if (status >= MultiXactStatusNoKeyUpdate)
                    elog(ERROR, "invalid lock mode in heap_lock_tuple");

                /* wait for multixact to end, or die trying  */
                switch (wait_policy)
                {
                    case LockWaitBlock:
                        MultiXactIdWait((MultiXactId) xwait, status, infomask,
                                        relation, &tuple->t_self, XLTW_Lock, NULL);
                        break;
                    case LockWaitSkip:
                        if (!ConditionalMultiXactIdWait((MultiXactId) xwait,
                                                        status, infomask, relation,
                                                        NULL))
                        {
                            result = HeapTupleWouldBlock;
                            /* recovery code expects to have buffer lock held */
                            LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
                            goto failed;
                        }
                        break;
                    case LockWaitError:
                        if (!ConditionalMultiXactIdWait((MultiXactId) xwait,
                                                        status, infomask, relation,
                                                        NULL))
                            ereport(ERROR,
                                    (errcode(ERRCODE_LOCK_NOT_AVAILABLE),
                                     errmsg("could not obtain lock on row in relation \"%s\"",
                                            RelationGetRelationName(relation))));

                        break;
                }

                /*
                 * Of course, the multixact might not be done here: if we're
                 * requesting a light lock mode, other transactions with light
                 * locks could still be alive, as well as locks owned by our
                 * own xact or other subxacts of this backend.  We need to
                 * preserve the surviving MultiXact members.  Note that it
                 * isn't absolutely necessary in the latter case, but doing so
                 * is simpler.
                 */
            }
            else
            {
                /* wait for regular transaction to end, or die trying */
                switch (wait_policy)
                {
                    case LockWaitBlock:
                        XactLockTableWait(xwait, relation, &tuple->t_self,
                                          XLTW_Lock);
                        break;
                    case LockWaitSkip:
                        if (!ConditionalXactLockTableWait(xwait))
                        {
                            result = HeapTupleWouldBlock;
                            /* recovery code expects to have buffer lock held */
                            LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
                            goto failed;
                        }
                        break;
                    case LockWaitError:
                        if (!ConditionalXactLockTableWait(xwait))
                            ereport(ERROR,
                                    (errcode(ERRCODE_LOCK_NOT_AVAILABLE),
                                     errmsg("could not obtain lock on row in relation \"%s\"",
                                            RelationGetRelationName(relation))));
                        break;
                }
            }

            /* if there are updates, follow the update chain */
            if (follow_updates && !HEAP_XMAX_IS_LOCKED_ONLY(infomask))
            {
                HTSU_Result res;

                res = heap_lock_updated_tuple(relation, tuple, &t_ctid,
                                              GetCurrentTransactionId(),
                                              mode);
                if (res != HeapTupleMayBeUpdated)
                {
                    result = res;
                    /* recovery code expects to have buffer lock held */
                    LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
                    goto failed;
                }
            }

            LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);

            /*
             * xwait is done, but if xwait had just locked the tuple then some
             * other xact could update this tuple before we get to this point.
             * Check for xmax change, and start over if so.
             */
            if (xmax_infomask_changed(tuple->t_data->t_infomask, infomask) ||
                !TransactionIdEquals(HeapTupleHeaderGetRawXmax(tuple->t_data),
                                     xwait))
                goto l3;

            if (!(infomask & HEAP_XMAX_IS_MULTI))
            {
                /*
                 * Otherwise check if it committed or aborted.  Note we cannot
                 * be here if the tuple was only locked by somebody who didn't
                 * conflict with us; that would have been handled above.  So
                 * that transaction must necessarily be gone by now.  But
                 * don't check for this in the multixact case, because some
                 * locker transactions might still be running.
                 */
                UpdateXmaxHintBits(tuple->t_data, *buffer, xwait);
            }
        }

        /* By here, we're certain that we hold buffer exclusive lock again */

        /*
         * We may lock if previous xmax aborted, or if it committed but only
         * locked the tuple without updating it; or if we didn't have to wait
         * at all for whatever reason.
         */
        if (!require_sleep ||
            (tuple->t_data->t_infomask & HEAP_XMAX_INVALID) ||
            HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_data->t_infomask) ||
            HeapTupleHeaderIsOnlyLocked(tuple->t_data))
            result = HeapTupleMayBeUpdated;
        else
            result = HeapTupleUpdated;
    }

failed:
    if (result != HeapTupleMayBeUpdated)
    {
        Assert(result == HeapTupleSelfUpdated || result == HeapTupleUpdated ||
               result == HeapTupleWouldBlock);
        Assert(!(tuple->t_data->t_infomask & HEAP_XMAX_INVALID));
        hufd->ctid = tuple->t_data->t_ctid;
        hufd->xmax = HeapTupleHeaderGetUpdateXid(tuple->t_data);
        if (result == HeapTupleSelfUpdated)
            hufd->cmax = HeapTupleHeaderGetCmax(tuple->t_data);
        else
            hufd->cmax = InvalidCommandId;
        goto out_locked;
    }

    /*
     * If we didn't pin the visibility map page and the page has become all
     * visible while we were busy locking the buffer, or during some
     * subsequent window during which we had it unlocked, we'll have to unlock
     * and re-lock, to avoid holding the buffer lock across I/O.  That's a bit
     * unfortunate, especially since we'll now have to recheck whether the
     * tuple has been locked or updated under us, but hopefully it won't
     * happen very often.
     */
    if (vmbuffer == InvalidBuffer && PageIsAllVisible(page))
    {
        LockBuffer(*buffer, BUFFER_LOCK_UNLOCK);
        visibilitymap_pin(relation, block, &vmbuffer);
        LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
        goto l3;
    }

    xmax = HeapTupleHeaderGetRawXmax(tuple->t_data);
    old_infomask = tuple->t_data->t_infomask;

    /*
     * If this is the first possibly-multixact-able operation in the current
     * transaction, set my per-backend OldestMemberMXactId setting. We can be
     * certain that the transaction will never become a member of any older
     * MultiXactIds than that.  (We have to do this even if we end up just
     * using our own TransactionId below, since some other backend could
     * incorporate our XID into a MultiXact immediately afterwards.)
     */
    MultiXactIdSetOldestMember();

    /*
     * Compute the new xmax and infomask to store into the tuple.  Note we do
     * not modify the tuple just yet, because that would leave it in the wrong
     * state if multixact.c elogs.
     */
    compute_new_xmax_infomask(xmax, old_infomask, tuple->t_data->t_infomask2,
                              GetCurrentTransactionId(), mode, false,
                              &xid, &new_infomask, &new_infomask2);

    START_CRIT_SECTION();

    /*
     * Store transaction information of xact locking the tuple.
     *
     * Note: Cmax is meaningless in this context, so don't set it; this avoids
     * possibly generating a useless combo CID.  Moreover, if we're locking a
     * previously updated tuple, it's important to preserve the Cmax.
     *
     * Also reset the HOT UPDATE bit, but only if there's no update; otherwise
     * we would break the HOT chain.
     */
    tuple->t_data->t_infomask &= ~HEAP_XMAX_BITS;
    tuple->t_data->t_infomask2 &= ~HEAP_KEYS_UPDATED;
    tuple->t_data->t_infomask |= new_infomask;
    tuple->t_data->t_infomask2 |= new_infomask2;
    if (HEAP_XMAX_IS_LOCKED_ONLY(new_infomask))
        HeapTupleHeaderClearHotUpdated(tuple->t_data);
    HeapTupleHeaderSetXmax(tuple->t_data, xid);

    /*
     * Make sure there is no forward chain link in t_ctid.  Note that in the
     * cases where the tuple has been updated, we must not overwrite t_ctid,
     * because it was set by the updater.  Moreover, if the tuple has been
     * updated, we need to follow the update chain to lock the new versions of
     * the tuple as well.
     */
    if (HEAP_XMAX_IS_LOCKED_ONLY(new_infomask))
        tuple->t_data->t_ctid = *tid;

    /* Clear only the all-frozen bit on visibility map if needed */
    if (PageIsAllVisible(page) &&
        visibilitymap_clear(relation, block, vmbuffer,
                            VISIBILITYMAP_ALL_FROZEN))
        cleared_all_frozen = true;


    MarkBufferDirty(*buffer);

    /*
     * XLOG stuff.  You might think that we don't need an XLOG record because
     * there is no state change worth restoring after a crash.  You would be
     * wrong however: we have just written either a TransactionId or a
     * MultiXactId that may never have been seen on disk before, and we need
     * to make sure that there are XLOG entries covering those ID numbers.
     * Else the same IDs might be re-used after a crash, which would be
     * disastrous if this page made it to disk before the crash.  Essentially
     * we have to enforce the WAL log-before-data rule even in this case.
     * (Also, in a PITR log-shipping or 2PC environment, we have to have XLOG
     * entries for everything anyway.)
     */
    if (RelationNeedsWAL(relation))
    {
        xl_heap_lock xlrec;
        XLogRecPtr  recptr;

        XLogBeginInsert();
        XLogRegisterBuffer(0, *buffer, REGBUF_STANDARD);

        xlrec.offnum = ItemPointerGetOffsetNumber(&tuple->t_self);
        xlrec.locking_xid = xid;
        xlrec.infobits_set = compute_infobits(new_infomask,
                                              tuple->t_data->t_infomask2);
        xlrec.flags = cleared_all_frozen ? XLH_LOCK_ALL_FROZEN_CLEARED : 0;
        XLogRegisterData((char *) &xlrec, SizeOfHeapLock);

        /* we don't decode row locks atm, so no need to log the origin */

        recptr = XLogInsert(RM_HEAP_ID, XLOG_HEAP_LOCK);

        PageSetLSN(page, recptr);
    }

    END_CRIT_SECTION();

    result = HeapTupleMayBeUpdated;

out_locked:
    LockBuffer(*buffer, BUFFER_LOCK_UNLOCK);

out_unlocked:
    if (BufferIsValid(vmbuffer))
        ReleaseBuffer(vmbuffer);

    /*
     * Don't update the visibility map here. Locking a tuple doesn't change
     * visibility info.
     */

    /*
     * Now that we have successfully marked the tuple as locked, we can
     * release the lmgr tuple lock, if we had it.
     */
    if (have_tuple_lock)
        UnlockTupleTuplock(relation, tid, mode);

    return result;
}

/*
 * Acquire heavyweight lock on the given tuple, in preparation for acquiring
 * its normal, Xmax-based tuple lock.
 *
 * have_tuple_lock is an input and output parameter: on input, it indicates
 * whether the lock has previously been acquired (and this function does
 * nothing in that case).  If this function returns success, have_tuple_lock
 * has been flipped to true.
 *
 * Returns false if it was unable to obtain the lock; this can only happen if
 * wait_policy is Skip.
 */
static bool
heap_acquire_tuplock(Relation relation, ItemPointer tid, LockTupleMode mode,
                     LockWaitPolicy wait_policy, bool *have_tuple_lock)
{
    if (*have_tuple_lock)
        return true;

    switch (wait_policy)
    {
        case LockWaitBlock:
            LockTupleTuplock(relation, tid, mode);
            break;

        case LockWaitSkip:
            if (!ConditionalLockTupleTuplock(relation, tid, mode))
                return false;
            break;

        case LockWaitError:
            if (!ConditionalLockTupleTuplock(relation, tid, mode))
                ereport(ERROR,
                        (errcode(ERRCODE_LOCK_NOT_AVAILABLE),
                         errmsg("could not obtain lock on row in relation \"%s\"",
                                RelationGetRelationName(relation))));
            break;
    }
    *have_tuple_lock = true;

    return true;
}

/*
 * Given an original set of Xmax and infomask, and a transaction (identified by
 * add_to_xmax) acquiring a new lock of some mode, compute the new Xmax and
 * corresponding infomasks to use on the tuple.
 *
 * Note that this might have side effects such as creating a new MultiXactId.
 *
 * Most callers will have called HeapTupleSatisfiesUpdate before this function;
 * that will have set the HEAP_XMAX_INVALID bit if the xmax was a MultiXactId
 * but it was not running anymore. There is a race condition, which is that the
 * MultiXactId may have finished since then, but that uncommon case is handled
 * either here, or within MultiXactIdExpand.
 *
 * There is a similar race condition possible when the old xmax was a regular
 * TransactionId.  We test TransactionIdIsInProgress again just to narrow the
 * window, but it's still possible to end up creating an unnecessary
 * MultiXactId.  Fortunately this is harmless.
 */
static void
compute_new_xmax_infomask(TransactionId xmax, uint16 old_infomask,
                          uint16 old_infomask2, TransactionId add_to_xmax,
                          LockTupleMode mode, bool is_update,
                          TransactionId *result_xmax, uint16 *result_infomask,
                          uint16 *result_infomask2)
{
    TransactionId new_xmax;
    uint16      new_infomask,
                new_infomask2;

    Assert(TransactionIdIsCurrentTransactionId(add_to_xmax));

l5:
    new_infomask = 0;
    new_infomask2 = 0;
    if (old_infomask & HEAP_XMAX_INVALID)
    {
        /*
         * No previous locker; we just insert our own TransactionId.
         *
         * Note that it's critical that this case be the first one checked,
         * because there are several blocks below that come back to this one
         * to implement certain optimizations; old_infomask might contain
         * other dirty bits in those cases, but we don't really care.
         */
        if (is_update)
        {
            new_xmax = add_to_xmax;
            if (mode == LockTupleExclusive)
                new_infomask2 |= HEAP_KEYS_UPDATED;
        }
        else
        {
            new_infomask |= HEAP_XMAX_LOCK_ONLY;
            switch (mode)
            {
                case LockTupleKeyShare:
                    new_xmax = add_to_xmax;
                    new_infomask |= HEAP_XMAX_KEYSHR_LOCK;
                    break;
                case LockTupleShare:
                    new_xmax = add_to_xmax;
                    new_infomask |= HEAP_XMAX_SHR_LOCK;
                    break;
                case LockTupleNoKeyExclusive:
                    new_xmax = add_to_xmax;
                    new_infomask |= HEAP_XMAX_EXCL_LOCK;
                    break;
                case LockTupleExclusive:
                    new_xmax = add_to_xmax;
                    new_infomask |= HEAP_XMAX_EXCL_LOCK;
                    new_infomask2 |= HEAP_KEYS_UPDATED;
                    break;
                default:
                    new_xmax = InvalidTransactionId;    /* silence compiler */
                    elog(ERROR, "invalid lock mode");
            }
        }
    }
    else if (old_infomask & HEAP_XMAX_IS_MULTI)
    {
        MultiXactStatus new_status;

        /*
         * Currently we don't allow XMAX_COMMITTED to be set for multis, so
         * cross-check.
         */
        Assert(!(old_infomask & HEAP_XMAX_COMMITTED));

        /*
         * A multixact together with LOCK_ONLY set but neither lock bit set
         * (i.e. a pg_upgraded share locked tuple) cannot possibly be running
         * anymore.  This check is critical for databases upgraded by
         * pg_upgrade; both MultiXactIdIsRunning and MultiXactIdExpand assume
         * that such multis are never passed.
         */
        if (HEAP_LOCKED_UPGRADED(old_infomask))
        {
            old_infomask &= ~HEAP_XMAX_IS_MULTI;
            old_infomask |= HEAP_XMAX_INVALID;
            goto l5;
        }

        /*
         * If the XMAX is already a MultiXactId, then we need to expand it to
         * include add_to_xmax; but if all the members were lockers and are
         * all gone, we can do away with the IS_MULTI bit and just set
         * add_to_xmax as the only locker/updater.  If all lockers are gone
         * and we have an updater that aborted, we can also do without a
         * multi.
         *
         * The cost of doing GetMultiXactIdMembers would be paid by
         * MultiXactIdExpand if we weren't to do this, so this check is not
         * incurring extra work anyhow.
         */
        if (!MultiXactIdIsRunning(xmax, HEAP_XMAX_IS_LOCKED_ONLY(old_infomask)))
        {
            if (HEAP_XMAX_IS_LOCKED_ONLY(old_infomask) ||
                !TransactionIdDidCommit(MultiXactIdGetUpdateXid(xmax,
                                                                old_infomask)))
            {
                /*
                 * Reset these bits and restart; otherwise fall through to
                 * create a new multi below.
                 */
                old_infomask &= ~HEAP_XMAX_IS_MULTI;
                old_infomask |= HEAP_XMAX_INVALID;
                goto l5;
            }
        }

        new_status = get_mxact_status_for_lock(mode, is_update);

        new_xmax = MultiXactIdExpand((MultiXactId) xmax, add_to_xmax,
                                     new_status);
        GetMultiXactIdHintBits(new_xmax, &new_infomask, &new_infomask2);
    }
    else if (old_infomask & HEAP_XMAX_COMMITTED)
    {
        /*
         * It's a committed update, so we need to preserve him as updater of
         * the tuple.
         */
        MultiXactStatus status;
        MultiXactStatus new_status;

        if (old_infomask2 & HEAP_KEYS_UPDATED)
            status = MultiXactStatusUpdate;
        else
            status = MultiXactStatusNoKeyUpdate;

        new_status = get_mxact_status_for_lock(mode, is_update);

        /*
         * since it's not running, it's obviously impossible for the old
         * updater to be identical to the current one, so we need not check
         * for that case as we do in the block above.
         */
        new_xmax = MultiXactIdCreate(xmax, status, add_to_xmax, new_status);
        GetMultiXactIdHintBits(new_xmax, &new_infomask, &new_infomask2);
    }
    else if (TransactionIdIsInProgress(xmax))
    {
        /*
         * If the XMAX is a valid, in-progress TransactionId, then we need to
         * create a new MultiXactId that includes both the old locker or
         * updater and our own TransactionId.
         */
        MultiXactStatus new_status;
        MultiXactStatus old_status;
        LockTupleMode old_mode;

        if (HEAP_XMAX_IS_LOCKED_ONLY(old_infomask))
        {
            if (HEAP_XMAX_IS_KEYSHR_LOCKED(old_infomask))
                old_status = MultiXactStatusForKeyShare;
            else if (HEAP_XMAX_IS_SHR_LOCKED(old_infomask))
                old_status = MultiXactStatusForShare;
            else if (HEAP_XMAX_IS_EXCL_LOCKED(old_infomask))
            {
                if (old_infomask2 & HEAP_KEYS_UPDATED)
                    old_status = MultiXactStatusForUpdate;
                else
                    old_status = MultiXactStatusForNoKeyUpdate;
            }
            else
            {
                /*
                 * LOCK_ONLY can be present alone only when a page has been
                 * upgraded by pg_upgrade.  But in that case,
                 * TransactionIdIsInProgress() should have returned false.  We
                 * assume it's no longer locked in this case.
                 */
                elog(WARNING, "LOCK_ONLY found for Xid in progress %u", xmax);
                old_infomask |= HEAP_XMAX_INVALID;
                old_infomask &= ~HEAP_XMAX_LOCK_ONLY;
                goto l5;
            }
        }
        else
        {
            /* it's an update, but which kind? */
            if (old_infomask2 & HEAP_KEYS_UPDATED)
                old_status = MultiXactStatusUpdate;
            else
                old_status = MultiXactStatusNoKeyUpdate;
        }

        old_mode = TUPLOCK_from_mxstatus(old_status);

        /*
         * If the lock to be acquired is for the same TransactionId as the
         * existing lock, there's an optimization possible: consider only the
         * strongest of both locks as the only one present, and restart.
         */
        if (xmax == add_to_xmax)
        {
            /*
             * Note that it's not possible for the original tuple to be
             * updated: we wouldn't be here because the tuple would have been
             * invisible and we wouldn't try to update it.  As a subtlety,
             * this code can also run when traversing an update chain to lock
             * future versions of a tuple.  But we wouldn't be here either,
             * because the add_to_xmax would be different from the original
             * updater.
             */
            Assert(HEAP_XMAX_IS_LOCKED_ONLY(old_infomask));

            /* acquire the strongest of both */
            if (mode < old_mode)
                mode = old_mode;
            /* mustn't touch is_update */

            old_infomask |= HEAP_XMAX_INVALID;
            goto l5;
        }

        /* otherwise, just fall back to creating a new multixact */
        new_status = get_mxact_status_for_lock(mode, is_update);
        new_xmax = MultiXactIdCreate(xmax, old_status,
                                     add_to_xmax, new_status);
        GetMultiXactIdHintBits(new_xmax, &new_infomask, &new_infomask2);
    }
    else if (!HEAP_XMAX_IS_LOCKED_ONLY(old_infomask) &&
             TransactionIdDidCommit(xmax))
    {
        /*
         * It's a committed update, so we gotta preserve him as updater of the
         * tuple.
         */
        MultiXactStatus status;
        MultiXactStatus new_status;

        if (old_infomask2 & HEAP_KEYS_UPDATED)
            status = MultiXactStatusUpdate;
        else
            status = MultiXactStatusNoKeyUpdate;

        new_status = get_mxact_status_for_lock(mode, is_update);

        /*
         * since it's not running, it's obviously impossible for the old
         * updater to be identical to the current one, so we need not check
         * for that case as we do in the block above.
         */
        new_xmax = MultiXactIdCreate(xmax, status, add_to_xmax, new_status);
        GetMultiXactIdHintBits(new_xmax, &new_infomask, &new_infomask2);
    }
    else
    {
        /*
         * Can get here iff the locking/updating transaction was running when
         * the infomask was extracted from the tuple, but finished before
         * TransactionIdIsInProgress got to run.  Deal with it as if there was
         * no locker at all in the first place.
         */
        old_infomask |= HEAP_XMAX_INVALID;
        goto l5;
    }

    *result_infomask = new_infomask;
    *result_infomask2 = new_infomask2;
    *result_xmax = new_xmax;
}

/*
 * Subroutine for heap_lock_updated_tuple_rec.
 *
 * Given a hypothetical multixact status held by the transaction identified
 * with the given xid, does the current transaction need to wait, fail, or can
 * it continue if it wanted to acquire a lock of the given mode?  "needwait"
 * is set to true if waiting is necessary; if it can continue, then
 * HeapTupleMayBeUpdated is returned.  If the lock is already held by the
 * current transaction, return HeapTupleSelfUpdated.  In case of a conflict
 * with another transaction, a different HeapTupleSatisfiesUpdate return code
 * is returned.
 *
 * The held status is said to be hypothetical because it might correspond to a
 * lock held by a single Xid, i.e. not a real MultiXactId; we express it this
 * way for simplicity of API.
 */
static HTSU_Result
test_lockmode_for_conflict(MultiXactStatus status, TransactionId xid,
                           LockTupleMode mode, bool *needwait)
{
    MultiXactStatus wantedstatus;

    *needwait = false;
    wantedstatus = get_mxact_status_for_lock(mode, false);

    /*
     * Note: we *must* check TransactionIdIsInProgress before
     * TransactionIdDidAbort/Commit; see comment at top of tqual.c for an
     * explanation.
     */
    if (TransactionIdIsCurrentTransactionId(xid))
    {
        /*
         * The tuple has already been locked by our own transaction.  This is
         * very rare but can happen if multiple transactions are trying to
         * lock an ancient version of the same tuple.
         */
        return HeapTupleSelfUpdated;
    }
    else if (TransactionIdIsInProgress(xid))
    {
        /*
         * If the locking transaction is running, what we do depends on
         * whether the lock modes conflict: if they do, then we must wait for
         * it to finish; otherwise we can fall through to lock this tuple
         * version without waiting.
         */
        if (DoLockModesConflict(LOCKMODE_from_mxstatus(status),
                                LOCKMODE_from_mxstatus(wantedstatus)))
        {
            *needwait = true;
        }

        /*
         * If we set needwait above, then this value doesn't matter;
         * otherwise, this value signals to caller that it's okay to proceed.
         */
        return HeapTupleMayBeUpdated;
    }
    else if (TransactionIdDidAbort(xid))
        return HeapTupleMayBeUpdated;
    else if (TransactionIdDidCommit(xid))
    {
        /*
         * The other transaction committed.  If it was only a locker, then the
         * lock is completely gone now and we can return success; but if it
         * was an update, then what we do depends on whether the two lock
         * modes conflict.  If they conflict, then we must report error to
         * caller. But if they don't, we can fall through to allow the current
         * transaction to lock the tuple.
         *
         * Note: the reason we worry about ISUPDATE here is because as soon as
         * a transaction ends, all its locks are gone and meaningless, and
         * thus we can ignore them; whereas its updates persist.  In the
         * TransactionIdIsInProgress case, above, we don't need to check
         * because we know the lock is still "alive" and thus a conflict needs
         * always be checked.
         */
        if (!ISUPDATE_from_mxstatus(status))
            return HeapTupleMayBeUpdated;

        if (DoLockModesConflict(LOCKMODE_from_mxstatus(status),
                                LOCKMODE_from_mxstatus(wantedstatus)))
            /* bummer */
            return HeapTupleUpdated;

        return HeapTupleMayBeUpdated;
    }

    /* Not in progress, not aborted, not committed -- must have crashed */
    return HeapTupleMayBeUpdated;
}


/*
 * Recursive part of heap_lock_updated_tuple
 *
 * Fetch the tuple pointed to by tid in rel, and mark it as locked by the given
 * xid with the given mode; if this tuple is updated, recurse to lock the new
 * version as well.
 */
static HTSU_Result
heap_lock_updated_tuple_rec(Relation rel, ItemPointer tid, TransactionId xid,
                            LockTupleMode mode)
{
    HTSU_Result result;
    ItemPointerData tupid;
    HeapTupleData mytup;
    Buffer      buf;
    uint16      new_infomask,
                new_infomask2,
                old_infomask,
                old_infomask2;
    TransactionId xmax,
                new_xmax;
    TransactionId priorXmax = InvalidTransactionId;
    bool        cleared_all_frozen = false;
    Buffer      vmbuffer = InvalidBuffer;
    BlockNumber block;

    ItemPointerCopy(tid, &tupid);

    for (;;)
    {
        new_infomask = 0;
        new_xmax = InvalidTransactionId;
        block = ItemPointerGetBlockNumber(&tupid);
        ItemPointerCopy(&tupid, &(mytup.t_self));

        if (!heap_fetch(rel, SnapshotAny, &mytup, &buf, false, NULL))
        {
            /*
             * if we fail to find the updated version of the tuple, it's
             * because it was vacuumed/pruned away after its creator
             * transaction aborted.  So behave as if we got to the end of the
             * chain, and there's no further tuple to lock: return success to
             * caller.
             */
            return HeapTupleMayBeUpdated;
        }

l4:
        CHECK_FOR_INTERRUPTS();

        /*
         * Before locking the buffer, pin the visibility map page if it
         * appears to be necessary.  Since we haven't got the lock yet,
         * someone else might be in the middle of changing this, so we'll need
         * to recheck after we have the lock.
         */
        if (PageIsAllVisible(BufferGetPage(buf)))
            visibilitymap_pin(rel, block, &vmbuffer);
        else
            vmbuffer = InvalidBuffer;

        LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);

        /*
         * If we didn't pin the visibility map page and the page has become
         * all visible while we were busy locking the buffer, we'll have to
         * unlock and re-lock, to avoid holding the buffer lock across I/O.
         * That's a bit unfortunate, but hopefully shouldn't happen often.
         */
        if (vmbuffer == InvalidBuffer && PageIsAllVisible(BufferGetPage(buf)))
        {
            LockBuffer(buf, BUFFER_LOCK_UNLOCK);
            visibilitymap_pin(rel, block, &vmbuffer);
            LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
        }

        /*
         * Check the tuple XMIN against prior XMAX, if any.  If we reached the
         * end of the chain, we're done, so return success.
         */
        if (TransactionIdIsValid(priorXmax) &&
            !TransactionIdEquals(HeapTupleHeaderGetXmin(mytup.t_data),
                                 priorXmax))
        {
            result = HeapTupleMayBeUpdated;
            goto out_locked;
        }

        /*
         * Also check Xmin: if this tuple was created by an aborted
         * (sub)transaction, then we already locked the last live one in the
         * chain, thus we're done, so return success.
         */
        if (TransactionIdDidAbort(HeapTupleHeaderGetXmin(mytup.t_data)))
        {
            UnlockReleaseBuffer(buf);
            return HeapTupleMayBeUpdated;
        }

        old_infomask = mytup.t_data->t_infomask;
        old_infomask2 = mytup.t_data->t_infomask2;
        xmax = HeapTupleHeaderGetRawXmax(mytup.t_data);

        /*
         * If this tuple version has been updated or locked by some concurrent
         * transaction(s), what we do depends on whether our lock mode
         * conflicts with what those other transactions hold, and also on the
         * status of them.
         */
        if (!(old_infomask & HEAP_XMAX_INVALID))
        {
            TransactionId rawxmax;
            bool        needwait;

            rawxmax = HeapTupleHeaderGetRawXmax(mytup.t_data);
            if (old_infomask & HEAP_XMAX_IS_MULTI)
            {
                int         nmembers;
                int         i;
                MultiXactMember *members;

                /*
                 * We don't need a test for pg_upgrade'd tuples: this is only
                 * applied to tuples after the first in an update chain.  Said
                 * first tuple in the chain may well be locked-in-9.2-and-
                 * pg_upgraded, but that one was already locked by our caller,
                 * not us; and any subsequent ones cannot be because our
                 * caller must necessarily have obtained a snapshot later than
                 * the pg_upgrade itself.
                 */
                Assert(!HEAP_LOCKED_UPGRADED(mytup.t_data->t_infomask));

                nmembers = GetMultiXactIdMembers(rawxmax, &members, false,
                                                 HEAP_XMAX_IS_LOCKED_ONLY(old_infomask));
                for (i = 0; i < nmembers; i++)
                {
                    result = test_lockmode_for_conflict(members[i].status,
                                                        members[i].xid,
                                                        mode, &needwait);

                    /*
                     * If the tuple was already locked by ourselves in a
                     * previous iteration of this (say heap_lock_tuple was
                     * forced to restart the locking loop because of a change
                     * in xmax), then we hold the lock already on this tuple
                     * version and we don't need to do anything; and this is
                     * not an error condition either.  We just need to skip
                     * this tuple and continue locking the next version in the
                     * update chain.
                     */
                    if (result == HeapTupleSelfUpdated)
                    {
                        pfree(members);
                        goto next;
                    }

                    if (needwait)
                    {
                        LockBuffer(buf, BUFFER_LOCK_UNLOCK);
                        XactLockTableWait(members[i].xid, rel,
                                          &mytup.t_self,
                                          XLTW_LockUpdated);
                        pfree(members);
                        goto l4;
                    }
                    if (result != HeapTupleMayBeUpdated)
                    {
                        pfree(members);
                        goto out_locked;
                    }
                }
                if (members)
                    pfree(members);
            }
            else
            {
                MultiXactStatus status;

                /*
                 * For a non-multi Xmax, we first need to compute the
                 * corresponding MultiXactStatus by using the infomask bits.
                 */
                if (HEAP_XMAX_IS_LOCKED_ONLY(old_infomask))
                {
                    if (HEAP_XMAX_IS_KEYSHR_LOCKED(old_infomask))
                        status = MultiXactStatusForKeyShare;
                    else if (HEAP_XMAX_IS_SHR_LOCKED(old_infomask))
                        status = MultiXactStatusForShare;
                    else if (HEAP_XMAX_IS_EXCL_LOCKED(old_infomask))
                    {
                        if (old_infomask2 & HEAP_KEYS_UPDATED)
                            status = MultiXactStatusForUpdate;
                        else
                            status = MultiXactStatusForNoKeyUpdate;
                    }
                    else
                    {
                        /*
                         * LOCK_ONLY present alone (a pg_upgraded tuple marked
                         * as share-locked in the old cluster) shouldn't be
                         * seen in the middle of an update chain.
                         */
                        elog(ERROR, "invalid lock status in tuple");
                    }
                }
                else
                {
                    /* it's an update, but which kind? */
                    if (old_infomask2 & HEAP_KEYS_UPDATED)
                        status = MultiXactStatusUpdate;
                    else
                        status = MultiXactStatusNoKeyUpdate;
                }

                result = test_lockmode_for_conflict(status, rawxmax, mode,
                                                    &needwait);

                /*
                 * If the tuple was already locked by ourselves in a previous
                 * iteration of this (say heap_lock_tuple was forced to
                 * restart the locking loop because of a change in xmax), then
                 * we hold the lock already on this tuple version and we don't
                 * need to do anything; and this is not an error condition
                 * either.  We just need to skip this tuple and continue
                 * locking the next version in the update chain.
                 */
                if (result == HeapTupleSelfUpdated)
                    goto next;

                if (needwait)
                {
                    LockBuffer(buf, BUFFER_LOCK_UNLOCK);
                    XactLockTableWait(rawxmax, rel, &mytup.t_self,
                                      XLTW_LockUpdated);
                    goto l4;
                }
                if (result !=