genam.c

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/*-------------------------------------------------------------------------
 *
 * genam.c
 *    general index access method routines
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/access/index/genam.c
 *
 * NOTES
 *    many of the old access method routines have been turned into
 *    macros and moved to genam.h -cim 4/30/91
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/genam.h"
#include "access/heapam.h"
#include "access/relscan.h"
#include "access/tableam.h"
#include "access/transam.h"
#include "catalog/index.h"
#include "lib/stringinfo.h"
#include "miscadmin.h"
#include "storage/bufmgr.h"
#include "storage/procarray.h"
#include "utils/acl.h"
#include "utils/injection_point.h"
#include "utils/lsyscache.h"
#include "utils/rel.h"
#include "utils/rls.h"
#include "utils/ruleutils.h"
#include "utils/snapmgr.h"
 
 
/* ----------------------------------------------------------------
 *      general access method routines
 *
 *      All indexed access methods use an identical scan structure.
 *      We don't know how the various AMs do locking, however, so we don't
 *      do anything about that here.
 *
 *      The intent is that an AM implementor will define a beginscan routine
 *      that calls RelationGetIndexScan, to fill in the scan, and then does
 *      whatever kind of locking he wants.
 *
 *      At the end of a scan, the AM's endscan routine undoes the locking,
 *      but does *not* call IndexScanEnd --- the higher-level index_endscan
 *      routine does that.  (We can't do it in the AM because index_endscan
 *      still needs to touch the IndexScanDesc after calling the AM.)
 *
 *      Because of this, the AM does not have a choice whether to call
 *      RelationGetIndexScan or not; its beginscan routine must return an
 *      object made by RelationGetIndexScan.  This is kinda ugly but not
 *      worth cleaning up now.
 * ----------------------------------------------------------------
 */
 
/* ----------------
 *  RelationGetIndexScan -- Create and fill an IndexScanDesc.
 *
 *      This routine creates an index scan structure and sets up initial
 *      contents for it.
 *
 *      Parameters:
 *              indexRelation -- index relation for scan.
 *              nkeys -- count of scan keys (index qual conditions).
 *              norderbys -- count of index order-by operators.
 *
 *      Returns:
 *              An initialized IndexScanDesc.
 * ----------------
 */
IndexScanDesc
RelationGetIndexScan(Relation indexRelation, int nkeys, int norderbys)
{
    IndexScanDesc scan;
 
    scan = palloc_object(IndexScanDescData);
 
    scan->heapRelation = NULL;  /* may be set later */
    scan->xs_heapfetch = NULL;
    scan->indexRelation = indexRelation;
    scan->xs_snapshot = InvalidSnapshot;    /* caller must initialize this */
    scan->numberOfKeys = nkeys;
    scan->numberOfOrderBys = norderbys;
 
    /*
     * We allocate key workspace here, but it won't get filled until amrescan.
     */
    if (nkeys > 0)
        scan->keyData = palloc_array(ScanKeyData, nkeys);
    else
        scan->keyData = NULL;
    if (norderbys > 0)
        scan->orderByData = palloc_array(ScanKeyData, norderbys);
    else
        scan->orderByData = NULL;
 
    scan->xs_want_itup = false; /* may be set later */
 
    /*
     * During recovery we ignore killed tuples and don't bother to kill them
     * either. We do this because the xmin on the primary node could easily be
     * later than the xmin on the standby node, so that what the primary
     * thinks is killed is supposed to be visible on standby. So for correct
     * MVCC for queries during recovery we must ignore these hints and check
     * all tuples. Do *not* set ignore_killed_tuples to true when running in a
     * transaction that was started during recovery. xactStartedInRecovery
     * should not be altered by index AMs.
     */
    scan->kill_prior_tuple = false;
    scan->xactStartedInRecovery = TransactionStartedDuringRecovery();
    scan->ignore_killed_tuples = !scan->xactStartedInRecovery;
 
    scan->opaque = NULL;
    scan->instrument = NULL;
 
    scan->xs_itup = NULL;
    scan->xs_itupdesc = NULL;
    scan->xs_hitup = NULL;
    scan->xs_hitupdesc = NULL;
 
    return scan;
}
 
/* ----------------
 *  IndexScanEnd -- End an index scan.
 *
 *      This routine just releases the storage acquired by
 *      RelationGetIndexScan().  Any AM-level resources are
 *      assumed to already have been released by the AM's
 *      endscan routine.
 *
 *  Returns:
 *      None.
 * ----------------
 */
void
IndexScanEnd(IndexScanDesc scan)
{
    if (scan->keyData != NULL)
        pfree(scan->keyData);
    if (scan->orderByData != NULL)
        pfree(scan->orderByData);
 
    pfree(scan);
}
 
/*
 * BuildIndexValueDescription
 *
 * Construct a string describing the contents of an index entry, in the
 * form "(key_name, ...)=(key_value, ...)".  This is currently used
 * for building unique-constraint, exclusion-constraint error messages, and
 * logical replication conflict error messages so only key columns of the index
 * are checked and printed.
 *
 * Note that if the user does not have permissions to view all of the
 * columns involved then a NULL is returned.  Returning a partial key seems
 * unlikely to be useful and we have no way to know which of the columns the
 * user provided (unlike in ExecBuildSlotValueDescription).
 *
 * The passed-in values/nulls arrays are the "raw" input to the index AM,
 * e.g. results of FormIndexDatum --- this is not necessarily what is stored
 * in the index, but it's what the user perceives to be stored.
 *
 * Note: if you change anything here, check whether
 * ExecBuildSlotPartitionKeyDescription() in execMain.c needs a similar
 * change.
 */
char *
BuildIndexValueDescription(Relation indexRelation,
                           const Datum *values, const bool *isnull)
{
    StringInfoData buf;
    Form_pg_index idxrec;
    int         indnkeyatts;
    int         i;
    int         keyno;
    Oid         indexrelid = RelationGetRelid(indexRelation);
    Oid         indrelid;
    AclResult   aclresult;
 
    indnkeyatts = IndexRelationGetNumberOfKeyAttributes(indexRelation);
 
    /*
     * Check permissions- if the user does not have access to view all of the
     * key columns then return NULL to avoid leaking data.
     *
     * First check if RLS is enabled for the relation.  If so, return NULL to
     * avoid leaking data.
     *
     * Next we need to check table-level SELECT access and then, if there is
     * no access there, check column-level permissions.
     */
    idxrec = indexRelation->rd_index;
    indrelid = idxrec->indrelid;
    Assert(indexrelid == idxrec->indexrelid);
 
    /* RLS check- if RLS is enabled then we don't return anything. */
    if (check_enable_rls(indrelid, InvalidOid, true) == RLS_ENABLED)
        return NULL;
 
    /* Table-level SELECT is enough, if the user has it */
    aclresult = pg_class_aclcheck(indrelid, GetUserId(), ACL_SELECT);
    if (aclresult != ACLCHECK_OK)
    {
        /*
         * No table-level access, so step through the columns in the index and
         * make sure the user has SELECT rights on all of them.
         */
        for (keyno = 0; keyno < indnkeyatts; keyno++)
        {
            AttrNumber  attnum = idxrec->indkey.values[keyno];
 
            /*
             * Note that if attnum == InvalidAttrNumber, then this is an index
             * based on an expression and we return no detail rather than try
             * to figure out what column(s) the expression includes and if the
             * user has SELECT rights on them.
             */
            if (attnum == InvalidAttrNumber ||
                pg_attribute_aclcheck(indrelid, attnum, GetUserId(),
                                      ACL_SELECT) != ACLCHECK_OK)
            {
                /* No access, so clean up and return */
                return NULL;
            }
        }
    }
 
    initStringInfo(&buf);
    appendStringInfo(&buf, "(%s)=(",
                     pg_get_indexdef_columns(indexrelid, true));
 
    for (i = 0; i < indnkeyatts; i++)
    {
        char       *val;
 
        if (isnull[i])
            val = "null";
        else
        {
            Oid         foutoid;
            bool        typisvarlena;
 
            /*
             * The provided data is not necessarily of the type stored in the
             * index; rather it is of the index opclass's input type. So look
             * at rd_opcintype not the index tupdesc.
             *
             * Note: this is a bit shaky for opclasses that have pseudotype
             * input types such as ANYARRAY or RECORD.  Currently, the
             * typoutput functions associated with the pseudotypes will work
             * okay, but we might have to try harder in future.
             */
            getTypeOutputInfo(indexRelation->rd_opcintype[i],
                              &foutoid, &typisvarlena);
            val = OidOutputFunctionCall(foutoid, values[i]);
        }
 
        if (i > 0)
            appendStringInfoString(&buf, ", ");
        appendStringInfoString(&buf, val);
    }
 
    appendStringInfoChar(&buf, ')');
 
    return buf.data;
}
 
/*
 * Get the snapshotConflictHorizon from the table entries pointed to by the
 * index tuples being deleted using an AM-generic approach.
 *
 * This is a table_index_delete_tuples() shim used by index AMs that only need
 * to consult the tableam to get a snapshotConflictHorizon value, and only
 * expect to delete index tuples that are already known deletable (typically
 * due to having LP_DEAD bits set).  When a snapshotConflictHorizon value
 * isn't needed in index AM's deletion WAL record, it is safe for it to skip
 * calling here entirely.
 *
 * We assume that caller index AM uses the standard IndexTuple representation,
 * with table TIDs stored in the t_tid field.  We also expect (and assert)
 * that the line pointers on page for 'itemnos' offsets are already marked
 * LP_DEAD.
 */
TransactionId
index_compute_xid_horizon_for_tuples(Relation irel,
                                     Relation hrel,
                                     Buffer ibuf,
                                     OffsetNumber *itemnos,
                                     int nitems)
{
    TM_IndexDeleteOp delstate;
    TransactionId snapshotConflictHorizon = InvalidTransactionId;
    Page        ipage = BufferGetPage(ibuf);
    IndexTuple  itup;
 
    Assert(nitems > 0);
 
    delstate.irel = irel;
    delstate.iblknum = BufferGetBlockNumber(ibuf);
    delstate.bottomup = false;
    delstate.bottomupfreespace = 0;
    delstate.ndeltids = 0;
    delstate.deltids = palloc_array(TM_IndexDelete, nitems);
    delstate.status = palloc_array(TM_IndexStatus, nitems);
 
    /* identify what the index tuples about to be deleted point to */
    for (int i = 0; i < nitems; i++)
    {
        OffsetNumber offnum = itemnos[i];
        ItemId      iitemid;
 
        iitemid = PageGetItemId(ipage, offnum);
        itup = (IndexTuple) PageGetItem(ipage, iitemid);
 
        Assert(ItemIdIsDead(iitemid));
 
        ItemPointerCopy(&itup->t_tid, &delstate.deltids[i].tid);
        delstate.deltids[i].id = delstate.ndeltids;
        delstate.status[i].idxoffnum = offnum;
        delstate.status[i].knowndeletable = true;   /* LP_DEAD-marked */
        delstate.status[i].promising = false;   /* unused */
        delstate.status[i].freespace = 0;   /* unused */
 
        delstate.ndeltids++;
    }
 
    /* determine the actual xid horizon */
    snapshotConflictHorizon = table_index_delete_tuples(hrel, &delstate);
 
    /* assert tableam agrees that all items are deletable */
    Assert(delstate.ndeltids == nitems);
 
    pfree(delstate.deltids);
    pfree(delstate.status);
 
    return snapshotConflictHorizon;
}
 
 
/* ----------------------------------------------------------------
 *      heap-or-index-scan access to system catalogs
 *
 *      These functions support system catalog accesses that normally use
 *      an index but need to be capable of being switched to heap scans
 *      if the system indexes are unavailable.
 *
 *      The specified scan keys must be compatible with the named index.
 *      Generally this means that they must constrain either all columns
 *      of the index, or the first K columns of an N-column index.
 *
 *      These routines could work with non-system tables, actually,
 *      but they're only useful when there is a known index to use with
 *      the given scan keys; so in practice they're only good for
 *      predetermined types of scans of system catalogs.
 * ----------------------------------------------------------------
 */
 
/*
 * systable_beginscan --- set up for heap-or-index scan
 *
 *  rel: catalog to scan, already opened and suitably locked
 *  indexId: OID of index to conditionally use
 *  indexOK: if false, forces a heap scan (see notes below)
 *  snapshot: time qual to use (NULL for a recent catalog snapshot)
 *  nkeys, key: scan keys
 *
 * The attribute numbers in the scan key should be set for the heap case.
 * If we choose to index, we convert them to 1..n to reference the index
 * columns.  Note this means there must be one scankey qualification per
 * index column!  This is checked by the Asserts in the normal, index-using
 * case, but won't be checked if the heapscan path is taken.
 *
 * The routine checks the normal cases for whether an indexscan is safe,
 * but caller can make additional checks and pass indexOK=false if needed.
 * In standard case indexOK can simply be constant TRUE.
 */
SysScanDesc
systable_beginscan(Relation heapRelation,
                   Oid indexId,
                   bool indexOK,
                   Snapshot snapshot,
                   int nkeys, ScanKey key)
{
    SysScanDesc sysscan;
    Relation    irel;
 
    /*
     * If this backend promised that it won't access shared catalogs during
     * logical decoding, this it the right place to verify.
     */
    Assert(!HistoricSnapshotActive() ||
           accessSharedCatalogsInDecoding ||
           !heapRelation->rd_rel->relisshared);
 
    if (indexOK &&
        !IgnoreSystemIndexes &&
        !ReindexIsProcessingIndex(indexId))
        irel = index_open(indexId, AccessShareLock);
    else
        irel = NULL;
 
    sysscan = palloc_object(SysScanDescData);
 
    sysscan->heap_rel = heapRelation;
    sysscan->irel = irel;
    sysscan->slot = table_slot_create(heapRelation, NULL);
 
    if (snapshot == NULL)
    {
        Oid         relid = RelationGetRelid(heapRelation);
 
        snapshot = RegisterSnapshot(GetCatalogSnapshot(relid));
        sysscan->snapshot = snapshot;
    }
    else
    {
        /* Caller is responsible for any snapshot. */
        sysscan->snapshot = NULL;
    }
 
    /*
     * If CheckXidAlive is set then set a flag to indicate that system table
     * scan is in-progress.  See detailed comments in xact.c where these
     * variables are declared.
     */
    if (TransactionIdIsValid(CheckXidAlive))
        bsysscan = true;
 
    if (irel)
    {
        int         i;
        ScanKey     idxkey;
 
        idxkey = palloc_array(ScanKeyData, nkeys);
 
        /* Convert attribute numbers to be index column numbers. */
        for (i = 0; i < nkeys; i++)
        {
            int         j;
 
            memcpy(&idxkey[i], &key[i], sizeof(ScanKeyData));
 
            for (j = 0; j < IndexRelationGetNumberOfAttributes(irel); j++)
            {
                if (key[i].sk_attno == irel->rd_index->indkey.values[j])
                {
                    idxkey[i].sk_attno = j + 1;
                    break;
                }
            }
            if (j == IndexRelationGetNumberOfAttributes(irel))
                elog(ERROR, "column is not in index");
        }
 
        sysscan->iscan = index_beginscan(heapRelation, irel,
                                         snapshot, NULL, nkeys, 0,
                                         SO_NONE);
        index_rescan(sysscan->iscan, idxkey, nkeys, NULL, 0);
        sysscan->scan = NULL;
 
        pfree(idxkey);
    }
    else
    {
        /*
         * We disallow synchronized scans when forced to use a heapscan on a
         * catalog.  In most cases the desired rows are near the front, so
         * that the unpredictable start point of a syncscan is a serious
         * disadvantage; and there are no compensating advantages, because
         * it's unlikely that such scans will occur in parallel.
         */
        sysscan->scan = table_beginscan_strat(heapRelation, snapshot,
                                              nkeys, key,
                                              true, false);
        sysscan->iscan = NULL;
    }
 
    return sysscan;
}
 
/*
 * HandleConcurrentAbort - Handle concurrent abort of the CheckXidAlive.
 *
 * Error out, if CheckXidAlive is aborted. We can't directly use
 * TransactionIdDidAbort as after crash such transaction might not have been
 * marked as aborted.  See detailed comments in xact.c where the variable
 * is declared.
 */
static inline void
HandleConcurrentAbort(void)
{
    if (TransactionIdIsValid(CheckXidAlive) &&
        !TransactionIdIsInProgress(CheckXidAlive) &&
        !TransactionIdDidCommit(CheckXidAlive))
        ereport(ERROR,
                (errcode(ERRCODE_TRANSACTION_ROLLBACK),
                 errmsg("transaction aborted during system catalog scan")));
}
 
/*
 * systable_getnext --- get next tuple in a heap-or-index scan
 *
 * Returns NULL if no more tuples available.
 *
 * Note that returned tuple is a reference to data in a disk buffer;
 * it must not be modified, and should be presumed inaccessible after
 * next getnext() or endscan() call.
 *
 * XXX: It'd probably make sense to offer a slot based interface, at least
 * optionally.
 */
HeapTuple
systable_getnext(SysScanDesc sysscan)
{
    HeapTuple   htup = NULL;
 
    if (sysscan->irel)
    {
        if (index_getnext_slot(sysscan->iscan, ForwardScanDirection, sysscan->slot))
        {
            bool        shouldFree;
 
            htup = ExecFetchSlotHeapTuple(sysscan->slot, false, &shouldFree);
            Assert(!shouldFree);
 
            /*
             * We currently don't need to support lossy index operators for
             * any system catalog scan.  It could be done here, using the scan
             * keys to drive the operator calls, if we arranged to save the
             * heap attnums during systable_beginscan(); this is practical
             * because we still wouldn't need to support indexes on
             * expressions.
             */
            if (sysscan->iscan->xs_recheck)
                elog(ERROR, "system catalog scans with lossy index conditions are not implemented");
        }
    }
    else
    {
        if (table_scan_getnextslot(sysscan->scan, ForwardScanDirection, sysscan->slot))
        {
            bool        shouldFree;
 
            htup = ExecFetchSlotHeapTuple(sysscan->slot, false, &shouldFree);
            Assert(!shouldFree);
        }
    }
 
    /*
     * Handle the concurrent abort while fetching the catalog tuple during
     * logical streaming of a transaction.
     */
    HandleConcurrentAbort();
 
    return htup;
}
 
/*
 * systable_recheck_tuple --- recheck visibility of most-recently-fetched tuple
 *
 * In particular, determine if this tuple would be visible to a catalog scan
 * that started now.  We don't handle the case of a non-MVCC scan snapshot,
 * because no caller needs that yet.
 *
 * This is useful to test whether an object was deleted while we waited to
 * acquire lock on it.
 *
 * Note: we don't actually *need* the tuple to be passed in, but it's a
 * good crosscheck that the caller is interested in the right tuple.
 */
bool
systable_recheck_tuple(SysScanDesc sysscan, HeapTuple tup)
{
    Snapshot    freshsnap;
    bool        result;
 
    Assert(tup == ExecFetchSlotHeapTuple(sysscan->slot, false, NULL));
 
    freshsnap = GetCatalogSnapshot(RelationGetRelid(sysscan->heap_rel));
    freshsnap = RegisterSnapshot(freshsnap);
 
    result = table_tuple_satisfies_snapshot(sysscan->heap_rel,
                                            sysscan->slot,
                                            freshsnap);
    UnregisterSnapshot(freshsnap);
 
    /*
     * Handle the concurrent abort while fetching the catalog tuple during
     * logical streaming of a transaction.
     */
    HandleConcurrentAbort();
 
    return result;
}
 
/*
 * systable_endscan --- close scan, release resources
 *
 * Note that it's still up to the caller to close the heap relation.
 */
void
systable_endscan(SysScanDesc sysscan)
{
    if (sysscan->slot)
    {
        ExecDropSingleTupleTableSlot(sysscan->slot);
        sysscan->slot = NULL;
    }
 
    if (sysscan->irel)
    {
        index_endscan(sysscan->iscan);
        index_close(sysscan->irel, AccessShareLock);
    }
    else
        table_endscan(sysscan->scan);
 
    if (sysscan->snapshot)
        UnregisterSnapshot(sysscan->snapshot);
 
    /*
     * Reset the bsysscan flag at the end of the systable scan.  See detailed
     * comments in xact.c where these variables are declared.
     */
    if (TransactionIdIsValid(CheckXidAlive))
        bsysscan = false;
 
    pfree(sysscan);
}
 
 
/*
 * systable_beginscan_ordered --- set up for ordered catalog scan
 *
 * These routines have essentially the same API as systable_beginscan etc,
 * except that they guarantee to return multiple matching tuples in
 * index order.  Also, for largely historical reasons, the index to use
 * is opened and locked by the caller, not here.
 *
 * Currently we do not support non-index-based scans here.  (In principle
 * we could do a heapscan and sort, but the uses are in places that
 * probably don't need to still work with corrupted catalog indexes.)
 * For the moment, therefore, these functions are merely the thinest of
 * wrappers around index_beginscan/index_getnext_slot.  The main reason for
 * their existence is to centralize possible future support of lossy operators
 * in catalog scans.
 */
SysScanDesc
systable_beginscan_ordered(Relation heapRelation,
                           Relation indexRelation,
                           Snapshot snapshot,
                           int nkeys, ScanKey key)
{
    SysScanDesc sysscan;
    int         i;
    ScanKey     idxkey;
 
    /* REINDEX can probably be a hard error here ... */
    if (ReindexIsProcessingIndex(RelationGetRelid(indexRelation)))
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("cannot access index \"%s\" while it is being reindexed",
                        RelationGetRelationName(indexRelation))));
    /* ... but we only throw a warning about violating IgnoreSystemIndexes */
    if (IgnoreSystemIndexes)
        elog(WARNING, "using index \"%s\" despite IgnoreSystemIndexes",
             RelationGetRelationName(indexRelation));
 
    sysscan = palloc_object(SysScanDescData);
 
    sysscan->heap_rel = heapRelation;
    sysscan->irel = indexRelation;
    sysscan->slot = table_slot_create(heapRelation, NULL);
 
    if (snapshot == NULL)
    {
        Oid         relid = RelationGetRelid(heapRelation);
 
        snapshot = RegisterSnapshot(GetCatalogSnapshot(relid));
        sysscan->snapshot = snapshot;
    }
    else
    {
        /* Caller is responsible for any snapshot. */
        sysscan->snapshot = NULL;
    }
 
    idxkey = palloc_array(ScanKeyData, nkeys);
 
    /* Convert attribute numbers to be index column numbers. */
    for (i = 0; i < nkeys; i++)
    {
        int         j;
 
        memcpy(&idxkey[i], &key[i], sizeof(ScanKeyData));
 
        for (j = 0; j < IndexRelationGetNumberOfAttributes(indexRelation); j++)
        {
            if (key[i].sk_attno == indexRelation->rd_index->indkey.values[j])
            {
                idxkey[i].sk_attno = j + 1;
                break;
            }
        }
        if (j == IndexRelationGetNumberOfAttributes(indexRelation))
            elog(ERROR, "column is not in index");
    }
 
    /*
     * If CheckXidAlive is set then set a flag to indicate that system table
     * scan is in-progress.  See detailed comments in xact.c where these
     * variables are declared.
     */
    if (TransactionIdIsValid(CheckXidAlive))
        bsysscan = true;
 
    sysscan->iscan = index_beginscan(heapRelation, indexRelation,
                                     snapshot, NULL, nkeys, 0,
                                     SO_NONE);
    index_rescan(sysscan->iscan, idxkey, nkeys, NULL, 0);
    sysscan->scan = NULL;
 
    pfree(idxkey);
 
    return sysscan;
}
 
/*
 * systable_getnext_ordered --- get next tuple in an ordered catalog scan
 */
HeapTuple
systable_getnext_ordered(SysScanDesc sysscan, ScanDirection direction)
{
    HeapTuple   htup = NULL;
 
    Assert(sysscan->irel);
    if (index_getnext_slot(sysscan->iscan, direction, sysscan->slot))
        htup = ExecFetchSlotHeapTuple(sysscan->slot, false, NULL);
 
    /* See notes in systable_getnext */
    if (htup && sysscan->iscan->xs_recheck)
        elog(ERROR, "system catalog scans with lossy index conditions are not implemented");
 
    /*
     * Handle the concurrent abort while fetching the catalog tuple during
     * logical streaming of a transaction.
     */
    HandleConcurrentAbort();
 
    return htup;
}
 
/*
 * systable_endscan_ordered --- close scan, release resources
 */
void
systable_endscan_ordered(SysScanDesc sysscan)
{
    if (sysscan->slot)
    {
        ExecDropSingleTupleTableSlot(sysscan->slot);
        sysscan->slot = NULL;
    }
 
    Assert(sysscan->irel);
    index_endscan(sysscan->iscan);
    if (sysscan->snapshot)
        UnregisterSnapshot(sysscan->snapshot);
 
    /*
     * Reset the bsysscan flag at the end of the systable scan.  See detailed
     * comments in xact.c where these variables are declared.
     */
    if (TransactionIdIsValid(CheckXidAlive))
        bsysscan = false;
 
    pfree(sysscan);
}
 
/*
 * systable_inplace_update_begin --- update a row "in place" (overwrite it)
 *
 * Overwriting violates both MVCC and transactional safety, so the uses of
 * this function in Postgres are extremely limited.  This makes no effort to
 * support updating cache key columns or other indexed columns.  Nonetheless
 * we find some places to use it.  See README.tuplock section "Locking to
 * write inplace-updated tables" and later sections for expectations of
 * readers and writers of a table that gets inplace updates.  Standard flow:
 *
 * ... [any slow preparation not requiring oldtup] ...
 * systable_inplace_update_begin([...], &tup, &inplace_state);
 * if (!HeapTupleIsValid(tup))
 *  elog(ERROR, [...]);
 * ... [buffer is exclusive-locked; mutate "tup"] ...
 * if (dirty)
 *  systable_inplace_update_finish(inplace_state, tup);
 * else
 *  systable_inplace_update_cancel(inplace_state);
 *
 * The first several params duplicate the systable_beginscan() param list.
 * "oldtupcopy" is an output parameter, assigned NULL if the key ceases to
 * find a live tuple.  (In PROC_IN_VACUUM, that is a low-probability transient
 * condition.)  If "oldtupcopy" gets non-NULL, you must pass output parameter
 * "state" to systable_inplace_update_finish() or
 * systable_inplace_update_cancel().
 */
void
systable_inplace_update_begin(Relation relation,
                              Oid indexId,
                              bool indexOK,
                              Snapshot snapshot,
                              int nkeys, const ScanKeyData *key,
                              HeapTuple *oldtupcopy,
                              void **state)
{
    int         retries = 0;
    SysScanDesc scan;
    HeapTuple   oldtup;
    BufferHeapTupleTableSlot *bslot;
 
    /*
     * For now, we don't allow parallel updates.  Unlike a regular update,
     * this should never create a combo CID, so it might be possible to relax
     * this restriction, but not without more thought and testing.  It's not
     * clear that it would be useful, anyway.
     */
    if (IsInParallelMode())
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TRANSACTION_STATE),
                 errmsg("cannot update tuples during a parallel operation")));
 
    /*
     * Accept a snapshot argument, for symmetry, but this function advances
     * its snapshot as needed to reach the tail of the updated tuple chain.
     */
    Assert(snapshot == NULL);
 
    Assert(IsInplaceUpdateRelation(relation) || !IsSystemRelation(relation));
 
    /* Loop for an exclusive-locked buffer of a non-updated tuple. */
    do
    {
        TupleTableSlot *slot;
 
        CHECK_FOR_INTERRUPTS();
 
        /*
         * Processes issuing heap_update (e.g. GRANT) at maximum speed could
         * drive us to this error.  A hostile table owner has stronger ways to
         * damage their own table, so that's minor.
         */
        if (retries++ > 10000)
            elog(ERROR, "giving up after too many tries to overwrite row");
 
        INJECTION_POINT("inplace-before-pin", NULL);
        scan = systable_beginscan(relation, indexId, indexOK, snapshot,
                                  nkeys, unconstify(ScanKeyData *, key));
        oldtup = systable_getnext(scan);
        if (!HeapTupleIsValid(oldtup))
        {
            systable_endscan(scan);
            *oldtupcopy = NULL;
            return;
        }
 
        slot = scan->slot;
        Assert(TTS_IS_BUFFERTUPLE(slot));
        bslot = (BufferHeapTupleTableSlot *) slot;
    } while (!heap_inplace_lock(scan->heap_rel,
                                bslot->base.tuple, bslot->buffer,
                                (void (*) (void *)) systable_endscan, scan));
 
    *oldtupcopy = heap_copytuple(oldtup);
    *state = scan;
}
 
/*
 * systable_inplace_update_finish --- second phase of inplace update
 *
 * The tuple cannot change size, and therefore its header fields and null
 * bitmap (if any) don't change either.
 */
void
systable_inplace_update_finish(void *state, HeapTuple tuple)
{
    SysScanDesc scan = (SysScanDesc) state;
    Relation    relation = scan->heap_rel;
    TupleTableSlot *slot = scan->slot;
    BufferHeapTupleTableSlot *bslot = (BufferHeapTupleTableSlot *) slot;
    HeapTuple   oldtup = bslot->base.tuple;
    Buffer      buffer = bslot->buffer;
 
    heap_inplace_update_and_unlock(relation, oldtup, tuple, buffer);
    systable_endscan(scan);
}
 
/*
 * systable_inplace_update_cancel --- abandon inplace update
 *
 * This is an alternative to making a no-op update.
 */
void
systable_inplace_update_cancel(void *state)
{
    SysScanDesc scan = (SysScanDesc) state;
    Relation    relation = scan->heap_rel;
    TupleTableSlot *slot = scan->slot;
    BufferHeapTupleTableSlot *bslot = (BufferHeapTupleTableSlot *) slot;
    HeapTuple   oldtup = bslot->base.tuple;
    Buffer      buffer = bslot->buffer;
 
    heap_inplace_unlock(relation, oldtup, buffer);
    systable_endscan(scan);
}