xid8funcs.c File Reference

#include "postgres.h"
#include "access/transam.h"
#include "access/xact.h"
#include "funcapi.h"
#include "lib/qunique.h"
#include "libpq/pqformat.h"
#include "miscadmin.h"
#include "storage/lwlock.h"
#include "storage/procarray.h"
#include "storage/procnumber.h"
#include "utils/builtins.h"
#include "utils/memutils.h"
#include "utils/snapmgr.h"
#include "utils/xid8.h"
#include "varatt.h"

Include dependency graph for xid8funcs.c:

Go to the source code of this file.

Data Structures
struct	pg_snapshot

Macros
#define	USE_BSEARCH_IF_NXIP_GREATER   30
#define	PG_SNAPSHOT_SIZE(nxip)    (offsetof(pg_snapshot, xip) + sizeof(FullTransactionId) * (nxip))
#define	PG_SNAPSHOT_MAX_NXIP    ((MaxAllocSize - offsetof(pg_snapshot, xip)) / sizeof(FullTransactionId))

Functions
	StaticAssertDecl (MAX_BACKENDS *2<=PG_SNAPSHOT_MAX_NXIP, "possible overflow in pg_current_snapshot()")
static bool	TransactionIdInRecentPast (FullTransactionId fxid, TransactionId *extracted_xid)
static int	cmp_fxid (const void *aa, const void *bb)
static void	sort_snapshot (pg_snapshot *snap)
static bool	is_visible_fxid (FullTransactionId value, const pg_snapshot *snap)
static StringInfo	buf_init (FullTransactionId xmin, FullTransactionId xmax)
static void	buf_add_txid (StringInfo buf, FullTransactionId fxid)
static pg_snapshot *	buf_finalize (StringInfo buf)
static pg_snapshot *	parse_snapshot (const char *str, Node *escontext)
Datum	pg_current_xact_id (PG_FUNCTION_ARGS)
Datum	pg_current_xact_id_if_assigned (PG_FUNCTION_ARGS)
Datum	pg_current_snapshot (PG_FUNCTION_ARGS)
Datum	pg_snapshot_in (PG_FUNCTION_ARGS)
Datum	pg_snapshot_out (PG_FUNCTION_ARGS)
Datum	pg_snapshot_recv (PG_FUNCTION_ARGS)
Datum	pg_snapshot_send (PG_FUNCTION_ARGS)
Datum	pg_visible_in_snapshot (PG_FUNCTION_ARGS)
Datum	pg_snapshot_xmin (PG_FUNCTION_ARGS)
Datum	pg_snapshot_xmax (PG_FUNCTION_ARGS)
Datum	pg_snapshot_xip (PG_FUNCTION_ARGS)
Datum	pg_xact_status (PG_FUNCTION_ARGS)

Macro Definition Documentation

PG_SNAPSHOT_MAX_NXIP

#define PG_SNAPSHOT_MAX_NXIP    ((MaxAllocSize - offsetof(pg_snapshot, xip)) / sizeof(FullTransactionId))

Definition at line 73 of file xid8funcs.c.

PG_SNAPSHOT_SIZE

#define PG_SNAPSHOT_SIZE

(

nxip

)

(offsetof(pg_snapshot, xip) + sizeof(FullTransactionId) * (nxip))

Definition at line 71 of file xid8funcs.c.

USE_BSEARCH_IF_NXIP_GREATER

#define USE_BSEARCH_IF_NXIP_GREATER   30

Definition at line 49 of file xid8funcs.c.

Function Documentation

buf_add_txid()

static void buf_add_txid ( StringInfo  buf, FullTransactionId  fxid  )

static

Definition at line 238 of file xid8funcs.c.

{
    pg_snapshot *snap = (pg_snapshot *) buf->data;
 
    /* do this before possible realloc */
    snap->nxip++;
 
    appendBinaryStringInfo(buf, &fxid, sizeof(fxid));
}

References appendBinaryStringInfo(), buf, and pg_snapshot::nxip.

Referenced by parse_snapshot().

buf_finalize()

static pg_snapshot * buf_finalize ( StringInfo  buf )

static

Definition at line 249 of file xid8funcs.c.

{
    pg_snapshot *snap = (pg_snapshot *) buf->data;
 
    SET_VARSIZE(snap, buf->len);
 
    /* buf is not needed anymore */
    buf->data = NULL;
    pfree(buf);
 
    return snap;
}

References buf, pfree(), and SET_VARSIZE().

Referenced by parse_snapshot().

buf_init()

static StringInfo buf_init ( FullTransactionId  xmin, FullTransactionId  xmax  )

static

Definition at line 223 of file xid8funcs.c.

{
    pg_snapshot snap;
    StringInfo  buf;
 
    snap.xmin = xmin;
    snap.xmax = xmax;
    snap.nxip = 0;
 
    buf = makeStringInfo();
    appendBinaryStringInfo(buf, &snap, PG_SNAPSHOT_SIZE(0));
    return buf;
}

References appendBinaryStringInfo(), buf, makeStringInfo(), pg_snapshot::nxip, PG_SNAPSHOT_SIZE, pg_snapshot::xmax, and pg_snapshot::xmin.

Referenced by parse_snapshot().

cmp_fxid()

static int cmp_fxid ( const void *  aa, const void *  bb  )

static

Definition at line 154 of file xid8funcs.c.

{
    FullTransactionId a = *(const FullTransactionId *) aa;
    FullTransactionId b = *(const FullTransactionId *) bb;
 
    if (FullTransactionIdPrecedes(a, b))
        return -1;
    if (FullTransactionIdPrecedes(b, a))
        return 1;
    return 0;
}

References a, b, and FullTransactionIdPrecedes.

Referenced by is_visible_fxid(), and sort_snapshot().

is_visible_fxid()

static bool is_visible_fxid ( FullTransactionId  value, const pg_snapshot *  snap  )

static

Definition at line 188 of file xid8funcs.c.

{
    if (FullTransactionIdPrecedes(value, snap->xmin))
        return true;
    else if (!FullTransactionIdPrecedes(value, snap->xmax))
        return false;
#ifdef USE_BSEARCH_IF_NXIP_GREATER
    else if (snap->nxip > USE_BSEARCH_IF_NXIP_GREATER)
    {
        void       *res;
 
        res = bsearch(&value, snap->xip, snap->nxip, sizeof(FullTransactionId),
                      cmp_fxid);
        /* if found, transaction is still in progress */
        return (res) ? false : true;
    }
#endif
    else
    {
        uint32      i;
 
        for (i = 0; i < snap->nxip; i++)
        {
            if (FullTransactionIdEquals(value, snap->xip[i]))
                return false;
        }
        return true;
    }
}

References cmp_fxid(), false, FullTransactionIdEquals, FullTransactionIdPrecedes, i, pg_snapshot::nxip, USE_BSEARCH_IF_NXIP_GREATER, value, pg_snapshot::xip, pg_snapshot::xmax, and pg_snapshot::xmin.

Referenced by pg_visible_in_snapshot().

parse_snapshot()

static pg_snapshot * parse_snapshot ( const char *  str, Node *  escontext  )

static

Definition at line 266 of file xid8funcs.c.

{
    FullTransactionId xmin;
    FullTransactionId xmax;
    FullTransactionId last_val = InvalidFullTransactionId;
    FullTransactionId val;
    const char *str_start = str;
    char       *endp;
    StringInfo  buf;
 
    xmin = FullTransactionIdFromU64(strtou64(str, &endp, 10));
    if (*endp != ':')
        goto bad_format;
    str = endp + 1;
 
    xmax = FullTransactionIdFromU64(strtou64(str, &endp, 10));
    if (*endp != ':')
        goto bad_format;
    str = endp + 1;
 
    /* it should look sane */
    if (!FullTransactionIdIsValid(xmin) ||
        !FullTransactionIdIsValid(xmax) ||
        FullTransactionIdPrecedes(xmax, xmin))
        goto bad_format;
 
    /* allocate buffer */
    buf = buf_init(xmin, xmax);
 
    /* loop over values */
    while (*str != '\0')
    {
        /* read next value */
        val = FullTransactionIdFromU64(strtou64(str, &endp, 10));
        str = endp;
 
        /* require the input to be in order */
        if (FullTransactionIdPrecedes(val, xmin) ||
            FullTransactionIdFollowsOrEquals(val, xmax) ||
            FullTransactionIdPrecedes(val, last_val))
            goto bad_format;
 
        /* skip duplicates */
        if (!FullTransactionIdEquals(val, last_val))
            buf_add_txid(buf, val);
        last_val = val;
 
        if (*str == ',')
            str++;
        else if (*str != '\0')
            goto bad_format;
    }
 
    return buf_finalize(buf);
 
bad_format:
    ereturn(escontext, NULL,
            (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
             errmsg("invalid input syntax for type %s: \"%s\"",
                    "pg_snapshot", str_start)));
}

References buf, buf_add_txid(), buf_finalize(), buf_init(), ereturn, errcode(), errmsg(), FullTransactionIdEquals, FullTransactionIdFollowsOrEquals, FullTransactionIdFromU64(), FullTransactionIdIsValid, FullTransactionIdPrecedes, InvalidFullTransactionId, str, and val.

Referenced by pg_snapshot_in().

pg_current_snapshot()

Datum pg_current_snapshot

(

PG_FUNCTION_ARGS

)

Definition at line 371 of file xid8funcs.c.

{
    pg_snapshot *snap;
    uint32      nxip,
                i;
    Snapshot    cur;
    FullTransactionId next_fxid = ReadNextFullTransactionId();
 
    cur = GetActiveSnapshot();
    if (cur == NULL)
        elog(ERROR, "no active snapshot set");
 
    /* allocate */
    nxip = cur->xcnt;
    snap = palloc(PG_SNAPSHOT_SIZE(nxip));
 
    /*
     * Fill.  This is the current backend's active snapshot, so MyProc->xmin
     * is <= all these XIDs.  As long as that remains so, oldestXid can't
     * advance past any of these XIDs.  Hence, these XIDs remain allowable
     * relative to next_fxid.
     */
    snap->xmin = FullTransactionIdFromAllowableAt(next_fxid, cur->xmin);
    snap->xmax = FullTransactionIdFromAllowableAt(next_fxid, cur->xmax);
    snap->nxip = nxip;
    for (i = 0; i < nxip; i++)
        snap->xip[i] =
            FullTransactionIdFromAllowableAt(next_fxid, cur->xip[i]);
 
    /*
     * We want them guaranteed to be in ascending order.  This also removes
     * any duplicate xids.  Normally, an XID can only be assigned to one
     * backend, but when preparing a transaction for two-phase commit, there
     * is a transient state when both the original backend and the dummy
     * PGPROC entry reserved for the prepared transaction hold the same XID.
     */
    sort_snapshot(snap);
 
    /* set size after sorting, because it may have removed duplicate xips */
    SET_VARSIZE(snap, PG_SNAPSHOT_SIZE(snap->nxip));
 
    PG_RETURN_POINTER(snap);
}

References cur, elog, ERROR, FullTransactionIdFromAllowableAt(), GetActiveSnapshot(), i, pg_snapshot::nxip, palloc(), PG_RETURN_POINTER, PG_SNAPSHOT_SIZE, ReadNextFullTransactionId(), SET_VARSIZE(), sort_snapshot(), pg_snapshot::xip, pg_snapshot::xmax, and pg_snapshot::xmin.

pg_current_xact_id()

Datum pg_current_xact_id

(

PG_FUNCTION_ARGS

)

Definition at line 335 of file xid8funcs.c.

{
    /*
     * Must prevent during recovery because if an xid is not assigned we try
     * to assign one, which would fail. Programs already rely on this function
     * to always return a valid current xid, so we should not change this to
     * return NULL or similar invalid xid.
     */
    PreventCommandDuringRecovery("pg_current_xact_id()");
 
    PG_RETURN_FULLTRANSACTIONID(GetTopFullTransactionId());
}

References GetTopFullTransactionId(), PG_RETURN_FULLTRANSACTIONID, and PreventCommandDuringRecovery().

pg_current_xact_id_if_assigned()

Datum pg_current_xact_id_if_assigned

(

PG_FUNCTION_ARGS

)

Definition at line 353 of file xid8funcs.c.

{
    FullTransactionId topfxid = GetTopFullTransactionIdIfAny();
 
    if (!FullTransactionIdIsValid(topfxid))
        PG_RETURN_NULL();
 
    PG_RETURN_FULLTRANSACTIONID(topfxid);
}

References FullTransactionIdIsValid, GetTopFullTransactionIdIfAny(), PG_RETURN_FULLTRANSACTIONID, and PG_RETURN_NULL.

pg_snapshot_in()

Datum pg_snapshot_in

(

PG_FUNCTION_ARGS

)

Definition at line 421 of file xid8funcs.c.

{
    char       *str = PG_GETARG_CSTRING(0);
    pg_snapshot *snap;
 
    snap = parse_snapshot(str, fcinfo->context);
 
    PG_RETURN_POINTER(snap);
}

References parse_snapshot(), PG_GETARG_CSTRING, PG_RETURN_POINTER, and str.

pg_snapshot_out()

Datum pg_snapshot_out

(

PG_FUNCTION_ARGS

)

Definition at line 437 of file xid8funcs.c.

{
    pg_snapshot *snap = (pg_snapshot *) PG_GETARG_VARLENA_P(0);
    StringInfoData str;
    uint32      i;
 
    initStringInfo(&str);
 
    appendStringInfo(&str, UINT64_FORMAT ":",
                     U64FromFullTransactionId(snap->xmin));
    appendStringInfo(&str, UINT64_FORMAT ":",
                     U64FromFullTransactionId(snap->xmax));
 
    for (i = 0; i < snap->nxip; i++)
    {
        if (i > 0)
            appendStringInfoChar(&str, ',');
        appendStringInfo(&str, UINT64_FORMAT,
                         U64FromFullTransactionId(snap->xip[i]));
    }
 
    PG_RETURN_CSTRING(str.data);
}

References appendStringInfo(), appendStringInfoChar(), i, initStringInfo(), pg_snapshot::nxip, PG_GETARG_VARLENA_P, PG_RETURN_CSTRING, str, U64FromFullTransactionId, UINT64_FORMAT, pg_snapshot::xip, pg_snapshot::xmax, and pg_snapshot::xmin.

pg_snapshot_recv()

Datum pg_snapshot_recv

(

PG_FUNCTION_ARGS

)

Definition at line 469 of file xid8funcs.c.

{
    StringInfo  buf = (StringInfo) PG_GETARG_POINTER(0);
    pg_snapshot *snap;
    FullTransactionId last = InvalidFullTransactionId;
    int         nxip;
    int         i;
    FullTransactionId xmin;
    FullTransactionId xmax;
 
    /* load and validate nxip */
    nxip = pq_getmsgint(buf, 4);
    if (nxip < 0 || nxip > PG_SNAPSHOT_MAX_NXIP)
        goto bad_format;
 
    xmin = FullTransactionIdFromU64((uint64) pq_getmsgint64(buf));
    xmax = FullTransactionIdFromU64((uint64) pq_getmsgint64(buf));
    if (!FullTransactionIdIsValid(xmin) ||
        !FullTransactionIdIsValid(xmax) ||
        FullTransactionIdPrecedes(xmax, xmin))
        goto bad_format;
 
    snap = palloc(PG_SNAPSHOT_SIZE(nxip));
    snap->xmin = xmin;
    snap->xmax = xmax;
 
    for (i = 0; i < nxip; i++)
    {
        FullTransactionId cur =
            FullTransactionIdFromU64((uint64) pq_getmsgint64(buf));
 
        if (FullTransactionIdPrecedes(cur, last) ||
            FullTransactionIdPrecedes(cur, xmin) ||
            FullTransactionIdPrecedes(xmax, cur))
            goto bad_format;
 
        /* skip duplicate xips */
        if (FullTransactionIdEquals(cur, last))
        {
            i--;
            nxip--;
            continue;
        }
 
        snap->xip[i] = cur;
        last = cur;
    }
    snap->nxip = nxip;
    SET_VARSIZE(snap, PG_SNAPSHOT_SIZE(nxip));
    PG_RETURN_POINTER(snap);
 
bad_format:
    ereport(ERROR,
            (errcode(ERRCODE_INVALID_BINARY_REPRESENTATION),
             errmsg("invalid external pg_snapshot data")));
    PG_RETURN_POINTER(NULL);    /* keep compiler quiet */
}

References buf, cur, ereport, errcode(), errmsg(), ERROR, FullTransactionIdEquals, FullTransactionIdFromU64(), FullTransactionIdIsValid, FullTransactionIdPrecedes, i, InvalidFullTransactionId, pg_snapshot::nxip, palloc(), PG_GETARG_POINTER, PG_RETURN_POINTER, PG_SNAPSHOT_MAX_NXIP, PG_SNAPSHOT_SIZE, pq_getmsgint(), pq_getmsgint64(), SET_VARSIZE(), pg_snapshot::xip, pg_snapshot::xmax, and pg_snapshot::xmin.

pg_snapshot_send()

Datum pg_snapshot_send

(

PG_FUNCTION_ARGS

)

Definition at line 535 of file xid8funcs.c.

{
    pg_snapshot *snap = (pg_snapshot *) PG_GETARG_VARLENA_P(0);
    StringInfoData buf;
    uint32      i;
 
    pq_begintypsend(&buf);
    pq_sendint32(&buf, snap->nxip);
    pq_sendint64(&buf, (int64) U64FromFullTransactionId(snap->xmin));
    pq_sendint64(&buf, (int64) U64FromFullTransactionId(snap->xmax));
    for (i = 0; i < snap->nxip; i++)
        pq_sendint64(&buf, (int64) U64FromFullTransactionId(snap->xip[i]));
    PG_RETURN_BYTEA_P(pq_endtypsend(&buf));
}

References buf, i, pg_snapshot::nxip, PG_GETARG_VARLENA_P, PG_RETURN_BYTEA_P, pq_begintypsend(), pq_endtypsend(), pq_sendint32(), pq_sendint64(), U64FromFullTransactionId, pg_snapshot::xip, pg_snapshot::xmax, and pg_snapshot::xmin.

pg_snapshot_xip()

Datum pg_snapshot_xip

(

PG_FUNCTION_ARGS

)

Definition at line 596 of file xid8funcs.c.

{
    FuncCallContext *fctx;
    pg_snapshot *snap;
    FullTransactionId value;
 
    /* on first call initialize fctx and get copy of snapshot */
    if (SRF_IS_FIRSTCALL())
    {
        pg_snapshot *arg = (pg_snapshot *) PG_GETARG_VARLENA_P(0);
 
        fctx = SRF_FIRSTCALL_INIT();
 
        /* make a copy of user snapshot */
        snap = MemoryContextAlloc(fctx->multi_call_memory_ctx, VARSIZE(arg));
        memcpy(snap, arg, VARSIZE(arg));
 
        fctx->user_fctx = snap;
    }
 
    /* return values one-by-one */
    fctx = SRF_PERCALL_SETUP();
    snap = fctx->user_fctx;
    if (fctx->call_cntr < snap->nxip)
    {
        value = snap->xip[fctx->call_cntr];
        SRF_RETURN_NEXT(fctx, FullTransactionIdGetDatum(value));
    }
    else
    {
        SRF_RETURN_DONE(fctx);
    }
}

References arg, FuncCallContext::call_cntr, FullTransactionIdGetDatum(), MemoryContextAlloc(), FuncCallContext::multi_call_memory_ctx, pg_snapshot::nxip, PG_GETARG_VARLENA_P, SRF_FIRSTCALL_INIT, SRF_IS_FIRSTCALL, SRF_PERCALL_SETUP, SRF_RETURN_DONE, SRF_RETURN_NEXT, FuncCallContext::user_fctx, value, VARSIZE(), and pg_snapshot::xip.

pg_snapshot_xmax()

Datum pg_snapshot_xmax

(

PG_FUNCTION_ARGS

)

Definition at line 583 of file xid8funcs.c.

{
    pg_snapshot *snap = (pg_snapshot *) PG_GETARG_VARLENA_P(0);
 
    PG_RETURN_FULLTRANSACTIONID(snap->xmax);
}

References PG_GETARG_VARLENA_P, PG_RETURN_FULLTRANSACTIONID, and pg_snapshot::xmax.

pg_snapshot_xmin()

Datum pg_snapshot_xmin

(

PG_FUNCTION_ARGS

)

Definition at line 570 of file xid8funcs.c.

{
    pg_snapshot *snap = (pg_snapshot *) PG_GETARG_VARLENA_P(0);
 
    PG_RETURN_FULLTRANSACTIONID(snap->xmin);
}

References PG_GETARG_VARLENA_P, PG_RETURN_FULLTRANSACTIONID, and pg_snapshot::xmin.

pg_visible_in_snapshot()

Datum pg_visible_in_snapshot

(

PG_FUNCTION_ARGS

)

Definition at line 556 of file xid8funcs.c.

{
    FullTransactionId value = PG_GETARG_FULLTRANSACTIONID(0);
    pg_snapshot *snap = (pg_snapshot *) PG_GETARG_VARLENA_P(1);
 
    PG_RETURN_BOOL(is_visible_fxid(value, snap));
}

References is_visible_fxid(), PG_GETARG_FULLTRANSACTIONID, PG_GETARG_VARLENA_P, PG_RETURN_BOOL, and value.

pg_xact_status()

Datum pg_xact_status

(

PG_FUNCTION_ARGS

)

Definition at line 641 of file xid8funcs.c.

{
    const char *status;
    FullTransactionId fxid = PG_GETARG_FULLTRANSACTIONID(0);
    TransactionId xid;
 
    /*
     * We must protect against concurrent truncation of clog entries to avoid
     * an I/O error on SLRU lookup.
     */
    LWLockAcquire(XactTruncationLock, LW_SHARED);
    if (TransactionIdInRecentPast(fxid, &xid))
    {
        Assert(TransactionIdIsValid(xid));
 
        /*
         * Like when doing visibility checks on a row, check whether the
         * transaction is still in progress before looking into the CLOG.
         * Otherwise we would incorrectly return "committed" for a transaction
         * that is committing and has already updated the CLOG, but hasn't
         * removed its XID from the proc array yet. (See comment on that race
         * condition at the top of heapam_visibility.c)
         */
        if (TransactionIdIsInProgress(xid))
            status = "in progress";
        else if (TransactionIdDidCommit(xid))
            status = "committed";
        else
        {
            /* it must have aborted or crashed */
            status = "aborted";
        }
    }
    else
    {
        status = NULL;
    }
    LWLockRelease(XactTruncationLock);
 
    if (status == NULL)
        PG_RETURN_NULL();
    else
        PG_RETURN_TEXT_P(cstring_to_text(status));
}

References Assert(), cstring_to_text(), LW_SHARED, LWLockAcquire(), LWLockRelease(), PG_GETARG_FULLTRANSACTIONID, PG_RETURN_NULL, PG_RETURN_TEXT_P, TransactionIdDidCommit(), TransactionIdInRecentPast(), TransactionIdIsInProgress(), and TransactionIdIsValid.

sort_snapshot()

static void sort_snapshot ( pg_snapshot *  snap )

static

Definition at line 174 of file xid8funcs.c.

{
    if (snap->nxip > 1)
    {
        qsort(snap->xip, snap->nxip, sizeof(FullTransactionId), cmp_fxid);
        snap->nxip = qunique(snap->xip, snap->nxip, sizeof(FullTransactionId),
                             cmp_fxid);
    }
}

References cmp_fxid(), pg_snapshot::nxip, qsort, qunique(), and pg_snapshot::xip.

Referenced by pg_current_snapshot().

StaticAssertDecl()

StaticAssertDecl	(	MAX_BACKENDS *2<=	PG_SNAPSHOT_MAX_NXIP,
		"possible overflow in pg_current_snapshot()"
	)

TransactionIdInRecentPast()

static bool TransactionIdInRecentPast ( FullTransactionId  fxid, TransactionId *  extracted_xid  )

static

Definition at line 98 of file xid8funcs.c.

{
    TransactionId xid = XidFromFullTransactionId(fxid);
    FullTransactionId now_fullxid;
    TransactionId oldest_clog_xid;
    FullTransactionId oldest_clog_fxid;
 
    now_fullxid = ReadNextFullTransactionId();
 
    if (extracted_xid != NULL)
        *extracted_xid = xid;
 
    if (!TransactionIdIsValid(xid))
        return false;
 
    /* For non-normal transaction IDs, we can ignore the epoch. */
    if (!TransactionIdIsNormal(xid))
        return true;
 
    /* If the transaction ID is in the future, throw an error. */
    if (!FullTransactionIdPrecedes(fxid, now_fullxid))
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("transaction ID %" PRIu64 " is in the future",
                        U64FromFullTransactionId(fxid))));
 
    /*
     * TransamVariables->oldestClogXid is protected by XactTruncationLock, but
     * we don't acquire that lock here.  Instead, we require the caller to
     * acquire it, because the caller is presumably going to look up the
     * returned XID.  If we took and released the lock within this function, a
     * CLOG truncation could occur before the caller finished with the XID.
     */
    Assert(LWLockHeldByMe(XactTruncationLock));
 
    /*
     * If fxid is not older than TransamVariables->oldestClogXid, the relevant
     * CLOG entry is guaranteed to still exist.
     *
     * TransamVariables->oldestXid governs allowable XIDs.  Usually,
     * oldestClogXid==oldestXid.  It's also possible for oldestClogXid to
     * follow oldestXid, in which case oldestXid might advance after our
     * ReadNextFullTransactionId() call.  If oldestXid has advanced, that
     * advancement reinstated the usual oldestClogXid==oldestXid.  Whether or
     * not that happened, oldestClogXid is allowable relative to now_fullxid.
     */
    oldest_clog_xid = TransamVariables->oldestClogXid;
    oldest_clog_fxid =
        FullTransactionIdFromAllowableAt(now_fullxid, oldest_clog_xid);
    return !FullTransactionIdPrecedes(fxid, oldest_clog_fxid);
}

References Assert(), ereport, errcode(), errmsg(), ERROR, FullTransactionIdFromAllowableAt(), FullTransactionIdPrecedes, LWLockHeldByMe(), TransamVariablesData::oldestClogXid, ReadNextFullTransactionId(), TransactionIdIsNormal, TransactionIdIsValid, TransamVariables, U64FromFullTransactionId, and XidFromFullTransactionId.

Referenced by pg_xact_status().