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Commit2ca19aa

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Close race condition between datfrozen and relfrozen updates.
vac_update_datfrozenxid() did multiple loads of relfrozenxid andrelminmxid from buffer memory, and it assumed each would get the samevalue. Not so if a concurrent vac_update_relstats() did an inplaceupdate. Commit2d2e40e fixed the samekind of bug in vac_truncate_clog(). Today's bug could cause therel-level field and XIDs in the rel's rows to precede the db-levelfield. A cluster having such values should VACUUM affected tables.Back-patch to v12 (all supported versions).Discussion:https://postgr.es/m/20240423003956.e7.nmisch@google.com
1 parent617a239 commit2ca19aa

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-12
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1 file changed

+16
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‎src/backend/commands/vacuum.c

Lines changed: 16 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -1480,6 +1480,8 @@ vac_update_datfrozenxid(void)
14801480
/*
14811481
* We must seqscan pg_class to find the minimum Xid, because there is no
14821482
* index that can help us here.
1483+
*
1484+
* See vac_truncate_clog() for the race condition to prevent.
14831485
*/
14841486
relation=table_open(RelationRelationId,AccessShareLock);
14851487

@@ -1488,7 +1490,9 @@ vac_update_datfrozenxid(void)
14881490

14891491
while ((classTup=systable_getnext(scan))!=NULL)
14901492
{
1491-
Form_pg_classclassForm= (Form_pg_class)GETSTRUCT(classTup);
1493+
volatileFormData_pg_class*classForm= (Form_pg_class)GETSTRUCT(classTup);
1494+
TransactionIdrelfrozenxid=classForm->relfrozenxid;
1495+
TransactionIdrelminmxid=classForm->relminmxid;
14921496

14931497
/*
14941498
* Only consider relations able to hold unfrozen XIDs (anything else
@@ -1498,8 +1502,8 @@ vac_update_datfrozenxid(void)
14981502
classForm->relkind!=RELKIND_MATVIEW&&
14991503
classForm->relkind!=RELKIND_TOASTVALUE)
15001504
{
1501-
Assert(!TransactionIdIsValid(classForm->relfrozenxid));
1502-
Assert(!MultiXactIdIsValid(classForm->relminmxid));
1505+
Assert(!TransactionIdIsValid(relfrozenxid));
1506+
Assert(!MultiXactIdIsValid(relminmxid));
15031507
continue;
15041508
}
15051509

@@ -1518,34 +1522,34 @@ vac_update_datfrozenxid(void)
15181522
* before those relations have been scanned and cleaned up.
15191523
*/
15201524

1521-
if (TransactionIdIsValid(classForm->relfrozenxid))
1525+
if (TransactionIdIsValid(relfrozenxid))
15221526
{
1523-
Assert(TransactionIdIsNormal(classForm->relfrozenxid));
1527+
Assert(TransactionIdIsNormal(relfrozenxid));
15241528

15251529
/* check for values in the future */
1526-
if (TransactionIdPrecedes(lastSaneFrozenXid,classForm->relfrozenxid))
1530+
if (TransactionIdPrecedes(lastSaneFrozenXid,relfrozenxid))
15271531
{
15281532
bogus= true;
15291533
break;
15301534
}
15311535

15321536
/* determine new horizon */
1533-
if (TransactionIdPrecedes(classForm->relfrozenxid,newFrozenXid))
1534-
newFrozenXid=classForm->relfrozenxid;
1537+
if (TransactionIdPrecedes(relfrozenxid,newFrozenXid))
1538+
newFrozenXid=relfrozenxid;
15351539
}
15361540

1537-
if (MultiXactIdIsValid(classForm->relminmxid))
1541+
if (MultiXactIdIsValid(relminmxid))
15381542
{
15391543
/* check for values in the future */
1540-
if (MultiXactIdPrecedes(lastSaneMinMulti,classForm->relminmxid))
1544+
if (MultiXactIdPrecedes(lastSaneMinMulti,relminmxid))
15411545
{
15421546
bogus= true;
15431547
break;
15441548
}
15451549

15461550
/* determine new horizon */
1547-
if (MultiXactIdPrecedes(classForm->relminmxid,newMinMulti))
1548-
newMinMulti=classForm->relminmxid;
1551+
if (MultiXactIdPrecedes(relminmxid,newMinMulti))
1552+
newMinMulti=relminmxid;
15491553
}
15501554
}
15511555

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