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Commit7c5915c

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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 parent9b41d1d commit7c5915c

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+16
-12
lines changed

1 file changed

+16
-12
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‎src/backend/commands/vacuum.c

Lines changed: 16 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -1532,6 +1532,8 @@ vac_update_datfrozenxid(void)
15321532
/*
15331533
* We must seqscan pg_class to find the minimum Xid, because there is no
15341534
* index that can help us here.
1535+
*
1536+
* See vac_truncate_clog() for the race condition to prevent.
15351537
*/
15361538
relation=table_open(RelationRelationId,AccessShareLock);
15371539

@@ -1540,7 +1542,9 @@ vac_update_datfrozenxid(void)
15401542

15411543
while ((classTup=systable_getnext(scan))!=NULL)
15421544
{
1543-
Form_pg_classclassForm= (Form_pg_class)GETSTRUCT(classTup);
1545+
volatileFormData_pg_class*classForm= (Form_pg_class)GETSTRUCT(classTup);
1546+
TransactionIdrelfrozenxid=classForm->relfrozenxid;
1547+
TransactionIdrelminmxid=classForm->relminmxid;
15441548

15451549
/*
15461550
* Only consider relations able to hold unfrozen XIDs (anything else
@@ -1550,8 +1554,8 @@ vac_update_datfrozenxid(void)
15501554
classForm->relkind!=RELKIND_MATVIEW&&
15511555
classForm->relkind!=RELKIND_TOASTVALUE)
15521556
{
1553-
Assert(!TransactionIdIsValid(classForm->relfrozenxid));
1554-
Assert(!MultiXactIdIsValid(classForm->relminmxid));
1557+
Assert(!TransactionIdIsValid(relfrozenxid));
1558+
Assert(!MultiXactIdIsValid(relminmxid));
15551559
continue;
15561560
}
15571561

@@ -1570,34 +1574,34 @@ vac_update_datfrozenxid(void)
15701574
* before those relations have been scanned and cleaned up.
15711575
*/
15721576

1573-
if (TransactionIdIsValid(classForm->relfrozenxid))
1577+
if (TransactionIdIsValid(relfrozenxid))
15741578
{
1575-
Assert(TransactionIdIsNormal(classForm->relfrozenxid));
1579+
Assert(TransactionIdIsNormal(relfrozenxid));
15761580

15771581
/* check for values in the future */
1578-
if (TransactionIdPrecedes(lastSaneFrozenXid,classForm->relfrozenxid))
1582+
if (TransactionIdPrecedes(lastSaneFrozenXid,relfrozenxid))
15791583
{
15801584
bogus= true;
15811585
break;
15821586
}
15831587

15841588
/* determine new horizon */
1585-
if (TransactionIdPrecedes(classForm->relfrozenxid,newFrozenXid))
1586-
newFrozenXid=classForm->relfrozenxid;
1589+
if (TransactionIdPrecedes(relfrozenxid,newFrozenXid))
1590+
newFrozenXid=relfrozenxid;
15871591
}
15881592

1589-
if (MultiXactIdIsValid(classForm->relminmxid))
1593+
if (MultiXactIdIsValid(relminmxid))
15901594
{
15911595
/* check for values in the future */
1592-
if (MultiXactIdPrecedes(lastSaneMinMulti,classForm->relminmxid))
1596+
if (MultiXactIdPrecedes(lastSaneMinMulti,relminmxid))
15931597
{
15941598
bogus= true;
15951599
break;
15961600
}
15971601

15981602
/* determine new horizon */
1599-
if (MultiXactIdPrecedes(classForm->relminmxid,newMinMulti))
1600-
newMinMulti=classForm->relminmxid;
1603+
if (MultiXactIdPrecedes(relminmxid,newMinMulti))
1604+
newMinMulti=relminmxid;
16011605
}
16021606
}
16031607

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