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Commitf222349

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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 parentcb0ccef commitf222349

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-12
lines changed

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
@@ -1330,6 +1330,8 @@ vac_update_datfrozenxid(void)
13301330
/*
13311331
* We must seqscan pg_class to find the minimum Xid, because there is no
13321332
* index that can help us here.
1333+
*
1334+
* See vac_truncate_clog() for the race condition to prevent.
13331335
*/
13341336
relation=table_open(RelationRelationId,AccessShareLock);
13351337

@@ -1338,7 +1340,9 @@ vac_update_datfrozenxid(void)
13381340

13391341
while ((classTup=systable_getnext(scan))!=NULL)
13401342
{
1341-
Form_pg_classclassForm= (Form_pg_class)GETSTRUCT(classTup);
1343+
volatileFormData_pg_class*classForm= (Form_pg_class)GETSTRUCT(classTup);
1344+
TransactionIdrelfrozenxid=classForm->relfrozenxid;
1345+
TransactionIdrelminmxid=classForm->relminmxid;
13421346

13431347
/*
13441348
* Only consider relations able to hold unfrozen XIDs (anything else
@@ -1348,8 +1352,8 @@ vac_update_datfrozenxid(void)
13481352
classForm->relkind!=RELKIND_MATVIEW&&
13491353
classForm->relkind!=RELKIND_TOASTVALUE)
13501354
{
1351-
Assert(!TransactionIdIsValid(classForm->relfrozenxid));
1352-
Assert(!MultiXactIdIsValid(classForm->relminmxid));
1355+
Assert(!TransactionIdIsValid(relfrozenxid));
1356+
Assert(!MultiXactIdIsValid(relminmxid));
13531357
continue;
13541358
}
13551359

@@ -1368,34 +1372,34 @@ vac_update_datfrozenxid(void)
13681372
* before those relations have been scanned and cleaned up.
13691373
*/
13701374

1371-
if (TransactionIdIsValid(classForm->relfrozenxid))
1375+
if (TransactionIdIsValid(relfrozenxid))
13721376
{
1373-
Assert(TransactionIdIsNormal(classForm->relfrozenxid));
1377+
Assert(TransactionIdIsNormal(relfrozenxid));
13741378

13751379
/* check for values in the future */
1376-
if (TransactionIdPrecedes(lastSaneFrozenXid,classForm->relfrozenxid))
1380+
if (TransactionIdPrecedes(lastSaneFrozenXid,relfrozenxid))
13771381
{
13781382
bogus= true;
13791383
break;
13801384
}
13811385

13821386
/* determine new horizon */
1383-
if (TransactionIdPrecedes(classForm->relfrozenxid,newFrozenXid))
1384-
newFrozenXid=classForm->relfrozenxid;
1387+
if (TransactionIdPrecedes(relfrozenxid,newFrozenXid))
1388+
newFrozenXid=relfrozenxid;
13851389
}
13861390

1387-
if (MultiXactIdIsValid(classForm->relminmxid))
1391+
if (MultiXactIdIsValid(relminmxid))
13881392
{
13891393
/* check for values in the future */
1390-
if (MultiXactIdPrecedes(lastSaneMinMulti,classForm->relminmxid))
1394+
if (MultiXactIdPrecedes(lastSaneMinMulti,relminmxid))
13911395
{
13921396
bogus= true;
13931397
break;
13941398
}
13951399

13961400
/* determine new horizon */
1397-
if (MultiXactIdPrecedes(classForm->relminmxid,newMinMulti))
1398-
newMinMulti=classForm->relminmxid;
1401+
if (MultiXactIdPrecedes(relminmxid,newMinMulti))
1402+
newMinMulti=relminmxid;
13991403
}
14001404
}
14011405

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