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Commit70cadfb

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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 parent440b625 commit70cadfb

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‎src/backend/commands/vacuum.c‎

Lines changed: 16 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -1379,6 +1379,8 @@ vac_update_datfrozenxid(void)
13791379
/*
13801380
* We must seqscan pg_class to find the minimum Xid, because there is no
13811381
* index that can help us here.
1382+
*
1383+
* See vac_truncate_clog() for the race condition to prevent.
13821384
*/
13831385
relation=table_open(RelationRelationId,AccessShareLock);
13841386

@@ -1387,7 +1389,9 @@ vac_update_datfrozenxid(void)
13871389

13881390
while ((classTup=systable_getnext(scan))!=NULL)
13891391
{
1390-
Form_pg_classclassForm= (Form_pg_class)GETSTRUCT(classTup);
1392+
volatileFormData_pg_class*classForm= (Form_pg_class)GETSTRUCT(classTup);
1393+
TransactionIdrelfrozenxid=classForm->relfrozenxid;
1394+
TransactionIdrelminmxid=classForm->relminmxid;
13911395

13921396
/*
13931397
* Only consider relations able to hold unfrozen XIDs (anything else
@@ -1397,8 +1401,8 @@ vac_update_datfrozenxid(void)
13971401
classForm->relkind!=RELKIND_MATVIEW&&
13981402
classForm->relkind!=RELKIND_TOASTVALUE)
13991403
{
1400-
Assert(!TransactionIdIsValid(classForm->relfrozenxid));
1401-
Assert(!MultiXactIdIsValid(classForm->relminmxid));
1404+
Assert(!TransactionIdIsValid(relfrozenxid));
1405+
Assert(!MultiXactIdIsValid(relminmxid));
14021406
continue;
14031407
}
14041408

@@ -1417,34 +1421,34 @@ vac_update_datfrozenxid(void)
14171421
* before those relations have been scanned and cleaned up.
14181422
*/
14191423

1420-
if (TransactionIdIsValid(classForm->relfrozenxid))
1424+
if (TransactionIdIsValid(relfrozenxid))
14211425
{
1422-
Assert(TransactionIdIsNormal(classForm->relfrozenxid));
1426+
Assert(TransactionIdIsNormal(relfrozenxid));
14231427

14241428
/* check for values in the future */
1425-
if (TransactionIdPrecedes(lastSaneFrozenXid,classForm->relfrozenxid))
1429+
if (TransactionIdPrecedes(lastSaneFrozenXid,relfrozenxid))
14261430
{
14271431
bogus= true;
14281432
break;
14291433
}
14301434

14311435
/* determine new horizon */
1432-
if (TransactionIdPrecedes(classForm->relfrozenxid,newFrozenXid))
1433-
newFrozenXid=classForm->relfrozenxid;
1436+
if (TransactionIdPrecedes(relfrozenxid,newFrozenXid))
1437+
newFrozenXid=relfrozenxid;
14341438
}
14351439

1436-
if (MultiXactIdIsValid(classForm->relminmxid))
1440+
if (MultiXactIdIsValid(relminmxid))
14371441
{
14381442
/* check for values in the future */
1439-
if (MultiXactIdPrecedes(lastSaneMinMulti,classForm->relminmxid))
1443+
if (MultiXactIdPrecedes(lastSaneMinMulti,relminmxid))
14401444
{
14411445
bogus= true;
14421446
break;
14431447
}
14441448

14451449
/* determine new horizon */
1446-
if (MultiXactIdPrecedes(classForm->relminmxid,newMinMulti))
1447-
newMinMulti=classForm->relminmxid;
1450+
if (MultiXactIdPrecedes(relminmxid,newMinMulti))
1451+
newMinMulti=relminmxid;
14481452
}
14491453
}
14501454

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