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Commit92685c3

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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 parentb19255c commit92685c3

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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
@@ -1633,6 +1633,8 @@ vac_update_datfrozenxid(void)
16331633
/*
16341634
* We must seqscan pg_class to find the minimum Xid, because there is no
16351635
* index that can help us here.
1636+
*
1637+
* See vac_truncate_clog() for the race condition to prevent.
16361638
*/
16371639
relation=table_open(RelationRelationId,AccessShareLock);
16381640

@@ -1641,7 +1643,9 @@ vac_update_datfrozenxid(void)
16411643

16421644
while ((classTup=systable_getnext(scan))!=NULL)
16431645
{
1644-
Form_pg_classclassForm= (Form_pg_class)GETSTRUCT(classTup);
1646+
volatileFormData_pg_class*classForm= (Form_pg_class)GETSTRUCT(classTup);
1647+
TransactionIdrelfrozenxid=classForm->relfrozenxid;
1648+
TransactionIdrelminmxid=classForm->relminmxid;
16451649

16461650
/*
16471651
* Only consider relations able to hold unfrozen XIDs (anything else
@@ -1651,8 +1655,8 @@ vac_update_datfrozenxid(void)
16511655
classForm->relkind!=RELKIND_MATVIEW&&
16521656
classForm->relkind!=RELKIND_TOASTVALUE)
16531657
{
1654-
Assert(!TransactionIdIsValid(classForm->relfrozenxid));
1655-
Assert(!MultiXactIdIsValid(classForm->relminmxid));
1658+
Assert(!TransactionIdIsValid(relfrozenxid));
1659+
Assert(!MultiXactIdIsValid(relminmxid));
16561660
continue;
16571661
}
16581662

@@ -1671,34 +1675,34 @@ vac_update_datfrozenxid(void)
16711675
* before those relations have been scanned and cleaned up.
16721676
*/
16731677

1674-
if (TransactionIdIsValid(classForm->relfrozenxid))
1678+
if (TransactionIdIsValid(relfrozenxid))
16751679
{
1676-
Assert(TransactionIdIsNormal(classForm->relfrozenxid));
1680+
Assert(TransactionIdIsNormal(relfrozenxid));
16771681

16781682
/* check for values in the future */
1679-
if (TransactionIdPrecedes(lastSaneFrozenXid,classForm->relfrozenxid))
1683+
if (TransactionIdPrecedes(lastSaneFrozenXid,relfrozenxid))
16801684
{
16811685
bogus= true;
16821686
break;
16831687
}
16841688

16851689
/* determine new horizon */
1686-
if (TransactionIdPrecedes(classForm->relfrozenxid,newFrozenXid))
1687-
newFrozenXid=classForm->relfrozenxid;
1690+
if (TransactionIdPrecedes(relfrozenxid,newFrozenXid))
1691+
newFrozenXid=relfrozenxid;
16881692
}
16891693

1690-
if (MultiXactIdIsValid(classForm->relminmxid))
1694+
if (MultiXactIdIsValid(relminmxid))
16911695
{
16921696
/* check for values in the future */
1693-
if (MultiXactIdPrecedes(lastSaneMinMulti,classForm->relminmxid))
1697+
if (MultiXactIdPrecedes(lastSaneMinMulti,relminmxid))
16941698
{
16951699
bogus= true;
16961700
break;
16971701
}
16981702

16991703
/* determine new horizon */
1700-
if (MultiXactIdPrecedes(classForm->relminmxid,newMinMulti))
1701-
newMinMulti=classForm->relminmxid;
1704+
if (MultiXactIdPrecedes(relminmxid,newMinMulti))
1705+
newMinMulti=relminmxid;
17021706
}
17031707
}
17041708

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