Close race condition between datfrozen and relfrozen updates.

nmisch · nmisch · commit f65ab862e3b8 · 2024-04-29T10:24:56.000-07:00
vac_update_datfrozenxid() did multiple loads of relfrozenxid and relminmxid from buffer memory, and it assumed each would get the same value. Not so if a concurrent vac_update_relstats() did an inplace update. Commit 2d2e40e fixed the same kind of bug in vac_truncate_clog(). Today's bug could cause the rel-level field and XIDs in the rel's rows to precede the db-level field. 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
diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
@@ -1611,6 +1611,8 @@ vac_update_datfrozenxid(void)
 	/*
 	 * We must seqscan pg_class to find the minimum Xid, because there is no
 	 * index that can help us here.
+	 *
+	 * See vac_truncate_clog() for the race condition to prevent.
 	 */
 	relation = table_open(RelationRelationId, AccessShareLock);
 
@@ -1619,7 +1621,9 @@ vac_update_datfrozenxid(void)
 
 	while ((classTup = systable_getnext(scan)) != NULL)
 	{
-		Form_pg_class classForm = (Form_pg_class) GETSTRUCT(classTup);
+		volatile FormData_pg_class *classForm = (Form_pg_class) GETSTRUCT(classTup);
+		TransactionId relfrozenxid = classForm->relfrozenxid;
+		TransactionId relminmxid = classForm->relminmxid;
 
 		/*
 		 * Only consider relations able to hold unfrozen XIDs (anything else
@@ -1629,8 +1633,8 @@ vac_update_datfrozenxid(void)
 			classForm->relkind != RELKIND_MATVIEW &&
 			classForm->relkind != RELKIND_TOASTVALUE)
 		{
-			Assert(!TransactionIdIsValid(classForm->relfrozenxid));
-			Assert(!MultiXactIdIsValid(classForm->relminmxid));
+			Assert(!TransactionIdIsValid(relfrozenxid));
+			Assert(!MultiXactIdIsValid(relminmxid));
 			continue;
 		}
 
@@ -1649,34 +1653,34 @@ vac_update_datfrozenxid(void)
 		 * before those relations have been scanned and cleaned up.
 		 */
 
-		if (TransactionIdIsValid(classForm->relfrozenxid))
+		if (TransactionIdIsValid(relfrozenxid))
 		{
-			Assert(TransactionIdIsNormal(classForm->relfrozenxid));
+			Assert(TransactionIdIsNormal(relfrozenxid));
 
 			/* check for values in the future */
-			if (TransactionIdPrecedes(lastSaneFrozenXid, classForm->relfrozenxid))
+			if (TransactionIdPrecedes(lastSaneFrozenXid, relfrozenxid))
 			{
 				bogus = true;
 				break;
 			}
 
 			/* determine new horizon */
-			if (TransactionIdPrecedes(classForm->relfrozenxid, newFrozenXid))
-				newFrozenXid = classForm->relfrozenxid;
+			if (TransactionIdPrecedes(relfrozenxid, newFrozenXid))
+				newFrozenXid = relfrozenxid;
 		}
 
-		if (MultiXactIdIsValid(classForm->relminmxid))
+		if (MultiXactIdIsValid(relminmxid))
 		{
 			/* check for values in the future */
-			if (MultiXactIdPrecedes(lastSaneMinMulti, classForm->relminmxid))
+			if (MultiXactIdPrecedes(lastSaneMinMulti, relminmxid))
 			{
 				bogus = true;
 				break;
 			}
 
 			/* determine new horizon */
-			if (MultiXactIdPrecedes(classForm->relminmxid, newMinMulti))
-				newMinMulti = classForm->relminmxid;
+			if (MultiXactIdPrecedes(relminmxid, newMinMulti))
+				newMinMulti = relminmxid;
 		}
 	}
 

Original file line number	Diff line number	Diff line change
`@@ -1611,6 +1611,8 @@ vac_update_datfrozenxid(void)`
`1611`	`1611`	`/*`
`1612`	`1612`	`* We must seqscan pg_class to find the minimum Xid, because there is no`
`1613`	`1613`	`* index that can help us here.`
	`1614`	`+ *`
	`1615`	`+ * See vac_truncate_clog() for the race condition to prevent.`
`1614`	`1616`	`*/`
`1615`	`1617`	`relation = table_open(RelationRelationId, AccessShareLock);`
`1616`	`1618`
`@@ -1619,7 +1621,9 @@ vac_update_datfrozenxid(void)`
`1619`	`1621`
`1620`	`1622`	`while ((classTup = systable_getnext(scan)) != NULL)`
`1621`	`1623`	`{`
`1622`		`- Form_pg_class classForm = (Form_pg_class) GETSTRUCT(classTup);`
	`1624`	`+ volatile FormData_pg_class *classForm = (Form_pg_class) GETSTRUCT(classTup);`
	`1625`	`+ TransactionId relfrozenxid = classForm->relfrozenxid;`
	`1626`	`+ TransactionId relminmxid = classForm->relminmxid;`
`1623`	`1627`
`1624`	`1628`	`/*`
`1625`	`1629`	`* Only consider relations able to hold unfrozen XIDs (anything else`
`@@ -1629,8 +1633,8 @@ vac_update_datfrozenxid(void)`
`1629`	`1633`	`classForm->relkind != RELKIND_MATVIEW &&`
`1630`	`1634`	`classForm->relkind != RELKIND_TOASTVALUE)`
`1631`	`1635`	`{`
`1632`		`- Assert(!TransactionIdIsValid(classForm->relfrozenxid));`
`1633`		`- Assert(!MultiXactIdIsValid(classForm->relminmxid));`
	`1636`	`+ Assert(!TransactionIdIsValid(relfrozenxid));`
	`1637`	`+ Assert(!MultiXactIdIsValid(relminmxid));`
`1634`	`1638`	`continue;`
`1635`	`1639`	`}`
`1636`	`1640`
`@@ -1649,34 +1653,34 @@ vac_update_datfrozenxid(void)`
`1649`	`1653`	`* before those relations have been scanned and cleaned up.`
`1650`	`1654`	`*/`
`1651`	`1655`
`1652`		`- if (TransactionIdIsValid(classForm->relfrozenxid))`
	`1656`	`+ if (TransactionIdIsValid(relfrozenxid))`
`1653`	`1657`	`{`
`1654`		`- Assert(TransactionIdIsNormal(classForm->relfrozenxid));`
	`1658`	`+ Assert(TransactionIdIsNormal(relfrozenxid));`
`1655`	`1659`
`1656`	`1660`	`/* check for values in the future */`
`1657`		`- if (TransactionIdPrecedes(lastSaneFrozenXid, classForm->relfrozenxid))`
	`1661`	`+ if (TransactionIdPrecedes(lastSaneFrozenXid, relfrozenxid))`
`1658`	`1662`	`{`
`1659`	`1663`	`bogus = true;`
`1660`	`1664`	`break;`
`1661`	`1665`	`}`
`1662`	`1666`
`1663`	`1667`	`/* determine new horizon */`
`1664`		`- if (TransactionIdPrecedes(classForm->relfrozenxid, newFrozenXid))`
`1665`		`- newFrozenXid = classForm->relfrozenxid;`
	`1668`	`+ if (TransactionIdPrecedes(relfrozenxid, newFrozenXid))`
	`1669`	`+ newFrozenXid = relfrozenxid;`
`1666`	`1670`	`}`
`1667`	`1671`
`1668`		`- if (MultiXactIdIsValid(classForm->relminmxid))`
	`1672`	`+ if (MultiXactIdIsValid(relminmxid))`
`1669`	`1673`	`{`
`1670`	`1674`	`/* check for values in the future */`
`1671`		`- if (MultiXactIdPrecedes(lastSaneMinMulti, classForm->relminmxid))`
	`1675`	`+ if (MultiXactIdPrecedes(lastSaneMinMulti, relminmxid))`
`1672`	`1676`	`{`
`1673`	`1677`	`bogus = true;`
`1674`	`1678`	`break;`
`1675`	`1679`	`}`
`1676`	`1680`
`1677`	`1681`	`/* determine new horizon */`
`1678`		`- if (MultiXactIdPrecedes(classForm->relminmxid, newMinMulti))`
`1679`		`- newMinMulti = classForm->relminmxid;`
	`1682`	`+ if (MultiXactIdPrecedes(relminmxid, newMinMulti))`
	`1683`	`+ newMinMulti = relminmxid;`
`1680`	`1684`	`}`
`1681`	`1685`	`}`
`1682`	`1686`