Close race condition between datfrozen and relfrozen updates.

nmisch · nmisch · commit f222349c4ec0 · 2024-04-29T10:25:00.000-07:00
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
@@ -1330,6 +1330,8 @@ 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);
 
@@ -1338,7 +1340,9 @@ 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
@@ -1348,8 +1352,8 @@ 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;
 		}
 
@@ -1368,34 +1372,34 @@ 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
`@@ -1330,6 +1330,8 @@ vac_update_datfrozenxid(void)`
`1330`	`1330`	`/*`
`1331`	`1331`	`* We must seqscan pg_class to find the minimum Xid, because there is no`
`1332`	`1332`	`* index that can help us here.`
	`1333`	`+ *`
	`1334`	`+ * See vac_truncate_clog() for the race condition to prevent.`
`1333`	`1335`	`*/`
`1334`	`1336`	`relation = table_open(RelationRelationId, AccessShareLock);`
`1335`	`1337`
`@@ -1338,7 +1340,9 @@ vac_update_datfrozenxid(void)`
`1338`	`1340`
`1339`	`1341`	`while ((classTup = systable_getnext(scan)) != NULL)`
`1340`	`1342`	`{`
`1341`		`- Form_pg_class classForm = (Form_pg_class) GETSTRUCT(classTup);`
	`1343`	`+ volatile FormData_pg_class *classForm = (Form_pg_class) GETSTRUCT(classTup);`
	`1344`	`+ TransactionId relfrozenxid = classForm->relfrozenxid;`
	`1345`	`+ TransactionId relminmxid = classForm->relminmxid;`
`1342`	`1346`
`1343`	`1347`	`/*`
`1344`	`1348`	`* Only consider relations able to hold unfrozen XIDs (anything else`
`@@ -1348,8 +1352,8 @@ vac_update_datfrozenxid(void)`
`1348`	`1352`	`classForm->relkind != RELKIND_MATVIEW &&`
`1349`	`1353`	`classForm->relkind != RELKIND_TOASTVALUE)`
`1350`	`1354`	`{`
`1351`		`- Assert(!TransactionIdIsValid(classForm->relfrozenxid));`
`1352`		`- Assert(!MultiXactIdIsValid(classForm->relminmxid));`
	`1355`	`+ Assert(!TransactionIdIsValid(relfrozenxid));`
	`1356`	`+ Assert(!MultiXactIdIsValid(relminmxid));`
`1353`	`1357`	`continue;`
`1354`	`1358`	`}`
`1355`	`1359`
`@@ -1368,34 +1372,34 @@ vac_update_datfrozenxid(void)`
`1368`	`1372`	`* before those relations have been scanned and cleaned up.`
`1369`	`1373`	`*/`
`1370`	`1374`
`1371`		`- if (TransactionIdIsValid(classForm->relfrozenxid))`
	`1375`	`+ if (TransactionIdIsValid(relfrozenxid))`
`1372`	`1376`	`{`
`1373`		`- Assert(TransactionIdIsNormal(classForm->relfrozenxid));`
	`1377`	`+ Assert(TransactionIdIsNormal(relfrozenxid));`
`1374`	`1378`
`1375`	`1379`	`/* check for values in the future */`
`1376`		`- if (TransactionIdPrecedes(lastSaneFrozenXid, classForm->relfrozenxid))`
	`1380`	`+ if (TransactionIdPrecedes(lastSaneFrozenXid, relfrozenxid))`
`1377`	`1381`	`{`
`1378`	`1382`	`bogus = true;`
`1379`	`1383`	`break;`
`1380`	`1384`	`}`
`1381`	`1385`
`1382`	`1386`	`/* determine new horizon */`
`1383`		`- if (TransactionIdPrecedes(classForm->relfrozenxid, newFrozenXid))`
`1384`		`- newFrozenXid = classForm->relfrozenxid;`
	`1387`	`+ if (TransactionIdPrecedes(relfrozenxid, newFrozenXid))`
	`1388`	`+ newFrozenXid = relfrozenxid;`
`1385`	`1389`	`}`
`1386`	`1390`
`1387`		`- if (MultiXactIdIsValid(classForm->relminmxid))`
	`1391`	`+ if (MultiXactIdIsValid(relminmxid))`
`1388`	`1392`	`{`
`1389`	`1393`	`/* check for values in the future */`
`1390`		`- if (MultiXactIdPrecedes(lastSaneMinMulti, classForm->relminmxid))`
	`1394`	`+ if (MultiXactIdPrecedes(lastSaneMinMulti, relminmxid))`
`1391`	`1395`	`{`
`1392`	`1396`	`bogus = true;`
`1393`	`1397`	`break;`
`1394`	`1398`	`}`
`1395`	`1399`
`1396`	`1400`	`/* determine new horizon */`
`1397`		`- if (MultiXactIdPrecedes(classForm->relminmxid, newMinMulti))`
`1398`		`- newMinMulti = classForm->relminmxid;`
	`1401`	`+ if (MultiXactIdPrecedes(relminmxid, newMinMulti))`
	`1402`	`+ newMinMulti = relminmxid;`
`1399`	`1403`	`}`
`1400`	`1404`	`}`
`1401`	`1405`