Close race condition between datfrozen and relfrozen updates.

nmisch · nmisch · commit 2ca19aa8165c · 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
@@ -1480,6 +1480,8 @@ 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);
 
@@ -1488,7 +1490,9 @@ 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
@@ -1498,8 +1502,8 @@ 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;
 		}
 
@@ -1518,34 +1522,34 @@ 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
`@@ -1480,6 +1480,8 @@ vac_update_datfrozenxid(void)`
`1480`	`1480`	`/*`
`1481`	`1481`	`* We must seqscan pg_class to find the minimum Xid, because there is no`
`1482`	`1482`	`* index that can help us here.`
	`1483`	`+ *`
	`1484`	`+ * See vac_truncate_clog() for the race condition to prevent.`
`1483`	`1485`	`*/`
`1484`	`1486`	`relation = table_open(RelationRelationId, AccessShareLock);`
`1485`	`1487`
`@@ -1488,7 +1490,9 @@ vac_update_datfrozenxid(void)`
`1488`	`1490`
`1489`	`1491`	`while ((classTup = systable_getnext(scan)) != NULL)`
`1490`	`1492`	`{`
`1491`		`- Form_pg_class classForm = (Form_pg_class) GETSTRUCT(classTup);`
	`1493`	`+ volatile FormData_pg_class *classForm = (Form_pg_class) GETSTRUCT(classTup);`
	`1494`	`+ TransactionId relfrozenxid = classForm->relfrozenxid;`
	`1495`	`+ TransactionId relminmxid = classForm->relminmxid;`
`1492`	`1496`
`1493`	`1497`	`/*`
`1494`	`1498`	`* Only consider relations able to hold unfrozen XIDs (anything else`
`@@ -1498,8 +1502,8 @@ vac_update_datfrozenxid(void)`
`1498`	`1502`	`classForm->relkind != RELKIND_MATVIEW &&`
`1499`	`1503`	`classForm->relkind != RELKIND_TOASTVALUE)`
`1500`	`1504`	`{`
`1501`		`- Assert(!TransactionIdIsValid(classForm->relfrozenxid));`
`1502`		`- Assert(!MultiXactIdIsValid(classForm->relminmxid));`
	`1505`	`+ Assert(!TransactionIdIsValid(relfrozenxid));`
	`1506`	`+ Assert(!MultiXactIdIsValid(relminmxid));`
`1503`	`1507`	`continue;`
`1504`	`1508`	`}`
`1505`	`1509`
`@@ -1518,34 +1522,34 @@ vac_update_datfrozenxid(void)`
`1518`	`1522`	`* before those relations have been scanned and cleaned up.`
`1519`	`1523`	`*/`
`1520`	`1524`
`1521`		`- if (TransactionIdIsValid(classForm->relfrozenxid))`
	`1525`	`+ if (TransactionIdIsValid(relfrozenxid))`
`1522`	`1526`	`{`
`1523`		`- Assert(TransactionIdIsNormal(classForm->relfrozenxid));`
	`1527`	`+ Assert(TransactionIdIsNormal(relfrozenxid));`
`1524`	`1528`
`1525`	`1529`	`/* check for values in the future */`
`1526`		`- if (TransactionIdPrecedes(lastSaneFrozenXid, classForm->relfrozenxid))`
	`1530`	`+ if (TransactionIdPrecedes(lastSaneFrozenXid, relfrozenxid))`
`1527`	`1531`	`{`
`1528`	`1532`	`bogus = true;`
`1529`	`1533`	`break;`
`1530`	`1534`	`}`
`1531`	`1535`
`1532`	`1536`	`/* determine new horizon */`
`1533`		`- if (TransactionIdPrecedes(classForm->relfrozenxid, newFrozenXid))`
`1534`		`- newFrozenXid = classForm->relfrozenxid;`
	`1537`	`+ if (TransactionIdPrecedes(relfrozenxid, newFrozenXid))`
	`1538`	`+ newFrozenXid = relfrozenxid;`
`1535`	`1539`	`}`
`1536`	`1540`
`1537`		`- if (MultiXactIdIsValid(classForm->relminmxid))`
	`1541`	`+ if (MultiXactIdIsValid(relminmxid))`
`1538`	`1542`	`{`
`1539`	`1543`	`/* check for values in the future */`
`1540`		`- if (MultiXactIdPrecedes(lastSaneMinMulti, classForm->relminmxid))`
	`1544`	`+ if (MultiXactIdPrecedes(lastSaneMinMulti, relminmxid))`
`1541`	`1545`	`{`
`1542`	`1546`	`bogus = true;`
`1543`	`1547`	`break;`
`1544`	`1548`	`}`
`1545`	`1549`
`1546`	`1550`	`/* determine new horizon */`
`1547`		`- if (MultiXactIdPrecedes(classForm->relminmxid, newMinMulti))`
`1548`		`- newMinMulti = classForm->relminmxid;`
	`1551`	`+ if (MultiXactIdPrecedes(relminmxid, newMinMulti))`
	`1552`	`+ newMinMulti = relminmxid;`
`1549`	`1553`	`}`
`1550`	`1554`	`}`
`1551`	`1555`