Close race condition between datfrozen and relfrozen updates.

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 2d2e40e3be 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

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;
 		}
 	}