Ignore nextOid when replaying an ONLINE checkpoint.

tglsfdc · tglsfdc · commit 6943fb9275a5 · 2018-04-11T18:11:30.000-04:00
The nextOid value is from the start of the checkpoint and may well be stale compared to values from more recent XLOG_NEXTOID records. Previously, we adopted it anyway, allowing the OID counter to go backwards during a crash. While this should be harmless, it contributed to the severity of the bug fixed in commit 0408e1e, by allowing duplicate TOAST OIDs to be assigned immediately following a crash. Without this error, that issue would only have arisen when TOAST objects just younger than a multiple of 2^32 OIDs were deleted and then not vacuumed in time to avoid a conflict. Pavan Deolasee Discussion: https://postgr.es/m/CABOikdOgWT2hHkYG3Wwo2cyZJq2zfs1FH0FgX-=h4OLosXHf9w@mail.gmail.com
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
@@ -9473,11 +9473,20 @@ xlog_redo(XLogRecPtr lsn, XLogRecord *record)
 								  checkPoint.nextXid))
 			ShmemVariableCache->nextXid = checkPoint.nextXid;
 		LWLockRelease(XidGenLock);
-		/* ... but still treat OID counter as exact */
-		LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
-		ShmemVariableCache->nextOid = checkPoint.nextOid;
-		ShmemVariableCache->oidCount = 0;
-		LWLockRelease(OidGenLock);
+
+		/*
+		 * We ignore the nextOid counter in an ONLINE checkpoint, preferring
+		 * to track OID assignment through XLOG_NEXTOID records.  The nextOid
+		 * counter is from the start of the checkpoint and might well be stale
+		 * compared to later XLOG_NEXTOID records.  We could try to take the
+		 * maximum of the nextOid counter and our latest value, but since
+		 * there's no particular guarantee about the speed with which the OID
+		 * counter wraps around, that's a risky thing to do.  In any case,
+		 * users of the nextOid counter are required to avoid assignment of
+		 * duplicates, so that a somewhat out-of-date value should be safe.
+		 */
+
+		/* Handle multixact */
 		MultiXactAdvanceNextMXact(checkPoint.nextMulti,
 								  checkPoint.nextMultiOffset);
 		if (TransactionIdPrecedes(ShmemVariableCache->oldestXid,
