Make XactLockTableWait work for transactions that are not yet self-locked

alvherre · alvherre · commit fe6bdc0a38c7 · 2018-01-03T14:38:39.000-03:00
XactLockTableWait assumed that its xid argument has already added itself to the lock table. That assumption led to another assumption that if locking the xid has succeeded but the xid is reported as still in progress, then the input xid must have been a subtransaction. These assumptions hold true for the original uses of this code in locking related to on-disk tuples, but they break down in logical replication slot snapshot building -- in particular, when a standby snapshot logged contains an xid that's already in ProcArray but not yet in the lock table. This leads to assertion failures that can be reproduced all the way back to 9.4, when logical decoding was introduced. To fix, change SubTransGetParent to SubTransGetTopmostTransaction which has a slightly different API: it returns the argument Xid if there is no parent, and it goes all the way to the top instead of moving up the levels one by one. Also, to avoid busy-waiting, add a 1ms sleep to give the other process time to register itself in the lock table. For consistency, change ConditionalXactLockTableWait the same way. Author: Petr Jelínek Discussion: https://postgr.es/m/1B3E32D8-FCF4-40B4-AEF9-5C0E3AC57969@postgrespro.ru Reported-by: Konstantin Knizhnik Diagnosed-by: Stas Kelvich, Petr Jelínek Reviewed-by: Andres Freund, Robert Haas
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
@@ -504,6 +504,7 @@ XactLockTableWait(TransactionId xid, Relation rel, ItemPointer ctid,
 	LOCKTAG		tag;
 	XactLockTableWaitInfo info;
 	ErrorContextCallback callback;
+	bool		first = true;
 
 	/*
 	 * If an operation is specified, set up our verbose error context
@@ -537,7 +538,26 @@ XactLockTableWait(TransactionId xid, Relation rel, ItemPointer ctid,
 
 		if (!TransactionIdIsInProgress(xid))
 			break;
-		xid = SubTransGetParent(xid);
+
+		/*
+		 * If the Xid belonged to a subtransaction, then the lock would have
+		 * gone away as soon as it was finished; for correct tuple visibility,
+		 * the right action is to wait on its parent transaction to go away.
+		 * But instead of going levels up one by one, we can just wait for the
+		 * topmost transaction to finish with the same end result, which also
+		 * incurs less locktable traffic.
+		 *
+		 * Some uses of this function don't involve tuple visibility -- such
+		 * as when building snapshots for logical decoding.  It is possible to
+		 * see a transaction in ProcArray before it registers itself in the
+		 * locktable.  The topmost transaction in that case is the same xid,
+		 * so we try again after a short sleep.  (Don't sleep the first time
+		 * through, to avoid slowing down the normal case.)
+		 */
+		if (!first)
+			pg_usleep(1000L);
+		first = false;
+		xid = SubTransGetTopmostTransaction(xid);
 	}
 
 	if (oper != XLTW_None)
@@ -554,6 +574,7 @@ bool
 ConditionalXactLockTableWait(TransactionId xid)
 {
 	LOCKTAG		tag;
+	bool		first = true;
 
 	for (;;)
 	{
@@ -569,7 +590,12 @@ ConditionalXactLockTableWait(TransactionId xid)
 
 		if (!TransactionIdIsInProgress(xid))
 			break;
-		xid = SubTransGetParent(xid);
+
+		/* See XactLockTableWait about this case */
+		if (!first)
+			pg_usleep(1000L);
+		first = false;
+		xid = SubTransGetTopmostTransaction(xid);
 	}
 
 	return true;
