Tighten up application of parallel mode checks.

robertmhaas · robertmhaas · commit 94b4f7e2a635 · 2015-10-16T09:59:57.000-04:00
Commit 924bcf4 failed to enforce parallel mode checks during the commit of a parallel worker, because we exited parallel mode prior to ending the transaction so that we could pop the active snapshot. Re-establish parallel mode during parallel worker commit. Without this, it's far too easy for unsafe actions during the pre-commit sequence to crash the server instead of hitting the error checks as intended. Just to be extra paranoid, adjust a couple of the sanity checks in xact.c to check not only IsInParallelMode() but also IsParallelWorker().
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
@@ -497,7 +497,7 @@ AssignTransactionId(TransactionState s)
 	 * Workers synchronize transaction state at the beginning of each parallel
 	 * operation, so we can't account for new XIDs at this point.
 	 */
-	if (IsInParallelMode())
+	if (IsInParallelMode() || IsParallelWorker())
 		elog(ERROR, "cannot assign XIDs during a parallel operation");
 
 	/*
@@ -931,7 +931,7 @@ CommandCounterIncrement(void)
 		 * parallel operation, so we can't account for new commands after that
 		 * point.
 		 */
-		if (IsInParallelMode())
+		if (IsInParallelMode() || IsParallelWorker())
 			elog(ERROR, "cannot start commands during a parallel operation");
 
 		currentCommandId += 1;
@@ -1927,6 +1927,10 @@ CommitTransaction(void)
 
 	is_parallel_worker = (s->blockState == TBLOCK_PARALLEL_INPROGRESS);
 
+	/* Enforce parallel mode restrictions during parallel worker commit. */
+	if (is_parallel_worker)
+		EnterParallelMode();
+
 	ShowTransactionState("CommitTransaction");
 
 	/*
@@ -1971,10 +1975,7 @@ CommitTransaction(void)
 
 	/* If we might have parallel workers, clean them up now. */
 	if (IsInParallelMode())
-	{
 		AtEOXact_Parallel(true);
-		s->parallelModeLevel = 0;
-	}
 
 	/* Shut down the deferred-trigger manager */
 	AfterTriggerEndXact(true);
@@ -2013,6 +2014,7 @@ CommitTransaction(void)
 	 * commit processing
 	 */
 	s->state = TRANS_COMMIT;
+	s->parallelModeLevel = 0;
 
 	if (!is_parallel_worker)
 	{
