Be more wary about shm_toc_lookup failure.

Commit 445dbd8 basically missed the point of commit d466335, which was that we shouldn't allow shm_toc_lookup() failure to lead to a core dump or assertion crash, because the odds of such a failure should never be considered negligible. It's correct that we can't expect the PARALLEL_KEY_ERROR_QUEUE TOC entry to be there if we have no workers. But if we have no workers, we're not going to do anything in this function with the lookup result anyway, so let's just skip it. That lets the code use the easy-to-prove-safe noError=false case, rather than anything requiring effort to review. Back-patch to v10, like the previous commit. Discussion: https://postgr.es/m/3647.1517601675@sss.pgh.pa.us
yati-sagade · Feb 2, 2018 · 957ff08 · 957ff08
1 parent bf641d3
commit 957ff08
Showing 1 changed file with 15 additions and 12 deletions.
diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
@@ -434,8 +434,6 @@ void
 ReinitializeParallelDSM(ParallelContext *pcxt)
 {
 	FixedParallelState *fps;
-	char	   *error_queue_space;
-	int			i;
 
 	/* Wait for any old workers to exit. */
 	if (pcxt->nworkers_launched > 0)
@@ -456,18 +454,23 @@ ReinitializeParallelDSM(ParallelContext *pcxt)
 	fps->last_xlog_end = 0;
 
 	/* Recreate error queues (if they exist). */
-	error_queue_space =
-		shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE, true);
-	Assert(pcxt->nworkers == 0 || error_queue_space != NULL);
-	for (i = 0; i < pcxt->nworkers; ++i)
+	if (pcxt->nworkers > 0)
 	{
-		char	   *start;
-		shm_mq	   *mq;
+		char	   *error_queue_space;
+		int			i;
 
-		start = error_queue_space + i * PARALLEL_ERROR_QUEUE_SIZE;
-		mq = shm_mq_create(start, PARALLEL_ERROR_QUEUE_SIZE);
-		shm_mq_set_receiver(mq, MyProc);
-		pcxt->worker[i].error_mqh = shm_mq_attach(mq, pcxt->seg, NULL);
+		error_queue_space =
+			shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE, false);
+		for (i = 0; i < pcxt->nworkers; ++i)
+		{
+			char	   *start;
+			shm_mq	   *mq;
+
+			start = error_queue_space + i * PARALLEL_ERROR_QUEUE_SIZE;
+			mq = shm_mq_create(start, PARALLEL_ERROR_QUEUE_SIZE);
+			shm_mq_set_receiver(mq, MyProc);
+			pcxt->worker[i].error_mqh = shm_mq_attach(mq, pcxt->seg, NULL);
+		}
 	}
 }