Fix ReinitializeParallelDSM to tolerate finding no error queues.

robertmhaas · robertmhaas · commit dba6e75c1113 · 2017-11-28T12:19:19.000-05:00
Commit d466335 changed things so that shm_toc_lookup would fail with an error rather than silently returning NULL in the hope that such failures would be reported in a useful way rather than via a system crash. However, it overlooked the fact that the lookup of PARALLEL_KEY_ERROR_QUEUE in ReinitializeParallelDSM is expected to fail when no DSM segment was created in the first place; in that case, we end up with a backend-private memory segment that still contains an entry for PARALLEL_KEY_FIXED but no others. Consequently a benign failure to initialize parallelism can escalate into an elog(ERROR); repair. Discussion: http://postgr.es/m/CA+Tgmob8LFw55DzH1QEREpBEA9RJ_W_amhBFCVZ6WMwUhVpOqg@mail.gmail.com
diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
@@ -399,9 +399,10 @@ ReinitializeParallelDSM(ParallelContext *pcxt)
 	fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED, false);
 	fps->last_xlog_end = 0;
 
-	/* Recreate error queues. */
+	/* Recreate error queues (if they exist). */
 	error_queue_space =
-		shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE, false);
+		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)
 	{
 		char	   *start;
