Fix ReinitializeParallelDSM to tolerate finding no error queues.

Commit d4663350646ca0c069a36d906155a0f7e3372eb7 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
index 1f542ed8d86497661f92cc2824beb85e1e1edf12..d3431a7c306581ac64129a76fc19ad4a1a3d8092 100644 (file)
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -428,9 +428,10 @@ 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;