Improve fix for not entering parallel mode when holding interrupts.

Commit ac04aa84a put the shutoff for this into the planner, which is
not ideal because it doesn't prevent us from re-using a previously
made parallel plan.  Revert the planner change and instead put the
shutoff into InitializeParallelDSM, modeling it on the existing code
there for recovering from failure to allocate a DSM segment.

However, that code path is mostly untested, and testing a bit harder
showed there's at least one bug: ExecHashJoinReInitializeDSM is not
prepared for us to have skipped doing parallel DSM setup.  I also
thought the Assert in ReinitializeParallelWorkers is pretty
ill-advised, and replaced it with a silent Min() operation.

The existing test case added by ac04aa84a serves fine to test this
version of the fix, so no change needed there.

Patch by me, but thanks to Noah Misch for the core idea that we
could shut off worker creation when !INTERRUPTS_CAN_BE_PROCESSED.
Back-patch to v12, as ac04aa84a was.

Discussion: https://postgr.es/m/CAC-SaSzHUKT=vZJ8MPxYdC_URPfax+yoA1hKTcF4ROz_Q6z0_Q@mail.gmail.com

diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
index f357804fda09005b6b2e43c062b431e3c2e5cb28..4164fcc5542236ee02980769901e247d63b052e7 100644 (file)
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -219,6 +219,15 @@ InitializeParallelDSM(ParallelContext *pcxt)
        shm_toc_estimate_chunk(&pcxt->estimator, sizeof(FixedParallelState));
        shm_toc_estimate_keys(&pcxt->estimator, 1);
 
+       /*
+        * If we manage to reach here while non-interruptible, it's unsafe to
+        * launch any workers: we would fail to process interrupts sent by them.
+        * We can deal with that edge case by pretending no workers were
+        * requested.
+        */
+       if (!INTERRUPTS_CAN_BE_PROCESSED())
+               pcxt->nworkers = 0;
+
        /*
         * Normally, the user will have requested at least one worker process, but
         * if by chance they have not, we can skip a bunch of things here.

diff --git a/src/backend/executor/nodeHashjoin.c b/src/backend/executor/nodeHashjoin.c
index 5808ef489004dadb70e7259d1480ce2aa6c5afad..b4b711e6c4ecab8371deef8ab68229f10605c024 100644 (file)
--- a/src/backend/executor/nodeHashjoin.c
+++ b/src/backend/executor/nodeHashjoin.c
@@ -1516,8 +1516,13 @@ void
 ExecHashJoinReInitializeDSM(HashJoinState *state, ParallelContext *cxt)
 {
        int                     plan_node_id = state->js.ps.plan->plan_node_id;
-       ParallelHashJoinState *pstate =
-       shm_toc_lookup(cxt->toc, plan_node_id, false);
+       ParallelHashJoinState *pstate;
+
+       /* Nothing to do if we failed to create a DSM segment. */
+       if (cxt->seg == NULL)
+               return;
+
+       pstate = shm_toc_lookup(cxt->toc, plan_node_id, false);
 
        /*
         * It would be possible to reuse the shared hash table in single-batch

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 0c058d0e7d9132b1aa5b88bbc38cc6faf88bac62..6428caa83b42f3642979e4daafd2d31a1c41c086 100644 (file)
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -331,11 +331,6 @@ standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
         * general; updates and deletes have additional problems especially around
         * combo CIDs.)
         *
-        * We don't try to use parallel mode unless interruptible.  The leader
-        * expects ProcessInterrupts() calls to reach HandleParallelMessages().
-        * Even if we called HandleParallelMessages() another way, starting a
-        * parallel worker is too delay-prone to be prudent when uncancellable.
-        *
         * For now, we don't try to use parallel mode if we're running inside a
         * parallel worker.  We might eventually be able to relax this
         * restriction, but for now it seems best not to have parallel workers
@@ -346,7 +341,6 @@ standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
                parse->commandType == CMD_SELECT &&
                !parse->hasModifyingCTE &&
                max_parallel_workers_per_gather > 0 &&
-               INTERRUPTS_CAN_BE_PROCESSED() &&
                !IsParallelWorker())
        {
                /* all the cheap tests pass, so scan the query tree */