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Commit06424e9

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Improve fix for not entering parallel mode when holding interrupts.
Commitac04aa8 put the shutoff for this into the planner, which isnot ideal because it doesn't prevent us from re-using a previouslymade parallel plan. Revert the planner change and instead put theshutoff into InitializeParallelDSM, modeling it on the existing codethere for recovering from failure to allocate a DSM segment.However, that code path is mostly untested, and testing a bit hardershowed there's at least one bug: ExecHashJoinReInitializeDSM is notprepared for us to have skipped doing parallel DSM setup. I alsothought the Assert in ReinitializeParallelWorkers is prettyill-advised, and replaced it with a silent Min() operation.The existing test case added byac04aa8 serves fine to test thisversion of the fix, so no change needed there.Patch by me, but thanks to Noah Misch for the core idea that wecould shut off worker creation when !INTERRUPTS_CAN_BE_PROCESSED.Back-patch to v12, asac04aa8 was.Discussion:https://postgr.es/m/CAC-SaSzHUKT=vZJ8MPxYdC_URPfax+yoA1hKTcF4ROz_Q6z0_Q@mail.gmail.com
1 parentf734b6b commit06424e9

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3 files changed

+23
-11
lines changed

3 files changed

+23
-11
lines changed

‎src/backend/access/transam/parallel.c

Lines changed: 16 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -228,6 +228,15 @@ InitializeParallelDSM(ParallelContext *pcxt)
228228
shm_toc_estimate_chunk(&pcxt->estimator,sizeof(FixedParallelState));
229229
shm_toc_estimate_keys(&pcxt->estimator,1);
230230

231+
/*
232+
* If we manage to reach here while non-interruptible, it's unsafe to
233+
* launch any workers: we would fail to process interrupts sent by them.
234+
* We can deal with that edge case by pretending no workers were
235+
* requested.
236+
*/
237+
if (!INTERRUPTS_CAN_BE_PROCESSED())
238+
pcxt->nworkers=0;
239+
231240
/*
232241
* Normally, the user will have requested at least one worker process, but
233242
* if by chance they have not, we can skip a bunch of things here.
@@ -474,6 +483,9 @@ InitializeParallelDSM(ParallelContext *pcxt)
474483
shm_toc_insert(pcxt->toc,PARALLEL_KEY_ENTRYPOINT,entrypointstate);
475484
}
476485

486+
/* Update nworkers_to_launch, in case we changed nworkers above. */
487+
pcxt->nworkers_to_launch=pcxt->nworkers;
488+
477489
/* Restore previous memory context. */
478490
MemoryContextSwitchTo(oldcontext);
479491
}
@@ -537,10 +549,11 @@ ReinitializeParallelWorkers(ParallelContext *pcxt, int nworkers_to_launch)
537549
{
538550
/*
539551
* The number of workers that need to be launched must be less than the
540-
* number of workers with which the parallel context is initialized.
552+
* number of workers with which the parallel context is initialized. But
553+
* the caller might not know that InitializeParallelDSM reduced nworkers,
554+
* so just silently trim the request.
541555
*/
542-
Assert(pcxt->nworkers >=nworkers_to_launch);
543-
pcxt->nworkers_to_launch=nworkers_to_launch;
556+
pcxt->nworkers_to_launch=Min(pcxt->nworkers,nworkers_to_launch);
544557
}
545558

546559
/*

‎src/backend/executor/nodeHashjoin.c

Lines changed: 7 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -1622,8 +1622,13 @@ void
16221622
ExecHashJoinReInitializeDSM(HashJoinState*state,ParallelContext*pcxt)
16231623
{
16241624
intplan_node_id=state->js.ps.plan->plan_node_id;
1625-
ParallelHashJoinState*pstate=
1626-
shm_toc_lookup(pcxt->toc,plan_node_id, false);
1625+
ParallelHashJoinState*pstate;
1626+
1627+
/* Nothing to do if we failed to create a DSM segment. */
1628+
if (pcxt->seg==NULL)
1629+
return;
1630+
1631+
pstate=shm_toc_lookup(pcxt->toc,plan_node_id, false);
16271632

16281633
/*
16291634
* It would be possible to reuse the shared hash table in single-batch

‎src/backend/optimizer/plan/planner.c

Lines changed: 0 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -338,11 +338,6 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
338338
* if we want to allow parallel inserts in general; updates and deletes
339339
* have additional problems especially around combo CIDs.)
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*
341-
* We don't try to use parallel mode unless interruptible. The leader
342-
* expects ProcessInterrupts() calls to reach HandleParallelMessages().
343-
* Even if we called HandleParallelMessages() another way, starting a
344-
* parallel worker is too delay-prone to be prudent when uncancellable.
345-
*
346341
* For now, we don't try to use parallel mode if we're running inside a
347342
* parallel worker. We might eventually be able to relax this
348343
* restriction, but for now it seems best not to have parallel workers
@@ -353,7 +348,6 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
353348
parse->commandType==CMD_SELECT&&
354349
!parse->hasModifyingCTE&&
355350
max_parallel_workers_per_gather>0&&
356-
INTERRUPTS_CAN_BE_PROCESSED()&&
357351
!IsParallelWorker())
358352
{
359353
/* all the cheap tests pass, so scan the query tree */

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