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Clean up shm_mq cleanup.

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The logic around shm_mq_detach was a few bricks shy of a load, because
(contrary to the comments for shm_mq_attach) all it did was update the
shared shm_mq state.  That left us leaking a bit of process-local
memory, but much worse, the on_dsm_detach callback for shm_mq_detach
was still armed.  That means that whenever we ultimately detach from
the DSM segment, we'd run shm_mq_detach again for already-detached,
possibly long-dead queues.  This accidentally fails to fail today,
because we only ever re-use a shm_mq's memory for another shm_mq, and
multiple detach attempts on the last such shm_mq are fairly harmless.
But it's gonna bite us someday, so let's clean it up.

To do that, change shm_mq_detach's API so it takes a shm_mq_handle
not the underlying shm_mq.  This makes the callers simpler in most
cases anyway.  Also fix a few places in parallel.c that were just
pfree'ing the handle structs rather than doing proper cleanup.

Back-patch to v10 because of the risk that the revenant shm_mq_detach
callbacks would cause a live bug sometime.  Since this is an API
change, it's too late to do it in 9.6.  (We could make a variant
patch that preserves API, but I'm not excited enough to do that.)

Discussion: https://postgr.es/m/8670.1504192177@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2017-08-31 15:10:24 -04:00
parent 4b1dd62a25
commit 6708e447ef
5 changed files with 51 additions and 21 deletions
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6
src/backend/access/transam/parallel.c
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@ -480,7 +480,7 @@ LaunchParallelWorkers(ParallelContext *pcxt)
*/
any_registrations_failed = true;
pcxt->worker[i].bgwhandle = NULL;
pfree(pcxt->worker[i].error_mqh);
shm_mq_detach(pcxt->worker[i].error_mqh);
pcxt->worker[i].error_mqh = NULL;
}
}
@ -612,7 +612,7 @@ DestroyParallelContext(ParallelContext *pcxt)
{
TerminateBackgroundWorker(pcxt->worker[i].bgwhandle);
pfree(pcxt->worker[i].error_mqh);
shm_mq_detach(pcxt->worker[i].error_mqh);
pcxt->worker[i].error_mqh = NULL;
}
}
@ -861,7 +861,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
case 'X': /* Terminate, indicating clean exit */
{
pfree(pcxt->worker[i].error_mqh);
shm_mq_detach(pcxt->worker[i].error_mqh);
pcxt->worker[i].error_mqh = NULL;
break;
}