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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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9
src/backend/executor/tqueue.c
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@ -578,7 +578,9 @@ tqueueShutdownReceiver(DestReceiver *self)
{
TQueueDestReceiver *tqueue = (TQueueDestReceiver *) self;
shm_mq_detach(shm_mq_get_queue(tqueue->queue));
if (tqueue->queue != NULL)
shm_mq_detach(tqueue->queue);
tqueue->queue = NULL;
}
/*
@ -589,6 +591,9 @@ tqueueDestroyReceiver(DestReceiver *self)
{
TQueueDestReceiver *tqueue = (TQueueDestReceiver *) self;
/* We probably already detached from queue, but let's be sure */
if (tqueue->queue != NULL)
shm_mq_detach(tqueue->queue);
if (tqueue->tmpcontext != NULL)
MemoryContextDelete(tqueue->tmpcontext);
if (tqueue->recordhtab != NULL)
@ -650,7 +655,7 @@ CreateTupleQueueReader(shm_mq_handle *handle, TupleDesc tupledesc)
void
DestroyTupleQueueReader(TupleQueueReader *reader)
{
shm_mq_detach(shm_mq_get_queue(reader->queue));
shm_mq_detach(reader->queue);
if (reader->typmodmap != NULL)
hash_destroy(reader->typmodmap);
/* Is it worth trying to free substructure of the remap tree? */