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shm_mq: Third attempt at fixing nowait behavior in shm_mq_receive.

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Commit a1480ec1d3 purported to fix the
problems with commit b2ccb5f4e6, but it
didn't completely fix them.  The problem is that the checks were
performed in the wrong order, leading to a race condition.  If the
sender attached, sent a message, and detached after the receiver
called shm_mq_get_sender and before the receiver called
shm_mq_counterparty_gone, we'd incorrectly return SHM_MQ_DETACHED
before all messages were read.  Repair by reversing the order of
operations, and add a long comment explaining why this new logic is
(hopefully) correct.
This commit is contained in:
Robert Haas 2015-11-03 09:12:52 -05:00
parent 0279f62fdc
commit 4efe26cbd3
1 changed files with 17 additions and 2 deletions
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19
src/backend/storage/ipc/shm_mq.c
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@ -501,11 +501,26 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
{
if (nowait)
{
int counterparty_gone;
/*
* We shouldn't return at this point at all unless the sender
* hasn't attached yet. However, the correct return value depends
* on whether the sender is still attached. If we first test
* whether the sender has ever attached and then test whether the
* sender has detached, there's a race condition: a sender that
* attaches and detaches very quickly might fool us into thinking
* the sender never attached at all. So, test whether our
* counterparty is definitively gone first, and only afterwards
* check whether the sender ever attached in the first place.
*/
counterparty_gone = shm_mq_counterparty_gone(mq, mqh->mqh_handle);
if (shm_mq_get_sender(mq) == NULL)
{
if (shm_mq_counterparty_gone(mq, mqh->mqh_handle))
if (counterparty_gone)
return SHM_MQ_DETACHED;
return SHM_MQ_WOULD_BLOCK;
else
return SHM_MQ_WOULD_BLOCK;
}
}
else if (!shm_mq_wait_internal(mq, &mq->mq_sender, mqh->mqh_handle)