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Commit4efe26c

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shm_mq: Third attempt at fixing nowait behavior in shm_mq_receive.
Commita1480ec purported to fix theproblems with commitb2ccb5f, but itdidn't completely fix them. The problem is that the checks wereperformed in the wrong order, leading to a race condition. If thesender attached, sent a message, and detached after the receivercalled shm_mq_get_sender and before the receiver calledshm_mq_counterparty_gone, we'd incorrectly return SHM_MQ_DETACHEDbefore all messages were read. Repair by reversing the order ofoperations, and add a long comment explaining why this new logic is(hopefully) correct.
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‎src/backend/storage/ipc/shm_mq.c

Lines changed: 17 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -501,11 +501,26 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
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{
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if (nowait)
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{
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intcounterparty_gone;
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/*
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* We shouldn't return at this point at all unless the sender
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* hasn't attached yet. However, the correct return value depends
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* on whether the sender is still attached. If we first test
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* whether the sender has ever attached and then test whether the
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* sender has detached, there's a race condition: a sender that
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* attaches and detaches very quickly might fool us into thinking
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* the sender never attached at all. So, test whether our
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* counterparty is definitively gone first, and only afterwards
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* check whether the sender ever attached in the first place.
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*/
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counterparty_gone=shm_mq_counterparty_gone(mq,mqh->mqh_handle);
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if (shm_mq_get_sender(mq)==NULL)
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{
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if (shm_mq_counterparty_gone(mq,mqh->mqh_handle))
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if (counterparty_gone)
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returnSHM_MQ_DETACHED;
508-
returnSHM_MQ_WOULD_BLOCK;
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else
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returnSHM_MQ_WOULD_BLOCK;
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}
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}
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elseif (!shm_mq_wait_internal(mq,&mq->mq_sender,mqh->mqh_handle)

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