index 1d0657f3858792bb472a67d9de70838904d17c9a..dafc8a7ca76ca03544209202e3353f80d952c9a8 100644 (file)
@@ -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)