shm_mq: Third attempt at fixing nowait behavior in shm_mq_receive.

robertmhaas · robertmhaas · commit 4efe26cbd3ef · 2015-11-03T09:12:52.000-05:00
Commit a1480ec purported to fix the problems with commit b2ccb5f, 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.
diff --git a/src/backend/storage/ipc/shm_mq.c b/src/backend/storage/ipc/shm_mq.c
@@ -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)

Original file line number	Diff line number	Diff line change
`@@ -501,11 +501,26 @@ shm_mq_receive(shm_mq_handle mqh, Size nbytesp, void **datap, bool nowait)`
`501`	`501`	`{`
`502`	`502`	`if (nowait)`
`503`	`503`	`{`
	`504`	`+ int counterparty_gone;`
	`505`	`+`
	`506`	`+ /*`
	`507`	`+ * We shouldn't return at this point at all unless the sender`
	`508`	`+ * hasn't attached yet. However, the correct return value depends`
	`509`	`+ * on whether the sender is still attached. If we first test`
	`510`	`+ * whether the sender has ever attached and then test whether the`
	`511`	`+ * sender has detached, there's a race condition: a sender that`
	`512`	`+ * attaches and detaches very quickly might fool us into thinking`
	`513`	`+ * the sender never attached at all. So, test whether our`
	`514`	`+ * counterparty is definitively gone first, and only afterwards`
	`515`	`+ * check whether the sender ever attached in the first place.`
	`516`	`+ */`
	`517`	`+ counterparty_gone = shm_mq_counterparty_gone(mq, mqh->mqh_handle);`
`504`	`518`	`if (shm_mq_get_sender(mq) == NULL)`
`505`	`519`	`{`
`506`		`- if (shm_mq_counterparty_gone(mq, mqh->mqh_handle))`
	`520`	`+ if (counterparty_gone)`
`507`	`521`	`return SHM_MQ_DETACHED;`
`508`		`- return SHM_MQ_WOULD_BLOCK;`
	`522`	`+ else`
	`523`	`+ return SHM_MQ_WOULD_BLOCK;`
`509`	`524`	`}`
`510`	`525`	`}`
`511`	`526`	`else if (!shm_mq_wait_internal(mq, &mq->mq_sender, mqh->mqh_handle)`