shm_mq: Fix detach race condition.

Commit 34db06ef adopted a lock-free
design for shm_mq.c, but it introduced a race condition that could
lose messages.  When shm_mq_receive_bytes() detects that the other end
has detached, it must make sure that it has seen the final version of
mq_bytes_written, or it might miss a message sent before detaching.

Thomas Munro

Discussion: https://postgr.es/m/CAEepm%3D2myZ4qxpt1a%3DC%2BwEv3o188K13K3UvD-44FK0SdAzHy%2Bw%40mail.gmail.com

src/backend/storage/ipc/shm_mq.c

@@ -1058,7 +1058,19 @@ shm_mq_receive_bytes(shm_mq_handle *mqh, Size bytes_needed, bool nowait,
 		 * detached.
 		 */
 		if (mq->mq_detached)
+		{
+			/*
+			 * If the writer advanced mq_bytes_written and then set
+			 * mq_detached, we might not have read the final value of
+			 * mq_bytes_written above.  Insert a read barrier and then check
+			 * again if mq_bytes_written has advanced.
+			 */
+			pg_read_barrier();
+			if (written != pg_atomic_read_u64(&mq->mq_bytes_written))
+				continue;
+
 			return SHM_MQ_DETACHED;
+		}
 
 		/*
 		 * We didn't get enough data to satisfy the request, so mark any data