Fix attach-related race condition in shm_mq_send_bytes.

Spotted by Antonin Houska.

src/backend/storage/ipc/shm_mq.c

@@ -777,16 +777,12 @@ shm_mq_send_bytes(shm_mq_handle *mqh, Size nbytes, const void *data,
 			return SHM_MQ_DETACHED;
 		}
 
-		if (available == 0)
+		if (available == 0 && !mqh->mqh_counterparty_attached)
 		{
-			shm_mq_result res;
-
 			/*
 			 * The queue is full, so if the receiver isn't yet known to be
 			 * attached, we must wait for that to happen.
 			 */
-			if (!mqh->mqh_counterparty_attached)
-			{
 			if (nowait)
 			{
 				if (shm_mq_get_receiver(mq) == NULL)
@@ -803,7 +799,15 @@ shm_mq_send_bytes(shm_mq_handle *mqh, Size nbytes, const void *data,
 				return SHM_MQ_DETACHED;
 			}
 			mqh->mqh_counterparty_attached = true;
+
+			/*
+			 * The receiver may have read some data after attaching, so we
+			 * must not wait without rechecking the queue state.
+			 */
 		}
+		else if (available == 0)
+		{
+			shm_mq_result res;
 
 			/* Let the receiver know that we need them to read some data. */
 			res = shm_mq_notify_receiver(mq);