Cannot use WL_SOCKET_WRITEABLE without WL_SOCKET_READABLE.

In copy-out mode, the frontend should not send any messages until the
backend has finished streaming, by sending a CopyDone message. I'm not sure
if it would be legal for the client to send a new query before receiving the
CopyDone message from the backend, but trying to support that would require
bigger changes to the backend code structure.

Fixes an assertion failure reported by Fujii Masao.

src/backend/replication/walsender.c

@@ -674,12 +674,7 @@ ProcessRepliesIfAny(void)
 	int			r;
 	bool		received = false;
 
-	/*
-	 * If we already received a CopyDone from the frontend, any subsequent
-	 * message is the beginning of a new command, and should be processed in
-	 * the main processing loop.
-	 */
-	while (!streamingDoneReceiving)
+	for (;;)
 	{
 		r = pq_getbyte_if_available(&firstchar);
 		if (r < 0)
@@ -696,6 +691,19 @@ ProcessRepliesIfAny(void)
 			break;
 		}
 
+		/*
+		 * If we already received a CopyDone from the frontend, the frontend
+		 * should not send us anything until we've closed our end of the COPY.
+		 * XXX: In theory, the frontend could already send the next command
+		 * before receiving the CopyDone, but libpq doesn't currently allow
+		 * that.
+		 */
+		if (streamingDoneReceiving && firstchar != 'X')
+			ereport(FATAL,
+					(errcode(ERRCODE_PROTOCOL_VIOLATION),
+					 errmsg("unexpected standby message type \"%c\", after receiving CopyDone",
+							firstchar)));
+
 		/* Handle the very limited subset of commands expected in this phase */
 		switch (firstchar)
 		{
@@ -1048,10 +1056,8 @@ WalSndLoop(void)
 			long		sleeptime = 10000;		/* 10 s */
 			int			wakeEvents;
 
-			wakeEvents = WL_LATCH_SET | WL_POSTMASTER_DEATH | WL_TIMEOUT;
-
-			if (!streamingDoneReceiving)
-				wakeEvents |= WL_SOCKET_READABLE;
+			wakeEvents = WL_LATCH_SET | WL_POSTMASTER_DEATH | WL_TIMEOUT |
+				WL_SOCKET_READABLE;
 
 			if (pq_is_send_pending())
 				wakeEvents |= WL_SOCKET_WRITEABLE;