Fix walsender to exit promptly if client requests shutdown.

It's possible for WalSndWaitForWal to be asked to wait for WAL that doesn't
exist yet.  That's fine, in fact it's the normal situation if we're caught
up; but when the client requests shutdown we should not keep waiting.
The previous coding could wait indefinitely if the source server was idle.

In passing, improve the rather weak comments in this area, and slightly
rearrange some related code for better readability.

Back-patch to 9.4 where this code was introduced.

Discussion: https://postgr.es/m/14154.1498781234@sss.pgh.pa.us

src/backend/replication/walsender.c

@@ -764,15 +764,14 @@ logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int req
 	/* make sure we have enough WAL available */
 	flushptr = WalSndWaitForWal(targetPagePtr + reqLen);
 
-	/* more than one block available */
-	if (targetPagePtr + XLOG_BLCKSZ <= flushptr)
-		count = XLOG_BLCKSZ;
-	/* not enough WAL synced, that can happen during shutdown */
-	else if (targetPagePtr + reqLen > flushptr)
+	/* fail if not (implies we are going to shut down) */
+	if (flushptr < targetPagePtr + reqLen)
 		return -1;
-	/* part of the page available */
+
+	if (targetPagePtr + XLOG_BLCKSZ <= flushptr)
+		count = XLOG_BLCKSZ;	/* more than one block available */
 	else
-		count = flushptr - targetPagePtr;
+		count = flushptr - targetPagePtr;	/* part of the page available */
 
 	/* now actually read the data, we know it's there */
 	XLogRead(cur_page, targetPagePtr, XLOG_BLCKSZ);
@@ -1266,7 +1265,11 @@ WalSndUpdateProgress(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId
 }
 
 /*
- * Wait till WAL < loc is flushed to disk so it can be safely read.
+ * Wait till WAL < loc is flushed to disk so it can be safely sent to client.
+ *
+ * Returns end LSN of flushed WAL.  Normally this will be >= loc, but
+ * if we detect a shutdown request (either from postmaster or client)
+ * we will return early, so caller must always check.
  */
 static XLogRecPtr
 WalSndWaitForWal(XLogRecPtr loc)
@@ -1333,9 +1336,7 @@ WalSndWaitForWal(XLogRecPtr loc)
 			RecentFlushPtr = GetXLogReplayRecPtr(NULL);
 
 		/*
-		 * If postmaster asked us to stop, don't wait here anymore. This will
-		 * cause the xlogreader to return without reading a full record, which
-		 * is the fastest way to reach the mainloop which then can quit.
+		 * If postmaster asked us to stop, don't wait anymore.
 		 *
 		 * It's important to do this check after the recomputation of
 		 * RecentFlushPtr, so we can send all remaining data before shutting
@@ -1366,14 +1367,20 @@ WalSndWaitForWal(XLogRecPtr loc)
 		WalSndCaughtUp = true;
 
 		/*
-		 * Try to flush pending output to the client. Also wait for the socket
-		 * becoming writable, if there's still pending output after an attempt
-		 * to flush. Otherwise we might just sit on output data while waiting
-		 * for new WAL being generated.
+		 * Try to flush any pending output to the client.
 		 */
 		if (pq_flush_if_writable() != 0)
 			WalSndShutdown();
 
+		/*
+		 * If we have received CopyDone from the client, sent CopyDone
+		 * ourselves, and the output buffer is empty, it's time to exit
+		 * streaming, so fail the current WAL fetch request.
+		 */
+		if (streamingDoneReceiving && streamingDoneSending &&
+			!pq_is_send_pending())
+			break;
+
 		now = GetCurrentTimestamp();
 
 		/* die if timeout was reached */
@@ -1382,6 +1389,13 @@ WalSndWaitForWal(XLogRecPtr loc)
 		/* Send keepalive if the time has come */
 		WalSndKeepaliveIfNecessary(now);
 
+		/*
+		 * Sleep until something happens or we time out.  Also wait for the
+		 * socket becoming writable, if there's still pending output.
+		 * Otherwise we might sit on sendable output data while waiting for
+		 * new WAL to be generated.  (But if we have nothing to send, we don't
+		 * want to wake on socket-writable.)
+		 */
 		sleeptime = WalSndComputeSleeptime(now);
 
 		wakeEvents = WL_LATCH_SET | WL_POSTMASTER_DEATH |
@@ -1390,7 +1404,6 @@ WalSndWaitForWal(XLogRecPtr loc)
 		if (pq_is_send_pending())
 			wakeEvents |= WL_SOCKET_WRITEABLE;
 
-		/* Sleep until something happens or we time out */
 		WaitLatchOrSocket(MyLatch, wakeEvents,
 						  MyProcPort->sock, sleeptime,
 						  WAIT_EVENT_WAL_SENDER_WAIT_WAL);
@@ -2115,7 +2128,8 @@ WalSndLoop(WalSndSendDataCallback send_data)
 		 * ourselves, and the output buffer is empty, it's time to exit
 		 * streaming.
 		 */
-		if (!pq_is_send_pending() && streamingDoneSending && streamingDoneReceiving)
+		if (streamingDoneReceiving && streamingDoneSending &&
+			!pq_is_send_pending())
 			break;
 
 		/*