Fix bug where walsender goes into a busy loop if connection is terminated.

The problem was that ResetLatch was not being called in the walsender loop
if the connection was terminated, so WaitLatch never sleeps until the
terminated connection is detected. In the master-branch, this was already
fixed as a side-effect of some refactoring of the loop. This commit
backports that refactoring to 9.1. 9.0 does not have this bug, because we
didn't use latches back then.

Fujii Masao

src/backend/replication/walsender.c

@@ -476,7 +476,7 @@ ProcessRepliesIfAny(void)
 {
 	unsigned char firstchar;
 	int			r;
-	int			received = false;
+	bool		received = false;
 
 	for (;;)
 	{
@@ -700,6 +700,9 @@ WalSndLoop(void)
 	/* Loop forever, unless we get an error */
 	for (;;)
 	{
+		/* Clear any already-pending wakeups */
+		ResetLatch(&MyWalSnd->latch);
+
 		/*
 		 * Emergency bailout if postmaster has died.  This is to avoid the
 		 * necessity for manual cleanup of all postmaster children.
@@ -718,60 +721,81 @@ WalSndLoop(void)
 		/* Normal exit from the walsender is here */
 		if (walsender_shutdown_requested)
 		{
-			/* Inform the standby that XLOG streaming was done */
+			/* Inform the standby that XLOG streaming is done */
 			pq_puttextmessage('C', "COPY 0");
 			pq_flush();
 
 			proc_exit(0);
 		}
 
+		/* Check for input from the client */
+		ProcessRepliesIfAny();
+
 		/*
 		 * If we don't have any pending data in the output buffer, try to send
-		 * some more.
+		 * some more.  If there is some, we don't bother to call XLogSend
+		 * again until we've flushed it ... but we'd better assume we are not
+		 * caught up.
 		 */
 		if (!pq_is_send_pending())
-		{
 			XLogSend(output_message, &caughtup);
+		else
+			caughtup = false;
 
+		/* Try to flush pending output to the client */
+		if (pq_flush_if_writable() != 0)
+			break;
+
+		/* If nothing remains to be sent right now ... */
+		if (caughtup && !pq_is_send_pending())
+		{
 			/*
-			 * Even if we wrote all the WAL that was available when we started
-			 * sending, more might have arrived while we were sending this
-			 * batch. We had the latch set while sending, so we have not
-			 * received any signals from that time. Let's arm the latch again,
-			 * and after that check that we're still up-to-date.
+			 * If we're in catchup state, move to streaming.  This is an
+			 * important state change for users to know about, since before
+			 * this point data loss might occur if the primary dies and we
+			 * need to failover to the standby. The state change is also
+			 * important for synchronous replication, since commits that
+			 * started to wait at that point might wait for some time.
 			 */
-			if (caughtup && !pq_is_send_pending())
+			if (MyWalSnd->state == WALSNDSTATE_CATCHUP)
 			{
-				ResetLatch(&MyWalSnd->latch);
-
-				XLogSend(output_message, &caughtup);
-			}
+				ereport(DEBUG1,
+						(errmsg("standby \"%s\" has now caught up with primary",
+								application_name)));
+				WalSndSetState(WALSNDSTATE_STREAMING);
 			}
 
-		/* Flush pending output to the client */
-		if (pq_flush_if_writable() != 0)
-			break;
-
 			/*
 			 * When SIGUSR2 arrives, we send any outstanding logs up to the
 			 * shutdown checkpoint record (i.e., the latest record) and exit.
+			 * This may be a normal termination at shutdown, or a promotion,
+			 * the walsender is not sure which.
 			 */
-		if (walsender_ready_to_stop && !pq_is_send_pending())
+			if (walsender_ready_to_stop)
 			{
+				/* ... let's just be real sure we're caught up ... */
 				XLogSend(output_message, &caughtup);
-			ProcessRepliesIfAny();
 				if (caughtup && !pq_is_send_pending())
+				{
 					walsender_shutdown_requested = true;
+					continue;		/* don't want to wait more */
+				}
+			}
 		}
 
-		if ((caughtup || pq_is_send_pending()) &&
-			!got_SIGHUP &&
-			!walsender_shutdown_requested)
+		/*
+		 * We don't block if not caught up, unless there is unsent data
+		 * pending in which case we'd better block until the socket is
+		 * write-ready.  This test is only needed for the case where XLogSend
+		 * loaded a subset of the available data but then pq_flush_if_writable
+		 * flushed it all --- we should immediately try to send more.
+		 */
+		if (caughtup || pq_is_send_pending())
 		{
 			TimestampTz finish_time = 0;
-			long		sleeptime;
+			long		sleeptime = -1;
 
-			/* Reschedule replication timeout */
+			/* Determine time until replication timeout */
 			if (replication_timeout > 0)
 			{
 				long		secs;
@@ -795,12 +819,16 @@ WalSndLoop(void)
 				sleeptime = WalSndDelay;
 			}
 
-			/* Sleep */
+			/* Sleep until something happens or replication timeout */
 			WaitLatchOrSocket(&MyWalSnd->latch, MyProcPort->sock,
 							  true, pq_is_send_pending(),
 							  sleeptime);
 
-			/* Check for replication timeout */
+			/*
+			 * Check for replication timeout.  Note we ignore the corner case
+			 * possibility that the client replied just as we reached the
+			 * timeout ... he's supposed to reply *before* that.
+			 */
 			if (replication_timeout > 0 &&
 				GetCurrentTimestamp() >= finish_time)
 			{
@@ -814,24 +842,6 @@ WalSndLoop(void)
 				break;
 			}
 		}
-
-		/*
-		 * If we're in catchup state, see if its time to move to streaming.
-		 * This is an important state change for users, since before this
-		 * point data loss might occur if the primary dies and we need to
-		 * failover to the standby. The state change is also important for
-		 * synchronous replication, since commits that started to wait at that
-		 * point might wait for some time.
-		 */
-		if (MyWalSnd->state == WALSNDSTATE_CATCHUP && caughtup)
-		{
-			ereport(DEBUG1,
-					(errmsg("standby \"%s\" has now caught up with primary",
-							application_name)));
-			WalSndSetState(WALSNDSTATE_STREAMING);
-		}
-
-		ProcessRepliesIfAny();
 	}
 
 	/*