Fix sloppiness in the timeline switch over streaming replication patch.

Here's another attempt at fixing the logic that decides how far the WAL can
be streamed, which was still broken if the timeline changed while streaming.
You would get an assertion failure. The way the logic is now written is more
readable, too.

Thom Brown reported the assertion failure.

src/backend/replication/walsender.c

@@ -169,7 +169,7 @@ static void WalSndLoop(void);
 static void InitWalSenderSlot(void);
 static void WalSndKill(int code, Datum arg);
 static void XLogSend(bool *caughtup);
-static XLogRecPtr GetStandbyFlushRecPtr(TimeLineID currentTLI);
+static XLogRecPtr GetStandbyFlushRecPtr(void);
 static void IdentifySystem(void);
 static void StartReplication(StartReplicationCmd *cmd);
 static void ProcessStandbyMessage(void);
@@ -250,7 +250,7 @@ IdentifySystem(void)
 	if (am_cascading_walsender)
 	{
 		/* this also updates ThisTimeLineID */
-		logptr = GetStandbyFlushRecPtr(0);
+		logptr = GetStandbyFlushRecPtr();
 	}
 	else
 		logptr = GetInsertRecPtr();
@@ -423,7 +423,7 @@ StartReplication(StartReplicationCmd *cmd)
 	if (am_cascading_walsender)
 	{
 		/* this also updates ThisTimeLineID */
-		FlushPtr = GetStandbyFlushRecPtr(0);
+		FlushPtr = GetStandbyFlushRecPtr();
 	}
 	else
 		FlushPtr = GetFlushRecPtr();
@@ -1310,7 +1310,6 @@ static void
 XLogSend(bool *caughtup)
 {
 	XLogRecPtr	SendRqstPtr;
-	XLogRecPtr	FlushPtr;
 	XLogRecPtr	startptr;
 	XLogRecPtr	endptr;
 	Size		nbytes;
@@ -1321,33 +1320,39 @@ XLogSend(bool *caughtup)
 		return;
 	}
 
-	/*
-	 * Attempt to send all data that's already been written out and fsync'd to
-	 * disk.  We cannot go further than what's been written out given the
-	 * current implementation of XLogRead().  And in any case it's unsafe to
-	 * send WAL that is not securely down to disk on the master: if the master
-	 * subsequently crashes and restarts, slaves must not have applied any WAL
-	 * that gets lost on the master.
-	 */
-	if (am_cascading_walsender)
-		FlushPtr = GetStandbyFlushRecPtr(sendTimeLine);
-	else
-		FlushPtr = GetFlushRecPtr();
-
-	/*
-	 * In a cascading standby, the current recovery target timeline can
-	 * change, or we can be promoted. In either case, the current timeline
-	 * becomes historic. We need to detect that so that we don't try to stream
-	 * past the point where we switched to another timeline. It's checked
-	 * after calculating FlushPtr, to avoid a race condition: if the timeline
-	 * becomes historic just after we checked that it was still current, it
-	 * should still be OK to stream it up to the FlushPtr that was calculated
-	 * before it became historic.
-	 */
-	if (!sendTimeLineIsHistoric && am_cascading_walsender)
+	/* Figure out how far we can safely send the WAL. */
+	if (sendTimeLineIsHistoric)
 	{
+		/*
+		 * Streaming an old timeline timeline that's in this server's history,
+		 * but is not the one we're currently inserting or replaying. It can
+		 * be streamed up to the point where we switched off that timeline.
+		 */
+		SendRqstPtr = sendTimeLineValidUpto;
+	}
+	else if (am_cascading_walsender)
+	{
+		/*
+		 * Streaming the latest timeline on a standby.
+		 *
+		 * Attempt to send all WAL that has already been replayed, so that
+		 * we know it's valid. If we're receiving WAL through streaming
+		 * replication, it's also OK to send any WAL that has been received
+		 * but not replayed.
+		 *
+		 * The timeline we're recovering from can change, or we can be
+		 * promoted. In either case, the current timeline becomes historic.
+		 * We need to detect that so that we don't try to stream past the
+		 * point where we switched to another timeline. We check for promotion
+		 * or timeline switch after calculating FlushPtr, to avoid a race
+		 * condition: if the timeline becomes historic just after we checked
+		 * that it was still current, it's still be OK to stream it up to the
+		 * FlushPtr that was calculated before it became historic.
+		 */
 		bool		becameHistoric = false;
 
+		SendRqstPtr = GetStandbyFlushRecPtr();
+
 		if (!RecoveryInProgress())
 		{
 			/*
@@ -1361,7 +1366,8 @@ XLogSend(bool *caughtup)
 		{
 			/*
 			 * Still a cascading standby. But is the timeline we're sending
-			 * still the one recovery is recovering from?
+			 * still the one recovery is recovering from? ThisTimeLineID was
+			 * updated by the GetStandbyFlushRecPtr() call above.
 			 */
 			if (sendTimeLine != ThisTimeLineID)
 				becameHistoric = true;
@@ -1391,8 +1397,24 @@ XLogSend(bool *caughtup)
 					 (uint32) sentPtr);
 
 			sendTimeLineIsHistoric = true;
+
+			SendRqstPtr = sendTimeLineValidUpto;
 		}
 	}
+	else
+	{
+		/*
+		 * Streaming the current timeline on a master.
+		 *
+		 * Attempt to send all data that's already been written out and
+		 * fsync'd to disk.  We cannot go further than what's been written out
+		 * given the current implementation of XLogRead().  And in any case
+		 * it's unsafe to send WAL that is not securely down to disk on the
+		 * master: if the master subsequently crashes and restarts, slaves
+		 * must not have applied any WAL that gets lost on the master.
+		 */
+		SendRqstPtr = GetFlushRecPtr();
+	}
 
 	/*
 	 * If this is a historic timeline and we've reached the point where we
@@ -1413,15 +1435,7 @@ XLogSend(bool *caughtup)
 		return;
 	}
 
-	/*
-	 * Stream up to the point known to be flushed to disk, or to the end of
-	 * this timeline, whichever comes first.
-	 */
-	if (sendTimeLineIsHistoric && XLByteLT(sendTimeLineValidUpto, FlushPtr))
-		SendRqstPtr = sendTimeLineValidUpto;
-	else
-		SendRqstPtr = FlushPtr;
-
+	/* Do we have any work to do? */
 	Assert(XLByteLE(sentPtr, SendRqstPtr));
 	if (XLByteLE(SendRqstPtr, sentPtr))
 	{
@@ -1522,15 +1536,11 @@ XLogSend(bool *caughtup)
  * can be sent to the standby. This should only be called when in recovery,
  * ie. we're streaming to a cascaded standby.
  *
- * If currentTLI is non-zero, the function returns the point that the WAL on
- * the given timeline has been flushed upto. If recovery has already switched
- * to a different timeline, InvalidXLogRecPtr is returned.
- *
  * As a side-effect, ThisTimeLineID is updated to the TLI of the last
  * replayed WAL record.
  */
 static XLogRecPtr
-GetStandbyFlushRecPtr(TimeLineID currentTLI)
+GetStandbyFlushRecPtr(void)
 {
 	XLogRecPtr replayPtr;
 	TimeLineID replayTLI;
@@ -1549,11 +1559,8 @@ GetStandbyFlushRecPtr(TimeLineID currentTLI)
 
 	ThisTimeLineID = replayTLI;
 
-	if (currentTLI != replayTLI && currentTLI != 0)
-		return InvalidXLogRecPtr;
-
 	result = replayPtr;
-	if (receiveTLI == currentTLI && receivePtr > replayPtr)
+	if (receiveTLI == ThisTimeLineID && receivePtr > replayPtr)
 		result = receivePtr;
 
 	return result;