[CF 5199] v2 - Fix standby may crash when switching-over in certain special cases

This branch was automatically generated by a robot using patches from an
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https://commitfest.postgresql.org/patch/5199

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Patch(es): https://www.postgresql.org/message-id/CAAccyYKzMZiq5xOjUkjhvxth8V+yiNuyX=9xcnLM_yBM9oYWFQ@mail.gmail.com
Author(s): pixian shi

Author: Commitfest Bot

src/backend/replication/walreceiverfuncs.c

@@ -312,10 +312,10 @@ RequestXLogStreaming(TimeLineID tli, XLogRecPtr recptr, const char *conninfo,
 	walrcv->startTime = now;
 
 	/*
-	 * If this is the first startup of walreceiver (on this timeline),
+	 * If this is the first startup of walreceiver (on this timeline) or recptr lt last flushedLsn,
 	 * initialize flushedUpto and latestChunkStart to the starting point.
 	 */
-	if (walrcv->receiveStart == 0 || walrcv->receivedTLI != tli)
+	if (walrcv->receiveStart == 0 || walrcv->receivedTLI != tli || recptr < walrcv->receiveStart)
 	{
 		walrcv->flushedUpto = recptr;
 		walrcv->receivedTLI = tli;