Teach xlogreader to follow timeline switches

Uses page-based mechanism to ensure we’re using the correct timeline.

Tests are included to exercise the functionality using a cold disk-level copy
of the master that's started up as a replica with slots intact, but the
intended use of the functionality is with later features.

Craig Ringer, reviewed by Simon Riggs and Andres Freund

src/backend/replication/walsender.c

@@ -48,6 +48,7 @@
 #include "access/transam.h"
 #include "access/xact.h"
 #include "access/xlog_internal.h"
+#include "access/xlogutils.h"
 
 #include "catalog/pg_type.h"
 #include "commands/dbcommands.h"
@@ -721,6 +722,12 @@ logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int req
 	XLogRecPtr	flushptr;
 	int			count;
 
+	XLogReadDetermineTimeline(state, targetPagePtr, reqLen);
+	sendTimeLineIsHistoric = (state->currTLI != ThisTimeLineID);
+	sendTimeLine = state->currTLI;
+	sendTimeLineValidUpto = state->currTLIValidUntil;
+	sendTimeLineNextTLI = state->nextTLI;
+
 	/* make sure we have enough WAL available */
 	flushptr = WalSndWaitForWal(targetPagePtr + reqLen);
 
@@ -974,10 +981,6 @@ StartLogicalReplication(StartReplicationCmd *cmd)
 	pq_endmessage(&buf);
 	pq_flush();
 
-	/* setup state for XLogReadPage */
-	sendTimeLineIsHistoric = false;
-	sendTimeLine = ThisTimeLineID;
-
 	/*
 	 * Initialize position to the last ack'ed one, then the xlog records begin
 	 * to be shipped from that position.