Enable logical slots to follow timeline switches

When decoding from a logical slot, it's necessary for xlog reading to be
able to read xlog from historical (i.e. not current) timelines;
otherwise, decoding fails after failover, because the archives are in
the historical timeline.  This is required to make "failover logical
slots" possible; it currently has no other use, although theoretically
it could be used by an extension that creates a slot on a standby and
continues to replay from the slot when the standby is promoted.

This commit includes a module in src/test/modules with functions to
manipulate the slots (which is not otherwise possible in SQL code) in
order to enable testing, and a new test in src/test/recovery to ensure
that the behavior is as expected.

Author: Craig Ringer
Reviewed-By: Oleksii Kliukin, Andres Freund, Petr Jelínek

src/include/access/xlogreader.h

@@ -27,6 +27,10 @@
 
 #include "access/xlogrecord.h"
 
+#ifndef FRONTEND
+#include "nodes/pg_list.h"
+#endif
+
 typedef struct XLogReaderState XLogReaderState;
 
 /* Function type definition for the read_page callback */
@@ -160,11 +164,25 @@ struct XLogReaderState
 
 	/* beginning of the WAL record being read. */
 	XLogRecPtr	currRecPtr;
+	/* timeline to read it from, 0 if a lookup is required */
+	TimeLineID	currTLI;
+	/*
+	 * Safe point to read to in currTLI.  If currTLI is historical, then this
+	 * is set to the end of the last whole segment that contains that TLI;
+	 * if currTLI is ThisTimeLineID, this is InvalidXLogRecPtr.  This is *not*
+	 * the tliSwitchPoint.
+	 */
+	XLogRecPtr	currTLIValidUntil;
 
 	/* Buffer for current ReadRecord result (expandable) */
 	char	   *readRecordBuf;
 	uint32		readRecordBufSize;
 
+#ifndef FRONTEND
+	/* cached timeline history, only available in backend */
+	List	   *timelineHistory;
+#endif
+
 	/* Buffer to hold error message */
 	char	   *errormsg_buf;
 };