Code review of recycling WAL segments in a restartpoint.

Seems cleaner to get the currently-replayed TLI in the same call to
GetXLogReplayRecPtr that we get the WAL position. Make it more clear in the
comment what the code does when recovery has already ended
(RecoveryInProgress() will set ThisTimeLineID in that case). Finally, make
resetting ThisTimeLineID afterwards more explicit.

src/backend/access/transam/xlog.c

@@ -7463,6 +7463,7 @@ CreateRestartPoint(int flags)
 	{
 		XLogRecPtr	receivePtr;
 		XLogRecPtr	replayPtr;
+		TimeLineID	replayTLI;
 		XLogRecPtr	endptr;
 
 		/*
@@ -7470,32 +7471,27 @@ CreateRestartPoint(int flags)
 		 * later.
 		 */
 		receivePtr = GetWalRcvWriteRecPtr(NULL, NULL);
-		replayPtr = GetXLogReplayRecPtr(NULL);
+		replayPtr = GetXLogReplayRecPtr(&replayTLI);
 		endptr = (receivePtr < replayPtr) ? replayPtr : receivePtr;
 
 		KeepLogSeg(endptr, &_logSegNo);
 		_logSegNo--;
 
 		/*
-		 * Update ThisTimeLineID to the timeline we're currently replaying, so
-		 * that we install any recycled segments on that timeline.
+		 * Try to recycle segments on a useful timeline. If we've been promoted
+		 * since the beginning of this restartpoint, use the new timeline
+		 * chosen at end of recovery (RecoveryInProgress() sets ThisTimeLineID
+		 * in that case). If we're still in recovery, use the timeline we're
+		 * currently replaying.
 		 *
 		 * There is no guarantee that the WAL segments will be useful on the
 		 * current timeline; if recovery proceeds to a new timeline right
 		 * after this, the pre-allocated WAL segments on this timeline will
 		 * not be used, and will go wasted until recycled on the next
 		 * restartpoint. We'll live with that.
-		 *
-		 * It's possible or perhaps even likely that we finish recovery while
-		 * a restartpoint is in progress. That means we may get to this point
-		 * some minutes afterwards. Setting ThisTimeLineID at that time would
-		 * actually set it backwards, so we don't want that to persist; if we
-		 * do reset it here, make sure to reset it back afterwards. This
-		 * doesn't look very clean or principled, but its the best of about
-		 * five different ways of handling this edge case.
 		 */
 		if (RecoveryInProgress())
-			(void) GetXLogReplayRecPtr(&ThisTimeLineID);
+			ThisTimeLineID = replayTLI;
 
 		RemoveOldXlogFiles(_logSegNo, endptr);
 
@@ -7506,10 +7502,14 @@ CreateRestartPoint(int flags)
 		PreallocXlogFiles(endptr);
 
 		/*
-		 * Reset this always, in case we set ThisTimeLineID backwards above.
-		 * Requires no locking; see InitXLOGAccess()
+		 * ThisTimeLineID is normally not set when we're still in recovery.
+		 * However, recycling/preallocating segments above needed
+		 * ThisTimeLineID to determine which timeline to install the segments
+		 * on. Reset it now, to restore the normal state of affairs for
+		 * debugging purposes.
 		 */
-		ThisTimeLineID = XLogCtl->ThisTimeLineID;
+		if (RecoveryInProgress())
+			ThisTimeLineID = 0;
 	}
 
 	/*