Change pg_last_xlog_receive_location() not to move backwards. That makes

it a lot more useful for determining which standby is most up-to-date,
for example. There was long discussions on whether overwriting existing
existing WAL makes sense to begin with, and whether we should do some more
extensive variable renaming, but this change nevertheless seems quite
uncontroversial.

Fujii Masao, reviewed by Jeff Janes, Robert Haas, Stephen Frost.

doc/src/sgml/func.sgml

@@ -14179,10 +14179,8 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
        <entry><type>text</type></entry>
        <entry>Get last transaction log location received and synced to disk by
         streaming replication. While streaming replication is in progress
-        this will increase monotonically. But when streaming replication is
-        restarted this will back off to the replication starting position,
-        typically the beginning of the WAL file containing the current
-        replay location.  If recovery has completed this will remain static at
+        this will increase monotonically. If recovery has completed this will
+        remain static at
         the value of the last WAL record received and synced to disk during
         recovery. If streaming replication is disabled, or if it has not yet
         started, the function returns NULL.

src/backend/replication/walreceiver.c

@@ -218,7 +218,7 @@ WalReceiverMain(void)
 
 	/* Fetch information required to start streaming */
 	strlcpy(conninfo, (char *) walrcv->conninfo, MAXCONNINFO);
-	startpoint = walrcv->receivedUpto;
+	startpoint = walrcv->receiveStart;
 	SpinLockRelease(&walrcv->mutex);
 
 	/* Arrange to clean up at walreceiver exit */
@@ -558,8 +558,11 @@ XLogWalRcvFlush(bool dying)
 
 		/* Update shared-memory status */
 		SpinLockAcquire(&walrcv->mutex);
-		walrcv->latestChunkStart = walrcv->receivedUpto;
-		walrcv->receivedUpto = LogstreamResult.Flush;
+		if (XLByteLT(walrcv->receivedUpto, LogstreamResult.Flush))
+		{
+			walrcv->latestChunkStart = walrcv->receivedUpto;
+			walrcv->receivedUpto = LogstreamResult.Flush;
+		}
 		SpinLockRelease(&walrcv->mutex);
 
 		/* Signal the startup process that new WAL has arrived */

src/backend/replication/walreceiverfuncs.c

@@ -199,8 +199,17 @@ RequestXLogStreaming(XLogRecPtr recptr, const char *conninfo)
 	walrcv->walRcvState = WALRCV_STARTING;
 	walrcv->startTime = now;
 
-	walrcv->receivedUpto = recptr;
-	walrcv->latestChunkStart = recptr;
+	/*
+	 * If this is the first startup of walreceiver, we initialize
+	 * receivedUpto and latestChunkStart to receiveStart.
+	 */
+	if (walrcv->receiveStart.xlogid == 0 &&
+		walrcv->receiveStart.xrecoff == 0)
+	{
+		walrcv->receivedUpto = recptr;
+		walrcv->latestChunkStart = recptr;
+	}
+	walrcv->receiveStart = recptr;
 
 	SpinLockRelease(&walrcv->mutex);
 

src/include/replication/walreceiver.h

@@ -51,12 +51,18 @@ typedef struct
 	WalRcvState walRcvState;
 	pg_time_t	startTime;
 
+	/*
+	 * receiveStart is the first byte position that will be received.
+	 * When startup process starts the walreceiver, it sets receiveStart
+	 * to the point where it wants the streaming to begin.
+	 */
+	XLogRecPtr	receiveStart;
+
 	/*
 	 * receivedUpto-1 is the last byte position that has already been
-	 * received.  When startup process starts the walreceiver, it sets
-	 * receivedUpto to the point where it wants the streaming to begin. After
-	 * that, walreceiver updates this whenever it flushes the received WAL to
-	 * disk.
+	 * received.  At the first startup of walreceiver, receivedUpto is
+	 * set to receiveStart. After that, walreceiver updates this whenever
+	 * it flushes the received WAL to disk.
 	 */
 	XLogRecPtr	receivedUpto;