Send status updates back from standby server to master, indicating how far

the standby has written, flushed, and applied the WAL. At the moment, this
is for informational purposes only, the values are only shown in
pg_stat_replication system view, but in the future they will also be needed
for synchronous replication.

Extracted from Simon riggs' synchronous replication patch by Robert Haas, with
some tweaking by me.

src/include/replication/walprotocol.h

@@ -39,6 +39,27 @@ typedef struct
 	TimestampTz sendTime;
 } WalDataMessageHeader;
 
+/*
+ * Reply message from standby (message type 'r').  This is wrapped within
+ * a CopyData message at the FE/BE protocol level.
+ *
+ * Note that the data length is not specified here.
+ */
+typedef struct
+{
+	/*
+	 * The xlog locations that have been written, flushed, and applied
+	 * by standby-side. These may be invalid if the standby-side is unable
+	 * to or chooses not to report these.
+	 */
+	XLogRecPtr	write;
+	XLogRecPtr	flush;
+	XLogRecPtr	apply;
+
+	/* Sender's system clock at the time of transmission */
+	TimestampTz sendTime;
+} StandbyReplyMessage;
+
 /*
  * Maximum data payload in a WAL data message.	Must be >= XLOG_BLCKSZ.
  *

src/include/replication/walreceiver.h

@@ -17,6 +17,7 @@
 #include "pgtime.h"
 
 extern bool am_walreceiver;
+extern int wal_receiver_status_interval;
 
 /*
  * MAXCONNINFO: maximum size of a connection string.

src/include/replication/walsender.h

@@ -35,7 +35,17 @@ typedef struct WalSnd
 	WalSndState state;			/* this walsender's state */
 	XLogRecPtr	sentPtr;		/* WAL has been sent up to this point */
 
-	slock_t		mutex;			/* locks shared variables shown above */
+	/*
+	 * The xlog locations that have been written, flushed, and applied
+	 * by standby-side. These may be invalid if the standby-side has not
+	 * offered values yet.
+	 */
+	XLogRecPtr	write;
+	XLogRecPtr	flush;
+	XLogRecPtr	apply;
+
+	/* Protects shared variables shown above. */
+	slock_t		mutex;
 
 	/*
 	 * Latch used by backends to wake up this walsender when it has work