Send new protocol keepalive messages to standby servers.

Allows streaming replication users to calculate transfer latency
and apply delay via internal functions. No external functions yet.

src/include/replication/walprotocol.h

@@ -16,6 +16,20 @@
 #include "datatype/timestamp.h"
 
 
+/*
+ * All messages from WalSender must contain these fields to allow us to
+ * correctly calculate the replication delay.
+ */
+typedef struct
+{
+	/* Current end of WAL on the sender */
+	XLogRecPtr	walEnd;
+
+	/* Sender's system clock at the time of transmission */
+	TimestampTz sendTime;
+} WalSndrMessage;
+
+
 /*
  * Header for a WAL data message (message type 'w').  This is wrapped within
  * a CopyData message at the FE/BE protocol level.
@@ -39,6 +53,14 @@ typedef struct
 	TimestampTz sendTime;
 } WalDataMessageHeader;
 
+/*
+ * Keepalive message from primary (message type 'k'). (lowercase k)
+ * This is wrapped within a CopyData message at the FE/BE protocol level.
+ *
+ * Note that the data length is not specified here.
+ */
+typedef WalSndrMessage	PrimaryKeepaliveMessage;
+
 /*
  * Reply message from standby (message type 'r').  This is wrapped within
  * a CopyData message at the FE/BE protocol level.

src/include/replication/walreceiver.h

@@ -78,6 +78,12 @@ typedef struct
 	 */
 	XLogRecPtr	latestChunkStart;
 
+	/*
+	 * Time of send and receive of any message received.
+	 */
+	TimestampTz lastMsgSendTime;
+	TimestampTz lastMsgReceiptTime;
+
 	/*
 	 * connection string; is used for walreceiver to connect with the primary.
 	 */
@@ -112,5 +118,7 @@ extern void ShutdownWalRcv(void);
 extern bool WalRcvInProgress(void);
 extern void RequestXLogStreaming(XLogRecPtr recptr, const char *conninfo);
 extern XLogRecPtr GetWalRcvWriteRecPtr(XLogRecPtr *latestChunkStart);
+extern int GetReplicationApplyDelay(void);
+extern int GetReplicationTransferLatency(void);
 
 #endif   /* _WALRECEIVER_H */