Add current WAL end (as seen by walsender, ie, GetWriteRecPtr() result)

and current server clock time to SR data messages.  These are not currently
used on the slave side but seem likely to be useful in future, and it'd be
better not to change the SR protocol after release.  Per discussion.
Also do some minor code review and cleanup on walsender.c, and improve the
protocol documentation.

src/include/replication/walprotocol.h

@@ -0,0 +1,53 @@
+/*-------------------------------------------------------------------------
+ *
+ * walprotocol.h
+ *	  Definitions relevant to the streaming WAL transmission protocol.
+ *
+ * Portions Copyright (c) 2010-2010, PostgreSQL Global Development Group
+ *
+ * $PostgreSQL: pgsql/src/include/replication/walprotocol.h,v 1.1 2010/06/03 22:17:32 tgl Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef _WALPROTOCOL_H
+#define _WALPROTOCOL_H
+
+#include "access/xlogdefs.h"
+#include "utils/timestamp.h"
+
+
+/*
+ * Header for a WAL data message (message type 'w').  This is wrapped within
+ * a CopyData message at the FE/BE protocol level.
+ *
+ * The header is followed by actual WAL data.  Note that the data length is
+ * not specified in the header --- it's just whatever remains in the message.
+ *
+ * walEnd and sendTime are not essential data, but are provided in case
+ * the receiver wants to adjust its behavior depending on how far behind
+ * it is.
+ */
+typedef struct
+{
+	/* WAL start location of the data included in this message */
+	XLogRecPtr	dataStart;
+
+	/* Current end of WAL on the sender */
+	XLogRecPtr	walEnd;
+
+	/* Sender's system clock at the time of transmission */
+	TimestampTz	sendTime;
+} WalDataMessageHeader;
+
+/*
+ * Maximum data payload in a WAL data message.  Must be >= XLOG_BLCKSZ.
+ *
+ * We don't have a good idea of what a good value would be; there's some
+ * overhead per message in both walsender and walreceiver, but on the other
+ * hand sending large batches makes walsender less responsive to signals
+ * because signals are checked only between messages.  128kB (with
+ * default 8k blocks) seems like a reasonable guess for now.
+ */
+#define MAX_SEND_SIZE (XLOG_BLCKSZ * 16)
+
+#endif   /* _WALPROTOCOL_H */

src/include/replication/walreceiver.h

@@ -5,7 +5,7 @@
  *
  * Portions Copyright (c) 2010-2010, PostgreSQL Global Development Group
  *
- * $PostgreSQL: pgsql/src/include/replication/walreceiver.h,v 1.8 2010/02/26 02:01:27 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/replication/walreceiver.h,v 1.9 2010/06/03 22:17:32 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -14,6 +14,7 @@
 
 #include "access/xlogdefs.h"
 #include "storage/spin.h"
+#include "pgtime.h"
 
 extern bool am_walreceiver;