Replace max_standby_delay with two parameters, max_standby_archive_delay and

max_standby_streaming_delay, and revise the implementation to avoid assuming
that timestamps found in WAL records can meaningfully be compared to clock
time on the standby server.  Instead, the delay limits are compared to the
elapsed time since we last obtained a new WAL segment from archive or since
we were last "caught up" to WAL data arriving via streaming replication.
This avoids problems with clock skew between primary and standby, as well
as other corner cases that the original coding would misbehave in, such
as the primary server having significant idle time between transactions.
Per my complaint some time ago and considerable ensuing discussion.

Do some desultory editing on the hot standby documentation, too.

1 files changed, 13 insertions, 4 deletions

diff --git a/src/backend/replication/walreceiverfuncs.c b/src/backend/replication/walreceiverfuncs.c
index 78ee7fb9f7e..4bc3bd875c0 100644
--- a/src/backend/replication/walreceiverfuncs.c
+++ b/src/backend/replication/walreceiverfuncs.c
@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/replication/walreceiverfuncs.c,v 1.5 2010/04/28 16:54:15 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/replication/walreceiverfuncs.c,v 1.6 2010/07/03 20:43:57 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -187,10 +187,11 @@ RequestXLogStreaming(XLogRecPtr recptr, const char *conninfo)
 	if (recptr.xrecoff % XLogSegSize != 0)
 		recptr.xrecoff -= recptr.xrecoff % XLogSegSize;
 
+	SpinLockAcquire(&walrcv->mutex);
+
 	/* It better be stopped before we try to restart it */
 	Assert(walrcv->walRcvState == WALRCV_STOPPED);
 
-	SpinLockAcquire(&walrcv->mutex);
 	if (conninfo != NULL)
 		strlcpy((char *) walrcv->conninfo, conninfo, MAXCONNINFO);
 	else
@@ -199,16 +200,22 @@ RequestXLogStreaming(XLogRecPtr recptr, const char *conninfo)
 	walrcv->startTime = now;
 
 	walrcv->receivedUpto = recptr;
+	walrcv->latestChunkStart = recptr;
+
 	SpinLockRelease(&walrcv->mutex);
 
 	SendPostmasterSignal(PMSIGNAL_START_WALRECEIVER);
 }
 
 /*
- * Returns the byte position that walreceiver has written
+ * Returns the last+1 byte position that walreceiver has written.
+ *
+ * Optionally, returns the previous chunk start, that is the first byte
+ * written in the most recent walreceiver flush cycle.  Callers not
+ * interested in that value may pass NULL for latestChunkStart.
  */
 XLogRecPtr
-GetWalRcvWriteRecPtr(void)
+GetWalRcvWriteRecPtr(XLogRecPtr *latestChunkStart)
 {
 	/* use volatile pointer to prevent code rearrangement */
 	volatile WalRcvData *walrcv = WalRcv;
@@ -216,6 +223,8 @@ GetWalRcvWriteRecPtr(void)
 
 	SpinLockAcquire(&walrcv->mutex);
 	recptr = walrcv->receivedUpto;
+	if (latestChunkStart)
+		*latestChunkStart = walrcv->latestChunkStart;
 	SpinLockRelease(&walrcv->mutex);
 
 	return recptr;