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.

1 files changed, 20 insertions, 5 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 8e9dc7ba92a..f5cb6576de4 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -9318,6 +9318,25 @@ pg_last_xlog_receive_location(PG_FUNCTION_ARGS)
 }
 
 /*
+ * Get latest redo apply position.
+ *
+ * Exported to allow WALReceiver to read the pointer directly.
+ */
+XLogRecPtr
+GetXLogReplayRecPtr(void)
+{
+	/* use volatile pointer to prevent code rearrangement */
+	volatile XLogCtlData *xlogctl = XLogCtl;
+	XLogRecPtr	recptr;
+
+	SpinLockAcquire(&xlogctl->info_lck);
+	recptr = xlogctl->recoveryLastRecPtr;
+	SpinLockRelease(&xlogctl->info_lck);
+
+	return recptr;
+}
+
+/*
  * Report the last WAL replay location (same format as pg_start_backup etc)
  *
  * This is useful for determining how much of WAL is visible to read-only
@@ -9326,14 +9345,10 @@ pg_last_xlog_receive_location(PG_FUNCTION_ARGS)
 Datum
 pg_last_xlog_replay_location(PG_FUNCTION_ARGS)
 {
-	/* use volatile pointer to prevent code rearrangement */
-	volatile XLogCtlData *xlogctl = XLogCtl;
 	XLogRecPtr	recptr;
 	char		location[MAXFNAMELEN];
 
-	SpinLockAcquire(&xlogctl->info_lck);
-	recptr = xlogctl->recoveryLastRecPtr;
-	SpinLockRelease(&xlogctl->info_lck);
+	recptr = GetXLogReplayRecPtr();
 
 	if (recptr.xlogid == 0 && recptr.xrecoff == 0)
 		PG_RETURN_NULL();