pg_stat_replication.sync_state was displayed incorrectly at page boundary.

XLogRecPtrIsInvalid() only checks the xrecoff field, which is correct when
checking if a WAL record could legally begin at the given position, but WAL
sending can legally be paused at a page boundary, in which case xrecoff is
0. Use XLByteEQ(..., InvalidXLogRecPtr) instead, which checks that both
xlogid and xrecoff are 0.

9.3 doesn't have this problem because XLogRecPtr is now a single 64-bit
integer, so XLogRecPtrIsInvalid() does the right thing. Apply to 9.2, and
9.1 where pg_stat_replication view was introduced.

Kyotaro HORIGUCHI, reviewed by Fujii Masao.

src/backend/replication/syncrep.c

@@ -375,6 +375,7 @@ SyncRepReleaseWaiters(void)
 	int			numprocs = 0;
 	int			priority = 0;
 	int			i;
+	XLogRecPtr	InvalidXLogRecPtr = {0, 0};
 
 	/*
 	 * If this WALSender is serving a standby that is not on the list of
@@ -384,7 +385,7 @@ SyncRepReleaseWaiters(void)
 	 */
 	if (MyWalSnd->sync_standby_priority == 0 ||
 		MyWalSnd->state < WALSNDSTATE_STREAMING ||
-		XLogRecPtrIsInvalid(MyWalSnd->flush))
+		XLByteEQ(MyWalSnd->flush, InvalidXLogRecPtr))
 		return;
 
 	/*
@@ -405,7 +406,7 @@ SyncRepReleaseWaiters(void)
 			walsnd->sync_standby_priority > 0 &&
 			(priority == 0 ||
 			 priority > walsnd->sync_standby_priority) &&
-			!XLogRecPtrIsInvalid(walsnd->flush))
+			!XLByteEQ(walsnd->flush, InvalidXLogRecPtr))
 		{
 			priority = walsnd->sync_standby_priority;
 			syncWalSnd = walsnd;

src/backend/replication/walsender.c

@@ -1403,6 +1403,7 @@ pg_stat_get_wal_senders(PG_FUNCTION_ARGS)
 	int			priority = 0;
 	int			sync_standby = -1;
 	int			i;
+	XLogRecPtr	InvalidXLogRecPtr = {0, 0};
 
 	/* check to see if caller supports us returning a tuplestore */
 	if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
@@ -1448,14 +1449,14 @@ pg_stat_get_wal_senders(PG_FUNCTION_ARGS)
 			 * which always returns an invalid flush location, as an
 			 * asynchronous standby.
 			 */
-			sync_priority[i] = XLogRecPtrIsInvalid(walsnd->flush) ?
+			sync_priority[i] = XLByteEQ(walsnd->flush, InvalidXLogRecPtr) ?
 				0 : walsnd->sync_standby_priority;
 
 			if (walsnd->state == WALSNDSTATE_STREAMING &&
 				walsnd->sync_standby_priority > 0 &&
 				(priority == 0 ||
 				 priority > walsnd->sync_standby_priority) &&
-				!XLogRecPtrIsInvalid(walsnd->flush))
+				!XLByteEQ(walsnd->flush, InvalidXLogRecPtr))
 			{
 				priority = walsnd->sync_standby_priority;
 				sync_standby = i;