report expected contrecord size

[Alvaro Herrera]

A pretty minor issue: when reporting that WAL appears invalid because
contrecord length doesn't match, we may as well print to the server log
the value that we're expecting.  Patch attached.

-- 
Álvaro Herrera                         http://www.flickr.com/photos/alvherre/

diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index 67996018da..b69cf80b59 100644
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -464,8 +464,9 @@ XLogReadRecord(XLogReaderState *state, char **errormsg)
 				total_len != (pageHeader->xlp_rem_len + gotlen))
 			{
 				report_invalid_record(state,
-									  "invalid contrecord length %u at %X/%X",
+									  "invalid contrecord length %u (expected %lld) at %X/%X",
 									  pageHeader->xlp_rem_len,
+									  (long long) total_len - gotlen,
 									  (uint32) (RecPtr >> 32), (uint32) RecPtr);
 				goto err;
 			}