Adjust assertion in XLogDecodeNextRecord.

As written, if you use XLogBeginRead() to position an xlogreader at
the beginning of a WAL page and then try to read WAL, this assertion
will fail. However, the header comment for XLogBeginRead() claims
that positioning an xlogreader at the beginning of a page is valid,
and the code here is perfectly able to cope with it. It's only the
assertion that causes trouble. So relax it.

This is formally a bug in all supported branches, but as it doesn't
seem to have any consequences for current uses of the xlogreader
facility, no back-patch, at least for now.

Dilip Kumar and Robert Haas

Discussion: http://postgr.es/m/CA+TgmoaJSs2_7WHW2GzFYe9+zfPtxBKvT3GW47+x=ptUE=cULw@mail.gmail.com

src/backend/access/transam/xlogreader.c

@@ -576,10 +576,11 @@ XLogDecodeNextRecord(XLogReaderState *state, bool nonblocking)
 		/*
 		 * Caller supplied a position to start at.
 		 *
-		 * In this case, NextRecPtr should already be pointing to a valid
-		 * record starting position.
+		 * In this case, NextRecPtr should already be pointing either to a
+		 * valid record starting position or alternatively to the beginning of
+		 * a page. See the header comments for XLogBeginRead.
 		 */
-		Assert(XRecOffIsValid(RecPtr));
+		Assert(RecPtr % XLOG_BLCKSZ == 0 || XRecOffIsValid(RecPtr));
 		randAccess = true;
 	}