pgsql: Fix pg_xlogdump so that it handles cross-page XLP_FIRST_IS_CONTR

Fix pg_xlogdump so that it handles cross-page XLP_FIRST_IS_CONTRECORD record.

Previously pg_xlogdump failed to dump the contents of the WAL file
if the file starts with the continuation WAL record which spans
more than one pages. Since pg_xlogdump assumed that the continuation
record always fits on a page, it could not find the valid WAL record to
start reading from in that case.

This patch changes pg_xlogdump so that it can handle a continuation
WAL record which crosses a page boundary and find the valid record
to start reading from.

Back-patch to 9.3 where pg_xlogdump was introduced.

Author: Pavan Deolasee
Reviewed-By: Michael Paquier and Craig Ringer
Discussion: CABOikdPsPByMiG6J01DKq6om2+BNkxHTPkOyqHM2a4oYwGKsqQ(at)mail(dot)gmail(dot)com

Branch
------
REL9_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/314a25fb3ae3ae2fc6007fd39ff01da7558565a4

Modified Files
--------------
src/backend/access/transam/xlogreader.c | 91 +++++++++++++++++++++++----------
1 file changed, 64 insertions(+), 27 deletions(-)