pgsql: Before removing backup_label and irrevocably changing pg_control

Before removing backup_label and irrevocably changing pg_control file, check
that WAL file containing the checkpoint redo-location can be found. This
avoids making the cluster irrecoverable if the redo location is in an earlie
WAL file than the checkpoint record.

Report, analysis and patch by Jeff Davis, with small changes by me.

Branch
------
REL8_4_STABLE

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=b7888758d85f77388ca0ffd26ee37655e90d695c

Modified Files
--------------
src/backend/access/transam/xlog.c |   19 +++++++++++++++++--
1 files changed, 17 insertions(+), 2 deletions(-)