pgsql: Use a fd opened for read/write when syncing slots during startup

Use a fd opened for read/write when syncing slots during startup.

Some operating systems, including the reporter's windows, return EBADFD
or similar when fsync() is invoked on a O_RDONLY file descriptor.
Unfortunately RestoreSlotFromDisk() does exactly that; which causes
failures after restarts in at least some scenarios.

If you hit the bug the error message will be something like
ERROR: could not fsync file "pg_replslot/$name/state": Bad file descriptor

Simply use O_RDWR instead of O_RDONLY when opening the relevant file
descriptor to fix the bug. Unfortunately I have no way of verifying the
fix, but we've seen similar problems in the past.

This bug goes back to 9.4 where slots were introduced. Backpatch
accordingly.

Reported-By: Patrice Drolet
Bug: #13143:
Discussion: 20150424101006(dot)2556(dot)60897(at)wrigleys(dot)postgresql(dot)org

Branch
------
REL9_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/fd3dfc236c155048caf21c35e23e8dadc89895ec

Modified Files
--------------
src/backend/replication/slot.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)