pgsql: Fetch XIDs atomically during vac_truncate_clog().

Fetch XIDs atomically during vac_truncate_clog().

Because vac_update_datfrozenxid() updates datfrozenxid and datminmxid
in-place, it's unsafe to assume that successive reads of those values will
give consistent results.  Fetch each one just once to ensure sane behavior
in the minimum calculation.  Noted while reviewing Alexander Korotkov's
patch in the same area.

Discussion: <8564.1464116473@sss.pgh.pa.us>

Branch
------
REL9_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/5551dac59c0601b49d57a3f897fb0a6d581e58ac

Modified Files
--------------
src/backend/commands/vacuum.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)