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
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/4cf0978ea2733548149ee7f11e588975daaf7609

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