pgsql: Fix race condition in multixact code: it's possible to try to

Does this need to be backpatched?

---------------------------------------------------------------------------

Tom Lane wrote:

Log Message:
-----------
Fix race condition in multixact code: it's possible to try to read a
multixact's starting offset before the offset has been stored into the
SLRU file. A simple fix would be to hold the MultiXactGenLock until the
offset has been stored, but that looks like a big concurrency hit. Instead
rely on knowledge that unset offsets will be zero, and loop when we see
a zero. This requires a little extra hacking to ensure that zero is never
a valid value for the offset. Problem reported by Matteo Beccati, fix
ideas from Martijn van Oosterhout, Alvaro Herrera, and Tom Lane.

Modified Files:
--------------
pgsql/src/backend/access/transam:
multixact.c (r1.9 -> r1.10)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/multixact.c.diff?r1=1.9&r2=1.10)

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Bruce Momjian <pgman@candle.pha.pa.us> writes:

Does this need to be backpatched?

No.

regards, tom lane