Re: [PATCH] Fix minor race in commit_ts SLRU truncation vs lookups

On Thu, Jan 19, 2017 at 10:06 AM, Alvaro Herrera
<alvherre(at)2ndquadrant(dot)com> wrote:
> Peter Eisentraut wrote:
>
>> Also, I wonder whether we should not in vacuum.c change the order of the
>> calls of SetTransactionIdLimit() and SetMultiXactIdLimit() as well, just
>> to keep everything consistent.
>
> I am wary of doing that. The current coding is well battle-tested by
> now, but doing things in the opposite order, not at all. Pending some
> testing to show that there is no problem with a change, I would leave
> things as they are. Probably said testing is too onerous for the
> benefit (which is just a little consistency). What I fear is: what
> happens if a concurrent checkpoint reads the values between the two
> operations, and a crash occurs? I think that the checkpoint might save
> the updated values, so after crash recovery the truncate would not be
> executed, possibly leaving files around. Leaving files around might be
> dangerous for multixacts at least (it's probably harmless for xids).

Don't both SLRUs eventually wrap? If so, leaving file around seems
dangerous for either in equal measure.

--
Robert Haas
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