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

On 23 January 2017 at 12:34, Craig Ringer <craig(at)2ndquadrant(dot)com> wrote:
> On 20 January 2017 at 21:40, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:
>
>> One option would be to add another limit Xid which advances before the
>> truncation but which is not used for other decisions other than limiting
>> what can users consult.
>
> This could be useful for other things, but it's probably heavier than needed.
>
> What I've done in the latest revision of the txid_status() patch is
> simply to advance OldestXid _before_ truncating the clog. The rest of
> the xid info is advanced after. Currently this is incorporated into
> the txid_status patch, but can be separated if desired.
>
> Relevant commit message portion:
>
> There was previously no way to look up an arbitrary xid without
> running the risk of having clog truncated out from under you. This
> hasn't been a problem because anything looking up xids in clog knows
> they're protected by datminxid, but that's not the case for arbitrary
> user-supplied XIDs. clog was truncated before we advance oldestXid so
> taking XidGenLock was insufficient. There's no way to look up a
> SLRU with soft-failure. To address this, increase oldestXid under XidGenLock
> before we trunate clog rather than after, so concurrent access is safe.
>
> Note that while oldestXid is advanced before clog truncation, the xid
> limits are advanced _after_ it. If we advanced the xid limits before
> truncation too, we'd theoretically run the risk of allocating an xid
> from the clog section we're about to truncate, which would be no fun.
> (In practice it can't really happen since we only use 1/2 the
> available space at a time).
>
> Moving the lower bound up, truncating, and moving the upper bound up
> is the way to go IMO.

Patch with explanation in commit msg:
https://www.postgresql.org/message-id/CAMsr+YHODEdUUA5vw1_rjPS_ASSvEMeJN_34rqd3pzHf5OFdJg@mail.gmail.com

--
Craig Ringer http://www.2ndQuadrant.com/
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