Re: [HACKERS] snapbuild woes

On 1 May 2017 at 09:54, Petr Jelinek <petr.jelinek@2ndquadrant.com> wrote:

> But, I still think we need to restart the tracking after new
> xl_running_xacts. Reason for that is afaics any of the catalog snapshots
> that we assigned to transactions at the end of SnapBuildCommitTxn might
> be corrupted otherwise as they were built before we knew one of the
> supposedly running txes was actually already committed and that
> transaction might have done catalog changes.

Due to the race where LogStandbySnapshot() collects running-xacts info
while a concurrent xact commits, such that the xl_xact_commit appears
before the xl_running_xacts, but the xl_running_xacts still has the
commited xact listed as running, right? Because we update PGXACT only
after we write the commit to WAL, so there's a window where an xact is
committed in WAL but not shown as committed in shmem.

-- Craig Ringer                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services