Re: pgsql: Avoid race in RelationBuildDesc() affecting CREATE INDEX CONCURR

On Tue, Feb 08, 2022 at 04:43:47PM -0800, Andres Freund wrote:
> On 2022-02-08 22:13:01 +0100, Tomas Vondra wrote:
> > On 10/24/21 03:40, Noah Misch wrote:
> > > Avoid race in RelationBuildDesc() affecting CREATE INDEX CONCURRENTLY.
> > >
> > > CIC and REINDEX CONCURRENTLY assume backends see their catalog changes
> > > no later than each backend's next transaction start. That failed to
> > > hold when a backend absorbed a relevant invalidation in the middle of
> > > running RelationBuildDesc() on the CIC index. Queries that use the
> > > resulting index can silently fail to find rows. Fix this for future
> > > index builds by making RelationBuildDesc() loop until it finishes
> > > without accepting a relevant invalidation. It may be necessary to
> > > reindex to recover from past occurrences; REINDEX CONCURRENTLY suffices.
> > > Back-patch to 9.6 (all supported versions).
> > >
> > > Noah Misch and Andrey Borodin, reviewed (in earlier versions) by Andres
> > > Freund.
> > >
> > > Discussion: https://postgr.es/m/20210730022548.GA1940096@gust.leadboat.com
> > >
> >
> > Unfortunately, this seems to have broken CLOBBER_CACHE_ALWAYS builds. Since
> > this commit, initdb never completes due to infinite retrying over and over
> > (on the first RelationBuildDesc call).

Thanks for the report. I had added the debug_discard arguments of
InvalidateSystemCachesExtended() and RelationCacheInvalidate() to make the new
code survive a CREATE TABLE at debug_discard_caches=5. Apparently that's not
enough for initdb. I'll queue a task to look at it.

It's a good reminder to set wait_timeout on buildfarm animals. (I should take
that advice, too.)

> Ugh. Do we need to do something about WRT the next set of minor releases?

No, given that this code already debuted in the November releases.