Re: postgres_fdw - make cached connection functions tests meaningful

On Thu, Jul 15, 2021 at 3:48 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com> writes:
> > While working on [1], I got to know that there is a new GUC
> > debug_invalidate_system_caches_always that has been introduced in v14.
> > It can be used to switch off cache invalidation in
> > CLOBBER_CACHE_ALWAYS builds which makes cache sensitive tests stable.
> > Using this GUC, it is quite possible to make cached connection
> > management function tests more meaningful by returning original
> > values(true/false, all the output columns) instead of SELECT 1.
>
> Note that this needs an update in the wake of d68a00391.
>
> More generally, though, I am not sure that I believe the premise of
> this patch. AFAICS it's assuming that forcing debug_discard_caches
> off guarantees zero cache flushes, which it does not. (If it could,
> we wouldn't need the whole thing; the point of that variable is to
> deterministically force flushes which would otherwise be
> nondeterministic, not nonexistent.)

Can the setting debug_discard_caches = 0 still make extra
flushes/discards (not the regular cache flushes/discards that happen
because of alters or changes in the cached elements)? My understanding
was that debug_discard_caches = 0, disables all the extra flushes with
clobber cache builds. If my understanding wasn't right, isn't it good
to mention it somewhere in the documentation or in the source code?

> Even in a contrib test that
> seemingly has nothing else running, background activity such as
> autovacuum could result in surprises. So I fear that what you have
> got here is a patch that will work 99% of the time; which is not
> good enough for the buildfarm.

If the setting debug_discard_caches = 0 makes at least a few extra
cache flushes, I don't mind withdrawing this patch.

Regards,
Bharath Rupireddy.