Re: [PATCH] Disable bgworkers during servers start in pg_upgrade

On Fri, Aug 27, 2021 at 10:02 AM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>
> On Fri, Aug 27, 2021 at 09:34:24AM +0800, Julien Rouhaud wrote:
> > That shouldn't lead to any problem right?
>
> Well, bgworker_should_start_now() does not exist for that, and
> RegisterBackgroundWorker() is the one doing the classification job, so
> it would be more consistent to keep everything under control in the
> same code path.

I'm not sure it's that uncontroversial. The way I see
RegisterBackgroundWorker() is that it's responsible for doing some
sanity checks to see if the module didn't make any error and if
ressources are available. Surely checking for IsBinaryUpgrade should
not be the responsibility of third-party code, so the question is
whether binary upgrade is seen as a resource and as such a reason to
forbid bgworker registration, in opposition to forbid the launch
itself.

Right now AFAICT there's no official API to check if a call to
RegisterBackgroundWorker() succeeded or not, but an extension could
check by itself using BackgroundWorkerList in bgworker_internals.h,
and error out or something if it didn't succeed, as a way to inform
users that they didn't configure the instance properly (like maybe
increasing max_worker_processes). Surely using a *_internals.h header
is a clear sign that you expose yourself to problems, but adding an
official way to check for bgworker registration doesn't seem
unreasonable to me. Is that worth the risk to have pg_upgrade
erroring out in this kind of scenario, or make the addition of a new
API to check for registration status more difficult?