Re: pgsql: pgstat: Bring up pgstat in BaseInit() to fix uninitialized use o

On Thu, Aug 12, 2021 at 6:18 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> Hi,
>
> On 2021-08-12 15:06:23 +0530, Amit Kapila wrote:
> > On Thu, Aug 12, 2021 at 1:52 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > > I'm not so sure. Why does sharedfileset have its own proc exit hook in the
> > > first place? ISTM that this should be dealt with using resowners, rathers than
> > > a sharedfileset specific mechanism?
> > >
>
> > The underlying temporary files need to be closed at xact end but need
> > to survive across transactions.
>
> Why do they need to be closed at xact end? To avoid wasting memory due to too
> many buffered files?
>

Yes.

>
> > These are registered with the resource owner via
> > PathNameOpenTemporaryFile/PathNameCreateTemporaryFile and then closed
> > at xact end. So, we need a way to remove the files used by the process
> > (apply worker in this particular case) before process exit and used
> > this proc_exit hook (possibly on the lines of AtProcExit_Files).
>
> What I'm wondering is why it is a good idea to have a SharedFileSet specific
> cleanup mechanism. One that only operates on process lifetime level, rather
> than something more granular. I get that the of the files here needs to be
> longer than a transaction, but that can easily be addressed by having a longer
> lived resource owner.
>
> Process lifetime may work well for the current worker.c, but even there it
> doesn't seem optimal. One e.g. could easily imagine that we'd want to handle
> connection errors or configuration changes without restarting the worker, in
> which case process lifetime obviously isn't a good idea anymore.
>

I don't deny that we can't make this at a more granular level. IIRC,
at that time, we tried to follow AtProcExit_Files which cleans up temp
files at proc exit and we needed something similar for temporary files
used via SharedFileSet. I think we can extend this API but I guess it
is better to then do it for dsm-based as well so that these get
tracked via resowner.

>
> I think SharedFileSetInit() needs a comment explaining that it needs to be
> called in a process-lifetime memory context if used without dsm
> segments.
>

We already have a comment about proc_exit clean up of files but will
extend that a bit about memory context.

--
With Regards,
Amit Kapila.