Re: Patch proposal: New hooks in the connection path - Mailing list pgsql-hackers

On Tue, Aug 16, 2022 at 3:16 AM Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:
>
> On Tue, Aug 16, 2022 at 1:55 PM Drouvot, Bertrand <bdrouvot@amazon.com> wrote:
> >
> > Hi,
> >
> > On 8/16/22 10:10 AM, Bharath Rupireddy wrote:
> > > On Tue, Aug 16, 2022 at 1:31 PM Drouvot, Bertrand <bdrouvot@amazon.com> wrote:
> > >> On 8/14/22 7:52 AM, Gurjeet Singh wrote:
> > >>> On Mon, Aug 8, 2022 at 3:51 AM Drouvot, Bertrand <bdrouvot@amazon.com> wrote:
> > >>> I think we can reduce the number of places the hook is called, if we
> > >>> call the hook from proc_exit(), and at all the other places we simply set
> > >>> a global variable to signify the reason for the failure. The case of
> > >>> _exit(1) from the signal-handler cannot use such a mechanism, but I
> > >>> think all the other cases of interest can simply register one of the
> > >>> FCET_* values, and let the call from proc_exit() pass that value
> > >>> to the hook.
> > >> That looks like a good idea to me. I'm tempted to rewrite the patch that
> > >> way (and addressing the first comment in the same time).
> > >>
> > >> Curious to hear about others hackers thoughts too.

I agree that we need feedback from long-timers here,  on the decision
of whether to use proc_exit() for this purpose.

> > > IMO, calling the hook from proc_exit() is not a good design as
> > > proc_exit() is a generic code called from many places in the source
> > > code, even the simple code of kind  if(call_failed_conn_hook) {
> > > falied_conn_hook(params);} can come in the way of many exit code paths
> > > which is undesirable, and the likelihood of introducing new bugs may
> > > increase.
> >
> > Thanks for the feedback.
> >
> > What do you think about calling the hook only if the new global variable
> > is not equal to its default value (which would mean don't trigger the
> > hook)?
>
> IMO, that's not a good design as explained above. Why should the
> failed connection hook related code get hit for each and every
> proc_exit() call? Here, the code duplication i.e. the number of places
> the failed connection hook gets called mustn't be the reason to move
> that code to proc_exit().

I agree, it doesn't feel _clean_, having to maintain a global
variable, pass it to hook at exit, etc. But the alternative feels less
cleaner.

This hook needs to be called when the process has decided to exit, so
it makes sense to place this call in stack above proc_exit(), whose
sole job is to let the process die gracefully, and take care of things
on the way out.

There are quite a few places in core that leverage proc_exit()'s
facilities (by registering on_proc_exit callbacks), so an
extension/hook doing so wouldn't be out of the ordinary; (apparently
contrib/sepgsql has already set the precedent on an extension using
the on_proc_exit callback). Admittedly, in this case the core will be
managing and passing it the additional global variable needed to
record the connection failure reason, FCET_*.

If we agree that proc_exit() is a good place to place this call, then
this hook can be converted into a on_proc_exit callback. If the global
variable is exported, then the extension(s) can access it in the
callback to ascertain why the process is exiting, and proc_exit()
won't have to know anything special about the extension, or hook, or
the global variable.

The on_proc_exit callback method wouldn't work for the _exit() called
in StartupPacketTimeoutHandler(), so that will need to be handled
separately.

Best regards,
Gurjeet
http://Gurje.et