Re: [Proposal] Adding callback support for custom statistics kinds

> > Also, I am now leaning towards creating a separate test module rather than
> > trying to do too much unrelated testing in injection points. It is definitely
> > convenient to use injection points, but I think we can do better testing with
> > a separate module. This module can also serve as an example for extension
> > developers.
>
> You are right that it may be cleaner this way. Do you think that it
> could make sense to move some of the existing "template" code of
> injection_points there?

By "template" code, do you mean Something like?

include/utils/custom_statkinds.h
backend/utils/misc/custom_statkinds.c

Where the template code here is PgStat_kind definition, callbacks, etc. for
injection_points or the new test module that is using a custom kind.

A few benefits I see for this is we can point extension developers to
this as an example in [0] and we are also maintaining the kind ids in
a single place. These may not be strong points, but may be worth while.

v2 attached is something that may be closer to what we've been discussing

v2-0001 are much simplified changes to pgstat.c that simply invoke the callbacks
and all the work is on the extension to implement what it needs to do.
This includes
a callback at the end of WRITE, READ, DISCARD with a flag passed to the caller
so they can perform the necessary clean-up actions.

v2-0002 implements a new test module that tests mainly that the recovery,
clean and crash, are working as expected.

I created a new tap test for this which performs a test similar to what is
done in recovery/029_stats_restart.pl. I could merge the new test there, but
I am reluctant to add a dependency on a new module to recovery. What
do you think?

[0] https://www.postgresql.org/docs/current/xfunc-c.html#XFUNC-ADDIN-CUSTOM-CUMULATIVE-STATISTICS

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Sami Imseih
Amazon Web Services (AWS)