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

> > 0002 fixes the symmetry of error handling which is a change the callbacks
> > that we could still do. This is also cleaner because when an extension signals
> > failure, pgstat_write_statsfile() discards the temporary file immediately rather
> > than writing bad data that would just be discarded on the next startup anyway.
> >
> > Note that I intentionally kept the core write path using deferred error
> > checking with ferror(). Core only writes to a single FILE handle, so keeping
> > it as one check at the end is sufficient and avoid adding if (!...) goto error
> > in about 12 different places.
>
> Hmm. I also see a point in not having to_serialized_data() return an
> error and keep processing all the entries: if we have multiple
> auxiliary files across multiple stats kinds, we would be able to let
> each stats kind notice that something is wrong and let their FILE
> handles be cleaned up in the finish() callback with a ferror() check.
> These errors run under the assumption that one error on a file handle
> would lost likely lead to other similar errors for the rest of the
> files written to. This means that we could miss opportunities to
> clean up auxiliary files with 0002.

When an error is returned from a callback, the error block in 0002 only
discards the core stats temp file and then moves on to call the finish
callback for every extension that registered one ( and they better register
one to perform cleanups ). So, the proposed will not cause any missed
cleanups of auxiliary files.

--
Sami