Re: new heapcheck contrib module

On Sat, Aug 29, 2020 at 10:48 AM Mark Dilger
<mark(dot)dilger(at)enterprisedb(dot)com> wrote:
> I had an earlier version of the verify_heapam patch that included a non-throwing interface to clog. Ultimately, I ripped that out. My reasoning was that a simpler patch submission was more likely to be acceptable to the community.

Isn't some kind of pragmatic compromise possible?

> But I don't want to make this patch dependent on that hypothetical patch getting written and accepted.

Fair enough, but if you're alluding to what I said then about
check_tuphdr_xids()/clog checking a while back then FWIW I didn't
intend to block progress on clog/xact status verification at all. I
just don't think that it is sensible to impose an iron clad guarantee
about having no assertion failures with corrupt clog data -- that
leads to far too much code duplication. But why should you need to
provide an absolute guarantee of that?

I for one would be fine with making the clog checks an optional extra,
that rescinds the no crash guarantee that you're keen on -- just like
with the TOAST checks that you have already in v15. It might make
sense to review how often crashes occur with simulated corruption, and
then to minimize the number of occurrences in the real world. Maybe we
could tolerate a usually-no-crash interface to clog -- if it could
still have assertion failures. Making a strong guarantee about
assertions seems unnecessary.

I don't see how verify_heapam will avoid raising an error during basic
validation from PageIsVerified(), which will violate the guarantee
about not throwing errors. I don't see that as a problem myself, but
presumably you will.

--
Peter Geoghegan