Re: HOT chain validation in verify_heapam()

On Wed, Mar 8, 2023 at 5:35 AM Aleksander Alekseev
<aleksander@timescale.com> wrote:
> I did, both with Meson and Autotools. All in all the patch looks very
> good, but I have a few little nitpicks.

Thank you for the nitpicks.

> +                /* HOT chains should not intersect. */
> +                if (predecessor[nextoffnum] != InvalidOffsetNumber)
> +                {
> +                    report_corruption(&ctx,
> +                                      psprintf("redirect line pointer
> points to offset %u, but offset %u also points there",
> +                                               (unsigned) nextoffnum,
> (unsigned) predecessor[nextoffnum]));
> +                    continue;
> +                }
> ```
>
> This type of corruption doesn't seem to be test-covered.

Himanshu, would you be able to try to write a test case for this? I
think you need something like this: update a tuple with a lower TID to
produce a tuple with a higher TID, e.g. (0,10) is updated to produce
(0,11). But then have a redirect line pointer that also points to the
result of the update, in this case (0,11).

> ```
> +            /*
> +             * If the next line pointer is a redirect, or if it's a tuple
> +             * but the XMAX of this tuple doesn't match the XMIN of the next
> +             * tuple, then the two aren't part of the same update chain and
> +             * there is nothing more to do.
> +             */
> +            if (ItemIdIsRedirected(next_lp))
> +                continue;
> ```
>
> lcov shows that the `continue` path is never executed. This is
> probably not a big deal however.

It might be good to have a negative test case for this, though. Let's
say we, e.g. update (0,1) to produce (0,2), but then abort. The page
is HOT-pruned. Then we add insert a new tuple at (0,2), HOT-update it
to produce (0,3), and commit. Then we HOT-prune again. Possibly we
could try to write a test case that verifies that this does NOT
produce any corruption indication.

> ```
> +$node->append_conf('postgresql.conf','max_prepared_transactions=100');
> ```
>
> From what I can tell this line is not needed.

That surprises me, because the new test cases involve preparing a
transaction, and by default max_prepared_transactions=0. So it seems
to me (without testing) that this ought to be required. Did you test
that it works without this setting?

The value of 100 seems a bit excessive, though. Most TAP tests seem to use 10.

-- 
Robert Haas
EDB: http://www.enterprisedb.com