Re: [PATCH] Improve amcheck to also check UNIQUE constraint in btree index.

Hi, Alexander!

On Mon, 13 May 2024 at 05:42, Alexander Korotkov <aekorotkov@gmail.com>
wrote:

> On Mon, May 13, 2024 at 12:23 AM Alexander Korotkov
> <aekorotkov@gmail.com> wrote:
> > On Sat, May 11, 2024 at 4:13 AM Mark Dilger
> > <mark.dilger@enterprisedb.com> wrote:
> > > > On May 10, 2024, at 12:05 PM, Alexander Korotkov <
> aekorotkov@gmail.com> wrote:
> > > > The only bt_target_page_check() caller is
> > > > bt_check_level_from_leftmost(), which overrides state->target in the
> > > > next iteration anyway.  I think the patch is just refactoring to
> > > > eliminate the confusion pointer by Peter Geoghegan upthread.
> > >
> > > I find your argument unconvincing.
> > >
> > > After bt_target_page_check() returns at line 919, and before
> bt_check_level_from_leftmost() overrides state->target in the next
> iteration, bt_check_level_from_leftmost() conditionally fetches an item
> from the page referenced by state->target.  See line 963.
> > >
> > > I'm left with four possibilities:
> > >
> > >
> > > 1)  bt_target_page_check() never gets to the code that uses
> "rightpage" rather than "state->target" in the same iteration where
> bt_check_level_from_leftmost() conditionally fetches an item from
> state->target, so the change you're making doesn't matter.
> > >
> > > 2)  The code prior to v2-0003 was wrong, having changed state->target
> in an inappropriate way, causing the wrong thing to happen at what is now
> line 963.  The patch fixes the bug, because state->target no longer gets
> overwritten where you are now using "rightpage" for the value.
> > >
> > > 3)  The code used to work, having set up state->target correctly in
> the place where you are now using "rightpage", but v2-0003 has broken that.
> > >
> > > 4)  It's been broken all along and your patch just changes from wrong
> to wrong.
> > >
> > >
> > > If you believe (1) is true, then I'm complaining that you are relying
> far to much on action at a distance, and that you are not documenting it.
> Even with documentation of this interrelationship, I'd be unhappy with how
> brittle the code is.  I cannot easily discern that the two don't ever
> happen in the same iteration, and I'm not at all convinced one way or the
> other.  I tried to set up some Asserts about that, but none of the test
> cases actually reach the new code, so adding Asserts doesn't help to
> investigate the question.
> > >
> > > If (2) is true, then I'm complaining that the commit message doesn't
> mention the fact that this is a bug fix.  Bug fixes should be clearly
> documented as such, otherwise future work might assume the commit can be
> reverted with only stylistic consequences.
> > >
> > > If (3) is true, then I'm complaining that the patch is flat busted.
> > >
> > > If (4) is true, then maybe we should revert the entire feature, or
> have a discussion of mitigation efforts that are needed.
> > >
> > > Regardless of which of 1..4 you pick, I think it could all do with
> more regression test coverage.
> > >
> > >
> > > For reference, I said something similar earlier today in another email
> to this thread:
> > >
> > > This patch introduces a change that stores a new page into variable
> "rightpage" rather than overwriting "state->target", which the old
> implementation most certainly did.  That means that after returning from
> bt_target_page_check() into the calling function
> bt_check_level_from_leftmost() the value in state->target is not what it
> would have been prior to this patch.  Now, that'd be irrelevant if nobody
> goes on to consult that value, but just 44 lines further down in
> bt_check_level_from_leftmost() state->target is clearly used.  So the
> behavior at that point is changing between the old and new versions of the
> code, and I think I'm within reason to ask if it was wrong before the
> patch, wrong after the patch, or something else?  Is this a bug being
> introduced, being fixed, or ... ?
> >
> > Thank you for your analysis.  I'm inclined to believe in 2, but not
> > yet completely sure.  It's really pity that our tests don't cover
> > this.  I'm investigating this area.
>
> It seems that I got to the bottom of this.  Changing
> BtreeCheckState.target for a cross-page unique constraint check is
> wrong, but that happens only for leaf pages.  After that
> BtreeCheckState.target is only used for setting the low key.  The low
> key is only used for non-leaf pages.  So, that didn't lead to any
> visible bug.  I've revised the commit message to reflect this.
>

I agree with your analysis regarding state->target:
- when the unique check is on, state->target was reassigned only for the
leaf pages (under P_ISLEAF(topaque) in bt_target_page_check).
- in this level (leaf) in bt_check_level_from_leftmost() this value of
state->target was used to get state->lowkey. Then it was reset (in the next
iteration of do loop in in bt_check_level_from_leftmost()
- state->lowkey lives until the end of pages level (leaf) iteration cycle.
Then, low-key is reset (state->lowkey = NULL in the end of
 bt_check_level_from_leftmost())
- state->lowkey is used only in bt_child_check/bt_child_highkey_check. Both
are called only from non-leaf pages iteration cycles (under
P_ISLEAF(topaque))
- Also there is a check (rightblock_number != P_NONE) in before getting
rightpage into state->target in bt_target_page_check() that ensures us that
rightpage indeed exists and getting this (unused) lowkey in
bt_check_level_from_leftmost will not invoke any page reading errors.

I'm pretty sure that there was no bug in this, not just the bug was hidden.

Indeed re-assigning state->target in leaf page iteration for cross-page
unique check was not beautiful, and Peter pointed out this. In my opinion
the patch 0003 is a pure code refactoring.

As for the cross-page check regression/TAP testing, this test had problems
since the btree page layout is not fixed (especially it's different on
32-bit arch). I had a variant for testing cross-page check when the test
was yet regression one upthread for both 32/64 bit architectures. I
remember it was decided not to include it due to complications and low
impact for testing the corner case of very rare cross-page duplicates.
(There were also suggestions to drop cross-page duplicates check at all,
which I didn't agree 2 years ago, but still it can make sense)

Separately, I propose to avoid getting state->lowkey for leaf pages at all
as it's unused. PFA is a simple patch for this. (I don't add it to the
current patch set as I believe it has nothing to do with UNIQUE constraint
check, rather it improves the previous btree amcheck code)

Best regards,
Pavel Borisov,
Supabase