Re: [PATCH] Improve amcheck to also check UNIQUE constraint in btree index. - Mailing list pgsql-hackers

> 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
thenext 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
iterationwhere bt_check_level_from_leftmost() conditionally fetches an item from state->target, so the change you're
makingdoesn't matter. 

2)  The code prior to v2-0003 was wrong, having changed state->target in an inappropriate way, causing the wrong thing
tohappen at what is now line 963.  The patch fixes the bug, because state->target no longer gets overwritten where you
arenow 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
v2-0003has 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
arenot 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
theother.  I tried to set up some Asserts about that, but none of the test cases actually reach the new code, so adding
Assertsdoesn'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
fixesshould be clearly documented as such, otherwise future work might assume the commit can be reverted with only
stylisticconsequences. 

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
itwould have been prior to this patch.  Now, that'd be irrelevant if nobody goes on to consult that value, but just 44
linesfurther down in bt_check_level_from_leftmost() state->target is clearly used.  So the behavior at that point is
changingbetween 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 ... ? 

—
Mark Dilger
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The Enterprise PostgreSQL Company