> On May 10, 2024, at 12:05 PM, Alexander Korotkov <aekorot...@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 ... ?
—
Mark Dilger
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
