On Mon, 13 May 2024 at 15:55, Pavel Borisov <pashkin.e...@gmail.com> wrote:
> Hi, Alexander! > > On Mon, 13 May 2024 at 05:42, Alexander Korotkov <aekorot...@gmail.com> > wrote: > >> On Mon, May 13, 2024 at 12:23 AM Alexander Korotkov >> <aekorot...@gmail.com> wrote: >> > On Sat, May 11, 2024 at 4:13 AM Mark Dilger >> > <mark.dil...@enterprisedb.com> wrote: >> > > > 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 ... ? >> > >> > 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) > A correction of a typo in previous message: non-leaf pages iteration cycles (under !P_ISLEAF(topaque)) -> non-leaf pages iteration cycles (under !P_ISLEAF(topaque))
