From: Derrick Stolee <derrickstolee@github.com>
To: Patrick Steinhardt <ps@pks.im>
Cc: Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>,
git@vger.kernel.org, me@ttaylorr.com, gitster@pobox.com,
abhishekkumar8222@gmail.com
Subject: Re: [PATCH 3/7] commit-graph: start parsing generation v2 (again)
Date: Tue, 1 Mar 2022 09:06:44 -0500 [thread overview]
Message-ID: <f50e74f0-9ffa-f4f2-4663-269801495ed3@github.com> (raw)
In-Reply-To: <Yh325v3RBDMxjFnD@ncase>
On 3/1/2022 5:35 AM, Patrick Steinhardt wrote:
> On Tue, Mar 01, 2022 at 10:46:14AM +0100, Patrick Steinhardt wrote:
>> On Mon, Feb 28, 2022 at 01:44:01PM -0500, Derrick Stolee wrote:
>>> On 2/28/2022 11:59 AM, Patrick Steinhardt wrote:
>>>> On Mon, Feb 28, 2022 at 11:23:38AM -0500, Derrick Stolee wrote:
>>>>> On 2/28/2022 10:18 AM, Patrick Steinhardt wrote:
>>>>>> I haven't yet found the time to dig deeper into why this is happening.
>>>>>> While the repository is publicly accessible at [1], unfortunately the
>>>>>> bug seems to be triggered by a commit that's only kept alive by an
>>>>>> internal reference.
>>>>>>
>>>>>> Patrick
>>>>>>
>>>>>> [1]: https://gitlab.com/gitlab-com/www-gitlab-com.git
>>>>>
>>>>> Thanks for including this information. Just to be clear: did you
>>>>> include patch 4 in your tests, or not? Patch 4 includes a fix
>>>>> related to overflow values, so it would be helpful to know if you
>>>>> found a _different_ bug or if it is the same one.
>>>>>
>>>>> Thanks,
>>>>> -Stolee
>>>>
>>>> I initially only applied the first three patches, but after having hit
>>>> the fatal error I also applied the rest of this series to have a look at
>>>> whether it is indeed fixed already by one of your later patches. The
>>>> error remains the same though.
>>>
>>> Thanks for this extra context. Is this a commit-graph that you wrote
>>> with the first three patches and then you get an error when reading it?
>>>
>>> Do you get the same error when deleting that file and rewriting it with
>>> all patches included?
>>>
>>> Thanks,
>>> -Stolee
>>
>> Yes, I do. I've applied all four patches from v2 on top of 715d08a9e5
>> (The eighth batch, 2022-02-25) and still get the same results:
>>
>> $ find objects/info/commit-graphs/
>> objects/info/commit-graphs/
>> objects/info/commit-graphs/graph-607e641165f3e83a82d5b14af4e611bf2a688f35.graph
>> objects/info/commit-graphs/commit-graph-chain
>> objects/info/commit-graphs/graph-5f357c7573c0075d42d82b28e660bc3eac01bfe8.graph
>> objects/info/commit-graphs/graph-e0c12ead1b61c7c30720ae372e8a9f98d95dfb2d.graph
>> objects/info/commit-graphs/graph-c96723b133c2d81106a01ecd7a8773bb2ef6c2e1.graph
>>
>> $ git commit-graph verify
>> fatal: commit-graph requires overflow generation data but has none
>>
>> $ git commit-graph write
>> Finding commits for commit graph among packed objects: 100% (10235119/10235119), done.
>> Expanding reachable commits in commit graph: 2197197, done.
>> Finding extra edges in commit graph: 100% (2197197/2197197), done.
>> fatal: commit-graph requires overflow generation data but has none
>>
>> $ rm -rf objects/info/commit-graphs/
>>
>> $ git commit-graph write
>> Finding commits for commit graph among packed objects: 100% (10235119/10235119), done.
>> Expanding reachable commits in commit graph: 2197197, done.
>> Finding extra edges in commit graph: 100% (2197197/2197197), done.
>> fatal: commit-graph requires overflow generation data but has none)
>>
>> So even generating them completely anew doesn't seem to generate the
>> overflow generation data.
>>
>> Patrick
>
> I stand corrected. I forgot that the repository at hand was connected to
> another one via `objects/info/alternates`. If I prune commit-graphs from
> that alternate, too, then it works alright with your patches.
OK, thanks. That clarifies the situation.
I ordered the patches such that the fix in patch 4 could be immediately
testable, which is not the case without patch 3. However, it does leave
this temporary state where information can be incorrect if only a subset
of the series is applied.
> This makes me wonder how such a bugfix should be handled though. As this
> series is right now, users will be faced with repository corruption as
> soon as they upgrade their Git version to one that contains this patch
> series. This corruption needs manual action: they have to go into the
> repository, delete the commit-graphs and then optionally create new
> ones.
>
> This is not a good user experience, and it's worse on the server-side
> where we now have a timeframe where all commit-graphs are potentially
> corrupt. This effectively leads to us being unable to serve those repos
> at all until we have rewritten the commit-graphs because all commands
> which make use of the commit-graph will now die:
>
> $ git log
> fatal: commit-graph requires overflow generation data but has none
>
> So the question is whether this is a change that needs to be rolled out
> over multiple releases. First we'd get in the bug fix such that we write
> correct commit-graphs, and after this fix has been released we can also
> release the fix that starts to actually parse the generation. This
> ensures there's a grace period during which we can hopefully correct the
> data on-disk such that users are not faced with failures.
You are right that we need to be careful here, but I also think that
previous versions of Git always wrote the correct data. Here is my
thought process:
1. To get this bug, we need to have parsed the corrected commit date
from an existing commit-graph in order to under-count the number
of overflow values.
2. Before this series, Git versions were not parsing the corrected
commit date, so they recompute the corrected commit date every
time the commit-graph is written, getting the proper count of
overflow values.
For these reasons, data written by previous versions of Git are
correct and can be trusted without a staged release.
Does this make sense? Or, do you experience a different result when
you build commit-graphs with a released Git version and then when
writing on top with all patches applied?
> The better alternative would probably be to just gracefully handle
> commit-graphs which are corrupted in such a way. Can we maybe just
> continue to not parse generations in case we find that the commit-graph
> doesn't have overflow generation data?
>
> This is more of a general issue though: commit-graphs are an auxiliary
> cache that is not required for proper operation at all. If we fail to
> parse it, then Git shouldn't die but instead fail gracefully just ignore
> it. Furthermore, if we notice that graphs are corrupt when we try to
> write new ones, we may just delete the corrupt versions automatically
> and generate completely new ones.
You are right that we can be better about failures here and report
and error instead of a die(). Especially in this case, we could just
revert to topological levels instead of throwing out the commit-graph
entirely.
This seems like something for another series, so we can be sure to
audit all cases of fatal errors when parsing the commit-graph so we
catch all of them and do the "best" thing in each case.
Thanks,
-Stolee
next prev parent reply other threads:[~2022-03-01 14:06 UTC|newest]
Thread overview: 70+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-24 20:38 [PATCH 0/7] Commit-graph: Generation Number v2 Fixes, v3 implementation Derrick Stolee via GitGitGadget
2022-02-24 20:38 ` [PATCH 1/7] test-read-graph: include extra post-parse info Derrick Stolee via GitGitGadget
2022-02-24 20:38 ` [PATCH 2/7] commit-graph: fix ordering bug in generation numbers Derrick Stolee via GitGitGadget
2022-02-24 22:15 ` Junio C Hamano
2022-02-25 13:51 ` Derrick Stolee
2022-02-25 17:35 ` Junio C Hamano
2022-02-24 20:38 ` [PATCH 3/7] commit-graph: start parsing generation v2 (again) Derrick Stolee via GitGitGadget
2022-02-28 15:18 ` Patrick Steinhardt
2022-02-28 16:23 ` Derrick Stolee
2022-02-28 16:59 ` Patrick Steinhardt
2022-02-28 18:44 ` Derrick Stolee
2022-03-01 9:46 ` Patrick Steinhardt
2022-03-01 10:35 ` Patrick Steinhardt
2022-03-01 14:06 ` Derrick Stolee [this message]
2022-03-01 14:53 ` Patrick Steinhardt
2022-03-01 15:25 ` Derrick Stolee
2022-03-02 13:57 ` Patrick Steinhardt
2022-03-02 14:57 ` Derrick Stolee
2022-03-02 18:15 ` Junio C Hamano
2022-03-02 18:46 ` Derrick Stolee
2022-03-02 22:42 ` Junio C Hamano
2022-03-03 11:19 ` Patrick Steinhardt
2022-03-03 16:00 ` Derrick Stolee
2022-03-04 14:03 ` Derrick Stolee
2022-03-07 10:34 ` Patrick Steinhardt
2022-03-07 13:45 ` Derrick Stolee
2022-03-07 17:22 ` Junio C Hamano
2022-03-10 13:58 ` Patrick Steinhardt
2022-03-10 17:18 ` Derrick Stolee
2022-02-24 20:38 ` [PATCH 4/7] commit-graph: fix generation number v2 overflow values Derrick Stolee via GitGitGadget
2022-02-24 22:35 ` Junio C Hamano
2022-02-25 13:53 ` Derrick Stolee
2022-02-25 17:38 ` Junio C Hamano
2022-02-24 20:38 ` [PATCH 5/7] commit-graph: document file format v2 Derrick Stolee via GitGitGadget
2022-02-24 22:55 ` Junio C Hamano
2022-02-25 22:31 ` Ævar Arnfjörð Bjarmason
2022-02-28 13:44 ` Derrick Stolee
2022-02-28 14:27 ` Ævar Arnfjörð Bjarmason
2022-02-28 16:39 ` Derrick Stolee
2022-02-28 21:14 ` Ævar Arnfjörð Bjarmason
2022-03-01 14:19 ` Derrick Stolee
2022-03-01 14:29 ` Ævar Arnfjörð Bjarmason
2022-03-01 15:59 ` Derrick Stolee
2022-02-24 20:38 ` [PATCH 6/7] commit-graph: parse " Derrick Stolee via GitGitGadget
2022-02-24 23:01 ` Junio C Hamano
2022-02-25 13:54 ` Derrick Stolee
2022-02-24 20:38 ` [PATCH 7/7] commit-graph: write " Derrick Stolee via GitGitGadget
2022-02-24 21:42 ` [PATCH 0/7] Commit-graph: Generation Number v2 Fixes, v3 implementation Junio C Hamano
2022-02-24 23:06 ` Junio C Hamano
2022-02-25 13:55 ` Derrick Stolee
2022-02-28 13:53 ` [PATCH v2 0/4] Commit-graph: Generation Number v2 Fixes Derrick Stolee via GitGitGadget
2022-02-28 13:53 ` [PATCH v2 1/4] test-read-graph: include extra post-parse info Derrick Stolee via GitGitGadget
2022-02-28 15:22 ` Ævar Arnfjörð Bjarmason
2022-02-28 13:53 ` [PATCH v2 2/4] commit-graph: fix ordering bug in generation numbers Derrick Stolee via GitGitGadget
2022-02-28 15:25 ` Ævar Arnfjörð Bjarmason
2022-02-28 13:53 ` [PATCH v2 3/4] commit-graph: start parsing generation v2 (again) Derrick Stolee via GitGitGadget
2022-02-28 15:30 ` Ævar Arnfjörð Bjarmason
2022-02-28 16:43 ` Derrick Stolee
2022-02-28 13:53 ` [PATCH v2 4/4] commit-graph: fix generation number v2 overflow values Derrick Stolee via GitGitGadget
2022-02-28 15:40 ` Ævar Arnfjörð Bjarmason
2022-03-01 17:23 ` [PATCH v2 0/4] Commit-graph: Generation Number v2 Fixes Ævar Arnfjörð Bjarmason
2022-03-01 19:48 ` [PATCH v3 0/5] " Derrick Stolee via GitGitGadget
2022-03-01 19:48 ` [PATCH v3 1/5] test-read-graph: include extra post-parse info Derrick Stolee via GitGitGadget
2022-03-01 19:48 ` [PATCH v3 2/5] t5318: extract helpers to lib-commit-graph.sh Derrick Stolee via GitGitGadget
2022-03-01 19:48 ` [PATCH v3 3/5] commit-graph: fix ordering bug in generation numbers Derrick Stolee via GitGitGadget
2022-03-01 20:13 ` Junio C Hamano
2022-03-01 20:30 ` Junio C Hamano
2022-03-02 14:13 ` Derrick Stolee
2022-03-01 19:48 ` [PATCH v3 4/5] commit-graph: start parsing generation v2 (again) Derrick Stolee via GitGitGadget
2022-03-01 19:48 ` [PATCH v3 5/5] commit-graph: fix generation number v2 overflow values Derrick Stolee via GitGitGadget
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