From: Derrick Stolee <derrickstolee@github.com>
To: Junio C Hamano <gitster@pobox.com>, Patrick Steinhardt <ps@pks.im>
Cc: git@vger.kernel.org, Taylor Blau <me@ttaylorr.com>
Subject: Re: [PATCH] commit-graph: fix truncated generation numbers
Date: Tue, 28 Mar 2023 14:32:40 -0400 [thread overview]
Message-ID: <ca1f77db-a483-8b32-8e34-9bce1ea8afc3@github.com> (raw)
In-Reply-To: <xmqqfs9og47y.fsf@gitster.g>
On 3/28/23 1:45 PM, Junio C Hamano wrote:
> Patrick Steinhardt <ps@pks.im> writes:
>
>> In 80c928d947 (commit-graph: simplify compute_generation_numbers(),
>> 2023-03-20), the code to compute generation numbers was simplified to
>> use the same infrastructure as is used to compute topological levels.
>> This refactoring introduced a bug where the generation numbers are
>> truncated when they exceed UINT32_MAX because we explicitly cast the
>> computed generation number to `uint32_t`. This is not required though:
>> both the computed value and the field of `struct commit_graph_data` are
>> of the same type `timestamp_t` already, so casting to `uint32_t` will
>> cause truncation.
>>
>> This cast can cause us to miscompute generation data overflows:
>>
>> 1. Given a commit with no parents and committer date
>> `UINT32_MAX + 1`.
>>
>> 2. We compute its generation number as `UINT32_MAX + 1`, but
>> truncate it to `1`.
>>
>> 3. We calculate the generation offset via `$generation - $date`,
>> which is thus `1 - (UINT32_MAX + 1)`. The computation underflows
>> and we thus end up with an offset that is bigger than the maximum
>> allowed offset.
>>
>> As a result, we'd be writing generation data overflow information into
>> the commit-graph that is bogus and ultimately not even required.
>>
>> Fix this bug by removing the needless cast.
>>
>> Signed-off-by: Patrick Steinhardt <ps@pks.im>
>> ---
>>
>> This commit applies on top of cbfe360b14 (commit-reach: add
>> tips_reachable_from_bases(), 2023-03-20), which has recently been merged
>> to next.
>
> The patch is clearly explained and the change looks quite
> straight-forward. Derrick, Ack?
Yes, looks good. What a silly mistake, but thanks for going
the extra mile to introduce a test that will prevent it in
the future.
Thanks,
-Stolee
next prev parent reply other threads:[~2023-03-28 18:32 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-27 8:08 [PATCH] commit-graph: fix truncated generation numbers Patrick Steinhardt
2023-03-28 17:45 ` Junio C Hamano
2023-03-28 18:32 ` Derrick Stolee [this message]
2023-03-28 18:39 ` Taylor Blau
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