Re: What's cooking in git.git (Apr 2017, #04; Wed, 19)

From: Johannes Sixt <j6t@kdbg.org>
Date: Apr 25, 2017, 05:51 UTC
Message-ID: <3bd4a2d7-ada9-6198-8cf1-573d80cae1b4@kdbg.org>
In-Reply-To: <CAP8UFD1r9TFE9ns5pobDOpugF8MBiJAYgrRALCqGVmgWud=QjQ@mail.gmail.com>

Am 25.04.2017 um 04:00 schrieb Christian Couder:

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> On Sat, Apr 22, 2017 at 3:37 PM, Johannes Sixt <j6t@kdbg.org> wrote:
>> Am 21.04.2017 um 14:29 schrieb Christian Couder:
>>>
>>> First bisect should ask you to test merge bases only if there are
>>> "good" commits that are not ancestors of the "bad" commit.
>>
>>
>> That's a tangent, but I have never understood why this needs to be so.
>> Consider this:
>>
>>     o--o--o--o--o--o--o--o--B
>>    /           /
>>  -o--o--o--o--g--o--o--o--o--G
>>
>> When I mark B as bad and G as good, why would g have to be tested first?
>
> It is because g could be bad if the bug has been fixed between g and G.
> If this happens and we don't test g, we would give a wrong result.

Gah! So, a typical messy workflow "requires" this behavior. A clean branchy workflow like Git's does not because we know that a breakage is either on a topic branch, B, or a (the?) bad commit is an ancester of (the integration branch) G.

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>
>> This is exactly what I do when I bisect in Git history: I mark the latest
>> commits on git-gui and gitk sub-histories as good, because I know they can't
>> possibly be bad. (In my setup, these two histories are ahead of pu and
>> next.)
>
> Yeah, it is safe to do that in this case as we test the merge bases.

The idea of marking git-gui and gitk histories that none of their commits is checked out: it erases all Git source code from the working directory, and a later bisection step places all code back and it requires a full build. Not a big deal with Git, but there are much larger code bases.

The current bisect behavior makes this idea unworkable. For me, it was a big step backwards when it was implemented. :-(

-- Hannes