From: Philip Oakley <philipoakley@iee.org>
Date: Sat, 18 Feb 2017 22:10:12 GMT
Subject: Re: Git bisect does not find commit introducing the bug
Message-ID: <6545FDC46D2741A289D0D5E9A84ADD75@PhilipOakley>
In-Reply-To: <CAMX8fZW2y+iPRfSbXVcHufbM+CsqgekS_0WnCEJ++=njy_TvKg@mail.gmail.com>

From: "Alex Hoffman" <spec@gal.ro>
>> But this is not how Git works. Git computes graph differences, i.e., it
>> subtracts from the commits reachable from v.bad those that are reachable
>> from v.good. This leaves more than just those on some path from v.good to
>> v.bad. And it should work this way. Consider this history:
>>
>> --o--o--o--G--X
>>    \           \
>>     x--x--x--x--X--B--
>>
>> When you find B bad and G good, than any one of the x or X may have
>> introduced the breakage, not just one of the X.
>>
>
> Thank you for clarifying how git bisect works. How did you find that
> out? Did you check the source code? If that is not documented in the
> man page may be it worth documenting it in order to avoid future
> confusion for other users.

Any suggestions for improving the documentation are always worthwhile. As 
someone who asked, what extra info would have helped?

Even beetter if it looks like a patch ;-)

>
> Let's consider your example with distinct names for nodes:
>
> --o1--o2--o3--G--X1
>    \                \
>     x1--x2--x3--x4--X1--B--
>
> It makes sense that git bisect is expecting a single transition, as
> this is a precondition for a binary search to work. My definition of
> "the transition" is a commit with at least one of its parents as a
> good version, but the commit itself a bad version. I hope we agree
> that git bisect's mission is to search for this transition (as I
> suppose that most of people need such a functionality from git, if not
> exactly from git bisect). How could be x1 or x3 be the transition, if
> chances are that o1 is not a good version? Of course it would make
> sense to me if bisect would check o1 whether good and only then to
> check also x1-x3, but this is not what git makes (at least not in my
> initial example).
>
> If you consider that git bisect's mission is different from finding
> the transition, could you please explain what exact commit git bisect
> is supposed to return (ideally with terms from the graph theory) and
> when it makes sense to return that? Because I do not see any sense in
> looking in the path x1-x3 without knowing that those commits may be a
> transition.
>
>
>> Oh, IMO git bisect was well thought through. If it considered just paths
>> from good to bad, it would not given the correct answer. See the example
>> history above. Bisect authors would not have deemed that sufficiently 
>> good
>
> You definitely convinced me that git MUST search more than only in the
> paths between good and bad commits, as the good commit G does not have
> to be the first good commit (thank you for that). My problem/confusion
> is that it returns something that does not make sense to me, because
> it does not make sure it returns a transition.
>
> VG
>
> PS: thank you for continuing this discussion.
>
--
Philip