Re: https://github.com/rust-lang/rust is not bisectable

["SZEDER Gábor" <szeder.dev@gmail.com>]

On Sun, Dec 18, 2022 at 01:03:54PM +0300, Askar Safin wrote:
> SZEDER Gábor, yes, in this particular case this helps.

Not just in this particular case, but this solves a general issue of
bisecting non-linear histories that is the file/testcase/whatever of
interest is not present in all candidate revisions in the range '^good
bad'.

> (But
> I still wonder how to tell git-bisect to ignore commits, which are not
> descendants of good commit.)

In general you do not want to tell that to bisect, because any one of
them could have introduced the change you are looking for.  Consider
the following piece of history, where commits 'm<N>' are on the master
branch, 'b<N>' are on a different branch, and the commit marked with C
made a buggy change, and commits marked with G and B are the ones you
marked as good and bad you started the bisection.

            (G)            (M)  (B)
  m1---m2---m3---m4---m5---m6---m7
        \                 /
         b1---b2---b3---b4
                   (C)

If bisect were to ignore commits "which are not descendants of good
commit", then it wouldn't look at any of the 'b<N>' commits made on
the side branch.  It would only look at commits m{3,4,5,6,7}, and thus
report m6 as the first bad commit, which is clearly not the case.