Re: [PATCH] bisect: test merge base if good rev is not an ancestor of bad rev

[Junio C Hamano <gitster@pobox.com>]

Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:

> You are opening a can of worms here, and I doubt that this is a good idea.
>
> git-bisect as-is has very precise, and _simple_ semantics, and users 
> should really know what they are doing (i.e. not marking something as 
> "good" which is on a branch containing a fix).
>
> Trying to be too clever here might just make the whole tool rather 
> useless.

Have you read the original thread yet?  I do not think this is trying to
be clever at all, but trying to be helpful.

As you explained, bisection *requires* that Good is ancestor of Bad.

     ---o---?---?---?---x
        Good            Bad

But in real life, things are more like "Today's master does not work, but
I am sure it used to work at 1.5.6.2".  I happen to always merge all of
'maint' to 'master' before pushing them out, so a good topology is
guaranteed, but not everybody does this (it takes careful planning what to
apply to 'maint' and where to fork topics from, and maintainers are not
perfect):

              Good
          ?---o (maint)
         /
     ---?---?---?---?---x (master)
        MB              Bad

People _will_ face such a topology.  If the users Know Better, they will
test MB=$(merge-base master maint) first to see if it is broken, and then
the world will have two possibilities:

    (1)       Good
          ?---o (maint)
         /
     ---o---?---?---?---x (master)
        Good            Bad

    (2)       Good
          ?---o (maint)
         /
     ---x---?---?---?---x (master)
        Bad             Bad


If (1), you go ahead with the usual bisection.  If (2), you cannot even
bisect.  Instead, you flip good and bad to find the "fix" in the side
branch (the answer has to be either the tip of maint or one previous in
the picture) to forward port to, either by merging 'maint' to 'master' or
cherry-picking.

The idea to check merge-base first is about automating this process (I
admit I still haven't looked at Christian's patch text yet).