Subject: Re: [PATCH] bisect: allow to run from subdirectories

From: Junio C Hamano <[email-masked]>
Date: Mon, 21 Jun 2021 12:43:32 +0900

Roland Hieber <[email-masked]> writes:

> Currently, calling 'git bisect' from a directory other than the top
> level of a repository only comes up with an error message:
>
>     You need to run this command from the toplevel of the working tree.
>
> After a glance through the bisect code, there seems to be nothing that
> relies on the current working directory, and a few hours of bisect usage
> also didn't turn up any problems. Set the appropriate flag for
> git-sh-setup to remove the error message.

Try to find a history in which to run a bisect that

 (1) has a directory T in the recent part of the history, and

 (2) does not have that directory in the older part of the history.
     Better yet, if the older part of the history has T as a regular
     file, that would be ideal.  Even better, if that old regular
     file T was added, then removed, and then recreated, before it
     got turned into a directory.

Now, using the two commits you used to satisfy conditions (1) and
(2) as "bad" and "good", and using "bad - has T as a directory" and
"good - does not have T as a directory", as the bisection criterion,
try to find where "good" turns "bad"---in other words, find the
commit that either creates T as a directory or turns the regular
file T into a directory.

Perform that bisect from the subdirectory T.  Would that work?  I
suspect it wouldn't; when trying to check out an old version that
does not have T in the directory, either the checkout would fail
because it cannot remove T which has an active process (i.e. your
terminal session) in it, or your process sitting in an orphaned
directory (i.e. your ".." may still be the original top-level
directory that used to have T subdirectory, but "cd T" from the
top-level will not reach where you are).  All sorts of things can
go wrong and that is why we forbid it.  Just a single "cd" upfront
would save the user a lot of headache.

This does not depend on "do we have T as a directory?" being the
bisection criteria.  The important thing is that the current
directory would appear and disappear as the bisection process makes
you jump around in the history.

Hope this helps understanding the issue.