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

From: Junio C Hamano <gitster@pobox.com>
Date: Jun 21, 2021, 03:43 UTC
Message-ID: <xmqqy2b3j317.fsf@gitster.g>
In-Reply-To: <20210620213836.10771-1-rhi@pengutronix.de>

Roland Hieber <rhi@pengutronix.de> writes:

Show 9 quoted lines

> 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.