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bug#23769: 25.0.95; Mode Line breakage in vc-git


From: Phillip Lord
Subject: bug#23769: 25.0.95; Mode Line breakage in vc-git
Date: Wed, 15 Jun 2016 23:09:48 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.0.95 (gnu/linux)

Dmitry Gutov <address@hidden> writes:

> On 06/15/2016 11:48 PM, Phillip Lord wrote:
>
>> I've noticed from instrumenting vc-git that it's not just the
>> symbolic-ref command that returns 128, but several others. So, it might
>> be that the best long term solution would be to make change vc-git--call
>> to set the default directory to the root, which would make GIT_DIR=.git
>> always correct.
>
> The patch looks wrong. Why does the problem script set GIT_DIR to '.git'?

Try it on a pre-commit hook. CWD is set to the root, and GIT_DIR is set
to .git. Other people have found this:

http://longair.net/blog/2011/04/09/missing-git-hooks-documentation/

AFAICT, it's not actually documented by git. Or it is but the
documentation is incomprehensible (hey, it's git, it could happen!).


> In all examples I've found, this variable is set to an absolute value. In
> general, its purpose, it seems, is to point to the '.git' directory when it's
> named otherwise and/or is situated somewhere outside of the current directory
> tree.
>
> The latter situation will break vc-git-root.

That situation does indeed happen. For example, I normally checkout
emacs into worktrees where you get this when commiting on emacs-25
branch which is a worktree off master.

/home/phillord/src/git/emacs-git/master/.git/worktrees/emacs-25

But in this case, I think vc-git-root will still work.

(defun vc-git-root (file)
  (or (vc-file-getprop file 'git-root)
      (vc-file-setprop file 'git-root (vc-find-root file ".git"))))

We still find ".git" because it is a *file* (not a directory) at the top
level of a worktree.

> As such, the submitted patch is only likely to work in the
> tautological case you've descried.

It's definately a risk. Git does many things, and people use it in many
ways.


> And it will add some performance penalty to each call, because
> vc-git-root, though usually fast, is not free.

Not free, but it is a constant time look up after the first. 


There is a simpler option. I am trying to solve the root cause of the
problem but, as you say, that might be fraught. Since the problem only
seems to cause an error with vc-git-mode-line-string, we could just
discard the error from vc-git-working-revision in this case.

Failing that, now I know what the problem is, at least I have a
workaround (unset GIT_DIR in the pre-commit hook).


Phil





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