bug#21559: 25.0.50; auto-revert-mode breaks git rebase

[Mitchel Humpherys]

On Sun, Feb 07 2016 at 11:55:07 AM, Ben Gamari <ben@smart-cactus.org> wrote:
> I've spoken with some folks in #git about this issue.
>
>  * the index lock is held "because it needs to read the index and
>    compare it to the worktree. If it doesn't take the index lock, other
>    things could change either the index or the worktree underneath it,
>    making git status lie (or even crash)"

Makes sense...

>  * It sounds as though a patch refactoring `git rebase` such that it
>    holds the index lock may be considered, although this is a
>    non-trivial refactoring as `rebase` is currently a shell script
>
>  * One option would be to check whether a rebase is in progress before
>    auto-reverting. In pseudo-shell,
>
>    if [ -e .git/rebase-apply ] || [ -e .git/rebase-merge ]; then
>       do_not_call_git_status_because_we_are_rebasing;
>    else
>       auto_revert
>    fi
>
>    Unfortunately it sounds like this wouldn't do the right thing when a
>    rebase pauses due to conflict (when you'd ideally want to
>    auto-revert).

Yeah that would definitely be a problem.  The other problem with this is
fix is that it wouldn't fix any other use case that locks the index,
like my repro earlier:

    $ cd /path/to/emacs
    $ ./src/emacs -Q lisp/*.el
    [ M-x global-auto-revert-mode ]
    $ for i in $(seq 30); do for f in lisp/*.el; do echo "; $i" >> $f; done; 
git commit -am "test $i"; done

There's no rebase there, so detecting a rebase to skip auto-revert
wouldn't help...

Hopefully we can get rid of the `git status' from the auto-revert path
altogether.  It looks like the only place we do a `git status' is in
`vc-git-conflicted-files'.  We call that from `vc-git-find-file-hook' to
see if we should start smerge.  But I don't see how we get there from
auto-revert.