Issues with existing index.lock

[The-Compiler]

Sometimes when I try to fetch merge a PR, this happens:

  0 git … fetch git\@github.com\:abbradar/qutebrowser.git pac-fix
From github.com:abbradar/qutebrowser
 * branch                pac-fix    -> FETCH_HEAD

  0 git … checkout -b abbradar/pac-fix master
Switched to a new branch 'abbradar/pac-fix'

  0 git … merge --no-edit 3d9729839d6d9b5ee5d38afdf6ddf410dfca2027
Auto-merging tests/unit/browser/webkit/network/test_pac.py
Auto-merging qutebrowser/browser/network/pac.py
Merge made by the 'recursive' strategy.
 qutebrowser/browser/network/pac.py            |  2 +-
 tests/unit/browser/webkit/network/test_pac.py | 45 ++++++++++++++++++++-------
 2 files changed, 35 insertions(+), 12 deletions(-)

128 git … checkout master
fatal: Unable to create '/home/florian/proj/qutebrowser/git/.git/index.lock': File exists.

Another git process seems to be running in this repository, e.g.
an editor opened by 'git commit'. Please make sure all processes
are terminated then try again. If it still fails, a git process
may have crashed in this repository earlier:
remove the file manually to continue.

  0 git … merge --no-edit abbradar/pac-fix
Already up-to-date.

  1 git … branch -D abbradar/pac-fix
error: Cannot delete branch 'abbradar/pac-fix' checked out at '/home/florian/proj/qutebrowser/git'

I don't know what is locking the index, and keeping the lock around. Is it magit? magit-gh-pulls? I don't do anything in the repo other than (with spacemacs) SPC g s (show git status), # g (reload PRs), # f (fetch) and # m (merge).

When I look for that lock file after the error I don't see it, and indeed simply retrying works fine usually (after checking out master and deleting the branch, that is).

Also, even if the lock exists for some other reason, magit-gh-pulls should probably abort trying to do things when some operation fails...

I'm using emacs 25.2 with spacemacs 0.200.9, magit 20170511.1544, magit-gh-pulls 20161020.249 and git 2.13.0 on Archlinux.