Re: fatal: unable to read after commit - deeper analysis

[Jeff King <peff@peff.net>]

On Sat, Apr 13, 2019 at 10:21:36AM +0100, Klaus Ethgen wrote:

> Finally, the error was a combination of 4 tools, git, vim, the mentioned
> vim-addon and task with a task-hook for committing pending.data.
> 
> When I do a git commit which invokes vim, then the following variables
> are set:
> - GIT_INDEX_FILE
> - GIT_AUTHOR_DATE
> - GIT_PREFIX
> - GIT_EXEC_PATH
> 
> And $GIT_INDEX_FILE is the source of trouble here. The task-hook clears
> respective sets the variables GIT_DIR and GIT_WORK_TREE. However, the
> GIT_INDEX_FILE environment is set (in some cases) to an absolute path
> pointing to .git/index.lock or .git/index (I have no idea when it is
> taking the .lock variant).
> 
> Now we have a mismatch of GIT_DIR and GIT_WORK_TREE on one hand and the
> absolute path of GIT_INDEX_FILE on the other. So the trouble is set. The
> following "git add pending.data" did break all. It does something to an
> index that does not belong to the git repo.
> 
> Mystery is when and why this changed in git. It was definitively changed
> in some recent version.

It was always the case that GIT_INDEX_FILE would sometimes be set. For a
partial commit like:

  echo changes >>one
  echo changes >>two
  git add .
  git commit two

we have to create a temporary index (so as not to commit the staged
changes from "one").

I was curious if we started setting it for the more vanilla case of
just "git commit" more recently. But trying to bisect on:

  GIT_EDITOR=set | grep ^GIT_INDEX; :' git commit

I couldn't find any version where we _didn't_ set $GIT_INDEX_FILE, going
back to v1.6.0. So I don't know of any recent changes in this area.

Presumably you've fixed it by unsetting $GIT_INDEX_FILE in your vim
task-hook when it switches to another repo. However, you may want to
switch to asking Git about the full list of repo-related variables you
should clear. In the shell that looks like:

  unset $(git rev-parse --local-env-vars)

That should be more thorough, and will future-proof you against new
variables being added.

-Peff