FR: restore missing `.git/HEAD` file

[BatmanAoD]

Is your feature request related to a problem? Please describe.

In general, .git/HEAD should never be missing in a colocated repo. But it can be useful to overwrite it manually, and sometimes people trying to make a manual change make mistakes.

If .git/HEAD is missing, but the .git repository is otherwise uncorrupted, jj has all the information it needs to restore the repository to a working state with jj git export, because jj git export overwrites HEAD with whatever jj thinks it should be.

This, however, is not what happens:

Internal error: The repository appears broken or inaccessible
Caused by:
1: Failed to open git repository
2: "/Users/kstrand/sandbox/git/.git" does not appear to be a git repository
3: Missing HEAD at '.git/HEAD'

Describe the solution you'd like In the case where the .git/HEAD file is missing, but .jj/ and .git/ exist, I would like jj git export to restore the last known git-head from the operation log. This would bring the repository back to a usable state.

The message above should be modified to indicate that jj git export may be able to fix the broken state.

Describe alternatives you've considered

• Do nothing; leave the behavior as-is.
  • This is arguably "correct" in the sense that, since jj has no idea why the file is missing, it has no idea what commit the user would expect to be the current HEAD. That said, the repository is completely broken and unusable (by both git and jj) until .git/HEAD is restored, so it's hard to see how restoring a somewhat-outdated HEAD is any worse.
• Automatically fix the git repo if any command fails to import from git.
  • This seems a little too...easy. Users shouldn't be deleting their .git/HEAD, and it should be surprising when it's missing. Silently pretending it was never gone would be a pretty strange behavior for, say, jj status. Even noisily fixing it, i.e. printing a warning and then fixing the repo, seems like something that probably shouldn't happen just from running jj status, IMO, much less something that is expected to change the working copy, such as jj new <foo>.
• Auto-detect multiple kinds of corruption, such as non-ASCII characters in .git/HEAD or corruption in other files.
  • ...maybe that would be nice some day? But it's certainly not necessary, IMO.
• Provide a new jj git fix command.
  • This would be fine, arguably better than changing the behavior of jj git export. However, I was surprised that jj git export didn't just write a new HEAD file in the first place, so if there's a new command, the "Failed to open git repository" error message should at least recommend trying the new command. Additionally, the existence of something like jj git fix seems...potentially overly ambitious; it could lead people to expect that jj can fix more types of corruption than it actually can.

Additional context

As far as I can tell, there is no git command that will restore .git/HEAD, even though the reflog should provide the means to do so. I don't know why git doesn't provide tools for basic fix-ups.

[arxanas]

Does Git automatically restore a missing .git/HEAD?

[yuja]

because jj git export overwrites HEAD with whatever jj thinks it should be.

That describes the current git::reset_head() behavior, but it's kinda bug. jj should update HEAD only if it matches the internal view.git_head record. https://github.com/martinvonz/jj/issues/3754

I'm not against adding git fsck-like command for recovery, but I wouldn't expect jj git export fixes corrupted repository.

[BatmanAoD]

Does Git automatically restore a missing .git/HEAD?

No; I mentioned this at the very end of my comment.