Pre-commit fails for git >=2.25 if repo is on a Windows subst drive

Cross reference for another issue with same apparent root cause: https://github.com/microsoft/vscode/issues/100274#issuecomment-646499795

Issue observed with pre-commit==2.7.1 and git 2.27. Issue resolved with downgrading git to 2.21 (I only have access to certain versions on my work machine).

Steps to recreate for pre-commit (some taken from the above cross-reference):

• Install git >= 2.25 on Windows

• Create a subst drive (mkdir C:\subst_dir && subst Z: C:\subst_dir)

• Create a git repo in there (mkdir Z:\repo && cd /d Z:\repo && git init)

• Add some python code, configure pre-commit, and run pre-commit.

Failure observed: An unexpected error has occurred: ValueError: path is on mount 'Z:', start on mount 'C:'

Diagnosis - it appears that the use of git rev-parse --show-toplevel in pre_commit.main.get_root() is suffering the same issue as seen in cross-referenced ticket; git will “see through” the subst command and rather than return a path on the subst-defined Z: drive, it will return the path from the C: drive. With this, after pre_commit.main._adjust_args_and_chdir() calls pre_commit.main.get_root() and does a chdir to the returned location, the following call to os.path.relpath(args.config) then fails with the ValueError as above, because it sees the path to the config file being on Z: but the current location being on C:.

Afraid I don’t have a suggested resolution but wanted to flag this up. I’m not too familiar with Windows systems and I’m a long way from Admin access on my work machine so opportunities for testing are limited; this was discovered as my scratch space for repos is a subst drive.

[mrogaski]

Using --show-cdup instead of --show-toplevel, as was suggested in the VSCode issue listed in the OP, is worth considering.

def get_root() -> str:
    try:
        root = cmd_output('git', 'rev-parse', '--show-cdup')[1].strip()
    except CalledProcessError:
        raise FatalError(
            'git failed. Is it installed, and are you in a Git repository '
            'directory?',
        )
    if not root:
        return os.getcwd()
    return root

The original failure mode (ie. issuing the command from in .git) will fail, but will be a non-zero return code from Git and be caught by CalledProcessError.

[jcameron73]

I’ve pulled this into the project where the issue first arose and it’s working beautifully - thank you!