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

[jcameron73]

Cross reference for another issue with same apparent root cause: microsoft/vscode#100274 (comment)

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.

[christopherdoyle]

I've come across this because I'm having a similar issue with mapped network drives (e.g. net use). I've replicated your issue with pre-commit 2.8.1 and git 2.29.1.

My fist stab at a solution that works in both git 2.17 and 2.29 is the following change to pre_commit.main._adjust_args_and_chdir:

from pathlib import Path
toplevel = Path(git.get_root()).resolve()
os.chdir(toplevel)

args.config = os.path.relpath(Path(args.config).resolve())

Probably should have if sys.platform == "win32" in there.

In the case of subst, Path.resolve will give the underlying location C:\subst_dir. For mapped drives it will return the network path, e.g. \\MyServer\Directory.

[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.

[asottile]

can probably be return os.path.abspath(root) to retain the original behaviour -- one thing to check would be the version in which --show-cdup was added in case that limits compatibility (but as long as it's sufficiently old it should be fine without needing a fallback)

[asottile]

yeah it's super old so that should be a fine patch!

$ git describe --contains 5f94c730f31862c7f500173ee3a9d141c4730f0b
v1.1.0~31

[jcameron73]

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