path.c: translate Windows paths recorded by Windows git on POSIX hosts

[johnnyshields]

The Problem

If I create a git worktree using Windows Git, then git inside WSL Linux cannot resolve its path, because the gitdir specifier will be a Windows-style path C:\repo\... rather than a POSIX-style /mnt/c/repo...

As background, I use a hybrid Windows and WSL Ubuntu setup on the same machine; it works nearly perfectly except for the above issue, specifically with worktrees.

Related Pull-Requests

The vice-versa issue also affects creating a worktree in WSL and accessing it via Windows Git; I've raised a patch to Windows Git here: git-for-windows#6234 but please let me know if I should upstream this to the master git repo.

Root Cause

Worktrees use a bi-directional link that include absolute paths in the gitdir value to point from the main repo to the worktree repo, and vice versa. It is specifically this gitdir value that needs Windows-to-POSIX path translation handling. (Nothing else besides this path requires additional logic for worktrees to work across WSL and Windows native.)

  <main-repo>/   ← e.g. C:\workspace\myrepo (or /mnt/c/workspace/myrepo from WSL)
  └── .git/
      └── worktrees/
          └── <name>/
              ├── gitdir      ← file. content: "<worktree>/.git\n"
              ├── commondir   ← file (sometimes). content: relative or absolute path back to main .git
              ├── HEAD
              ├── index
              └── config.worktree

  <worktree>/    ← e.g. C:\workspace\myrepo-wt
  └── .git       ← FILE (not a dir). content: "gitdir: <main-repo>/.git/worktrees/<name>\n"

When git worktree add is run from native Windows, git writes absolute paths into the worktree's .git file, into <commondir>/worktrees/<id>/gitdir, and (when present) into <commondir>/commondir, in <x>:/... or <x>:\... form. Reading those files back from a non-Windows-native build of git fails because neither form is meaningful on POSIX, so the worktree appears broken even though every byte of it is reachable - the most common scenario being a worktree on a Windows drive opened from inside WSL2 (where the Windows filesystem is mounted at /mnt/<x>/) or from Cygwin/MSYS (where it is /cygdrive/<x>/).

The Solution (What this Patch Does)

This patch adds a small helper translate_windows_path() that recognises this shape at the start of a path and rewrites it to the POSIX mount form appropriate for the current build (/cygdrive/<x>/ on Cygwin, /mnt/<x>/ everywhere else), converting any backslashes in the remainder to forward slashes. Call it at the three places where non-Windows-native git reads a recorded worktree-related path back from disk:

• read_gitfile_gently() - the gitdir: line in a worktree's .git file.
• get_common_dir_noenv() - the commondir file inside a worktree's git directory, which points at the main repo.
• get_linked_worktree() - the gitdir file inside <commondir>/worktrees/<id>/, which points at the worktree's .git link.

Translation only happens for <x>:/ or <x>:\ where <x> is a single ASCII letter; anything else is left alone. The helper is a no-op on GIT_WINDOWS_NATIVE builds, where the input is already in native form. On non-WSL Linux hosts the translation still produces a syntactically valid POSIX path; if the corresponding /mnt/<x>/ mount does not exist, the next stat()/open() fails as it would have without translation - i.e. the change cannot make a working configuration stop working.

Why this is Safe

This PR is safe because:

• The logic is POSIX-only: the helper compiles to a no-op if GIT_WINDOWS_NATIVE, so a /mnt/c/... path on a Linux host - where it might really exist - is still treated literally.
  • I made a related patch for Git-for-Windows here: path.c: translate WSL and Cygwin paths when resolving worktrees git-for-windows/git#6234. Let me know if it should be upstreamed to be part of this PR.
• The logic is read-only: it only affects how Windows-style paths are read from POSIX; it does NOT mutate how the paths are stored in .git's definitions.
• Path traversal safety: the path translation only happens for the left-anchored part of the string <x>:\ where <x> is a single ASCII letter (e.g. C:\). Anything else (e.g. http://) is left alone. The helper also doesn't evaluate/collapse relative paths, so it doesn't introduce any path-traversal vectors that aren't existing today.

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[gitgitgadget]

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[gitgitgadget]

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