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0000-long-paths-windows.md

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  • Feature Name: long_windows_paths
  • Start Date: 2017-10-24
  • RFC PR:
  • Rust Issue:

Summary

Rust programs currently cannot access files on Windows 8 and below with paths longer than MAX_PATH. This is a bug and should be fixed.

Motivation

Windows 8 and below make it difficult but not impossible for a program to access a file with a name longer than MAX_PATH. The kernel APIs have no problem with such paths at all, but the user-mode APIs do. They require that the path be prefixed with \\?\, which disables all normalization.

Fortunately, there is a solution that gets back normalization: if (and only if) the file name does not begin with \??\ or \\?\, one can pass it to the Windows API function GetFullPathNameW, which supports long paths just fine and can easily be called from Rust. The resulting path can safely have \\?\ prepended and then be passed to the underlying Windows API calls.  GetFullPathNameW’s documentation claims that it does not support long paths, but it in fact does.

Guide-level explanation

This should require at most a brief mention in the Rust book. All that is needed is a note that while many programs do not support long paths on Windows 8 and below (or Windows 10 without proper manifest/registry settings), Rust’s standard library has no such problems, and will work on any path that the underlying NT API supports.

Reference-level explanation

Whenever Rust sends a path to an underlying NT API, it should use first check if the path length is less than 260 characters or begins with either \\?\ or \??\, then pass it to the underlying Windows API function directly. Otherwise, pass it to GetFullPathNameW. If it fails, return; if not, pass the result to the underlying Windows API call.

Drawbacks

This adds a small amount of complexity to the standard library. This complexity should be more than compensated by the ability to handle long paths generated by e.g. NPM.

Rationale and alternatives

The alternatives are:

  • Do nothing. This is bad, because tools like NPM already support and generate long paths.
  • Try to implement GetFullPathNameW in pure Rust. This is not easy, and does not guarantee compatibility with the Windows implementation.
  • Use GetFullPathNameW to allow for long paths on Windows. This is the approach chosen here.

Unresolved questions

None. This is a bug fix, pure and simple.