subprocess.run() and subprocess.Popen() accepts a cwd= parameter to change directory before running the subprocess.

Unfortunately it's not possible to use a file descriptor to run the subprocess in a directory already opened.

For example:

    import os
    import subprocess
    with os.open('/usr/bin', os.O_RDONLY) as f:
        subprocess.run(["ls", "-l"], cwd=f, check=True)

fails with

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/usr/lib/python3.9/subprocess.py", line 505, in run
        with Popen(*popenargs, **kwargs) as process:
      File "/usr/lib/python3.9/subprocess.py", line 951, in __init__
        self._execute_child(args, executable, preexec_fn, close_fds,
      File "/usr/lib/python3.9/subprocess.py", line 1754, in _execute_child
        self.pid = _posixsubprocess.fork_exec(

Using a file descriptor instead of path is useful to address TOCTOU issues.

Maybe a mean to convert a file descriptor to a Path-like object would do the trick.

Basically you want it to call fchdir() instead of chdir() when passed a fd (integer) instead of a string/Path-like.  That makes sense and should be a reasonably straight forward set of changes to _posixsubprocess.c.

(A way to convert a fd into a Path-like object would _not_ work as that'd reintroduce the TOCTOU on the directory - that'd be a pathlib feature request anyways, not a subprocess one)

I looked at posixmodule: os.chdir() accepts a file descriptor.
Maybe it can be possible to invoke it from _posixsubprocess.c instead of calling chdir().

this mostly requires plumbing to accept an int as the cwd and plumb that through to the between fork and exec code to call `fchdir(cwd_fd)` on the `int` instead of chdir(cwd) on the `char*`.

the Modules/_posixsubprocess.c internals are a bit of a mess today with a bazillion parameters passed to internal functions which makes this a pain... I want to refactor things to use a struct and not need that, but adding this feature is doable regardless.