path: Force-remove files in unlink()

[kvmajo]

Fixes #128.

Path.unlink() removes a file with a plain rm:

self.host.exec0("rm", self)

If the file is write-protected (e.g. -r--r--r--) but the containing
directory still permits deletion, GNU rm asks an interactive
confirmation question before removing it:

rm: remove write-protected regular file '...'?

Since tbot's channel has no way to answer that question, exec0()
never sees a shell prompt again and hangs indefinitely instead of
raising an error.

unlink() already resolves existence/missing_ok itself before
running rm, so nothing is lost by making the actual removal
non-interactive as well, via -f.

Testing

• pre-commit's pinned black, flake8, and mypy all pass on the
  changed file.
• selftest/tests/test_path.py has the same pass/fail counts before
  and after this change (two pre-existing, unrelated failures in
  test_rglob_error reproduce identically on unmodified master).
• Reproduced the actual bug: unlinking a chmod 400 file hangs
  (confirmed via timeout) on unmodified master, and completes cleanly
  on this branch.

[Rahix]

Hi, thanks for the contribution!

I just confirmed, Python pathlib's .unlink() also removes a 0400 file without complaining so we should do the same. I think your change is fine, but please add a new testcase that creates such an 0400 file and then unlinks it.

The -f makes me a bit anxious, I think we should add some more safeguards here. This would have been a good idea regardless, I wonder why I didn't do it back then already.

I just checked, busybox rm also supports -- to signal end of flags so maybe we should just add a -- before the self argument to ensure you can't pass filenames that will be interpreted as flags? Alternatively we could add a ./ to any non-absolute paths which may be a bit more portable.

[Rahix]

two pre-existing, unrelated failures in test_rglob_error reproduce identically on unmodified master

What were those about? I cannot reproduce them here and CI also seems happy... Can you open a new issue for this topic, please?