Consider adding a Dir::into_fd() method

[linkmauve]

Currently, once an OwnedFd has been passed to rustix::fs::Dir, there is no way to reuse it.

My usecase is to call rustix::fs::openat() and rustix::fs::statat() on it after I’m done with the iteration (or even during, I don’t think there is any issue with that since getdents64(2) is stateless).

My current solution is to call rustix::io::dup() on the fd before creating the Dir, but that shouldn’t be needed and wastes one fd. Another solution would be to rustix::fs::open() it again once the iteration is done, but that’s wasteful too.

rustix::fs::Dir::read_from has the same issue, it implements dup(2) manually by calling fcntl(4, F_GETFL) then openat(4, ".", …) with 4 being the fd being passed to it.

And since getdents64(2) is stateless, I believe there is no unsoundness issue with e.g. multiple rustix::fs::Dir being created around the same fd.

[sunfishcode]

The tricky thing here is that rustix is aiming to have the same API across both the libc and linux-raw backends, and in the libc backend, it uses opendir/readdir/closedir. libc doesn't have a way to free a DIR object without closing the fd.

One option might be to have the libc backend implement into_fd by calling dup before calling closedir, however I'm not sure if it's too awkward if into_fd() has to be fallible.