fix: allow safer PosixSpawnFileActions usage

[CameronNemo]

Many functions used for PosixSpawnFileActions were demanding fds passed implement the AsFd trait, but because these actions are meant to be taken in the child process, that trait doesn't offer much benefit and actually often leads to the caller needing to do an unsafe operation: instantiating an OwnedFd from a RawFd. All of these functions need a RawFd anyway, so just let the caller pass a RawFd directly rather than have to unsafely create an OwnedFd first, which itself could have unintended side effects like closing the FD in the parent when no parent-side actions were intended.

What does this PR do

Followup to #2475, which added the API and has not been in a released version yet.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API (NOTE: changelog was added in Add wrappers for posix_spawn related functions #2475, and the changelog does not need any updates)

[CameronNemo]

For some context, right now I am being steered toward writing this code:

    let mut file_actions = spawn::PosixSpawnFileActions::init()?;
    let dev_null = CString::new("/dev/null")?;
    let dev_null = dev_null.as_c_str();
    let fd0 = unsafe { std::os::fd::OwnedFd::from_raw_fd(0) };
    let fd1 = unsafe { std::os::fd::OwnedFd::from_raw_fd(1) };
    file_actions.add_open(fd0, dev_null, OFlag::O_RDONLY, Mode::empty())?;
    file_actions.add_dup2(fd0, fd1)?;

Besides the unsafe usage, this code is wrong -- it will close the stdin and stdout file descriptors in the parent when the fd0 and fd1 variables get dropped. That is not at all what we are trying to accomplish; we only want actions to be taken in the child, thus I think AsRawFds are a better API choice for these functions.

[SteveLauC]

Thanks for the feedback!

I agree that I/O-safe interfaces do not make sense here.

Regarding your example, you can actually use the Stdin and Stdout from std as they have AsFd implemented, though I think we should use RawFd in these interfaces.

[SteveLauC]

Taking a look at the types that implement AsRawFd, they are mostly owned file descriptors, e.g., File, passing them by value would have them closed within these add_xxx() functions, and we don't have an impl like:

impl<T: AsRawFd> AsRawFd for &T

what about:

add_xxx<Fd: AsRawFd>(_: &AsRawFd)

Though one drawback with this interface is that when passing RawFds, we still need to add a layer of reference to it:

let stdin_raw_fd: RawFd = 0;

add_xxx(&stdin_raw_fd);

thoughts on this?

[SteveLauC]

What about removing the generic type and just using RawFd?

[CameronNemo]

Hmm yeah just using RawFd might be simpler.

[CameronNemo]

Alright pushed with the direct RawFd type.