Filedescriptor crate to support dup2

[robinbernon]

Hi Wez. Am currently looking into tweaking the gag crate so that it works cross platform and wanted to use your filedescriptor crate to do it. Was wandering whether your crate supports dup2? Need this feature in order to redirect stdout/stderr into file. Happy to submit a PR for this, although I don't do much low level stuff so might need a couple pointers here..

[wez]

There's no direct support for dup2, but it wouldn't be difficult to add; I'd be happy to accept a PR for that.

This is the code used for dup:
https://github.com/wez/wezterm/blob/main/filedescriptor/src/unix.rs#L107-L147

There are two flavors: one that atomically dup's with CLOEXEC set, and one that non-atomically does the same.
To add dup2 you'll repeat that pattern but using dup3 (pass O_CLOEXEC as the third parameter) for the atomic case, and dup2 for the non-atomic case.

I'd suggest making the method have an unsafe signature to represent that there is potential danger in re-assigning the fd:

#[cfg(unix)]
unsafe pub fn dup2<F: AsRawFileDescriptor>(f: &F, dest_fd: RawFd) -> Result<Self>;

If you wanted to portably redirect stdio streams on windows you can't literally use dup2 there, but instead would need to look at using SetStdHandle and passing the result of calling https://docs.rs/filedescriptor/0.7.3/i686-pc-windows-msvc/filedescriptor/trait.IntoRawFileDescriptor.html to that API.

If you also wanted to add a portable API for that to filedescriptor I'm open to it; I think you'd need to add an enum representing stdin, stdout and stderr and then have it call the appropriate dup2 or SetStdHandle call depending on the OS.

[robinbernon]

I've opened up a premature PR with only windows directs supported for now: #788
Are these sorts of changes what you had in mind? (Gonna now work on the unix equivalent methods)

[wez]

I'd suggest something like:

#[derive(Debug, Clone, Copy, Eq, PartialEq)]
pub enum StdioDescriptor {
   Stdin,
   Stdout,
   Stderr,
}

impl FileDescriptor {
   /// Duplicates the descriptor representing `stdio`, then re-assigns that descriptor
   /// by duplicating `f`.
   /// Returns the original decriptor.
   pub fn redirect_stdio<F: AsRawFileDescriptor>(f: &F, stdio: StdioDescriptor) -> Result<Self>;
}

Under the covers that would use FileDescriptor::dup to get the original stream, the use dup2 on unix, or SetStdHandle on windows to assign it.

I'd prefer not to include the restore-on-drop functionality in this crate; gag could cleanly and portably implement that functionality on top of this interface:

struct TemporaryRedirection {
   stdio: StdioDescriptor,
   fd: FileDescriptor
}

impl TemporaryRedirection {
  pub fn new<F: AsRawFileDescriptor>(f: &F, stdio: StdioDescriptor) -> Result<Self> {
    let fd = FileDescriptor::redirect_stdio(f, stdio)?;
    Ok(Self { fd, stdio }}
  }
}

impl Drop for TemporaryRedirection {
   fn drop(&mut self) {
      let _ = FileDescriptor::redirect_stdio(self.fd, self.stdio);
   }
}

[robinbernon]

Sounds good - will just return the original file descriptor in the redirect_stdio method like you mentioned above then 👍
(will aim to submit a finalised PR by the end of tomorrow then)

[robinbernon]

@wez have also made a PR for the gag crate now: Stebalien/gag-rs#7

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