dd: remove unsafe from stdin_as_file() by rustix::{io::stdin,stdio::stdin}

[oech3]

This fn is used just for avoiding overreading from stdin. But the unsafe can be logically removed by rustix's read syscall wrapper (at least on unix and wasi).
So we don't need have both of unexpected closing of fd and additional syscall overhead caused by clone.

coreutils/src/uu/dd/src/dd.rs

Lines 230 to 242 in f65350a

	/// Create a source from stdin using its raw file descriptor.
	///
	/// This returns an instance of the `Source::StdinFile` variant,
	/// using the raw file descriptor of [`io::Stdin`] to create
	/// the [`File`] parameter. You can use this instead of
	/// `Source::Stdin` to allow reading from stdin without consuming
	/// the entire contents of stdin when this process terminates.
	#[cfg(unix)]
	fn stdin_as_file() -> Self {
	let fd = io::stdin().as_raw_fd();
	let f = unsafe { File::from_raw_fd(fd) };
	Self::StdinFile(f)
	}

[xtqqczze]

#12210 removes stdin_as_file and wraps the unsafe in a safe StdinRaw.

[oech3]

It does not remove unsafe from our code base. Just wraps.

[xtqqczze]

I mean we can wrap the unsafe to avoid any possible unsoundness, e.g.:

pub struct StdinRaw(ManuallyDrop<File>);

#[cfg(not(windows))]
pub fn stdin_raw() -> StdinRaw {
    // SAFETY: We ensure that the file descriptor is never closed by
    // wrapping the `File` in `ManuallyDrop`.
    let fd = unsafe { rustix::stdio::take_stdin() };
    let f = File::from(fd);
    StdinRaw(ManuallyDrop::new(f))
}

We can't use the safe rustix::io::stdin because we need an OwnedFd not a BorrowedFd to initialize the File.

[oech3]

Bt why do we exactly need File? seek only?

[xtqqczze]

File has the convenient Read, Write and Seek traits that a BorrowedFd lacks.

[oech3]

I could add definition of seek.

[xtqqczze]

Specifically in dd, we call File::metadata on stdin, so we need &File.

[oech3]

We can use rustix's API to AsFd instead of std's API