Need to check if the std{in,err,out} file descriptors status

[anastygnome]

Hello,

I was faced with a difficulty earlier today as I needed to port an application whose behaviour depends upon the status of those three file descriptors.

I have tracked the issue down to the following function :

rust/library/std/src/sys/unix/mod.rs

Lines 81 to 133 in 247e44e

	unsafe fn sanitize_standard_fds() {
	// fast path with a single syscall for systems with poll()
	#[cfg(not(any(
	miri,
	target_os = "emscripten",
	target_os = "fuchsia",
	target_os = "vxworks",
	// The poll on Darwin doesn't set POLLNVAL for closed fds.
	target_os = "macos",
	target_os = "ios",
	target_os = "watchos",
	target_os = "redox",
	target_os = "l4re",
	target_os = "horizon",
	)))]
	'poll: {
	use crate::sys::os::errno;
	#[cfg(not(all(target_os = "linux", target_env = "gnu")))]
	use libc::open as open64;
	#[cfg(all(target_os = "linux", target_env = "gnu"))]
	use libc::open64;
	let pfds: &mut [_] = &mut [
	libc::pollfd { fd: 0, events: 0, revents: 0 },
	libc::pollfd { fd: 1, events: 0, revents: 0 },
	libc::pollfd { fd: 2, events: 0, revents: 0 },
	];
	while libc::poll(pfds.as_mut_ptr(), 3, 0) == -1 {
	match errno() {
	libc::EINTR => continue,
	libc::EINVAL | libc::EAGAIN | libc::ENOMEM => {
	// RLIMIT_NOFILE or temporary allocation failures
	// may be preventing use of poll(), fall back to fcntl
	break 'poll;
	}
	_ => libc::abort(),
	}
	}
	for pfd in pfds {
	if pfd.revents & libc::POLLNVAL == 0 {
	continue;
	}
	if open64("/dev/null\0".as_ptr().cast(), libc::O_RDWR, 0) == -1 {
	// If the stream is closed but we failed to reopen it, abort the
	// process. Otherwise we wouldn't preserve the safety of
	// operations on the corresponding Rust object Stdin, Stdout, or
	// Stderr.
	libc::abort();
	}
	}
	return;
	}

I am of the opinion that, even if the std library needs to reopen those descriptors, we should be able to get an indication that those are not the original ones. Maybe through some kind of booleans or a dedicated structure?
Related: #47271
This has also affected the uutils project.
Any hope this will get fixed ?

[anastygnome]

uutils/coreutils#2873

For example, some coreutils behaviour need to take into account this fact too.

[the8472]

See also #88825. posix requires that descriptors 0-2 are open at program startup. So any program that wants to detect a closed descriptor is non-compliant.

[clubby789]

@rustbot label +T-libs +C-enhancement

[anastygnome]

See also #88825. posix requires that descriptors 0-2 are open at program startup. So any program that wants to detect a closed descriptor is non-compliant.

Sometimes, one needs to be non-compliant. Not all environments are, and I don't believe we should prevent people from doing it.