add std::os::fd::CommandExt::fd

Author: Qelxiros

library/std/src/os/fd/mod.rs

@@ -12,6 +12,11 @@ mod raw;
 // `OwnedFd`, `AsFd`, etc.
 mod owned;
 
+// `CommandExt`, etc.
+#[cfg(unix)]
+#[unstable(feature = "command_pass_fds", issue = "144989")]
+pub mod process;
+
 // Implementations for `AsRawFd` etc. for network types.
 #[cfg(not(target_os = "trusty"))]
 mod net;

library/std/src/os/fd/process.rs

@@ -0,0 +1,67 @@
+use libc::{F_GETFD, F_SETFD, FD_CLOEXEC};
+
+use crate::os::fd::raw::AsRawFd;
+use crate::os::fd::{OwnedFd, RawFd};
+use crate::process::Command;
+use crate::sealed::Sealed;
+use crate::sys::{cvt, cvt_r};
+use crate::sys_common::AsInnerMut;
+
+/// Extensions to the [`crate::process::Command`] builder for Unix and WASI, platforms that support file
+/// descriptors.
+///
+/// This trait is sealed: it cannot be implemented outside the standard library.
+/// This is so that future additional methods are not breaking changes.
+#[unstable(feature = "command_pass_fds", issue = "144989")]
+pub trait CommandExt: Sealed {
+    /// Pass a file descriptor to a child process.
+    ///
+    /// Getting this right is tricky. It is recommended to provide further information to the child
+    /// process by some other mechanism. This could be an argument confirming file descriptors that
+    /// the child can use, device/inode numbers to allow for sanity checks, or something similar.
+    ///
+    /// If `new_fd` is an open file descriptor and closing it would produce one or more errors,
+    /// those errors will be lost when this function is called. See
+    /// [`man 2 dup`](https://www.man7.org/linux/man-pages/man2/dup.2.html#NOTES) for more information.
+    ///
+    /// If this method is called multiple times with the same `new_fd`, all but one file descriptor
+    /// will be lost.
+    ///
+    /// ```
+    /// #![feature(command_pass_fds)]
+    ///
+    /// use std::fs::OpenOptions;
+    /// use std::process::Command;
+    /// use std::os::fd::process::CommandExt;
+    ///
+    /// eprintln!("chom");
+    /// let file = OpenOptions::new().read(true).write(true).create(true).open("/tmp/fd_doctest.rs").unwrap();
+    /// eprintln!("chom");
+    ///
+    /// let mut command = Command::new("ls");
+    /// command.arg("/proc/self/fd").fd(5, file);
+    /// eprintln!("chom");
+    ///
+    /// let output = command.output().inspect(|o| eprintln!("{o:?}")).unwrap();
+    /// assert_eq!(String::from_utf8_lossy(&output.stdout).split_whitespace().map(|s| s.parse::<usize>().unwrap()).collect::<Vec<_>>(), vec![0,1,2,3,5]);
+    /// ```
+    fn fd(&mut self, new_fd: RawFd, old_fd: impl Into<OwnedFd>) -> &mut Self;
+}
+
+#[unstable(feature = "command_pass_fds", issue = "144989")]
+impl CommandExt for Command {
+    fn fd(&mut self, new_fd: RawFd, old_fd: impl Into<OwnedFd>) -> &mut Self {
+        let old = old_fd.into().as_raw_fd();
+        unsafe {
+            self.as_inner_mut().pre_exec(Box::new(move || {
+                cvt_r(|| libc::dup2(old, new_fd))?;
+                let flags = cvt(libc::fcntl(new_fd, F_GETFD))?;
+                cvt(libc::fcntl(new_fd, F_SETFD, flags & !FD_CLOEXEC))?;
+                cvt_r(|| libc::close(old))?;
+                Ok(())
+            }))
+        }
+
+        self
+    }
+}