File descriptors passed to Command::stdout/err/in are not closed after dup2

File descriptors passed to Command::stdout/err/in are not closed after dup2 to replace stdout/stderr/stdin. This means the file descriptors passed to Command::stdout/err/in are leaked in the child process.

Example:

use std::fs::File;
use std::process::{Command, Stdio};

fn main() -> std::io::Result<()> {
    let file = File::create("output.txt")?;

    let mut child = Command::new("echo")
        .arg("Hello from the child process!")
        .stdout(Stdio::from(file))
        .spawn()?;

    let status = child.wait()?;

    println!("Child exited with status: {}", status);

    Ok(())
}

strace -f output (full output is attached):

3903339 dup2(3, 1)                      = 1
3903339 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
3903339 execve("/home/i509vcb/.cargo/bin/echo", ["echo", "Hello from the child process!"], 0x7fffc66d0a58 /* 75 vars */) = -1 ENOENT (No such file or directory)
3903339 execve("/home/i509vcb/.local/bin/echo", ["echo", "Hello from the child process!"], 0x7fffc66d0a58 /* 75 vars */) = -1 ENOENT (No such file or directory)
3903339 execve("/home/i509vcb/bin/echo", ["echo", "Hello from the child process!"], 0x7fffc66d0a58 /* 75 vars */) = -1 ENOENT (No such file or directory)
3903339 execve("/usr/lib64/ccache/echo", ["echo", "Hello from the child process!"], 0x7fffc66d0a58 /* 75 vars */) = -1 ENOENT (No such file or directory)
3903339 execve("/usr/local/bin/echo", ["echo", "Hello from the child process!"], 0x7fffc66d0a58 /* 75 vars */) = -1 ENOENT (No such file or directory)
3903339 execve("/usr/bin/echo", ["echo", "Hello from the child process!"], 0x7fffc66d0a58 /* 75 vars */ <unfinished ...>
3903338 <... clone3 resumed>)           = 3903339

This will leak fd 3 on the child process. Ideally do_exec would close the stdio file descriptors which were overriden after dup2 on the child: https://github.com/rust-lang/rust/blob/2300c2aef7dbc2a7bbbeaa9894d07d459abd9bc6/library/std/src/sys/process/unix/unix.rs#L282

strace.txt

Start in library/std/src/sys/process/unix/unix.rs at the referenced do_exec implementation and compare the dup2 handling with the attached strace output. Reproduce the example on a Unix system and verify that overridden stdio descriptors are closed in the child after dup2, including the failed-exec path.