Use rustix for cmd extension

This is safer and may actually fix a race condition I've seen sometimes
in CI runs.  Part of investigating using rustix (and cap-std) in
our section of the ecosystem (xref rust-lang/rfcs#2610).

lib/Cargo.toml

@@ -26,6 +26,7 @@ indicatif = "0.16.0"
 once_cell = "1.9"
 libc = "0.2.92"
 nix = "0.23"
+rustix = "0.31.3"
 oci-spec = "0.5.0"
 openat = "0.1.20"
 openat-ext = "0.2.0"

lib/src/cmdext.rs

@@ -1,19 +1,23 @@
-use std::os::unix::prelude::{CommandExt, RawFd};
+use rustix::fd::{FromRawFd, IntoRawFd};
+use rustix::io::OwnedFd;
+use std::os::unix::prelude::CommandExt;
+use std::sync::Arc;
 
 pub(crate) trait CommandRedirectionExt {
     /// Pass a file descriptor into the target process.
-    /// IMPORTANT: `fd` must be valid (i.e. cannot be closed) until after [`std::Process::Command::spawn`] or equivalent is invoked.
-    fn take_fd_n(&mut self, fd: i32, target: i32) -> &mut Self;
+    fn take_fd_n(&mut self, fd: Arc<OwnedFd>, target: i32) -> &mut Self;
 }
 
 #[allow(unsafe_code)]
 impl CommandRedirectionExt for std::process::Command {
-    fn take_fd_n(&mut self, fd: i32, target: i32) -> &mut Self {
+    fn take_fd_n(&mut self, fd: Arc<OwnedFd>, target: i32) -> &mut Self {
         unsafe {
             self.pre_exec(move || {
-                nix::unistd::dup2(fd, target as RawFd)
-                    .map(|_r| ())
-                    .map_err(|e| std::io::Error::new(std::io::ErrorKind::Other, format!("{}", e)))
+                let target = rustix::io::OwnedFd::from_raw_fd(target);
+                rustix::io::dup2(&*fd, &target)?;
+                // Intentionally leak into the child.
+                let _ = target.into_raw_fd();
+                Ok(())
             });
         }
         self

lib/src/tar/write.rs

@@ -13,12 +13,13 @@ use anyhow::{anyhow, Context};
 use camino::{Utf8Component, Utf8Path, Utf8PathBuf};
 use ostree::gio;
 use ostree::prelude::FileExt;
+use rustix::fd::FromFd;
 use std::collections::BTreeMap;
 use std::convert::TryInto;
 use std::io::{BufWriter, Write};
-use std::os::unix::prelude::AsRawFd;
 use std::path::Path;
 use std::process::Stdio;
+use std::sync::Arc;
 use tokio::io::{AsyncRead, AsyncReadExt, AsyncWrite};
 use tracing::instrument;
 
@@ -197,13 +198,14 @@ pub async fn write_tar(
     };
     let mut c = std::process::Command::new("ostree");
     let repofd = repo.dfd_as_file()?;
+    let repofd = Arc::new(rustix::io::OwnedFd::from_into_fd(repofd));
     {
         let c = c
             .stdin(Stdio::piped())
             .stdout(Stdio::piped())
             .stderr(Stdio::piped())
             .args(&["commit"]);
-        c.take_fd_n(repofd.as_raw_fd(), 3);
+        c.take_fd_n(repofd.clone(), 3);
         c.arg("--repo=/proc/self/fd/3");
         if let Some(sepolicy) = sepolicy.as_ref() {
             c.arg("--selinux-policy");