Auto merge of rust-lang#81825 - voidc:pidfd, r=joshtriplett

Add Linux-specific pidfd process extensions (take 2)

Continuation of rust-lang#77168.
I addressed the following concerns from the original PR:

- make `CommandExt` and `ChildExt` sealed traits
- wrap file descriptors in `PidFd` struct representing ownership over the fd
- add `take_pidfd` to take the fd out of `Child`
- close fd when dropped

Tracking Issue: rust-lang#82971

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

@@ -4,4 +4,5 @@
 #![doc(cfg(target_os = "linux"))]
 
 pub mod fs;
+pub mod process;
 pub mod raw;

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

@@ -0,0 +1,154 @@
+//! Linux-specific extensions to primitives in the `std::process` module.
+
+#![unstable(feature = "linux_pidfd", issue = "82971")]
+
+use crate::io::Result;
+use crate::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
+use crate::process;
+#[cfg(not(doc))]
+use crate::sys::fd::FileDesc;
+use crate::sys_common::{AsInner, AsInnerMut, FromInner, IntoInner};
+
+#[cfg(doc)]
+struct FileDesc;
+
+/// This type represents a file descriptor that refers to a process.
+///
+/// A `PidFd` can be obtained by setting the corresponding option on [`Command`]
+/// with [`create_pidfd`]. Subsequently, the created pidfd can be retrieved
+/// from the [`Child`] by calling [`pidfd`] or [`take_pidfd`].
+///
+/// Example:
+/// ```no_run
+/// #![feature(linux_pidfd)]
+/// use std::os::linux::process::{CommandExt, ChildExt};
+/// use std::process::Command;
+///
+/// let mut child = Command::new("echo")
+///     .create_pidfd(true)
+///     .spawn()
+///     .expect("Failed to spawn child");
+///
+/// let pidfd = child
+///     .take_pidfd()
+///     .expect("Failed to retrieve pidfd");
+///
+/// // The file descriptor will be closed when `pidfd` is dropped.
+/// ```
+/// Refer to the man page of [`pidfd_open(2)`] for further details.
+///
+/// [`Command`]: process::Command
+/// [`create_pidfd`]: CommandExt::create_pidfd
+/// [`Child`]: process::Child
+/// [`pidfd`]: fn@ChildExt::pidfd
+/// [`take_pidfd`]: ChildExt::take_pidfd
+/// [`pidfd_open(2)`]: https://man7.org/linux/man-pages/man2/pidfd_open.2.html
+#[derive(Debug)]
+pub struct PidFd {
+    inner: FileDesc,
+}
+
+impl AsInner<FileDesc> for PidFd {
+    fn as_inner(&self) -> &FileDesc {
+        &self.inner
+    }
+}
+
+impl FromInner<FileDesc> for PidFd {
+    fn from_inner(inner: FileDesc) -> PidFd {
+        PidFd { inner }
+    }
+}
+
+impl IntoInner<FileDesc> for PidFd {
+    fn into_inner(self) -> FileDesc {
+        self.inner
+    }
+}
+
+impl AsRawFd for PidFd {
+    fn as_raw_fd(&self) -> RawFd {
+        self.as_inner().raw()
+    }
+}
+
+impl FromRawFd for PidFd {
+    unsafe fn from_raw_fd(fd: RawFd) -> Self {
+        Self::from_inner(FileDesc::new(fd))
+    }
+}
+
+impl IntoRawFd for PidFd {
+    fn into_raw_fd(self) -> RawFd {
+        self.into_inner().into_raw()
+    }
+}
+
+mod private_child_ext {
+    pub trait Sealed {}
+    impl Sealed for crate::process::Child {}
+}
+
+/// Os-specific extensions for [`Child`]
+///
+/// [`Child`]: process::Child
+pub trait ChildExt: private_child_ext::Sealed {
+    /// Obtains a reference to the [`PidFd`] created for this [`Child`], if available.
+    ///
+    /// A pidfd will only be available if its creation was requested with
+    /// [`create_pidfd`] when the corresponding [`Command`] was created.
+    ///
+    /// Even if requested, a pidfd may not be available due to an older
+    /// version of Linux being in use, or if some other error occurred.
+    ///
+    /// [`Command`]: process::Command
+    /// [`create_pidfd`]: CommandExt::create_pidfd
+    /// [`Child`]: process::Child
+    fn pidfd(&self) -> Result<&PidFd>;
+
+    /// Takes ownership of the [`PidFd`] created for this [`Child`], if available.
+    ///
+    /// A pidfd will only be available if its creation was requested with
+    /// [`create_pidfd`] when the corresponding [`Command`] was created.
+    ///
+    /// Even if requested, a pidfd may not be available due to an older
+    /// version of Linux being in use, or if some other error occurred.
+    ///
+    /// [`Command`]: process::Command
+    /// [`create_pidfd`]: CommandExt::create_pidfd
+    /// [`Child`]: process::Child
+    fn take_pidfd(&mut self) -> Result<PidFd>;
+}
+
+mod private_command_ext {
+    pub trait Sealed {}
+    impl Sealed for crate::process::Command {}
+}
+
+/// Os-specific extensions for [`Command`]
+///
+/// [`Command`]: process::Command
+pub trait CommandExt: private_command_ext::Sealed {
+    /// Sets whether a [`PidFd`](struct@PidFd) should be created for the [`Child`]
+    /// spawned by this [`Command`].
+    /// By default, no pidfd will be created.
+    ///
+    /// The pidfd can be retrieved from the child with [`pidfd`] or [`take_pidfd`].
+    ///
+    /// A pidfd will only be created if it is possible to do so
+    /// in a guaranteed race-free manner (e.g. if the `clone3` system call
+    /// is supported). Otherwise, [`pidfd`] will return an error.
+    ///
+    /// [`Command`]: process::Command
+    /// [`Child`]: process::Child
+    /// [`pidfd`]: fn@ChildExt::pidfd
+    /// [`take_pidfd`]: ChildExt::take_pidfd
+    fn create_pidfd(&mut self, val: bool) -> &mut process::Command;
+}
+
+impl CommandExt for process::Command {
+    fn create_pidfd(&mut self, val: bool) -> &mut process::Command {
+        self.as_inner_mut().create_pidfd(val);
+        self
+    }
+}

library/std/src/process.rs

@@ -166,7 +166,7 @@ use crate::sys_common::{AsInner, AsInnerMut, FromInner, IntoInner};
 /// [`wait`]: Child::wait
 #[stable(feature = "process", since = "1.0.0")]
 pub struct Child {
-    handle: imp::Process,
+    pub(crate) handle: imp::Process,
 
     /// The handle for writing to the child's standard input (stdin), if it has
     /// been captured. To avoid partially moving

library/std/src/sys/unix/process/process_common.rs

@@ -79,6 +79,8 @@ pub struct Command {
     stdin: Option<Stdio>,
     stdout: Option<Stdio>,
     stderr: Option<Stdio>,
+    #[cfg(target_os = "linux")]
+    create_pidfd: bool,
 }
 
 // Create a new type for argv, so that we can make it `Send` and `Sync`
@@ -124,6 +126,7 @@ pub enum Stdio {
 }
 
 impl Command {
+    #[cfg(not(target_os = "linux"))]
     pub fn new(program: &OsStr) -> Command {
         let mut saw_nul = false;
         let program = os2c(program, &mut saw_nul);
@@ -144,6 +147,28 @@ impl Command {
         }
     }
 
+    #[cfg(target_os = "linux")]
+    pub fn new(program: &OsStr) -> Command {
+        let mut saw_nul = false;
+        let program = os2c(program, &mut saw_nul);
+        Command {
+            argv: Argv(vec![program.as_ptr(), ptr::null()]),
+            args: vec![program.clone()],
+            program,
+            env: Default::default(),
+            cwd: None,
+            uid: None,
+            gid: None,
+            saw_nul,
+            closures: Vec::new(),
+            groups: None,
+            stdin: None,
+            stdout: None,
+            stderr: None,
+            create_pidfd: false,
+        }
+    }
+
     pub fn set_arg_0(&mut self, arg: &OsStr) {
         // Set a new arg0
         let arg = os2c(arg, &mut self.saw_nul);
@@ -177,6 +202,22 @@ impl Command {
         self.groups = Some(Box::from(groups));
     }
 
+    #[cfg(target_os = "linux")]
+    pub fn create_pidfd(&mut self, val: bool) {
+        self.create_pidfd = val;
+    }
+
+    #[cfg(not(target_os = "linux"))]
+    #[allow(dead_code)]
+    pub fn get_create_pidfd(&self) -> bool {
+        false
+    }
+
+    #[cfg(target_os = "linux")]
+    pub fn get_create_pidfd(&self) -> bool {
+        self.create_pidfd
+    }
+
     pub fn saw_nul(&self) -> bool {
         self.saw_nul
     }