Add PidFd type and seal traits

Improve docs

Split do_fork into two

Make do_fork unsafe

Add target attribute to create_pidfd field in Command

Add method to get create_pidfd value

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

@@ -2,40 +2,142 @@
 
 #![unstable(feature = "linux_pidfd", issue = "none")]
 
-use crate::process;
-use crate::sys_common::AsInnerMut;
 use crate::io::Result;
+use crate::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
+use crate::process;
+use crate::sys::fd::FileDesc;
+use crate::sys_common::{AsInner, AsInnerMut, FromInner, IntoInner};
 
-/// Os-specific extensions to [`process::Child`]
+/// This type represents a file descriptor that refers to a process.
 ///
-/// [`process::Child`]: crate::process::Child
-pub trait ChildExt {
-    /// Obtains the pidfd created for this child process, if available.
-    ///
-    /// A pidfd will only ever be available if `create_pidfd(true)` was called
-    /// when the corresponding `Command` was created.
-    ///
-    /// Even if `create_pidfd(true)` is called, a pidfd may not be available
-    /// due to an older version of Linux being in use, or if
-    /// some other error occured.
-    ///
-    /// See `man pidfd_open` for more details about pidfds.
-    fn pidfd(&self) -> Result<i32>;
+/// 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
+#[derive(Debug)]
+pub struct PidFd {
+    inner: FileDesc,
 }
 
-/// Os-specific extensions to [`process::Command`]
+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`]
 ///
-/// [`process::Command`]: crate::process::Command
-pub trait CommandExt {
-    /// Sets whether or this `Command` will attempt to create a pidfd
-    /// for the child. If this method is never called, a pidfd will
-    /// not be crated.
+/// [`Child`]: process::Child
+pub trait ChildExt: private_child_ext::Sealed {
+    /// Obtains a reference to the [`PidFd`] created for this [`Child`], if available.
     ///
-    /// The pidfd can be retrieved from the child via [`ChildExt::pidfd`]
+    /// 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, [`ChildExit::pidfd`] will return an error.
+    /// 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;
 }