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Auto merge of rust-lang#124101 - the8472:pidfd-methods, r=cuviper
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Add PidFd::{kill, wait, try_wait}

rust-lang#117957 changed `Child` kill/wait/try_wait to use its pidfd instead of the pid, when one is available.
This PR extracts those implementations and makes them available on `PidFd` directly.

The `PidFd` implementations differ significantly from the corresponding `Child` methods:

* the methods can be called after the child has been reaped, which will result in an error but will be safe. This state is not observable in `Child` unless something stole the zombie child
* the `ExitStatus` is not kept, meaning that only the first time a wait succeeds it will be returned
* `wait` does not close stdin
* `wait` only requires `&self` instead of `&mut self` since there is no state to maintain and subsequent calls are safe

Tracking issue: rust-lang#82971
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bors committed Jun 22, 2024
2 parents fcae626 + 8abf149 commit 10e1f5d
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Showing 7 changed files with 265 additions and 124 deletions.
89 changes: 66 additions & 23 deletions library/std/src/os/linux/process.rs
Original file line number Diff line number Diff line change
Expand Up @@ -6,20 +6,20 @@

use crate::io::Result;
use crate::os::unix::io::{AsFd, AsRawFd, BorrowedFd, FromRawFd, IntoRawFd, OwnedFd, RawFd};
use crate::process;
use crate::process::{self, ExitStatus};
use crate::sealed::Sealed;
#[cfg(not(doc))]
use crate::sys::fd::FileDesc;
use crate::sys::{fd::FileDesc, linux::pidfd::PidFd as InnerPidFd};
use crate::sys_common::{AsInner, AsInnerMut, FromInner, IntoInner};

#[cfg(doc)]
struct FileDesc;
struct InnerPidFd;

/// 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`].
/// from the [`Child`] by calling [`pidfd`] or [`into_pidfd`].
///
/// Example:
/// ```no_run
Expand All @@ -33,7 +33,7 @@ struct FileDesc;
/// .expect("Failed to spawn child");
///
/// let pidfd = child
/// .take_pidfd()
/// .into_pidfd()
/// .expect("Failed to retrieve pidfd");
///
/// // The file descriptor will be closed when `pidfd` is dropped.
Expand All @@ -44,66 +44,101 @@ struct FileDesc;
/// [`create_pidfd`]: CommandExt::create_pidfd
/// [`Child`]: process::Child
/// [`pidfd`]: fn@ChildExt::pidfd
/// [`take_pidfd`]: ChildExt::take_pidfd
/// [`into_pidfd`]: ChildExt::into_pidfd
/// [`pidfd_open(2)`]: https://man7.org/linux/man-pages/man2/pidfd_open.2.html
#[derive(Debug)]
#[repr(transparent)]
pub struct PidFd {
inner: FileDesc,
inner: InnerPidFd,
}

impl AsInner<FileDesc> for PidFd {
impl PidFd {
/// Forces the child process to exit.
///
/// Unlike [`Child::kill`] it is possible to attempt to kill
/// reaped children since PidFd does not suffer from pid recycling
/// races. But doing so will return an Error.
///
/// [`Child::kill`]: process::Child::kill
pub fn kill(&self) -> Result<()> {
self.inner.kill()
}

/// Waits for the child to exit completely, returning the status that it exited with.
///
/// Unlike [`Child::wait`] it does not ensure that the stdin handle is closed.
/// Additionally it will not return an `ExitStatus` if the child
/// has already been reaped. Instead an error will be returned.
///
/// [`Child::wait`]: process::Child::wait
pub fn wait(&self) -> Result<ExitStatus> {
self.inner.wait().map(FromInner::from_inner)
}

/// Attempts to collect the exit status of the child if it has already exited.
///
/// Unlike [`Child::try_wait`] this method will return an Error
/// if the child has already been reaped.
///
/// [`Child::try_wait`]: process::Child::try_wait
pub fn try_wait(&self) -> Result<Option<ExitStatus>> {
Ok(self.inner.try_wait()?.map(FromInner::from_inner))
}
}

impl AsInner<InnerPidFd> for PidFd {
#[inline]
fn as_inner(&self) -> &FileDesc {
fn as_inner(&self) -> &InnerPidFd {
&self.inner
}
}

impl FromInner<FileDesc> for PidFd {
fn from_inner(inner: FileDesc) -> PidFd {
impl FromInner<InnerPidFd> for PidFd {
fn from_inner(inner: InnerPidFd) -> PidFd {
PidFd { inner }
}
}

impl IntoInner<FileDesc> for PidFd {
fn into_inner(self) -> FileDesc {
impl IntoInner<InnerPidFd> for PidFd {
fn into_inner(self) -> InnerPidFd {
self.inner
}
}

impl AsRawFd for PidFd {
#[inline]
fn as_raw_fd(&self) -> RawFd {
self.as_inner().as_raw_fd()
self.as_inner().as_inner().as_raw_fd()
}
}

impl FromRawFd for PidFd {
unsafe fn from_raw_fd(fd: RawFd) -> Self {
Self::from_inner(FileDesc::from_raw_fd(fd))
Self::from_inner(InnerPidFd::from_raw_fd(fd))
}
}

impl IntoRawFd for PidFd {
fn into_raw_fd(self) -> RawFd {
self.into_inner().into_raw_fd()
self.into_inner().into_inner().into_raw_fd()
}
}

impl AsFd for PidFd {
fn as_fd(&self) -> BorrowedFd<'_> {
self.as_inner().as_fd()
self.as_inner().as_inner().as_fd()
}
}

impl From<OwnedFd> for PidFd {
fn from(fd: OwnedFd) -> Self {
Self::from_inner(FileDesc::from_inner(fd))
Self::from_inner(InnerPidFd::from_inner(FileDesc::from_inner(fd)))
}
}

impl From<PidFd> for OwnedFd {
fn from(pid_fd: PidFd) -> Self {
pid_fd.into_inner().into_inner()
pid_fd.into_inner().into_inner().into_inner()
}
}

Expand All @@ -124,18 +159,26 @@ pub trait ChildExt: Sealed {
/// [`Child`]: process::Child
fn pidfd(&self) -> Result<&PidFd>;

/// Takes ownership of the [`PidFd`] created for this [`Child`], if available.
/// Returns the [`PidFd`] created for this [`Child`], if available.
/// Otherwise self is returned.
///
/// A pidfd will only be available if its creation was requested with
/// [`create_pidfd`] when the corresponding [`Command`] was created.
///
/// Taking ownership of the PidFd consumes the Child to avoid pid reuse
/// races. Use [`pidfd`] and [`BorrowedFd::try_clone_to_owned`] if
/// you don't want to disassemble the Child yet.
///
/// 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
/// [`pidfd`]: ChildExt::pidfd
/// [`Child`]: process::Child
fn take_pidfd(&mut self) -> Result<PidFd>;
fn into_pidfd(self) -> crate::result::Result<PidFd, Self>
where
Self: Sized;
}

/// Os-specific extensions for [`Command`]
Expand All @@ -146,7 +189,7 @@ pub trait CommandExt: Sealed {
/// spawned by this [`Command`].
/// By default, no pidfd will be created.
///
/// The pidfd can be retrieved from the child with [`pidfd`] or [`take_pidfd`].
/// The pidfd can be retrieved from the child with [`pidfd`] or [`into_pidfd`].
///
/// A pidfd will only be created if it is possible to do so
/// in a guaranteed race-free manner. Otherwise, [`pidfd`] will return an error.
Expand All @@ -160,7 +203,7 @@ pub trait CommandExt: Sealed {
/// [`Command`]: process::Command
/// [`Child`]: process::Child
/// [`pidfd`]: fn@ChildExt::pidfd
/// [`take_pidfd`]: ChildExt::take_pidfd
/// [`into_pidfd`]: ChildExt::into_pidfd
fn create_pidfd(&mut self, val: bool) -> &mut process::Command;
}

Expand Down
1 change: 1 addition & 0 deletions library/std/src/sys/pal/unix/linux/mod.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
pub mod pidfd;
76 changes: 76 additions & 0 deletions library/std/src/sys/pal/unix/linux/pidfd.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,76 @@
use crate::io;
use crate::os::fd::{AsRawFd, FromRawFd, RawFd};
use crate::sys::cvt;
use crate::sys::pal::unix::fd::FileDesc;
use crate::sys::process::ExitStatus;
use crate::sys_common::{AsInner, FromInner, IntoInner};

#[cfg(test)]
mod tests;

#[derive(Debug)]
pub(crate) struct PidFd(FileDesc);

impl PidFd {
pub fn kill(&self) -> io::Result<()> {
return cvt(unsafe {
libc::syscall(
libc::SYS_pidfd_send_signal,
self.0.as_raw_fd(),
libc::SIGKILL,
crate::ptr::null::<()>(),
0,
)
})
.map(drop);
}

pub fn wait(&self) -> io::Result<ExitStatus> {
let mut siginfo: libc::siginfo_t = unsafe { crate::mem::zeroed() };
cvt(unsafe {
libc::waitid(libc::P_PIDFD, self.0.as_raw_fd() as u32, &mut siginfo, libc::WEXITED)
})?;
return Ok(ExitStatus::from_waitid_siginfo(siginfo));
}

pub fn try_wait(&self) -> io::Result<Option<ExitStatus>> {
let mut siginfo: libc::siginfo_t = unsafe { crate::mem::zeroed() };

cvt(unsafe {
libc::waitid(
libc::P_PIDFD,
self.0.as_raw_fd() as u32,
&mut siginfo,
libc::WEXITED | libc::WNOHANG,
)
})?;
if unsafe { siginfo.si_pid() } == 0 {
return Ok(None);
}
return Ok(Some(ExitStatus::from_waitid_siginfo(siginfo)));
}
}

impl AsInner<FileDesc> for PidFd {
fn as_inner(&self) -> &FileDesc {
&self.0
}
}

impl IntoInner<FileDesc> for PidFd {
fn into_inner(self) -> FileDesc {
self.0
}
}

impl FromInner<FileDesc> for PidFd {
fn from_inner(inner: FileDesc) -> Self {
Self(inner)
}
}

impl FromRawFd for PidFd {
unsafe fn from_raw_fd(fd: RawFd) -> Self {
Self(FileDesc::from_raw_fd(fd))
}
}
87 changes: 87 additions & 0 deletions library/std/src/sys/pal/unix/linux/pidfd/tests.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,87 @@
use crate::assert_matches::assert_matches;
use crate::os::fd::{AsRawFd, RawFd};
use crate::os::linux::process::{ChildExt, CommandExt};
use crate::os::unix::process::ExitStatusExt;
use crate::process::Command;

#[test]
fn test_command_pidfd() {
let pidfd_open_available = probe_pidfd_support();

// always exercise creation attempts
let mut child = Command::new("false").create_pidfd(true).spawn().unwrap();

// but only check if we know that the kernel supports pidfds.
// We don't assert the precise value, since the standard library
// might have opened other file descriptors before our code runs.
if pidfd_open_available {
assert!(child.pidfd().is_ok());
}
if let Ok(pidfd) = child.pidfd() {
let flags = super::cvt(unsafe { libc::fcntl(pidfd.as_raw_fd(), libc::F_GETFD) }).unwrap();
assert!(flags & libc::FD_CLOEXEC != 0);
}
let status = child.wait().expect("error waiting on pidfd");
assert_eq!(status.code(), Some(1));

let mut child = Command::new("sleep").arg("1000").create_pidfd(true).spawn().unwrap();
assert_matches!(child.try_wait(), Ok(None));
child.kill().expect("failed to kill child");
let status = child.wait().expect("error waiting on pidfd");
assert_eq!(status.signal(), Some(libc::SIGKILL));

let _ = Command::new("echo")
.create_pidfd(false)
.spawn()
.unwrap()
.pidfd()
.expect_err("pidfd should not have been created when create_pid(false) is set");

let _ = Command::new("echo")
.spawn()
.unwrap()
.pidfd()
.expect_err("pidfd should not have been created");
}

#[test]
fn test_pidfd() {
if !probe_pidfd_support() {
return;
}

let child = Command::new("sleep")
.arg("1000")
.create_pidfd(true)
.spawn()
.expect("executing 'sleep' failed");

let fd = child.into_pidfd().unwrap();

assert_matches!(fd.try_wait(), Ok(None));
fd.kill().expect("kill failed");
fd.kill().expect("sending kill twice failed");
let status = fd.wait().expect("1st wait failed");
assert_eq!(status.signal(), Some(libc::SIGKILL));

// Trying to wait again for a reaped child is safe since there's no pid-recycling race.
// But doing so will return an error.
let res = fd.wait();
assert_matches!(res, Err(e) if e.raw_os_error() == Some(libc::ECHILD));

// Ditto for additional attempts to kill an already-dead child.
let res = fd.kill();
assert_matches!(res, Err(e) if e.raw_os_error() == Some(libc::ESRCH));
}

fn probe_pidfd_support() -> bool {
// pidfds require the pidfd_open syscall
let our_pid = crate::process::id();
let pidfd = unsafe { libc::syscall(libc::SYS_pidfd_open, our_pid, 0) };
if pidfd >= 0 {
unsafe { libc::close(pidfd as RawFd) };
true
} else {
false
}
}
2 changes: 2 additions & 0 deletions library/std/src/sys/pal/unix/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,8 @@ pub mod io;
pub mod kernel_copy;
#[cfg(target_os = "l4re")]
mod l4re;
#[cfg(target_os = "linux")]
pub mod linux;
#[cfg(not(target_os = "l4re"))]
pub mod net;
#[cfg(target_os = "l4re")]
Expand Down
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