Repository: rust
Issue: Tracking Issue for linux_pidfd
Labels: T-libs-api,C-tracking-issue,F-linux_pidfd

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Feature gate: `#![feature(linux_pidfd)]`

This is a tracking issue for Linux-specific extension methods allowing to obtain [process file descriptors](https://man.archlinux.org/man/pidfd_open.2.en) for processes spawned with the standard Command API.

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### Public API

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```rust
// std::os::linux::process

pub struct PidFd;

impl PidFd {
   pub fn kill(&self) -> Result<()> {...}
   pub fn wait(&self) -> Result<ExitStatus> {...}
   pub fn try_wait(&self) -> Result<Option<ExitStatus>> {...}
}

impl AsRawFd for PidFd {}
impl FromRawFd for PidFd {}
impl IntoRawFd for PidFd {}

// sealed trait, implemented for std::process::Child
pub trait ChildExt {
    fn pidfd(&self) -> Result<&PidFd>;
    fn into_pidfd(self) -> Result<PidFd, Self>;
}

// sealed trait, implemented for std::process::Command
pub trait CommandExt {
    fn create_pidfd(&mut self, val: bool) -> &mut process::Command;
}
```

### Steps / History

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- [x] Implementation: #77168, #81825
- [x] replaced use of clone3: #113939
- [x] #117957
- [x] #124101
- [x] #126827
- [ ] Final commenting period (FCP)
- [ ] Stabilization PR

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### Unresolved Questions

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- [x] How can we properly open a pidfd for the new process? The current implementation using a manual `clone3` means we can't safely call libc in the child: https://github.com/rust-lang/rust/issues/89522#issuecomment-944636262
  - [`pidfd_open`](https://man7.org/linux/man-pages/man2/pidfd_open.2.html) may work, but it has conditions on avoiding pid-recycling races.
- [ ] should `Child::pidfd(&self)` be removed? It can lead to `Child::wait` returning errors instead of a saved exit status if `PidFd::wait` obtains the exit status first, which may be surprising behavior.