feat: pid_open

Author: JonathanWoollett-Light

CHANGELOG.md

@@ -13,6 +13,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
   ([#1912](https://github.com/nix-rust/nix/pull/1912))
 - Added `pidfd_getfd` on Linux.
   ([#1868](https://github.com/nix-rust/nix/pull/1868))
+- Added `pid_open` on Linux.
+  ([#1868](https://github.com/nix-rust/nix/pull/1868))
 
 ### Changed
 

src/sys/mod.rs

@@ -227,6 +227,6 @@ feature! {
     pub mod timer;
 }
 
-#[cfg(target_os = "linux")]
+#[cfg(all(target_os = "linux", feature = "process"))]
 /// pidfd related functionality
 pub mod pidfd;

src/sys/pidfd.rs

@@ -1,4 +1,5 @@
 use crate::errno::Errno;
+use crate::unistd::Pid;
 use crate::Result;
 use std::convert::TryFrom;
 use std::os::unix::io::{AsRawFd, FromRawFd, OwnedFd};
@@ -40,4 +41,36 @@ pub fn pidfd_getfd<PFd: AsRawFd, TFd: AsRawFd>(
         }
         _ => unreachable!(),
     }
-}
+}
+
+/// Creates a file descriptor that refers to the process whose PID is specified in `pid`.  The file
+/// descriptor is returned as the function result; the close-on-exec flag is set on the file
+/// descriptor.
+///
+/// If `nonblock == true` returns a nonblocking file descriptor.  If the process
+/// referred to by the file descriptor has not yet terminated,
+/// then an attempt to wait on the file descriptor using
+/// waitid(2) will immediately return the error EAGAIN rather
+/// than blocking.
+pub fn pid_open(pid: Pid, nonblock: bool) -> Result<OwnedFd> {
+    #[allow(clippy::useless_conversion)] // Not useless on all OSes
+    match unsafe {
+        libc::syscall(
+            libc::SYS_pidfd_open,
+            pid,
+            if nonblock {
+                // MUSL is missing the `PIDFD_NONBLOCK` alias, so the underlying `O_NONBLOCK` needs
+                // to be used for compatibility.
+                libc::PIDFD_NONBLOCK
+            } else {
+                0
+            },
+        )
+    } {
+        -1 => Err(Errno::last()),
+        fd @ 0.. => {
+            Ok(unsafe { OwnedFd::from_raw_fd(i32::try_from(fd).unwrap()) })
+        }
+        _ => unreachable!(),
+    }
+}