Add a convenience method .pid() for WaitStatus.

CHANGELOG.md

@@ -12,6 +12,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ([#672](https://github.com/nix-rust/nix/pull/672))
 - Added protocol families in `AddressFamily` enum.
   ([#647](https://github.com/nix-rust/nix/pull/647))
+- Added the `pid()` method to `WaitStatus` for extracting the PID.
+  ([#722](https://github.com/nix-rust/nix/pull/722))
 
 ### Changed
 - Renamed existing `ptrace` wrappers to encourage namespacing ([#692](https://github.com/nix-rust/nix/pull/692))

src/sys/wait.rs

@@ -93,6 +93,25 @@ pub enum WaitStatus {
     StillAlive
 }
 
+impl WaitStatus {
+    /// Extracts the PID from the WaitStatus unless it equals StillAlive.
+    pub fn pid(&self) -> Option<Pid> {
+        use self::WaitStatus::*;
+        match *self {
+            Exited(p, _) => Some(p),
+            Signaled(p, _, _) => Some(p),
+            Stopped(p, _) => Some(p),
+            Continued(p) => Some(p),
+            StillAlive => None,
+
+            #[cfg(any(target_os = "linux", target_os = "android"))]
+            PtraceEvent(p, _, _) => Some(p),
+            #[cfg(any(target_os = "linux", target_os = "android"))]
+            PtraceSyscall(p) => Some(p),
+        }
+    }
+}
+
 #[cfg(any(target_os = "linux",
           target_os = "android"))]
 mod status {

test/sys/test_wait.rs

@@ -37,6 +37,19 @@ fn test_wait_exit() {
     }
 }
 
+#[test]
+fn test_waitstatus_pid() {
+    let _m = ::FORK_MTX.lock().expect("Mutex got poisoned by another test");
+
+    match fork().unwrap() {
+        Child => unsafe { _exit(0) },
+        Parent { child } => {
+            let status = waitpid(child, None).unwrap();
+            assert_eq!(status.pid(), Some(child));
+        }
+    }
+}
+
 #[cfg(any(target_os = "linux", target_os = "android"))]
 // FIXME: qemu-user doesn't implement ptrace on most arches
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
@@ -47,7 +60,7 @@ mod ptrace {
     use nix::sys::wait::*;
     use nix::unistd::*;
     use nix::unistd::ForkResult::*;
-    use std::{ptr, process};
+    use std::ptr;
     use libc::_exit;
 
     fn ptrace_child() -> ! {