process: add Child::raw_handle() on windows (#3998)

Fixes #3987

tokio/src/process/mod.rs

@@ -199,6 +199,8 @@ use std::io;
 #[cfg(unix)]
 use std::os::unix::process::CommandExt;
 #[cfg(windows)]
+use std::os::windows::io::{AsRawHandle, RawHandle};
+#[cfg(windows)]
 use std::os::windows::process::CommandExt;
 use std::path::Path;
 use std::pin::Pin;
@@ -952,6 +954,16 @@ impl Child {
         }
     }
 
+    /// Extracts the raw handle of the process associated with this child while
+    /// it is still running. Returns `None` if the child has exited.
+    #[cfg(windows)]
+    pub fn raw_handle(&self) -> Option<RawHandle> {
+        match &self.child {
+            FusedChild::Child(c) => Some(c.inner.as_raw_handle()),
+            FusedChild::Done(_) => None,
+        }
+    }
+
     /// Attempts to force the child to exit, but does not wait for the request
     /// to take effect.
     ///

tokio/src/process/windows.rs

@@ -24,7 +24,7 @@ use mio::windows::NamedPipe;
 use std::fmt;
 use std::future::Future;
 use std::io;
-use std::os::windows::prelude::{AsRawHandle, FromRawHandle, IntoRawHandle};
+use std::os::windows::prelude::{AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle};
 use std::pin::Pin;
 use std::process::Stdio;
 use std::process::{Child as StdChild, Command as StdCommand, ExitStatus};
@@ -144,6 +144,12 @@ impl Future for Child {
     }
 }
 
+impl AsRawHandle for Child {
+    fn as_raw_handle(&self) -> RawHandle {
+        self.child.as_raw_handle()
+    }
+}
+
 impl Drop for Waiting {
     fn drop(&mut self) {
         unsafe {

tokio/tests/process_raw_handle.rs

@@ -0,0 +1,23 @@
+#![warn(rust_2018_idioms)]
+#![cfg(feature = "full")]
+#![cfg(windows)]
+
+use tokio::process::Command;
+use winapi::um::processthreadsapi::GetProcessId;
+
+#[tokio::test]
+async fn obtain_raw_handle() {
+    let mut cmd = Command::new("cmd");
+    cmd.kill_on_drop(true);
+    cmd.arg("/c");
+    cmd.arg("pause");
+
+    let child = cmd.spawn().unwrap();
+
+    let orig_id = child.id().expect("missing id");
+    assert!(orig_id > 0);
+
+    let handle = child.raw_handle().expect("process stopped");
+    let handled_id = unsafe { GetProcessId(handle as _) };
+    assert_eq!(handled_id, orig_id);
+}