process: add Child::{wait,try_wait} (#2796)

* add Child::try_wait to mirror the std API
* replace Future impl on Child with `.wait()` method to bring our
  APIs closer to those in std and it allow us to
  internally fuse the future so that repeated calls to `wait` result in
  the same value (similar to std) without forcing the caller to fuse the
  outer future
* Also change `Child::id` to return an Option result to avoid
  allowing the caller to accidentally use the pid on Unix systems after
  the child has been reaped
* Also remove deprecated Child methods

1 files changed, 4 insertions, 2 deletions

diff --git a/tokio/tests/process_issue_42.rs b/tokio/tests/process_issue_42.rs
index aa70af3b..569c122e 100644
--- a/tokio/tests/process_issue_42.rs
+++ b/tokio/tests/process_issue_42.rs
@@ -18,14 +18,16 @@ async fn issue_42() {
     let join_handles = (0..10usize).map(|_| {
         task::spawn(async {
             let processes = (0..10usize).map(|i| {
-                Command::new("echo")
+                let mut child = Command::new("echo")
                     .arg(format!("I am spawned process #{}", i))
                     .stdin(Stdio::null())
                     .stdout(Stdio::null())
                     .stderr(Stdio::null())
                     .kill_on_drop(true)
                     .spawn()
-                    .unwrap()
+                    .unwrap();
+
+                async move { child.wait().await }
             });
 
             join_all(processes).await;