rt_common.rs

#![allow(clippy::needless_range_loop)]
#![warn(rust_2018_idioms)]
#![cfg(feature = "full")]

// Tests to run on both current-thread & thread-pool runtime variants.

macro_rules! rt_test {
    ($($t:tt)*) => {
        mod current_thread_scheduler {
            $($t)*

            fn rt() -> Arc<Runtime> {
                tokio::runtime::Builder::new_current_thread()
                    .enable_all()
                    .build()
                    .unwrap()
                    .into()
            }
        }

        mod threaded_scheduler_4_threads {
            $($t)*

            fn rt() -> Arc<Runtime> {
                tokio::runtime::Builder::new_multi_thread()
                    .worker_threads(4)
                    .enable_all()
                    .build()
                    .unwrap()
                    .into()
            }
        }

        mod threaded_scheduler_1_thread {
            $($t)*

            fn rt() -> Arc<Runtime> {
                tokio::runtime::Builder::new_multi_thread()
                    .worker_threads(1)
                    .enable_all()
                    .build()
                    .unwrap()
                    .into()
            }
        }
    }
}

#[test]
fn send_sync_bound() {
    use tokio::runtime::Runtime;
    fn is_send<T: Send + Sync>() {}

    is_send::<Runtime>();
}

rt_test! {
    use tokio::net::{TcpListener, TcpStream, UdpSocket};
    use tokio::io::{AsyncReadExt, AsyncWriteExt};
    use tokio::runtime::Runtime;
    use tokio::sync::oneshot;
    use tokio::{task, time};
    use tokio_test::{assert_err, assert_ok};

    use futures::future::poll_fn;
    use std::future::Future;
    use std::pin::Pin;
    use std::sync::{mpsc, Arc};
    use std::task::{Context, Poll};
    use std::thread;
    use std::time::{Duration, Instant};

    #[test]
    fn block_on_sync() {
        let rt = rt();

        let mut win = false;
        rt.block_on(async {
            win = true;
        });

        assert!(win);
    }


    #[test]
    fn block_on_async() {
        let rt = rt();

        let out = rt.block_on(async {
            let (tx, rx) = oneshot::channel();

            thread::spawn(move || {
                thread::sleep(Duration::from_millis(50));
                tx.send("ZOMG").unwrap();
            });

            assert_ok!(rx.await)
        });

        assert_eq!(out, "ZOMG");
    }

    #[test]
    fn spawn_one_bg() {
        let rt = rt();

        let out = rt.block_on(async {
            let (tx, rx) = oneshot::channel();

            tokio::spawn(async move {
                tx.send("ZOMG").unwrap();
            });

            assert_ok!(rx.await)
        });

        assert_eq!(out, "ZOMG");
    }

    #[test]
    fn spawn_one_join() {
        let rt = rt();

        let out = rt.block_on(async {
            let (tx, rx) = oneshot::channel();

            let handle = tokio::spawn(async move {
                tx.send("ZOMG").unwrap();
                "DONE"
            });

            let msg = assert_ok!(rx.await);

            let out = assert_ok!(handle.await);
            assert_eq!(out, "DONE");

            msg
        });

        assert_eq!(out, "ZOMG");
    }

    #[test]
    fn spawn_two() {
        let rt = rt();

        let out = rt.block_on(async {
            let (tx1, rx1) = oneshot::channel();
            let (tx2, rx2) = oneshot::channel();

            tokio::spawn(async move {
                assert_ok!(tx1.send("ZOMG"));
            });

            tokio::spawn(async move {
                let msg = assert_ok!(rx1.await);
                assert_ok!(tx2.send(msg));
            });

            assert_ok!(rx2.await)
        });

        assert_eq!(out, "ZOMG");
    }

    #[test]
    fn spawn_many_from_block_on() {
        use tokio::sync::mpsc;

        const ITER: usize = 200;

        let rt = rt();

        let out = rt.block_on(async {
            let (done_tx, mut done_rx) = mpsc::unbounded_channel();

            let mut txs = (0..ITER)
                .map(|i| {
                    let (tx, rx) = oneshot::channel();
                    let done_tx = done_tx.clone();

                    tokio::spawn(async move {
                        let msg = assert_ok!(rx.await);
                        assert_eq!(i, msg);
                        assert_ok!(done_tx.send(msg));
                    });

                    tx
                })
                .collect::<Vec<_>>();

            drop(done_tx);

            thread::spawn(move || {
                for (i, tx) in txs.drain(..).enumerate() {
                    assert_ok!(tx.send(i));
                }
            });

            let mut out = vec![];
            while let Some(i) = done_rx.recv().await {
                out.push(i);
            }

            out.sort_unstable();
            out
        });

        assert_eq!(ITER, out.len());

        for i in 0..ITER {
            assert_eq!(i, out[i]);
        }
    }

    #[test]
    fn spawn_many_from_task() {
        use tokio::sync::mpsc;

        const ITER: usize = 500;

        let rt = rt();

        let out = rt.block_on(async {
            tokio::spawn(async move {
                let (done_tx, mut done_rx) = mpsc::unbounded_channel();

                /*
                for _ in 0..100 {
                    tokio::spawn(async move { });
                }

                tokio::task::yield_now().await;
                */

                let mut txs = (0..ITER)
                    .map(|i| {
                        let (tx, rx) = oneshot::channel();
                        let done_tx = done_tx.clone();

                        tokio::spawn(async move {
                            let msg = assert_ok!(rx.await);
                            assert_eq!(i, msg);
                            assert_ok!(done_tx.send(msg));
                        });

                        tx
                    })
                    .collect::<Vec<_>>();

                drop(done_tx);

                thread::spawn(move || {
                    for (i, tx) in txs.drain(..).enumerate() {
                        assert_ok!(tx.send(i));
                    }
                });

                let mut out = vec![];
                while let Some(i) = done_rx.recv().await {
                    out.push(i);
                }

                out.sort_unstable();
                out
            }).await.unwrap()
        });

        assert_eq!(ITER, out.len());

        for i in 0..ITER {
            assert_eq!(i, out[i]);
        }
    }

    #[test]
    fn spawn_await_chain() {
        let rt = rt();

        let out = rt.block_on(async {
            assert_ok!(tokio::spawn(async {
                assert_ok!(tokio::spawn(async {
                    "hello"
                }).await)
            }).await)
        });

        assert_eq!(out, "hello");
    }

    #[test]
    fn outstanding_tasks_dropped() {
        let rt = rt();

        let cnt = Arc::new(());

        rt.block_on(async {
            let cnt = cnt.clone();

            tokio::spawn(poll_fn(move |_| {
                assert_eq!(2, Arc::strong_count(&cnt));
                Poll::<()>::Pending
            }));
        });

        assert_eq!(2, Arc::strong_count(&cnt));

        drop(rt);

        assert_eq!(1, Arc::strong_count(&cnt));
    }

    #[test]
    #[should_panic]
    fn nested_rt() {
        let rt1 = rt();
        let rt2 = rt();

        rt1.block_on(async { rt2.block_on(async { "hello" }) });
    }

    #[test]
    fn create_rt_in_block_on() {
        let rt1 = rt();
        let rt2 = rt1.block_on(async { rt() });
        let out = rt2.block_on(async { "ZOMG" });

        assert_eq!(out, "ZOMG");
    }

    #[test]
    fn complete_block_on_under_load() {
        let rt = rt();

        rt.block_on(async {
            let (tx, rx) = oneshot::channel();

            // Spin hard
            tokio::spawn(async {
                loop {
                    yield_once().await;
                }
            });

            thread::spawn(move || {
                thread::sleep(Duration::from_millis(50));
                assert_ok!(tx.send(()));
            });

            assert_ok!(rx.await);
        });
    }

    #[test]
    fn complete_task_under_load() {
        let rt = rt();

        rt.block_on(async {
            let (tx1, rx1) = oneshot::channel();
            let (tx2, rx2) = oneshot::channel();

            // Spin hard
            tokio::spawn(async {
                loop {
                    yield_once().await;
                }
            });

            thread::spawn(move || {
                thread::sleep(Duration::from_millis(50));
                assert_ok!(tx1.send(()));
            });

            tokio::spawn(async move {
                assert_ok!(rx1.await);
                assert_ok!(tx2.send(()));
            });

            assert_ok!(rx2.await);
        });
    }

    #[test]
    fn spawn_from_other_thread_idle() {
        let rt = rt();
        let handle = rt.clone();

        let (tx, rx) = oneshot::channel();

        thread::spawn(move || {
            thread::sleep(Duration::from_millis(50));

            handle.spawn(async move {
                assert_ok!(tx.send(()));
            });
        });

        rt.block_on(async move {
            assert_ok!(rx.await);
        });
    }

    #[test]
    fn spawn_from_other_thread_under_load() {
        let rt = rt();
        let handle = rt.clone();

        let (tx, rx) = oneshot::channel();

        thread::spawn(move || {
            handle.spawn(async move {
                assert_ok!(tx.send(()));
            });
        });

        rt.block_on(async move {
            // Spin hard
            tokio::spawn(async {
                loop {
                    yield_once().await;
                }
            });

            assert_ok!(rx.await);
        });
    }

    #[test]
    fn sleep_at_root() {
        let rt = rt();

        let now = Instant::now();
        let dur = Duration::from_millis(50);

        rt.block_on(async move {
            time::sleep(dur).await;
        });

        assert!(now.elapsed() >= dur);
    }

    #[test]
    fn sleep_in_spawn() {
        let rt = rt();

        let now = Instant::now();
        let dur = Duration::from_millis(50);

        rt.block_on(async move {
            let (tx, rx) = oneshot::channel();

            tokio::spawn(async move {
                time::sleep(dur).await;
                assert_ok!(tx.send(()));
            });

            assert_ok!(rx.await);
        });

        assert!(now.elapsed() >= dur);
    }

    #[test]
    fn block_on_socket() {
        let rt = rt();

        rt.block_on(async move {
            let (tx, rx) = oneshot::channel();

            let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
            let addr = listener.local_addr().unwrap();

            tokio::spawn(async move {
                let _ = listener.accept().await;
                tx.send(()).unwrap();
            });

            TcpStream::connect(&addr).await.unwrap();
            rx.await.unwrap();
        });
    }

    #[test]
    fn spawn_from_blocking() {
        let rt = rt();

        let out = rt.block_on(async move {
            let inner = assert_ok!(tokio::task::spawn_blocking(|| {
                tokio::spawn(async move { "hello" })
            }).await);

            assert_ok!(inner.await)
        });

        assert_eq!(out, "hello")
    }

    #[test]
    fn spawn_blocking_from_blocking() {
        let rt = rt();

        let out = rt.block_on(async move {
            let inner = assert_ok!(tokio::task::spawn_blocking(|| {
                tokio::task::spawn_blocking(|| "hello")
            }).await);

            assert_ok!(inner.await)
        });

        assert_eq!(out, "hello")
    }

    #[test]
    fn sleep_from_blocking() {
        let rt = rt();

        rt.block_on(async move {
            assert_ok!(tokio::task::spawn_blocking(|| {
                let now = std::time::Instant::now();
                let dur = Duration::from_millis(1);

                // use the futures' block_on fn to make sure we aren't setting
                // any Tokio context
                futures::executor::block_on(async {
                    tokio::time::sleep(dur).await;
                });

                assert!(now.elapsed() >= dur);
            }).await);
        });
    }

    #[test]
    fn socket_from_blocking() {
        let rt = rt();

        rt.block_on(async move {
            let listener = assert_ok!(TcpListener::bind("127.0.0.1:0").await);
            let addr = assert_ok!(listener.local_addr());

            let peer = tokio::task::spawn_blocking(move || {
                // use the futures' block_on fn to make sure we aren't setting
                // any Tokio context
                futures::executor::block_on(async {
                    assert_ok!(TcpStream::connect(addr).await);
                });
            });

            // Wait for the client to connect
            let _ = assert_ok!(listener.accept().await);

            assert_ok!(peer.await);
        });
    }

    #[test]
    fn always_active_parker() {
        // This test it to show that we will always have
        // an active parker even if we call block_on concurrently

        let rt = rt();
        let rt2 = rt.clone();

        let (tx1, rx1) = oneshot::channel();
        let (tx2, rx2) = oneshot::channel();

        let jh1 = thread::spawn(move || {
                rt.block_on(async move {
                    rx2.await.unwrap();
                    time::sleep(Duration::from_millis(5)).await;
                    tx1.send(()).unwrap();
                });
        });

        let jh2 = thread::spawn(move || {
            rt2.block_on(async move {
                tx2.send(()).unwrap();
                time::sleep(Duration::from_millis(5)).await;
                rx1.await.unwrap();
                time::sleep(Duration::from_millis(5)).await;
            });
        });

        jh1.join().unwrap();
        jh2.join().unwrap();
    }

    #[test]
    // IOCP requires setting the "max thread" concurrency value. The sane,
    // default, is to set this to the number of cores. Threads that poll I/O
    // become associated with the IOCP handle. Once those threads sleep for any
    // reason (mutex), they yield their ownership.
    //
    // This test hits an edge case on windows where more threads than cores are
    // created, none of those threads ever yield due to being at capacity, so
    // IOCP gets "starved".
    //
    // For now, this is a very edge case that is probably not a real production
    // concern. There also isn't a great/obvious solution to take. For now, the
    // test is disabled.
    #[cfg(not(windows))]
    fn io_driver_called_when_under_load() {
        let rt = rt();

        // Create a lot of constant load. The scheduler will always be busy.
        for _ in 0..100 {
            rt.spawn(async {
                loop {
                    tokio::task::yield_now().await;
                }
            });
        }

        // Do some I/O work
        rt.block_on(async {
            let listener = assert_ok!(TcpListener::bind("127.0.0.1:0").await);
            let addr = assert_ok!(listener.local_addr());

            let srv = tokio::spawn(async move {
                let (mut stream, _) = assert_ok!(listener.accept().await);
                assert_ok!(stream.write_all(b"hello world").await);
            });

            let cli = tokio::spawn(async move {
                let mut stream = assert_ok!(TcpStream::connect(addr).await);
                let mut dst = vec![0; 11];

                assert_ok!(stream.read_exact(&mut dst).await);
                assert_eq!(dst, b"hello world");
            });

            assert_ok!(srv.await);
            assert_ok!(cli.await);
        });
    }

    #[test]
    fn client_server_block_on() {
        let rt = rt();
        let (tx, rx) = mpsc::channel();

        rt.block_on(async move { client_server(tx).await });

        assert_ok!(rx.try_recv());
        assert_err!(rx.try_recv());
    }

    #[test]
    fn panic_in_task() {
        let rt = rt();
        let (tx, rx) = oneshot::channel();

        struct Boom(Option<oneshot::Sender<()>>);

        impl Future for Boom {
            type Output = ();

            fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<()> {
                panic!();
            }
        }

        impl Drop for Boom {
            fn drop(&mut self) {
                assert!(std::thread::panicking());
                self.0.take().unwrap().send(()).unwrap();
            }
        }

        rt.spawn(Boom(Some(tx)));
        assert_ok!(rt.block_on(rx));
    }

    #[test]
    #[should_panic]
    fn panic_in_block_on() {
        let rt = rt();
        rt.block_on(async { panic!() });
    }

    async fn yield_once() {
        let mut yielded = false;
        poll_fn(|cx| {
            if yielded {
                Poll::Ready(())
            } else {
                yielded = true;
                cx.waker().wake_by_ref();
                Poll::Pending
            }
        })
        .await
    }

    #[test]
    fn enter_and_spawn() {
        let rt = rt();
        let handle = {
            let _enter = rt.enter();
            tokio::spawn(async {})
        };

        assert_ok!(rt.block_on(handle));
    }

    #[test]
    fn eagerly_drops_futures_on_shutdown() {
        use std::sync::mpsc;

        struct Never {
            drop_tx: mpsc::Sender<()>,
        }

        impl Future for Never {
            type Output = ();

            fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<()> {
                Poll::Pending
            }
        }

        impl Drop for Never {
            fn drop(&mut self) {
                self.drop_tx.send(()).unwrap();
            }
        }

        let rt = rt();

        let (drop_tx, drop_rx) = mpsc::channel();
        let (run_tx, run_rx) = oneshot::channel();

        rt.block_on(async move {
            tokio::spawn(async move {
                assert_ok!(run_tx.send(()));

                Never { drop_tx }.await
            });

            assert_ok!(run_rx.await);
        });

        drop(rt);

        assert_ok!(drop_rx.recv());
    }

    #[test]
    fn wake_while_rt_is_dropping() {
        use tokio::task;

        struct OnDrop<F: FnMut()>(F);

        impl<F: FnMut()> Drop for OnDrop<F> {
            fn drop(&mut self) {
                (self.0)()
            }
        }

        let (tx1, rx1) = oneshot::channel();
        let (tx2, rx2) = oneshot::channel();
        let (tx3, rx3) = oneshot::channel();

        let rt = rt();

        let h1 = rt.clone();

        rt.spawn(async move {
            // Ensure a waker gets stored in oneshot 1.
            let _ = rx1.await;
            tx3.send(()).unwrap();
        });

        rt.spawn(async move {
            // When this task is dropped, we'll be "closing remotes".
            // We spawn a new task that owns the `tx1`, to move its Drop
            // out of here.
            //
            // Importantly, the oneshot 1 has a waker already stored, so
            // the eventual drop here will try to re-schedule again.
            let mut opt_tx1 = Some(tx1);
            let _d = OnDrop(move || {
                let tx1 = opt_tx1.take().unwrap();
                h1.spawn(async move {
                    tx1.send(()).unwrap();
                });
            });
            let _ = rx2.await;
        });

        rt.spawn(async move {
            let _ = rx3.await;
            // We'll never get here, but once task 3 drops, this will
            // force task 2 to re-schedule since it's waiting on oneshot 2.
            tx2.send(()).unwrap();
        });

        // Tick the loop
        rt.block_on(async {
            task::yield_now().await;
        });

        // Drop the rt
        drop(rt);
    }

    #[test]
    fn io_notify_while_shutting_down() {
        use std::net::Ipv6Addr;
        use std::sync::Arc;

        for _ in 1..10 {
            let runtime = rt();

            runtime.block_on(async {
                let socket = UdpSocket::bind((Ipv6Addr::LOCALHOST, 0)).await.unwrap();
                let addr = socket.local_addr().unwrap();
                let send_half = Arc::new(socket);
                let recv_half = send_half.clone();

                tokio::spawn(async move {
                    let mut buf = [0];
                    loop {
                        recv_half.recv_from(&mut buf).await.unwrap();
                        std::thread::sleep(Duration::from_millis(2));
                    }
                });

                tokio::spawn(async move {
                    let buf = [0];
                    loop {
                        send_half.send_to(&buf, &addr).await.unwrap();
                        tokio::time::sleep(Duration::from_millis(1)).await;
                    }
                });

                tokio::time::sleep(Duration::from_millis(5)).await;
            });
        }
    }

    #[test]
    fn shutdown_timeout() {
        let (tx, rx) = oneshot::channel();
        let runtime = rt();

        runtime.block_on(async move {
            task::spawn_blocking(move || {
                tx.send(()).unwrap();
                thread::sleep(Duration::from_secs(10_000));
            });

            rx.await.unwrap();
        });

        Arc::try_unwrap(runtime).unwrap().shutdown_timeout(Duration::from_millis(100));
    }

    #[test]
    fn shutdown_timeout_0() {
        let runtime = rt();

        runtime.block_on(async move {
            task::spawn_blocking(move || {
                thread::sleep(Duration::from_secs(10_000));
            });
        });

        let now = Instant::now();
        Arc::try_unwrap(runtime).unwrap().shutdown_timeout(Duration::from_nanos(0));
        assert!(now.elapsed().as_secs() < 1);
    }

    #[test]
    fn shutdown_wakeup_time() {
        let runtime = rt();

        runtime.block_on(async move {
            tokio::time::sleep(std::time::Duration::from_millis(100)).await;
        });

        Arc::try_unwrap(runtime).unwrap().shutdown_timeout(Duration::from_secs(10_000));
    }

    // This test is currently ignored on Windows because of a
    // rust-lang issue in thread local storage destructors.
    // See https://github.com/rust-lang/rust/issues/74875
    #[test]
    #[cfg(not(windows))]
    fn runtime_in_thread_local() {
        use std::cell::RefCell;
        use std::thread;

        thread_local!(
            static R: RefCell<Option<Runtime>> = RefCell::new(None);
        );

        thread::spawn(|| {
            R.with(|cell| {
                let rt = rt();
                let rt = Arc::try_unwrap(rt).unwrap();
                *cell.borrow_mut() = Some(rt);
            });

            let _rt = rt();
        }).join().unwrap();
    }

    async fn client_server(tx: mpsc::Sender<()>) {
        let server = assert_ok!(TcpListener::bind("127.0.0.1:0").await);

        // Get the assigned address
        let addr = assert_ok!(server.local_addr());

        // Spawn the server
        tokio::spawn(async move {
            // Accept a socket
            let (mut socket, _) = server.accept().await.unwrap();

            // Write some data
            socket.write_all(b"hello").await.unwrap();
        });

        let mut client = TcpStream::connect(&addr).await.unwrap();

        let mut buf = vec![];
        client.read_to_end(&mut buf).await.unwrap();

        assert_eq!(buf, b"hello");
        tx.send(()).unwrap();
    }

    #[test]
    fn local_set_block_on_socket() {
        let rt = rt();
        let local = task::LocalSet::new();

        local.block_on(&rt, async move {
            let (tx, rx) = oneshot::channel();

            let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
            let addr = listener.local_addr().unwrap();

            task::spawn_local(async move {
                let _ = listener.accept().await;
                tx.send(()).unwrap();
            });

            TcpStream::connect(&addr).await.unwrap();
            rx.await.unwrap();
        });
    }

    #[test]
    fn local_set_client_server_block_on() {
        let rt = rt();
        let (tx, rx) = mpsc::channel();

        let local = task::LocalSet::new();

        local.block_on(&rt, async move { client_server_local(tx).await });

        assert_ok!(rx.try_recv());
        assert_err!(rx.try_recv());
    }

    async fn client_server_local(tx: mpsc::Sender<()>) {
        let server = assert_ok!(TcpListener::bind("127.0.0.1:0").await);

        // Get the assigned address
        let addr = assert_ok!(server.local_addr());

        // Spawn the server
        task::spawn_local(async move {
            // Accept a socket
            let (mut socket, _) = server.accept().await.unwrap();

            // Write some data
            socket.write_all(b"hello").await.unwrap();
        });

        let mut client = TcpStream::connect(&addr).await.unwrap();

        let mut buf = vec![];
        client.read_to_end(&mut buf).await.unwrap();

        assert_eq!(buf, b"hello");
        tx.send(()).unwrap();
    }

    #[test]
    fn coop() {
        use std::task::Poll::Ready;

        let rt = rt();

        rt.block_on(async {
            // Create a bunch of tasks
            let mut tasks = (0..1_000).map(|_| {
                tokio::spawn(async { })
            }).collect::<Vec<_>>();

            // Hope that all the tasks complete...
            time::sleep(Duration::from_millis(100)).await;

            poll_fn(|cx| {
                // At least one task should not be ready
                for task in &mut tasks {
                    if Pin::new(task).poll(cx).is_pending() {
                        return Ready(());
                    }
                }

                panic!("did not yield");
            }).await;
        });
    }

    #[test]
    fn coop_unconstrained() {
        use std::task::Poll::Ready;

        let rt = rt();

        rt.block_on(async {
            // Create a bunch of tasks
            let mut tasks = (0..1_000).map(|_| {
                tokio::spawn(async { })
            }).collect::<Vec<_>>();

            // Hope that all the tasks complete...
            time::sleep(Duration::from_millis(100)).await;

            tokio::task::unconstrained(poll_fn(|cx| {
                // All the tasks should be ready
                for task in &mut tasks {
                    assert!(Pin::new(task).poll(cx).is_ready());
                }

                Ready(())
            })).await;
        });
    }

    #[test]
    fn coop_consume_budget() {
        let rt = rt();

        rt.block_on(async {
            poll_fn(|cx| {
                let counter = Arc::new(std::sync::Mutex::new(0));
                let counter_clone = Arc::clone(&counter);
                let mut worker = Box::pin(async move {
                    // Consume the budget until a yield happens
                    for _ in 0..1000 {
                        *counter.lock().unwrap() += 1;
                        task::consume_budget().await
                    }
                });
                // Assert that the worker was yielded and it didn't manage
                // to finish the whole work (assuming the total budget of 128)
                assert!(Pin::new(&mut worker).poll(cx).is_pending());
                assert!(*counter_clone.lock().unwrap() < 1000);
                std::task::Poll::Ready(())
            }).await;
        });
    }

    // Tests that the "next task" scheduler optimization is not able to starve
    // other tasks.
    #[test]
    fn ping_pong_saturation() {
        use std::sync::atomic::{Ordering, AtomicBool};
        use tokio::sync::mpsc;

        const NUM: usize = 100;

        let rt = rt();

        let running = Arc::new(AtomicBool::new(true));

        rt.block_on(async {
            let (spawned_tx, mut spawned_rx) = mpsc::unbounded_channel();

            let mut tasks = vec![];
            // Spawn a bunch of tasks that ping ping between each other to
            // saturate the runtime.
            for _ in 0..NUM {
                let (tx1, mut rx1) = mpsc::unbounded_channel();
                let (tx2, mut rx2) = mpsc::unbounded_channel();
                let spawned_tx = spawned_tx.clone();
                let running = running.clone();
                tasks.push(task::spawn(async move {
                    spawned_tx.send(()).unwrap();


                    while running.load(Ordering::Relaxed) {
                        tx1.send(()).unwrap();
                        rx2.recv().await.unwrap();
                    }

                    // Close the channel and wait for the other task to exit.
                    drop(tx1);
                    assert!(rx2.recv().await.is_none());
                }));

                tasks.push(task::spawn(async move {
                    while rx1.recv().await.is_some() {
                        tx2.send(()).unwrap();
                    }
                }));
            }

            for _ in 0..NUM {
                spawned_rx.recv().await.unwrap();
            }

            // spawn another task and wait for it to complete
            let handle = task::spawn(async {
                for _ in 0..5 {
                    // Yielding forces it back into the local queue.
                    task::yield_now().await;
                }
            });
            handle.await.unwrap();
            running.store(false, Ordering::Relaxed);
            for t in tasks {
                t.await.unwrap();
            }
        });
    }
}