sync_mpsc.rs

#![allow(clippy::redundant_clone)]
#![warn(rust_2018_idioms)]
#![cfg(feature = "sync")]

#[cfg(tokio_wasm_not_wasi)]
use wasm_bindgen_test::wasm_bindgen_test as test;
#[cfg(tokio_wasm_not_wasi)]
use wasm_bindgen_test::wasm_bindgen_test as maybe_tokio_test;

#[cfg(not(tokio_wasm_not_wasi))]
use tokio::test as maybe_tokio_test;

use tokio::sync::mpsc::error::{TryRecvError, TrySendError};
use tokio::sync::mpsc::{self, channel};
use tokio::sync::oneshot;
use tokio_test::*;

use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::{Acquire, Release};
use std::sync::Arc;

#[cfg(not(tokio_wasm))]
mod support {
    pub(crate) mod mpsc_stream;
}

trait AssertSend: Send {}
impl AssertSend for mpsc::Sender<i32> {}
impl AssertSend for mpsc::Receiver<i32> {}

#[maybe_tokio_test]
async fn send_recv_with_buffer() {
    let (tx, mut rx) = mpsc::channel::<i32>(16);

    // Using poll_ready / try_send
    // let permit assert_ready_ok!(tx.reserve());
    let permit = tx.reserve().await.unwrap();
    permit.send(1);

    // Without poll_ready
    tx.try_send(2).unwrap();

    drop(tx);

    let val = rx.recv().await;
    assert_eq!(val, Some(1));

    let val = rx.recv().await;
    assert_eq!(val, Some(2));

    let val = rx.recv().await;
    assert!(val.is_none());
}

#[tokio::test]
#[cfg(feature = "full")]
async fn reserve_disarm() {
    let (tx, mut rx) = mpsc::channel::<i32>(2);
    let tx1 = tx.clone();
    let tx2 = tx.clone();
    let tx3 = tx.clone();
    let tx4 = tx;

    // We should be able to `poll_ready` two handles without problem
    let permit1 = assert_ok!(tx1.reserve().await);
    let permit2 = assert_ok!(tx2.reserve().await);

    // But a third should not be ready
    let mut r3 = tokio_test::task::spawn(tx3.reserve());
    assert_pending!(r3.poll());

    let mut r4 = tokio_test::task::spawn(tx4.reserve());
    assert_pending!(r4.poll());

    // Using one of the reserved slots should allow a new handle to become ready
    permit1.send(1);

    // We also need to receive for the slot to be free
    assert!(!r3.is_woken());
    rx.recv().await.unwrap();
    // Now there's a free slot!
    assert!(r3.is_woken());
    assert!(!r4.is_woken());

    // Dropping a permit should also open up a slot
    drop(permit2);
    assert!(r4.is_woken());

    let mut r1 = tokio_test::task::spawn(tx1.reserve());
    assert_pending!(r1.poll());
}

#[tokio::test]
#[cfg(all(feature = "full", not(tokio_wasi)))] // Wasi doesn't support threads
async fn send_recv_stream_with_buffer() {
    use tokio_stream::StreamExt;

    let (tx, rx) = support::mpsc_stream::channel_stream::<i32>(16);
    let mut rx = Box::pin(rx);

    tokio::spawn(async move {
        assert_ok!(tx.send(1).await);
        assert_ok!(tx.send(2).await);
    });

    assert_eq!(Some(1), rx.next().await);
    assert_eq!(Some(2), rx.next().await);
    assert_eq!(None, rx.next().await);
}

#[tokio::test]
#[cfg(feature = "full")]
async fn async_send_recv_with_buffer() {
    let (tx, mut rx) = mpsc::channel(16);

    tokio::spawn(async move {
        assert_ok!(tx.send(1).await);
        assert_ok!(tx.send(2).await);
    });

    assert_eq!(Some(1), rx.recv().await);
    assert_eq!(Some(2), rx.recv().await);
    assert_eq!(None, rx.recv().await);
}

#[tokio::test]
#[cfg(feature = "full")]
async fn start_send_past_cap() {
    use std::future::Future;

    let mut t1 = tokio_test::task::spawn(());

    let (tx1, mut rx) = mpsc::channel(1);
    let tx2 = tx1.clone();

    assert_ok!(tx1.try_send(()));

    let mut r1 = Box::pin(tx1.reserve());
    t1.enter(|cx, _| assert_pending!(r1.as_mut().poll(cx)));

    {
        let mut r2 = tokio_test::task::spawn(tx2.reserve());
        assert_pending!(r2.poll());

        drop(r1);

        assert!(rx.recv().await.is_some());

        assert!(r2.is_woken());
        assert!(!t1.is_woken());
    }

    drop(tx1);
    drop(tx2);

    assert!(rx.recv().await.is_none());
}

#[test]
#[should_panic]
#[cfg(not(tokio_wasm))] // wasm currently doesn't support unwinding
fn buffer_gteq_one() {
    mpsc::channel::<i32>(0);
}

#[maybe_tokio_test]
async fn send_recv_unbounded() {
    let (tx, mut rx) = mpsc::unbounded_channel::<i32>();

    // Using `try_send`
    assert_ok!(tx.send(1));
    assert_ok!(tx.send(2));

    assert_eq!(rx.recv().await, Some(1));
    assert_eq!(rx.recv().await, Some(2));

    drop(tx);

    assert!(rx.recv().await.is_none());
}

#[tokio::test]
#[cfg(feature = "full")]
async fn async_send_recv_unbounded() {
    let (tx, mut rx) = mpsc::unbounded_channel();

    tokio::spawn(async move {
        assert_ok!(tx.send(1));
        assert_ok!(tx.send(2));
    });

    assert_eq!(Some(1), rx.recv().await);
    assert_eq!(Some(2), rx.recv().await);
    assert_eq!(None, rx.recv().await);
}

#[tokio::test]
#[cfg(all(feature = "full", not(tokio_wasi)))] // Wasi doesn't support threads
async fn send_recv_stream_unbounded() {
    use tokio_stream::StreamExt;

    let (tx, rx) = support::mpsc_stream::unbounded_channel_stream::<i32>();

    let mut rx = Box::pin(rx);

    tokio::spawn(async move {
        assert_ok!(tx.send(1));
        assert_ok!(tx.send(2));
    });

    assert_eq!(Some(1), rx.next().await);
    assert_eq!(Some(2), rx.next().await);
    assert_eq!(None, rx.next().await);
}

#[maybe_tokio_test]
async fn no_t_bounds_buffer() {
    struct NoImpls;

    let (tx, mut rx) = mpsc::channel(100);

    // sender should be Debug even though T isn't Debug
    println!("{:?}", tx);
    // same with Receiver
    println!("{:?}", rx);
    // and sender should be Clone even though T isn't Clone
    assert!(tx.clone().try_send(NoImpls).is_ok());

    assert!(rx.recv().await.is_some());
}

#[maybe_tokio_test]
async fn no_t_bounds_unbounded() {
    struct NoImpls;

    let (tx, mut rx) = mpsc::unbounded_channel();

    // sender should be Debug even though T isn't Debug
    println!("{:?}", tx);
    // same with Receiver
    println!("{:?}", rx);
    // and sender should be Clone even though T isn't Clone
    assert!(tx.clone().send(NoImpls).is_ok());

    assert!(rx.recv().await.is_some());
}

#[tokio::test]
#[cfg(feature = "full")]
async fn send_recv_buffer_limited() {
    let (tx, mut rx) = mpsc::channel::<i32>(1);

    // Reserve capacity
    let p1 = assert_ok!(tx.reserve().await);

    // Send first message
    p1.send(1);

    // Not ready
    let mut p2 = tokio_test::task::spawn(tx.reserve());
    assert_pending!(p2.poll());

    // Take the value
    assert!(rx.recv().await.is_some());

    // Notified
    assert!(p2.is_woken());

    // Trying to send fails
    assert_err!(tx.try_send(1337));

    // Send second
    let permit = assert_ready_ok!(p2.poll());
    permit.send(2);

    assert!(rx.recv().await.is_some());
}

#[maybe_tokio_test]
async fn recv_close_gets_none_idle() {
    let (tx, mut rx) = mpsc::channel::<i32>(10);

    rx.close();

    assert!(rx.recv().await.is_none());

    assert_err!(tx.send(1).await);
}

#[tokio::test]
#[cfg(feature = "full")]
async fn recv_close_gets_none_reserved() {
    let (tx1, mut rx) = mpsc::channel::<i32>(1);
    let tx2 = tx1.clone();

    let permit1 = assert_ok!(tx1.reserve().await);
    let mut permit2 = tokio_test::task::spawn(tx2.reserve());
    assert_pending!(permit2.poll());

    rx.close();

    assert!(permit2.is_woken());
    assert_ready_err!(permit2.poll());

    {
        let mut recv = tokio_test::task::spawn(rx.recv());
        assert_pending!(recv.poll());

        permit1.send(123);
        assert!(recv.is_woken());

        let v = assert_ready!(recv.poll());
        assert_eq!(v, Some(123));
    }

    assert!(rx.recv().await.is_none());
}

#[maybe_tokio_test]
async fn tx_close_gets_none() {
    let (_, mut rx) = mpsc::channel::<i32>(10);
    assert!(rx.recv().await.is_none());
}

#[maybe_tokio_test]
async fn try_send_fail() {
    let (tx, mut rx) = mpsc::channel(1);

    tx.try_send("hello").unwrap();

    // This should fail
    match assert_err!(tx.try_send("fail")) {
        TrySendError::Full(..) => {}
        _ => panic!(),
    }

    assert_eq!(rx.recv().await, Some("hello"));

    assert_ok!(tx.try_send("goodbye"));
    drop(tx);

    assert_eq!(rx.recv().await, Some("goodbye"));
    assert!(rx.recv().await.is_none());
}

#[maybe_tokio_test]
async fn try_send_fail_with_try_recv() {
    let (tx, mut rx) = mpsc::channel(1);

    tx.try_send("hello").unwrap();

    // This should fail
    match assert_err!(tx.try_send("fail")) {
        TrySendError::Full(..) => {}
        _ => panic!(),
    }

    assert_eq!(rx.try_recv(), Ok("hello"));

    assert_ok!(tx.try_send("goodbye"));
    drop(tx);

    assert_eq!(rx.try_recv(), Ok("goodbye"));
    assert_eq!(rx.try_recv(), Err(TryRecvError::Disconnected));
}

#[maybe_tokio_test]
async fn try_reserve_fails() {
    let (tx, mut rx) = mpsc::channel(1);

    let permit = tx.try_reserve().unwrap();

    // This should fail
    match assert_err!(tx.try_reserve()) {
        TrySendError::Full(()) => {}
        _ => panic!(),
    }

    permit.send("foo");

    assert_eq!(rx.recv().await, Some("foo"));

    // Dropping permit releases the slot.
    let permit = tx.try_reserve().unwrap();
    drop(permit);

    let _permit = tx.try_reserve().unwrap();
}

#[tokio::test]
#[cfg(feature = "full")]
async fn drop_permit_releases_permit() {
    // poll_ready reserves capacity, ensure that the capacity is released if tx
    // is dropped w/o sending a value.
    let (tx1, _rx) = mpsc::channel::<i32>(1);
    let tx2 = tx1.clone();

    let permit = assert_ok!(tx1.reserve().await);

    let mut reserve2 = tokio_test::task::spawn(tx2.reserve());
    assert_pending!(reserve2.poll());

    drop(permit);

    assert!(reserve2.is_woken());
    assert_ready_ok!(reserve2.poll());
}

#[maybe_tokio_test]
async fn dropping_rx_closes_channel() {
    let (tx, rx) = mpsc::channel(100);

    let msg = Arc::new(());
    assert_ok!(tx.try_send(msg.clone()));

    drop(rx);
    assert_err!(tx.reserve().await);
    assert_eq!(1, Arc::strong_count(&msg));
}

#[test]
fn dropping_rx_closes_channel_for_try() {
    let (tx, rx) = mpsc::channel(100);

    let msg = Arc::new(());
    tx.try_send(msg.clone()).unwrap();

    drop(rx);

    assert!(matches!(
        tx.try_send(msg.clone()),
        Err(TrySendError::Closed(_))
    ));
    assert!(matches!(tx.try_reserve(), Err(TrySendError::Closed(_))));
    assert!(matches!(
        tx.try_reserve_owned(),
        Err(TrySendError::Closed(_))
    ));

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

#[test]
fn unconsumed_messages_are_dropped() {
    let msg = Arc::new(());

    let (tx, rx) = mpsc::channel(100);

    tx.try_send(msg.clone()).unwrap();

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

    drop((tx, rx));

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

#[test]
#[cfg(all(feature = "full", not(tokio_wasi)))] // Wasi doesn't support threads
fn blocking_recv() {
    let (tx, mut rx) = mpsc::channel::<u8>(1);

    let sync_code = std::thread::spawn(move || {
        assert_eq!(Some(10), rx.blocking_recv());
    });

    tokio::runtime::Runtime::new()
        .unwrap()
        .block_on(async move {
            let _ = tx.send(10).await;
        });
    sync_code.join().unwrap()
}

#[tokio::test]
#[should_panic]
#[cfg(not(tokio_wasm))] // wasm currently doesn't support unwinding
async fn blocking_recv_async() {
    let (_tx, mut rx) = mpsc::channel::<()>(1);
    let _ = rx.blocking_recv();
}

#[test]
#[cfg(all(feature = "full", not(tokio_wasi)))] // Wasi doesn't support threads
fn blocking_send() {
    let (tx, mut rx) = mpsc::channel::<u8>(1);

    let sync_code = std::thread::spawn(move || {
        tx.blocking_send(10).unwrap();
    });

    tokio::runtime::Runtime::new()
        .unwrap()
        .block_on(async move {
            assert_eq!(Some(10), rx.recv().await);
        });
    sync_code.join().unwrap()
}

#[tokio::test]
#[should_panic]
#[cfg(not(tokio_wasm))] // wasm currently doesn't support unwinding
async fn blocking_send_async() {
    let (tx, _rx) = mpsc::channel::<()>(1);
    let _ = tx.blocking_send(());
}

#[tokio::test]
#[cfg(feature = "full")]
async fn ready_close_cancel_bounded() {
    let (tx, mut rx) = mpsc::channel::<()>(100);
    let _tx2 = tx.clone();

    let permit = assert_ok!(tx.reserve().await);

    rx.close();

    let mut recv = tokio_test::task::spawn(rx.recv());
    assert_pending!(recv.poll());

    drop(permit);

    assert!(recv.is_woken());
    let val = assert_ready!(recv.poll());
    assert!(val.is_none());
}

#[tokio::test]
#[cfg(feature = "full")]
async fn permit_available_not_acquired_close() {
    let (tx1, mut rx) = mpsc::channel::<()>(1);
    let tx2 = tx1.clone();

    let permit1 = assert_ok!(tx1.reserve().await);

    let mut permit2 = tokio_test::task::spawn(tx2.reserve());
    assert_pending!(permit2.poll());

    rx.close();

    drop(permit1);
    assert!(permit2.is_woken());

    drop(permit2);
    assert!(rx.recv().await.is_none());
}

#[test]
fn try_recv_bounded() {
    let (tx, mut rx) = mpsc::channel(5);

    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    assert!(tx.try_send("hello").is_err());

    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Err(TryRecvError::Empty), rx.try_recv());

    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    assert_eq!(Ok("hello"), rx.try_recv());
    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    assert!(tx.try_send("hello").is_err());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Err(TryRecvError::Empty), rx.try_recv());

    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    drop(tx);
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Err(TryRecvError::Disconnected), rx.try_recv());
}

#[test]
fn try_recv_unbounded() {
    for num in 0..100 {
        let (tx, mut rx) = mpsc::unbounded_channel();

        for i in 0..num {
            tx.send(i).unwrap();
        }

        for i in 0..num {
            assert_eq!(rx.try_recv(), Ok(i));
        }

        assert_eq!(rx.try_recv(), Err(TryRecvError::Empty));
        drop(tx);
        assert_eq!(rx.try_recv(), Err(TryRecvError::Disconnected));
    }
}

#[test]
fn try_recv_close_while_empty_bounded() {
    let (tx, mut rx) = mpsc::channel::<()>(5);

    assert_eq!(Err(TryRecvError::Empty), rx.try_recv());
    drop(tx);
    assert_eq!(Err(TryRecvError::Disconnected), rx.try_recv());
}

#[test]
fn try_recv_close_while_empty_unbounded() {
    let (tx, mut rx) = mpsc::unbounded_channel::<()>();

    assert_eq!(Err(TryRecvError::Empty), rx.try_recv());
    drop(tx);
    assert_eq!(Err(TryRecvError::Disconnected), rx.try_recv());
}

#[tokio::test(start_paused = true)]
#[cfg(feature = "full")]
async fn recv_timeout() {
    use tokio::sync::mpsc::error::SendTimeoutError::{Closed, Timeout};
    use tokio::time::Duration;

    let (tx, rx) = mpsc::channel(5);

    assert_eq!(tx.send_timeout(10, Duration::from_secs(1)).await, Ok(()));
    assert_eq!(tx.send_timeout(20, Duration::from_secs(1)).await, Ok(()));
    assert_eq!(tx.send_timeout(30, Duration::from_secs(1)).await, Ok(()));
    assert_eq!(tx.send_timeout(40, Duration::from_secs(1)).await, Ok(()));
    assert_eq!(tx.send_timeout(50, Duration::from_secs(1)).await, Ok(()));
    assert_eq!(
        tx.send_timeout(60, Duration::from_secs(1)).await,
        Err(Timeout(60))
    );

    drop(rx);
    assert_eq!(
        tx.send_timeout(70, Duration::from_secs(1)).await,
        Err(Closed(70))
    );
}

#[test]
#[should_panic = "there is no reactor running, must be called from the context of a Tokio 1.x runtime"]
#[cfg(not(tokio_wasm))] // wasm currently doesn't support unwinding
fn recv_timeout_panic() {
    use futures::future::FutureExt;
    use tokio::time::Duration;

    let (tx, _rx) = mpsc::channel(5);
    tx.send_timeout(10, Duration::from_secs(1)).now_or_never();
}

#[tokio::test]
async fn weak_sender() {
    let (tx, mut rx) = channel(11);

    let tx_weak = tokio::spawn(async move {
        let tx_weak = tx.clone().downgrade();

        for i in 0..10 {
            if tx.send(i).await.is_err() {
                return None;
            }
        }

        let tx2 = tx_weak
            .upgrade()
            .expect("expected to be able to upgrade tx_weak");
        let _ = tx2.send(20).await;
        let tx_weak = tx2.downgrade();

        Some(tx_weak)
    })
    .await
    .unwrap();

    for i in 0..12 {
        let recvd = rx.recv().await;

        match recvd {
            Some(msg) => {
                if i == 10 {
                    assert_eq!(msg, 20);
                }
            }
            None => {
                assert_eq!(i, 11);
                break;
            }
        }
    }

    let tx_weak = tx_weak.unwrap();
    let upgraded = tx_weak.upgrade();
    assert!(upgraded.is_none());
}

#[tokio::test]
async fn actor_weak_sender() {
    pub struct MyActor {
        receiver: mpsc::Receiver<ActorMessage>,
        sender: mpsc::WeakSender<ActorMessage>,
        next_id: u32,
        pub received_self_msg: bool,
    }

    enum ActorMessage {
        GetUniqueId { respond_to: oneshot::Sender<u32> },
        SelfMessage {},
    }

    impl MyActor {
        fn new(
            receiver: mpsc::Receiver<ActorMessage>,
            sender: mpsc::WeakSender<ActorMessage>,
        ) -> Self {
            MyActor {
                receiver,
                sender,
                next_id: 0,
                received_self_msg: false,
            }
        }

        fn handle_message(&mut self, msg: ActorMessage) {
            match msg {
                ActorMessage::GetUniqueId { respond_to } => {
                    self.next_id += 1;

                    // The `let _ =` ignores any errors when sending.
                    //
                    // This can happen if the `select!` macro is used
                    // to cancel waiting for the response.
                    let _ = respond_to.send(self.next_id);
                }
                ActorMessage::SelfMessage { .. } => {
                    self.received_self_msg = true;
                }
            }
        }

        async fn send_message_to_self(&mut self) {
            let msg = ActorMessage::SelfMessage {};

            let sender = self.sender.clone();

            // cannot move self.sender here
            if let Some(sender) = sender.upgrade() {
                let _ = sender.send(msg).await;
                self.sender = sender.downgrade();
            }
        }

        async fn run(&mut self) {
            let mut i = 0;
            while let Some(msg) = self.receiver.recv().await {
                self.handle_message(msg);

                if i == 0 {
                    self.send_message_to_self().await;
                }

                i += 1
            }

            assert!(self.received_self_msg);
        }
    }

    #[derive(Clone)]
    pub struct MyActorHandle {
        sender: mpsc::Sender<ActorMessage>,
    }

    impl MyActorHandle {
        pub fn new() -> (Self, MyActor) {
            let (sender, receiver) = mpsc::channel(8);
            let actor = MyActor::new(receiver, sender.clone().downgrade());

            (Self { sender }, actor)
        }

        pub async fn get_unique_id(&self) -> u32 {
            let (send, recv) = oneshot::channel();
            let msg = ActorMessage::GetUniqueId { respond_to: send };

            // Ignore send errors. If this send fails, so does the
            // recv.await below. There's no reason to check the
            // failure twice.
            let _ = self.sender.send(msg).await;
            recv.await.expect("Actor task has been killed")
        }
    }

    let (handle, mut actor) = MyActorHandle::new();

    let actor_handle = tokio::spawn(async move { actor.run().await });

    let _ = tokio::spawn(async move {
        let _ = handle.get_unique_id().await;
        drop(handle);
    })
    .await;

    let _ = actor_handle.await;
}

static NUM_DROPPED: AtomicUsize = AtomicUsize::new(0);

#[derive(Debug)]
struct Msg;

impl Drop for Msg {
    fn drop(&mut self) {
        NUM_DROPPED.fetch_add(1, Release);
    }
}

// Tests that no pending messages are put onto the channel after `Rx` was
// dropped.
//
// Note: After the introduction of `WeakSender`, which internally
// used `Arc` and doesn't call a drop of the channel after the last strong
// `Sender` was dropped while more than one `WeakSender` remains, we want to
// ensure that no messages are kept in the channel, which were sent after
// the receiver was dropped.
#[tokio::test]
async fn test_msgs_dropped_on_rx_drop() {
    let (tx, mut rx) = mpsc::channel(3);

    let _ = tx.send(Msg {}).await.unwrap();
    let _ = tx.send(Msg {}).await.unwrap();

    // This msg will be pending and should be dropped when `rx` is dropped
    let sent_fut = tx.send(Msg {});

    let _ = rx.recv().await.unwrap();
    let _ = rx.recv().await.unwrap();

    let _ = sent_fut.await.unwrap();

    drop(rx);

    assert_eq!(NUM_DROPPED.load(Acquire), 3);

    // This msg will not be put onto `Tx` list anymore, since `Rx` is closed.
    assert!(tx.send(Msg {}).await.is_err());

    assert_eq!(NUM_DROPPED.load(Acquire), 4);
}

// Tests that a `WeakSender` is upgradeable when other `Sender`s exist.
#[tokio::test]
async fn downgrade_upgrade_sender_success() {
    let (tx, _rx) = mpsc::channel::<i32>(1);
    let weak_tx = tx.downgrade();
    assert!(weak_tx.upgrade().is_some());
}

// Tests that a `WeakSender` fails to upgrade when no other `Sender` exists.
#[tokio::test]
async fn downgrade_upgrade_sender_failure() {
    let (tx, _rx) = mpsc::channel::<i32>(1);
    let weak_tx = tx.downgrade();
    drop(tx);
    assert!(weak_tx.upgrade().is_none());
}

// Tests that a `WeakSender` cannot be upgraded after a `Sender` was dropped,
// which existed at the time of the `downgrade` call.
#[tokio::test]
async fn downgrade_drop_upgrade() {
    let (tx, _rx) = mpsc::channel::<i32>(1);

    // the cloned `Tx` is dropped right away
    let weak_tx = tx.clone().downgrade();
    drop(tx);
    assert!(weak_tx.upgrade().is_none());
}

// Tests that we can upgrade a weak sender with an outstanding permit
// but no other strong senders.
#[tokio::test]
async fn downgrade_get_permit_upgrade_no_senders() {
    let (tx, _rx) = mpsc::channel::<i32>(1);
    let weak_tx = tx.downgrade();
    let _permit = tx.reserve_owned().await.unwrap();
    assert!(weak_tx.upgrade().is_some());
}

// Tests that you can downgrade and upgrade a sender with an outstanding permit
// but no other senders left.
#[tokio::test]
async fn downgrade_upgrade_get_permit_no_senders() {
    let (tx, _rx) = mpsc::channel::<i32>(1);
    let tx2 = tx.clone();
    let _permit = tx.reserve_owned().await.unwrap();
    let weak_tx = tx2.downgrade();
    drop(tx2);
    assert!(weak_tx.upgrade().is_some());
}

// Tests that `downgrade` does not change the `tx_count` of the channel.
#[tokio::test]
async fn test_tx_count_weak_sender() {
    let (tx, _rx) = mpsc::channel::<i32>(1);
    let tx_weak = tx.downgrade();
    let tx_weak2 = tx.downgrade();
    drop(tx);

    assert!(tx_weak.upgrade().is_none() && tx_weak2.upgrade().is_none());
}

// Tests that channel `capacity` changes and `max_capacity` stays the same
#[tokio::test]
async fn test_tx_capacity() {
    let (tx, _rx) = mpsc::channel::<()>(10);
    // both capacities are same before
    assert_eq!(tx.capacity(), 10);
    assert_eq!(tx.max_capacity(), 10);

    let _permit = tx.reserve().await.unwrap();
    // after reserve, only capacity should drop by one
    assert_eq!(tx.capacity(), 9);
    assert_eq!(tx.max_capacity(), 10);

    let _sent = tx.send(()).await.unwrap();
    // after send, capacity should drop by one again
    assert_eq!(tx.capacity(), 8);
    assert_eq!(tx.max_capacity(), 10);
}