Priority channel using binary heap

embassy-sync/src/lib.rs

@@ -15,6 +15,7 @@ pub mod blocking_mutex;
 pub mod channel;
 pub mod mutex;
 pub mod pipe;
+pub mod priority_channel;
 pub mod pubsub;
 pub mod signal;
 pub mod waitqueue;

embassy-sync/src/priority_channel.rs

@@ -0,0 +1,744 @@
+//! A queue for sending values between asynchronous tasks.
+//!
+//! Similar to a [`Channel`](crate::channel::Channel), however [`PriorityChannel`] sifts higher priority items to the front of the queue.
+
+use core::cell::RefCell;
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+use heapless::BinaryHeap;
+
+use crate::blocking_mutex::raw::RawMutex;
+use crate::blocking_mutex::Mutex;
+use crate::waitqueue::WakerRegistration;
+
+/// Send-only access to a [`PriorityChannel`].
+pub struct Sender<'ch, M, T, K, const N: usize>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+    channel: &'ch PriorityChannel<M, T, K, N>,
+}
+
+impl<'ch, M, T, K, const N: usize> Clone for Sender<'ch, M, T, K, N>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+    fn clone(&self) -> Self {
+        Sender { channel: self.channel }
+    }
+}
+
+impl<'ch, M, T, K, const N: usize> Copy for Sender<'ch, M, T, K, N>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+}
+
+impl<'ch, M, T, K, const N: usize> Sender<'ch, M, T, K, N>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+    /// Sends a value.
+    ///
+    /// See [`PriorityChannel::send()`]
+    pub fn send(&self, message: T) -> SendFuture<'ch, M, T, K, N> {
+        self.channel.send(message)
+    }
+
+    /// Attempt to immediately send a message.
+    ///
+    /// See [`PriorityChannel::send()`]
+    pub fn try_send(&self, message: T) -> Result<(), TrySendError<T>> {
+        self.channel.try_send(message)
+    }
+
+    /// Allows a poll_fn to poll until the channel is ready to send
+    ///
+    /// See [`PriorityChannel::poll_ready_to_send()`]
+    pub fn poll_ready_to_send(&self, cx: &mut Context<'_>) -> Poll<()> {
+        self.channel.poll_ready_to_send(cx)
+    }
+}
+
+/// Send-only access to a [`PriorityChannel`] without knowing channel size.
+pub struct DynamicSender<'ch, T> {
+    channel: &'ch dyn DynamicChannel<T>,
+}
+
+impl<'ch, T> Clone for DynamicSender<'ch, T> {
+    fn clone(&self) -> Self {
+        DynamicSender { channel: self.channel }
+    }
+}
+
+impl<'ch, T> Copy for DynamicSender<'ch, T> {}
+
+impl<'ch, M, T, K, const N: usize> From<Sender<'ch, M, T, K, N>> for DynamicSender<'ch, T>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+    fn from(s: Sender<'ch, M, T, K, N>) -> Self {
+        Self { channel: s.channel }
+    }
+}
+
+impl<'ch, T> DynamicSender<'ch, T> {
+    /// Sends a value.
+    ///
+    /// See [`PriorityChannel::send()`]
+    pub fn send(&self, message: T) -> DynamicSendFuture<'ch, T> {
+        DynamicSendFuture {
+            channel: self.channel,
+            message: Some(message),
+        }
+    }
+
+    /// Attempt to immediately send a message.
+    ///
+    /// See [`PriorityChannel::send()`]
+    pub fn try_send(&self, message: T) -> Result<(), TrySendError<T>> {
+        self.channel.try_send_with_context(message, None)
+    }
+
+    /// Allows a poll_fn to poll until the channel is ready to send
+    ///
+    /// See [`PriorityChannel::poll_ready_to_send()`]
+    pub fn poll_ready_to_send(&self, cx: &mut Context<'_>) -> Poll<()> {
+        self.channel.poll_ready_to_send(cx)
+    }
+}
+
+/// Receive-only access to a [`PriorityChannel`].
+pub struct Receiver<'ch, M, T, K, const N: usize>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+    channel: &'ch PriorityChannel<M, T, K, N>,
+}
+
+impl<'ch, M, T, K, const N: usize> Clone for Receiver<'ch, M, T, K, N>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+    fn clone(&self) -> Self {
+        Receiver { channel: self.channel }
+    }
+}
+
+impl<'ch, M, T, K, const N: usize> Copy for Receiver<'ch, M, T, K, N>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+}
+
+impl<'ch, M, T, K, const N: usize> Receiver<'ch, M, T, K, N>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+    /// Receive the next value.
+    ///
+    /// See [`PriorityChannel::receive()`].
+    pub fn receive(&self) -> ReceiveFuture<'_, M, T, K, N> {
+        self.channel.receive()
+    }
+
+    /// Attempt to immediately receive the next value.
+    ///
+    /// See [`PriorityChannel::try_receive()`]
+    pub fn try_receive(&self) -> Result<T, TryReceiveError> {
+        self.channel.try_receive()
+    }
+
+    /// Allows a poll_fn to poll until the channel is ready to receive
+    ///
+    /// See [`PriorityChannel::poll_ready_to_receive()`]
+    pub fn poll_ready_to_receive(&self, cx: &mut Context<'_>) -> Poll<()> {
+        self.channel.poll_ready_to_receive(cx)
+    }
+
+    /// Poll the channel for the next item
+    ///
+    /// See [`PriorityChannel::poll_receive()`]
+    pub fn poll_receive(&self, cx: &mut Context<'_>) -> Poll<T> {
+        self.channel.poll_receive(cx)
+    }
+}
+
+/// Receive-only access to a [`PriorityChannel`] without knowing channel size.
+pub struct DynamicReceiver<'ch, T> {
+    channel: &'ch dyn DynamicChannel<T>,
+}
+
+impl<'ch, T> Clone for DynamicReceiver<'ch, T> {
+    fn clone(&self) -> Self {
+        DynamicReceiver { channel: self.channel }
+    }
+}
+
+impl<'ch, T> Copy for DynamicReceiver<'ch, T> {}
+
+impl<'ch, T> DynamicReceiver<'ch, T> {
+    /// Receive the next value.
+    ///
+    /// See [`PriorityChannel::receive()`].
+    pub fn receive(&self) -> DynamicReceiveFuture<'_, T> {
+        DynamicReceiveFuture { channel: self.channel }
+    }
+
+    /// Attempt to immediately receive the next value.
+    ///
+    /// See [`PriorityChannel::try_receive()`]
+    pub fn try_receive(&self) -> Result<T, TryReceiveError> {
+        self.channel.try_receive_with_context(None)
+    }
+
+    /// Allows a poll_fn to poll until the channel is ready to receive
+    ///
+    /// See [`PriorityChannel::poll_ready_to_receive()`]
+    pub fn poll_ready_to_receive(&self, cx: &mut Context<'_>) -> Poll<()> {
+        self.channel.poll_ready_to_receive(cx)
+    }
+
+    /// Poll the channel for the next item
+    ///
+    /// See [`PriorityChannel::poll_receive()`]
+    pub fn poll_receive(&self, cx: &mut Context<'_>) -> Poll<T> {
+        self.channel.poll_receive(cx)
+    }
+}
+
+impl<'ch, M, T, K, const N: usize> From<Receiver<'ch, M, T, K, N>> for DynamicReceiver<'ch, T>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+    fn from(s: Receiver<'ch, M, T, K, N>) -> Self {
+        Self { channel: s.channel }
+    }
+}
+
+/// Future returned by [`PriorityChannel::receive`] and  [`Receiver::receive`].
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct ReceiveFuture<'ch, M, T, K, const N: usize>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+    channel: &'ch PriorityChannel<M, T, K, N>,
+}
+
+impl<'ch, M, T, K, const N: usize> Future for ReceiveFuture<'ch, M, T, K, N>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+    type Output = T;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
+        self.channel.poll_receive(cx)
+    }
+}
+
+/// Future returned by [`DynamicReceiver::receive`].
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct DynamicReceiveFuture<'ch, T> {
+    channel: &'ch dyn DynamicChannel<T>,
+}
+
+impl<'ch, T> Future for DynamicReceiveFuture<'ch, T> {
+    type Output = T;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
+        match self.channel.try_receive_with_context(Some(cx)) {
+            Ok(v) => Poll::Ready(v),
+            Err(TryReceiveError::Empty) => Poll::Pending,
+        }
+    }
+}
+
+/// Future returned by [`PriorityChannel::send`] and  [`Sender::send`].
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct SendFuture<'ch, M, T, K, const N: usize>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+    channel: &'ch PriorityChannel<M, T, K, N>,
+    message: Option<T>,
+}
+
+impl<'ch, M, T, K, const N: usize> Future for SendFuture<'ch, M, T, K, N>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+    type Output = ();
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match self.message.take() {
+            Some(m) => match self.channel.try_send_with_context(m, Some(cx)) {
+                Ok(..) => Poll::Ready(()),
+                Err(TrySendError::Full(m)) => {
+                    self.message = Some(m);
+                    Poll::Pending
+                }
+            },
+            None => panic!("Message cannot be None"),
+        }
+    }
+}
+
+impl<'ch, M, T, K, const N: usize> Unpin for SendFuture<'ch, M, T, K, N>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+}
+
+/// Future returned by [`DynamicSender::send`].
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct DynamicSendFuture<'ch, T> {
+    channel: &'ch dyn DynamicChannel<T>,
+    message: Option<T>,
+}
+
+impl<'ch, T> Future for DynamicSendFuture<'ch, T> {
+    type Output = ();
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match self.message.take() {
+            Some(m) => match self.channel.try_send_with_context(m, Some(cx)) {
+                Ok(..) => Poll::Ready(()),
+                Err(TrySendError::Full(m)) => {
+                    self.message = Some(m);
+                    Poll::Pending
+                }
+            },
+            None => panic!("Message cannot be None"),
+        }
+    }
+}
+
+impl<'ch, T> Unpin for DynamicSendFuture<'ch, T> {}
+
+trait DynamicChannel<T> {
+    fn try_send_with_context(&self, message: T, cx: Option<&mut Context<'_>>) -> Result<(), TrySendError<T>>;
+
+    fn try_receive_with_context(&self, cx: Option<&mut Context<'_>>) -> Result<T, TryReceiveError>;
+
+    fn poll_ready_to_send(&self, cx: &mut Context<'_>) -> Poll<()>;
+    fn poll_ready_to_receive(&self, cx: &mut Context<'_>) -> Poll<()>;
+
+    fn poll_receive(&self, cx: &mut Context<'_>) -> Poll<T>;
+}
+
+/// Error returned by [`try_receive`](PriorityChannel::try_receive).
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub enum TryReceiveError {
+    /// A message could not be received because the channel is empty.
+    Empty,
+}
+
+/// Error returned by [`try_send`](PriorityChannel::try_send).
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub enum TrySendError<T> {
+    /// The data could not be sent on the channel because the channel is
+    /// currently full and sending would require blocking.
+    Full(T),
+}
+
+struct ChannelState<T, K, const N: usize> {
+    queue: BinaryHeap<T, K, N>,
+    receiver_waker: WakerRegistration,
+    senders_waker: WakerRegistration,
+}
+
+impl<T, K, const N: usize> ChannelState<T, K, N>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+{
+    const fn new() -> Self {
+        ChannelState {
+            queue: BinaryHeap::new(),
+            receiver_waker: WakerRegistration::new(),
+            senders_waker: WakerRegistration::new(),
+        }
+    }
+
+    fn try_receive(&mut self) -> Result<T, TryReceiveError> {
+        self.try_receive_with_context(None)
+    }
+
+    fn try_receive_with_context(&mut self, cx: Option<&mut Context<'_>>) -> Result<T, TryReceiveError> {
+        if self.queue.len() == self.queue.capacity() {
+            self.senders_waker.wake();
+        }
+
+        if let Some(message) = self.queue.pop() {
+            Ok(message)
+        } else {
+            if let Some(cx) = cx {
+                self.receiver_waker.register(cx.waker());
+            }
+            Err(TryReceiveError::Empty)
+        }
+    }
+
+    fn poll_receive(&mut self, cx: &mut Context<'_>) -> Poll<T> {
+        if self.queue.len() == self.queue.capacity() {
+            self.senders_waker.wake();
+        }
+
+        if let Some(message) = self.queue.pop() {
+            Poll::Ready(message)
+        } else {
+            self.receiver_waker.register(cx.waker());
+            Poll::Pending
+        }
+    }
+
+    fn poll_ready_to_receive(&mut self, cx: &mut Context<'_>) -> Poll<()> {
+        self.receiver_waker.register(cx.waker());
+
+        if !self.queue.is_empty() {
+            Poll::Ready(())
+        } else {
+            Poll::Pending
+        }
+    }
+
+    fn try_send(&mut self, message: T) -> Result<(), TrySendError<T>> {
+        self.try_send_with_context(message, None)
+    }
+
+    fn try_send_with_context(&mut self, message: T, cx: Option<&mut Context<'_>>) -> Result<(), TrySendError<T>> {
+        match self.queue.push(message) {
+            Ok(()) => {
+                self.receiver_waker.wake();
+                Ok(())
+            }
+            Err(message) => {
+                if let Some(cx) = cx {
+                    self.senders_waker.register(cx.waker());
+                }
+                Err(TrySendError::Full(message))
+            }
+        }
+    }
+
+    fn poll_ready_to_send(&mut self, cx: &mut Context<'_>) -> Poll<()> {
+        self.senders_waker.register(cx.waker());
+
+        if !self.queue.len() == self.queue.capacity() {
+            Poll::Ready(())
+        } else {
+            Poll::Pending
+        }
+    }
+}
+
+/// A bounded channel for communicating between asynchronous tasks
+/// with backpressure.
+///
+/// The channel will buffer up to the provided number of messages.  Once the
+/// buffer is full, attempts to `send` new messages will wait until a message is
+/// received from the channel.
+///
+/// All data sent will become available in the same order as it was sent.
+pub struct PriorityChannel<M, T, K, const N: usize>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+    inner: Mutex<M, RefCell<ChannelState<T, K, N>>>,
+}
+
+impl<M, T, K, const N: usize> PriorityChannel<M, T, K, N>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+    /// Establish a new bounded channel. For example, to create one with a NoopMutex:
+    ///
+    /// ```
+    /// use embassy_sync::channel::PriorityChannel;
+    /// use embassy_sync::blocking_mutex::raw::NoopRawMutex;
+    ///
+    /// // Declare a bounded channel of 3 u32s.
+    /// let mut channel = PriorityChannel::<NoopRawMutex, u32, 3>::new();
+    /// ```
+    pub const fn new() -> Self {
+        Self {
+            inner: Mutex::new(RefCell::new(ChannelState::new())),
+        }
+    }
+
+    fn lock<R>(&self, f: impl FnOnce(&mut ChannelState<T, K, N>) -> R) -> R {
+        self.inner.lock(|rc| f(&mut *unwrap!(rc.try_borrow_mut())))
+    }
+
+    fn try_receive_with_context(&self, cx: Option<&mut Context<'_>>) -> Result<T, TryReceiveError> {
+        self.lock(|c| c.try_receive_with_context(cx))
+    }
+
+    /// Poll the channel for the next message
+    pub fn poll_receive(&self, cx: &mut Context<'_>) -> Poll<T> {
+        self.lock(|c| c.poll_receive(cx))
+    }
+
+    fn try_send_with_context(&self, m: T, cx: Option<&mut Context<'_>>) -> Result<(), TrySendError<T>> {
+        self.lock(|c| c.try_send_with_context(m, cx))
+    }
+
+    /// Allows a poll_fn to poll until the channel is ready to receive
+    pub fn poll_ready_to_receive(&self, cx: &mut Context<'_>) -> Poll<()> {
+        self.lock(|c| c.poll_ready_to_receive(cx))
+    }
+
+    /// Allows a poll_fn to poll until the channel is ready to send
+    pub fn poll_ready_to_send(&self, cx: &mut Context<'_>) -> Poll<()> {
+        self.lock(|c| c.poll_ready_to_send(cx))
+    }
+
+    /// Get a sender for this channel.
+    pub fn sender(&self) -> Sender<'_, M, T, K, N> {
+        Sender { channel: self }
+    }
+
+    /// Get a receiver for this channel.
+    pub fn receiver(&self) -> Receiver<'_, M, T, K, N> {
+        Receiver { channel: self }
+    }
+
+    /// Send a value, waiting until there is capacity.
+    ///
+    /// Sending completes when the value has been pushed to the channel's queue.
+    /// This doesn't mean the value has been received yet.
+    pub fn send(&self, message: T) -> SendFuture<'_, M, T, K, N> {
+        SendFuture {
+            channel: self,
+            message: Some(message),
+        }
+    }
+
+    /// Attempt to immediately send a message.
+    ///
+    /// This method differs from [`send`](PriorityChannel::send) by returning immediately if the channel's
+    /// buffer is full, instead of waiting.
+    ///
+    /// # Errors
+    ///
+    /// If the channel capacity has been reached, i.e., the channel has `n`
+    /// buffered values where `n` is the argument passed to [`PriorityChannel`], then an
+    /// error is returned.
+    pub fn try_send(&self, message: T) -> Result<(), TrySendError<T>> {
+        self.lock(|c| c.try_send(message))
+    }
+
+    /// Receive the next value.
+    ///
+    /// If there are no messages in the channel's buffer, this method will
+    /// wait until a message is sent.
+    pub fn receive(&self) -> ReceiveFuture<'_, M, T, K, N> {
+        ReceiveFuture { channel: self }
+    }
+
+    /// Attempt to immediately receive a message.
+    ///
+    /// This method will either receive a message from the channel immediately or return an error
+    /// if the channel is empty.
+    pub fn try_receive(&self) -> Result<T, TryReceiveError> {
+        self.lock(|c| c.try_receive())
+    }
+}
+
+/// Implements the DynamicChannel to allow creating types that are unaware of the queue size with the
+/// tradeoff cost of dynamic dispatch.
+impl<M, T, K, const N: usize> DynamicChannel<T> for PriorityChannel<M, T, K, N>
+where
+    T: Ord,
+    K: heapless::binary_heap::Kind,
+    M: RawMutex,
+{
+    fn try_send_with_context(&self, m: T, cx: Option<&mut Context<'_>>) -> Result<(), TrySendError<T>> {
+        PriorityChannel::try_send_with_context(self, m, cx)
+    }
+
+    fn try_receive_with_context(&self, cx: Option<&mut Context<'_>>) -> Result<T, TryReceiveError> {
+        PriorityChannel::try_receive_with_context(self, cx)
+    }
+
+    fn poll_ready_to_send(&self, cx: &mut Context<'_>) -> Poll<()> {
+        PriorityChannel::poll_ready_to_send(self, cx)
+    }
+
+    fn poll_ready_to_receive(&self, cx: &mut Context<'_>) -> Poll<()> {
+        PriorityChannel::poll_ready_to_receive(self, cx)
+    }
+
+    fn poll_receive(&self, cx: &mut Context<'_>) -> Poll<T> {
+        PriorityChannel::poll_receive(self, cx)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use core::time::Duration;
+
+    use futures_executor::ThreadPool;
+    use futures_timer::Delay;
+    use futures_util::task::SpawnExt;
+    use static_cell::StaticCell;
+
+    use super::*;
+    use crate::blocking_mutex::raw::{CriticalSectionRawMutex, NoopRawMutex};
+
+    fn capacity<T, const N: usize>(c: &ChannelState<T, N>) -> usize {
+        c.queue.capacity() - c.queue.len()
+    }
+
+    #[test]
+    fn sending_once() {
+        let mut c = ChannelState::<u32, 3>::new();
+        assert!(c.try_send(1).is_ok());
+        assert_eq!(capacity(&c), 2);
+    }
+
+    #[test]
+    fn sending_when_full() {
+        let mut c = ChannelState::<u32, 3>::new();
+        let _ = c.try_send(1);
+        let _ = c.try_send(1);
+        let _ = c.try_send(1);
+        match c.try_send(2) {
+            Err(TrySendError::Full(2)) => assert!(true),
+            _ => assert!(false),
+        }
+        assert_eq!(capacity(&c), 0);
+    }
+
+    #[test]
+    fn receiving_once_with_one_send() {
+        let mut c = ChannelState::<u32, 3>::new();
+        assert!(c.try_send(1).is_ok());
+        assert_eq!(c.try_receive().unwrap(), 1);
+        assert_eq!(capacity(&c), 3);
+    }
+
+    #[test]
+    fn receiving_when_empty() {
+        let mut c = ChannelState::<u32, 3>::new();
+        match c.try_receive() {
+            Err(TryReceiveError::Empty) => assert!(true),
+            _ => assert!(false),
+        }
+        assert_eq!(capacity(&c), 3);
+    }
+
+    #[test]
+    fn simple_send_and_receive() {
+        let c = PriorityChannel::<NoopRawMutex, u32, 3>::new();
+        assert!(c.try_send(1).is_ok());
+        assert_eq!(c.try_receive().unwrap(), 1);
+    }
+
+    #[test]
+    fn cloning() {
+        let c = PriorityChannel::<NoopRawMutex, u32, 3>::new();
+        let r1 = c.receiver();
+        let s1 = c.sender();
+
+        let _ = r1.clone();
+        let _ = s1.clone();
+    }
+
+    #[test]
+    fn dynamic_dispatch() {
+        let c = PriorityChannel::<NoopRawMutex, u32, 3>::new();
+        let s: DynamicSender<'_, u32> = c.sender().into();
+        let r: DynamicReceiver<'_, u32> = c.receiver().into();
+
+        assert!(s.try_send(1).is_ok());
+        assert_eq!(r.try_receive().unwrap(), 1);
+    }
+
+    #[futures_test::test]
+    async fn receiver_receives_given_try_send_async() {
+        let executor = ThreadPool::new().unwrap();
+
+        static CHANNEL: StaticCell<PriorityChannel<CriticalSectionRawMutex, u32, 3>> = StaticCell::new();
+        let c = &*CHANNEL.init(PriorityChannel::new());
+        let c2 = c;
+        assert!(executor
+            .spawn(async move {
+                assert!(c2.try_send(1).is_ok());
+            })
+            .is_ok());
+        assert_eq!(c.receive().await, 1);
+    }
+
+    #[futures_test::test]
+    async fn sender_send_completes_if_capacity() {
+        let c = PriorityChannel::<CriticalSectionRawMutex, u32, 1>::new();
+        c.send(1).await;
+        assert_eq!(c.receive().await, 1);
+    }
+
+    #[futures_test::test]
+    async fn senders_sends_wait_until_capacity() {
+        let executor = ThreadPool::new().unwrap();
+
+        static CHANNEL: StaticCell<PriorityChannel<CriticalSectionRawMutex, u32, 1>> = StaticCell::new();
+        let c = &*CHANNEL.init(PriorityChannel::new());
+        assert!(c.try_send(1).is_ok());
+
+        let c2 = c;
+        let send_task_1 = executor.spawn_with_handle(async move { c2.send(2).await });
+        let c2 = c;
+        let send_task_2 = executor.spawn_with_handle(async move { c2.send(3).await });
+        // Wish I could think of a means of determining that the async send is waiting instead.
+        // However, I've used the debugger to observe that the send does indeed wait.
+        Delay::new(Duration::from_millis(500)).await;
+        assert_eq!(c.receive().await, 1);
+        assert!(executor
+            .spawn(async move {
+                loop {
+                    c.receive().await;
+                }
+            })
+            .is_ok());
+        send_task_1.unwrap().await;
+        send_task_2.unwrap().await;
+    }
+}