Merge pull request #226 from huntc/mpsc

Multi Producer Single Consumer channel

5 files changed, 994 insertions, 0 deletions

diff --git a/embassy/Cargo.toml b/embassy/Cargo.toml
index b2ad80495..c03fc0df5 100644
--- a/embassy/Cargo.toml
+++ b/embassy/Cargo.toml
@@ -3,6 +3,7 @@ name = "embassy"
 version = "0.1.0"
 authors = ["Dario Nieuwenhuis <[email protected]>"]
 edition = "2018"
+resolver = "2"
 
 [features]
 default = []
@@ -36,3 +37,10 @@ embedded-hal = "0.2.5"
 
 # Workaround https://github.com/japaric/cast.rs/pull/27
 cast = { version = "=0.2.3", default-features = false }
+
+[dev-dependencies]
+embassy = { path = ".", features = ["executor-agnostic"] }
+futures-executor = { version = "0.3", features = [ "thread-pool" ] }
+futures-test = "0.3"
+futures-timer = "0.3"
+futures-util = { version = "0.3", features = [ "channel" ] }
diff --git a/embassy/src/util/mod.rs b/embassy/src/util/mod.rs
index 88ae5c285..87d313e28 100644
--- a/embassy/src/util/mod.rs
+++ b/embassy/src/util/mod.rs
@@ -11,6 +11,7 @@ mod waker;
 
 pub use drop_bomb::*;
 pub use forever::*;
+pub mod mpsc;
 pub use mutex::*;
 pub use on_drop::*;
 pub use portal::*;
diff --git a/embassy/src/util/mpsc.rs b/embassy/src/util/mpsc.rs
new file mode 100644
index 000000000..cc9e2a5dd
--- /dev/null
+++ b/embassy/src/util/mpsc.rs
@@ -0,0 +1,854 @@
+//! A multi-producer, single-consumer queue for sending values between
+//! asynchronous tasks. This queue takes a Mutex type so that various
+//! targets can be attained. For example, a ThreadModeMutex can be used
+//! for single-core Cortex-M targets where messages are only passed
+//! between tasks running in thread mode. Similarly, a CriticalSectionMutex
+//! can also be used for single-core targets where messages are to be
+//! passed from exception mode e.g. out of an interrupt handler.
+//!
+//! This module provides a bounded channel that has a limit on the number of
+//! messages that it can store, and if this limit is reached, trying to send
+//! another message will result in an error being returned.
+//!
+//! Similar to the `mpsc` channels provided by `std`, the channel constructor
+//! functions provide separate send and receive handles, [`Sender`] and
+//! [`Receiver`]. If there is no message to read, the current task will be
+//! notified when a new value is sent. [`Sender`] allows sending values into
+//! the channel. If the bounded channel is at capacity, the send is rejected.
+//!
+//! # Disconnection
+//!
+//! When all [`Sender`] handles have been dropped, it is no longer
+//! possible to send values into the channel. This is considered the termination
+//! event of the stream.
+//!
+//! If the [`Receiver`] handle is dropped, then messages can no longer
+//! be read out of the channel. In this case, all further attempts to send will
+//! result in an error.
+//!
+//! # Clean Shutdown
+//!
+//! When the [`Receiver`] is dropped, it is possible for unprocessed messages to
+//! remain in the channel. Instead, it is usually desirable to perform a "clean"
+//! shutdown. To do this, the receiver first calls `close`, which will prevent
+//! any further messages to be sent into the channel. Then, the receiver
+//! consumes the channel to completion, at which point the receiver can be
+//! dropped.
+//!
+//! This channel and its associated types were derived from https://docs.rs/tokio/0.1.22/tokio/sync/mpsc/fn.channel.html
+
+use core::cell::UnsafeCell;
+use core::fmt;
+use core::marker::PhantomData;
+use core::mem::MaybeUninit;
+use core::pin::Pin;
+use core::ptr;
+use core::task::Context;
+use core::task::Poll;
+use core::task::Waker;
+
+use futures::Future;
+
+use super::CriticalSectionMutex;
+use super::Mutex;
+use super::NoopMutex;
+use super::ThreadModeMutex;
+use super::WakerRegistration;
+
+/// Send values to the associated `Receiver`.
+///
+/// Instances are created by the [`split`](split) function.
+pub struct Sender<'ch, M, T, const N: usize>
+where
+    M: Mutex<Data = ()>,
+{
+    channel_cell: &'ch UnsafeCell<ChannelCell<M, T, N>>,
+}
+
+// Safe to pass the sender around
+unsafe impl<'ch, M, T, const N: usize> Send for Sender<'ch, M, T, N> where M: Mutex<Data = ()> + Sync
+{}
+unsafe impl<'ch, M, T, const N: usize> Sync for Sender<'ch, M, T, N> where M: Mutex<Data = ()> + Sync
+{}
+
+/// Receive values from the associated `Sender`.
+///
+/// Instances are created by the [`split`](split) function.
+pub struct Receiver<'ch, M, T, const N: usize>
+where
+    M: Mutex<Data = ()>,
+{
+    channel_cell: &'ch UnsafeCell<ChannelCell<M, T, N>>,
+    _receiver_consumed: &'ch mut PhantomData<()>,
+}
+
+// Safe to pass the receiver around
+unsafe impl<'ch, M, T, const N: usize> Send for Receiver<'ch, M, T, N> where
+    M: Mutex<Data = ()> + Sync
+{
+}
+unsafe impl<'ch, M, T, const N: usize> Sync for Receiver<'ch, M, T, N> where
+    M: Mutex<Data = ()> + Sync
+{
+}
+
+/// Splits a bounded mpsc channel into a `Sender` and `Receiver`.
+///
+/// All data sent on `Sender` will become available on `Receiver` in the same
+/// order as it was sent.
+///
+/// The `Sender` can be cloned to `send` to the same channel from multiple code
+/// locations. Only one `Receiver` is valid.
+///
+/// If the `Receiver` is disconnected while trying to `send`, the `send` method
+/// will return a `SendError`. Similarly, if `Sender` is disconnected while
+/// trying to `recv`, the `recv` method will return a `RecvError`.
+///
+/// Note that when splitting the channel, the sender and receiver cannot outlive
+/// their channel. The following will therefore fail compilation:
+////
+/// ```compile_fail
+/// use embassy::util::mpsc;
+/// use embassy::util::mpsc::{Channel, WithThreadModeOnly};
+///
+/// let (sender, receiver) = {
+///    let mut channel = Channel::<WithThreadModeOnly, u32, 3>::with_thread_mode_only();
+///     mpsc::split(&mut channel)
+/// };
+/// ```
+pub fn split<M, T, const N: usize>(
+    channel: &mut Channel<M, T, N>,
+) -> (Sender<M, T, N>, Receiver<M, T, N>)
+where
+    M: Mutex<Data = ()>,
+{
+    let sender = Sender {
+        channel_cell: &channel.channel_cell,
+    };
+    let receiver = Receiver {
+        channel_cell: &channel.channel_cell,
+        _receiver_consumed: &mut channel.receiver_consumed,
+    };
+    Channel::lock(&channel.channel_cell, |c| {
+        c.register_receiver();
+        c.register_sender();
+    });
+    (sender, receiver)
+}
+
+impl<'ch, M, T, const N: usize> Receiver<'ch, M, T, N>
+where
+    M: Mutex<Data = ()>,
+{
+    /// Receives the next value for this receiver.
+    ///
+    /// This method returns `None` if the channel has been closed and there are
+    /// no remaining messages in the channel's buffer. This indicates that no
+    /// further values can ever be received from this `Receiver`. The channel is
+    /// closed when all senders have been dropped, or when [`close`] is called.
+    ///
+    /// If there are no messages in the channel's buffer, but the channel has
+    /// not yet been closed, this method will sleep until a message is sent or
+    /// the channel is closed.
+    ///
+    /// Note that if [`close`] is called, but there are still outstanding
+    /// messages from before it was closed, the channel is not considered
+    /// closed by `recv` until they are all consumed.
+    ///
+    /// [`close`]: Self::close
+    pub async fn recv(&mut self) -> Option<T> {
+        futures::future::poll_fn(|cx| self.recv_poll(cx)).await
+    }
+
+    fn recv_poll(&mut self, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        Channel::lock(self.channel_cell, |c| {
+            match c.try_recv_with_context(Some(cx)) {
+                Ok(v) => Poll::Ready(Some(v)),
+                Err(TryRecvError::Closed) => Poll::Ready(None),
+                Err(TryRecvError::Empty) => Poll::Pending,
+            }
+        })
+    }
+
+    /// Attempts to immediately receive a message on this `Receiver`
+    ///
+    /// This method will either receive a message from the channel immediately or return an error
+    /// if the channel is empty.
+    pub fn try_recv(&self) -> Result<T, TryRecvError> {
+        Channel::lock(self.channel_cell, |c| c.try_recv())
+    }
+
+    /// Closes the receiving half of a channel without dropping it.
+    ///
+    /// This prevents any further messages from being sent on the channel while
+    /// still enabling the receiver to drain messages that are buffered.
+    ///
+    /// To guarantee that no messages are dropped, after calling `close()`,
+    /// `recv()` must be called until `None` is returned. If there are
+    /// outstanding messages, the `recv` method will not return `None`
+    /// until those are released.
+    ///
+    pub fn close(&mut self) {
+        Channel::lock(self.channel_cell, |c| c.close())
+    }
+}
+
+impl<'ch, M, T, const N: usize> Drop for Receiver<'ch, M, T, N>
+where
+    M: Mutex<Data = ()>,
+{
+    fn drop(&mut self) {
+        Channel::lock(self.channel_cell, |c| c.deregister_receiver())
+    }
+}
+
+impl<'ch, M, T, const N: usize> Sender<'ch, M, T, N>
+where
+    M: Mutex<Data = ()>,
+{
+    /// Sends a value, waiting until there is capacity.
+    ///
+    /// A successful send occurs when it is determined that the other end of the
+    /// channel has not hung up already. An unsuccessful send would be one where
+    /// the corresponding receiver has already been closed. Note that a return
+    /// value of `Err` means that the data will never be received, but a return
+    /// value of `Ok` does not mean that the data will be received. It is
+    /// possible for the corresponding receiver to hang up immediately after
+    /// this function returns `Ok`.
+    ///
+    /// # Errors
+    ///
+    /// If the receive half of the channel is closed, either due to [`close`]
+    /// being called or the [`Receiver`] handle dropping, the function returns
+    /// an error. The error includes the value passed to `send`.
+    ///
+    /// [`close`]: Receiver::close
+    /// [`Receiver`]: Receiver
+    pub async fn send(&self, message: T) -> Result<(), SendError<T>> {
+        SendFuture {
+            sender: self.clone(),
+            message: Some(message),
+        }
+        .await
+    }
+
+    /// Attempts to immediately send a message on this `Sender`
+    ///
+    /// This method differs from [`send`] by returning immediately if the channel's
+    /// buffer is full or no receiver is waiting to acquire some data. Compared
+    /// with [`send`], this function has two failure cases instead of one (one for
+    /// disconnection, one for a full buffer).
+    ///
+    /// # Errors
+    ///
+    /// If the channel capacity has been reached, i.e., the channel has `n`
+    /// buffered values where `n` is the argument passed to [`channel`], then an
+    /// error is returned.
+    ///
+    /// If the receive half of the channel is closed, either due to [`close`]
+    /// being called or the [`Receiver`] handle dropping, the function returns
+    /// an error. The error includes the value passed to `send`.
+    ///
+    /// [`send`]: Sender::send
+    /// [`channel`]: channel
+    /// [`close`]: Receiver::close
+    pub fn try_send(&self, message: T) -> Result<(), TrySendError<T>> {
+        Channel::lock(self.channel_cell, |c| c.try_send(message))
+    }
+
+    /// Completes when the receiver has dropped.
+    ///
+    /// This allows the producers to get notified when interest in the produced
+    /// values is canceled and immediately stop doing work.
+    pub async fn closed(&self) {
+        CloseFuture {
+            sender: self.clone(),
+        }
+        .await
+    }
+
+    /// Checks if the channel has been closed. This happens when the
+    /// [`Receiver`] is dropped, or when the [`Receiver::close`] method is
+    /// called.
+    ///
+    /// [`Receiver`]: crate::sync::mpsc::Receiver
+    /// [`Receiver::close`]: crate::sync::mpsc::Receiver::close
+    pub fn is_closed(&self) -> bool {
+        Channel::lock(self.channel_cell, |c| c.is_closed())
+    }
+}
+
+struct SendFuture<'ch, M, T, const N: usize>
+where
+    M: Mutex<Data = ()>,
+{
+    sender: Sender<'ch, M, T, N>,
+    message: Option<T>,
+}
+
+impl<'ch, M, T, const N: usize> Future for SendFuture<'ch, M, T, N>
+where
+    M: Mutex<Data = ()>,
+{
+    type Output = Result<(), SendError<T>>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match self.message.take() {
+            Some(m) => match Channel::lock(self.sender.channel_cell, |c| {
+                c.try_send_with_context(m, Some(cx))
+            }) {
+                Ok(..) => Poll::Ready(Ok(())),
+                Err(TrySendError::Closed(m)) => Poll::Ready(Err(SendError(m))),
+                Err(TrySendError::Full(m)) => {
+                    self.message.insert(m);
+                    Poll::Pending
+                }
+            },
+            None => panic!("Message cannot be None"),
+        }
+    }
+}
+
+impl<'ch, M, T, const N: usize> Unpin for SendFuture<'ch, M, T, N> where M: Mutex<Data = ()> {}
+
+struct CloseFuture<'ch, M, T, const N: usize>
+where
+    M: Mutex<Data = ()>,
+{
+    sender: Sender<'ch, M, T, N>,
+}
+
+impl<'ch, M, T, const N: usize> Future for CloseFuture<'ch, M, T, N>
+where
+    M: Mutex<Data = ()>,
+{
+    type Output = ();
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        if Channel::lock(self.sender.channel_cell, |c| {
+            c.is_closed_with_context(Some(cx))
+        }) {
+            Poll::Ready(())
+        } else {
+            Poll::Pending
+        }
+    }
+}
+
+impl<'ch, M, T, const N: usize> Drop for Sender<'ch, M, T, N>
+where
+    M: Mutex<Data = ()>,
+{
+    fn drop(&mut self) {
+        Channel::lock(self.channel_cell, |c| c.deregister_sender())
+    }
+}
+
+impl<'ch, M, T, const N: usize> Clone for Sender<'ch, M, T, N>
+where
+    M: Mutex<Data = ()>,
+{
+    #[allow(clippy::clone_double_ref)]
+    fn clone(&self) -> Self {
+        Channel::lock(self.channel_cell, |c| c.register_sender());
+        Sender {
+            channel_cell: self.channel_cell.clone(),
+        }
+    }
+}
+
+/// An error returned from the [`try_recv`] method.
+///
+/// [`try_recv`]: super::Receiver::try_recv
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub enum TryRecvError {
+    /// A message could not be received because the channel is empty.
+    Empty,
+
+    /// The message could not be received because the channel is empty and closed.
+    Closed,
+}
+
+/// Error returned by the `Sender`.
+#[derive(Debug)]
+pub struct SendError<T>(pub T);
+
+impl<T> fmt::Display for SendError<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(fmt, "channel closed")
+    }
+}
+
+/// This enumeration is the list of the possible error outcomes for the
+/// [try_send](super::Sender::try_send) method.
+#[derive(Debug)]
+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),
+
+    /// The receive half of the channel was explicitly closed or has been
+    /// dropped.
+    Closed(T),
+}
+
+impl<T> fmt::Display for TrySendError<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            fmt,
+            "{}",
+            match self {
+                TrySendError::Full(..) => "no available capacity",
+                TrySendError::Closed(..) => "channel closed",
+            }
+        )
+    }
+}
+
+struct ChannelState<T, const N: usize> {
+    buf: [MaybeUninit<UnsafeCell<T>>; N],
+    read_pos: usize,
+    write_pos: usize,
+    full: bool,
+    closed: bool,
+    receiver_registered: bool,
+    senders_registered: u32,
+    receiver_waker: WakerRegistration,
+    senders_waker: WakerRegistration,
+}
+
+impl<T, const N: usize> ChannelState<T, N> {
+    const INIT: MaybeUninit<UnsafeCell<T>> = MaybeUninit::uninit();
+
+    const fn new() -> Self {
+        ChannelState {
+            buf: [Self::INIT; N],
+            read_pos: 0,
+            write_pos: 0,
+            full: false,
+            closed: false,
+            receiver_registered: false,
+            senders_registered: 0,
+            receiver_waker: WakerRegistration::new(),
+            senders_waker: WakerRegistration::new(),
+        }
+    }
+
+    fn try_recv(&mut self) -> Result<T, TryRecvError> {
+        self.try_recv_with_context(None)
+    }
+
+    fn try_recv_with_context(&mut self, cx: Option<&mut Context<'_>>) -> Result<T, TryRecvError> {
+        if self.read_pos != self.write_pos || self.full {
+            if self.full {
+                self.full = false;
+                self.senders_waker.wake();
+            }
+            let message = unsafe { (self.buf[self.read_pos]).assume_init_mut().get().read() };
+            self.read_pos = (self.read_pos + 1) % self.buf.len();
+            Ok(message)
+        } else if !self.closed {
+            cx.into_iter()
+                .for_each(|cx| self.set_receiver_waker(&cx.waker()));
+            Err(TryRecvError::Empty)
+        } else {
+            Err(TryRecvError::Closed)
+        }
+    }
+
+    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>> {
+        if !self.closed {
+            if !self.full {
+                self.buf[self.write_pos] = MaybeUninit::new(message.into());
+                self.write_pos = (self.write_pos + 1) % self.buf.len();
+                if self.write_pos == self.read_pos {
+                    self.full = true;
+                }
+                self.receiver_waker.wake();
+                Ok(())
+            } else {
+                cx.into_iter()
+                    .for_each(|cx| self.set_senders_waker(&cx.waker()));
+                Err(TrySendError::Full(message))
+            }
+        } else {
+            Err(TrySendError::Closed(message))
+        }
+    }
+
+    fn close(&mut self) {
+        self.receiver_waker.wake();
+        self.closed = true;
+    }
+
+    fn is_closed(&mut self) -> bool {
+        self.is_closed_with_context(None)
+    }
+
+    fn is_closed_with_context(&mut self, cx: Option<&mut Context<'_>>) -> bool {
+        if self.closed {
+            cx.into_iter()
+                .for_each(|cx| self.set_senders_waker(&cx.waker()));
+            true
+        } else {
+            false
+        }
+    }
+
+    fn register_receiver(&mut self) {
+        assert!(!self.receiver_registered);
+        self.receiver_registered = true;
+    }
+
+    fn deregister_receiver(&mut self) {
+        if self.receiver_registered {
+            self.closed = true;
+            self.senders_waker.wake();
+        }
+        self.receiver_registered = false;
+    }
+
+    fn register_sender(&mut self) {
+        self.senders_registered += 1;
+    }
+
+    fn deregister_sender(&mut self) {
+        assert!(self.senders_registered > 0);
+        self.senders_registered -= 1;
+        if self.senders_registered == 0 {
+            self.receiver_waker.wake();
+            self.closed = true;
+        }
+    }
+
+    fn set_receiver_waker(&mut self, receiver_waker: &Waker) {
+        self.receiver_waker.register(receiver_waker);
+    }
+
+    fn set_senders_waker(&mut self, senders_waker: &Waker) {
+        // Dispose of any existing sender causing them to be polled again.
+        // This could cause a spin given multiple concurrent senders, however given that
+        // most sends only block waiting for the receiver to become active, this should
+        // be a short-lived activity. The upside is a greatly simplified implementation
+        // that avoids the need for intrusive linked-lists and unsafe operations on pinned
+        // pointers.
+        self.senders_waker.wake();
+        self.senders_waker.register(senders_waker);
+    }
+}
+
+impl<T, const N: usize> Drop for ChannelState<T, N> {
+    fn drop(&mut self) {
+        while self.read_pos != self.write_pos || self.full {
+            self.full = false;
+            unsafe { ptr::drop_in_place(self.buf[self.read_pos].as_mut_ptr()) };
+            self.read_pos = (self.read_pos + 1) % N;
+        }
+    }
+}
+
+/// A a bounded mpsc 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 Channel<M, T, const N: usize>
+where
+    M: Mutex<Data = ()>,
+{
+    channel_cell: UnsafeCell<ChannelCell<M, T, N>>,
+    receiver_consumed: PhantomData<()>,
+}
+
+struct ChannelCell<M, T, const N: usize>
+where
+    M: Mutex<Data = ()>,
+{
+    mutex: M,
+    state: ChannelState<T, N>,
+}
+
+pub type WithCriticalSections = CriticalSectionMutex<()>;
+
+pub type WithThreadModeOnly = ThreadModeMutex<()>;
+
+pub type WithNoThreads = NoopMutex<()>;
+
+impl<M, T, const N: usize> Channel<M, T, N>
+where
+    M: Mutex<Data = ()>,
+{
+    /// Establish a new bounded channel. For example, to create one with a NoopMutex:
+    ///
+    /// ```
+    /// use embassy::util::mpsc;
+    /// use embassy::util::mpsc::{Channel, WithNoThreads};
+    ///
+    /// // Declare a bounded channel of 3 u32s.
+    /// let mut channel = Channel::<WithNoThreads, u32, 3>::new();
+    /// // once we have a channel, obtain its sender and receiver
+    /// let (sender, receiver) = mpsc::split(&mut channel);
+    /// ```
+    pub fn new() -> Self {
+        let mutex = M::new(());
+        let state = ChannelState::new();
+        let channel_cell = ChannelCell { mutex, state };
+        Channel {
+            channel_cell: UnsafeCell::new(channel_cell),
+            receiver_consumed: PhantomData,
+        }
+    }
+
+    fn lock<R>(
+        channel_cell: &UnsafeCell<ChannelCell<M, T, N>>,
+        f: impl FnOnce(&mut ChannelState<T, N>) -> R,
+    ) -> R {
+        unsafe {
+            let channel_cell = &mut *(channel_cell.get());
+            let mutex = &mut channel_cell.mutex;
+            let mut state = &mut channel_cell.state;
+            mutex.lock(|_| f(&mut state))
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use core::time::Duration;
+
+    use futures::task::SpawnExt;
+    use futures_executor::ThreadPool;
+    use futures_timer::Delay;
+
+    use crate::util::Forever;
+
+    use super::*;
+
+    fn capacity<T, const N: usize>(c: &ChannelState<T, N>) -> usize {
+        if !c.full {
+            if c.write_pos > c.read_pos {
+                (c.buf.len() - c.write_pos) + c.read_pos
+            } else {
+                (c.buf.len() - c.read_pos) + c.write_pos
+            }
+        } else {
+            0
+        }
+    }
+
+    #[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 sending_when_closed() {
+        let mut c = ChannelState::<u32, 3>::new();
+        c.closed = true;
+        match c.try_send(2) {
+            Err(TrySendError::Closed(2)) => assert!(true),
+            _ => assert!(false),
+        }
+    }
+
+    #[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_recv().unwrap(), 1);
+        assert_eq!(capacity(&c), 3);
+    }
+
+    #[test]
+    fn receiving_when_empty() {
+        let mut c = ChannelState::<u32, 3>::new();
+        match c.try_recv() {
+            Err(TryRecvError::Empty) => assert!(true),
+            _ => assert!(false),
+        }
+        assert_eq!(capacity(&c), 3);
+    }
+
+    #[test]
+    fn receiving_when_closed() {
+        let mut c = ChannelState::<u32, 3>::new();
+        c.closed = true;
+        match c.try_recv() {
+            Err(TryRecvError::Closed) => assert!(true),
+            _ => assert!(false),
+        }
+    }
+
+    #[test]
+    fn simple_send_and_receive() {
+        let mut c = Channel::<WithNoThreads, u32, 3>::new();
+        let (s, r) = split(&mut c);
+        assert!(s.clone().try_send(1).is_ok());
+        assert_eq!(r.try_recv().unwrap(), 1);
+    }
+
+    #[test]
+    fn should_close_without_sender() {
+        let mut c = Channel::<WithNoThreads, u32, 3>::new();
+        let (s, r) = split(&mut c);
+        drop(s);
+        match r.try_recv() {
+            Err(TryRecvError::Closed) => assert!(true),
+            _ => assert!(false),
+        }
+    }
+
+    #[test]
+    fn should_close_once_drained() {
+        let mut c = Channel::<WithNoThreads, u32, 3>::new();
+        let (s, r) = split(&mut c);
+        assert!(s.try_send(1).is_ok());
+        drop(s);
+        assert_eq!(r.try_recv().unwrap(), 1);
+        match r.try_recv() {
+            Err(TryRecvError::Closed) => assert!(true),
+            _ => assert!(false),
+        }
+    }
+
+    #[test]
+    fn should_reject_send_when_receiver_dropped() {
+        let mut c = Channel::<WithNoThreads, u32, 3>::new();
+        let (s, r) = split(&mut c);
+        drop(r);
+        match s.try_send(1) {
+            Err(TrySendError::Closed(1)) => assert!(true),
+            _ => assert!(false),
+        }
+    }
+
+    #[test]
+    fn should_reject_send_when_channel_closed() {
+        let mut c = Channel::<WithNoThreads, u32, 3>::new();
+        let (s, mut r) = split(&mut c);
+        assert!(s.try_send(1).is_ok());
+        r.close();
+        assert_eq!(r.try_recv().unwrap(), 1);
+        match r.try_recv() {
+            Err(TryRecvError::Closed) => assert!(true),
+            _ => assert!(false),
+        }
+        assert!(s.is_closed());
+    }
+
+    #[futures_test::test]
+    async fn receiver_closes_when_sender_dropped_async() {
+        let executor = ThreadPool::new().unwrap();
+
+        static CHANNEL: Forever<Channel<WithCriticalSections, u32, 3>> = Forever::new();
+        let c = CHANNEL.put(Channel::new());
+        let (s, mut r) = split(c);
+        assert!(executor
+            .spawn(async move {
+                drop(s);
+            })
+            .is_ok());
+        assert_eq!(r.recv().await, None);
+    }
+
+    #[futures_test::test]
+    async fn receiver_receives_given_try_send_async() {
+        let executor = ThreadPool::new().unwrap();
+
+        static CHANNEL: Forever<Channel<WithCriticalSections, u32, 3>> = Forever::new();
+        let c = CHANNEL.put(Channel::new());
+        let (s, mut r) = split(c);
+        assert!(executor
+            .spawn(async move {
+                assert!(s.try_send(1).is_ok());
+            })
+            .is_ok());
+        assert_eq!(r.recv().await, Some(1));
+    }
+
+    #[futures_test::test]
+    async fn sender_send_completes_if_capacity() {
+        let mut c = Channel::<WithCriticalSections, u32, 1>::new();
+        let (s, mut r) = split(&mut c);
+        assert!(s.send(1).await.is_ok());
+        assert_eq!(r.recv().await, Some(1));
+    }
+
+    #[futures_test::test]
+    async fn sender_send_completes_if_closed() {
+        static CHANNEL: Forever<Channel<WithCriticalSections, u32, 1>> = Forever::new();
+        let c = CHANNEL.put(Channel::new());
+        let (s, r) = split(c);
+        drop(r);
+        match s.send(1).await {
+            Err(SendError(1)) => assert!(true),
+            _ => assert!(false),
+        }
+    }
+
+    #[futures_test::test]
+    async fn senders_sends_wait_until_capacity() {
+        let executor = ThreadPool::new().unwrap();
+
+        static CHANNEL: Forever<Channel<WithCriticalSections, u32, 1>> = Forever::new();
+        let c = CHANNEL.put(Channel::new());
+        let (s0, mut r) = split(c);
+        assert!(s0.try_send(1).is_ok());
+        let s1 = s0.clone();
+        let send_task_1 = executor.spawn_with_handle(async move { s0.send(2).await });
+        let send_task_2 = executor.spawn_with_handle(async move { s1.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.
+        assert!(Delay::new(Duration::from_millis(500)).await.is_ok());
+        assert_eq!(r.recv().await, Some(1));
+        assert!(executor
+            .spawn(async move { while let Some(_) = r.recv().await {} })
+            .is_ok());
+        assert!(send_task_1.unwrap().await.is_ok());
+        assert!(send_task_2.unwrap().await.is_ok());
+    }
+
+    #[futures_test::test]
+    async fn sender_close_completes_if_closing() {
+        static CHANNEL: Forever<Channel<WithCriticalSections, u32, 1>> = Forever::new();
+        let c = CHANNEL.put(Channel::new());
+        let (s, mut r) = split(c);
+        r.close();
+        s.closed().await;
+    }
+
+    #[futures_test::test]
+    async fn sender_close_completes_if_closed() {
+        static CHANNEL: Forever<Channel<WithCriticalSections, u32, 1>> = Forever::new();
+        let c = CHANNEL.put(Channel::new());
+        let (s, r) = split(c);
+        drop(r);
+        s.closed().await;
+    }
+}
diff --git a/embassy/src/util/mutex.rs b/embassy/src/util/mutex.rs
index e4b7764ce..0506ffe6f 100644
--- a/embassy/src/util/mutex.rs
+++ b/embassy/src/util/mutex.rs
@@ -1,6 +1,19 @@
 use core::cell::UnsafeCell;
 use critical_section::CriticalSection;
 
+/// Any object implementing this trait guarantees exclusive access to the data contained
+/// within the mutex for the duration of the lock.
+/// Adapted from https://github.com/rust-embedded/mutex-trait.
+pub trait Mutex {
+    /// Data protected by the mutex.
+    type Data;
+
+    fn new(data: Self::Data) -> Self;
+
+    /// Creates a critical section and grants temporary access to the protected data.
+    fn lock<R>(&mut self, f: impl FnOnce(&Self::Data) -> R) -> R;
+}
+
 /// A "mutex" based on critical sections
 ///
 /// # Safety
@@ -33,6 +46,18 @@ impl<T> CriticalSectionMutex<T> {
     }
 }
 
+impl<T> Mutex for CriticalSectionMutex<T> {
+    type Data = T;
+
+    fn new(data: T) -> Self {
+        Self::new(data)
+    }
+
+    fn lock<R>(&mut self, f: impl FnOnce(&Self::Data) -> R) -> R {
+        critical_section::with(|cs| f(self.borrow(cs)))
+    }
+}
+
 /// A "mutex" that only allows borrowing from thread mode.
 ///
 /// # Safety
@@ -70,6 +95,18 @@ impl<T> ThreadModeMutex<T> {
     }
 }
 
+impl<T> Mutex for ThreadModeMutex<T> {
+    type Data = T;
+
+    fn new(data: T) -> Self {
+        Self::new(data)
+    }
+
+    fn lock<R>(&mut self, f: impl FnOnce(&Self::Data) -> R) -> R {
+        f(self.borrow())
+    }
+}
+
 pub fn in_thread_mode() -> bool {
     #[cfg(feature = "std")]
     return Some("main") == std::thread::current().name();
@@ -78,3 +115,32 @@ pub fn in_thread_mode() -> bool {
     return cortex_m::peripheral::SCB::vect_active()
         == cortex_m::peripheral::scb::VectActive::ThreadMode;
 }
+
+/// A "mutex" that does nothing and cannot be shared between threads.
+pub struct NoopMutex<T> {
+    inner: T,
+}
+
+impl<T> NoopMutex<T> {
+    pub const fn new(value: T) -> Self {
+        NoopMutex { inner: value }
+    }
+}
+
+impl<T> NoopMutex<T> {
+    pub fn borrow(&self) -> &T {
+        &self.inner
+    }
+}
+
+impl<T> Mutex for NoopMutex<T> {
+    type Data = T;
+
+    fn new(data: T) -> Self {
+        Self::new(data)
+    }
+
+    fn lock<R>(&mut self, f: impl FnOnce(&Self::Data) -> R) -> R {
+        f(self.borrow())
+    }
+}
diff --git a/examples/nrf/src/bin/mpsc.rs b/examples/nrf/src/bin/mpsc.rs
new file mode 100644
index 000000000..443955239
--- /dev/null
+++ b/examples/nrf/src/bin/mpsc.rs
@@ -0,0 +1,65 @@
+#![no_std]
+#![no_main]
+#![feature(min_type_alias_impl_trait)]
+#![feature(impl_trait_in_bindings)]
+#![feature(type_alias_impl_trait)]
+#![allow(incomplete_features)]
+
+#[path = "../example_common.rs"]
+mod example_common;
+
+use defmt::panic;
+use embassy::executor::Spawner;
+use embassy::time::{Duration, Timer};
+use embassy::util::mpsc::TryRecvError;
+use embassy::util::{mpsc, Forever};
+use embassy_nrf::gpio::{Level, Output, OutputDrive};
+use embassy_nrf::Peripherals;
+use embedded_hal::digital::v2::OutputPin;
+use mpsc::{Channel, Sender, WithNoThreads};
+
+enum LedState {
+    On,
+    Off,
+}
+
+static CHANNEL: Forever<Channel<WithNoThreads, LedState, 1>> = Forever::new();
+
+#[embassy::task(pool_size = 1)]
+async fn my_task(sender: Sender<'static, WithNoThreads, LedState, 1>) {
+    loop {
+        let _ = sender.send(LedState::On).await;
+        Timer::after(Duration::from_secs(1)).await;
+        let _ = sender.send(LedState::Off).await;
+        Timer::after(Duration::from_secs(1)).await;
+    }
+}
+
+#[embassy::main]
+async fn main(spawner: Spawner, p: Peripherals) {
+    
+    let mut led = Output::new(p.P0_13, Level::Low, OutputDrive::Standard);
+
+    let channel = CHANNEL.put(Channel::new());
+    let (sender, mut receiver) = mpsc::split(channel);
+
+    spawner.spawn(my_task(sender)).unwrap();
+
+    // We could just loop on `receiver.recv()` for simplicity. The code below
+    // is optimized to drain the queue as fast as possible in the spirit of
+    // handling events as fast as possible. This optimization is benign when in
+    // thread mode, but can be useful when interrupts are sending messages
+    // with the channel having been created via with_critical_sections.
+    loop {
+        let maybe_message = match receiver.try_recv() {
+            m @ Ok(..) => m.ok(),
+            Err(TryRecvError::Empty) => receiver.recv().await,
+            Err(TryRecvError::Closed) => break,
+        };
+        match maybe_message {
+            Some(LedState::On) => led.set_high().unwrap(),
+            Some(LedState::Off) => led.set_low().unwrap(),
+            _ => (),
+        }
+    }
+}