2 files changed, 286 insertions, 0 deletions

diff --git a/embassy-sync/src/lib.rs b/embassy-sync/src/lib.rs
index d88c76db5..f02985564 100644
--- a/embassy-sync/src/lib.rs
+++ b/embassy-sync/src/lib.rs
@@ -12,6 +12,7 @@ mod ring_buffer;
 
 pub mod blocking_mutex;
 pub mod channel;
+pub mod multi_signal;
 pub mod mutex;
 pub mod pipe;
 pub mod priority_channel;
diff --git a/embassy-sync/src/multi_signal.rs b/embassy-sync/src/multi_signal.rs
new file mode 100644
index 000000000..db858f269
--- /dev/null
+++ b/embassy-sync/src/multi_signal.rs
@@ -0,0 +1,285 @@
+//! A synchronization primitive for passing the latest value to **multiple** tasks.
+use core::{
+    cell::RefCell,
+    marker::PhantomData,
+    ops::{Deref, DerefMut},
+    pin::Pin,
+    task::{Context, Poll},
+};
+
+use futures_util::Future;
+
+use crate::{
+    blocking_mutex::{raw::RawMutex, Mutex},
+    waitqueue::MultiWakerRegistration,
+};
+
+/// A `MultiSignal` is a single-slot signaling primitive, which can awake `N` separate [`Receiver`]s.
+///
+/// Similar to a [`Signal`](crate::signal::Signal), except `MultiSignal` allows for multiple tasks to
+/// `.await` the latest value, and all receive it.
+///
+/// This is similar to a [`PubSubChannel`](crate::pubsub::PubSubChannel) with a buffer size of 1, except
+/// "sending" to it (calling [`MultiSignal::write`]) will immediately overwrite the previous value instead
+/// of waiting for the receivers to pop the previous value.
+///
+/// `MultiSignal` is useful when a single task is responsible for updating a value or "state", which multiple other
+/// tasks are interested in getting notified about changes to the latest value of. It is therefore fine for
+/// [`Receiver`]s to "lose" stale values.
+///
+/// Anyone with a reference to the MultiSignal can update or peek the value. MultiSignals are generally declared
+/// as `static`s and then borrowed as required to either [`MultiSignal::peek`] the value or obtain a [`Receiver`]
+/// with [`MultiSignal::receiver`] which has async methods.
+/// ```
+///
+/// use futures_executor::block_on;
+/// use embassy_sync::multi_signal::MultiSignal;
+/// use embassy_sync::blocking_mutex::raw::CriticalSectionRawMutex;
+///
+/// let f = async {
+///
+/// static SOME_SIGNAL: MultiSignal<CriticalSectionRawMutex, u8, 2> = MultiSignal::new(0);
+///
+/// // Obtain Receivers
+/// let mut rcv0 = SOME_SIGNAL.receiver().unwrap();
+/// let mut rcv1 = SOME_SIGNAL.receiver().unwrap();
+/// assert!(SOME_SIGNAL.receiver().is_err());
+///
+/// SOME_SIGNAL.write(10);
+///     
+/// // Receive the new value
+/// assert_eq!(rcv0.changed().await, 10);
+/// assert_eq!(rcv1.try_changed(), Some(10));
+///     
+/// // No update
+/// assert_eq!(rcv0.try_changed(), None);
+/// assert_eq!(rcv1.try_changed(), None);
+///
+/// SOME_SIGNAL.write(20);
+///
+/// // Receive new value with predicate
+/// assert_eq!(rcv0.changed_and(|x|x>&10).await, 20);
+/// assert_eq!(rcv1.try_changed_and(|x|x>&30), None);
+///
+/// // Anyone can peek the current value
+/// assert_eq!(rcv0.peek(), 20);
+/// assert_eq!(rcv1.peek(), 20);
+/// assert_eq!(SOME_SIGNAL.peek(), 20);
+/// assert_eq!(SOME_SIGNAL.peek_and(|x|x>&30), None);
+/// };
+/// block_on(f);
+/// ```
+pub struct MultiSignal<'a, M: RawMutex, T: Clone, const N: usize> {
+    mutex: Mutex<M, RefCell<MultiSignalState<N, T>>>,
+    _phantom: PhantomData<&'a ()>,
+}
+
+struct MultiSignalState<const N: usize, T: Clone> {
+    data: T,
+    current_id: u64,
+    wakers: MultiWakerRegistration<N>,
+    receiver_count: usize,
+}
+
+#[derive(Debug)]
+/// An error that can occur when a `MultiSignal` returns a `Result`.
+pub enum Error {
+    /// The maximum number of [`Receiver`](crate::multi_signal::Receiver) has been reached.
+    MaximumReceiversReached,
+}
+
+impl<'a, M: RawMutex, T: Clone, const N: usize> MultiSignal<'a, M, T, N> {
+    /// Create a new `MultiSignal` initialized with the given value.
+    pub const fn new(init: T) -> Self {
+        Self {
+            mutex: Mutex::new(RefCell::new(MultiSignalState {
+                data: init,
+                current_id: 1,
+                wakers: MultiWakerRegistration::new(),
+                receiver_count: 0,
+            })),
+            _phantom: PhantomData,
+        }
+    }
+
+    /// Get a [`Receiver`] for the `MultiSignal`.
+    pub fn receiver(&'a self) -> Result<Receiver<'a, M, T, N>, Error> {
+        self.mutex.lock(|state| {
+            let mut s = state.borrow_mut();
+            if s.receiver_count < N {
+                s.receiver_count += 1;
+                Ok(Receiver(Rcv::new(self)))
+            } else {
+                Err(Error::MaximumReceiversReached)
+            }
+        })
+    }
+
+    /// Update the value of the `MultiSignal`.
+    pub fn write(&self, data: T) {
+        self.mutex.lock(|state| {
+            let mut s = state.borrow_mut();
+            s.data = data;
+            s.current_id += 1;
+            s.wakers.wake();
+        })
+    }
+
+    /// Peek the current value of the `MultiSignal`.
+    pub fn peek(&self) -> T {
+        self.mutex.lock(|state| state.borrow().data.clone())
+    }
+
+    /// Peek the current value of the `MultiSignal` and check if it satisfies the predicate `f`.
+    pub fn peek_and(&self, f: fn(&T) -> bool) -> Option<T> {
+        self.mutex.lock(|state| {
+            let s = state.borrow();
+            if f(&s.data) {
+                Some(s.data.clone())
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Get the ID of the current value of the `MultiSignal`.
+    /// This method is mostly for testing purposes.
+    #[allow(dead_code)]
+    fn get_id(&self) -> u64 {
+        self.mutex.lock(|state| state.borrow().current_id)
+    }
+
+    /// Poll the `MultiSignal` with an optional context.
+    fn get_with_context(&'a self, waker: &mut Rcv<'a, M, T, N>, cx: Option<&mut Context>) -> Poll<T> {
+        self.mutex.lock(|state| {
+            let mut s = state.borrow_mut();
+            match (s.current_id > waker.at_id, waker.predicate) {
+                (true, None) => {
+                    waker.at_id = s.current_id;
+                    Poll::Ready(s.data.clone())
+                }
+                (true, Some(f)) if f(&s.data) => {
+                    waker.at_id = s.current_id;
+                    Poll::Ready(s.data.clone())
+                }
+                _ => {
+                    if let Some(cx) = cx {
+                        s.wakers.register(cx.waker());
+                    }
+                    Poll::Pending
+                }
+            }
+        })
+    }
+}
+
+/// A receiver is able to `.await` a changed `MultiSignal` value.
+pub struct Rcv<'a, M: RawMutex, T: Clone, const N: usize> {
+    multi_sig: &'a MultiSignal<'a, M, T, N>,
+    predicate: Option<fn(&T) -> bool>,
+    at_id: u64,
+}
+
+// f: Option<impl FnMut(&T) -> bool>
+impl<'a, M: RawMutex, T: Clone, const N: usize> Rcv<'a, M, T, N> {
+    /// Create a new `Receiver` with a reference the given `MultiSignal`.
+    fn new(multi_sig: &'a MultiSignal<'a, M, T, N>) -> Self {
+        Self {
+            multi_sig,
+            predicate: None,
+            at_id: 0,
+        }
+    }
+
+    /// Wait for a change to the value of the corresponding `MultiSignal`.
+    pub fn changed<'s>(&'s mut self) -> ReceiverFuture<'s, 'a, M, T, N> {
+        self.predicate = None;
+        ReceiverFuture { subscriber: self }
+    }
+
+    /// Wait for a change to the value of the corresponding `MultiSignal` which matches the predicate `f`.
+    pub fn changed_and<'s>(&'s mut self, f: fn(&T) -> bool) -> ReceiverFuture<'s, 'a, M, T, N> {
+        self.predicate = Some(f);
+        ReceiverFuture { subscriber: self }
+    }
+
+    /// Try to get a changed value of the corresponding `MultiSignal`.
+    pub fn try_changed(&mut self) -> Option<T> {
+        self.multi_sig.mutex.lock(|state| {
+            let s = state.borrow();
+            match s.current_id > self.at_id {
+                true => {
+                    self.at_id = s.current_id;
+                    Some(s.data.clone())
+                }
+                false => None,
+            }
+        })
+    }
+
+    /// Try to get a changed value of the corresponding `MultiSignal` which matches the predicate `f`.
+    pub fn try_changed_and(&mut self, f: fn(&T) -> bool) -> Option<T> {
+        self.multi_sig.mutex.lock(|state| {
+            let s = state.borrow();
+            match s.current_id > self.at_id && f(&s.data) {
+                true => {
+                    self.at_id = s.current_id;
+                    Some(s.data.clone())
+                }
+                false => None,
+            }
+        })
+    }
+
+    /// Peek the current value of the corresponding `MultiSignal`.
+    pub fn peek(&self) -> T {
+        self.multi_sig.peek()
+    }
+
+    /// Peek the current value of the corresponding `MultiSignal` and check if it satisfies the predicate `f`.
+    pub fn peek_and(&self, f: fn(&T) -> bool) -> Option<T> {
+        self.multi_sig.peek_and(f)
+    }
+
+    /// Check if the value of the corresponding `MultiSignal` has changed.
+    pub fn has_changed(&mut self) -> bool {
+        self.multi_sig
+            .mutex
+            .lock(|state| state.borrow().current_id > self.at_id)
+    }
+}
+
+/// A `Receiver` is able to `.await` a change to the corresponding [`MultiSignal`] value.
+pub struct Receiver<'a, M: RawMutex, T: Clone, const N: usize>(Rcv<'a, M, T, N>);
+
+impl<'a, M: RawMutex, T: Clone, const N: usize> Deref for Receiver<'a, M, T, N> {
+    type Target = Rcv<'a, M, T, N>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<'a, M: RawMutex, T: Clone, const N: usize> DerefMut for Receiver<'a, M, T, N> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+/// Future for the `Receiver` wait action
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct ReceiverFuture<'s, 'a, M: RawMutex, T: Clone, const N: usize> {
+    subscriber: &'s mut Rcv<'a, M, T, N>,
+}
+
+impl<'s, 'a, M: RawMutex, T: Clone, const N: usize> Future for ReceiverFuture<'s, 'a, M, T, N> {
+    type Output = T;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        self.subscriber
+            .multi_sig
+            .get_with_context(&mut self.subscriber, Some(cx))
+    }
+}
+
+impl<'s, 'a, M: RawMutex, T: Clone, const N: usize> Unpin for ReceiverFuture<'s, 'a, M, T, N> {}