embassy_sync: Sink adapter for pubsub::Pub

Corresponding to the `Stream` impl for `pubsub::Sub`. Notable
difference is that we need a separate adapter type to store
the pending item, i.e. we can't `impl Sink for Pub` directly.
Instead a method `Pub::sink(&self)` is exposed, which
constructs a `PubSink`.

embassy-sync/Cargo.toml

@@ -27,6 +27,7 @@ turbowakers = []
 defmt = { version = "0.3", optional = true }
 log = { version = "0.4.14", optional = true }
 
+futures-sink = { version = "0.3", default-features = false, features = [] }
 futures-util = { version = "0.3.17", default-features = false }
 critical-section = "1.1"
 heapless = "0.8"
@@ -37,7 +38,7 @@ embedded-io-async = { version = "0.6.1" }
 futures-executor = { version = "0.3.17", features = [ "thread-pool" ] }
 futures-test = "0.3.17"
 futures-timer = "3.0.2"
-futures-util = { version = "0.3.17", features = [ "channel" ] }
+futures-util = { version = "0.3.17", features = [ "channel", "sink" ] }
 
 # Enable critical-section implementation for std, for tests
 critical-section = { version = "1.1", features = ["std"] }

embassy-sync/src/pubsub/mod.rs

@@ -755,4 +755,30 @@ mod tests {
         assert_eq!(1, sub0.try_next_message_pure().unwrap().0);
         assert_eq!(0, sub1.try_next_message_pure().unwrap().0);
     }
+
+    #[futures_test::test]
+    async fn publisher_sink() {
+        use futures_util::{SinkExt, StreamExt};
+
+        let channel = PubSubChannel::<NoopRawMutex, u32, 4, 4, 4>::new();
+
+        let mut sub = channel.subscriber().unwrap();
+
+        let publ = channel.publisher().unwrap();
+        let mut sink = publ.sink();
+
+        sink.send(0).await.unwrap();
+        assert_eq!(0, sub.try_next_message_pure().unwrap());
+
+        sink.send(1).await.unwrap();
+        assert_eq!(1, sub.try_next_message_pure().unwrap());
+
+        sink.send_all(&mut futures_util::stream::iter(0..4).map(Ok))
+            .await
+            .unwrap();
+        assert_eq!(0, sub.try_next_message_pure().unwrap());
+        assert_eq!(1, sub.try_next_message_pure().unwrap());
+        assert_eq!(2, sub.try_next_message_pure().unwrap());
+        assert_eq!(3, sub.try_next_message_pure().unwrap());
+    }
 }

embassy-sync/src/pubsub/publisher.rs

@@ -74,6 +74,12 @@ impl<'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> Pub<'a, PSB, T> {
     pub fn is_full(&self) -> bool {
         self.channel.is_full()
     }
+
+    /// Create a [`futures::Sink`] adapter for this publisher.
+    #[inline]
+    pub const fn sink(&self) -> PubSink<'a, '_, PSB, T> {
+        PubSink { publ: self, fut: None }
+    }
 }
 
 impl<'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> Drop for Pub<'a, PSB, T> {
@@ -221,6 +227,67 @@ impl<'a, M: RawMutex, T: Clone, const CAP: usize, const SUBS: usize, const PUBS:
     }
 }
 
+#[must_use = "Sinks do nothing unless polled"]
+/// [`futures_sink::Sink`] adapter for [`Pub`].
+pub struct PubSink<'a, 'p, PSB, T>
+where
+    T: Clone,
+    PSB: PubSubBehavior<T> + ?Sized,
+{
+    publ: &'p Pub<'a, PSB, T>,
+    fut: Option<PublisherWaitFuture<'p, 'a, PSB, T>>,
+}
+
+impl<'a, 'p, PSB, T> PubSink<'a, 'p, PSB, T>
+where
+    PSB: PubSubBehavior<T> + ?Sized,
+    T: Clone,
+{
+    /// Try to make progress on the pending future if we have one.
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
+        let Some(mut fut) = self.fut.take() else {
+            return Poll::Ready(());
+        };
+
+        if Pin::new(&mut fut).poll(cx).is_pending() {
+            self.fut = Some(fut);
+            return Poll::Pending;
+        }
+
+        Poll::Ready(())
+    }
+}
+
+impl<'a, 'p, PSB, T> futures_sink::Sink<T> for PubSink<'a, 'p, PSB, T>
+where
+    PSB: PubSubBehavior<T> + ?Sized,
+    T: Clone,
+{
+    type Error = core::convert::Infallible;
+
+    #[inline]
+    fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.poll(cx).map(Ok)
+    }
+
+    #[inline]
+    fn start_send(mut self: Pin<&mut Self>, item: T) -> Result<(), Self::Error> {
+        self.fut = Some(self.publ.publish(item));
+
+        Ok(())
+    }
+
+    #[inline]
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.poll(cx).map(Ok)
+    }
+
+    #[inline]
+    fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.poll(cx).map(Ok)
+    }
+}
+
 /// Future for the publisher wait action
 #[must_use = "futures do nothing unless you `.await` or poll them"]
 pub struct PublisherWaitFuture<'s, 'a, PSB: PubSubBehavior<T> + ?Sized, T: Clone> {