Add blocking functionality to channels

src/lib.rs

@@ -242,6 +242,35 @@ impl<T> Sender<T> {
         }
     }
 
+    /// Sends a message into this channel using the blocking strategy.
+    ///
+    /// If the channel is full, this method will block until there is room.
+    /// If the channel is closed, this method returns an error.
+    ///
+    /// # Blocking
+    ///
+    /// Rather than using asynchronous waiting, like the [`send`] method,
+    /// this method will block the current thread until the message is sent.
+    ///
+    /// This method should not be used in an asynchronous context. It is intended
+    /// to be used such that a channel can be used in both asynchronous and synchronous contexts.
+    /// Calling this method in an asynchronous context may result in deadlocks.
+    ///  
+    /// # Examples
+    ///
+    /// ```
+    /// use async_channel::{unbounded, SendError};
+    ///
+    /// let (s, r) = unbounded();
+    ///
+    /// assert_eq!(s.send_blocking(1), Ok(()));
+    /// drop(r);
+    /// assert_eq!(s.send_blocking(2), Err(SendError(2)));
+    /// ```
+    pub fn send_blocking(&self, msg: T) -> Result<(), SendError<T>> {
+        self.send(msg).wait()
+    }
+
     /// Closes the channel.
     ///
     /// Returns `true` if this call has closed the channel and it was not closed already.
@@ -511,6 +540,38 @@ impl<T> Receiver<T> {
         }
     }
 
+    /// Receives a message from the channel using the blocking strategy.
+    ///
+    /// If the channel is empty, this method waits until there is a message.
+    /// If the channel is closed, this method receives a message or returns an error if there are
+    /// no more messages.
+    ///
+    /// # Blocking
+    ///
+    /// Rather than using asynchronous waiting, like the [`recv`] method,
+    /// this method will block the current thread until the message is sent.
+    ///
+    /// This method should not be used in an asynchronous context. It is intended
+    /// to be used such that a channel can be used in both asynchronous and synchronous contexts.
+    /// Calling this method in an `async` block may result in deadlocks.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_channel::{unbounded, RecvError};
+    ///
+    /// let (s, r) = unbounded();
+    ///
+    /// assert_eq!(s.send_blocking(1), Ok(()));
+    /// drop(s);
+    ///
+    /// assert_eq!(r.recv_blocking(), Ok(1));
+    /// assert_eq!(r.recv_blocking(), Err(RecvError));
+    /// ```
+    pub fn recv_blocking(&self) -> Result<T, RecvError> {
+        self.recv().wait()
+    }
+
     /// Closes the channel.
     ///
     /// Returns `true` if this call has closed the channel and it was not closed already.
@@ -895,50 +956,62 @@ pub struct Send<'a, T> {
     msg: Option<T>,
 }
 
-impl<'a, T> Unpin for Send<'a, T> {}
-
-impl<'a, T> Future for Send<'a, T> {
-    type Output = Result<(), SendError<T>>;
-
-    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        let mut this = Pin::new(self);
-
+impl<'a, T> Send<'a, T> {
+    /// Run this future with the given `Strategy`.
+    fn run_with_strategy<S: Strategy>(
+        &mut self,
+        cx: &mut S::Context,
+    ) -> Poll<Result<(), SendError<T>>> {
         loop {
-            let msg = this.msg.take().unwrap();
+            let msg = self.msg.take().unwrap();
             // Attempt to send a message.
-            match this.sender.try_send(msg) {
+            match self.sender.try_send(msg) {
                 Ok(()) => {
                     // If the capacity is larger than 1, notify another blocked send operation.
-                    match this.sender.channel.queue.capacity() {
+                    match self.sender.channel.queue.capacity() {
                         Some(1) => {}
-                        Some(_) | None => this.sender.channel.send_ops.notify(1),
+                        Some(_) | None => self.sender.channel.send_ops.notify(1),
                     }
                     return Poll::Ready(Ok(()));
                 }
                 Err(TrySendError::Closed(msg)) => return Poll::Ready(Err(SendError(msg))),
-                Err(TrySendError::Full(m)) => this.msg = Some(m),
+                Err(TrySendError::Full(m)) => self.msg = Some(m),
             }
 
             // Sending failed - now start listening for notifications or wait for one.
-            match &mut this.listener {
+            match self.listener.take() {
                 None => {
                     // Start listening and then try sending again.
-                    this.listener = Some(this.sender.channel.send_ops.listen());
+                    self.listener = Some(self.sender.channel.send_ops.listen());
                 }
                 Some(l) => {
-                    // Wait for a notification.
-                    match Pin::new(l).poll(cx) {
-                        Poll::Ready(_) => {
-                            this.listener = None;
-                            continue;
-                        }
-
-                        Poll::Pending => return Poll::Pending,
+                    // Poll using the given strategy
+                    if let Err(l) = S::poll(l, cx) {
+                        self.listener = Some(l);
+                        return Poll::Pending;
                     }
                 }
             }
         }
     }
+
+    /// Run using the blocking strategy.
+    fn wait(mut self) -> Result<(), SendError<T>> {
+        match self.run_with_strategy::<Blocking>(&mut ()) {
+            Poll::Ready(res) => res,
+            Poll::Pending => unreachable!(),
+        }
+    }
+}
+
+impl<'a, T> Unpin for Send<'a, T> {}
+
+impl<'a, T> Future for Send<'a, T> {
+    type Output = Result<(), SendError<T>>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        self.run_with_strategy::<NonBlocking<'_>>(cx)
+    }
 }
 
 /// A future returned by [`Receiver::recv()`].
@@ -951,22 +1024,22 @@ pub struct Recv<'a, T> {
 
 impl<'a, T> Unpin for Recv<'a, T> {}
 
-impl<'a, T> Future for Recv<'a, T> {
-    type Output = Result<T, RecvError>;
-
-    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        let mut this = Pin::new(self);
-
+impl<'a, T> Recv<'a, T> {
+    /// Run this future with the given `Strategy`.
+    fn run_with_strategy<S: Strategy>(
+        &mut self,
+        cx: &mut S::Context,
+    ) -> Poll<Result<T, RecvError>> {
         loop {
             // Attempt to receive a message.
-            match this.receiver.try_recv() {
+            match self.receiver.try_recv() {
                 Ok(msg) => {
                     // If the capacity is larger than 1, notify another blocked receive operation.
                     // There is no need to notify stream operations because all of them get
                     // notified every time a message is sent into the channel.
-                    match this.receiver.channel.queue.capacity() {
+                    match self.receiver.channel.queue.capacity() {
                         Some(1) => {}
-                        Some(_) | None => this.receiver.channel.recv_ops.notify(1),
+                        Some(_) | None => self.receiver.channel.recv_ops.notify(1),
                     }
                     return Poll::Ready(Ok(msg));
                 }
@@ -975,23 +1048,73 @@ impl<'a, T> Future for Recv<'a, T> {
             }
 
             // Receiving failed - now start listening for notifications or wait for one.
-            match &mut this.listener {
+            match self.listener.take() {
                 None => {
                     // Start listening and then try receiving again.
-                    this.listener = Some(this.receiver.channel.recv_ops.listen());
+                    self.listener = Some(self.receiver.channel.recv_ops.listen());
                 }
                 Some(l) => {
-                    // Wait for a notification.
-                    match Pin::new(l).poll(cx) {
-                        Poll::Ready(_) => {
-                            this.listener = None;
-                            continue;
-                        }
-
-                        Poll::Pending => return Poll::Pending,
+                    // Poll using the given strategy.
+                    if let Err(l) = S::poll(l, cx) {
+                        self.listener = Some(l);
+                        return Poll::Pending;
                     }
                 }
             }
         }
     }
+
+    /// Run with the blocking strategy.
+    fn wait(mut self) -> Result<T, RecvError> {
+        match self.run_with_strategy::<Blocking>(&mut ()) {
+            Poll::Ready(res) => res,
+            Poll::Pending => unreachable!(),
+        }
+    }
+}
+
+impl<'a, T> Future for Recv<'a, T> {
+    type Output = Result<T, RecvError>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        self.run_with_strategy::<NonBlocking<'_>>(cx)
+    }
+}
+
+/// A strategy used to poll an `EventListener`.
+trait Strategy {
+    /// Context needed to be provided to the `poll` method.
+    type Context;
+
+    /// Polls the given `EventListener`.
+    ///
+    /// Returns the `EventListener` back if it was not completed; otherwise,
+    /// returns `Ok(())`.
+    fn poll(evl: EventListener, cx: &mut Self::Context) -> Result<(), EventListener>;
+}
+
+/// Non-blocking strategy for use in asynchronous code.
+struct NonBlocking<'a>(&'a mut ());
+
+impl<'a> Strategy for NonBlocking<'a> {
+    type Context = Context<'a>;
+
+    fn poll(mut evl: EventListener, cx: &mut Context<'a>) -> Result<(), EventListener> {
+        match Pin::new(&mut evl).poll(cx) {
+            Poll::Ready(()) => Ok(()),
+            Poll::Pending => Err(evl),
+        }
+    }
+}
+
+/// Blocking strategy for use in synchronous code.
+struct Blocking;
+
+impl Strategy for Blocking {
+    type Context = ();
+
+    fn poll(evl: EventListener, _cx: &mut ()) -> Result<(), EventListener> {
+        evl.wait();
+        Ok(())
+    }
 }

tests/bounded.rs

@@ -23,6 +23,22 @@ fn smoke() {
     assert_eq!(r.try_recv(), Err(TryRecvError::Empty));
 }
 
+#[test]
+fn smoke_blocking() {
+    let (s, r) = bounded(1);
+
+    s.send_blocking(7).unwrap();
+    assert_eq!(r.try_recv(), Ok(7));
+
+    s.send_blocking(8).unwrap();
+    assert_eq!(future::block_on(r.recv()), Ok(8));
+
+    future::block_on(s.send(9)).unwrap();
+    assert_eq!(r.recv_blocking(), Ok(9));
+
+    assert_eq!(r.try_recv(), Err(TryRecvError::Empty));
+}
+
 #[test]
 fn capacity() {
     for i in 1..10 {

tests/unbounded.rs

@@ -22,6 +22,22 @@ fn smoke() {
     assert_eq!(r.try_recv(), Err(TryRecvError::Empty));
 }
 
+#[test]
+fn smoke_blocking() {
+    let (s, r) = unbounded();
+
+    s.send_blocking(7).unwrap();
+    assert_eq!(r.try_recv(), Ok(7));
+
+    s.send_blocking(8).unwrap();
+    assert_eq!(future::block_on(r.recv()), Ok(8));
+
+    future::block_on(s.send(9)).unwrap();
+    assert_eq!(r.recv_blocking(), Ok(9));
+
+    assert_eq!(r.try_recv(), Err(TryRecvError::Empty));
+}
+
 #[test]
 fn capacity() {
     let (s, r) = unbounded::<()>();