sync: add blocking_recv and blocking_send in mpsc (#2684)

Fixes: #2629

tokio/src/sync/mpsc/bounded.rs

@@ -155,6 +155,42 @@ impl<T> Receiver<T> {
         self.chan.recv(cx)
     }
 
+    /// Blocking receive to call outside of asynchronous contexts.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if called within an asynchronous execution
+    /// context.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::thread;
+    /// use tokio::runtime::Runtime;
+    /// use tokio::sync::mpsc;
+    ///
+    /// fn main() {
+    ///     let (mut tx, mut rx) = mpsc::channel::<u8>(10);
+    ///
+    ///     let sync_code = thread::spawn(move || {
+    ///         assert_eq!(Some(10), rx.blocking_recv());
+    ///     });
+    ///
+    ///     Runtime::new()
+    ///         .unwrap()
+    ///         .block_on(async move {
+    ///             let _ = tx.send(10).await;
+    ///         });
+    ///     sync_code.join().unwrap()
+    /// }
+    /// ```
+    #[cfg(feature = "rt-core")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "rt-core")))]
+    pub fn blocking_recv(&mut self) -> Option<T> {
+        let mut enter_handle = crate::runtime::enter::enter(false);
+        enter_handle.block_on(self.recv()).unwrap()
+    }
+
     /// Attempts to return a pending value on this receiver without blocking.
     ///
     /// This method will never block the caller in order to wait for data to
@@ -393,6 +429,40 @@ impl<T> Sender<T> {
         }
     }
 
+    /// Blocking send to call outside of asynchronous contexts.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if called within an asynchronous execution
+    /// context.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::thread;
+    /// use tokio::runtime::Runtime;
+    /// use tokio::sync::mpsc;
+    ///
+    /// fn main() {
+    ///     let (mut tx, mut rx) = mpsc::channel::<u8>(1);
+    ///
+    ///     let sync_code = thread::spawn(move || {
+    ///         tx.blocking_send(10).unwrap();
+    ///     });
+    ///
+    ///     Runtime::new().unwrap().block_on(async move {
+    ///         assert_eq!(Some(10), rx.recv().await);
+    ///     });
+    ///     sync_code.join().unwrap()
+    /// }
+    /// ```
+    #[cfg(feature = "rt-core")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "rt-core")))]
+    pub fn blocking_send(&mut self, value: T) -> Result<(), SendError<T>> {
+        let mut enter_handle = crate::runtime::enter::enter(false);
+        enter_handle.block_on(self.send(value)).unwrap()
+    }
+
     /// Returns `Poll::Ready(Ok(()))` when the channel is able to accept another item.
     ///
     /// If the channel is full, then `Poll::Pending` is returned and the task is notified when a

tokio/src/sync/mpsc/mod.rs

@@ -43,11 +43,10 @@
 //! are two situations to consider:
 //!
 //! **Bounded channel**: If you need a bounded channel, you should use a bounded
-//! Tokio `mpsc` channel for both directions of communication. To call the async
-//! [`send`][bounded-send] or [`recv`][bounded-recv] methods in sync code, you
-//! will need to use [`Handle::block_on`], which allow you to execute an async
-//! method in synchronous code. This is necessary because a bounded channel may
-//! need to wait for additional capacity to become available.
+//! Tokio `mpsc` channel for both directions of communication. Instead of calling
+//! the async [`send`][bounded-send] or [`recv`][bounded-recv] methods, in
+//! synchronous code you will need to use the [`blocking_send`][blocking-send] or
+//! [`blocking_recv`][blocking-recv] methods.
 //!
 //! **Unbounded channel**: You should use the kind of channel that matches where
 //! the receiver is. So for sending a message _from async to sync_, you should
@@ -59,6 +58,8 @@
 //! [`Receiver`]: crate::sync::mpsc::Receiver
 //! [bounded-send]: crate::sync::mpsc::Sender::send()
 //! [bounded-recv]: crate::sync::mpsc::Receiver::recv()
+//! [blocking-send]: crate::sync::mpsc::Sender::blocking_send()
+//! [blocking-recv]: crate::sync::mpsc::Receiver::blocking_recv()
 //! [`UnboundedSender`]: crate::sync::mpsc::UnboundedSender
 //! [`Handle::block_on`]: crate::runtime::Handle::block_on()
 //! [std-unbounded]: std::sync::mpsc::channel

tokio/tests/sync_mpsc.rs

@@ -2,6 +2,8 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "full")]
 
+use std::thread;
+use tokio::runtime::Runtime;
 use tokio::sync::mpsc;
 use tokio::sync::mpsc::error::{TryRecvError, TrySendError};
 use tokio_test::task;
@@ -490,3 +492,45 @@ fn try_recv_unbounded() {
         _ => panic!(),
     }
 }
+
+#[test]
+fn blocking_recv() {
+    let (mut tx, mut rx) = mpsc::channel::<u8>(1);
+
+    let sync_code = thread::spawn(move || {
+        assert_eq!(Some(10), rx.blocking_recv());
+    });
+
+    Runtime::new().unwrap().block_on(async move {
+        let _ = tx.send(10).await;
+    });
+    sync_code.join().unwrap()
+}
+
+#[tokio::test]
+#[should_panic]
+async fn blocking_recv_async() {
+    let (_tx, mut rx) = mpsc::channel::<()>(1);
+    let _ = rx.blocking_recv();
+}
+
+#[test]
+fn blocking_send() {
+    let (mut tx, mut rx) = mpsc::channel::<u8>(1);
+
+    let sync_code = thread::spawn(move || {
+        tx.blocking_send(10).unwrap();
+    });
+
+    Runtime::new().unwrap().block_on(async move {
+        assert_eq!(Some(10), rx.recv().await);
+    });
+    sync_code.join().unwrap()
+}
+
+#[tokio::test]
+#[should_panic]
+async fn blocking_send_async() {
+    let (mut tx, _rx) = mpsc::channel::<()>(1);
+    let _ = tx.blocking_send(());
+}