sync: implement Weak version of mpsc::UnboundedSender

tokio/src/sync/mpsc/mod.rs

@@ -97,7 +97,9 @@ mod chan;
 pub(super) mod list;
 
 mod unbounded;
-pub use self::unbounded::{unbounded_channel, UnboundedReceiver, UnboundedSender};
+pub use self::unbounded::{
+    unbounded_channel, UnboundedReceiver, UnboundedSender, WeakUnboundedSender,
+};
 
 pub mod error;
 

tokio/src/sync/mpsc/unbounded.rs

@@ -1,4 +1,4 @@
-use crate::loom::sync::atomic::AtomicUsize;
+use crate::loom::sync::{atomic::AtomicUsize, Arc};
 use crate::sync::mpsc::chan;
 use crate::sync::mpsc::error::{SendError, TryRecvError};
 
@@ -13,6 +13,40 @@ pub struct UnboundedSender<T> {
     chan: chan::Tx<T, Semaphore>,
 }
 
+/// An unbounded sender that does not prevent the channel from being closed.
+///
+/// If all [`UnboundedSender`] instances of a channel were dropped and only
+/// `WeakUnboundedSender` instances remain, the channel is closed.
+///
+/// In order to send messages, the `WeakUnboundedSender` needs to be upgraded using
+/// [`WeakUnboundedSender::upgrade`], which returns `Option<UnboundedSender>`. It returns `None`
+/// if all `UnboundedSender`s have been dropped, and otherwise it returns an `UnboundedSender`.
+///
+/// [`UnboundedSender`]: UnboundedSender
+/// [`WeakUnboundedSender::upgrade`]: WeakUnboundedSender::upgrade
+///
+/// #Examples
+///
+/// ```
+/// use tokio::sync::mpsc::unbounded_channel;
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let (tx, _rx) = unbounded_channel::<i32>();
+///     let tx_weak = tx.downgrade();
+///   
+///     // Upgrading will succeed because `tx` still exists.
+///     assert!(tx_weak.upgrade().is_some());
+///   
+///     // If we drop `tx`, then it will fail.
+///     drop(tx);
+///     assert!(tx_weak.clone().upgrade().is_none());
+/// }
+/// ```
+pub struct WeakUnboundedSender<T> {
+    chan: Arc<chan::Chan<T, Semaphore>>,
+}
+
 impl<T> Clone for UnboundedSender<T> {
     fn clone(&self) -> Self {
         UnboundedSender {
@@ -384,4 +418,37 @@ impl<T> UnboundedSender<T> {
     pub fn same_channel(&self, other: &Self) -> bool {
         self.chan.same_channel(&other.chan)
     }
+
+    /// Converts the `UnboundedSender` to a [`WeakUnboundedSender`] that does not count
+    /// towards RAII semantics, i.e. if all `UnboundedSender` instances of the
+    /// channel were dropped and only `WeakUnboundedSender` instances remain,
+    /// the channel is closed.
+    pub fn downgrade(&self) -> WeakUnboundedSender<T> {
+        WeakUnboundedSender {
+            chan: self.chan.downgrade(),
+        }
+    }
+}
+
+impl<T> Clone for WeakUnboundedSender<T> {
+    fn clone(&self) -> Self {
+        WeakUnboundedSender {
+            chan: self.chan.clone(),
+        }
+    }
+}
+
+impl<T> WeakUnboundedSender<T> {
+    /// Tries to convert a WeakUnboundedSender into an [`UnboundedSender`].
+    /// This will return `Some` if there are other `Sender` instances alive and
+    /// the channel wasn't previously dropped, otherwise `None` is returned.
+    pub fn upgrade(&self) -> Option<UnboundedSender<T>> {
+        chan::Tx::upgrade(self.chan.clone()).map(UnboundedSender::new)
+    }
+}
+
+impl<T> fmt::Debug for WeakUnboundedSender<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("WeakUnboundedSender").finish()
+    }
 }

tokio/tests/sync_mpsc.rs

@@ -11,7 +11,7 @@ use wasm_bindgen_test::wasm_bindgen_test as maybe_tokio_test;
 use tokio::test as maybe_tokio_test;
 
 use tokio::sync::mpsc::error::{TryRecvError, TrySendError};
-use tokio::sync::mpsc::{self, channel};
+use tokio::sync::mpsc::{self, channel, unbounded_channel};
 use tokio::sync::oneshot;
 use tokio_test::*;
 
@@ -943,3 +943,233 @@ async fn test_tx_capacity() {
 }
 
 fn is_debug<T: fmt::Debug>(_: &T) {}
+
+#[tokio::test]
+async fn weak_unbounded_sender() {
+    let (tx, mut rx) = unbounded_channel();
+
+    let tx_weak = tokio::spawn(async move {
+        let tx_weak = tx.clone().downgrade();
+
+        for i in 0..10 {
+            if tx.send(i).is_err() {
+                return None;
+            }
+        }
+
+        let tx2 = tx_weak
+            .upgrade()
+            .expect("expected to be able to upgrade tx_weak");
+        let _ = tx2.send(20);
+        let tx_weak = tx2.downgrade();
+
+        Some(tx_weak)
+    })
+    .await
+    .unwrap();
+
+    for i in 0..12 {
+        let recvd = rx.recv().await;
+
+        match recvd {
+            Some(msg) => {
+                if i == 10 {
+                    assert_eq!(msg, 20);
+                }
+            }
+            None => {
+                assert_eq!(i, 11);
+                break;
+            }
+        }
+    }
+
+    let tx_weak = tx_weak.unwrap();
+    let upgraded = tx_weak.upgrade();
+    assert!(upgraded.is_none());
+}
+
+#[tokio::test]
+async fn actor_weak_unbounded_sender() {
+    pub struct MyActor {
+        receiver: mpsc::UnboundedReceiver<ActorMessage>,
+        sender: mpsc::WeakUnboundedSender<ActorMessage>,
+        next_id: u32,
+        pub received_self_msg: bool,
+    }
+
+    enum ActorMessage {
+        GetUniqueId { respond_to: oneshot::Sender<u32> },
+        SelfMessage {},
+    }
+
+    impl MyActor {
+        fn new(
+            receiver: mpsc::UnboundedReceiver<ActorMessage>,
+            sender: mpsc::WeakUnboundedSender<ActorMessage>,
+        ) -> Self {
+            MyActor {
+                receiver,
+                sender,
+                next_id: 0,
+                received_self_msg: false,
+            }
+        }
+
+        fn handle_message(&mut self, msg: ActorMessage) {
+            match msg {
+                ActorMessage::GetUniqueId { respond_to } => {
+                    self.next_id += 1;
+
+                    // The `let _ =` ignores any errors when sending.
+                    //
+                    // This can happen if the `select!` macro is used
+                    // to cancel waiting for the response.
+                    let _ = respond_to.send(self.next_id);
+                }
+                ActorMessage::SelfMessage { .. } => {
+                    self.received_self_msg = true;
+                }
+            }
+        }
+
+        async fn send_message_to_self(&mut self) {
+            let msg = ActorMessage::SelfMessage {};
+
+            let sender = self.sender.clone();
+
+            // cannot move self.sender here
+            if let Some(sender) = sender.upgrade() {
+                let _ = sender.send(msg);
+                self.sender = sender.downgrade();
+            }
+        }
+
+        async fn run(&mut self) {
+            let mut i = 0;
+            while let Some(msg) = self.receiver.recv().await {
+                self.handle_message(msg);
+
+                if i == 0 {
+                    self.send_message_to_self().await;
+                }
+
+                i += 1
+            }
+
+            assert!(self.received_self_msg);
+        }
+    }
+
+    #[derive(Clone)]
+    pub struct MyActorHandle {
+        sender: mpsc::UnboundedSender<ActorMessage>,
+    }
+
+    impl MyActorHandle {
+        pub fn new() -> (Self, MyActor) {
+            let (sender, receiver) = mpsc::unbounded_channel();
+            let actor = MyActor::new(receiver, sender.clone().downgrade());
+
+            (Self { sender }, actor)
+        }
+
+        pub async fn get_unique_id(&self) -> u32 {
+            let (send, recv) = oneshot::channel();
+            let msg = ActorMessage::GetUniqueId { respond_to: send };
+
+            // Ignore send errors. If this send fails, so does the
+            // recv.await below. There's no reason to check the
+            // failure twice.
+            let _ = self.sender.send(msg);
+            recv.await.expect("Actor task has been killed")
+        }
+    }
+
+    let (handle, mut actor) = MyActorHandle::new();
+
+    let actor_handle = tokio::spawn(async move { actor.run().await });
+
+    let _ = tokio::spawn(async move {
+        let _ = handle.get_unique_id().await;
+        drop(handle);
+    })
+    .await;
+
+    let _ = actor_handle.await;
+}
+
+// Tests that no pending messages are put onto the channel after `Rx` was
+// dropped.
+//
+// Note: After the introduction of `UnboundedWeakSender`, which internally
+// used `Arc` and doesn't call a drop of the channel after the last strong
+// `UnboundedSender` was dropped while more than one `UnboundedWeakSender`
+// remains, we want to ensure that no messages are kept in the channel, which
+// were sent after the receiver was dropped.
+#[tokio::test]
+async fn test_msgs_dropped_on_unbounded_rx_drop() {
+    let (tx, mut rx) = mpsc::unbounded_channel();
+
+    tx.send(Msg {}).unwrap();
+    tx.send(Msg {}).unwrap();
+
+    // This msg will be pending and should be dropped when `rx` is dropped
+    let sent = tx.send(Msg {});
+
+    let _ = rx.recv().await.unwrap();
+    let _ = rx.recv().await.unwrap();
+
+    sent.unwrap();
+
+    drop(rx);
+
+    assert_eq!(NUM_DROPPED.load(Acquire), 3);
+
+    // This msg will not be put onto `Tx` list anymore, since `Rx` is closed.
+    assert!(tx.send(Msg {}).is_err());
+
+    assert_eq!(NUM_DROPPED.load(Acquire), 4);
+}
+
+// Tests that an `WeakUnboundedSender` is upgradeable when other
+// `UnboundedSender`s exist.
+#[tokio::test]
+async fn downgrade_upgrade_unbounded_sender_success() {
+    let (tx, _rx) = mpsc::unbounded_channel::<i32>();
+    let weak_tx = tx.downgrade();
+    assert!(weak_tx.upgrade().is_some());
+}
+
+// Tests that a `WeakUnboundedSender` fails to upgrade when no other
+// `UnboundedSender` exists.
+#[tokio::test]
+async fn downgrade_upgrade_unbounded_sender_failure() {
+    let (tx, _rx) = mpsc::unbounded_channel::<i32>();
+    let weak_tx = tx.downgrade();
+    drop(tx);
+    assert!(weak_tx.upgrade().is_none());
+}
+
+// Tests that an `WeakUnboundedSender` cannot be upgraded after an
+// `UnboundedSender` was dropped, which existed at the time of the `downgrade` call.
+#[tokio::test]
+async fn downgrade_drop_upgrade_unbounded() {
+    let (tx, _rx) = mpsc::unbounded_channel::<i32>();
+
+    // the cloned `Tx` is dropped right away
+    let weak_tx = tx.clone().downgrade();
+    drop(tx);
+    assert!(weak_tx.upgrade().is_none());
+}
+
+// Tests that `downgrade` does not change the `tx_count` of the channel.
+#[tokio::test]
+async fn test_tx_count_weak_unbounded_sender() {
+    let (tx, _rx) = mpsc::unbounded_channel::<i32>();
+    let tx_weak = tx.downgrade();
+    let tx_weak2 = tx.downgrade();
+    drop(tx);
+
+    assert!(tx_weak.upgrade().is_none() && tx_weak2.upgrade().is_none());
+}