watch: make watch::Sender::subscribe public (#3800)

Co-authored-by: Alice Ryhl <aliceryhl@google.com>

tokio/src/sync/watch.rs

@@ -165,6 +165,9 @@ mod state {
 
     /// Snapshot of the state. The first bit is used as the CLOSED bit.
     /// The remaining bits are used as the version.
+    ///
+    /// The CLOSED bit tracks whether the Sender has been dropped. Dropping all
+    /// receivers does not set it.
     #[derive(Copy, Clone, Debug)]
     pub(super) struct StateSnapshot(usize);
 
@@ -427,7 +430,7 @@ impl<T> Sender<T> {
     /// every receiver has been dropped.
     pub fn send(&self, value: T) -> Result<(), error::SendError<T>> {
         // This is pretty much only useful as a hint anyway, so synchronization isn't critical.
-        if 0 == self.shared.ref_count_rx.load(Relaxed) {
+        if 0 == self.receiver_count() {
             return Err(error::SendError { inner: value });
         }
 
@@ -484,7 +487,7 @@ impl<T> Sender<T> {
     /// assert!(tx.is_closed());
     /// ```
     pub fn is_closed(&self) -> bool {
-        self.shared.ref_count_rx.load(Relaxed) == 0
+        self.receiver_count() == 0
     }
 
     /// Completes when all receivers have dropped.
@@ -517,23 +520,81 @@ impl<T> Sender<T> {
     /// }
     /// ```
     pub async fn closed(&self) {
-        let notified = self.shared.notify_tx.notified();
+        while self.receiver_count() > 0 {
+            let notified = self.shared.notify_tx.notified();
 
-        if self.shared.ref_count_rx.load(Relaxed) == 0 {
-            return;
+            if self.receiver_count() == 0 {
+                return;
+            }
+
+            notified.await;
+            // The channel could have been reopened in the meantime by calling
+            // `subscribe`, so we loop again.
         }
-
-        notified.await;
-        debug_assert_eq!(0, self.shared.ref_count_rx.load(Relaxed));
     }
 
-    cfg_signal_internal! {
-        pub(crate) fn subscribe(&self) -> Receiver<T> {
-            let shared = self.shared.clone();
-            let version = shared.state.load().version();
+    /// Creates a new [`Receiver`] connected to this `Sender`.
+    ///
+    /// All messages sent before this call to `subscribe` are initially marked
+    /// as seen by the new `Receiver`.
+    ///
+    /// This method can be called even if there are no other receivers. In this
+    /// case, the channel is reopened.
+    ///
+    /// # Examples
+    ///
+    /// The new channel will receive messages sent on this `Sender`.
+    ///
+    /// ```
+    /// use tokio::sync::watch;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, _rx) = watch::channel(0u64);
+    ///
+    ///     tx.send(5).unwrap();
+    ///
+    ///     let rx = tx.subscribe();
+    ///     assert_eq!(5, *rx.borrow());
+    ///
+    ///     tx.send(10).unwrap();
+    ///     assert_eq!(10, *rx.borrow());
+    /// }
+    /// ```
+    ///
+    /// The most recent message is considered seen by the channel, so this test
+    /// is guaranteed to pass.
+    ///
+    /// ```
+    /// use tokio::sync::watch;
+    /// use tokio::time::Duration;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, _rx) = watch::channel(0u64);
+    ///     tx.send(5).unwrap();
+    ///     let mut rx = tx.subscribe();
+    ///
+    ///     tokio::spawn(async move {
+    ///         // by spawning and sleeping, the message is sent after `main`
+    ///         // hits the call to `changed`.
+    ///         # if false {
+    ///         tokio::time::sleep(Duration::from_millis(10)).await;
+    ///         # }
+    ///         tx.send(100).unwrap();
+    ///     });
+    ///
+    ///     rx.changed().await.unwrap();
+    ///     assert_eq!(100, *rx.borrow());
+    /// }
+    /// ```
+    pub fn subscribe(&self) -> Receiver<T> {
+        let shared = self.shared.clone();
+        let version = shared.state.load().version();
 
-            Receiver::from_shared(version, shared)
-        }
+        // The CLOSED bit in the state tracks only whether the sender is
+        // dropped, so we do not need to unset it if this reopens the channel.
+        Receiver::from_shared(version, shared)
     }
 
     /// Returns the number of receivers that currently exist

tokio/tests/sync_watch.rs

@@ -186,3 +186,18 @@ fn borrow_and_update() {
     assert_eq!(*rx.borrow_and_update(), "three");
     assert_ready!(spawn(rx.changed()).poll()).unwrap_err();
 }
+
+#[test]
+fn reopened_after_subscribe() {
+    let (tx, rx) = watch::channel("one");
+    assert!(!tx.is_closed());
+
+    drop(rx);
+    assert!(tx.is_closed());
+
+    let rx = tx.subscribe();
+    assert!(!tx.is_closed());
+
+    drop(rx);
+    assert!(tx.is_closed());
+}