sync: add watch::Receiver::wait_for (#5611)

tokio/src/sync/tests/loom_watch.rs

@@ -2,6 +2,7 @@ use crate::sync::watch;
 
 use loom::future::block_on;
 use loom::thread;
+use std::sync::Arc;
 
 #[test]
 fn smoke() {
@@ -34,3 +35,56 @@ fn smoke() {
         th.join().unwrap();
     })
 }
+
+#[test]
+fn wait_for_test() {
+    loom::model(move || {
+        let (tx, mut rx) = watch::channel(false);
+
+        let tx_arc = Arc::new(tx);
+        let tx1 = tx_arc.clone();
+        let tx2 = tx_arc.clone();
+
+        let th1 = thread::spawn(move || {
+            for _ in 0..2 {
+                tx1.send_modify(|_x| {});
+            }
+        });
+
+        let th2 = thread::spawn(move || {
+            tx2.send(true).unwrap();
+        });
+
+        assert_eq!(*block_on(rx.wait_for(|x| *x)).unwrap(), true);
+
+        th1.join().unwrap();
+        th2.join().unwrap();
+    });
+}
+
+#[test]
+fn wait_for_returns_correct_value() {
+    loom::model(move || {
+        let (tx, mut rx) = watch::channel(0);
+
+        let jh = thread::spawn(move || {
+            tx.send(1).unwrap();
+            tx.send(2).unwrap();
+            tx.send(3).unwrap();
+        });
+
+        // Stop at the first value we are called at.
+        let mut stopped_at = usize::MAX;
+        let returned = *block_on(rx.wait_for(|x| {
+            stopped_at = *x;
+            true
+        }))
+        .unwrap();
+
+        // Check that it returned the same value as the one we returned
+        // `true` for.
+        assert_eq!(stopped_at, returned);
+
+        jh.join().unwrap();
+    });
+}

tokio/src/sync/watch.rs

@@ -595,18 +595,93 @@ impl<T> Receiver<T> {
     /// }
     /// ```
     pub async fn changed(&mut self) -> Result<(), error::RecvError> {
-        loop {
-            // In order to avoid a race condition, we first request a notification,
-            // **then** check the current value's version. If a new version exists,
-            // the notification request is dropped.
-            let notified = self.shared.notify_rx.notified();
+        changed_impl(&self.shared, &mut self.version).await
+    }
 
-            if let Some(ret) = maybe_changed(&self.shared, &mut self.version) {
-                return ret;
+    /// Waits for a value that satisifes the provided condition.
+    ///
+    /// This method will call the provided closure whenever something is sent on
+    /// the channel. Once the closure returns `true`, this method will return a
+    /// reference to the value that was passed to the closure.
+    ///
+    /// Before `wait_for` starts waiting for changes, it will call the closure
+    /// on the current value. If the closure returns `true` when given the
+    /// current value, then `wait_for` will immediately return a reference to
+    /// the current value. This is the case even if the current value is already
+    /// considered seen.
+    ///
+    /// The watch channel only keeps track of the most recent value, so if
+    /// several messages are sent faster than `wait_for` is able to call the
+    /// closure, then it may skip some updates. Whenever the closure is called,
+    /// it will be called with the most recent value.
+    ///
+    /// When this function returns, the value that was passed to the closure
+    /// when it returned `true` will be considered seen.
+    ///
+    /// If the channel is closed, then `wait_for` will return a `RecvError`.
+    /// Once this happens, no more messages can ever be sent on the channel.
+    /// When an error is returned, it is guaranteed that the closure has been
+    /// called on the last value, and that it returned `false` for that value.
+    /// (If the closure returned `true`, then the last value would have been
+    /// returned instead of the error.)
+    ///
+    /// Like the `borrow` method, the returned borrow holds a read lock on the
+    /// inner value. This means that long-lived borrows could cause the producer
+    /// half to block. It is recommended to keep the borrow as short-lived as
+    /// possible. See the documentation of `borrow` for more information on
+    /// this.
+    ///
+    /// [`Receiver::changed()`]: crate::sync::watch::Receiver::changed
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::watch;
+    ///
+    /// #[tokio::main]
+    ///
+    /// async fn main() {
+    ///     let (tx, _rx) = watch::channel("hello");
+    ///
+    ///     tx.send("goodbye").unwrap();
+    ///
+    ///     // here we subscribe to a second receiver
+    ///     // now in case of using `changed` we would have
+    ///     // to first check the current value and then wait
+    ///     // for changes or else `changed` would hang.
+    ///     let mut rx2 = tx.subscribe();
+    ///
+    ///     // in place of changed we have use `wait_for`
+    ///     // which would automatically check the current value
+    ///     // and wait for changes until the closure returns true.
+    ///     assert!(rx2.wait_for(|val| *val == "goodbye").await.is_ok());
+    ///     assert_eq!(*rx2.borrow(), "goodbye");
+    /// }
+    /// ```
+    pub async fn wait_for(
+        &mut self,
+        mut f: impl FnMut(&T) -> bool,
+    ) -> Result<Ref<'_, T>, error::RecvError> {
+        let mut closed = false;
+        loop {
+            {
+                let inner = self.shared.value.read().unwrap();
+
+                let new_version = self.shared.state.load().version();
+                let has_changed = self.version != new_version;
+                self.version = new_version;
+
+                if (!closed || has_changed) && f(&inner) {
+                    return Ok(Ref { inner, has_changed });
+                }
             }
 
-            notified.await;
-            // loop around again in case the wake-up was spurious
+            if closed {
+                return Err(error::RecvError(()));
+            }
+
+            // Wait for the value to change.
+            closed = changed_impl(&self.shared, &mut self.version).await.is_err();
         }
     }
 
@@ -655,6 +730,25 @@ fn maybe_changed<T>(
     None
 }
 
+async fn changed_impl<T>(
+    shared: &Shared<T>,
+    version: &mut Version,
+) -> Result<(), error::RecvError> {
+    loop {
+        // In order to avoid a race condition, we first request a notification,
+        // **then** check the current value's version. If a new version exists,
+        // the notification request is dropped.
+        let notified = shared.notify_rx.notified();
+
+        if let Some(ret) = maybe_changed(shared, version) {
+            return ret;
+        }
+
+        notified.await;
+        // loop around again in case the wake-up was spurious
+    }
+}
+
 impl<T> Clone for Receiver<T> {
     fn clone(&self) -> Self {
         let version = self.version;