sync: Add has_changed method to watch::Ref (#4758)

tokio/src/sync/watch.rs

@@ -114,6 +114,55 @@ pub struct Sender<T> {
 #[derive(Debug)]
 pub struct Ref<'a, T> {
     inner: RwLockReadGuard<'a, T>,
+    has_changed: bool,
+}
+
+impl<'a, T> Ref<'a, T> {
+    /// Indicates if the borrowed value is considered as _changed_ since the last
+    /// time it has been marked as seen.
+    ///
+    /// Unlike [`Receiver::has_changed()`], this method does not fail if the channel is closed.
+    ///
+    /// When borrowed from the [`Sender`] this function will always return `false`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::watch;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, mut rx) = watch::channel("hello");
+    ///
+    ///     tx.send("goodbye").unwrap();
+    ///     // The sender does never consider the value as changed.
+    ///     assert!(!tx.borrow().has_changed());
+    ///
+    ///     // Drop the sender immediately, just for testing purposes.
+    ///     drop(tx);
+    ///
+    ///     // Even if the sender has already been dropped...
+    ///     assert!(rx.has_changed().is_err());
+    ///     // ...the modified value is still readable and detected as changed.
+    ///     assert_eq!(*rx.borrow(), "goodbye");
+    ///     assert!(rx.borrow().has_changed());
+    ///
+    ///     // Read the changed value and mark it as seen.
+    ///     {
+    ///         let received = rx.borrow_and_update();
+    ///         assert_eq!(*received, "goodbye");
+    ///         assert!(received.has_changed());
+    ///         // Release the read lock when leaving this scope.
+    ///     }
+    ///
+    ///     // Now the value has already been marked as seen and could
+    ///     // never be modified again (after the sender has been dropped).
+    ///     assert!(!rx.borrow().has_changed());
+    /// }
+    /// ```
+    pub fn has_changed(&self) -> bool {
+        self.has_changed
+    }
 }
 
 #[derive(Debug)]
@@ -336,15 +385,21 @@ impl<T> Receiver<T> {
     /// ```
     pub fn borrow(&self) -> Ref<'_, T> {
         let inner = self.shared.value.read().unwrap();
-        Ref { inner }
+
+        // After obtaining a read-lock no concurrent writes could occur
+        // and the loaded version matches that of the borrowed reference.
+        let new_version = self.shared.state.load().version();
+        let has_changed = self.version != new_version;
+
+        Ref { inner, has_changed }
     }
 
-    /// Returns a reference to the most recently sent value and mark that value
+    /// Returns a reference to the most recently sent value and marks that value
     /// as seen.
     ///
-    /// This method marks the value as seen, so [`changed`] will not return
-    /// immediately if the newest value is one previously returned by
-    /// `borrow_and_update`.
+    /// This method marks the current value as seen. Subsequent calls to [`changed`]
+    /// will not return immediately until the [`Sender`] has modified the shared
+    /// value again.
     ///
     /// Outstanding borrows hold a read lock. This means that long lived borrows
     /// could cause the send half to block. It is recommended to keep the borrow
@@ -372,8 +427,16 @@ impl<T> Receiver<T> {
     /// [`changed`]: Receiver::changed
     pub fn borrow_and_update(&mut self) -> Ref<'_, T> {
         let inner = self.shared.value.read().unwrap();
-        self.version = self.shared.state.load().version();
-        Ref { inner }
+
+        // After obtaining a read-lock no concurrent writes could occur
+        // and the loaded version matches that of the borrowed reference.
+        let new_version = self.shared.state.load().version();
+        let has_changed = self.version != new_version;
+
+        // Mark the shared value as seen by updating the version
+        self.version = new_version;
+
+        Ref { inner, has_changed }
     }
 
     /// Checks if this channel contains a message that this receiver has not yet
@@ -731,7 +794,11 @@ impl<T> Sender<T> {
     /// ```
     pub fn borrow(&self) -> Ref<'_, T> {
         let inner = self.shared.value.read().unwrap();
-        Ref { inner }
+
+        // The sender/producer always sees the current version
+        let has_changed = false;
+
+        Ref { inner, has_changed }
     }
 
     /// Checks if the channel has been closed. This happens when all receivers