mpsc: Add Sender::try_reserve function (#3418)

* mpsc: Add `Sender::try_reserve` function

* Update tokio/src/sync/mpsc/bounded.rs

Co-authored-by: Alice Ryhl <alice@ryhl.io>

* Fix doc links

Co-authored-by: Alice Ryhl <alice@ryhl.io>

tokio/src/sync/mpsc/bounded.rs

@@ -603,6 +603,58 @@ impl<T> Sender<T> {
 
         Ok(Permit { chan: &self.chan })
     }
+
+    /// Try to acquire a slot in the channel without waiting for the slot to become
+    /// available.
+    ///
+    /// If the channel is full this function will return [`TrySendError`], otherwise
+    /// if there is a slot available it will return a [`Permit`] that will then allow you
+    /// to [`send`] on the channel with a guaranteed slot. This function is similar to
+    /// [`reserve`] execpt it does not await for the slot to become available.
+    ///
+    /// Dropping [`Permit`] without sending a message releases the capacity back
+    /// to the channel.
+    ///
+    /// [`Permit`]: Permit
+    /// [`send`]: Permit::send
+    /// [`reserve`]: Sender::reserve
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::mpsc;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, mut rx) = mpsc::channel(1);
+    ///
+    ///     // Reserve capacity
+    ///     let permit = tx.try_reserve().unwrap();
+    ///
+    ///     // Trying to send directly on the `tx` will fail due to no
+    ///     // available capacity.
+    ///     assert!(tx.try_send(123).is_err());
+    ///
+    ///     // Trying to reserve an additional slot on the `tx` will
+    ///     // fail because there is no capacity.
+    ///     assert!(tx.try_reserve().is_err());
+    ///
+    ///     // Sending on the permit succeeds
+    ///     permit.send(456);
+    ///
+    ///     // The value sent on the permit is received
+    ///     assert_eq!(rx.recv().await.unwrap(), 456);
+    ///
+    /// }
+    /// ```
+    pub fn try_reserve(&self) -> Result<Permit<'_, T>, TrySendError<()>> {
+        match self.chan.semaphore().0.try_acquire(1) {
+            Ok(_) => {}
+            Err(_) => return Err(TrySendError::Full(())),
+        }
+
+        Ok(Permit { chan: &self.chan })
+    }
 }
 
 impl<T> Clone for Sender<T> {

tokio/tests/sync_mpsc.rs

@@ -327,6 +327,29 @@ async fn try_send_fail() {
     assert!(rx.recv().await.is_none());
 }
 
+#[tokio::test]
+async fn try_reserve_fails() {
+    let (tx, mut rx) = mpsc::channel(1);
+
+    let permit = tx.try_reserve().unwrap();
+
+    // This should fail
+    match assert_err!(tx.try_reserve()) {
+        TrySendError::Full(()) => {}
+        _ => panic!(),
+    }
+
+    permit.send("foo");
+
+    assert_eq!(rx.recv().await, Some("foo"));
+
+    // Dropping permit releases the slot.
+    let permit = tx.try_reserve().unwrap();
+    drop(permit);
+
+    let _permit = tx.try_reserve().unwrap();
+}
+
 #[tokio::test]
 async fn drop_permit_releases_permit() {
     // poll_ready reserves capacity, ensure that the capacity is released if tx