implement try_join_next

tokio/src/task/join_set.rs

@@ -306,6 +306,22 @@ impl<T: 'static> JoinSet<T> {
         crate::future::poll_fn(|cx| self.poll_join_next_with_id(cx)).await
     }
 
+    /// Tries to join one of the tasks in the set that has completed and return its output.
+    ///
+    /// Returns `None` if the set is empty.    
+    pub fn try_join_next(&mut self) -> Option<Result<T, JoinError>> {
+        let mut entry = self.inner.try_pop_notified()?;
+
+        let res = entry.with_value_and_context(|jh, ctx| Pin::new(jh).poll(ctx));
+
+        if let Poll::Ready(res) = res {
+            let _entry = entry.remove();
+            Some(res)
+        } else {
+            None
+        }
+    }
+
     /// Aborts all tasks and waits for them to finish shutting down.
     ///
     /// Calling this method is equivalent to calling [`abort_all`] and then calling [`join_next`] in

tokio/src/util/idle_notified_set.rs

@@ -203,6 +203,34 @@ impl<T> IdleNotifiedSet<T> {
         Some(EntryInOneOfTheLists { entry, set: self })
     }
 
+    /// Tries to pop an entry from the notified list to poll it. The entry is moved to
+    /// the idle list atomically.
+    pub(crate) fn try_pop_notified(&mut self) -> Option<EntryInOneOfTheLists<'_, T>> {
+        // We don't decrement the length because this call moves the entry to
+        // the idle list rather than removing it.
+        if self.length == 0 {
+            // Fast path.
+            return None;
+        }
+
+        let mut lock = self.lists.lock();
+
+        // Pop the entry, returning None if empty.
+        let entry = lock.notified.pop_back()?;
+
+        lock.idle.push_front(entry.clone());
+
+        // Safety: We are holding the lock.
+        entry.my_list.with_mut(|ptr| unsafe {
+            *ptr = List::Idle;
+        });
+
+        drop(lock);
+
+        // Safety: We just put the entry in the idle list, so it is in one of the lists.
+        Some(EntryInOneOfTheLists { entry, set: self })
+    }
+
     /// Call a function on every element in this list.
     pub(crate) fn for_each<F: FnMut(&mut T)>(&mut self, mut func: F) {
         fn get_ptrs<T>(list: &mut LinkedList<T>, ptrs: &mut Vec<*mut T>) {

tokio/tests/task_join_set.rs

@@ -227,3 +227,42 @@ async fn join_set_coop() {
     assert!(coop_count >= 1);
     assert_eq!(count, TASK_NUM);
 }
+
+#[tokio::test(flavor = "current_thread")]
+async fn try_join_next() {
+    // Large enough to trigger coop.
+    const TASK_NUM: u32 = 1000;
+
+    static SEM: tokio::sync::Semaphore = tokio::sync::Semaphore::const_new(0);
+
+    let mut set = JoinSet::new();
+
+    for _ in 0..TASK_NUM {
+        set.spawn(async {
+            SEM.add_permits(1);
+        });
+    }
+
+    // Wait for all tasks to complete.
+    //
+    // Since this is a `current_thread` runtime, there's no race condition
+    // between the last permit being added and the task completing.
+    let _ = SEM.acquire_many(TASK_NUM).await.unwrap();
+
+    let mut count = 0;
+    let mut coop_count = 0;
+    while count != TASK_NUM {
+        match set.try_join_next() {
+            Some(Ok(())) => {
+                count += 1;
+            }
+            Some(Err(err)) => panic!("failed: {}", err),
+            None => {
+                coop_count += 1;
+                tokio::task::yield_now().await;
+            }
+        }
+    }
+    assert!(coop_count >= 1);
+    assert_eq!(count, TASK_NUM);
+}