Add FuturesUnordered::clear (#2415)

futures-util/src/stream/futures_unordered/mod.rs

@@ -236,7 +236,7 @@ impl<Fut> FuturesUnordered<Fut> {
         (task, len)
     }
 
-    /// Releases the task. It destorys the future inside and either drops
+    /// Releases the task. It destroys the future inside and either drops
     /// the `Arc<Task>` or transfers ownership to the ready to run queue.
     /// The task this method is called on must have been unlinked before.
     fn release_task(&mut self, task: Arc<Task<Fut>>) {
@@ -553,19 +553,33 @@ impl<Fut> Debug for FuturesUnordered<Fut> {
     }
 }
 
+impl<Fut> FuturesUnordered<Fut> {
+    /// Clears the set, removing all futures.
+    pub fn clear(&mut self) {
+        self.clear_head_all();
+
+        // we just cleared all the tasks, and we have &mut self, so this is safe.
+        unsafe { self.ready_to_run_queue.clear() };
+
+        self.is_terminated.store(false, Relaxed);
+    }
+
+    fn clear_head_all(&mut self) {
+        while !self.head_all.get_mut().is_null() {
+            let head = *self.head_all.get_mut();
+            let task = unsafe { self.unlink(head) };
+            self.release_task(task);
+        }
+    }
+}
+
 impl<Fut> Drop for FuturesUnordered<Fut> {
     fn drop(&mut self) {
         // When a `FuturesUnordered` is dropped we want to drop all futures
         // associated with it. At the same time though there may be tons of
         // wakers flying around which contain `Task<Fut>` references
         // inside them. We'll let those naturally get deallocated.
-        unsafe {
-            while !self.head_all.get_mut().is_null() {
-                let head = *self.head_all.get_mut();
-                let task = self.unlink(head);
-                self.release_task(task);
-            }
-        }
+        self.clear_head_all();
 
         // Note that at this point we could still have a bunch of tasks in the
         // ready to run queue. None of those tasks, however, have futures

futures-util/src/stream/futures_unordered/ready_to_run_queue.rs

@@ -85,25 +85,38 @@ impl<Fut> ReadyToRunQueue<Fut> {
     pub(super) fn stub(&self) -> *const Task<Fut> {
         &*self.stub
     }
+
+    // Clear the queue of tasks.
+    //
+    // Note that each task has a strong reference count associated with it
+    // which is owned by the ready to run queue. This method just pulls out
+    // tasks and drops their refcounts.
+    //
+    // # Safety
+    //
+    // - All tasks **must** have had their futures dropped already (by FuturesUnordered::clear)
+    // - The caller **must** guarantee unique access to `self`
+    pub(crate) unsafe fn clear(&self) {
+        loop {
+            // SAFETY: We have the guarantee of mutual exclusion required by `dequeue`.
+            match self.dequeue() {
+                Dequeue::Empty => break,
+                Dequeue::Inconsistent => abort("inconsistent in drop"),
+                Dequeue::Data(ptr) => drop(Arc::from_raw(ptr)),
+            }
+        }
+    }
 }
 
 impl<Fut> Drop for ReadyToRunQueue<Fut> {
     fn drop(&mut self) {
         // Once we're in the destructor for `Inner<Fut>` we need to clear out
         // the ready to run queue of tasks if there's anything left in there.
-        //
-        // Note that each task has a strong reference count associated with it
-        // which is owned by the ready to run queue. All tasks should have had
-        // their futures dropped already by the `FuturesUnordered` destructor
-        // above, so we're just pulling out tasks and dropping their refcounts.
+
+        // All tasks have had their futures dropped already by the `FuturesUnordered`
+        // destructor above, and we have &mut self, so this is safe.
         unsafe {
-            loop {
-                match self.dequeue() {
-                    Dequeue::Empty => break,
-                    Dequeue::Inconsistent => abort("inconsistent in drop"),
-                    Dequeue::Data(ptr) => drop(Arc::from_raw(ptr)),
-                }
-            }
+            self.clear();
         }
     }
 }

futures/tests/stream_futures_unordered.rs

@@ -345,3 +345,25 @@ fn polled_only_once_at_most_per_iteration() {
     let mut tasks = FuturesUnordered::<F>::new();
     assert_eq!(Poll::Ready(None), tasks.poll_next_unpin(cx));
 }
+
+#[test]
+fn clear() {
+    let mut tasks = FuturesUnordered::from_iter(vec![future::ready(1), future::ready(2)]);
+
+    assert_eq!(block_on(tasks.next()), Some(1));
+    assert!(!tasks.is_empty());
+
+    tasks.clear();
+    assert!(tasks.is_empty());
+
+    tasks.push(future::ready(3));
+    assert!(!tasks.is_empty());
+
+    tasks.clear();
+    assert!(tasks.is_empty());
+
+    assert_eq!(block_on(tasks.next()), None);
+    assert!(tasks.is_terminated());
+    tasks.clear();
+    assert!(!tasks.is_terminated());
+}