task: fix leak in LocalSet (#3978)

tokio/src/runtime/tests/loom_local.rs

@@ -0,0 +1,47 @@
+use crate::runtime::tests::loom_oneshot as oneshot;
+use crate::runtime::Builder;
+use crate::task::LocalSet;
+
+use std::task::Poll;
+
+/// Waking a runtime will attempt to push a task into a queue of notifications
+/// in the runtime, however the tasks in such a queue usually have a reference
+/// to the runtime itself. This means that if they are not properly removed at
+/// runtime shutdown, this will cause a memory leak.
+///
+/// This test verifies that waking something during shutdown of a LocalSet does
+/// not result in tasks lingering in the queue once shutdown is complete. This
+/// is verified using loom's leak finder.
+#[test]
+fn wake_during_shutdown() {
+    loom::model(|| {
+        let rt = Builder::new_current_thread().build().unwrap();
+        let ls = LocalSet::new();
+
+        let (send, recv) = oneshot::channel();
+
+        ls.spawn_local(async move {
+            let mut send = Some(send);
+
+            let () = futures::future::poll_fn(|cx| {
+                if let Some(send) = send.take() {
+                    send.send(cx.waker().clone());
+                }
+
+                Poll::Pending
+            })
+            .await;
+        });
+
+        let handle = loom::thread::spawn(move || {
+            let waker = recv.recv();
+            waker.wake();
+        });
+
+        ls.block_on(&rt, crate::task::yield_now());
+
+        drop(ls);
+        handle.join().unwrap();
+        drop(rt);
+    });
+}

tokio/src/runtime/tests/mod.rs

@@ -21,6 +21,7 @@ mod joinable_wrapper {
 
 cfg_loom! {
     mod loom_basic_scheduler;
+    mod loom_local;
     mod loom_blocking;
     mod loom_oneshot;
     mod loom_pool;

tokio/src/task/local.rs

@@ -242,7 +242,7 @@ struct Tasks {
 /// LocalSet state shared between threads.
 struct Shared {
     /// Remote run queue sender
-    queue: Mutex<VecDeque<task::Notified<Arc<Shared>>>>,
+    queue: Mutex<Option<VecDeque<task::Notified<Arc<Shared>>>>>,
 
     /// Wake the `LocalSet` task
     waker: AtomicWaker,
@@ -338,7 +338,7 @@ impl LocalSet {
                     queue: VecDeque::with_capacity(INITIAL_CAPACITY),
                 }),
                 shared: Arc::new(Shared {
-                    queue: Mutex::new(VecDeque::with_capacity(INITIAL_CAPACITY)),
+                    queue: Mutex::new(Some(VecDeque::with_capacity(INITIAL_CAPACITY))),
                     waker: AtomicWaker::new(),
                 }),
             },
@@ -538,15 +538,23 @@ impl LocalSet {
                 .shared
                 .queue
                 .lock()
-                .pop_front()
+                .as_mut()
+                .and_then(|queue| queue.pop_front())
                 .or_else(|| self.context.tasks.borrow_mut().queue.pop_front())
         } else {
             self.context
                 .tasks
                 .borrow_mut()
                 .queue
                 .pop_front()
-                .or_else(|| self.context.shared.queue.lock().pop_front())
+                .or_else(|| {
+                    self.context
+                        .shared
+                        .queue
+                        .lock()
+                        .as_mut()
+                        .and_then(|queue| queue.pop_front())
+                })
         }
     }
 
@@ -610,7 +618,10 @@ impl Drop for LocalSet {
                 task.shutdown();
             }
 
-            for task in self.context.shared.queue.lock().drain(..) {
+            // Take the queue from the Shared object to prevent pushing
+            // notifications to it in the future.
+            let queue = self.context.shared.queue.lock().take().unwrap();
+            for task in queue {
                 task.shutdown();
             }
 
@@ -660,8 +671,16 @@ impl Shared {
                 cx.tasks.borrow_mut().queue.push_back(task);
             }
             _ => {
-                self.queue.lock().push_back(task);
-                self.waker.wake();
+                // First check whether the queue is still there (if not, the
+                // LocalSet is dropped). Then push to it if so, and if not,
+                // do nothing.
+                let mut lock = self.queue.lock();
+
+                if let Some(queue) = lock.as_mut() {
+                    queue.push_back(task);
+                    drop(lock);
+                    self.waker.wake();
+                }
             }
         });
     }