rt: avoid dropping a task in calls to wake() (#1972)

Calls to tasks should not be nested. Currently, while a task is being
executed and the runtime is shutting down, a call to wake() can result
in the wake target to be dropped. This, in turn, results in the drop
handler being called.

If the user holds a ref cell borrow, a mutex guard, or any such value,
dropping the task inline can result in a deadlock.

The fix is to permit tasks to be scheduled during the shutdown process
and dropping the tasks once they are popped from the queue.

Fixes #1929, #1886

tokio/src/runtime/basic_scheduler.rs

@@ -84,7 +84,7 @@ where
         F::Output: Send + 'static,
     {
         let (task, handle) = task::joinable(future);
-        self.scheduler.schedule(task);
+        self.scheduler.schedule(task, true);
         handle
     }
 
@@ -161,7 +161,7 @@ impl Spawner {
         F::Output: Send + 'static,
     {
         let (task, handle) = task::joinable(future);
-        self.scheduler.schedule(task);
+        self.scheduler.schedule(task, true);
         handle
     }
 
@@ -230,6 +230,27 @@ impl SchedulerPriv {
         self.queues.push_local(task);
         handle
     }
+
+    fn schedule(&self, task: Task<Self>, spawn: bool) {
+        let is_current = ACTIVE.with(|cell| cell.get() == self as *const SchedulerPriv);
+
+        if is_current {
+            unsafe {
+                // safety: this function is safe to call only from the
+                // thread the basic scheduler is running on. If `is_current` is
+                // then we are on that thread.
+                self.queues.push_local(task)
+            };
+        } else {
+            let mut lock = self.queues.remote();
+            lock.schedule(task, spawn);
+
+            // while locked, call unpark
+            self.unpark.unpark();
+
+            drop(lock);
+        }
+    }
 }
 
 impl Schedule for SchedulerPriv {
@@ -260,24 +281,7 @@ impl Schedule for SchedulerPriv {
     }
 
     fn schedule(&self, task: Task<Self>) {
-        let is_current = ACTIVE.with(|cell| cell.get() == self as *const SchedulerPriv);
+        SchedulerPriv::schedule(self, task, false);
-
-        if is_current {
-            unsafe {
-                // safety: this function is safe to call only from the
-                // thread the basic scheduler is running on. If `is_current` is
-                // then we are on that thread.
-                self.queues.push_local(task)
-            };
-        } else {
-            let mut lock = self.queues.remote();
-            lock.schedule(task);
-
-            // while locked, call unpark
-            self.unpark.unpark();
-
-            drop(lock);
-        }
     }
 }
 
@@ -298,10 +302,16 @@ where
         }
 
         // Wait until all tasks have been released.
-        while unsafe { self.scheduler.queues.has_tasks_remaining() } {
+        loop {
-            self.local.park.park().ok().expect("park failed");
             unsafe {
                 self.scheduler.queues.drain_pending_drop();
+                self.scheduler.queues.drain_queues();
+
+                if !self.scheduler.queues.has_tasks_remaining() {
+                    break;
+                }
+
+                self.local.park.park().ok().expect("park failed");
             }
         }
     }

tokio/src/task/local.rs

@@ -345,7 +345,7 @@ impl Schedule for Scheduler {
             unsafe { self.queues.push_local(task) };
         } else {
             let mut lock = self.queues.remote();
-            lock.schedule(task);
+            lock.schedule(task, false);
 
             self.waker.wake();
 
@@ -437,8 +437,15 @@ impl Drop for Scheduler {
             // Wait until all tasks have been released.
             // XXX: this is a busy loop, but we don't really have any way to park
             // the thread here?
-            while self.queues.has_tasks_remaining() {
+            loop {
                 self.queues.drain_pending_drop();
+                self.queues.drain_queues();
+
+                if !self.queues.has_tasks_remaining() {
+                    break;
+                }
+
+                std::thread::yield_now();
             }
         }
     }

tokio/src/task/queue.rs

@@ -233,6 +233,16 @@ where
         self.drain_pending_drop();
     }
 
+    /// Drain both the local and remote run queues, shutting down any tasks.
+    ///
+    /// # Safety
+    ///
+    /// This *must* be called only from the thread that owns the scheduler.
+    pub(crate) unsafe fn drain_queues(&self) {
+        self.close_local();
+        self.close_remote();
+    }
+
     /// Shut down the scheduler's owned task list.
     ///
     /// # Safety
@@ -300,8 +310,10 @@ where
     /// If the queue is open to accept new tasks, the task is pushed to the back
     /// of the queue. Otherwise, if the queue is closed (the scheduler is
     /// shutting down), the new task will be shut down immediately.
-    pub(crate) fn schedule(&mut self, task: Task<S>) {
+    ///
-        if self.open {
+    /// `spawn` should be set if the caller is spawning a new task.
+    pub(crate) fn schedule(&mut self, task: Task<S>, spawn: bool) {
+        if !spawn || self.open {
             self.queue.push_back(task);
         } else {
             task.shutdown();

tokio/tests/rt_basic.rs

@@ -27,6 +27,42 @@ fn spawned_task_does_not_progress_without_block_on() {
     assert_eq!(out, "hello");
 }
 
+#[test]
+fn acquire_mutex_in_drop() {
+    use futures::future::pending;
+    use tokio::task;
+
+    let (tx1, rx1) = oneshot::channel();
+    let (tx2, rx2) = oneshot::channel();
+
+    let mut rt = rt();
+
+    rt.spawn(async move {
+        let _ = rx2.await;
+        unreachable!();
+    });
+
+    rt.spawn(async move {
+        let _ = rx1.await;
+        let _ = tx2.send(()).unwrap();
+        unreachable!();
+    });
+
+    // Spawn a task that will never notify
+    rt.spawn(async move {
+        pending::<()>().await;
+        tx1.send(()).unwrap();
+    });
+
+    // Tick the loop
+    rt.block_on(async {
+        task::yield_now().await;
+    });
+
+    // Drop the rt
+    drop(rt);
+}
+
 fn rt() -> Runtime {
     tokio::runtime::Builder::new()
         .basic_scheduler()

tokio/tests/task_local_set.rs

@@ -0,0 +1,50 @@
+#![warn(rust_2018_idioms)]
+#![cfg(feature = "full")]
+
+use tokio::runtime::Runtime;
+use tokio::sync::oneshot;
+use tokio::task::{self, LocalSet};
+
+#[test]
+fn acquire_mutex_in_drop() {
+    use futures::future::pending;
+
+    let (tx1, rx1) = oneshot::channel();
+    let (tx2, rx2) = oneshot::channel();
+
+    let mut rt = rt();
+    let local = LocalSet::new();
+
+    local.spawn_local(async move {
+        let _ = rx2.await;
+        unreachable!();
+    });
+
+    local.spawn_local(async move {
+        let _ = rx1.await;
+        let _ = tx2.send(()).unwrap();
+        unreachable!();
+    });
+
+    // Spawn a task that will never notify
+    local.spawn_local(async move {
+        pending::<()>().await;
+        tx1.send(()).unwrap();
+    });
+
+    // Tick the loop
+    local.block_on(&mut rt, async {
+        task::yield_now().await;
+    });
+
+    // Drop the LocalSet
+    drop(local);
+}
+
+fn rt() -> Runtime {
+    tokio::runtime::Builder::new()
+        .basic_scheduler()
+        .enable_all()
+        .build()
+        .unwrap()
+}