libasync: pin tasks to memory

libasync/src/executor.rs

@@ -34,8 +34,13 @@ static VTABLE: RawWakerVTable = {
 ///
 /// This is a singleton
 pub struct Executor {
+    // Entered block_on() already?
     in_block_on: RefCell<bool>,
-    tasks: RefCell<Deque<Task>>,
+
+    /// Tasks reside on the heap, so that we just queue pointers. They
+    /// must also be pinned in memory because our RawWaker is a pointer
+    /// to their `ready` field.
+    tasks: RefCell<Deque<Pin<Box<Task>>>>,
 }
 
 impl Executor {
@@ -97,17 +102,14 @@ impl Executor {
                     let mut cx = Context::from_waker(&waker);
                     // this points into a `static` memory so it's already pinned
                     // println!("run task");
-                    let ready = unsafe {
-                        Pin::new_unchecked(&mut *task.f)
-                            .poll(&mut cx)
-                            .is_ready()
-                    };
-                    // println!("ran task, ready={:?}", r);
-                    if !ready {
-                        // Task is not finished, requeue
-                        self.tasks.borrow_mut().push_back(task);
+                    let ready = task.f.as_mut().poll(&mut cx).is_ready();
+                    if ready {
+                        // Task is finished, do not requeue
+                        continue;
                     }
                 }
+                // Requeue
+                self.tasks.borrow_mut().push_back(task);
             }
 
             // // try to sleep; this will be a no-op if any of the previous tasks generated a SEV or an
@@ -118,26 +120,22 @@ impl Executor {
         val
     }
 
-    // NOTE CAREFUL! this method can overlap with `block_on`
-    // FIXME we want to use `Future<Output = !>` here but the never type (`!`) is unstable; so as a
-    // workaround we'll "abort" if the task / future terminates (see `Task::new`)
     pub fn spawn(&self, f: impl Future + 'static) {
-        // NOTE(unsafe) only safe as long as `spawn` is never re-entered and this does not overlap
-        // with operation `(A)` (see `Task::block_on`)
-        self.tasks.borrow_mut().push_back(Task::new(f));
+        let task = Box::pin(Task::new(f));
+        self.tasks.borrow_mut().push_back(task);
     }
 }
 
 pub struct Task {
     ready: AtomicBool,
-    f: Box<dyn Future<Output = ()>>,
+    f: Pin<Box<dyn Future<Output = ()>>>,
 }
 
 impl Task {
     fn new(f: impl Future + 'static) -> Self {
         Task {
             ready: AtomicBool::new(true),
-            f: Box::new(async { f.await; }),
+            f: Box::pin(async { f.await; }),
         }
     }
 }

libasync/src/task.rs

@@ -17,10 +17,7 @@ pub fn block_on<T>(f: impl Future<Output = T>) -> T {
 /// Spawns a task onto the executor
 ///
 /// The spawned task will not make any progress until `block_on` is called.
-///
-/// The future `f` must never terminate. The program will *abort* if `f` (the async code) returns.
-/// The right signature here would be `f: impl Future<Output = !>` but that requires nightly
-pub fn spawn<T>(f: impl Future<Output = T> + 'static) {
+pub fn spawn(f: impl Future + 'static) {
     executor::current().spawn(f)
 }