Web: async improvements (#3805)

- Internal: Fix dropping `Notifier` without sending a result causing `Future`s to never complete. This should never happen anyway, but now we get a panic instead of nothing if we hit a bug.
- Internal: Remove a bunch of `unwrap()`s that aren't required when correctly using `MainThreadMarker`.
- `Window::canvas()` is now able to return a reference instead of an owned value.

Extracted from #3801.

src/changelog/unreleased.md

@@ -70,6 +70,7 @@ changelog entry.
 - Change signature of `EventLoop::run_app`, `EventLoopExtPumpEvents::pump_app_events` and
   `EventLoopExtRunOnDemand::run_app_on_demand` to accept a `impl ApplicationHandler` directly,
   instead of requiring a `&mut` reference to it.
+- On Web, `Window::canvas()` now returns a reference.
 
 ### Removed
 

src/platform/web.rs

@@ -42,6 +42,7 @@
 //! [`WindowEvent::Touch`]: crate::event::WindowEvent::Touch
 //! [`Window::set_outer_position()`]: crate::window::Window::set_outer_position
 
+use std::cell::Ref;
 use std::error::Error;
 use std::fmt::{self, Display, Formatter};
 use std::future::Future;
@@ -67,7 +68,7 @@ pub struct HtmlCanvasElement;
 pub trait WindowExtWeb {
     /// Only returns the canvas if called from inside the window context (the
     /// main thread).
-    fn canvas(&self) -> Option<HtmlCanvasElement>;
+    fn canvas(&self) -> Option<Ref<'_, HtmlCanvasElement>>;
 
     /// Returns [`true`] if calling `event.preventDefault()` is enabled.
     ///
@@ -87,7 +88,7 @@ pub trait WindowExtWeb {
 
 impl WindowExtWeb for Window {
     #[inline]
-    fn canvas(&self) -> Option<HtmlCanvasElement> {
+    fn canvas(&self) -> Option<Ref<'_, HtmlCanvasElement>> {
         self.window.canvas()
     }
 
@@ -175,7 +176,7 @@ pub trait EventLoopExtWeb {
         not(all(web_platform, target_feature = "exception-handling")),
         doc = "[`run_app()`]: EventLoop::run_app()"
     )]
-    /// [^1]: `run_app()` is _not_ available on WASM when the target supports `exception-handling`.
+    /// [^1]: `run_app()` is _not_ available on Wasm when the target supports `exception-handling`.
     fn spawn_app<A: ApplicationHandler + 'static>(self, app: A);
 
     /// Sets the strategy for [`ControlFlow::Poll`].
@@ -398,7 +399,7 @@ impl fmt::Display for BadAnimation {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             Self::Empty => write!(f, "No cursors supplied"),
-            Self::Animation => write!(f, "A supplied cursor is an animtion"),
+            Self::Animation => write!(f, "A supplied cursor is an animation"),
         }
     }
 }

src/platform_impl/web/async/dispatcher.rs

@@ -10,13 +10,12 @@ pub struct Dispatcher<T: 'static>(Wrapper<T, Arc<Sender<Closure<T>>>, Closure<T>
 struct Closure<T>(Box<dyn FnOnce(&T) + Send>);
 
 impl<T> Dispatcher<T> {
-    #[track_caller]
-    pub fn new(main_thread: MainThreadMarker, value: T) -> Option<(Self, DispatchRunner<T>)> {
+    pub fn new(main_thread: MainThreadMarker, value: T) -> (Self, DispatchRunner<T>) {
         let (sender, receiver) = channel::<Closure<T>>();
         let sender = Arc::new(sender);
         let receiver = Rc::new(receiver);
 
-        Wrapper::new(
+        let wrapper = Wrapper::new(
             main_thread,
             value,
             |value, Closure(closure)| {
@@ -29,8 +28,7 @@ impl<T> Dispatcher<T> {
                 move |value| async move {
                     while let Ok(Closure(closure)) = receiver.next().await {
                         // SAFETY: The given `Closure` here isn't really `'static`, so we shouldn't
-                        // do anything funny with it here. See
-                        // `Self::queue()`.
+                        // do anything funny with it here. See `Self::queue()`.
                         closure(value.borrow().as_ref().unwrap())
                     }
                 }
@@ -41,25 +39,25 @@ impl<T> Dispatcher<T> {
                 // anything funny with it here. See `Self::queue()`.
                 sender.send(closure).unwrap()
             },
-        )
-        .map(|wrapper| (Self(wrapper.clone()), DispatchRunner { wrapper, receiver }))
+        );
+        (Self(wrapper.clone()), DispatchRunner { wrapper, receiver })
     }
 
-    pub fn value(&self) -> Option<Ref<'_, T>> {
-        self.0.value()
+    pub fn value(&self, main_thread: MainThreadMarker) -> Ref<'_, T> {
+        self.0.value(main_thread)
     }
 
     pub fn dispatch(&self, f: impl 'static + FnOnce(&T) + Send) {
-        if let Some(value) = self.0.value() {
-            f(&value)
+        if let Some(main_thread) = MainThreadMarker::new() {
+            f(&self.0.value(main_thread))
         } else {
             self.0.send(Closure(Box::new(f)))
         }
     }
 
     pub fn queue<R: Send>(&self, f: impl FnOnce(&T) -> R + Send) -> R {
-        if let Some(value) = self.0.value() {
-            f(&value)
+        if let Some(main_thread) = MainThreadMarker::new() {
+            f(&self.0.value(main_thread))
         } else {
             let pair = Arc::new((Mutex::new(None), Condvar::new()));
             let closure = Box::new({
@@ -98,13 +96,13 @@ pub struct DispatchRunner<T: 'static> {
 }
 
 impl<T> DispatchRunner<T> {
-    pub fn run(&self) {
+    pub fn run(&self, main_thread: MainThreadMarker) {
         while let Some(Closure(closure)) =
             self.receiver.try_recv().expect("should only be closed when `Dispatcher` is dropped")
         {
             // SAFETY: The given `Closure` here isn't really `'static`, so we shouldn't do anything
             // funny with it here. See `Self::queue()`.
-            closure(&self.wrapper.value().expect("don't call this outside the main thread"))
+            closure(&self.wrapper.value(main_thread))
         }
     }
 }

src/platform_impl/web/async/notifier.rs

@@ -17,24 +17,28 @@ impl<T: Clone> Notifier<T> {
         if self.0.value.set(value).is_err() {
             unreachable!("value set before")
         }
+    }
+
+    pub fn notified(&self) -> Notified<T> {
+        Notified(Some(Arc::clone(&self.0)))
+    }
+}
 
+impl<T: Clone> Drop for Notifier<T> {
+    fn drop(&mut self) {
         self.0.queue.close();
 
         while let Ok(waker) = self.0.queue.pop() {
             waker.wake()
         }
     }
-
-    pub fn notified(&self) -> Notified<T> {
-        Notified(Some(Arc::clone(&self.0)))
-    }
 }
 
 #[derive(Clone, Debug)]
 pub struct Notified<T: Clone>(Option<Arc<Inner<T>>>);
 
 impl<T: Clone> Future for Notified<T> {
-    type Output = T;
+    type Output = Option<T>;
 
     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
         let this = self.0.take().expect("`Receiver` polled after completion");
@@ -54,14 +58,13 @@ impl<T: Clone> Future for Notified<T> {
             }
         }
 
-        let (Ok(Some(value)) | Err(Some(value))) = Arc::try_unwrap(this)
+        match Arc::try_unwrap(this)
             .map(|mut inner| inner.value.take())
             .map_err(|this| this.value.get().cloned())
-        else {
-            unreachable!("found no value despite being ready")
-        };
-
-        Poll::Ready(value)
+        {
+            Ok(Some(value)) | Err(Some(value)) => Poll::Ready(Some(value)),
+            _ => Poll::Ready(None),
+        }
     }
 }
 

src/platform_impl/web/async/waker.rs

@@ -19,8 +19,7 @@ struct Handler<T> {
 struct Sender(Arc<Inner>);
 
 impl<T> WakerSpawner<T> {
-    #[track_caller]
-    pub fn new(main_thread: MainThreadMarker, value: T, handler: fn(&T, bool)) -> Option<Self> {
+    pub fn new(main_thread: MainThreadMarker, value: T, handler: fn(&T, bool)) -> Self {
         let inner = Arc::new(Inner {
             awoken: AtomicBool::new(false),
             waker: AtomicWaker::new(),
@@ -31,7 +30,7 @@ impl<T> WakerSpawner<T> {
 
         let sender = Sender(Arc::clone(&inner));
 
-        let wrapper = Wrapper::new(
+        Self(Wrapper::new(
             main_thread,
             handler,
             |handler, _| {
@@ -73,9 +72,7 @@ impl<T> WakerSpawner<T> {
                 inner.0.awoken.store(true, Ordering::Relaxed);
                 inner.0.waker.wake();
             },
-        )?;
-
-        Some(Self(wrapper))
+        ))
     }
 
     pub fn waker(&self) -> Waker<T> {

src/platform_impl/web/async/wrapper.rs

@@ -33,15 +33,14 @@ unsafe impl<V> Send for Value<V> {}
 unsafe impl<V> Sync for Value<V> {}
 
 impl<V, S: Clone + Send, E> Wrapper<V, S, E> {
-    #[track_caller]
     pub fn new<R: Future<Output = ()>>(
         _: MainThreadMarker,
         value: V,
         handler: fn(&RefCell<Option<V>>, E),
         receiver: impl 'static + FnOnce(Arc<RefCell<Option<V>>>) -> R,
         sender_data: S,
         sender_handler: fn(&S, E),
-    ) -> Option<Self> {
+    ) -> Self {
         let value = Arc::new(RefCell::new(Some(value)));
 
         wasm_bindgen_futures::spawn_local({
@@ -52,12 +51,7 @@ impl<V, S: Clone + Send, E> Wrapper<V, S, E> {
             }
         });
 
-        Some(Self {
-            value: Value { value, local: PhantomData },
-            handler,
-            sender_data,
-            sender_handler,
-        })
+        Self { value: Value { value, local: PhantomData }, handler, sender_data, sender_handler }
     }
 
     pub fn send(&self, event: E) {
@@ -68,9 +62,8 @@ impl<V, S: Clone + Send, E> Wrapper<V, S, E> {
         }
     }
 
-    pub fn value(&self) -> Option<Ref<'_, V>> {
-        MainThreadMarker::new()
-            .map(|_| Ref::map(self.value.value.borrow(), |value| value.as_ref().unwrap()))
+    pub fn value(&self, _: MainThreadMarker) -> Ref<'_, V> {
+        Ref::map(self.value.value.borrow(), |value| value.as_ref().unwrap())
     }
 
     pub fn with_sender_data<T>(&self, f: impl FnOnce(&S) -> T) -> T {

src/platform_impl/web/cursor.rs

@@ -178,7 +178,7 @@ impl Future for CustomCursorFuture {
             panic!("`CustomCursorFuture` polled after completion")
         }
 
-        let result = ready!(Pin::new(&mut self.notified).poll(cx));
+        let result = ready!(Pin::new(&mut self.notified).poll(cx)).unwrap();
         let state = self.state.take().expect("`CustomCursorFuture` polled after completion");
 
         Poll::Ready(result.map(|_| CustomCursor { animation: self.animation, state }))
@@ -662,7 +662,7 @@ async fn from_animation(
             ImageState::Loading { notifier, .. } => {
                 let notified = notifier.notified();
                 drop(state);
-                notified.await?;
+                notified.await.unwrap()?;
             },
             ImageState::Failed(error) => return Err(error.clone()),
             ImageState::Image(_) => drop(state),

src/platform_impl/web/event_loop/proxy.rs

@@ -1,15 +1,13 @@
-use std::rc::Weak;
-
-use super::runner::Execution;
+use super::runner::WeakShared;
 use crate::platform_impl::platform::r#async::Waker;
 
 #[derive(Clone)]
 pub struct EventLoopProxy {
-    runner: Waker<Weak<Execution>>,
+    runner: Waker<WeakShared>,
 }
 
 impl EventLoopProxy {
-    pub fn new(runner: Waker<Weak<Execution>>) -> Self {
+    pub fn new(runner: Waker<WeakShared>) -> Self {
         Self { runner }
     }