Make Tasks functional on WASM (#13889)

# Objective

Right not bevy's task pool abstraction is kind of useless on wasm, since
it returns a `FakeTask` which can't be interacted with. This is only
good for fire-and-forget it tasks, and isn't even that useful since it's
just a thin wrapper around `wasm-bindgen-futures::spawn_local`

## Solution

Add a simple `Task<T>` handler type to wasm targets that allow waiting
for a task's output or periodically checking for its completion. This PR
aims to give the wasm version of these tasks feature parity with the
native, multi-threaded version of the task

## Testing

- Did you test these changes? *Not yet*

---------

Co-authored-by: Periwink <charlesbour@gmail.com>
Co-authored-by: Jan Hohenheim <jan@hohenheim.ch>

crates/bevy_tasks/Cargo.toml

@@ -20,6 +20,8 @@ concurrent-queue = { version = "2.0.0", optional = true }
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 wasm-bindgen-futures = "0.4"
+pin-project = "1"
+futures-channel = "0.3"
 
 [dev-dependencies]
 web-time = { version = "1.1" }

crates/bevy_tasks/src/lib.rs

@@ -8,7 +8,9 @@
 mod slice;
 pub use slice::{ParallelSlice, ParallelSliceMut};
 
+#[cfg_attr(target_arch = "wasm32", path = "wasm_task.rs")]
 mod task;
+
 pub use task::Task;
 
 #[cfg(all(not(target_arch = "wasm32"), feature = "multi_threaded"))]
@@ -19,7 +21,7 @@ pub use task_pool::{Scope, TaskPool, TaskPoolBuilder};
 #[cfg(any(target_arch = "wasm32", not(feature = "multi_threaded")))]
 mod single_threaded_task_pool;
 #[cfg(any(target_arch = "wasm32", not(feature = "multi_threaded")))]
-pub use single_threaded_task_pool::{FakeTask, Scope, TaskPool, TaskPoolBuilder, ThreadExecutor};
+pub use single_threaded_task_pool::{Scope, TaskPool, TaskPoolBuilder, ThreadExecutor};
 
 mod usages;
 #[cfg(not(target_arch = "wasm32"))]

crates/bevy_tasks/src/single_threaded_task_pool.rs

@@ -1,6 +1,8 @@
 use std::sync::Arc;
 use std::{cell::RefCell, future::Future, marker::PhantomData, mem, rc::Rc};
 
+use crate::Task;
+
 thread_local! {
     static LOCAL_EXECUTOR: async_executor::LocalExecutor<'static> = const { async_executor::LocalExecutor::new() };
 }
@@ -145,34 +147,33 @@ impl TaskPool {
             .collect()
     }
 
-    /// Spawns a static future onto the thread pool. The returned Task is a future. It can also be
-    /// cancelled and "detached" allowing it to continue running without having to be polled by the
+    /// Spawns a static future onto the thread pool. The returned Task is a future, which can be polled
+    /// to retrieve the output of the original future. Dropping the task will attempt to cancel it.
+    /// It can also be "detached", allowing it to continue running without having to be polled by the
     /// end-user.
     ///
     /// If the provided future is non-`Send`, [`TaskPool::spawn_local`] should be used instead.
-    pub fn spawn<T>(&self, future: impl Future<Output = T> + 'static) -> FakeTask
+    pub fn spawn<T>(&self, future: impl Future<Output = T> + 'static) -> Task<T>
     where
         T: 'static,
     {
         #[cfg(target_arch = "wasm32")]
-        wasm_bindgen_futures::spawn_local(async move {
-            future.await;
-        });
+        return Task::wrap_future(future);
 
         #[cfg(not(target_arch = "wasm32"))]
         {
             LOCAL_EXECUTOR.with(|executor| {
-                let _task = executor.spawn(future);
+                let task = executor.spawn(future);
                 // Loop until all tasks are done
                 while executor.try_tick() {}
-            });
-        }
 
-        FakeTask
+                Task::new(task)
+            })
+        }
     }
 
     /// Spawns a static future on the JS event loop. This is exactly the same as [`TaskPool::spawn`].
-    pub fn spawn_local<T>(&self, future: impl Future<Output = T> + 'static) -> FakeTask
+    pub fn spawn_local<T>(&self, future: impl Future<Output = T> + 'static) -> Task<T>
     where
         T: 'static,
     {
@@ -198,17 +199,6 @@ impl TaskPool {
     }
 }
 
-/// An empty task used in single-threaded contexts.
-///
-/// This does nothing and is therefore safe, and recommended, to ignore.
-#[derive(Debug)]
-pub struct FakeTask;
-
-impl FakeTask {
-    /// No op on the single threaded task pool
-    pub fn detach(self) {}
-}
-
 /// A `TaskPool` scope for running one or more non-`'static` futures.
 ///
 /// For more information, see [`TaskPool::scope`].

crates/bevy_tasks/src/wasm_task.rs

@@ -0,0 +1,82 @@
+use std::{
+    any::Any,
+    future::{Future, IntoFuture},
+    panic::{AssertUnwindSafe, UnwindSafe},
+    pin::Pin,
+    task::Poll,
+};
+
+use futures_channel::oneshot;
+
+/// Wraps an asynchronous task, a spawned future.
+///
+/// Tasks are also futures themselves and yield the output of the spawned future.
+#[derive(Debug)]
+pub struct Task<T>(oneshot::Receiver<Result<T, Panic>>);
+
+impl<T: 'static> Task<T> {
+    pub(crate) fn wrap_future(future: impl Future<Output = T> + 'static) -> Self {
+        let (sender, receiver) = oneshot::channel();
+        wasm_bindgen_futures::spawn_local(async move {
+            // Catch any panics that occur when polling the future so they can
+            // be propagated back to the task handle.
+            let value = CatchUnwind(AssertUnwindSafe(future)).await;
+            let _ = sender.send(value);
+        });
+        Self(receiver.into_future())
+    }
+
+    /// When building for Wasm, this method has no effect.
+    /// This is only included for feature parity with other platforms.
+    pub fn detach(self) {}
+
+    /// Requests a task to be cancelled and returns a future that suspends until it completes.
+    /// Returns the output of the future if it has already completed.
+    ///
+    /// # Implementation
+    ///
+    /// When building for Wasm, it is not possible to cancel tasks, which means this is the same
+    /// as just awaiting the task. This method is only included for feature parity with other platforms.
+    pub async fn cancel(self) -> Option<T> {
+        match self.0.await {
+            Ok(Ok(value)) => Some(value),
+            Err(_) => None,
+            Ok(Err(panic)) => {
+                // drop this to prevent the panic payload from resuming the panic on drop.
+                // this also leaks the box but I'm not sure how to avoid that
+                std::mem::forget(panic);
+                None
+            }
+        }
+    }
+}
+
+impl<T> Future for Task<T> {
+    type Output = T;
+    fn poll(
+        mut self: std::pin::Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Self::Output> {
+        match Pin::new(&mut self.0).poll(cx) {
+            Poll::Ready(Ok(Ok(value))) => Poll::Ready(value),
+            // NOTE: Propagating the panic here sorta has parity with the async_executor behavior.
+            // For those tasks, polling them after a panic returns a `None` which gets `unwrap`ed, so
+            // using `resume_unwind` here is essentially keeping the same behavior while adding more information.
+            Poll::Ready(Ok(Err(panic))) => std::panic::resume_unwind(panic),
+            Poll::Ready(Err(_)) => panic!("Polled a task after it was cancelled"),
+            Poll::Pending => Poll::Pending,
+        }
+    }
+}
+
+type Panic = Box<dyn Any + Send + 'static>;
+
+#[pin_project::pin_project]
+struct CatchUnwind<F: UnwindSafe>(#[pin] F);
+
+impl<F: Future + UnwindSafe> Future for CatchUnwind<F> {
+    type Output = Result<F::Output, Panic>;
+    fn poll(self: std::pin::Pin<&mut Self>, cx: &mut std::task::Context) -> Poll<Self::Output> {
+        std::panic::catch_unwind(AssertUnwindSafe(|| self.project().0.poll(cx)))?.map(Ok)
+    }
+}