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https://github.com/bevyengine/bevy
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Make Task
s 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>
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c3057d4353
commit
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4 changed files with 99 additions and 23 deletions
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@ -20,6 +20,8 @@ concurrent-queue = { version = "2.0.0", optional = true }
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[target.'cfg(target_arch = "wasm32")'.dependencies]
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wasm-bindgen-futures = "0.4"
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pin-project = "1"
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futures-channel = "0.3"
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[dev-dependencies]
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web-time = { version = "1.1" }
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@ -8,7 +8,9 @@
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mod slice;
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pub use slice::{ParallelSlice, ParallelSliceMut};
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#[cfg_attr(target_arch = "wasm32", path = "wasm_task.rs")]
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mod task;
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pub use task::Task;
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#[cfg(all(not(target_arch = "wasm32"), feature = "multi_threaded"))]
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@ -19,7 +21,7 @@ pub use task_pool::{Scope, TaskPool, TaskPoolBuilder};
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#[cfg(any(target_arch = "wasm32", not(feature = "multi_threaded")))]
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mod single_threaded_task_pool;
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#[cfg(any(target_arch = "wasm32", not(feature = "multi_threaded")))]
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pub use single_threaded_task_pool::{FakeTask, Scope, TaskPool, TaskPoolBuilder, ThreadExecutor};
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pub use single_threaded_task_pool::{Scope, TaskPool, TaskPoolBuilder, ThreadExecutor};
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mod usages;
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#[cfg(not(target_arch = "wasm32"))]
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@ -1,6 +1,8 @@
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use std::sync::Arc;
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use std::{cell::RefCell, future::Future, marker::PhantomData, mem, rc::Rc};
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use crate::Task;
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thread_local! {
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static LOCAL_EXECUTOR: async_executor::LocalExecutor<'static> = const { async_executor::LocalExecutor::new() };
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}
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@ -145,34 +147,33 @@ impl TaskPool {
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.collect()
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}
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/// Spawns a static future onto the thread pool. The returned Task is a future. It can also be
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/// cancelled and "detached" allowing it to continue running without having to be polled by the
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/// Spawns a static future onto the thread pool. The returned Task is a future, which can be polled
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/// to retrieve the output of the original future. Dropping the task will attempt to cancel it.
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/// It can also be "detached", allowing it to continue running without having to be polled by the
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/// end-user.
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///
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/// If the provided future is non-`Send`, [`TaskPool::spawn_local`] should be used instead.
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pub fn spawn<T>(&self, future: impl Future<Output = T> + 'static) -> FakeTask
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pub fn spawn<T>(&self, future: impl Future<Output = T> + 'static) -> Task<T>
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where
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T: 'static,
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{
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#[cfg(target_arch = "wasm32")]
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wasm_bindgen_futures::spawn_local(async move {
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future.await;
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});
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return Task::wrap_future(future);
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#[cfg(not(target_arch = "wasm32"))]
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{
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LOCAL_EXECUTOR.with(|executor| {
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let _task = executor.spawn(future);
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let task = executor.spawn(future);
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// Loop until all tasks are done
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while executor.try_tick() {}
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});
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}
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FakeTask
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Task::new(task)
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})
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}
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}
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/// Spawns a static future on the JS event loop. This is exactly the same as [`TaskPool::spawn`].
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pub fn spawn_local<T>(&self, future: impl Future<Output = T> + 'static) -> FakeTask
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pub fn spawn_local<T>(&self, future: impl Future<Output = T> + 'static) -> Task<T>
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where
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T: 'static,
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{
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@ -198,17 +199,6 @@ impl TaskPool {
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}
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}
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/// An empty task used in single-threaded contexts.
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///
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/// This does nothing and is therefore safe, and recommended, to ignore.
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#[derive(Debug)]
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pub struct FakeTask;
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impl FakeTask {
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/// No op on the single threaded task pool
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pub fn detach(self) {}
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}
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/// A `TaskPool` scope for running one or more non-`'static` futures.
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///
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/// For more information, see [`TaskPool::scope`].
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82
crates/bevy_tasks/src/wasm_task.rs
Normal file
82
crates/bevy_tasks/src/wasm_task.rs
Normal file
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@ -0,0 +1,82 @@
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use std::{
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any::Any,
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future::{Future, IntoFuture},
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panic::{AssertUnwindSafe, UnwindSafe},
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pin::Pin,
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task::Poll,
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};
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use futures_channel::oneshot;
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/// Wraps an asynchronous task, a spawned future.
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///
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/// Tasks are also futures themselves and yield the output of the spawned future.
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#[derive(Debug)]
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pub struct Task<T>(oneshot::Receiver<Result<T, Panic>>);
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impl<T: 'static> Task<T> {
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pub(crate) fn wrap_future(future: impl Future<Output = T> + 'static) -> Self {
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let (sender, receiver) = oneshot::channel();
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wasm_bindgen_futures::spawn_local(async move {
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// Catch any panics that occur when polling the future so they can
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// be propagated back to the task handle.
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let value = CatchUnwind(AssertUnwindSafe(future)).await;
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let _ = sender.send(value);
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});
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Self(receiver.into_future())
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}
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/// When building for Wasm, this method has no effect.
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/// This is only included for feature parity with other platforms.
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pub fn detach(self) {}
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/// Requests a task to be cancelled and returns a future that suspends until it completes.
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/// Returns the output of the future if it has already completed.
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///
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/// # Implementation
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///
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/// When building for Wasm, it is not possible to cancel tasks, which means this is the same
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/// as just awaiting the task. This method is only included for feature parity with other platforms.
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pub async fn cancel(self) -> Option<T> {
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match self.0.await {
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Ok(Ok(value)) => Some(value),
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Err(_) => None,
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Ok(Err(panic)) => {
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// drop this to prevent the panic payload from resuming the panic on drop.
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// this also leaks the box but I'm not sure how to avoid that
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std::mem::forget(panic);
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None
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}
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}
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}
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}
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impl<T> Future for Task<T> {
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type Output = T;
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fn poll(
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mut self: std::pin::Pin<&mut Self>,
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cx: &mut std::task::Context<'_>,
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) -> std::task::Poll<Self::Output> {
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match Pin::new(&mut self.0).poll(cx) {
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Poll::Ready(Ok(Ok(value))) => Poll::Ready(value),
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// NOTE: Propagating the panic here sorta has parity with the async_executor behavior.
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// For those tasks, polling them after a panic returns a `None` which gets `unwrap`ed, so
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// using `resume_unwind` here is essentially keeping the same behavior while adding more information.
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Poll::Ready(Ok(Err(panic))) => std::panic::resume_unwind(panic),
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Poll::Ready(Err(_)) => panic!("Polled a task after it was cancelled"),
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Poll::Pending => Poll::Pending,
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}
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}
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}
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type Panic = Box<dyn Any + Send + 'static>;
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#[pin_project::pin_project]
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struct CatchUnwind<F: UnwindSafe>(#[pin] F);
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impl<F: Future + UnwindSafe> Future for CatchUnwind<F> {
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type Output = Result<F::Output, Panic>;
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fn poll(self: std::pin::Pin<&mut Self>, cx: &mut std::task::Context) -> Poll<Self::Output> {
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std::panic::catch_unwind(AssertUnwindSafe(|| self.project().0.poll(cx)))?.map(Ok)
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}
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}
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