ee15be8549
# 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>
83 lines
3.1 KiB
Rust
83 lines
3.1 KiB
Rust
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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