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>
This commit is contained in:
Joseph 2024-07-15 18:15:03 -07:00 committed by GitHub
parent c3057d4353
commit ee15be8549
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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 }
[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" }

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@ -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"))]

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@ -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`].

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@ -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)
}
}