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//! A single-threaded executor where join handles propagate panics from tasks.
use std::future::Future;
use std::panic::{resume_unwind, AssertUnwindSafe};
use std::pin::Pin;
use std::task::{Context, Poll};
use std::thread;
use crossbeam::channel::{unbounded, Sender};
use futures::executor;
use futures::future::FutureExt;
use lazy_static::lazy_static;
type Task = async_task::Task<()>;
/// Spawns a future on the executor.
fn spawn<F, R>(future: F) -> JoinHandle<R>
where
F: Future<Output = R> + Send + 'static,
R: Send + 'static,
{
lazy_static! {
// A channel that holds scheduled tasks.
static ref QUEUE: Sender<Task> = {
let (sender, receiver) = unbounded::<Task>();
// Start the executor thread.
thread::spawn(|| {
for task in receiver {
// No need for `catch_unwind()` here because panics are already caught.
task.run();
}
});
sender
};
}
// Create a future that catches panics within itself.
let future = AssertUnwindSafe(future).catch_unwind();
// Create a task that is scheduled by sending itself into the channel.
let schedule = |t| QUEUE.send(t).unwrap();
let (task, handle) = async_task::spawn(future, schedule, ());
// Schedule the task by sending it into the channel.
task.schedule();
// Wrap the handle into one that propagates panics.
JoinHandle(handle)
}
/// A join handle that propagates panics inside the task.
struct JoinHandle<R>(async_task::JoinHandle<thread::Result<R>, ()>);
impl<R> Future for JoinHandle<R> {
type Output = Option<R>;
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
match Pin::new(&mut self.0).poll(cx) {
Poll::Pending => Poll::Pending,
Poll::Ready(None) => Poll::Ready(None),
Poll::Ready(Some(Ok(val))) => Poll::Ready(Some(val)),
Poll::Ready(Some(Err(err))) => resume_unwind(err),
}
}
}
fn main() {
// Spawn a future that panics and block on it.
let handle = spawn(async {
panic!("Ooops!");
});
executor::block_on(handle);
}
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