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// Copyright Materialize, Inc. and contributors. All rights reserved.
//
// Use of this software is governed by the Business Source License
// included in the LICENSE file.
//
// As of the Change Date specified in that file, in accordance with
// the Business Source License, use of this software will be governed
// by the Apache License, Version 2.0.
//! Extensions for `OperatorBuilder` to create async operators.
use std::cell::{Cell, RefCell};
use std::future::Future;
use std::pin::Pin;
use std::rc::Rc;
use std::sync::Arc;
use std::task::{Context, Poll};
use futures_util::future::FusedFuture;
use futures_util::FutureExt;
use timely::dataflow::operators::generic::builder_rc::OperatorBuilder;
use timely::dataflow::operators::Capability;
use timely::dataflow::Scope;
use timely::progress::{Antichain, Timestamp};
use timely::PartialOrder;
/// A type that is not inhabited by any value. Should be redefined as the
/// [never](https://doc.rust-lang.org/std/primitive.never.html) type once it stabilizes
pub enum Never {}
/// A helper type to integrate timely notifications with async futures. Its intended to be used
/// with a top level while loop. Timely will automatically make the futures returned by
/// `scheduler.notified.await()` resolve when there are progress or data updates to be processed by
/// the operator
///
/// ```ignore
/// async fn example(s: Scheduler) {
/// while scheduler.notified().await {
/// }
/// }
/// ```
#[derive(Debug, Default, Clone)]
pub struct Scheduler {
inner: Rc<Cell<bool>>,
}
impl Scheduler {
/// Notifies a waiting task, returning a boolean indicating whether or not there was a pending
/// notification already stored in the scheduler.
///
/// If a task is currently waiting, that task is notified. Otherwise, a permit is stored in
/// this `Scheduler` and the next call to `notified().await` will complete immediately
/// consuming the permit made available by this call to `notify()`.
fn notify(&self) -> bool {
self.inner.replace(true)
}
/// Returns a boolean indicating whether a notification is pending
fn is_notified(&self) -> bool {
self.inner.get()
}
/// Wait for a notification.
#[allow(dead_code)]
pub fn notified(&self) -> Notified<'_> {
Notified {
notified: &*self.inner,
}
}
}
/// The future returned by [Scheduler::notified]
#[derive(Debug)]
pub struct Notified<'a> {
notified: &'a Cell<bool>,
}
impl Future for Notified<'_> {
type Output = bool;
fn poll(self: Pin<&mut Self>, _cx: &mut Context) -> Poll<Self::Output> {
if self.notified.replace(false) {
Poll::Ready(true)
} else {
Poll::Pending
}
}
}
/// Extension trait for `OperatorBuilder`.
pub trait OperatorBuilderExt<G: Scope> {
/// Creates an operator implementation from supplied async logic constructor.
///
/// The logic constructror is expected to return a future that will never return and that
/// follows the following pattern:
///
/// ```ignore
/// op.build_async(scope, move |capabilities, frontier, scheduler| async move {
/// while scheduler.yield_now().await {
/// // async operator logic here
/// }
/// });
/// ```
///
/// Since timely's input handles and frontier notifications are not integrated with the async
/// ecosystem the only way to yield control back to timely is by awaiting on
/// `scheduler.yield_now()`. The operator will ensure that this call resolves when there is
/// more work to do.
fn build_async<B, Fut>(self, scope: G, constructor: B)
where
B: FnOnce(
Vec<Capability<G::Timestamp>>,
Rc<RefCell<Vec<Antichain<G::Timestamp>>>>,
Scheduler,
) -> Fut,
Fut: Future + 'static;
}
impl<G: Scope> OperatorBuilderExt<G> for OperatorBuilder<G> {
fn build_async<B, Fut>(self, scope: G, constructor: B)
where
B: FnOnce(
Vec<Capability<G::Timestamp>>,
Rc<RefCell<Vec<Antichain<G::Timestamp>>>>,
Scheduler,
) -> Fut,
Fut: Future + 'static,
{
let activator = scope.sync_activator_for(&self.operator_info().address[..]);
let waker = futures_util::task::waker(Arc::new(activator));
let shared_frontiers = Rc::new(RefCell::new(vec![
Antichain::from_elem(Timestamp::minimum());
self.shape().inputs()
]));
self.build_reschedule(move |capabilities| {
let scheduler = Scheduler::default();
let mut logic_fut = Box::pin(
constructor(
capabilities,
Rc::clone(&shared_frontiers),
scheduler.clone(),
)
.fuse(),
);
move |frontiers| {
// Attempt to update the shared frontier before polling the future. This operation
// can fail if the future also borrowed the frontier and kept the borrow active
// across an await point. It is fine for this operation to fail because we will
// poll the future right afterwards and will come back to this point once the
// operator gets rescheduled. At that future moment we will be able to update the
// frontier and poll the future with fresh data in the handle.
if let Ok(mut shared_frontiers) = shared_frontiers.try_borrow_mut() {
for (shared, new) in shared_frontiers.iter_mut().zip(frontiers) {
if !PartialOrder::less_equal(&new.frontier(), &shared.borrow()) {
*shared = new.frontier().to_owned();
}
}
}
if logic_fut.is_terminated() {
return false;
}
let had_pending_notify = scheduler.notify();
let operator_incomplete = Pin::new(&mut logic_fut)
.poll(&mut Context::from_waker(&waker))
.is_pending();
// Here we check that:
// 1. the scheduler had been notified in some previous run of the closure
// 2. the future just went past a `scheduler.notified().await` point
//
// These can be true at the same time only if the logic future awaited on something
// other then `scheduler.notified()`. We care about this case because while the
// logic future is awaiting on some other future, more data might become available
// in the input handles or the frontier might have progressed. Therefore this check
// will ensure that `scheduler.notified()` will complete at least one more time
// which will give a chance to the logic future to check its input and frontier.
if had_pending_notify && !scheduler.is_notified() {
waker.wake_by_ref();
}
operator_incomplete
}
});
}
}
/// Convenience macro used to wrap what might otherwise be the argument to `build_reschedule`.
#[macro_export]
macro_rules! async_op {
(|$capabilities:ident, $frontiers:ident| $body:block) => {
move |mut capabilities, frontiers, scheduler| async move {
loop {
scheduler.notified().await;
// rebind to mutable references to make sure they can't be accidentally dropped
#[allow(unused_mut)]
let mut $capabilities = &mut capabilities;
let $frontiers = (*frontiers.borrow()).clone();
if !async { $body }.await && $frontiers.iter().all(|f| f.is_empty()) {
break;
}
}
}
};
}
#[cfg(test)]
mod test {
use super::*;
use std::time::Duration;
use timely::dataflow::channels::pact::Pipeline;
use timely::dataflow::operators::capture::Extract;
use timely::dataflow::operators::{Capture, ToStream};
#[tokio::test]
async fn async_operator() {
// Run timely in a separate thread
#[allow(clippy::disallowed_method)]
let extracted = tokio::task::spawn_blocking(|| {
let capture = timely::example(|scope| {
let input = (0..10).to_stream(scope);
let mut op = OperatorBuilder::new("async_passthru".to_string(), input.scope());
let mut input_handle = op.new_input(&input, Pipeline);
let (mut output, output_stream) = op.new_output();
op.build_async(
input.scope(),
async_op!(|capabilities, _frontiers| {
// Drop initial capabilities
capabilities.clear();
let mut output_handle = output.activate();
while let Some((cap, data)) = input_handle.next() {
let cap = cap.retain();
let mut session = output_handle.session(&cap);
for item in data.iter().copied() {
tokio::time::sleep(Duration::from_millis(10)).await;
session.give(item);
}
}
false
}),
);
output_stream.capture()
});
capture.extract()
})
.await
.expect("timely panicked");
assert_eq!(extracted, vec![(0, vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9])]);
}
}