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// Copyright 2020 The Chromium OS Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
use std::cell::Cell;
use std::collections::VecDeque;
use std::future::Future;
use std::pin::Pin;
use std::rc::Rc;
use std::task::Waker;
use std::task::{Context, Poll};
use crate::waker::create_waker;
/// Represents a future executor that can be run. Implementers of the trait will take a list of
/// futures and poll them until completed.
pub trait Executor {
/// The type returned by the executor. This is normally `()` or a combination of the output the
/// futures produce.
type Output;
/// Run the executor, this will return once the exit criteria is met. The exit criteria is
/// specified when the executor is created, for example running until all futures are complete.
fn run(&mut self) -> Self::Output;
}
// Tracks if a future needs to be polled and the waker to use.
pub(crate) struct FutureState {
pub needs_poll: Rc<Cell<bool>>,
pub waker: Waker,
}
impl FutureState {
pub fn new() -> FutureState {
let needs_poll = Rc::new(Cell::new(true));
// Safe because a valid pointer is passed to `create_waker` and the valid result is
// passed to `Waker::from_raw`. And because the reference count to needs_poll is
// incremented by cloning it so it can't be dropped before the waker.
let waker = unsafe {
let clone = needs_poll.clone();
let raw_waker = create_waker(Rc::into_raw(clone) as *const _);
Waker::from_raw(raw_waker)
};
FutureState { needs_poll, waker }
}
}
// Couples a future owned by the executor with a flag that indicates the future is ready to be
// polled. Futures will start with the flag set. After blocking by returning `Poll::Pending`, the
// flag will be false until the waker is triggered and sets the flag to true, signalling the
// executor to poll the future again.
pub(crate) struct ExecutableFuture<T> {
future: Pin<Box<dyn Future<Output = T>>>,
state: FutureState,
}
impl<T> ExecutableFuture<T> {
// Creates an `ExecutableFuture` from the future. The returned struct is used to track when the
// future should be polled again.
pub fn new(future: Pin<Box<dyn Future<Output = T>>>) -> ExecutableFuture<T> {
ExecutableFuture {
future,
state: FutureState::new(),
}
}
// Polls the future if needed and returns the result.
// Covers setting up the waker and context before calling the future.
fn poll(&mut self) -> Poll<T> {
let mut ctx = Context::from_waker(&self.state.waker);
let f = self.future.as_mut();
f.poll(&mut ctx)
}
}
// Private trait used to allow one executor to behave differently. Using FutureList allows the
// executor code to be common across different collections of crates and different termination
// behavior. For example, one list can decide to exit after the first trait completes, others can
// wait until all are complete.
pub(crate) trait FutureList {
type Output;
// Return a mutable reference to the list of futures that can be added or removed from this
// List.
fn futures_mut(&mut self) -> &mut UnitFutures;
// Polls all futures that are ready. Returns the results if this list has completed.
fn poll_results(&mut self) -> Option<Self::Output>;
// Returns true if any future in the list is ready to be polled.
fn any_ready(&self) -> bool;
}
// `UnitFutures` is the simplest implementor of `FutureList`. It runs all futures added to it until
// there are none left to poll. The futures must all return `()`.
pub(crate) struct UnitFutures {
futures: VecDeque<ExecutableFuture<()>>,
}
impl UnitFutures {
// Creates a new, empty list of futures.
pub fn new() -> UnitFutures {
UnitFutures {
futures: VecDeque::new(),
}
}
// Adds a future to the list of futures to be polled.
pub fn append(&mut self, futures: &mut VecDeque<ExecutableFuture<()>>) {
self.futures.append(futures);
}
// Polls all futures that are ready to be polled. Removes any futures that indicate they are
// completed.
pub fn poll_all(&mut self) {
let mut i = 0;
while i < self.futures.len() {
let fut = &mut self.futures[i];
let remove = if fut.state.needs_poll.replace(false) {
fut.poll().is_ready()
} else {
false
};
if remove {
self.futures.remove(i);
} else {
i += 1;
}
}
}
}
impl FutureList for UnitFutures {
type Output = ();
fn futures_mut(&mut self) -> &mut UnitFutures {
self
}
fn poll_results(&mut self) -> Option<Self::Output> {
self.poll_all();
if self.futures.is_empty() {
Some(())
} else {
None
}
}
fn any_ready(&self) -> bool {
self.futures.iter().any(|fut| fut.state.needs_poll.get())
}
}
#[cfg(test)]
mod tests {
use super::*;
use std::sync::atomic::{AtomicUsize, Ordering};
#[test]
fn basic_run() {
async fn f(called: Rc<AtomicUsize>) {
called.fetch_add(1, Ordering::Relaxed);
}
let f1_called = Rc::new(AtomicUsize::new(0));
let f2_called = Rc::new(AtomicUsize::new(0));
let fut1 = Box::pin(f(f1_called.clone()));
let fut2 = Box::pin(f(f2_called.clone()));
let mut futures = VecDeque::new();
futures.push_back(ExecutableFuture::new(fut1));
futures.push_back(ExecutableFuture::new(fut2));
let mut uf = UnitFutures::new();
uf.append(&mut futures);
assert!(uf.poll_results().is_some());
assert_eq!(f1_called.load(Ordering::Relaxed), 1);
assert_eq!(f2_called.load(Ordering::Relaxed), 1);
}
}
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