use crate::enter;
use futures_core::future::Future;
use futures_core::stream::Stream;
use futures_core::task::{Context, Poll};
use futures_task::{waker_ref, ArcWake};
use futures_task::{FutureObj, LocalFutureObj, LocalSpawn, Spawn, SpawnError};
use futures_util::pin_mut;
use futures_util::stream::FuturesUnordered;
use futures_util::stream::StreamExt;
use std::cell::RefCell;
use std::ops::{Deref, DerefMut};
use std::rc::{Rc, Weak};
use std::sync::{Arc, atomic::{AtomicBool, Ordering}};
use std::thread::{self, Thread};
/// A single-threaded task pool for polling futures to completion.
///
/// This executor allows you to multiplex any number of tasks onto a single
/// thread. It's appropriate to poll strictly I/O-bound futures that do very
/// little work in between I/O actions.
///
/// To get a handle to the pool that implements
/// [`Spawn`](futures_task::Spawn), use the
/// [`spawner()`](LocalPool::spawner) method. Because the executor is
/// single-threaded, it supports a special form of task spawning for non-`Send`
/// futures, via [`spawn_local_obj`](futures_task::LocalSpawn::spawn_local_obj).
#[derive(Debug)]
pub struct LocalPool {
pool: FuturesUnordered<LocalFutureObj<'static, ()>>,
incoming: Rc<Incoming>,
}
/// A handle to a [`LocalPool`](LocalPool) that implements
/// [`Spawn`](futures_task::Spawn).
#[derive(Clone, Debug)]
pub struct LocalSpawner {
incoming: Weak<Incoming>,
}
type Incoming = RefCell<Vec<LocalFutureObj<'static, ()>>>;
pub(crate) struct ThreadNotify {
/// The (single) executor thread.
thread: Thread,
/// A flag to ensure a wakeup (i.e. `unpark()`) is not "forgotten"
/// before the next `park()`, which may otherwise happen if the code
/// being executed as part of the future(s) being polled makes use of
/// park / unpark calls of its own, i.e. we cannot assume that no other
/// code uses park / unpark on the executing `thread`.
unparked: AtomicBool,
}
thread_local! {
static CURRENT_THREAD_NOTIFY: Arc<ThreadNotify> = Arc::new(ThreadNotify {
thread: thread::current(),
unparked: AtomicBool::new(false),
});
}
impl ArcWake for ThreadNotify {
fn wake_by_ref(arc_self: &Arc<Self>) {
// Make sure the wakeup is remembered until the next `park()`.
let unparked = arc_self.unparked.swap(true, Ordering::Relaxed);
if !unparked {
// If the thread has not been unparked yet, it must be done
// now. If it was actually parked, it will run again,
// otherwise the token made available by `unpark`
// may be consumed before reaching `park()`, but `unparked`
// ensures it is not forgotten.
arc_self.thread.unpark();
}
}
}
// Set up and run a basic single-threaded spawner loop, invoking `f` on each
// turn.
fn run_executor<T, F: FnMut(&mut Context<'_>) -> Poll<T>>(mut f: F) -> T {
let _enter = enter().expect(
"cannot execute `LocalPool` executor from within \
another executor",
);
CURRENT_THREAD_NOTIFY.with(|thread_notify| {
let waker = waker_ref(thread_notify);
let mut cx = Context::from_waker(&waker);
loop {
if let Poll::Ready(t) = f(&mut cx) {
return t;
}
// Consume the wakeup that occurred while executing `f`, if any.
let unparked = thread_notify.unparked.swap(false, Ordering::Acquire);
if !unparked {
// No wakeup occurred. It may occur now, right before parking,
// but in that case the token made available by `unpark()`
// is guaranteed to still be available and `park()` is a no-op.
thread::park();
// When the thread is unparked, `unparked` will have been set
// and needs to be unset before the next call to `f` to avoid
// a redundant loop iteration.
thread_notify.unparked.store(false, Ordering::Release);
}
}
})
}
fn poll_executor<T, F: FnMut(&mut Context<'_>) -> T>(mut f: F) -> T {
let _enter = enter().expect(
"cannot execute `LocalPool` executor from within \
another executor",
);
CURRENT_THREAD_NOTIFY.with(|thread_notify| {
let waker = waker_ref(thread_notify);
let mut cx = Context::from_waker(&waker);
f(&mut cx)
})
}
impl LocalPool {
/// Create a new, empty pool of tasks.
pub fn new() -> LocalPool {
LocalPool {
pool: FuturesUnordered::new(),
incoming: Default::default(),
}
}
/// Get a clonable handle to the pool as a [`Spawn`].
pub fn spawner(&self) -> LocalSpawner {
LocalSpawner {
incoming: Rc::downgrade(&self.incoming),
}
}
/// Run all tasks in the pool to completion.
///
/// ```
/// use futures::executor::LocalPool;
///
/// let mut pool = LocalPool::new();
///
/// // ... spawn some initial tasks using `spawn.spawn()` or `spawn.spawn_local()`
///
/// // run *all* tasks in the pool to completion, including any newly-spawned ones.
/// pool.run();
/// ```
///
/// The function will block the calling thread until *all* tasks in the pool
/// are complete, including any spawned while running existing tasks.
pub fn run(&mut self) {
run_executor(|cx| self.poll_pool(cx))
}
/// Runs all the tasks in the pool until the given future completes.
///
/// ```
/// use futures::executor::LocalPool;
///
/// let mut pool = LocalPool::new();
/// # let my_app = async {};
///
/// // run tasks in the pool until `my_app` completes
/// pool.run_until(my_app);
/// ```
///
/// The function will block the calling thread *only* until the future `f`
/// completes; there may still be incomplete tasks in the pool, which will
/// be inert after the call completes, but can continue with further use of
/// one of the pool's run or poll methods. While the function is running,
/// however, all tasks in the pool will try to make progress.
pub fn run_until<F: Future>(&mut self, future: F) -> F::Output {
pin_mut!(future);
run_executor(|cx| {
{
// if our main task is done, so are we
let result = future.as_mut().poll(cx);
if let Poll::Ready(output) = result {
return Poll::Ready(output);
}
}
let _ = self.poll_pool(cx);
Poll::Pending
})
}
/// Runs all tasks and returns after completing one future or until no more progress
/// can be made. Returns `true` if one future was completed, `false` otherwise.
///
/// ```
/// use futures::executor::LocalPool;
/// use futures::task::LocalSpawnExt;
/// use futures::future::{ready, pending};
///
/// let mut pool = LocalPool::new();
/// let spawner = pool.spawner();
///
/// spawner.spawn_local(ready(())).unwrap();
/// spawner.spawn_local(ready(())).unwrap();
/// spawner.spawn_local(pending()).unwrap();
///
/// // Run the two ready tasks and return true for them.
/// pool.try_run_one(); // returns true after completing one of the ready futures
/// pool.try_run_one(); // returns true after completing the other ready future
///
/// // the remaining task can not be completed
/// assert!(!pool.try_run_one()); // returns false
/// ```
///
/// This function will not block the calling thread and will return the moment
/// that there are no tasks left for which progress can be made or after exactly one
/// task was completed; Remaining incomplete tasks in the pool can continue with
/// further use of one of the pool's run or poll methods.
/// Though only one task will be completed, progress may be made on multiple tasks.
pub fn try_run_one(&mut self) -> bool {
poll_executor(|ctx| {
loop {
let ret = self.poll_pool_once(ctx);
// return if we have executed a future
if let Poll::Ready(Some(_)) = ret {
return true;
}
// if there are no new incoming futures
// then there is no feature that can make progress
// and we can return without having completed a single future
if self.incoming.borrow().is_empty() {
return false;
}
}
})
}
/// Runs all tasks in the pool and returns if no more progress can be made
/// on any task.
///
/// ```
/// use futures::executor::LocalPool;
/// use futures::task::LocalSpawnExt;
/// use futures::future::{ready, pending};
///
/// let mut pool = LocalPool::new();
/// let spawner = pool.spawner();
///
/// spawner.spawn_local(ready(())).unwrap();
/// spawner.spawn_local(ready(())).unwrap();
/// spawner.spawn_local(pending()).unwrap();
///
/// // Runs the two ready task and returns.
/// // The empty task remains in the pool.
/// pool.run_until_stalled();
/// ```
///
/// This function will not block the calling thread and will return the moment
/// that there are no tasks left for which progress can be made;
/// remaining incomplete tasks in the pool can continue with further use of one
/// of the pool's run or poll methods. While the function is running, all tasks
/// in the pool will try to make progress.
pub fn run_until_stalled(&mut self) {
poll_executor(|ctx| {
let _ = self.poll_pool(ctx);
});
}
// Make maximal progress on the entire pool of spawned task, returning `Ready`
// if the pool is empty and `Pending` if no further progress can be made.
fn poll_pool(&mut self, cx: &mut Context<'_>) -> Poll<()> {
// state for the FuturesUnordered, which will never be used
loop {
let ret = self.poll_pool_once(cx);
// we queued up some new tasks; add them and poll again
if !self.incoming.borrow().is_empty() {
continue;
}
// no queued tasks; we may be done
match ret {
Poll::Pending => return Poll::Pending,
Poll::Ready(None) => return Poll::Ready(()),
_ => {}
}
}
}
// Try make minimal progress on the pool of spawned tasks
fn poll_pool_once(&mut self, cx: &mut Context<'_>) -> Poll<Option<()>> {
// empty the incoming queue of newly-spawned tasks
{
let mut incoming = self.incoming.borrow_mut();
for task in incoming.drain(..) {
self.pool.push(task)
}
}
// try to execute the next ready future
self.pool.poll_next_unpin(cx)
}
}
impl Default for LocalPool {
fn default() -> Self {
Self::new()
}
}
/// Run a future to completion on the current thread.
///
/// This function will block the caller until the given future has completed.
///
/// Use a [`LocalPool`](LocalPool) if you need finer-grained control over
/// spawned tasks.
pub fn block_on<F: Future>(f: F) -> F::Output {
pin_mut!(f);
run_executor(|cx| f.as_mut().poll(cx))
}
/// Turn a stream into a blocking iterator.
///
/// When `next` is called on the resulting `BlockingStream`, the caller
/// will be blocked until the next element of the `Stream` becomes available.
pub fn block_on_stream<S: Stream + Unpin>(stream: S) -> BlockingStream<S> {
BlockingStream { stream }
}
/// An iterator which blocks on values from a stream until they become available.
#[derive(Debug)]
pub struct BlockingStream<S: Stream + Unpin> {
stream: S,
}
impl<S: Stream + Unpin> Deref for BlockingStream<S> {
type Target = S;
fn deref(&self) -> &Self::Target {
&self.stream
}
}
impl<S: Stream + Unpin> DerefMut for BlockingStream<S> {
fn deref_mut(&mut self) -> &mut Self::Target {
&mut self.stream
}
}
impl<S: Stream + Unpin> BlockingStream<S> {
/// Convert this `BlockingStream` into the inner `Stream` type.
pub fn into_inner(self) -> S {
self.stream
}
}
impl<S: Stream + Unpin> Iterator for BlockingStream<S> {
type Item = S::Item;
fn next(&mut self) -> Option<Self::Item> {
LocalPool::new().run_until(self.stream.next())
}
fn size_hint(&self) -> (usize, Option<usize>) {
self.stream.size_hint()
}
}
impl Spawn for LocalSpawner {
fn spawn_obj(&self, future: FutureObj<'static, ()>) -> Result<(), SpawnError> {
if let Some(incoming) = self.incoming.upgrade() {
incoming.borrow_mut().push(future.into());
Ok(())
} else {
Err(SpawnError::shutdown())
}
}
fn status(&self) -> Result<(), SpawnError> {
if self.incoming.upgrade().is_some() {
Ok(())
} else {
Err(SpawnError::shutdown())
}
}
}
impl LocalSpawn for LocalSpawner {
fn spawn_local_obj(&self, future: LocalFutureObj<'static, ()>) -> Result<(), SpawnError> {
if let Some(incoming) = self.incoming.upgrade() {
incoming.borrow_mut().push(future);
Ok(())
} else {
Err(SpawnError::shutdown())
}
}
fn status_local(&self) -> Result<(), SpawnError> {
if self.incoming.upgrade().is_some() {
Ok(())
} else {
Err(SpawnError::shutdown())
}
}
}