futures-executor/src/local_pool.rs

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::{
    atomic::{AtomicBool, Ordering},
    Arc,
};
use std::thread::{self, Thread};
use std::vec::Vec;

/// 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`] 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,
}

std::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::Release);
        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;
            }

            // Wait for a wakeup.
            while !thread_notify.unparked.swap(false, Ordering::Acquire) {
                // 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();
            }
        }
    })
}

/// Check for a wakeup, but don't consume it.
fn woken() -> bool {
    CURRENT_THREAD_NOTIFY.with(|thread_notify| thread_notify.unparked.load(Ordering::Acquire))
}

impl LocalPool {
    /// Create a new, empty pool of tasks.
    pub fn new() -> Self {
        Self { 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 {
        run_executor(|cx| {
            loop {
                self.drain_incoming();

                match self.pool.poll_next_unpin(cx) {
                    // Success!
                    Poll::Ready(Some(())) => return Poll::Ready(true),
                    // The pool was empty.
                    Poll::Ready(None) => return Poll::Ready(false),
                    Poll::Pending => (),
                }

                if !self.incoming.borrow().is_empty() {
                    // New tasks were spawned; try again.
                    continue;
                } else if woken() {
                    // The pool yielded to us, but there's more progress to be made.
                    return Poll::Pending;
                } else {
                    return Poll::Ready(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) {
        run_executor(|cx| match self.poll_pool(cx) {
            // The pool is empty.
            Poll::Ready(()) => Poll::Ready(()),
            Poll::Pending => {
                if woken() {
                    Poll::Pending
                } else {
                    // We're stalled for now.
                    Poll::Ready(())
                }
            }
        });
    }

    /// Poll `self.pool`, re-filling it with any newly-spawned tasks.
    /// Repeat until either the pool is empty, or it returns `Pending`.
    ///
    /// Returns `Ready` if the pool was empty, and `Pending` otherwise.
    ///
    /// NOTE: the pool may call `wake`, so `Pending` doesn't necessarily
    /// mean that the pool can't make progress.
    fn poll_pool(&mut self, cx: &mut Context<'_>) -> Poll<()> {
        loop {
            self.drain_incoming();

            let pool_ret = self.pool.poll_next_unpin(cx);

            // We queued up some new tasks; add them and poll again.
            if !self.incoming.borrow().is_empty() {
                continue;
            }

            match pool_ret {
                Poll::Ready(Some(())) => continue,
                Poll::Ready(None) => return Poll::Ready(()),
                Poll::Pending => return Poll::Pending,
            }
        }
    }

    /// Empty the incoming queue of newly-spawned tasks.
    fn drain_incoming(&mut self) {
        let mut incoming = self.incoming.borrow_mut();
        for task in incoming.drain(..) {
            self.pool.push(task)
        }
    }
}

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`] 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())
        }
    }
}