local_pool_with_id/

lib.rs

//! # local-pool-with-id
//! A minor variation on a [LocalPool](https://docs.rs/futures/0.3/futures/executor/struct.LocalPool.html) executor which exposes unique IDs for tracking future completion.
//!
//! This should almost be a drop in replacement for the existing LocalPool. All existing traits are still implemented. There are two API differences:
//! 1. New `(Local)SpawnWithId` traits have been implemented. These accept the same arguments as their non-ID counterparts but return a unique ID that can be used to identify whether a spawned future has been completed.
//! 2. `try_run_one` now returns an `Option<usize>` instead of a boolean. This usize will correspond to the ID received from the previous APIs and can be used with external tracking mechanism to know if a future is complete.
//!
//! ## Motivation
//! The existing `LocalPool` allowed you to run all futures, opaquely, in a non-blocking way or to, blockingly, run a single future to completion and retrieve it's output. By providing tracking IDs, we can use an external lookup to infer which futures are finished and ask them for their results directly.
//!
//! ## Example
//! ```rust
//! use local_pool_with_id::SpawnWithIdExt;
//! use futures::prelude::*;
//!
//! let mut spawned_ids = std::collections::HashSet::new();
//! let mut pool = local_pool_with_id::LocalPool::new();
//! let spawner = pool.spawner();
//!
//! let (id1, handle1) = spawner
//!     .spawn_with_handle(futures::future::ready(1i32))
//!     .unwrap();
//! let (id2, handle2) = spawner
//!     .spawn_with_handle(futures::future::ready(2u32))
//!     .unwrap();
//!
//! spawned_ids.insert(id1);
//! spawned_ids.insert(id2);
//!
//! while !spawned_ids.is_empty() {
//!     if let Some(completed) = pool.try_run_one() {
//!         assert!(spawned_ids.remove(&completed))
//!     }
//! }
//!
//! assert_eq!(handle1.now_or_never().unwrap(), 1);
//! assert_eq!(handle2.now_or_never().unwrap(), 2);
//! ```

use futures::executor::enter;
use futures::future::{FutureObj, LocalFutureObj, RemoteHandle};
use futures::prelude::*;
use futures::stream::FuturesUnordered;
use futures::task::{waker_ref, ArcWake, LocalSpawn, Spawn, SpawnError};
use std::cell::RefCell;
use std::pin::Pin;
use std::rc::{Rc, Weak};
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;
use std::task::{Context, Poll};
use std::thread;
use std::thread::Thread;

#[must_use = "futures do nothing unless you `.await` or poll them"]
#[derive(Debug)]
struct IndexWrapper<T> {
    data: T, // A future or a future's output
    index: usize,
}

impl<T> IndexWrapper<T> {
    pin_utils::unsafe_pinned!(data: T);
}

impl<T> Future for IndexWrapper<T>
where
    T: Future,
{
    type Output = IndexWrapper<T::Output>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        self.as_mut()
            .data()
            .as_mut()
            .poll(cx)
            .map(|output| IndexWrapper {
                data: output,
                index: self.index,
            })
    }
}

/// 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<IndexWrapper<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>,
}

#[derive(Debug, Default)]
struct IncomingTracking {
    queue: Vec<(usize, LocalFutureObj<'static, ()>)>,
    index: usize,
}

type Incoming = RefCell<IncomingTracking>;

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 local_pool_with_id::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 local_pool_with_id::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_utils::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 the associated ID if one future was completed, `None` otherwise.
    ///
    /// ```
    /// use local_pool_with_id::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 returns the IDs for them.
    /// assert!(pool.try_run_one().is_some());
    /// assert!(pool.try_run_one().is_some());
    ///
    /// // the remaining task can not be completed
    /// assert!(pool.try_run_one().is_none()); // 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) -> Option<usize> {
        poll_executor(|ctx| {
            loop {
                let ret = self.poll_pool_once(ctx);

                // return if we have executed a future
                if let Poll::Ready(Some(key)) = ret {
                    return Some(key);
                }

                // 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().queue.is_empty() {
                    return None;
                }
            }
        })
    }

    /// Runs all tasks in the pool and returns if no more progress can be made
    /// on any task.
    ///
    /// ```
    /// use local_pool_with_id::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().queue.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<usize>> {
        // empty the incoming queue of newly-spawned tasks
        {
            let mut incoming = self.incoming.borrow_mut();
            for (key, task) in incoming.queue.drain(..) {
                self.pool.push(IndexWrapper {
                    data: task,
                    index: key,
                })
            }
        }

        // try to execute the next ready future
        self.pool
            .poll_next_unpin(cx)
            .map(|poll| poll.map(|wrapper| wrapper.index))
    }
}

impl Default for LocalPool {
    fn default() -> Self {
        Self::new()
    }
}

impl Spawn for LocalSpawner {
    fn spawn_obj(&self, future: FutureObj<'static, ()>) -> Result<(), SpawnError> {
        self.spawn_obj_with_id(future).map(|_| ())
    }

    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> {
        self.spawn_local_obj_with_id(future).map(|_| ())
    }

    fn status_local(&self) -> Result<(), SpawnError> {
        if self.incoming.upgrade().is_some() {
            Ok(())
        } else {
            Err(SpawnError::shutdown())
        }
    }
}

impl SpawnWithId for LocalSpawner {
    fn spawn_obj_with_id(&self, future: FutureObj<'static, ()>) -> Result<usize, SpawnError> {
        if let Some(incoming) = self.incoming.upgrade() {
            let mut incoming = incoming.borrow_mut();
            let id = incoming.index;
            incoming.index += 1;
            incoming.queue.push((id, future.into()));
            Ok(id)
        } else {
            Err(SpawnError::shutdown())
        }
    }
}

impl LocalSpawnWithId for LocalSpawner {
    fn spawn_local_obj_with_id(
        &self,
        future: LocalFutureObj<'static, ()>,
    ) -> Result<usize, SpawnError> {
        if let Some(incoming) = self.incoming.upgrade() {
            let mut incoming = incoming.borrow_mut();
            let id = incoming.index;
            incoming.index += 1;
            incoming.queue.push((id, future));
            Ok(id)
        } else {
            Err(SpawnError::shutdown())
        }
    }
}

pub trait SpawnWithId {
    fn spawn_obj_with_id(&self, future: FutureObj<'static, ()>) -> Result<usize, SpawnError>;
}

pub trait LocalSpawnWithId {
    fn spawn_local_obj_with_id(
        &self,
        future: LocalFutureObj<'static, ()>,
    ) -> Result<usize, SpawnError>;
}

impl<Sp: ?Sized> SpawnWithIdExt for Sp where Sp: SpawnWithId {}
impl<Sp: ?Sized> LocalSpawnWithIdExt for Sp where Sp: LocalSpawnWithId {}

/// Extension trait for `Spawn`.
pub trait SpawnWithIdExt: SpawnWithId {
    /// Spawns a task that polls the given future with output `()` to
    /// completion.
    ///
    /// This method returns a [`Result`] that contains a [`SpawnError`] if
    /// spawning fails.
    ///
    /// You can use [`spawn_with_handle`](SpawnWithIdExt::spawn_with_handle) if
    /// you want to spawn a future with output other than `()` or if you want
    /// to be able to await its completion.
    ///
    /// Note this method will eventually be replaced with the upcoming
    /// `Spawn::spawn` method which will take a `dyn Future` as input.
    /// Technical limitations prevent `Spawn::spawn` from being implemented
    /// today. Feel free to use this method in the meantime.
    ///
    /// ```
    /// use local_pool_with_id::LocalPool;
    /// use local_pool_with_id::SpawnWithIdExt;
    ///
    /// let executor = LocalPool::new();
    /// let spawner = executor.spawner();
    ///
    /// let future = async { /* ... */ };
    /// spawner.spawn(future).unwrap();
    /// ```
    fn spawn<Fut>(&self, future: Fut) -> Result<usize, SpawnError>
    where
        Fut: Future<Output = ()> + Send + 'static,
    {
        self.spawn_obj_with_id(FutureObj::new(Box::new(future)))
    }

    /// Spawns a task that polls the given future to completion and returns a
    /// future that resolves to the spawned future's output.
    ///
    /// This method returns a [`Result`] that contains a [`RemoteHandle`](futures::future::RemoteHandle), or, if
    /// spawning fails, a [`SpawnError`]. [`RemoteHandle`](futures::future::RemoteHandle) is a future that
    /// resolves to the output of the spawned future.
    ///
    /// ```
    /// use futures::executor::block_on;
    /// use futures::future;
    /// use local_pool_with_id::LocalPool;
    /// use local_pool_with_id::SpawnWithIdExt;
    ///
    /// let mut executor = LocalPool::new();
    /// let spawner = executor.spawner();
    ///
    /// let future = future::ready(1);
    /// let (id, join_handle_fut) = spawner.spawn_with_handle(future).unwrap();
    /// assert_eq!(executor.run_until(join_handle_fut), 1);
    /// ```
    fn spawn_with_handle<Fut>(
        &self,
        future: Fut,
    ) -> Result<(usize, RemoteHandle<Fut::Output>), SpawnError>
    where
        Fut: Future + Send + 'static,
        Fut::Output: Send,
    {
        let (future, handle) = future.remote_handle();
        let id = self.spawn(future)?;
        Ok((id, handle))
    }
}

/// Extension trait for `LocalSpawn`.
pub trait LocalSpawnWithIdExt: LocalSpawnWithId {
    /// Spawns a task that polls the given future with output `()` to
    /// completion.
    ///
    /// This method returns a [`Result`] that contains a [`SpawnError`] if
    /// spawning fails.
    ///
    /// You can use [`spawn_with_handle`](SpawnWithIdExt::spawn_with_handle) if
    /// you want to spawn a future with output other than `()` or if you want
    /// to be able to await its completion.
    ///
    /// Note this method will eventually be replaced with the upcoming
    /// `Spawn::spawn` method which will take a `dyn Future` as input.
    /// Technical limitations prevent `Spawn::spawn` from being implemented
    /// today. Feel free to use this method in the meantime.
    ///
    /// ```
    /// use local_pool_with_id::LocalPool;
    /// use local_pool_with_id::LocalSpawnWithIdExt;
    ///
    /// let executor = LocalPool::new();
    /// let spawner = executor.spawner();
    ///
    /// let future = async { /* ... */ };
    /// spawner.spawn_local(future).unwrap();
    /// ```
    fn spawn_local<Fut>(&self, future: Fut) -> Result<usize, SpawnError>
    where
        Fut: Future<Output = ()> + 'static,
    {
        self.spawn_local_obj_with_id(LocalFutureObj::new(Box::new(future)))
    }

    /// Spawns a task that polls the given future to completion and returns a
    /// future that resolves to the spawned future's output.
    ///
    /// This method returns a [`Result`] that contains a [`RemoteHandle`](futures::future::RemoteHandle), or, if
    /// spawning fails, a [`SpawnError`]. [`RemoteHandle`](futures::future::RemoteHandle) is a future that
    /// resolves to the output of the spawned future.
    ///
    /// ```
    /// use local_pool_with_id::LocalPool;
    /// use local_pool_with_id::LocalSpawnWithIdExt;
    ///
    /// let mut executor = LocalPool::new();
    /// let spawner = executor.spawner();
    ///
    /// let future = async { 1 };
    /// let (id, join_handle_fut) = spawner.spawn_local_with_handle(future).unwrap();
    /// assert_eq!(executor.run_until(join_handle_fut), 1);
    /// ```
    fn spawn_local_with_handle<Fut>(
        &self,
        future: Fut,
    ) -> Result<(usize, RemoteHandle<Fut::Output>), SpawnError>
    where
        Fut: Future + 'static,
    {
        let (future, handle) = future.remote_handle();
        let id = self.spawn_local(future)?;
        Ok((id, handle))
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn tracking() {
        let mut spawned_ids = std::collections::HashSet::new();
        let mut pool = LocalPool::new();
        let spawner = pool.spawner();

        let (id1, handle1) = spawner
            .spawn_with_handle(futures::future::ready(1i32))
            .unwrap();
        let (id2, handle2) = spawner
            .spawn_with_handle(futures::future::ready(2u32))
            .unwrap();

        spawned_ids.insert(id1);
        spawned_ids.insert(id2);

        while !spawned_ids.is_empty() {
            if let Some(completed) = pool.try_run_one() {
                assert!(spawned_ids.remove(&completed))
            }
        }

        assert_eq!(handle1.now_or_never().unwrap(), 1);
        assert_eq!(handle2.now_or_never().unwrap(), 2);
    }
}