Import tokio-0.2.22

Test: None
Change-Id: Iea7ee5e62819c9b16dbfad05a6146775df72506a

4 files changed, 1197 insertions, 0 deletions

diff --git a/src/runtime/thread_pool/atomic_cell.rs b/src/runtime/thread_pool/atomic_cell.rs
new file mode 100644
index 0000000..2bda0fc
--- /dev/null
+++ b/src/runtime/thread_pool/atomic_cell.rs
@@ -0,0 +1,52 @@
+use crate::loom::sync::atomic::AtomicPtr;
+
+use std::ptr;
+use std::sync::atomic::Ordering::AcqRel;
+
+pub(super) struct AtomicCell<T> {
+    data: AtomicPtr<T>,
+}
+
+unsafe impl<T: Send> Send for AtomicCell<T> {}
+unsafe impl<T: Send> Sync for AtomicCell<T> {}
+
+impl<T> AtomicCell<T> {
+    pub(super) fn new(data: Option<Box<T>>) -> AtomicCell<T> {
+        AtomicCell {
+            data: AtomicPtr::new(to_raw(data)),
+        }
+    }
+
+    pub(super) fn swap(&self, val: Option<Box<T>>) -> Option<Box<T>> {
+        let old = self.data.swap(to_raw(val), AcqRel);
+        from_raw(old)
+    }
+
+    #[cfg(feature = "blocking")]
+    pub(super) fn set(&self, val: Box<T>) {
+        let _ = self.swap(Some(val));
+    }
+
+    pub(super) fn take(&self) -> Option<Box<T>> {
+        self.swap(None)
+    }
+}
+
+fn to_raw<T>(data: Option<Box<T>>) -> *mut T {
+    data.map(Box::into_raw).unwrap_or(ptr::null_mut())
+}
+
+fn from_raw<T>(val: *mut T) -> Option<Box<T>> {
+    if val.is_null() {
+        None
+    } else {
+        Some(unsafe { Box::from_raw(val) })
+    }
+}
+
+impl<T> Drop for AtomicCell<T> {
+    fn drop(&mut self) {
+        // Free any data still held by the cell
+        let _ = self.take();
+    }
+}
diff --git a/src/runtime/thread_pool/idle.rs b/src/runtime/thread_pool/idle.rs
new file mode 100644
index 0000000..ae87ca4
--- /dev/null
+++ b/src/runtime/thread_pool/idle.rs
@@ -0,0 +1,222 @@
+//! Coordinates idling workers
+
+use crate::loom::sync::atomic::AtomicUsize;
+use crate::loom::sync::Mutex;
+
+use std::fmt;
+use std::sync::atomic::Ordering::{self, SeqCst};
+
+pub(super) struct Idle {
+    /// Tracks both the number of searching workers and the number of unparked
+    /// workers.
+    ///
+    /// Used as a fast-path to avoid acquiring the lock when needed.
+    state: AtomicUsize,
+
+    /// Sleeping workers
+    sleepers: Mutex<Vec<usize>>,
+
+    /// Total number of workers.
+    num_workers: usize,
+}
+
+const UNPARK_SHIFT: usize = 16;
+const UNPARK_MASK: usize = !SEARCH_MASK;
+const SEARCH_MASK: usize = (1 << UNPARK_SHIFT) - 1;
+
+#[derive(Copy, Clone)]
+struct State(usize);
+
+impl Idle {
+    pub(super) fn new(num_workers: usize) -> Idle {
+        let init = State::new(num_workers);
+
+        Idle {
+            state: AtomicUsize::new(init.into()),
+            sleepers: Mutex::new(Vec::with_capacity(num_workers)),
+            num_workers,
+        }
+    }
+
+    /// If there are no workers actively searching, returns the index of a
+    /// worker currently sleeping.
+    pub(super) fn worker_to_notify(&self) -> Option<usize> {
+        // If at least one worker is spinning, work being notified will
+        // eventully be found. A searching thread will find **some** work and
+        // notify another worker, eventually leading to our work being found.
+        //
+        // For this to happen, this load must happen before the thread
+        // transitioning `num_searching` to zero. Acquire / Relese does not
+        // provide sufficient guarantees, so this load is done with `SeqCst` and
+        // will pair with the `fetch_sub(1)` when transitioning out of
+        // searching.
+        if !self.notify_should_wakeup() {
+            return None;
+        }
+
+        // Acquire the lock
+        let mut sleepers = self.sleepers.lock().unwrap();
+
+        // Check again, now that the lock is acquired
+        if !self.notify_should_wakeup() {
+            return None;
+        }
+
+        // A worker should be woken up, atomically increment the number of
+        // searching workers as well as the number of unparked workers.
+        State::unpark_one(&self.state);
+
+        // Get the worker to unpark
+        let ret = sleepers.pop();
+        debug_assert!(ret.is_some());
+
+        ret
+    }
+
+    /// Returns `true` if the worker needs to do a final check for submitted
+    /// work.
+    pub(super) fn transition_worker_to_parked(&self, worker: usize, is_searching: bool) -> bool {
+        // Acquire the lock
+        let mut sleepers = self.sleepers.lock().unwrap();
+
+        // Decrement the number of unparked threads
+        let ret = State::dec_num_unparked(&self.state, is_searching);
+
+        // Track the sleeping worker
+        sleepers.push(worker);
+
+        ret
+    }
+
+    pub(super) fn transition_worker_to_searching(&self) -> bool {
+        let state = State::load(&self.state, SeqCst);
+        if 2 * state.num_searching() >= self.num_workers {
+            return false;
+        }
+
+        // It is possible for this routine to allow more than 50% of the workers
+        // to search. That is OK. Limiting searchers is only an optimization to
+        // prevent too much contention.
+        State::inc_num_searching(&self.state, SeqCst);
+        true
+    }
+
+    /// A lightweight transition from searching -> running.
+    ///
+    /// Returns `true` if this is the final searching worker. The caller
+    /// **must** notify a new worker.
+    pub(super) fn transition_worker_from_searching(&self) -> bool {
+        State::dec_num_searching(&self.state)
+    }
+
+    /// Unpark a specific worker. This happens if tasks are submitted from
+    /// within the worker's park routine.
+    pub(super) fn unpark_worker_by_id(&self, worker_id: usize) {
+        let mut sleepers = self.sleepers.lock().unwrap();
+
+        for index in 0..sleepers.len() {
+            if sleepers[index] == worker_id {
+                sleepers.swap_remove(index);
+
+                // Update the state accordingly whle the lock is held.
+                State::unpark_one(&self.state);
+
+                return;
+            }
+        }
+    }
+
+    /// Returns `true` if `worker_id` is contained in the sleep set
+    pub(super) fn is_parked(&self, worker_id: usize) -> bool {
+        let sleepers = self.sleepers.lock().unwrap();
+        sleepers.contains(&worker_id)
+    }
+
+    fn notify_should_wakeup(&self) -> bool {
+        let state = State(self.state.fetch_add(0, SeqCst));
+        state.num_searching() == 0 && state.num_unparked() < self.num_workers
+    }
+}
+
+impl State {
+    fn new(num_workers: usize) -> State {
+        // All workers start in the unparked state
+        let ret = State(num_workers << UNPARK_SHIFT);
+        debug_assert_eq!(num_workers, ret.num_unparked());
+        debug_assert_eq!(0, ret.num_searching());
+        ret
+    }
+
+    fn load(cell: &AtomicUsize, ordering: Ordering) -> State {
+        State(cell.load(ordering))
+    }
+
+    fn unpark_one(cell: &AtomicUsize) {
+        cell.fetch_add(1 | (1 << UNPARK_SHIFT), SeqCst);
+    }
+
+    fn inc_num_searching(cell: &AtomicUsize, ordering: Ordering) {
+        cell.fetch_add(1, ordering);
+    }
+
+    /// Returns `true` if this is the final searching worker
+    fn dec_num_searching(cell: &AtomicUsize) -> bool {
+        let state = State(cell.fetch_sub(1, SeqCst));
+        state.num_searching() == 1
+    }
+
+    /// Track a sleeping worker
+    ///
+    /// Returns `true` if this is the final searching worker.
+    fn dec_num_unparked(cell: &AtomicUsize, is_searching: bool) -> bool {
+        let mut dec = 1 << UNPARK_SHIFT;
+
+        if is_searching {
+            dec += 1;
+        }
+
+        let prev = State(cell.fetch_sub(dec, SeqCst));
+        is_searching && prev.num_searching() == 1
+    }
+
+    /// Number of workers currently searching
+    fn num_searching(self) -> usize {
+        self.0 & SEARCH_MASK
+    }
+
+    /// Number of workers currently unparked
+    fn num_unparked(self) -> usize {
+        (self.0 & UNPARK_MASK) >> UNPARK_SHIFT
+    }
+}
+
+impl From<usize> for State {
+    fn from(src: usize) -> State {
+        State(src)
+    }
+}
+
+impl From<State> for usize {
+    fn from(src: State) -> usize {
+        src.0
+    }
+}
+
+impl fmt::Debug for State {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("worker::State")
+            .field("num_unparked", &self.num_unparked())
+            .field("num_searching", &self.num_searching())
+            .finish()
+    }
+}
+
+#[test]
+fn test_state() {
+    assert_eq!(0, UNPARK_MASK & SEARCH_MASK);
+    assert_eq!(0, !(UNPARK_MASK | SEARCH_MASK));
+
+    let state = State::new(10);
+    assert_eq!(10, state.num_unparked());
+    assert_eq!(0, state.num_searching());
+}
diff --git a/src/runtime/thread_pool/mod.rs b/src/runtime/thread_pool/mod.rs
new file mode 100644
index 0000000..ced9712
--- /dev/null
+++ b/src/runtime/thread_pool/mod.rs
@@ -0,0 +1,117 @@
+//! Threadpool
+
+mod atomic_cell;
+use atomic_cell::AtomicCell;
+
+mod idle;
+use self::idle::Idle;
+
+mod worker;
+pub(crate) use worker::Launch;
+
+cfg_blocking! {
+    pub(crate) use worker::block_in_place;
+}
+
+use crate::loom::sync::Arc;
+use crate::runtime::task::{self, JoinHandle};
+use crate::runtime::Parker;
+
+use std::fmt;
+use std::future::Future;
+
+/// Work-stealing based thread pool for executing futures.
+pub(crate) struct ThreadPool {
+    spawner: Spawner,
+}
+
+/// Submit futures to the associated thread pool for execution.
+///
+/// A `Spawner` instance is a handle to a single thread pool that allows the owner
+/// of the handle to spawn futures onto the thread pool.
+///
+/// The `Spawner` handle is *only* used for spawning new futures. It does not
+/// impact the lifecycle of the thread pool in any way. The thread pool may
+/// shutdown while there are outstanding `Spawner` instances.
+///
+/// `Spawner` instances are obtained by calling [`ThreadPool::spawner`].
+///
+/// [`ThreadPool::spawner`]: method@ThreadPool::spawner
+#[derive(Clone)]
+pub(crate) struct Spawner {
+    shared: Arc<worker::Shared>,
+}
+
+// ===== impl ThreadPool =====
+
+impl ThreadPool {
+    pub(crate) fn new(size: usize, parker: Parker) -> (ThreadPool, Launch) {
+        let (shared, launch) = worker::create(size, parker);
+        let spawner = Spawner { shared };
+        let thread_pool = ThreadPool { spawner };
+
+        (thread_pool, launch)
+    }
+
+    /// Returns reference to `Spawner`.
+    ///
+    /// The `Spawner` handle can be cloned and enables spawning tasks from other
+    /// threads.
+    pub(crate) fn spawner(&self) -> &Spawner {
+        &self.spawner
+    }
+
+    /// Spawns a task
+    pub(crate) fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
+    where
+        F: Future + Send + 'static,
+        F::Output: Send + 'static,
+    {
+        self.spawner.spawn(future)
+    }
+
+    /// Blocks the current thread waiting for the future to complete.
+    ///
+    /// The future will execute on the current thread, but all spawned tasks
+    /// will be executed on the thread pool.
+    pub(crate) fn block_on<F>(&self, future: F) -> F::Output
+    where
+        F: Future,
+    {
+        let mut enter = crate::runtime::enter(true);
+        enter.block_on(future).expect("failed to park thread")
+    }
+}
+
+impl fmt::Debug for ThreadPool {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("ThreadPool").finish()
+    }
+}
+
+impl Drop for ThreadPool {
+    fn drop(&mut self) {
+        self.spawner.shared.close();
+    }
+}
+
+// ==== impl Spawner =====
+
+impl Spawner {
+    /// Spawns a future onto the thread pool
+    pub(crate) fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
+    where
+        F: Future + Send + 'static,
+        F::Output: Send + 'static,
+    {
+        let (task, handle) = task::joinable(future);
+        self.shared.schedule(task, false);
+        handle
+    }
+}
+
+impl fmt::Debug for Spawner {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("Spawner").finish()
+    }
+}
diff --git a/src/runtime/thread_pool/worker.rs b/src/runtime/thread_pool/worker.rs
new file mode 100644
index 0000000..abe20da
--- /dev/null
+++ b/src/runtime/thread_pool/worker.rs
@@ -0,0 +1,806 @@
+//! A scheduler is initialized with a fixed number of workers. Each worker is
+//! driven by a thread. Each worker has a "core" which contains data such as the
+//! run queue and other state. When `block_in_place` is called, the worker's
+//! "core" is handed off to a new thread allowing the scheduler to continue to
+//! make progress while the originating thread blocks.
+
+use crate::coop;
+use crate::loom::rand::seed;
+use crate::loom::sync::{Arc, Mutex};
+use crate::park::{Park, Unpark};
+use crate::runtime;
+use crate::runtime::park::{Parker, Unparker};
+use crate::runtime::thread_pool::{AtomicCell, Idle};
+use crate::runtime::{queue, task};
+use crate::util::linked_list::LinkedList;
+use crate::util::FastRand;
+
+use std::cell::RefCell;
+use std::time::Duration;
+
+/// A scheduler worker
+pub(super) struct Worker {
+    /// Reference to shared state
+    shared: Arc<Shared>,
+
+    /// Index holding this worker's remote state
+    index: usize,
+
+    /// Used to hand-off a worker's core to another thread.
+    core: AtomicCell<Core>,
+}
+
+/// Core data
+struct Core {
+    /// Used to schedule bookkeeping tasks every so often.
+    tick: u8,
+
+    /// When a task is scheduled from a worker, it is stored in this slot. The
+    /// worker will check this slot for a task **before** checking the run
+    /// queue. This effectively results in the **last** scheduled task to be run
+    /// next (LIFO). This is an optimization for message passing patterns and
+    /// helps to reduce latency.
+    lifo_slot: Option<Notified>,
+
+    /// The worker-local run queue.
+    run_queue: queue::Local<Arc<Worker>>,
+
+    /// True if the worker is currently searching for more work. Searching
+    /// involves attempting to steal from other workers.
+    is_searching: bool,
+
+    /// True if the scheduler is being shutdown
+    is_shutdown: bool,
+
+    /// Tasks owned by the core
+    tasks: LinkedList<Task>,
+
+    /// Parker
+    ///
+    /// Stored in an `Option` as the parker is added / removed to make the
+    /// borrow checker happy.
+    park: Option<Parker>,
+
+    /// Fast random number generator.
+    rand: FastRand,
+}
+
+/// State shared across all workers
+pub(super) struct Shared {
+    /// Per-worker remote state. All other workers have access to this and is
+    /// how they communicate between each other.
+    remotes: Box<[Remote]>,
+
+    /// Submit work to the scheduler while **not** currently on a worker thread.
+    inject: queue::Inject<Arc<Worker>>,
+
+    /// Coordinates idle workers
+    idle: Idle,
+
+    /// Workers have have observed the shutdown signal
+    ///
+    /// The core is **not** placed back in the worker to avoid it from being
+    /// stolen by a thread that was spawned as part of `block_in_place`.
+    shutdown_workers: Mutex<Vec<(Box<Core>, Arc<Worker>)>>,
+}
+
+/// Used to communicate with a worker from other threads.
+struct Remote {
+    /// Steal tasks from this worker.
+    steal: queue::Steal<Arc<Worker>>,
+
+    /// Transfers tasks to be released. Any worker pushes tasks, only the owning
+    /// worker pops.
+    pending_drop: task::TransferStack<Arc<Worker>>,
+
+    /// Unparks the associated worker thread
+    unpark: Unparker,
+}
+
+/// Thread-local context
+struct Context {
+    /// Worker
+    worker: Arc<Worker>,
+
+    /// Core data
+    core: RefCell<Option<Box<Core>>>,
+}
+
+/// Starts the workers
+pub(crate) struct Launch(Vec<Arc<Worker>>);
+
+/// Running a task may consume the core. If the core is still available when
+/// running the task completes, it is returned. Otherwise, the worker will need
+/// to stop processing.
+type RunResult = Result<Box<Core>, ()>;
+
+/// A task handle
+type Task = task::Task<Arc<Worker>>;
+
+/// A notified task handle
+type Notified = task::Notified<Arc<Worker>>;
+
+// Tracks thread-local state
+scoped_thread_local!(static CURRENT: Context);
+
+pub(super) fn create(size: usize, park: Parker) -> (Arc<Shared>, Launch) {
+    let mut cores = vec![];
+    let mut remotes = vec![];
+
+    // Create the local queues
+    for _ in 0..size {
+        let (steal, run_queue) = queue::local();
+
+        let park = park.clone();
+        let unpark = park.unpark();
+
+        cores.push(Box::new(Core {
+            tick: 0,
+            lifo_slot: None,
+            run_queue,
+            is_searching: false,
+            is_shutdown: false,
+            tasks: LinkedList::new(),
+            park: Some(park),
+            rand: FastRand::new(seed()),
+        }));
+
+        remotes.push(Remote {
+            steal,
+            pending_drop: task::TransferStack::new(),
+            unpark,
+        });
+    }
+
+    let shared = Arc::new(Shared {
+        remotes: remotes.into_boxed_slice(),
+        inject: queue::Inject::new(),
+        idle: Idle::new(size),
+        shutdown_workers: Mutex::new(vec![]),
+    });
+
+    let mut launch = Launch(vec![]);
+
+    for (index, core) in cores.drain(..).enumerate() {
+        launch.0.push(Arc::new(Worker {
+            shared: shared.clone(),
+            index,
+            core: AtomicCell::new(Some(core)),
+        }));
+    }
+
+    (shared, launch)
+}
+
+cfg_blocking! {
+    use crate::runtime::enter::EnterContext;
+
+    pub(crate) fn block_in_place<F, R>(f: F) -> R
+    where
+        F: FnOnce() -> R,
+    {
+        // Try to steal the worker core back
+        struct Reset(coop::Budget);
+
+        impl Drop for Reset {
+            fn drop(&mut self) {
+                CURRENT.with(|maybe_cx| {
+                    if let Some(cx) = maybe_cx {
+                        let core = cx.worker.core.take();
+                        let mut cx_core = cx.core.borrow_mut();
+                        assert!(cx_core.is_none());
+                        *cx_core = core;
+
+                        // Reset the task budget as we are re-entering the
+                        // runtime.
+                        coop::set(self.0);
+                    }
+                });
+            }
+        }
+
+        let mut had_core = false;
+        let mut had_entered = false;
+
+        CURRENT.with(|maybe_cx| {
+            match (crate::runtime::enter::context(),  maybe_cx.is_some()) {
+                (EnterContext::Entered { .. }, true) => {
+                    // We are on a thread pool runtime thread, so we just need to set up blocking.
+                    had_entered = true;
+                }
+                (EnterContext::Entered { allow_blocking }, false) => {
+                    // We are on an executor, but _not_ on the thread pool.
+                    // That is _only_ okay if we are in a thread pool runtime's block_on method:
+                    if allow_blocking {
+                        had_entered = true;
+                        return;
+                    } else {
+                        // This probably means we are on the basic_scheduler or in a LocalSet,
+                        // where it is _not_ okay to block.
+                        panic!("can call blocking only when running on the multi-threaded runtime");
+                    }
+                }
+                (EnterContext::NotEntered, true) => {
+                    // This is a nested call to block_in_place (we already exited).
+                    // All the necessary setup has already been done.
+                    return;
+                }
+                (EnterContext::NotEntered, false) => {
+                    // We are outside of the tokio runtime, so blocking is fine.
+                    // We can also skip all of the thread pool blocking setup steps.
+                    return;
+                }
+            }
+
+            let cx = maybe_cx.expect("no .is_some() == false cases above should lead here");
+
+            // Get the worker core. If none is set, then blocking is fine!
+            let core = match cx.core.borrow_mut().take() {
+                Some(core) => core,
+                None => return,
+            };
+
+            // The parker should be set here
+            assert!(core.park.is_some());
+
+            // In order to block, the core must be sent to another thread for
+            // execution.
+            //
+            // First, move the core back into the worker's shared core slot.
+            cx.worker.core.set(core);
+            had_core = true;
+
+            // Next, clone the worker handle and send it to a new thread for
+            // processing.
+            //
+            // Once the blocking task is done executing, we will attempt to
+            // steal the core back.
+            let worker = cx.worker.clone();
+            runtime::spawn_blocking(move || run(worker));
+        });
+
+        if had_core {
+            // Unset the current task's budget. Blocking sections are not
+            // constrained by task budgets.
+            let _reset = Reset(coop::stop());
+
+            crate::runtime::enter::exit(f)
+        } else if had_entered {
+            crate::runtime::enter::exit(f)
+        } else {
+            f()
+        }
+    }
+}
+
+/// After how many ticks is the global queue polled. This helps to ensure
+/// fairness.
+///
+/// The number is fairly arbitrary. I believe this value was copied from golang.
+const GLOBAL_POLL_INTERVAL: u8 = 61;
+
+impl Launch {
+    pub(crate) fn launch(mut self) {
+        for worker in self.0.drain(..) {
+            runtime::spawn_blocking(move || run(worker));
+        }
+    }
+}
+
+fn run(worker: Arc<Worker>) {
+    // Acquire a core. If this fails, then another thread is running this
+    // worker and there is nothing further to do.
+    let core = match worker.core.take() {
+        Some(core) => core,
+        None => return,
+    };
+
+    // Set the worker context.
+    let cx = Context {
+        worker,
+        core: RefCell::new(None),
+    };
+
+    let _enter = crate::runtime::enter(true);
+
+    CURRENT.set(&cx, || {
+        // This should always be an error. It only returns a `Result` to support
+        // using `?` to short circuit.
+        assert!(cx.run(core).is_err());
+    });
+}
+
+impl Context {
+    fn run(&self, mut core: Box<Core>) -> RunResult {
+        while !core.is_shutdown {
+            // Increment the tick
+            core.tick();
+
+            // Run maintenance, if needed
+            core = self.maintenance(core);
+
+            // First, check work available to the current worker.
+            if let Some(task) = core.next_task(&self.worker) {
+                core = self.run_task(task, core)?;
+                continue;
+            }
+
+            // There is no more **local** work to process, try to steal work
+            // from other workers.
+            if let Some(task) = core.steal_work(&self.worker) {
+                core = self.run_task(task, core)?;
+            } else {
+                // Wait for work
+                core = self.park(core);
+            }
+        }
+
+        // Signal shutdown
+        self.worker.shared.shutdown(core, self.worker.clone());
+        Err(())
+    }
+
+    fn run_task(&self, task: Notified, mut core: Box<Core>) -> RunResult {
+        // Make sure thew orker is not in the **searching** state. This enables
+        // another idle worker to try to steal work.
+        core.transition_from_searching(&self.worker);
+
+        // Make the core available to the runtime context
+        *self.core.borrow_mut() = Some(core);
+
+        // Run the task
+        coop::budget(|| {
+            task.run();
+
+            // As long as there is budget remaining and a task exists in the
+            // `lifo_slot`, then keep running.
+            loop {
+                // Check if we still have the core. If not, the core was stolen
+                // by another worker.
+                let mut core = match self.core.borrow_mut().take() {
+                    Some(core) => core,
+                    None => return Err(()),
+                };
+
+                // Check for a task in the LIFO slot
+                let task = match core.lifo_slot.take() {
+                    Some(task) => task,
+                    None => return Ok(core),
+                };
+
+                if coop::has_budget_remaining() {
+                    // Run the LIFO task, then loop
+                    *self.core.borrow_mut() = Some(core);
+                    task.run();
+                } else {
+                    // Not enough budget left to run the LIFO task, push it to
+                    // the back of the queue and return.
+                    core.run_queue.push_back(task, self.worker.inject());
+                    return Ok(core);
+                }
+            }
+        })
+    }
+
+    fn maintenance(&self, mut core: Box<Core>) -> Box<Core> {
+        if core.tick % GLOBAL_POLL_INTERVAL == 0 {
+            // Call `park` with a 0 timeout. This enables the I/O driver, timer, ...
+            // to run without actually putting the thread to sleep.
+            core = self.park_timeout(core, Some(Duration::from_millis(0)));
+
+            // Run regularly scheduled maintenance
+            core.maintenance(&self.worker);
+        }
+
+        core
+    }
+
+    fn park(&self, mut core: Box<Core>) -> Box<Core> {
+        core.transition_to_parked(&self.worker);
+
+        while !core.is_shutdown {
+            core = self.park_timeout(core, None);
+
+            // Run regularly scheduled maintenance
+            core.maintenance(&self.worker);
+
+            if core.transition_from_parked(&self.worker) {
+                return core;
+            }
+        }
+
+        core
+    }
+
+    fn park_timeout(&self, mut core: Box<Core>, duration: Option<Duration>) -> Box<Core> {
+        // Take the parker out of core
+        let mut park = core.park.take().expect("park missing");
+
+        // Store `core` in context
+        *self.core.borrow_mut() = Some(core);
+
+        // Park thread
+        if let Some(timeout) = duration {
+            park.park_timeout(timeout).expect("park failed");
+        } else {
+            park.park().expect("park failed");
+        }
+
+        // Remove `core` from context
+        core = self.core.borrow_mut().take().expect("core missing");
+
+        // Place `park` back in `core`
+        core.park = Some(park);
+
+        // If there are tasks available to steal, notify a worker
+        if core.run_queue.is_stealable() {
+            self.worker.shared.notify_parked();
+        }
+
+        core
+    }
+}
+
+impl Core {
+    /// Increment the tick
+    fn tick(&mut self) {
+        self.tick = self.tick.wrapping_add(1);
+    }
+
+    /// Return the next notified task available to this worker.
+    fn next_task(&mut self, worker: &Worker) -> Option<Notified> {
+        if self.tick % GLOBAL_POLL_INTERVAL == 0 {
+            worker.inject().pop().or_else(|| self.next_local_task())
+        } else {
+            self.next_local_task().or_else(|| worker.inject().pop())
+        }
+    }
+
+    fn next_local_task(&mut self) -> Option<Notified> {
+        self.lifo_slot.take().or_else(|| self.run_queue.pop())
+    }
+
+    fn steal_work(&mut self, worker: &Worker) -> Option<Notified> {
+        if !self.transition_to_searching(worker) {
+            return None;
+        }
+
+        let num = worker.shared.remotes.len();
+        // Start from a random worker
+        let start = self.rand.fastrand_n(num as u32) as usize;
+
+        for i in 0..num {
+            let i = (start + i) % num;
+
+            // Don't steal from ourself! We know we don't have work.
+            if i == worker.index {
+                continue;
+            }
+
+            let target = &worker.shared.remotes[i];
+            if let Some(task) = target.steal.steal_into(&mut self.run_queue) {
+                return Some(task);
+            }
+        }
+
+        // Fallback on checking the global queue
+        worker.shared.inject.pop()
+    }
+
+    fn transition_to_searching(&mut self, worker: &Worker) -> bool {
+        if !self.is_searching {
+            self.is_searching = worker.shared.idle.transition_worker_to_searching();
+        }
+
+        self.is_searching
+    }
+
+    fn transition_from_searching(&mut self, worker: &Worker) {
+        if !self.is_searching {
+            return;
+        }
+
+        self.is_searching = false;
+        worker.shared.transition_worker_from_searching();
+    }
+
+    /// Prepare the worker state for parking
+    fn transition_to_parked(&mut self, worker: &Worker) {
+        // When the final worker transitions **out** of searching to parked, it
+        // must check all the queues one last time in case work materialized
+        // between the last work scan and transitioning out of searching.
+        let is_last_searcher = worker
+            .shared
+            .idle
+            .transition_worker_to_parked(worker.index, self.is_searching);
+
+        // The worker is no longer searching. Setting this is the local cache
+        // only.
+        self.is_searching = false;
+
+        if is_last_searcher {
+            worker.shared.notify_if_work_pending();
+        }
+    }
+
+    /// Returns `true` if the transition happened.
+    fn transition_from_parked(&mut self, worker: &Worker) -> bool {
+        // If a task is in the lifo slot, then we must unpark regardless of
+        // being notified
+        if self.lifo_slot.is_some() {
+            worker.shared.idle.unpark_worker_by_id(worker.index);
+            self.is_searching = true;
+            return true;
+        }
+
+        if worker.shared.idle.is_parked(worker.index) {
+            return false;
+        }
+
+        // When unparked, the worker is in the searching state.
+        self.is_searching = true;
+        true
+    }
+
+    /// Runs maintenance work such as free pending tasks and check the pool's
+    /// state.
+    fn maintenance(&mut self, worker: &Worker) {
+        self.drain_pending_drop(worker);
+
+        if !self.is_shutdown {
+            // Check if the scheduler has been shutdown
+            self.is_shutdown = worker.inject().is_closed();
+        }
+    }
+
+    // Shutdown the core
+    fn shutdown(&mut self, worker: &Worker) {
+        // Take the core
+        let mut park = self.park.take().expect("park missing");
+
+        // Signal to all tasks to shut down.
+        for header in self.tasks.iter() {
+            header.shutdown();
+        }
+
+        loop {
+            self.drain_pending_drop(worker);
+
+            if self.tasks.is_empty() {
+                break;
+            }
+
+            // Wait until signalled
+            park.park().expect("park failed");
+        }
+
+        // Drain the queue
+        while self.next_local_task().is_some() {}
+    }
+
+    fn drain_pending_drop(&mut self, worker: &Worker) {
+        use std::mem::ManuallyDrop;
+
+        for task in worker.remote().pending_drop.drain() {
+            let task = ManuallyDrop::new(task);
+
+            // safety: tasks are only pushed into the `pending_drop` stacks that
+            // are associated with the list they are inserted into. When a task
+            // is pushed into `pending_drop`, the ref-inc is skipped, so we must
+            // not ref-dec here.
+            //
+            // See `bind` and `release` implementations.
+            unsafe {
+                self.tasks.remove(task.header().into());
+            }
+        }
+    }
+}
+
+impl Worker {
+    /// Returns a reference to the scheduler's injection queue
+    fn inject(&self) -> &queue::Inject<Arc<Worker>> {
+        &self.shared.inject
+    }
+
+    /// Return a reference to this worker's remote data
+    fn remote(&self) -> &Remote {
+        &self.shared.remotes[self.index]
+    }
+
+    fn eq(&self, other: &Worker) -> bool {
+        self.shared.ptr_eq(&other.shared) && self.index == other.index
+    }
+}
+
+impl task::Schedule for Arc<Worker> {
+    fn bind(task: Task) -> Arc<Worker> {
+        CURRENT.with(|maybe_cx| {
+            let cx = maybe_cx.expect("scheduler context missing");
+
+            // Track the task
+            cx.core
+                .borrow_mut()
+                .as_mut()
+                .expect("scheduler core missing")
+                .tasks
+                .push_front(task);
+
+            // Return a clone of the worker
+            cx.worker.clone()
+        })
+    }
+
+    fn release(&self, task: &Task) -> Option<Task> {
+        use std::ptr::NonNull;
+
+        CURRENT.with(|maybe_cx| {
+            let cx = maybe_cx.expect("scheduler context missing");
+
+            if self.eq(&cx.worker) {
+                let mut maybe_core = cx.core.borrow_mut();
+
+                if let Some(core) = &mut *maybe_core {
+                    // Directly remove the task
+                    //
+                    // safety: the task is inserted in the list in `bind`.
+                    unsafe {
+                        let ptr = NonNull::from(task.header());
+                        return core.tasks.remove(ptr);
+                    }
+                }
+            }
+
+            // Track the task to be released by the worker that owns it
+            //
+            // Safety: We get a new handle without incrementing the ref-count.
+            // A ref-count is held by the "owned" linked list and it is only
+            // ever removed from that list as part of the release process: this
+            // method or popping the task from `pending_drop`. Thus, we can rely
+            // on the ref-count held by the linked-list to keep the memory
+            // alive.
+            //
+            // When the task is removed from the stack, it is forgotten instead
+            // of dropped.
+            let task = unsafe { Task::from_raw(task.header().into()) };
+
+            self.remote().pending_drop.push(task);
+
+            if cx.core.borrow().is_some() {
+                return None;
+            }
+
+            // The worker core has been handed off to another thread. In the
+            // event that the scheduler is currently shutting down, the thread
+            // that owns the task may be waiting on the release to complete
+            // shutdown.
+            if self.inject().is_closed() {
+                self.remote().unpark.unpark();
+            }
+
+            None
+        })
+    }
+
+    fn schedule(&self, task: Notified) {
+        self.shared.schedule(task, false);
+    }
+
+    fn yield_now(&self, task: Notified) {
+        self.shared.schedule(task, true);
+    }
+}
+
+impl Shared {
+    pub(super) fn schedule(&self, task: Notified, is_yield: bool) {
+        CURRENT.with(|maybe_cx| {
+            if let Some(cx) = maybe_cx {
+                // Make sure the task is part of the **current** scheduler.
+                if self.ptr_eq(&cx.worker.shared) {
+                    // And the current thread still holds a core
+                    if let Some(core) = cx.core.borrow_mut().as_mut() {
+                        self.schedule_local(core, task, is_yield);
+                        return;
+                    }
+                }
+            }
+
+            // Otherwise, use the inject queue
+            self.inject.push(task);
+            self.notify_parked();
+        });
+    }
+
+    fn schedule_local(&self, core: &mut Core, task: Notified, is_yield: bool) {
+        // Spawning from the worker thread. If scheduling a "yield" then the
+        // task must always be pushed to the back of the queue, enabling other
+        // tasks to be executed. If **not** a yield, then there is more
+        // flexibility and the task may go to the front of the queue.
+        let should_notify = if is_yield {
+            core.run_queue.push_back(task, &self.inject);
+            true
+        } else {
+            // Push to the LIFO slot
+            let prev = core.lifo_slot.take();
+            let ret = prev.is_some();
+
+            if let Some(prev) = prev {
+                core.run_queue.push_back(prev, &self.inject);
+            }
+
+            core.lifo_slot = Some(task);
+
+            ret
+        };
+
+        // Only notify if not currently parked. If `park` is `None`, then the
+        // scheduling is from a resource driver. As notifications often come in
+        // batches, the notification is delayed until the park is complete.
+        if should_notify && core.park.is_some() {
+            self.notify_parked();
+        }
+    }
+
+    pub(super) fn close(&self) {
+        if self.inject.close() {
+            self.notify_all();
+        }
+    }
+
+    fn notify_parked(&self) {
+        if let Some(index) = self.idle.worker_to_notify() {
+            self.remotes[index].unpark.unpark();
+        }
+    }
+
+    fn notify_all(&self) {
+        for remote in &self.remotes[..] {
+            remote.unpark.unpark();
+        }
+    }
+
+    fn notify_if_work_pending(&self) {
+        for remote in &self.remotes[..] {
+            if !remote.steal.is_empty() {
+                self.notify_parked();
+                return;
+            }
+        }
+
+        if !self.inject.is_empty() {
+            self.notify_parked();
+        }
+    }
+
+    fn transition_worker_from_searching(&self) {
+        if self.idle.transition_worker_from_searching() {
+            // We are the final searching worker. Because work was found, we
+            // need to notify another worker.
+            self.notify_parked();
+        }
+    }
+
+    /// Signals that a worker has observed the shutdown signal and has replaced
+    /// its core back into its handle.
+    ///
+    /// If all workers have reached this point, the final cleanup is performed.
+    fn shutdown(&self, core: Box<Core>, worker: Arc<Worker>) {
+        let mut workers = self.shutdown_workers.lock().unwrap();
+        workers.push((core, worker));
+
+        if workers.len() != self.remotes.len() {
+            return;
+        }
+
+        for (mut core, worker) in workers.drain(..) {
+            core.shutdown(&worker);
+        }
+
+        // Drain the injection queue
+        while self.inject.pop().is_some() {}
+    }
+
+    fn ptr_eq(&self, other: &Shared) -> bool {
+        self as *const _ == other as *const _
+    }
+}