src/runtime/park.rs

#![cfg_attr(not(feature = "full"), allow(dead_code))]

use crate::loom::sync::atomic::AtomicUsize;
use crate::loom::sync::{Arc, Condvar, Mutex};

use std::sync::atomic::Ordering::SeqCst;
use std::time::Duration;

#[derive(Debug)]
pub(crate) struct ParkThread {
    inner: Arc<Inner>,
}

/// Unblocks a thread that was blocked by `ParkThread`.
#[derive(Clone, Debug)]
pub(crate) struct UnparkThread {
    inner: Arc<Inner>,
}

#[derive(Debug)]
struct Inner {
    state: AtomicUsize,
    mutex: Mutex<()>,
    condvar: Condvar,
}

const EMPTY: usize = 0;
const PARKED: usize = 1;
const NOTIFIED: usize = 2;

tokio_thread_local! {
    static CURRENT_PARKER: ParkThread = ParkThread::new();
}

// Bit of a hack, but it is only for loom
#[cfg(loom)]
tokio_thread_local! {
    pub(crate) static CURRENT_THREAD_PARK_COUNT: AtomicUsize = AtomicUsize::new(0);
}

// ==== impl ParkThread ====

impl ParkThread {
    pub(crate) fn new() -> Self {
        Self {
            inner: Arc::new(Inner {
                state: AtomicUsize::new(EMPTY),
                mutex: Mutex::new(()),
                condvar: Condvar::new(),
            }),
        }
    }

    pub(crate) fn unpark(&self) -> UnparkThread {
        let inner = self.inner.clone();
        UnparkThread { inner }
    }

    pub(crate) fn park(&mut self) {
        #[cfg(loom)]
        CURRENT_THREAD_PARK_COUNT.with(|count| count.fetch_add(1, SeqCst));
        self.inner.park();
    }

    pub(crate) fn park_timeout(&mut self, duration: Duration) {
        #[cfg(loom)]
        CURRENT_THREAD_PARK_COUNT.with(|count| count.fetch_add(1, SeqCst));

        // Wasm doesn't have threads, so just sleep.
        #[cfg(not(target_family = "wasm"))]
        self.inner.park_timeout(duration);
        #[cfg(target_family = "wasm")]
        std::thread::sleep(duration);
    }

    pub(crate) fn shutdown(&mut self) {
        self.inner.shutdown();
    }
}

// ==== impl Inner ====

impl Inner {
    fn park(&self) {
        // If we were previously notified then we consume this notification and
        // return quickly.
        if self
            .state
            .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
            .is_ok()
        {
            return;
        }

        // Otherwise we need to coordinate going to sleep
        let mut m = self.mutex.lock();

        match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
            Ok(_) => {}
            Err(NOTIFIED) => {
                // We must read here, even though we know it will be `NOTIFIED`.
                // This is because `unpark` may have been called again since we read
                // `NOTIFIED` in the `compare_exchange` above. We must perform an
                // acquire operation that synchronizes with that `unpark` to observe
                // any writes it made before the call to unpark. To do that we must
                // read from the write it made to `state`.
                let old = self.state.swap(EMPTY, SeqCst);
                debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly");

                return;
            }
            Err(actual) => panic!("inconsistent park state; actual = {actual}"),
        }

        loop {
            m = self.condvar.wait(m).unwrap();

            if self
                .state
                .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
                .is_ok()
            {
                // got a notification
                return;
            }

            // spurious wakeup, go back to sleep
        }
    }

    /// Parks the current thread for at most `dur`.
    fn park_timeout(&self, dur: Duration) {
        // Like `park` above we have a fast path for an already-notified thread,
        // and afterwards we start coordinating for a sleep. Return quickly.
        if self
            .state
            .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
            .is_ok()
        {
            return;
        }

        if dur == Duration::from_millis(0) {
            return;
        }

        let m = self.mutex.lock();

        match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
            Ok(_) => {}
            Err(NOTIFIED) => {
                // We must read again here, see `park`.
                let old = self.state.swap(EMPTY, SeqCst);
                debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly");

                return;
            }
            Err(actual) => panic!("inconsistent park_timeout state; actual = {actual}"),
        }

        // Wait with a timeout, and if we spuriously wake up or otherwise wake up
        // from a notification, we just want to unconditionally set the state back to
        // empty, either consuming a notification or un-flagging ourselves as
        // parked.
        let (_m, _result) = self.condvar.wait_timeout(m, dur).unwrap();

        match self.state.swap(EMPTY, SeqCst) {
            NOTIFIED => {} // got a notification, hurray!
            PARKED => {}   // no notification, alas
            n => panic!("inconsistent park_timeout state: {n}"),
        }
    }

    fn unpark(&self) {
        // To ensure the unparked thread will observe any writes we made before
        // this call, we must perform a release operation that `park` can
        // synchronize with. To do that we must write `NOTIFIED` even if `state`
        // is already `NOTIFIED`. That is why this must be a swap rather than a
        // compare-and-swap that returns if it reads `NOTIFIED` on failure.
        match self.state.swap(NOTIFIED, SeqCst) {
            EMPTY => return,    // no one was waiting
            NOTIFIED => return, // already unparked
            PARKED => {}        // gotta go wake someone up
            _ => panic!("inconsistent state in unpark"),
        }

        // There is a period between when the parked thread sets `state` to
        // `PARKED` (or last checked `state` in the case of a spurious wake
        // up) and when it actually waits on `cvar`. If we were to notify
        // during this period it would be ignored and then when the parked
        // thread went to sleep it would never wake up. Fortunately, it has
        // `lock` locked at this stage so we can acquire `lock` to wait until
        // it is ready to receive the notification.
        //
        // Releasing `lock` before the call to `notify_one` means that when the
        // parked thread wakes it doesn't get woken only to have to wait for us
        // to release `lock`.
        drop(self.mutex.lock());

        self.condvar.notify_one();
    }

    fn shutdown(&self) {
        self.condvar.notify_all();
    }
}

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

// ===== impl UnparkThread =====

impl UnparkThread {
    pub(crate) fn unpark(&self) {
        self.inner.unpark();
    }
}

use crate::loom::thread::AccessError;
use std::future::Future;
use std::marker::PhantomData;
use std::rc::Rc;
use std::task::{RawWaker, RawWakerVTable, Waker};

/// Blocks the current thread using a condition variable.
#[derive(Debug)]
pub(crate) struct CachedParkThread {
    _anchor: PhantomData<Rc<()>>,
}

impl CachedParkThread {
    /// Creates a new `ParkThread` handle for the current thread.
    ///
    /// This type cannot be moved to other threads, so it should be created on
    /// the thread that the caller intends to park.
    pub(crate) fn new() -> CachedParkThread {
        CachedParkThread {
            _anchor: PhantomData,
        }
    }

    pub(crate) fn waker(&self) -> Result<Waker, AccessError> {
        self.unpark().map(UnparkThread::into_waker)
    }

    fn unpark(&self) -> Result<UnparkThread, AccessError> {
        self.with_current(ParkThread::unpark)
    }

    pub(crate) fn park(&mut self) {
        self.with_current(|park_thread| park_thread.inner.park())
            .unwrap();
    }

    pub(crate) fn park_timeout(&mut self, duration: Duration) {
        self.with_current(|park_thread| park_thread.inner.park_timeout(duration))
            .unwrap();
    }

    /// Gets a reference to the `ParkThread` handle for this thread.
    fn with_current<F, R>(&self, f: F) -> Result<R, AccessError>
    where
        F: FnOnce(&ParkThread) -> R,
    {
        CURRENT_PARKER.try_with(|inner| f(inner))
    }

    pub(crate) fn block_on<F: Future>(&mut self, f: F) -> Result<F::Output, AccessError> {
        use std::task::Context;
        use std::task::Poll::Ready;

        let waker = self.waker()?;
        let mut cx = Context::from_waker(&waker);

        pin!(f);

        loop {
            if let Ready(v) = crate::runtime::coop::budget(|| f.as_mut().poll(&mut cx)) {
                return Ok(v);
            }

            self.park();
        }
    }
}

impl UnparkThread {
    pub(crate) fn into_waker(self) -> Waker {
        unsafe {
            let raw = unparker_to_raw_waker(self.inner);
            Waker::from_raw(raw)
        }
    }
}

impl Inner {
    #[allow(clippy::wrong_self_convention)]
    fn into_raw(this: Arc<Inner>) -> *const () {
        Arc::into_raw(this) as *const ()
    }

    unsafe fn from_raw(ptr: *const ()) -> Arc<Inner> {
        Arc::from_raw(ptr as *const Inner)
    }
}

unsafe fn unparker_to_raw_waker(unparker: Arc<Inner>) -> RawWaker {
    RawWaker::new(
        Inner::into_raw(unparker),
        &RawWakerVTable::new(clone, wake, wake_by_ref, drop_waker),
    )
}

unsafe fn clone(raw: *const ()) -> RawWaker {
    Arc::increment_strong_count(raw as *const Inner);
    unparker_to_raw_waker(Inner::from_raw(raw))
}

unsafe fn drop_waker(raw: *const ()) {
    drop(Inner::from_raw(raw));
}

unsafe fn wake(raw: *const ()) {
    let unparker = Inner::from_raw(raw);
    unparker.unpark();
}

unsafe fn wake_by_ref(raw: *const ()) {
    let raw = raw as *const Inner;
    (*raw).unpark();
}

#[cfg(loom)]
pub(crate) fn current_thread_park_count() -> usize {
    CURRENT_THREAD_PARK_COUNT.with(|count| count.load(SeqCst))
}