1 files changed, 316 insertions, 0 deletions
diff --git a/tokio/src/park/thread.rs b/tokio/src/park/thread.rs
new file mode 100644
index 00000000..dc844871
--- /dev/null
+++ b/tokio/src/park/thread.rs
@@ -0,0 +1,316 @@
+use crate::loom::sync::atomic::AtomicUsize;
+use crate::loom::sync::{Arc, Condvar, Mutex};
+use crate::park::{Park, Unpark};
+
+use std::sync::atomic::Ordering::SeqCst;
+use std::time::Duration;
+
+#[derive(Debug)]
+pub(crate) struct ParkThread {
+    inner: Arc<Inner>,
+}
+
+/// Error returned by [`ParkThread`]
+///
+/// This currently is never returned, but might at some point in the future.
+///
+/// [`ParkThread`]: struct.ParkThread.html
+#[derive(Debug)]
+pub(crate) struct ParkError {
+    _p: (),
+}
+
+/// 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;
+
+thread_local! {
+    static CURRENT_PARKER: ParkThread = ParkThread::new();
+}
+
+// ==== impl ParkThread ====
+
+impl ParkThread {
+    pub(crate) fn new() -> Self {
+        Self {
+            inner: Arc::new(Inner {
+                state: AtomicUsize::new(EMPTY),
+                mutex: Mutex::new(()),
+                condvar: Condvar::new(),
+            }),
+        }
+    }
+}
+
+impl Park for ParkThread {
+    type Unpark = UnparkThread;
+    type Error = ParkError;
+
+    fn unpark(&self) -> Self::Unpark {
+        let inner = self.inner.clone();
+        UnparkThread { inner }
+    }
+
+    fn park(&mut self) -> Result<(), Self::Error> {
+        self.inner.park();
+        Ok(())
+    }
+
+    fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> {
+        self.inner.park_timeout(duration);
+        Ok(())
+    }
+}
+
+// ==== impl Inner ====
+
+impl Inner {
+    /// Park the current thread for at most `dur`.
+    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().unwrap();
+
+        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
+        }
+    }
+
+    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;
+        }
+
+        let m = self.mutex.lock().unwrap();
+
+        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().unwrap());
+
+        self.condvar.notify_one()
+    }
+}
+
+impl Default for ParkThread {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+// ===== impl UnparkThread =====
+
+impl Unpark for UnparkThread {
+    fn unpark(&self) {
+        self.inner.unpark();
+    }
+}
+
+cfg_blocking_impl! {
+    use std::marker::PhantomData;
+    use std::rc::Rc;
+
+    use std::mem;
+    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 {
+        /// Create 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,
+            }
+        }
+
+        /// Get a reference to the `ParkThread` handle for this thread.
+        fn with_current<F, R>(&self, f: F) -> R
+        where
+            F: FnOnce(&ParkThread) -> R,
+        {
+            CURRENT_PARKER.with(|inner| f(inner))
+        }
+    }
+
+    impl Park for CachedParkThread {
+        type Unpark = UnparkThread;
+        type Error = ParkError;
+
+        fn unpark(&self) -> Self::Unpark {
+            self.with_current(|park_thread| park_thread.unpark())
+        }
+
+        fn park(&mut self) -> Result<(), Self::Error> {
+            self.with_current(|park_thread| park_thread.inner.park());
+            Ok(())
+        }
+
+        fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> {
+            self.with_current(|park_thread| park_thread.inner.park_timeout(duration));
+            Ok(())
+        }
+    }
+
+
+    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 {
+        let unparker = Inner::from_raw(raw);
+
+        // Increment the ref count
+        mem::forget(unparker.clone());
+
+        unparker_to_raw_waker(unparker)
+    }
+
+    unsafe fn drop_waker(raw: *const ()) {
+        let _ = 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 unparker = Inner::from_raw(raw);
+        unparker.unpark();
+
+        // We don't actually own a reference to the unparker
+        mem::forget(unparker);
+    }
+}