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diff --git a/third_party/rust/tokio/src/park/mod.rs b/third_party/rust/tokio/src/park/mod.rs
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+//! Abstraction over blocking and unblocking the current thread.
+//!
+//! Provides an abstraction over blocking the current thread. This is similar to
+//! the park / unpark constructs provided by `std` but made generic. This allows
+//! embedding custom functionality to perform when the thread is blocked.
+//!
+//! A blocked `Park` instance is unblocked by calling `unpark` on its
+//! `Unpark` handle.
+//!
+//! The `ParkThread` struct implements `Park` using `thread::park` to put the
+//! thread to sleep. The Tokio reactor also implements park, but uses
+//! `mio::Poll` to block the thread instead.
+//!
+//! The `Park` trait is composable. A timer implementation might decorate a
+//! `Park` implementation by checking if any timeouts have elapsed after the
+//! inner `Park` implementation unblocks.
+//!
+//! # Model
+//!
+//! Conceptually, each `Park` instance has an associated token, which is
+//! initially not present:
+//!
+//! * The `park` method blocks the current thread unless or until the token is
+//!   available, at which point it atomically consumes the token.
+//! * The `unpark` method atomically makes the token available if it wasn't
+//!   already.
+//!
+//! Some things to note:
+//!
+//! * If `unpark` is called before `park`, the next call to `park` will
+//!   **not** block the thread.
+//! * **Spurious** wakeups are permitted, i.e., the `park` method may unblock
+//!   even if `unpark` was not called.
+//! * `park_timeout` does the same as `park` but allows specifying a maximum
+//!   time to block the thread for.
+
+cfg_resource_drivers! {
+    mod either;
+    pub(crate) use self::either::Either;
+}
+
+mod thread;
+pub(crate) use self::thread::ParkThread;
+
+cfg_blocking_impl! {
+    pub(crate) use self::thread::{CachedParkThread, ParkError};
+}
+
+use std::sync::Arc;
+use std::time::Duration;
+
+/// Block the current thread.
+pub(crate) trait Park {
+    /// Unpark handle type for the `Park` implementation.
+    type Unpark: Unpark;
+
+    /// Error returned by `park`
+    type Error;
+
+    /// Gets a new `Unpark` handle associated with this `Park` instance.
+    fn unpark(&self) -> Self::Unpark;
+
+    /// Blocks the current thread unless or until the token is available.
+    ///
+    /// A call to `park` does not guarantee that the thread will remain blocked
+    /// forever, and callers should be prepared for this possibility. This
+    /// function may wakeup spuriously for any reason.
+    ///
+    /// # Panics
+    ///
+    /// This function **should** not panic, but ultimately, panics are left as
+    /// an implementation detail. Refer to the documentation for the specific
+    /// `Park` implementation
+    fn park(&mut self) -> Result<(), Self::Error>;
+
+    /// Parks the current thread for at most `duration`.
+    ///
+    /// This function is the same as `park` but allows specifying a maximum time
+    /// to block the thread for.
+    ///
+    /// Same as `park`, there is no guarantee that the thread will remain
+    /// blocked for any amount of time. Spurious wakeups are permitted for any
+    /// reason.
+    ///
+    /// # Panics
+    ///
+    /// This function **should** not panic, but ultimately, panics are left as
+    /// an implementation detail. Refer to the documentation for the specific
+    /// `Park` implementation
+    fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error>;
+}
+
+/// Unblock a thread blocked by the associated `Park` instance.
+pub(crate) trait Unpark: Sync + Send + 'static {
+    /// Unblocks a thread that is blocked by the associated `Park` handle.
+    ///
+    /// Calling `unpark` atomically makes available the unpark token, if it is
+    /// not already available.
+    ///
+    /// # Panics
+    ///
+    /// This function **should** not panic, but ultimately, panics are left as
+    /// an implementation detail. Refer to the documentation for the specific
+    /// `Unpark` implementation
+    fn unpark(&self);
+}
+
+impl Unpark for Box<dyn Unpark> {
+    fn unpark(&self) {
+        (**self).unpark()
+    }
+}
+
+impl Unpark for Arc<dyn Unpark> {
+    fn unpark(&self) {
+        (**self).unpark()
+    }
+}