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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`][p] instance is unblocked by calling [`unpark`] on its
//! [`Unpark`][up] handle.
//!
//! The [`ParkThread`] struct implements [`Park`][p] using
//! [`thread::park`][`std`] to put the thread to sleep. The Tokio reactor also
//! implements park, but uses [`mio::Poll`][mio] to block the thread instead.
//!
//! The [`Park`][p] trait is composable. A timer implementation might decorate a
//! [`Park`][p] implementation by checking if any timeouts have elapsed after
//! the inner [`Park`][p] implementation unblocks.
//!
//! # Model
//!
//! Conceptually, each [`Park`][p] 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.
//!
//! [`std`]: https://doc.rust-lang.org/std/thread/fn.park.html
//! [`thread::park`]: https://doc.rust-lang.org/std/thread/fn.park.html
//! [`ParkThread`]: struct.ParkThread.html
//! [p]: trait.Park.html
//! [`park`]: trait.Park.html#tymethod.park
//! [`park_timeout`]: trait.Park.html#tymethod.park_timeout
//! [`unpark`]: trait.Unpark.html#tymethod.unpark
//! [up]: trait.Unpark.html
//! [mio]: https://docs.rs/mio/0.6/mio/struct.Poll.html

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;
}

use std::sync::Arc;
use std::time::Duration;

/// Block the current thread.
///
/// See [module documentation][mod] for more details.
///
/// [mod]: ../index.html
pub(crate) trait Park {
    /// Unpark handle type for the `Park` implementation.
    type Unpark: Unpark;

    /// Error returned by `park`
    type Error;

    /// Get a new `Unpark` handle associated with this `Park` instance.
    fn unpark(&self) -> Self::Unpark;

    /// Block 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.
    ///
    /// See [module documentation][mod] for more details.
    ///
    /// # Panics
    ///
    /// This function **should** not panic, but ultimately, panics are left as
    /// an implementation detail. Refer to the documentation for the specific
    /// `Park` implementation
    ///
    /// [mod]: ../index.html
    fn park(&mut self) -> Result<(), Self::Error>;

    /// Park 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.
    ///
    /// See [module documentation][mod] for more details.
    ///
    /// # Panics
    ///
    /// This function **should** not panic, but ultimately, panics are left as
    /// an implementation detail. Refer to the documentation for the specific
    /// `Park` implementation
    ///
    /// [mod]: ../index.html
    fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error>;
}

/// Unblock a thread blocked by the associated [`Park`] instance.
///
/// See [module documentation][mod] for more details.
///
/// [mod]: ../index.html
/// [`Park`]: trait.Park.html
pub(crate) trait Unpark: Sync + Send + 'static {
    /// Unblock a thread that is blocked by the associated `Park` handle.
    ///
    /// Calling `unpark` atomically makes available the unpark token, if it is
    /// not already available.
    ///
    /// See [module documentation][mod] for more details.
    ///
    /// # Panics
    ///
    /// This function **should** not panic, but ultimately, panics are left as
    /// an implementation detail. Refer to the documentation for the specific
    /// `Unpark` implementation
    ///
    /// [mod]: ../index.html
    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()
    }
}