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use core::mem;
use core::task::Waker;
/// Utility struct to register and wake a waker.
#[derive(Debug, Default)]
pub struct WakerRegistration {
waker: Option<Waker>,
}
impl WakerRegistration {
/// Create a new `WakerRegistration`.
pub const fn new() -> Self {
Self { waker: None }
}
/// Register a waker. Overwrites the previous waker, if any.
pub fn register(&mut self, w: &Waker) {
match self.waker {
// Optimization: If both the old and new Wakers wake the same task, we can simply
// keep the old waker, skipping the clone. (In most executor implementations,
// cloning a waker is somewhat expensive, comparable to cloning an Arc).
Some(ref w2) if (w2.will_wake(w)) => {}
_ => {
// clone the new waker and store it
if let Some(old_waker) = mem::replace(&mut self.waker, Some(w.clone())) {
// We had a waker registered for another task. Wake it, so the other task can
// reregister itself if it's still interested.
//
// If two tasks are waiting on the same thing concurrently, this will cause them
// to wake each other in a loop fighting over this WakerRegistration. This wastes
// CPU but things will still work.
//
// If the user wants to have two tasks waiting on the same thing they should use
// a more appropriate primitive that can store multiple wakers.
old_waker.wake()
}
}
}
}
/// Wake the registered waker, if any.
pub fn wake(&mut self) {
if let Some(w) = self.waker.take() {
w.wake()
}
}
/// Returns true if a waker is currently registered
pub fn occupied(&self) -> bool {
self.waker.is_some()
}
}