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#![unstable(feature = "wake_trait", issue = "69912")]
//! Types and Traits for working with asynchronous tasks.
use core::mem::{self, ManuallyDrop};
use core::task::{RawWaker, RawWakerVTable, Waker};
use crate::sync::Arc;
/// The implementation of waking a task on an executor.
///
/// This trait can be used to create a [`Waker`]. An executor can define an
/// implementation of this trait, and use that to construct a Waker to pass
/// to the tasks that are executed on that executor.
///
/// This trait is a memory-safe and ergonomic alternative to constructing a
/// [`RawWaker`]. It supports the common executor design in which the data
/// used to wake up a task is stored in an [`Arc`]. Some executors (especially
/// those for embedded systems) cannot use this API, which is why [`RawWaker`]
/// exists as an alternative for those systems.
#[unstable(feature = "wake_trait", issue = "69912")]
pub trait Wake {
/// Wake this task.
#[unstable(feature = "wake_trait", issue = "69912")]
fn wake(self: Arc<Self>);
/// Wake this task without consuming the waker.
///
/// If an executor supports a cheaper way to wake without consuming the
/// waker, it should override this method. By default, it clones the
/// [`Arc`] and calls `wake` on the clone.
#[unstable(feature = "wake_trait", issue = "69912")]
fn wake_by_ref(self: &Arc<Self>) {
self.clone().wake();
}
}
#[unstable(feature = "wake_trait", issue = "69912")]
impl<W: Wake + Send + Sync + 'static> From<Arc<W>> for Waker {
fn from(waker: Arc<W>) -> Waker {
// SAFETY: This is safe because raw_waker safely constructs
// a RawWaker from Arc<W>.
unsafe { Waker::from_raw(raw_waker(waker)) }
}
}
#[unstable(feature = "wake_trait", issue = "69912")]
impl<W: Wake + Send + Sync + 'static> From<Arc<W>> for RawWaker {
fn from(waker: Arc<W>) -> RawWaker {
raw_waker(waker)
}
}
// NB: This private function for constructing a RawWaker is used, rather than
// inlining this into the `From<Arc<W>> for RawWaker` impl, to ensure that
// the safety of `From<Arc<W>> for Waker` does not depend on the correct
// trait dispatch - instead both impls call this function directly and
// explicitly.
#[inline(always)]
fn raw_waker<W: Wake + Send + Sync + 'static>(waker: Arc<W>) -> RawWaker {
// Increment the reference count of the arc to clone it.
unsafe fn clone_waker<W: Wake + Send + Sync + 'static>(waker: *const ()) -> RawWaker {
let waker: Arc<W> = Arc::from_raw(waker as *const W);
mem::forget(Arc::clone(&waker));
raw_waker(waker)
}
// Wake by value, moving the Arc into the Wake::wake function
unsafe fn wake<W: Wake + Send + Sync + 'static>(waker: *const ()) {
let waker: Arc<W> = Arc::from_raw(waker as *const W);
<W as Wake>::wake(waker);
}
// Wake by reference, wrap the waker in ManuallyDrop to avoid dropping it
unsafe fn wake_by_ref<W: Wake + Send + Sync + 'static>(waker: *const ()) {
let waker: ManuallyDrop<Arc<W>> = ManuallyDrop::new(Arc::from_raw(waker as *const W));
<W as Wake>::wake_by_ref(&waker);
}
// Decrement the reference count of the Arc on drop
unsafe fn drop_waker<W: Wake + Send + Sync + 'static>(waker: *const ()) {
mem::drop(Arc::from_raw(waker as *const W));
}
RawWaker::new(
Arc::into_raw(waker) as *const (),
&RawWakerVTable::new(clone_waker::<W>, wake::<W>, wake_by_ref::<W>, drop_waker::<W>),
)
}