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#![stable(feature = "wake_trait", since = "1.51.0")]
//! Types and Traits for working with asynchronous tasks.
//!
//! **Note**: This module is only available on platforms that support atomic
//! loads and stores of pointers. This may be detected at compile time using
//! `#[cfg(target_has_atomic = "ptr")]`.
use core::mem::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.
///
/// [arc]: ../../std/sync/struct.Arc.html
///
/// # Examples
///
/// A basic `block_on` function that takes a future and runs it to completion on
/// the current thread.
///
/// **Note:** This example trades correctness for simplicity. In order to prevent
/// deadlocks, production-grade implementations will also need to handle
/// intermediate calls to `thread::unpark` as well as nested invocations.
///
/// ```rust
/// use std::future::Future;
/// use std::sync::Arc;
/// use std::task::{Context, Poll, Wake};
/// use std::thread::{self, Thread};
///
/// /// A waker that wakes up the current thread when called.
/// struct ThreadWaker(Thread);
///
/// impl Wake for ThreadWaker {
/// fn wake(self: Arc<Self>) {
/// self.0.unpark();
/// }
/// }
///
/// /// Run a future to completion on the current thread.
/// fn block_on<T>(fut: impl Future<Output = T>) -> T {
/// // Pin the future so it can be polled.
/// let mut fut = Box::pin(fut);
///
/// // Create a new context to be passed to the future.
/// let t = thread::current();
/// let waker = Arc::new(ThreadWaker(t)).into();
/// let mut cx = Context::from_waker(&waker);
///
/// // Run the future to completion.
/// loop {
/// match fut.as_mut().poll(&mut cx) {
/// Poll::Ready(res) => return res,
/// Poll::Pending => thread::park(),
/// }
/// }
/// }
///
/// block_on(async {
/// println!("Hi from inside a future!");
/// });
/// ```
#[stable(feature = "wake_trait", since = "1.51.0")]
pub trait Wake {
/// Wake this task.
#[stable(feature = "wake_trait", since = "1.51.0")]
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.
///
/// [`wake`]: Wake::wake
#[stable(feature = "wake_trait", since = "1.51.0")]
fn wake_by_ref(self: &Arc<Self>) {
self.clone().wake();
}
}
#[stable(feature = "wake_trait", since = "1.51.0")]
impl<W: Wake + Send + Sync + 'static> From<Arc<W>> for Waker {
/// Use a `Wake`-able type as a `Waker`.
///
/// No heap allocations or atomic operations are used for this conversion.
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)) }
}
}
#[stable(feature = "wake_trait", since = "1.51.0")]
impl<W: Wake + Send + Sync + 'static> From<Arc<W>> for RawWaker {
/// Use a `Wake`-able type as a `RawWaker`.
///
/// No heap allocations or atomic operations are used for this conversion.
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 {
unsafe { Arc::increment_strong_count(waker as *const W) };
RawWaker::new(
waker as *const (),
&RawWakerVTable::new(clone_waker::<W>, wake::<W>, wake_by_ref::<W>, drop_waker::<W>),
)
}
// Wake by value, moving the Arc into the Wake::wake function
unsafe fn wake<W: Wake + Send + Sync + 'static>(waker: *const ()) {
let waker = unsafe { 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 = unsafe { 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 ()) {
unsafe { Arc::decrement_strong_count(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>),
)
}
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