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#![stable(feature = "futures_api", since = "1.36.0")]
use crate::fmt;
use crate::marker::{PhantomData, Unpin};
/// A `RawWaker` allows the implementor of a task executor to create a [`Waker`]
/// which provides customized wakeup behavior.
///
/// [vtable]: https://en.wikipedia.org/wiki/Virtual_method_table
///
/// It consists of a data pointer and a [virtual function pointer table (vtable)][vtable]
/// that customizes the behavior of the `RawWaker`.
#[derive(PartialEq, Debug)]
#[stable(feature = "futures_api", since = "1.36.0")]
pub struct RawWaker {
/// A data pointer, which can be used to store arbitrary data as required
/// by the executor. This could be e.g. a type-erased pointer to an `Arc`
/// that is associated with the task.
/// The value of this field gets passed to all functions that are part of
/// the vtable as the first parameter.
data: *const (),
/// Virtual function pointer table that customizes the behavior of this waker.
vtable: &'static RawWakerVTable,
}
impl RawWaker {
/// Creates a new `RawWaker` from the provided `data` pointer and `vtable`.
///
/// The `data` pointer can be used to store arbitrary data as required
/// by the executor. This could be e.g. a type-erased pointer to an `Arc`
/// that is associated with the task.
/// The value of this pointer will get passed to all functions that are part
/// of the `vtable` as the first parameter.
///
/// The `vtable` customizes the behavior of a `Waker` which gets created
/// from a `RawWaker`. For each operation on the `Waker`, the associated
/// function in the `vtable` of the underlying `RawWaker` will be called.
#[inline]
#[rustc_promotable]
#[stable(feature = "futures_api", since = "1.36.0")]
#[rustc_const_stable(feature = "futures_api", since = "1.36.0")]
#[must_use]
pub const fn new(data: *const (), vtable: &'static RawWakerVTable) -> RawWaker {
RawWaker { data, vtable }
}
/// Get the `data` pointer used to create this `RawWaker`.
#[inline]
#[must_use]
#[unstable(feature = "waker_getters", issue = "87021")]
pub fn data(&self) -> *const () {
self.data
}
/// Get the `vtable` pointer used to create this `RawWaker`.
#[inline]
#[must_use]
#[unstable(feature = "waker_getters", issue = "87021")]
pub fn vtable(&self) -> &'static RawWakerVTable {
self.vtable
}
}
/// A virtual function pointer table (vtable) that specifies the behavior
/// of a [`RawWaker`].
///
/// The pointer passed to all functions inside the vtable is the `data` pointer
/// from the enclosing [`RawWaker`] object.
///
/// The functions inside this struct are only intended to be called on the `data`
/// pointer of a properly constructed [`RawWaker`] object from inside the
/// [`RawWaker`] implementation. Calling one of the contained functions using
/// any other `data` pointer will cause undefined behavior.
#[stable(feature = "futures_api", since = "1.36.0")]
#[derive(PartialEq, Copy, Clone, Debug)]
pub struct RawWakerVTable {
/// This function will be called when the [`RawWaker`] gets cloned, e.g. when
/// the [`Waker`] in which the [`RawWaker`] is stored gets cloned.
///
/// The implementation of this function must retain all resources that are
/// required for this additional instance of a [`RawWaker`] and associated
/// task. Calling `wake` on the resulting [`RawWaker`] should result in a wakeup
/// of the same task that would have been awoken by the original [`RawWaker`].
clone: unsafe fn(*const ()) -> RawWaker,
/// This function will be called when `wake` is called on the [`Waker`].
/// It must wake up the task associated with this [`RawWaker`].
///
/// The implementation of this function must make sure to release any
/// resources that are associated with this instance of a [`RawWaker`] and
/// associated task.
wake: unsafe fn(*const ()),
/// This function will be called when `wake_by_ref` is called on the [`Waker`].
/// It must wake up the task associated with this [`RawWaker`].
///
/// This function is similar to `wake`, but must not consume the provided data
/// pointer.
wake_by_ref: unsafe fn(*const ()),
/// This function gets called when a [`RawWaker`] gets dropped.
///
/// The implementation of this function must make sure to release any
/// resources that are associated with this instance of a [`RawWaker`] and
/// associated task.
drop: unsafe fn(*const ()),
}
impl RawWakerVTable {
/// Creates a new `RawWakerVTable` from the provided `clone`, `wake`,
/// `wake_by_ref`, and `drop` functions.
///
/// # `clone`
///
/// This function will be called when the [`RawWaker`] gets cloned, e.g. when
/// the [`Waker`] in which the [`RawWaker`] is stored gets cloned.
///
/// The implementation of this function must retain all resources that are
/// required for this additional instance of a [`RawWaker`] and associated
/// task. Calling `wake` on the resulting [`RawWaker`] should result in a wakeup
/// of the same task that would have been awoken by the original [`RawWaker`].
///
/// # `wake`
///
/// This function will be called when `wake` is called on the [`Waker`].
/// It must wake up the task associated with this [`RawWaker`].
///
/// The implementation of this function must make sure to release any
/// resources that are associated with this instance of a [`RawWaker`] and
/// associated task.
///
/// # `wake_by_ref`
///
/// This function will be called when `wake_by_ref` is called on the [`Waker`].
/// It must wake up the task associated with this [`RawWaker`].
///
/// This function is similar to `wake`, but must not consume the provided data
/// pointer.
///
/// # `drop`
///
/// This function gets called when a [`RawWaker`] gets dropped.
///
/// The implementation of this function must make sure to release any
/// resources that are associated with this instance of a [`RawWaker`] and
/// associated task.
#[rustc_promotable]
#[stable(feature = "futures_api", since = "1.36.0")]
#[rustc_const_stable(feature = "futures_api", since = "1.36.0")]
pub const fn new(
clone: unsafe fn(*const ()) -> RawWaker,
wake: unsafe fn(*const ()),
wake_by_ref: unsafe fn(*const ()),
drop: unsafe fn(*const ()),
) -> Self {
Self { clone, wake, wake_by_ref, drop }
}
}
/// The `Context` of an asynchronous task.
///
/// Currently, `Context` only serves to provide access to a `&Waker`
/// which can be used to wake the current task.
#[stable(feature = "futures_api", since = "1.36.0")]
pub struct Context<'a> {
waker: &'a Waker,
// Ensure we future-proof against variance changes by forcing
// the lifetime to be invariant (argument-position lifetimes
// are contravariant while return-position lifetimes are
// covariant).
_marker: PhantomData<fn(&'a ()) -> &'a ()>,
}
impl<'a> Context<'a> {
/// Create a new `Context` from a `&Waker`.
#[stable(feature = "futures_api", since = "1.36.0")]
#[must_use]
#[inline]
pub fn from_waker(waker: &'a Waker) -> Self {
Context { waker, _marker: PhantomData }
}
/// Returns a reference to the `Waker` for the current task.
#[stable(feature = "futures_api", since = "1.36.0")]
#[must_use]
#[inline]
pub fn waker(&self) -> &'a Waker {
&self.waker
}
}
#[stable(feature = "futures_api", since = "1.36.0")]
impl fmt::Debug for Context<'_> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Context").field("waker", &self.waker).finish()
}
}
/// A `Waker` is a handle for waking up a task by notifying its executor that it
/// is ready to be run.
///
/// This handle encapsulates a [`RawWaker`] instance, which defines the
/// executor-specific wakeup behavior.
///
/// Implements [`Clone`], [`Send`], and [`Sync`].
#[repr(transparent)]
#[stable(feature = "futures_api", since = "1.36.0")]
pub struct Waker {
waker: RawWaker,
}
#[stable(feature = "futures_api", since = "1.36.0")]
impl Unpin for Waker {}
#[stable(feature = "futures_api", since = "1.36.0")]
unsafe impl Send for Waker {}
#[stable(feature = "futures_api", since = "1.36.0")]
unsafe impl Sync for Waker {}
impl Waker {
/// Wake up the task associated with this `Waker`.
///
/// As long as the runtime keeps running and the task is not finished, it is
/// guaranteed that each invocation of `wake` (or `wake_by_ref`) will be followed
/// by at least one `poll` of the task to which this `Waker` belongs. This makes
/// it possible to temporarily yield to other tasks while running potentially
/// unbounded processing loops.
///
/// Note that the above implies that multiple wake-ups may be coalesced into a
/// single `poll` invocation by the runtime.
///
/// Also note that yielding to competing tasks is not guaranteed: it is the
/// executor’s choice which task to run and the executor may choose to run the
/// current task again.
#[inline]
#[stable(feature = "futures_api", since = "1.36.0")]
pub fn wake(self) {
// The actual wakeup call is delegated through a virtual function call
// to the implementation which is defined by the executor.
let wake = self.waker.vtable.wake;
let data = self.waker.data;
// Don't call `drop` -- the waker will be consumed by `wake`.
crate::mem::forget(self);
// SAFETY: This is safe because `Waker::from_raw` is the only way
// to initialize `wake` and `data` requiring the user to acknowledge
// that the contract of `RawWaker` is upheld.
unsafe { (wake)(data) };
}
/// Wake up the task associated with this `Waker` without consuming the `Waker`.
///
/// This is similar to `wake`, but may be slightly less efficient in the case
/// where an owned `Waker` is available. This method should be preferred to
/// calling `waker.clone().wake()`.
#[inline]
#[stable(feature = "futures_api", since = "1.36.0")]
pub fn wake_by_ref(&self) {
// The actual wakeup call is delegated through a virtual function call
// to the implementation which is defined by the executor.
// SAFETY: see `wake`
unsafe { (self.waker.vtable.wake_by_ref)(self.waker.data) }
}
/// Returns `true` if this `Waker` and another `Waker` have awoken the same task.
///
/// This function works on a best-effort basis, and may return false even
/// when the `Waker`s would awaken the same task. However, if this function
/// returns `true`, it is guaranteed that the `Waker`s will awaken the same task.
///
/// This function is primarily used for optimization purposes.
#[inline]
#[must_use]
#[stable(feature = "futures_api", since = "1.36.0")]
pub fn will_wake(&self, other: &Waker) -> bool {
self.waker == other.waker
}
/// Creates a new `Waker` from [`RawWaker`].
///
/// The behavior of the returned `Waker` is undefined if the contract defined
/// in [`RawWaker`]'s and [`RawWakerVTable`]'s documentation is not upheld.
/// Therefore this method is unsafe.
#[inline]
#[must_use]
#[stable(feature = "futures_api", since = "1.36.0")]
pub unsafe fn from_raw(waker: RawWaker) -> Waker {
Waker { waker }
}
/// Get a reference to the underlying [`RawWaker`].
#[inline]
#[must_use]
#[unstable(feature = "waker_getters", issue = "87021")]
pub fn as_raw(&self) -> &RawWaker {
&self.waker
}
}
#[stable(feature = "futures_api", since = "1.36.0")]
impl Clone for Waker {
#[inline]
fn clone(&self) -> Self {
Waker {
// SAFETY: This is safe because `Waker::from_raw` is the only way
// to initialize `clone` and `data` requiring the user to acknowledge
// that the contract of [`RawWaker`] is upheld.
waker: unsafe { (self.waker.vtable.clone)(self.waker.data) },
}
}
}
#[stable(feature = "futures_api", since = "1.36.0")]
impl Drop for Waker {
#[inline]
fn drop(&mut self) {
// SAFETY: This is safe because `Waker::from_raw` is the only way
// to initialize `drop` and `data` requiring the user to acknowledge
// that the contract of `RawWaker` is upheld.
unsafe { (self.waker.vtable.drop)(self.waker.data) }
}
}
#[stable(feature = "futures_api", since = "1.36.0")]
impl fmt::Debug for Waker {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let vtable_ptr = self.waker.vtable as *const RawWakerVTable;
f.debug_struct("Waker")
.field("data", &self.waker.data)
.field("vtable", &vtable_ptr)
.finish()
}
}