1 #![stable(feature = "futures_api", since = "1.36.0")]
4 use crate::marker::{PhantomData, Unpin};
6 /// A `RawWaker` allows the implementor of a task executor to create a [`Waker`]
7 /// which provides customized wakeup behavior.
9 /// [vtable]: https://en.wikipedia.org/wiki/Virtual_method_table
11 /// It consists of a data pointer and a [virtual function pointer table (vtable)][vtable]
12 /// that customizes the behavior of the `RawWaker`.
13 #[derive(PartialEq, Debug)]
14 #[stable(feature = "futures_api", since = "1.36.0")]
16 /// A data pointer, which can be used to store arbitrary data as required
17 /// by the executor. This could be e.g. a type-erased pointer to an `Arc`
18 /// that is associated with the task.
19 /// The value of this field gets passed to all functions that are part of
20 /// the vtable as the first parameter.
22 /// Virtual function pointer table that customizes the behavior of this waker.
23 vtable: &'static RawWakerVTable,
27 /// Creates a new `RawWaker` from the provided `data` pointer and `vtable`.
29 /// The `data` pointer can be used to store arbitrary data as required
30 /// by the executor. This could be e.g. a type-erased pointer to an `Arc`
31 /// that is associated with the task.
32 /// The value of this pointer will get passed to all functions that are part
33 /// of the `vtable` as the first parameter.
35 /// The `vtable` customizes the behavior of a `Waker` which gets created
36 /// from a `RawWaker`. For each operation on the `Waker`, the associated
37 /// function in the `vtable` of the underlying `RawWaker` will be called.
40 #[stable(feature = "futures_api", since = "1.36.0")]
41 #[rustc_const_stable(feature = "futures_api", since = "1.36.0")]
42 pub const fn new(data: *const (), vtable: &'static RawWakerVTable) -> RawWaker {
43 RawWaker { data, vtable }
47 /// A virtual function pointer table (vtable) that specifies the behavior
48 /// of a [`RawWaker`].
50 /// The pointer passed to all functions inside the vtable is the `data` pointer
51 /// from the enclosing [`RawWaker`] object.
53 /// The functions inside this struct are only intended to be called on the `data`
54 /// pointer of a properly constructed [`RawWaker`] object from inside the
55 /// [`RawWaker`] implementation. Calling one of the contained functions using
56 /// any other `data` pointer will cause undefined behavior.
57 #[stable(feature = "futures_api", since = "1.36.0")]
58 #[derive(PartialEq, Copy, Clone, Debug)]
59 pub struct RawWakerVTable {
60 /// This function will be called when the [`RawWaker`] gets cloned, e.g. when
61 /// the [`Waker`] in which the [`RawWaker`] is stored gets cloned.
63 /// The implementation of this function must retain all resources that are
64 /// required for this additional instance of a [`RawWaker`] and associated
65 /// task. Calling `wake` on the resulting [`RawWaker`] should result in a wakeup
66 /// of the same task that would have been awoken by the original [`RawWaker`].
67 clone: unsafe fn(*const ()) -> RawWaker,
69 /// This function will be called when `wake` is called on the [`Waker`].
70 /// It must wake up the task associated with this [`RawWaker`].
72 /// The implementation of this function must make sure to release any
73 /// resources that are associated with this instance of a [`RawWaker`] and
75 wake: unsafe fn(*const ()),
77 /// This function will be called when `wake_by_ref` is called on the [`Waker`].
78 /// It must wake up the task associated with this [`RawWaker`].
80 /// This function is similar to `wake`, but must not consume the provided data
82 wake_by_ref: unsafe fn(*const ()),
84 /// This function gets called when a [`RawWaker`] gets dropped.
86 /// The implementation of this function must make sure to release any
87 /// resources that are associated with this instance of a [`RawWaker`] and
89 drop: unsafe fn(*const ()),
93 /// Creates a new `RawWakerVTable` from the provided `clone`, `wake`,
94 /// `wake_by_ref`, and `drop` functions.
98 /// This function will be called when the [`RawWaker`] gets cloned, e.g. when
99 /// the [`Waker`] in which the [`RawWaker`] is stored gets cloned.
101 /// The implementation of this function must retain all resources that are
102 /// required for this additional instance of a [`RawWaker`] and associated
103 /// task. Calling `wake` on the resulting [`RawWaker`] should result in a wakeup
104 /// of the same task that would have been awoken by the original [`RawWaker`].
108 /// This function will be called when `wake` is called on the [`Waker`].
109 /// It must wake up the task associated with this [`RawWaker`].
111 /// The implementation of this function must make sure to release any
112 /// resources that are associated with this instance of a [`RawWaker`] and
117 /// This function will be called when `wake_by_ref` is called on the [`Waker`].
118 /// It must wake up the task associated with this [`RawWaker`].
120 /// This function is similar to `wake`, but must not consume the provided data
125 /// This function gets called when a [`RawWaker`] gets dropped.
127 /// The implementation of this function must make sure to release any
128 /// resources that are associated with this instance of a [`RawWaker`] and
131 #[stable(feature = "futures_api", since = "1.36.0")]
132 // `rustc_allow_const_fn_ptr` is a hack that should not be used anywhere else
133 // without first consulting with T-Lang.
135 // FIXME: remove whenever we have a stable way to accept fn pointers from const fn
136 // (see https://github.com/rust-rfcs/const-eval/issues/19#issuecomment-472799062)
137 #[rustc_allow_const_fn_ptr]
138 #[rustc_const_stable(feature = "futures_api", since = "1.36.0")]
140 clone: unsafe fn(*const ()) -> RawWaker,
141 wake: unsafe fn(*const ()),
142 wake_by_ref: unsafe fn(*const ()),
143 drop: unsafe fn(*const ()),
145 Self { clone, wake, wake_by_ref, drop }
149 /// The `Context` of an asynchronous task.
151 /// Currently, `Context` only serves to provide access to a `&Waker`
152 /// which can be used to wake the current task.
153 #[stable(feature = "futures_api", since = "1.36.0")]
154 pub struct Context<'a> {
156 // Ensure we future-proof against variance changes by forcing
157 // the lifetime to be invariant (argument-position lifetimes
158 // are contravariant while return-position lifetimes are
160 _marker: PhantomData<fn(&'a ()) -> &'a ()>,
163 impl<'a> Context<'a> {
164 /// Create a new `Context` from a `&Waker`.
165 #[stable(feature = "futures_api", since = "1.36.0")]
167 pub fn from_waker(waker: &'a Waker) -> Self {
168 Context { waker, _marker: PhantomData }
171 /// Returns a reference to the `Waker` for the current task.
172 #[stable(feature = "futures_api", since = "1.36.0")]
174 pub fn waker(&self) -> &'a Waker {
179 #[stable(feature = "futures_api", since = "1.36.0")]
180 impl fmt::Debug for Context<'_> {
181 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
182 f.debug_struct("Context").field("waker", &self.waker).finish()
186 /// A `Waker` is a handle for waking up a task by notifying its executor that it
187 /// is ready to be run.
189 /// This handle encapsulates a [`RawWaker`] instance, which defines the
190 /// executor-specific wakeup behavior.
192 /// Implements [`Clone`], [`Send`], and [`Sync`].
194 #[stable(feature = "futures_api", since = "1.36.0")]
199 #[stable(feature = "futures_api", since = "1.36.0")]
200 impl Unpin for Waker {}
201 #[stable(feature = "futures_api", since = "1.36.0")]
202 unsafe impl Send for Waker {}
203 #[stable(feature = "futures_api", since = "1.36.0")]
204 unsafe impl Sync for Waker {}
207 /// Wake up the task associated with this `Waker`.
209 #[stable(feature = "futures_api", since = "1.36.0")]
211 // The actual wakeup call is delegated through a virtual function call
212 // to the implementation which is defined by the executor.
213 let wake = self.waker.vtable.wake;
214 let data = self.waker.data;
216 // Don't call `drop` -- the waker will be consumed by `wake`.
217 crate::mem::forget(self);
219 // SAFETY: This is safe because `Waker::from_raw` is the only way
220 // to initialize `wake` and `data` requiring the user to acknowledge
221 // that the contract of `RawWaker` is upheld.
222 unsafe { (wake)(data) };
225 /// Wake up the task associated with this `Waker` without consuming the `Waker`.
227 /// This is similar to `wake`, but may be slightly less efficient in the case
228 /// where an owned `Waker` is available. This method should be preferred to
229 /// calling `waker.clone().wake()`.
231 #[stable(feature = "futures_api", since = "1.36.0")]
232 pub fn wake_by_ref(&self) {
233 // The actual wakeup call is delegated through a virtual function call
234 // to the implementation which is defined by the executor.
236 // SAFETY: see `wake`
237 unsafe { (self.waker.vtable.wake_by_ref)(self.waker.data) }
240 /// Returns `true` if this `Waker` and another `Waker` have awoken the same task.
242 /// This function works on a best-effort basis, and may return false even
243 /// when the `Waker`s would awaken the same task. However, if this function
244 /// returns `true`, it is guaranteed that the `Waker`s will awaken the same task.
246 /// This function is primarily used for optimization purposes.
248 #[stable(feature = "futures_api", since = "1.36.0")]
249 pub fn will_wake(&self, other: &Waker) -> bool {
250 self.waker == other.waker
253 /// Creates a new `Waker` from [`RawWaker`].
255 /// The behavior of the returned `Waker` is undefined if the contract defined
256 /// in [`RawWaker`]'s and [`RawWakerVTable`]'s documentation is not upheld.
257 /// Therefore this method is unsafe.
259 #[stable(feature = "futures_api", since = "1.36.0")]
260 pub unsafe fn from_raw(waker: RawWaker) -> Waker {
265 #[stable(feature = "futures_api", since = "1.36.0")]
266 impl Clone for Waker {
268 fn clone(&self) -> Self {
270 // SAFETY: This is safe because `Waker::from_raw` is the only way
271 // to initialize `clone` and `data` requiring the user to acknowledge
272 // that the contract of [`RawWaker`] is upheld.
273 waker: unsafe { (self.waker.vtable.clone)(self.waker.data) },
278 #[stable(feature = "futures_api", since = "1.36.0")]
279 impl Drop for Waker {
282 // SAFETY: This is safe because `Waker::from_raw` is the only way
283 // to initialize `drop` and `data` requiring the user to acknowledge
284 // that the contract of `RawWaker` is upheld.
285 unsafe { (self.waker.vtable.drop)(self.waker.data) }
289 #[stable(feature = "futures_api", since = "1.36.0")]
290 impl fmt::Debug for Waker {
291 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
292 let vtable_ptr = self.waker.vtable as *const RawWakerVTable;
293 f.debug_struct("Waker")
294 .field("data", &self.waker.data)
295 .field("vtable", &vtable_ptr)