1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

#![unstable(feature = "futures_api",
            reason = "futures in libcore are unstable",
            issue = "50547")]

use fmt;
use marker::Unpin;
use ptr::NonNull;

/// A `Waker` is a handle for waking up a task by notifying its executor that it
/// is ready to be run.
///
/// This handle contains a trait object pointing to an instance of the `UnsafeWake`
/// trait, allowing notifications to get routed through it.
#[repr(transparent)]
pub struct Waker {
    inner: NonNull<UnsafeWake>,
}

impl Unpin for Waker {}
unsafe impl Send for Waker {}
unsafe impl Sync for Waker {}

impl Waker {
    /// Constructs a new `Waker` directly.
    ///
    /// Note that most code will not need to call this. Implementers of the
    /// `UnsafeWake` trait will typically provide a wrapper that calls this
    /// but you otherwise shouldn't call it directly.
    ///
    /// If you're working with the standard library then it's recommended to
    /// use the `Waker::from` function instead which works with the safe
    /// `Arc` type and the safe `Wake` trait.
    #[inline]
    pub unsafe fn new(inner: NonNull<UnsafeWake>) -> Self {
        Waker { inner: inner }
    }

    /// Wake up the task associated with this `Waker`.
    #[inline]
    pub fn wake(&self) {
        unsafe { self.inner.as_ref().wake() }
    }

    /// Returns whether or not this `Waker` and `other` awaken 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]
    pub fn will_wake(&self, other: &Waker) -> bool {
        self.inner == other.inner
    }
}

impl Clone for Waker {
    #[inline]
    fn clone(&self) -> Self {
        unsafe {
            self.inner.as_ref().clone_raw()
        }
    }
}

impl fmt::Debug for Waker {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("Waker")
            .finish()
    }
}

impl Drop for Waker {
    #[inline]
    fn drop(&mut self) {
        unsafe {
            self.inner.as_ref().drop_raw()
        }
    }
}

/// A `LocalWaker` is a handle for waking up a task by notifying its executor that it
/// is ready to be run.
///
/// This is similar to the `Waker` type, but cannot be sent across threads.
/// Task executors can use this type to implement more optimized singlethreaded wakeup
/// behavior.
#[repr(transparent)]
pub struct LocalWaker {
    inner: NonNull<UnsafeWake>,
}

impl Unpin for LocalWaker {}
impl !Send for LocalWaker {}
impl !Sync for LocalWaker {}

impl LocalWaker {
    /// Constructs a new `LocalWaker` directly.
    ///
    /// Note that most code will not need to call this. Implementers of the
    /// `UnsafeWake` trait will typically provide a wrapper that calls this
    /// but you otherwise shouldn't call it directly.
    ///
    /// If you're working with the standard library then it's recommended to
    /// use the `local_waker_from_nonlocal` or `local_waker` to convert a `Waker`
    /// into a `LocalWaker`.
    ///
    /// For this function to be used safely, it must be sound to call `inner.wake_local()`
    /// on the current thread.
    #[inline]
    pub unsafe fn new(inner: NonNull<UnsafeWake>) -> Self {
        LocalWaker { inner: inner }
    }

    /// Wake up the task associated with this `LocalWaker`.
    #[inline]
    pub fn wake(&self) {
        unsafe { self.inner.as_ref().wake_local() }
    }

    /// Returns whether or not this `LocalWaker` and `other` `LocalWaker` awaken the same task.
    ///
    /// This function works on a best-effort basis, and may return false even
    /// when the `LocalWaker`s would awaken the same task. However, if this function
    /// returns true, it is guaranteed that the `LocalWaker`s will awaken the same
    /// task.
    ///
    /// This function is primarily used for optimization purposes.
    #[inline]
    pub fn will_wake(&self, other: &LocalWaker) -> bool {
        self.inner == other.inner
    }

    /// Returns whether or not this `LocalWaker` and `other` `Waker` awaken 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 `LocalWaker`s will awaken the same
    /// task.
    ///
    /// This function is primarily used for optimization purposes.
    #[inline]
    pub fn will_wake_nonlocal(&self, other: &Waker) -> bool {
        self.inner == other.inner
    }
}

impl From<LocalWaker> for Waker {
    #[inline]
    fn from(local_waker: LocalWaker) -> Self {
        Waker { inner: local_waker.inner }
    }
}

impl Clone for LocalWaker {
    #[inline]
    fn clone(&self) -> Self {
        unsafe {
            LocalWaker { inner: self.inner.as_ref().clone_raw().inner }
        }
    }
}

impl fmt::Debug for LocalWaker {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("Waker")
            .finish()
    }
}

impl Drop for LocalWaker {
    #[inline]
    fn drop(&mut self) {
        unsafe {
            self.inner.as_ref().drop_raw()
        }
    }
}

/// An unsafe trait for implementing custom memory management for a `Waker` or `LocalWaker`.
///
/// A `Waker` conceptually is a cloneable trait object for `Wake`, and is
/// most often essentially just `Arc<dyn Wake>`. However, in some contexts
/// (particularly `no_std`), it's desirable to avoid `Arc` in favor of some
/// custom memory management strategy. This trait is designed to allow for such
/// customization.
///
/// When using `std`, a default implementation of the `UnsafeWake` trait is provided for
/// `Arc<T>` where `T: Wake`.
pub unsafe trait UnsafeWake: Send + Sync {
    /// Creates a clone of this `UnsafeWake` and stores it behind a `Waker`.
    ///
    /// This function will create a new uniquely owned handle that under the
    /// hood references the same notification instance. In other words calls
    /// to `wake` on the returned handle should be equivalent to calls to
    /// `wake` on this handle.
    ///
    /// # Unsafety
    ///
    /// This function is unsafe to call because it's asserting the `UnsafeWake`
    /// value is in a consistent state, i.e. hasn't been dropped.
    unsafe fn clone_raw(&self) -> Waker;

    /// Drops this instance of `UnsafeWake`, deallocating resources
    /// associated with it.
    ///
    /// FIXME(cramertj)
    /// This method is intended to have a signature such as:
    ///
    /// ```ignore (not-a-doctest)
    /// fn drop_raw(self: *mut Self);
    /// ```
    ///
    /// Unfortunately in Rust today that signature is not object safe.
    /// Nevertheless it's recommended to implement this function *as if* that
    /// were its signature. As such it is not safe to call on an invalid
    /// pointer, nor is the validity of the pointer guaranteed after this
    /// function returns.
    ///
    /// # Unsafety
    ///
    /// This function is unsafe to call because it's asserting the `UnsafeWake`
    /// value is in a consistent state, i.e. hasn't been dropped.
    unsafe fn drop_raw(&self);

    /// Indicates that the associated task is ready to make progress and should
    /// be `poll`ed.
    ///
    /// Executors generally maintain a queue of "ready" tasks; `wake` should place
    /// the associated task onto this queue.
    ///
    /// # Panics
    ///
    /// Implementations should avoid panicking, but clients should also be prepared
    /// for panics.
    ///
    /// # Unsafety
    ///
    /// This function is unsafe to call because it's asserting the `UnsafeWake`
    /// value is in a consistent state, i.e. hasn't been dropped.
    unsafe fn wake(&self);

    /// Indicates that the associated task is ready to make progress and should
    /// be `poll`ed. This function is the same as `wake`, but can only be called
    /// from the thread that this `UnsafeWake` is "local" to. This allows for
    /// implementors to provide specialized wakeup behavior specific to the current
    /// thread. This function is called by `LocalWaker::wake`.
    ///
    /// Executors generally maintain a queue of "ready" tasks; `wake_local` should place
    /// the associated task onto this queue.
    ///
    /// # Panics
    ///
    /// Implementations should avoid panicking, but clients should also be prepared
    /// for panics.
    ///
    /// # Unsafety
    ///
    /// This function is unsafe to call because it's asserting the `UnsafeWake`
    /// value is in a consistent state, i.e. hasn't been dropped, and that the
    /// `UnsafeWake` hasn't moved from the thread on which it was created.
    unsafe fn wake_local(&self) {
        self.wake()
    }
}