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
277
278
279
use futures_core::future::Future;
use futures_core::task::{LocalWaker, Poll, Wake, Waker};
use slab::Slab;
use std::fmt;
use std::cell::UnsafeCell;
use std::marker::Unpin;
use std::pin::Pin;
use std::sync::{Arc, Mutex};
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::SeqCst;
use std::task::local_waker_from_nonlocal;

/// A future that is cloneable and can be polled in multiple threads.
/// Use `Future::shared()` method to convert any future into a `Shared` future.
#[must_use = "futures do nothing unless polled"]
pub struct Shared<Fut: Future> {
    inner: Arc<Inner<Fut>>,
    waker_key: usize,
}

struct Inner<Fut: Future> {
    future_or_output: UnsafeCell<FutureOrOutput<Fut>>,
    notifier: Arc<Notifier>,
}

struct Notifier {
    state: AtomicUsize,
    wakers: Mutex<Option<Slab<Option<Waker>>>>,
}

// The future itself is polled behind the `Arc`, so it won't be moved
// when `Shared` is moved.
impl<Fut: Future> Unpin for Shared<Fut> {}

impl<Fut: Future> fmt::Debug for Shared<Fut> {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        fmt.debug_struct("Shared")
            .field("inner", &self.inner)
            .field("waker_key", &self.waker_key)
            .finish()
    }
}

impl<Fut: Future> fmt::Debug for Inner<Fut> {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        fmt.debug_struct("Inner")
            .finish()
    }
}

enum FutureOrOutput<Fut: Future> {
    Future(Fut),
    Output(Fut::Output),
}

unsafe impl<Fut> Send for Inner<Fut>
where
    Fut: Future + Send,
    Fut::Output: Send + Sync,
{}

unsafe impl<Fut> Sync for Inner<Fut>
where
    Fut: Future + Send,
    Fut::Output: Send + Sync,
{}

const IDLE: usize = 0;
const POLLING: usize = 1;
const REPOLL: usize = 2;
const COMPLETE: usize = 3;
const POISONED: usize = 4;

const NULL_WAKER_KEY: usize = usize::max_value();

impl<Fut: Future> Shared<Fut> {
    pub(super) fn new(future: Fut) -> Shared<Fut> {
        Shared {
            inner: Arc::new(Inner {
                future_or_output: UnsafeCell::new(FutureOrOutput::Future(future)),
                notifier: Arc::new(Notifier {
                    state: AtomicUsize::new(IDLE),
                    wakers: Mutex::new(Some(Slab::new())),
                }),
            }),
            waker_key: NULL_WAKER_KEY,
        }
    }
}

impl<Fut> Shared<Fut>
where
    Fut: Future,
    Fut::Output: Clone,
{
    /// If any clone of this `Shared` has completed execution, returns its result immediately
    /// without blocking. Otherwise, returns None without triggering the work represented by
    /// this `Shared`.
    pub fn peek(&self) -> Option<Fut::Output> {
        match self.inner.notifier.state.load(SeqCst) {
            COMPLETE => Some(unsafe { self.clone_output() }),
            POISONED => panic!("inner future panicked during poll"),
            _ => None,
        }
    }

    /// Registers the current task to receive a wakeup when `Inner` is awoken.
    fn set_waker(&mut self, lw: &LocalWaker) {
        // Acquire the lock first before checking COMPLETE to ensure there
        // isn't a race.
        let mut wakers = self.inner.notifier.wakers.lock().unwrap();
        let wakers = if let Some(wakers) = wakers.as_mut() {
            wakers
        } else {
            // The value is already available, so there's no need to set the waker.
            return
        };
        if self.waker_key == NULL_WAKER_KEY {
            self.waker_key = wakers.insert(Some(lw.clone().into_waker()));
        } else {
            let waker_slot = &mut wakers[self.waker_key];
            let needs_replacement = if let Some(old_waker) = waker_slot {
                // If there's still an unwoken waker in the slot, only replace
                // if the current one wouldn't wake the same task.
                !lw.will_wake_nonlocal(old_waker)
            } else {
                true
            };
            if needs_replacement {
                *waker_slot = Some(lw.clone().into_waker());
            }
        }
        debug_assert!(self.waker_key != NULL_WAKER_KEY);
    }

    /// Safety: callers must first ensure that `self.inner.state`
    /// is `COMPLETE`
    unsafe fn clone_output(&self) -> Fut::Output {
        if let FutureOrOutput::Output(item) = &*self.inner.future_or_output.get() {
            item.clone()
        } else {
            unreachable!()
        }
    }
}

impl<Fut: Future> Future for Shared<Fut>
where
    Fut::Output: Clone,
{
    type Output = Fut::Output;

    fn poll(mut self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<Self::Output> {
        let this = &mut *self;

        this.set_waker(lw);

        match this.inner.notifier.state.compare_and_swap(IDLE, POLLING, SeqCst) {
            IDLE => {
                // Lock acquired, fall through
            }
            POLLING | REPOLL => {
                // Another task is currently polling, at this point we just want
                // to ensure that our task handle is currently registered

                return Poll::Pending
            }
            COMPLETE => {
                return unsafe { Poll::Ready(this.clone_output()) };
            }
            POISONED => panic!("inner future panicked during poll"),
            _ => unreachable!(),
        }

        let waker = local_waker_from_nonlocal(this.inner.notifier.clone());
        let lw = &waker;

        loop {
            struct Reset<'a>(&'a AtomicUsize);

            impl<'a> Drop for Reset<'a> {
                fn drop(&mut self) {
                    use std::thread;

                    if thread::panicking() {
                        self.0.store(POISONED, SeqCst);
                    }
                }
            }

            let _reset = Reset(&this.inner.notifier.state);

            // Poll the future
            let res = unsafe {
                if let FutureOrOutput::Future(future) = &mut *this.inner.future_or_output.get() {
                    Pin::new_unchecked(future).poll(lw)
                } else {
                    unreachable!()
                }
            };
            match res {
                Poll::Pending => {
                    // Not ready, try to release the handle
                    match this.inner.notifier.state.compare_and_swap(POLLING, IDLE, SeqCst) {
                        POLLING => {
                            // Success
                            return Poll::Pending;
                        }
                        REPOLL => {
                            // Gotta poll again!
                            let prev = this.inner.notifier.state.swap(POLLING, SeqCst);
                            assert_eq!(prev, REPOLL);
                        }
                        _ => unreachable!(),
                    }
                }
                Poll::Ready(output) => {
                    unsafe {
                        *this.inner.future_or_output.get() =
                            FutureOrOutput::Output(output.clone());
                    }

                    // Complete the future
                    let mut lock = this.inner.notifier.wakers.lock().unwrap();
                    this.inner.notifier.state.store(COMPLETE, SeqCst);
                    let wakers = &mut lock.take().unwrap();
                    for (_key, opt_waker) in wakers {
                        if let Some(waker) = opt_waker.take() {
                            waker.wake();
                        }
                    }
                    return Poll::Ready(output);
                }
            }
        }
    }
}

impl<Fut> Clone for Shared<Fut>
where
    Fut: Future,
{
    fn clone(&self) -> Self {
        Shared {
            inner: self.inner.clone(),
            waker_key: NULL_WAKER_KEY,
        }
    }
}

impl<Fut> Drop for Shared<Fut>
where
    Fut: Future,
{
    fn drop(&mut self) {
        if self.waker_key != NULL_WAKER_KEY {
            if let Ok(mut wakers) = self.inner.notifier.wakers.lock() {
                if let Some(wakers) = wakers.as_mut() {
                    wakers.remove(self.waker_key);
                }
            }
        }
    }
}

impl Wake for Notifier {
    fn wake(arc_self: &Arc<Self>) {
        arc_self.state.compare_and_swap(POLLING, REPOLL, SeqCst);

        let wakers = &mut *arc_self.wakers.lock().unwrap();
        if let Some(wakers) = wakers.as_mut() {
            for (_key, opt_waker) in wakers {
                if let Some(waker) = opt_waker.take() {
                    waker.wake();
                }
            }
        }
    }
}