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
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
// Copyright 2020 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

//! An event that can be signaled and waited on by multiple consumers.

use fuchsia_sync::Mutex;
use futures::future::{FusedFuture, Future};
use slab::Slab;
use std::fmt;
use std::pin::Pin;
use std::sync::Arc;
use std::task::{Context, Poll, Waker};

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

/// An `Event` is a clonable object that can be signaled once. Calls to `.wait()` produce a future,
/// `EventWait`, that can wait on that signal. Once the `Event` has been signaled, all futures will
/// complete immediately.
#[derive(Clone)]
pub struct Event {
    inner: Arc<EventSignaler>,
}

impl Event {
    /// Create a new `Event` that has not yet been signaled.
    pub fn new() -> Self {
        Self {
            inner: Arc::new(EventSignaler {
                inner: Arc::new(Mutex::new(EventState {
                    state: State::Waiting,
                    wakers: Slab::new(),
                })),
            }),
        }
    }

    /// Signal the `Event`. Once this is done, it cannot be undone. Any tasks waiting on this
    /// `Event` will be notified and its `Future` implementation will complete.
    ///
    /// Returns true if this `Event` was the one that performed the signal operation.
    pub fn signal(&self) -> bool {
        self.inner.set(State::Signaled)
    }

    /// Return true if `Event::signal` has already been called.
    pub fn signaled(&self) -> bool {
        self.inner.inner.lock().state == State::Signaled
    }

    /// Create a new `EventWait` future that will complete after this event has been signaled.
    /// If all signalers are dropped, this future will continue to return `Poll::Pending`. To be
    /// notified when all signalers are dropped without signaling, use `wait_or_dropped`.
    pub fn wait(&self) -> EventWait {
        EventWait { inner: self.wait_or_dropped() }
    }

    /// Create a new `EventWaitResult` future that will complete after this event has been
    /// signaled or all `Event` clones have been dropped.
    ///
    /// This future will output a `Result<(), Dropped>` to indicate what has occurred.
    pub fn wait_or_dropped(&self) -> EventWaitResult {
        EventWaitResult {
            inner: (*self.inner).inner.clone(),
            waker_key: NULL_WAKER_KEY,
            terminated: false,
        }
    }
}

impl fmt::Debug for Event {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Event {{ state: {:?} }}", self.inner.inner.lock().state)
    }
}

/// `Event` state tracking enum.
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
enum State {
    /// The `Event` has not yet been signaled. This is the initial state of an `Event`.
    Waiting,
    /// The `signal` method has been called on an `Event`.
    Signaled,
    /// All clones of an `Event` have been dropped without the `signal` method being called. An
    /// `Event` can never move out of the `Dropped` state.
    Dropped,
}

/// Tracks state shared by all Event clones and futures.
struct EventState {
    pub state: State,
    pub wakers: Slab<Waker>,
}

/// A handle shared between all `Event` structs for a given event. Once all `Event`s are dropped,
/// this will be dropped and will notify the `EventState` that it is unreachable by any signalers
/// and will never be signaled if it hasn't been already.
struct EventSignaler {
    inner: Arc<Mutex<EventState>>,
}

impl EventSignaler {
    /// Internal function to set the self.inner.state value if it has not already been set to
    /// `State::Signaled`. Returns true if this function call changed the value of self.inner.state.
    fn set(&self, state: State) -> bool {
        assert!(state != State::Waiting, "Cannot reset the state to Waiting");
        let mut guard = self.inner.lock();
        if let State::Signaled = guard.state {
            // Avoid double panicking.
            if !std::thread::panicking() {
                assert!(
                    guard.wakers.is_empty(),
                    "If there are wakers, a race condition is present"
                );
            }
            false
        } else {
            let mut wakers = std::mem::replace(&mut guard.wakers, Slab::new());
            guard.state = state;
            drop(guard);
            for waker in wakers.drain() {
                waker.wake();
            }
            true
        }
    }
}

impl Drop for EventSignaler {
    fn drop(&mut self) {
        // Indicate that all `Event` clones have been dropped. This does not set the value if it
        // has already been set to `State::Signaled`.
        let _: bool = self.set(State::Dropped);
    }
}

/// Future implementation for `Event::wait_or_dropped`.
#[must_use = "futures do nothing unless polled"]
pub struct EventWaitResult {
    inner: Arc<Mutex<EventState>>,
    waker_key: usize,
    terminated: bool,
}

impl Future for EventWaitResult {
    type Output = Result<(), Dropped>;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        // `this: &mut Self` allows the compiler to track access to individual fields of Self as
        // distinct borrows.
        let this = self.get_mut();
        let mut guard = this.inner.lock();

        match guard.state {
            State::Waiting => {
                let mut new_key = None;
                if this.waker_key == NULL_WAKER_KEY || !guard.wakers.contains(this.waker_key) {
                    new_key = Some(guard.wakers.insert(cx.waker().clone()));
                } else {
                    guard.wakers[this.waker_key] = cx.waker().clone();
                }

                if let Some(key) = new_key {
                    this.waker_key = key;
                }

                Poll::Pending
            }
            State::Signaled => {
                this.terminated = true;
                this.waker_key = NULL_WAKER_KEY;
                Poll::Ready(Ok(()))
            }
            State::Dropped => {
                this.terminated = true;
                this.waker_key = NULL_WAKER_KEY;
                Poll::Ready(Err(Dropped))
            }
        }
    }
}

impl FusedFuture for EventWaitResult {
    fn is_terminated(&self) -> bool {
        self.terminated
    }
}

impl Unpin for EventWaitResult {}

impl Drop for EventWaitResult {
    fn drop(&mut self) {
        if self.waker_key != NULL_WAKER_KEY {
            // Cleanup the EventWaitResult's waker one is present in the wakers slab.
            let mut guard = self.inner.lock();
            if guard.wakers.contains(self.waker_key) {
                let _ = guard.wakers.remove(self.waker_key);
            }
        }
    }
}

/// Future implementation for `Event::wait`. This future only completes when the event is signaled.
/// If all signalers are dropped, `EventWait` continues to return `Poll::Pending`.
#[must_use = "futures do nothing unless polled"]
pub struct EventWait {
    inner: EventWaitResult,
}

impl Future for EventWait {
    type Output = ();

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        match Pin::new(&mut self.inner).poll(cx) {
            Poll::Ready(Ok(())) => Poll::Ready(()),
            _ => Poll::Pending,
        }
    }
}

impl FusedFuture for EventWait {
    fn is_terminated(&self) -> bool {
        self.inner.is_terminated()
    }
}

impl Unpin for EventWait {}

/// Error returned from an `EventWait` when the Event is dropped.
#[derive(Debug, Eq, PartialEq, Clone, Copy)]
pub struct Dropped;

impl fmt::Display for Dropped {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "event dropped")
    }
}

impl std::error::Error for Dropped {}

#[cfg(test)]
mod tests {
    use super::*;
    use fuchsia_async as fasync;

    // TODO: Add tests to check waker count in EventWait and EventWaitResult.

    #[test]
    fn signaled_method_respects_signaling() {
        let event = Event::new();
        let event_clone = event.clone();

        assert!(!event.signaled());
        assert!(!event_clone.signaled());

        assert!(event.signal());

        assert!(event.signaled());
        assert!(event_clone.signaled());
    }

    #[test]
    fn unsignaled_event_is_pending() {
        let mut ex = fasync::TestExecutor::new();

        let event = Event::new();
        let mut wait = event.wait();
        let mut wait_or_dropped = event.wait_or_dropped();
        assert!(ex.run_until_stalled(&mut wait).is_pending());
        assert!(ex.run_until_stalled(&mut wait_or_dropped).is_pending());
    }

    #[test]
    fn signaled_event_is_ready() {
        let mut ex = fasync::TestExecutor::new();

        let event = Event::new();
        let mut wait = event.wait();
        let mut wait_or_dropped = event.wait_or_dropped();
        assert!(event.signal());
        assert!(ex.run_until_stalled(&mut wait).is_ready());
        assert!(ex.run_until_stalled(&mut wait_or_dropped).is_ready());
    }

    #[test]
    fn event_is_ready_and_wakes_after_stalled() {
        let mut ex = fasync::TestExecutor::new();

        let event = Event::new();
        let mut wait = event.wait();
        let mut wait_or_dropped = event.wait_or_dropped();
        assert!(ex.run_until_stalled(&mut wait).is_pending());
        assert!(ex.run_until_stalled(&mut wait_or_dropped).is_pending());
        assert!(event.signal());
        assert!(ex.run_until_stalled(&mut wait).is_ready());
        assert!(ex.run_until_stalled(&mut wait_or_dropped).is_ready());
    }

    #[test]
    fn signaling_event_registers_and_wakes_multiple_waiters_properly() {
        let mut ex = fasync::TestExecutor::new();

        let event = Event::new();
        let mut wait_1 = event.wait();
        let mut wait_2 = event.wait();
        let mut wait_3 = event.wait();

        // Multiple waiters events are pending correctly.
        assert!(ex.run_until_stalled(&mut wait_1).is_pending());
        assert!(ex.run_until_stalled(&mut wait_2).is_pending());

        assert!(event.signal());

        // Both previously registered and unregistered event waiters complete correctly.
        assert!(ex.run_until_stalled(&mut wait_1).is_ready());
        assert!(ex.run_until_stalled(&mut wait_2).is_ready());
        assert!(ex.run_until_stalled(&mut wait_3).is_ready());
    }

    #[test]
    fn event_is_terminated_after_complete() {
        let mut ex = fasync::TestExecutor::new();

        let event = Event::new();
        let mut wait = event.wait();
        let mut wait_or_dropped = event.wait_or_dropped();
        assert!(ex.run_until_stalled(&mut wait).is_pending());
        assert!(ex.run_until_stalled(&mut wait_or_dropped).is_pending());
        assert!(!wait.is_terminated());
        assert!(!wait_or_dropped.is_terminated());
        assert!(event.signal());
        assert!(ex.run_until_stalled(&mut wait).is_ready());
        assert!(ex.run_until_stalled(&mut wait_or_dropped).is_ready());
        assert!(wait.is_terminated());
        assert!(wait_or_dropped.is_terminated());
    }

    #[test]
    fn waiter_drops_gracefully() {
        let mut ex = fasync::TestExecutor::new();

        let event = Event::new();
        let mut wait = event.wait();
        let mut wait_or_dropped = event.wait();
        assert!(ex.run_until_stalled(&mut wait).is_pending());
        assert!(ex.run_until_stalled(&mut wait_or_dropped).is_pending());
        assert!(!wait.is_terminated());
        assert!(!wait_or_dropped.is_terminated());
        drop(wait);
        drop(wait_or_dropped);
        assert!(event.signal());
    }

    #[test]
    fn waiter_completes_after_all_events_drop() {
        let mut ex = fasync::TestExecutor::new();

        let event = Event::new();
        let event_clone = Event::new();
        let mut wait = event.wait();
        let mut wait_or_dropped = event.wait_or_dropped();
        assert!(ex.run_until_stalled(&mut wait).is_pending());
        assert!(ex.run_until_stalled(&mut wait_or_dropped).is_pending());
        assert!(!wait.is_terminated());
        assert!(!wait_or_dropped.is_terminated());
        drop(event);
        drop(event_clone);
        assert!(ex.run_until_stalled(&mut wait).is_pending());
        assert!(ex.run_until_stalled(&mut wait_or_dropped).is_ready());
    }

    #[test]
    fn drop_receiver_after_poll_without_event_signal() {
        let mut exec = fasync::TestExecutor::new();
        let event = Event::new();
        let mut waiter = event.wait_or_dropped();
        assert!(exec.run_until_stalled(&mut waiter).is_pending());
        drop(event);
        drop(waiter);
    }

    #[test]
    fn drop_receiver_after_event_signal_without_repoll() {
        let mut exec = fasync::TestExecutor::new();
        let event = Event::new();
        let mut waiter = event.wait_or_dropped();
        assert_eq!(event.inner.inner.lock().wakers.len(), 0);

        // Polling the waiter will register a new waker.
        assert!(exec.run_until_stalled(&mut waiter).is_pending());
        assert_eq!(event.inner.inner.lock().wakers.len(), 1);

        // The waiter's waker is used.
        assert!(event.signal());
        assert_eq!(event.inner.inner.lock().wakers.len(), 0);

        // Dropping a waiter without polling it is valid.
        drop(waiter);
    }
}