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
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
use crate::loom::cell::UnsafeCell;
use crate::loom::future::AtomicWaker;
use crate::loom::sync::atomic::AtomicUsize;
use crate::loom::sync::Arc;
use crate::runtime::park::CachedParkThread;
use crate::sync::mpsc::error::TryRecvError;
use crate::sync::mpsc::{bounded, list, unbounded};
use crate::sync::notify::Notify;

use std::fmt;
use std::process;
use std::sync::atomic::Ordering::{AcqRel, Acquire, Relaxed, Release};
use std::task::Poll::{Pending, Ready};
use std::task::{Context, Poll};

/// Channel sender.
pub(crate) struct Tx<T, S> {
    inner: Arc<Chan<T, S>>,
}

impl<T, S: fmt::Debug> fmt::Debug for Tx<T, S> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_struct("Tx").field("inner", &self.inner).finish()
    }
}

/// Channel receiver.
pub(crate) struct Rx<T, S: Semaphore> {
    inner: Arc<Chan<T, S>>,
}

impl<T, S: Semaphore + fmt::Debug> fmt::Debug for Rx<T, S> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_struct("Rx").field("inner", &self.inner).finish()
    }
}

pub(crate) trait Semaphore {
    fn is_idle(&self) -> bool;

    fn add_permit(&self);

    fn close(&self);

    fn is_closed(&self) -> bool;
}

pub(super) struct Chan<T, S> {
    /// Notifies all tasks listening for the receiver being dropped.
    notify_rx_closed: Notify,

    /// Handle to the push half of the lock-free list.
    tx: list::Tx<T>,

    /// Coordinates access to channel's capacity.
    semaphore: S,

    /// Receiver waker. Notified when a value is pushed into the channel.
    rx_waker: AtomicWaker,

    /// Tracks the number of outstanding sender handles.
    ///
    /// When this drops to zero, the send half of the channel is closed.
    tx_count: AtomicUsize,

    /// Only accessed by `Rx` handle.
    rx_fields: UnsafeCell<RxFields<T>>,
}

impl<T, S> fmt::Debug for Chan<T, S>
where
    S: fmt::Debug,
{
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_struct("Chan")
            .field("tx", &self.tx)
            .field("semaphore", &self.semaphore)
            .field("rx_waker", &self.rx_waker)
            .field("tx_count", &self.tx_count)
            .field("rx_fields", &"...")
            .finish()
    }
}

/// Fields only accessed by `Rx` handle.
struct RxFields<T> {
    /// Channel receiver. This field is only accessed by the `Receiver` type.
    list: list::Rx<T>,

    /// `true` if `Rx::close` is called.
    rx_closed: bool,
}

impl<T> fmt::Debug for RxFields<T> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_struct("RxFields")
            .field("list", &self.list)
            .field("rx_closed", &self.rx_closed)
            .finish()
    }
}

unsafe impl<T: Send, S: Send> Send for Chan<T, S> {}
unsafe impl<T: Send, S: Sync> Sync for Chan<T, S> {}

pub(crate) fn channel<T, S: Semaphore>(semaphore: S) -> (Tx<T, S>, Rx<T, S>) {
    let (tx, rx) = list::channel();

    let chan = Arc::new(Chan {
        notify_rx_closed: Notify::new(),
        tx,
        semaphore,
        rx_waker: AtomicWaker::new(),
        tx_count: AtomicUsize::new(1),
        rx_fields: UnsafeCell::new(RxFields {
            list: rx,
            rx_closed: false,
        }),
    });

    (Tx::new(chan.clone()), Rx::new(chan))
}

// ===== impl Tx =====

impl<T, S> Tx<T, S> {
    fn new(chan: Arc<Chan<T, S>>) -> Tx<T, S> {
        Tx { inner: chan }
    }

    pub(super) fn downgrade(&self) -> Arc<Chan<T, S>> {
        self.inner.clone()
    }

    // Returns the upgraded channel or None if the upgrade failed.
    pub(super) fn upgrade(chan: Arc<Chan<T, S>>) -> Option<Self> {
        let mut tx_count = chan.tx_count.load(Acquire);

        loop {
            if tx_count == 0 {
                // channel is closed
                return None;
            }

            match chan
                .tx_count
                .compare_exchange_weak(tx_count, tx_count + 1, AcqRel, Acquire)
            {
                Ok(_) => return Some(Tx { inner: chan }),
                Err(prev_count) => tx_count = prev_count,
            }
        }
    }

    pub(super) fn semaphore(&self) -> &S {
        &self.inner.semaphore
    }

    /// Send a message and notify the receiver.
    pub(crate) fn send(&self, value: T) {
        self.inner.send(value);
    }

    /// Wake the receive half
    pub(crate) fn wake_rx(&self) {
        self.inner.rx_waker.wake();
    }

    /// Returns `true` if senders belong to the same channel.
    pub(crate) fn same_channel(&self, other: &Self) -> bool {
        Arc::ptr_eq(&self.inner, &other.inner)
    }
}

impl<T, S: Semaphore> Tx<T, S> {
    pub(crate) fn is_closed(&self) -> bool {
        self.inner.semaphore.is_closed()
    }

    pub(crate) async fn closed(&self) {
        // In order to avoid a race condition, we first request a notification,
        // **then** check whether the semaphore is closed. If the semaphore is
        // closed the notification request is dropped.
        let notified = self.inner.notify_rx_closed.notified();

        if self.inner.semaphore.is_closed() {
            return;
        }
        notified.await;
    }
}

impl<T, S> Clone for Tx<T, S> {
    fn clone(&self) -> Tx<T, S> {
        // Using a Relaxed ordering here is sufficient as the caller holds a
        // strong ref to `self`, preventing a concurrent decrement to zero.
        self.inner.tx_count.fetch_add(1, Relaxed);

        Tx {
            inner: self.inner.clone(),
        }
    }
}

impl<T, S> Drop for Tx<T, S> {
    fn drop(&mut self) {
        if self.inner.tx_count.fetch_sub(1, AcqRel) != 1 {
            return;
        }

        // Close the list, which sends a `Close` message
        self.inner.tx.close();

        // Notify the receiver
        self.wake_rx();
    }
}

// ===== impl Rx =====

impl<T, S: Semaphore> Rx<T, S> {
    fn new(chan: Arc<Chan<T, S>>) -> Rx<T, S> {
        Rx { inner: chan }
    }

    pub(crate) fn close(&mut self) {
        self.inner.rx_fields.with_mut(|rx_fields_ptr| {
            let rx_fields = unsafe { &mut *rx_fields_ptr };

            if rx_fields.rx_closed {
                return;
            }

            rx_fields.rx_closed = true;
        });

        self.inner.semaphore.close();
        self.inner.notify_rx_closed.notify_waiters();
    }

    /// Receive the next value
    pub(crate) fn recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<T>> {
        use super::block::Read::*;

        // Keep track of task budget
        let coop = ready!(crate::runtime::coop::poll_proceed(cx));

        self.inner.rx_fields.with_mut(|rx_fields_ptr| {
            let rx_fields = unsafe { &mut *rx_fields_ptr };

            macro_rules! try_recv {
                () => {
                    match rx_fields.list.pop(&self.inner.tx) {
                        Some(Value(value)) => {
                            self.inner.semaphore.add_permit();
                            coop.made_progress();
                            return Ready(Some(value));
                        }
                        Some(Closed) => {
                            // TODO: This check may not be required as it most
                            // likely can only return `true` at this point. A
                            // channel is closed when all tx handles are
                            // dropped. Dropping a tx handle releases memory,
                            // which ensures that if dropping the tx handle is
                            // visible, then all messages sent are also visible.
                            assert!(self.inner.semaphore.is_idle());
                            coop.made_progress();
                            return Ready(None);
                        }
                        None => {} // fall through
                    }
                };
            }

            try_recv!();

            self.inner.rx_waker.register_by_ref(cx.waker());

            // It is possible that a value was pushed between attempting to read
            // and registering the task, so we have to check the channel a
            // second time here.
            try_recv!();

            if rx_fields.rx_closed && self.inner.semaphore.is_idle() {
                coop.made_progress();
                Ready(None)
            } else {
                Pending
            }
        })
    }

    /// Try to receive the next value.
    pub(crate) fn try_recv(&mut self) -> Result<T, TryRecvError> {
        use super::list::TryPopResult;

        self.inner.rx_fields.with_mut(|rx_fields_ptr| {
            let rx_fields = unsafe { &mut *rx_fields_ptr };

            macro_rules! try_recv {
                () => {
                    match rx_fields.list.try_pop(&self.inner.tx) {
                        TryPopResult::Ok(value) => {
                            self.inner.semaphore.add_permit();
                            return Ok(value);
                        }
                        TryPopResult::Closed => return Err(TryRecvError::Disconnected),
                        TryPopResult::Empty => return Err(TryRecvError::Empty),
                        TryPopResult::Busy => {} // fall through
                    }
                };
            }

            try_recv!();

            // If a previous `poll_recv` call has set a waker, we wake it here.
            // This allows us to put our own CachedParkThread waker in the
            // AtomicWaker slot instead.
            //
            // This is not a spurious wakeup to `poll_recv` since we just got a
            // Busy from `try_pop`, which only happens if there are messages in
            // the queue.
            self.inner.rx_waker.wake();

            // Park the thread until the problematic send has completed.
            let mut park = CachedParkThread::new();
            let waker = park.waker().unwrap();
            loop {
                self.inner.rx_waker.register_by_ref(&waker);
                // It is possible that the problematic send has now completed,
                // so we have to check for messages again.
                try_recv!();
                park.park();
            }
        })
    }
}

impl<T, S: Semaphore> Drop for Rx<T, S> {
    fn drop(&mut self) {
        use super::block::Read::Value;

        self.close();

        self.inner.rx_fields.with_mut(|rx_fields_ptr| {
            let rx_fields = unsafe { &mut *rx_fields_ptr };

            while let Some(Value(_)) = rx_fields.list.pop(&self.inner.tx) {
                self.inner.semaphore.add_permit();
            }
        })
    }
}

// ===== impl Chan =====

impl<T, S> Chan<T, S> {
    fn send(&self, value: T) {
        // Push the value
        self.tx.push(value);

        // Notify the rx task
        self.rx_waker.wake();
    }
}

impl<T, S> Drop for Chan<T, S> {
    fn drop(&mut self) {
        use super::block::Read::Value;

        // Safety: the only owner of the rx fields is Chan, and being
        // inside its own Drop means we're the last ones to touch it.
        self.rx_fields.with_mut(|rx_fields_ptr| {
            let rx_fields = unsafe { &mut *rx_fields_ptr };

            while let Some(Value(_)) = rx_fields.list.pop(&self.tx) {}
            unsafe { rx_fields.list.free_blocks() };
        });
    }
}

// ===== impl Semaphore for (::Semaphore, capacity) =====

impl Semaphore for bounded::Semaphore {
    fn add_permit(&self) {
        self.semaphore.release(1)
    }

    fn is_idle(&self) -> bool {
        self.semaphore.available_permits() == self.bound
    }

    fn close(&self) {
        self.semaphore.close();
    }

    fn is_closed(&self) -> bool {
        self.semaphore.is_closed()
    }
}

// ===== impl Semaphore for AtomicUsize =====

impl Semaphore for unbounded::Semaphore {
    fn add_permit(&self) {
        let prev = self.0.fetch_sub(2, Release);

        if prev >> 1 == 0 {
            // Something went wrong
            process::abort();
        }
    }

    fn is_idle(&self) -> bool {
        self.0.load(Acquire) >> 1 == 0
    }

    fn close(&self) {
        self.0.fetch_or(1, Release);
    }

    fn is_closed(&self) -> bool {
        self.0.load(Acquire) & 1 == 1
    }
}