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
426
427
428
429
430
431
//! A single-producer, multi-consumer channel that only retains the *last* sent
//! value.
//!
//! This channel is useful for watching for changes to a value from multiple
//! points in the code base, for example, changes to configuration values.
//!
//! # Usage
//!
//! [`channel`] returns a [`Sender`] / [`Receiver`] pair. These are
//! the producer and sender halves of the channel. The channel is
//! created with an initial value. [`Receiver::recv`] will always
//! be ready upon creation and will yield either this initial value or
//! the latest value that has been sent by `Sender`.
//!
//! Calls to [`Receiver::recv`] will always yield the latest value.
//!
//! # Examples
//!
//! ```
//! use tokio::sync::watch;
//!
//! # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
//!     let (tx, mut rx) = watch::channel("hello");
//!
//!     tokio::spawn(async move {
//!         while let Some(value) = rx.recv().await {
//!             println!("received = {:?}", value);
//!         }
//!     });
//!
//!     tx.broadcast("world")?;
//! # Ok(())
//! # }
//! ```
//!
//! # Closing
//!
//! [`Sender::closed`] allows the producer to detect when all [`Receiver`]
//! handles have been dropped. This indicates that there is no further interest
//! in the values being produced and work can be stopped.
//!
//! # Thread safety
//!
//! Both [`Sender`] and [`Receiver`] are thread safe. They can be moved to other
//! threads and can be used in a concurrent environment. Clones of [`Receiver`]
//! handles may be moved to separate threads and also used concurrently.
//!
//! [`Sender`]: crate::sync::watch::Sender
//! [`Receiver`]: crate::sync::watch::Receiver
//! [`Receiver::recv`]: crate::sync::watch::Receiver::recv
//! [`channel`]: crate::sync::watch::channel
//! [`Sender::closed`]: crate::sync::watch::Sender::closed

use crate::future::poll_fn;
use crate::sync::task::AtomicWaker;

use fnv::FnvHashSet;
use std::ops;
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::{Relaxed, SeqCst};
use std::sync::{Arc, Mutex, RwLock, RwLockReadGuard, Weak};
use std::task::Poll::{Pending, Ready};
use std::task::{Context, Poll};

/// Receives values from the associated [`Sender`](struct@Sender).
///
/// Instances are created by the [`channel`](fn@channel) function.
#[derive(Debug)]
pub struct Receiver<T> {
    /// Pointer to the shared state
    shared: Arc<Shared<T>>,

    /// Pointer to the watcher's internal state
    inner: Watcher,
}

/// Sends values to the associated [`Receiver`](struct@Receiver).
///
/// Instances are created by the [`channel`](fn@channel) function.
#[derive(Debug)]
pub struct Sender<T> {
    shared: Weak<Shared<T>>,
}

/// Returns a reference to the inner value
///
/// Outstanding borrows hold a read lock on the inner value. This means that
/// long lived borrows could cause the produce half to block. It is recommended
/// to keep the borrow as short lived as possible.
#[derive(Debug)]
pub struct Ref<'a, T> {
    inner: RwLockReadGuard<'a, T>,
}

pub mod error {
    //! Watch error types

    use std::fmt;

    /// Error produced when sending a value fails.
    #[derive(Debug)]
    pub struct SendError<T> {
        pub(crate) inner: T,
    }

    // ===== impl SendError =====

    impl<T: fmt::Debug> fmt::Display for SendError<T> {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(fmt, "channel closed")
        }
    }

    impl<T: fmt::Debug> std::error::Error for SendError<T> {}
}

#[derive(Debug)]
struct Shared<T> {
    /// The most recent value
    value: RwLock<T>,

    /// The current version
    ///
    /// The lowest bit represents a "closed" state. The rest of the bits
    /// represent the current version.
    version: AtomicUsize,

    /// All watchers
    watchers: Mutex<Watchers>,

    /// Task to notify when all watchers drop
    cancel: AtomicWaker,
}

type Watchers = FnvHashSet<Watcher>;

/// The watcher's ID is based on the Arc's pointer.
#[derive(Clone, Debug)]
struct Watcher(Arc<WatchInner>);

#[derive(Debug)]
struct WatchInner {
    /// Last observed version
    version: AtomicUsize,
    waker: AtomicWaker,
}

const CLOSED: usize = 1;

/// Creates a new watch channel, returning the "send" and "receive" handles.
///
/// All values sent by [`Sender`] will become visible to the [`Receiver`] handles.
/// Only the last value sent is made available to the [`Receiver`] half. All
/// intermediate values are dropped.
///
/// # Examples
///
/// ```
/// use tokio::sync::watch;
///
/// # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
///     let (tx, mut rx) = watch::channel("hello");
///
///     tokio::spawn(async move {
///         while let Some(value) = rx.recv().await {
///             println!("received = {:?}", value);
///         }
///     });
///
///     tx.broadcast("world")?;
/// # Ok(())
/// # }
/// ```
///
/// [`Sender`]: struct@Sender
/// [`Receiver`]: struct@Receiver
pub fn channel<T: Clone>(init: T) -> (Sender<T>, Receiver<T>) {
    const VERSION_0: usize = 0;
    const VERSION_1: usize = 2;

    // We don't start knowing VERSION_1
    let inner = Watcher::new_version(VERSION_0);

    // Insert the watcher
    let mut watchers = FnvHashSet::with_capacity_and_hasher(0, Default::default());
    watchers.insert(inner.clone());

    let shared = Arc::new(Shared {
        value: RwLock::new(init),
        version: AtomicUsize::new(VERSION_1),
        watchers: Mutex::new(watchers),
        cancel: AtomicWaker::new(),
    });

    let tx = Sender {
        shared: Arc::downgrade(&shared),
    };

    let rx = Receiver { shared, inner };

    (tx, rx)
}

impl<T> Receiver<T> {
    /// Returns a reference to the most recently sent value
    ///
    /// Outstanding borrows hold a read lock. This means that long lived borrows
    /// could cause the send half to block. It is recommended to keep the borrow
    /// as short lived as possible.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::watch;
    ///
    /// let (_, rx) = watch::channel("hello");
    /// assert_eq!(*rx.borrow(), "hello");
    /// ```
    pub fn borrow(&self) -> Ref<'_, T> {
        let inner = self.shared.value.read().unwrap();
        Ref { inner }
    }

    // TODO: document
    #[doc(hidden)]
    pub fn poll_recv_ref<'a>(&'a mut self, cx: &mut Context<'_>) -> Poll<Option<Ref<'a, T>>> {
        // Make sure the task is up to date
        self.inner.waker.register_by_ref(cx.waker());

        let state = self.shared.version.load(SeqCst);
        let version = state & !CLOSED;

        if self.inner.version.swap(version, Relaxed) != version {
            let inner = self.shared.value.read().unwrap();

            return Ready(Some(Ref { inner }));
        }

        if CLOSED == state & CLOSED {
            // The `Store` handle has been dropped.
            return Ready(None);
        }

        Pending
    }
}

impl<T: Clone> Receiver<T> {
    /// Attempts to clone the latest value sent via the channel.
    ///
    /// If this is the first time the function is called on a `Receiver`
    /// instance, then the function completes immediately with the **current**
    /// value held by the channel. On the next call, the function waits until
    /// a new value is sent in the channel.
    ///
    /// `None` is returned if the `Sender` half is dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::watch;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = watch::channel("hello");
    ///
    ///     let v = rx.recv().await.unwrap();
    ///     assert_eq!(v, "hello");
    ///
    ///     tokio::spawn(async move {
    ///         tx.broadcast("goodbye").unwrap();
    ///     });
    ///
    ///     // Waits for the new task to spawn and send the value.
    ///     let v = rx.recv().await.unwrap();
    ///     assert_eq!(v, "goodbye");
    ///
    ///     let v = rx.recv().await;
    ///     assert!(v.is_none());
    /// }
    /// ```
    pub async fn recv(&mut self) -> Option<T> {
        poll_fn(|cx| {
            let v_ref = ready!(self.poll_recv_ref(cx));
            Poll::Ready(v_ref.map(|v_ref| (*v_ref).clone()))
        })
        .await
    }
}

#[cfg(feature = "stream")]
impl<T: Clone> crate::stream::Stream for Receiver<T> {
    type Item = T;

    fn poll_next(mut self: std::pin::Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
        let v_ref = ready!(self.poll_recv_ref(cx));

        Poll::Ready(v_ref.map(|v_ref| (*v_ref).clone()))
    }
}

impl<T> Clone for Receiver<T> {
    fn clone(&self) -> Self {
        let ver = self.inner.version.load(Relaxed);
        let inner = Watcher::new_version(ver);
        let shared = self.shared.clone();

        shared.watchers.lock().unwrap().insert(inner.clone());

        Receiver { shared, inner }
    }
}

impl<T> Drop for Receiver<T> {
    fn drop(&mut self) {
        self.shared.watchers.lock().unwrap().remove(&self.inner);
    }
}

impl<T> Sender<T> {
    /// Broadcasts a new value via the channel, notifying all receivers.
    pub fn broadcast(&self, value: T) -> Result<(), error::SendError<T>> {
        let shared = match self.shared.upgrade() {
            Some(shared) => shared,
            // All `Watch` handles have been canceled
            None => return Err(error::SendError { inner: value }),
        };

        // Replace the value
        {
            let mut lock = shared.value.write().unwrap();
            *lock = value;
        }

        // Update the version. 2 is used so that the CLOSED bit is not set.
        shared.version.fetch_add(2, SeqCst);

        // Notify all watchers
        notify_all(&*shared);

        Ok(())
    }

    /// Completes when all receivers have dropped.
    ///
    /// This allows the producer to get notified when interest in the produced
    /// values is canceled and immediately stop doing work.
    pub async fn closed(&mut self) {
        poll_fn(|cx| self.poll_close(cx)).await
    }

    fn poll_close(&mut self, cx: &mut Context<'_>) -> Poll<()> {
        match self.shared.upgrade() {
            Some(shared) => {
                shared.cancel.register_by_ref(cx.waker());
                Pending
            }
            None => Ready(()),
        }
    }
}

/// Notifies all watchers of a change
fn notify_all<T>(shared: &Shared<T>) {
    let watchers = shared.watchers.lock().unwrap();

    for watcher in watchers.iter() {
        // Notify the task
        watcher.waker.wake();
    }
}

impl<T> Drop for Sender<T> {
    fn drop(&mut self) {
        if let Some(shared) = self.shared.upgrade() {
            shared.version.fetch_or(CLOSED, SeqCst);
            notify_all(&*shared);
        }
    }
}

// ===== impl Ref =====

impl<T> ops::Deref for Ref<'_, T> {
    type Target = T;

    fn deref(&self) -> &T {
        self.inner.deref()
    }
}

// ===== impl Shared =====

impl<T> Drop for Shared<T> {
    fn drop(&mut self) {
        self.cancel.wake();
    }
}

// ===== impl Watcher =====

impl Watcher {
    fn new_version(version: usize) -> Self {
        Watcher(Arc::new(WatchInner {
            version: AtomicUsize::new(version),
            waker: AtomicWaker::new(),
        }))
    }
}

impl std::cmp::PartialEq for Watcher {
    fn eq(&self, other: &Watcher) -> bool {
        Arc::ptr_eq(&self.0, &other.0)
    }
}

impl std::cmp::Eq for Watcher {}

impl std::hash::Hash for Watcher {
    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
        (&*self.0 as *const WatchInner).hash(state)
    }
}

impl std::ops::Deref for Watcher {
    type Target = WatchInner;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}