summaryrefslogtreecommitdiffstats
path: root/tokio/src/sync/watch.rs
blob: b377ca7f77b6b69f8dbf5c16bb3ab8e5425b22f3 (plain)
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
//! 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. The **latest** value stored in the channel is accessed with
//! [`Receiver::borrow()`]. Awaiting [`Receiver::changed()`] waits for a new
//! value to sent by the [`Sender`] half.
//!
//! # 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 rx.changed().await.is_ok() {
//!             println!("received = {:?}", *rx.borrow());
//!         }
//!     });
//!
//!     tx.send("world")?;
//! # Ok(())
//! # }
//! ```
//!
//! # Closing
//!
//! [`Sender::is_closed`] and [`Sender::closed`] allow 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::changed()`]: crate::sync::watch::Receiver::changed
//! [`Receiver::borrow()`]: crate::sync::watch::Receiver::borrow
//! [`channel`]: crate::sync::watch::channel
//! [`Sender::is_closed`]: crate::sync::watch::Sender::is_closed
//! [`Sender::closed`]: crate::sync::watch::Sender::closed

use crate::sync::Notify;

use std::ops;
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::{Relaxed, SeqCst};
use std::sync::{Arc, RwLock, RwLockReadGuard};

/// 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>>,

    /// Last observed version
    version: usize,
}

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

#[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,

    /// Tracks the number of `Receiver` instances
    ref_count_rx: AtomicUsize,

    /// Notifies waiting receivers that the value changed.
    notify_rx: Notify,

    /// Notifies any task listening for `Receiver` dropped events
    notify_tx: Notify,
}

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> {}

    /// Error produced when receiving a change notification.
    #[derive(Debug)]
    pub struct RecvError(pub(super) ());

    // ===== impl RecvError =====

    impl fmt::Display for RecvError {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(fmt, "channel closed")
        }
    }

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

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 rx.changed().await.is_ok() {
///             println!("received = {:?}", *rx.borrow());
///         }
///     });
///
///     tx.send("world")?;
/// # Ok(())
/// # }
/// ```
///
/// [`Sender`]: struct@Sender
/// [`Receiver`]: struct@Receiver
pub fn channel<T>(init: T) -> (Sender<T>, Receiver<T>) {
    let shared = Arc::new(Shared {
        value: RwLock::new(init),
        version: AtomicUsize::new(0),
        ref_count_rx: AtomicUsize::new(1),
        notify_rx: Notify::new(),
        notify_tx: Notify::new(),
    });

    let tx = Sender {
        shared: shared.clone(),
    };

    let rx = Receiver { shared, version: 0 };

    (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 }
    }

    /// Wait for a change notification
    ///
    /// Returns when a new value has been sent by the [`Sender`] since the last
    /// time `changed()` was called. When the `Sender` half is dropped, `Err` is
    /// returned.
    ///
    /// [`Sender`]: struct@Sender
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::watch;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = watch::channel("hello");
    ///
    ///     tokio::spawn(async move {
    ///         tx.send("goodbye").unwrap();
    ///     });
    ///
    ///     assert!(rx.changed().await.is_ok());
    ///     assert_eq!(*rx.borrow(), "goodbye");
    ///
    ///     // The `tx` handle has been dropped
    ///     assert!(rx.changed().await.is_err());
    /// }
    /// ```
    pub async fn changed(&mut self) -> Result<(), error::RecvError> {
        // In order to avoid a race condition, we first request a notification,
        // **then** check the current value's version. If a new version exists,
        // the notification request is dropped.
        let notified = self.shared.notify_rx.notified();

        if let Some(ret) = maybe_changed(&self.shared, &mut self.version) {
            return ret;
        }

        notified.await;

        maybe_changed(&self.shared, &mut self.version)
            .expect("[bug] failed to observe change after notificaton.")
    }
}

fn maybe_changed<T>(
    shared: &Shared<T>,
    version: &mut usize,
) -> Option<Result<(), error::RecvError>> {
    // Load the version from the state
    let state = shared.version.load(SeqCst);
    let new_version = state & !CLOSED;

    if *version != new_version {
        // Observe the new version and return
        *version = new_version;
        return Some(Ok(()));
    }

    if CLOSED == state & CLOSED {
        // All receivers have dropped.
        return Some(Err(error::RecvError(())));
    }

    None
}

impl<T> Clone for Receiver<T> {
    fn clone(&self) -> Self {
        let version = self.version;
        let shared = self.shared.clone();

        // No synchronization necessary as this is only used as a counter and
        // not memory access.
        shared.ref_count_rx.fetch_add(1, Relaxed);

        Receiver { version, shared }
    }
}

impl<T> Drop for Receiver<T> {
    fn drop(&mut self) {
        // No synchronization necessary as this is only used as a counter and
        // not memory access.
        if 1 == self.shared.ref_count_rx.fetch_sub(1, Relaxed) {
            // This is the last `Receiver` handle, tasks waiting on `Sender::closed()`
            self.shared.notify_tx.notify_waiters();
        }
    }
}

impl<T> Sender<T> {
    /// Sends a new value via the channel, notifying all receivers.
    pub fn send(&self, value: T) -> Result<(), error::SendError<T>> {
        // This is pretty much only useful as a hint anyway, so synchronization isn't critical.
        if 0 == self.shared.ref_count_rx.load(Relaxed) {
            return Err(error::SendError { inner: value });
        }

        *self.shared.value.write().unwrap() = value;

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

        // Notify all watchers
        self.shared.notify_rx.notify_waiters();

        Ok(())
    }

    /// Checks if the channel has been closed. This happens when all receivers
    /// have dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// let (tx, rx) = tokio::sync::watch::channel(());
    /// assert!(!tx.is_closed());
    ///
    /// drop(rx);
    /// assert!(tx.is_closed());
    /// ```
    pub fn is_closed(&self) -> bool {
        self.shared.ref_count_rx.load(Relaxed) == 0
    }

    /// 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.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::watch;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, rx) = watch::channel("hello");
    ///
    ///     tokio::spawn(async move {
    ///         // use `rx`
    ///         drop(rx);
    ///     });
    ///
    ///     // Waits for `rx` to drop
    ///     tx.closed().await;
    ///     println!("the `rx` handles dropped")
    /// }
    /// ```
    pub async fn closed(&self) {
        let notified = self.shared.notify_tx.notified();

        if self.shared.ref_count_rx.load(Relaxed) == 0 {
            return;
        }

        notified.await;
        debug_assert_eq!(0, self.shared.ref_count_rx.load(Relaxed));
    }
}

impl<T> Drop for Sender<T> {
    fn drop(&mut self) {
        self.shared.version.fetch_or(CLOSED, SeqCst);
        self.shared.notify_rx.notify_waiters();
    }
}

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

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

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