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
use crate::primitive::sync::atomic::{AtomicUsize, Ordering::SeqCst};
use crate::primitive::sync::{Arc, Condvar, Mutex};
use std::fmt;
use std::marker::PhantomData;
use std::time::{Duration, Instant};

/// A thread parking primitive.
///
/// Conceptually, each `Parker` has an associated token which is initially not present:
///
/// * The [`park`] method blocks the current thread unless or until the token is available, at
///   which point it automatically consumes the token.
///
/// * The [`park_timeout`] and [`park_deadline`] methods work the same as [`park`], but block for
///   a specified maximum time.
///
/// * The [`unpark`] method atomically makes the token available if it wasn't already. Because the
///   token is initially absent, [`unpark`] followed by [`park`] will result in the second call
///   returning immediately.
///
/// In other words, each `Parker` acts a bit like a spinlock that can be locked and unlocked using
/// [`park`] and [`unpark`].
///
/// # Examples
///
/// ```
/// use std::thread;
/// use std::time::Duration;
/// use crossbeam_utils::sync::Parker;
///
/// let p = Parker::new();
/// let u = p.unparker().clone();
///
/// // Make the token available.
/// u.unpark();
/// // Wakes up immediately and consumes the token.
/// p.park();
///
/// thread::spawn(move || {
///     thread::sleep(Duration::from_millis(500));
///     u.unpark();
/// });
///
/// // Wakes up when `u.unpark()` provides the token.
/// p.park();
/// # std::thread::sleep(std::time::Duration::from_millis(500)); // wait for background threads closed: https://github.com/rust-lang/miri/issues/1371
/// ```
///
/// [`park`]: Parker::park
/// [`park_timeout`]: Parker::park_timeout
/// [`park_deadline`]: Parker::park_deadline
/// [`unpark`]: Unparker::unpark
pub struct Parker {
    unparker: Unparker,
    _marker: PhantomData<*const ()>,
}

unsafe impl Send for Parker {}

impl Default for Parker {
    fn default() -> Self {
        Self {
            unparker: Unparker {
                inner: Arc::new(Inner {
                    state: AtomicUsize::new(EMPTY),
                    lock: Mutex::new(()),
                    cvar: Condvar::new(),
                }),
            },
            _marker: PhantomData,
        }
    }
}

impl Parker {
    /// Creates a new `Parker`.
    ///
    /// # Examples
    ///
    /// ```
    /// use crossbeam_utils::sync::Parker;
    ///
    /// let p = Parker::new();
    /// ```
    ///
    pub fn new() -> Parker {
        Self::default()
    }

    /// Blocks the current thread until the token is made available.
    ///
    /// # Examples
    ///
    /// ```
    /// use crossbeam_utils::sync::Parker;
    ///
    /// let p = Parker::new();
    /// let u = p.unparker().clone();
    ///
    /// // Make the token available.
    /// u.unpark();
    ///
    /// // Wakes up immediately and consumes the token.
    /// p.park();
    /// ```
    pub fn park(&self) {
        self.unparker.inner.park(None);
    }

    /// Blocks the current thread until the token is made available, but only for a limited time.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::time::Duration;
    /// use crossbeam_utils::sync::Parker;
    ///
    /// let p = Parker::new();
    ///
    /// // Waits for the token to become available, but will not wait longer than 500 ms.
    /// p.park_timeout(Duration::from_millis(500));
    /// ```
    pub fn park_timeout(&self, timeout: Duration) {
        match Instant::now().checked_add(timeout) {
            Some(deadline) => self.park_deadline(deadline),
            None => self.park(),
        }
    }

    /// Blocks the current thread until the token is made available, or until a certain deadline.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::time::{Duration, Instant};
    /// use crossbeam_utils::sync::Parker;
    ///
    /// let p = Parker::new();
    /// let deadline = Instant::now() + Duration::from_millis(500);
    ///
    /// // Waits for the token to become available, but will not wait longer than 500 ms.
    /// p.park_deadline(deadline);
    /// ```
    pub fn park_deadline(&self, deadline: Instant) {
        self.unparker.inner.park(Some(deadline))
    }

    /// Returns a reference to an associated [`Unparker`].
    ///
    /// The returned [`Unparker`] doesn't have to be used by reference - it can also be cloned.
    ///
    /// # Examples
    ///
    /// ```
    /// use crossbeam_utils::sync::Parker;
    ///
    /// let p = Parker::new();
    /// let u = p.unparker().clone();
    ///
    /// // Make the token available.
    /// u.unpark();
    /// // Wakes up immediately and consumes the token.
    /// p.park();
    /// ```
    ///
    /// [`park`]: Parker::park
    /// [`park_timeout`]: Parker::park_timeout
    pub fn unparker(&self) -> &Unparker {
        &self.unparker
    }

    /// Converts a `Parker` into a raw pointer.
    ///
    /// # Examples
    ///
    /// ```
    /// use crossbeam_utils::sync::Parker;
    ///
    /// let p = Parker::new();
    /// let raw = Parker::into_raw(p);
    /// # let _ = unsafe { Parker::from_raw(raw) };
    /// ```
    pub fn into_raw(this: Parker) -> *const () {
        Unparker::into_raw(this.unparker)
    }

    /// Converts a raw pointer into a `Parker`.
    ///
    /// # Safety
    ///
    /// This method is safe to use only with pointers returned by [`Parker::into_raw`].
    ///
    /// # Examples
    ///
    /// ```
    /// use crossbeam_utils::sync::Parker;
    ///
    /// let p = Parker::new();
    /// let raw = Parker::into_raw(p);
    /// let p = unsafe { Parker::from_raw(raw) };
    /// ```
    pub unsafe fn from_raw(ptr: *const ()) -> Parker {
        Parker {
            unparker: Unparker::from_raw(ptr),
            _marker: PhantomData,
        }
    }
}

impl fmt::Debug for Parker {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.pad("Parker { .. }")
    }
}

/// Unparks a thread parked by the associated [`Parker`].
pub struct Unparker {
    inner: Arc<Inner>,
}

unsafe impl Send for Unparker {}
unsafe impl Sync for Unparker {}

impl Unparker {
    /// Atomically makes the token available if it is not already.
    ///
    /// This method will wake up the thread blocked on [`park`] or [`park_timeout`], if there is
    /// any.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    /// use std::time::Duration;
    /// use crossbeam_utils::sync::Parker;
    ///
    /// let p = Parker::new();
    /// let u = p.unparker().clone();
    ///
    /// thread::spawn(move || {
    ///     thread::sleep(Duration::from_millis(500));
    ///     u.unpark();
    /// });
    ///
    /// // Wakes up when `u.unpark()` provides the token.
    /// p.park();
    /// # std::thread::sleep(std::time::Duration::from_millis(500)); // wait for background threads closed: https://github.com/rust-lang/miri/issues/1371
    /// ```
    ///
    /// [`park`]: Parker::park
    /// [`park_timeout`]: Parker::park_timeout
    pub fn unpark(&self) {
        self.inner.unpark()
    }

    /// Converts an `Unparker` into a raw pointer.
    ///
    /// # Examples
    ///
    /// ```
    /// use crossbeam_utils::sync::{Parker, Unparker};
    ///
    /// let p = Parker::new();
    /// let u = p.unparker().clone();
    /// let raw = Unparker::into_raw(u);
    /// # let _ = unsafe { Unparker::from_raw(raw) };
    /// ```
    pub fn into_raw(this: Unparker) -> *const () {
        Arc::into_raw(this.inner).cast::<()>()
    }

    /// Converts a raw pointer into an `Unparker`.
    ///
    /// # Safety
    ///
    /// This method is safe to use only with pointers returned by [`Unparker::into_raw`].
    ///
    /// # Examples
    ///
    /// ```
    /// use crossbeam_utils::sync::{Parker, Unparker};
    ///
    /// let p = Parker::new();
    /// let u = p.unparker().clone();
    ///
    /// let raw = Unparker::into_raw(u);
    /// let u = unsafe { Unparker::from_raw(raw) };
    /// ```
    pub unsafe fn from_raw(ptr: *const ()) -> Unparker {
        Unparker {
            inner: Arc::from_raw(ptr.cast::<Inner>()),
        }
    }
}

impl fmt::Debug for Unparker {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.pad("Unparker { .. }")
    }
}

impl Clone for Unparker {
    fn clone(&self) -> Unparker {
        Unparker {
            inner: self.inner.clone(),
        }
    }
}

const EMPTY: usize = 0;
const PARKED: usize = 1;
const NOTIFIED: usize = 2;

struct Inner {
    state: AtomicUsize,
    lock: Mutex<()>,
    cvar: Condvar,
}

impl Inner {
    fn park(&self, deadline: Option<Instant>) {
        // If we were previously notified then we consume this notification and return quickly.
        if self
            .state
            .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
            .is_ok()
        {
            return;
        }

        // If the timeout is zero, then there is no need to actually block.
        if let Some(deadline) = deadline {
            if deadline <= Instant::now() {
                return;
            }
        }

        // Otherwise we need to coordinate going to sleep.
        let mut m = self.lock.lock().unwrap();

        match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
            Ok(_) => {}
            // Consume this notification to avoid spurious wakeups in the next park.
            Err(NOTIFIED) => {
                // We must read `state` here, even though we know it will be `NOTIFIED`. This is
                // because `unpark` may have been called again since we read `NOTIFIED` in the
                // `compare_exchange` above. We must perform an acquire operation that synchronizes
                // with that `unpark` to observe any writes it made before the call to `unpark`. To
                // do that we must read from the write it made to `state`.
                let old = self.state.swap(EMPTY, SeqCst);
                assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
                return;
            }
            Err(n) => panic!("inconsistent park_timeout state: {}", n),
        }

        loop {
            // Block the current thread on the conditional variable.
            m = match deadline {
                None => self.cvar.wait(m).unwrap(),
                Some(deadline) => {
                    let now = Instant::now();
                    if now < deadline {
                        // We could check for a timeout here, in the return value of wait_timeout,
                        // but in the case that a timeout and an unpark arrive simultaneously, we
                        // prefer to report the former.
                        self.cvar.wait_timeout(m, deadline - now).unwrap().0
                    } else {
                        // We've timed out; swap out the state back to empty on our way out
                        match self.state.swap(EMPTY, SeqCst) {
                            NOTIFIED | PARKED => return,
                            n => panic!("inconsistent park_timeout state: {}", n),
                        };
                    }
                }
            };

            if self
                .state
                .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
                .is_ok()
            {
                // got a notification
                return;
            }

            // Spurious wakeup, go back to sleep. Alternatively, if we timed out, it will be caught
            // in the branch above, when we discover the deadline is in the past
        }
    }

    pub(crate) fn unpark(&self) {
        // To ensure the unparked thread will observe any writes we made before this call, we must
        // perform a release operation that `park` can synchronize with. To do that we must write
        // `NOTIFIED` even if `state` is already `NOTIFIED`. That is why this must be a swap rather
        // than a compare-and-swap that returns if it reads `NOTIFIED` on failure.
        match self.state.swap(NOTIFIED, SeqCst) {
            EMPTY => return,    // no one was waiting
            NOTIFIED => return, // already unparked
            PARKED => {}        // gotta go wake someone up
            _ => panic!("inconsistent state in unpark"),
        }

        // There is a period between when the parked thread sets `state` to `PARKED` (or last
        // checked `state` in the case of a spurious wakeup) and when it actually waits on `cvar`.
        // If we were to notify during this period it would be ignored and then when the parked
        // thread went to sleep it would never wake up. Fortunately, it has `lock` locked at this
        // stage so we can acquire `lock` to wait until it is ready to receive the notification.
        //
        // Releasing `lock` before the call to `notify_one` means that when the parked thread wakes
        // it doesn't get woken only to have to wait for us to release `lock`.
        drop(self.lock.lock().unwrap());
        self.cvar.notify_one();
    }
}