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
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
//! An async mutex.
//!
//! The locking mechanism uses eventual fairness to ensure locking will be fair on average without
//! sacrificing performance. This is done by forcing a fair lock whenever a lock operation is
//! starved for longer than 0.5 milliseconds.
//!
//! # Examples
//!
//! ```
//! # futures_lite::future::block_on(async {
//! use async_mutex::Mutex;
//!
//! let m = Mutex::new(1);
//!
//! let mut guard = m.lock().await;
//! *guard = 2;
//!
//! assert!(m.try_lock().is_none());
//! drop(guard);
//! assert_eq!(*m.try_lock().unwrap(), 2);
//! # })
//! ```

#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]

use std::cell::UnsafeCell;
use std::fmt;
use std::ops::{Deref, DerefMut};
use std::process;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::sync::Arc;
use std::time::{Duration, Instant};
use std::usize;

use event_listener::Event;

/// An async mutex.
pub struct Mutex<T: ?Sized> {
    /// Current state of the mutex.
    ///
    /// The least significant bit is set to 1 if the mutex is locked.
    /// The other bits hold the number of starved lock operations.
    state: AtomicUsize,

    /// Lock operations waiting for the mutex to be released.
    lock_ops: Event,

    /// The value inside the mutex.
    data: UnsafeCell<T>,
}

unsafe impl<T: Send + ?Sized> Send for Mutex<T> {}
unsafe impl<T: Send + ?Sized> Sync for Mutex<T> {}

impl<T> Mutex<T> {
    /// Creates a new async mutex.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_mutex::Mutex;
    ///
    /// let mutex = Mutex::new(0);
    /// ```
    pub const fn new(data: T) -> Mutex<T> {
        Mutex {
            state: AtomicUsize::new(0),
            lock_ops: Event::new(),
            data: UnsafeCell::new(data),
        }
    }

    /// Consumes the mutex, returning the underlying data.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_mutex::Mutex;
    ///
    /// let mutex = Mutex::new(10);
    /// assert_eq!(mutex.into_inner(), 10);
    /// ```
    pub fn into_inner(self) -> T {
        self.data.into_inner()
    }
}

impl<T: ?Sized> Mutex<T> {
    /// Acquires the mutex.
    ///
    /// Returns a guard that releases the mutex when dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_mutex::Mutex;
    ///
    /// let mutex = Mutex::new(10);
    /// let guard = mutex.lock().await;
    /// assert_eq!(*guard, 10);
    /// # })
    /// ```
    #[inline]
    pub async fn lock(&self) -> MutexGuard<'_, T> {
        if let Some(guard) = self.try_lock() {
            return guard;
        }
        self.acquire_slow().await;
        MutexGuard(self)
    }

    /// Slow path for acquiring the mutex.
    #[cold]
    async fn acquire_slow(&self) {
        // Get the current time.
        let start = Instant::now();

        loop {
            // Start listening for events.
            let listener = self.lock_ops.listen();

            // Try locking if nobody is being starved.
            match self.state.compare_and_swap(0, 1, Ordering::Acquire) {
                // Lock acquired!
                0 => return,

                // Lock is held and nobody is starved.
                1 => {}

                // Somebody is starved.
                _ => break,
            }

            // Wait for a notification.
            listener.await;

            // Try locking if nobody is being starved.
            match self.state.compare_and_swap(0, 1, Ordering::Acquire) {
                // Lock acquired!
                0 => return,

                // Lock is held and nobody is starved.
                1 => {}

                // Somebody is starved.
                _ => {
                    // Notify the first listener in line because we probably received a
                    // notification that was meant for a starved task.
                    self.lock_ops.notify(1);
                    break;
                }
            }

            // If waiting for too long, fall back to a fairer locking strategy that will prevent
            // newer lock operations from starving us forever.
            if start.elapsed() > Duration::from_micros(500) {
                break;
            }
        }

        // Increment the number of starved lock operations.
        if self.state.fetch_add(2, Ordering::Release) > usize::MAX / 2 {
            // In case of potential overflow, abort.
            process::abort();
        }

        // Decrement the counter when exiting this function.
        let _call = CallOnDrop(|| {
            self.state.fetch_sub(2, Ordering::Release);
        });

        loop {
            // Start listening for events.
            let listener = self.lock_ops.listen();

            // Try locking if nobody else is being starved.
            match self.state.compare_and_swap(2, 2 | 1, Ordering::Acquire) {
                // Lock acquired!
                2 => return,

                // Lock is held by someone.
                s if s % 2 == 1 => {}

                // Lock is available.
                _ => {
                    // Be fair: notify the first listener and then go wait in line.
                    self.lock_ops.notify(1);
                }
            }

            // Wait for a notification.
            listener.await;

            // Try acquiring the lock without waiting for others.
            if self.state.fetch_or(1, Ordering::Acquire) % 2 == 0 {
                return;
            }
        }
    }

    /// Attempts to acquire the mutex.
    ///
    /// If the mutex could not be acquired at this time, then [`None`] is returned. Otherwise, a
    /// guard is returned that releases the mutex when dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_mutex::Mutex;
    ///
    /// let mutex = Mutex::new(10);
    /// if let Some(guard) = mutex.try_lock() {
    ///     assert_eq!(*guard, 10);
    /// }
    /// # ;
    /// ```
    #[inline]
    pub fn try_lock(&self) -> Option<MutexGuard<'_, T>> {
        if self.state.compare_and_swap(0, 1, Ordering::Acquire) == 0 {
            Some(MutexGuard(self))
        } else {
            None
        }
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// Since this call borrows the mutex mutably, no actual locking takes place -- the mutable
    /// borrow statically guarantees the mutex is not already acquired.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_mutex::Mutex;
    ///
    /// let mut mutex = Mutex::new(0);
    /// *mutex.get_mut() = 10;
    /// assert_eq!(*mutex.lock().await, 10);
    /// # })
    /// ```
    pub fn get_mut(&mut self) -> &mut T {
        unsafe { &mut *self.data.get() }
    }
}

impl<T: ?Sized> Mutex<T> {
    /// Acquires the mutex and clones a reference to it.
    ///
    /// Returns an owned guard that releases the mutex when dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_mutex::Mutex;
    /// use std::sync::Arc;
    ///
    /// let mutex = Arc::new(Mutex::new(10));
    /// let guard = mutex.lock_arc().await;
    /// assert_eq!(*guard, 10);
    /// # })
    /// ```
    #[inline]
    pub async fn lock_arc(self: &Arc<Self>) -> MutexGuardArc<T> {
        if let Some(guard) = self.try_lock_arc() {
            return guard;
        }
        self.acquire_slow().await;
        MutexGuardArc(self.clone())
    }

    /// Attempts to acquire the mutex and clone a reference to it.
    ///
    /// If the mutex could not be acquired at this time, then [`None`] is returned. Otherwise, an
    /// owned guard is returned that releases the mutex when dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_mutex::Mutex;
    /// use std::sync::Arc;
    ///
    /// let mutex = Arc::new(Mutex::new(10));
    /// if let Some(guard) = mutex.try_lock() {
    ///     assert_eq!(*guard, 10);
    /// }
    /// # ;
    /// ```
    #[inline]
    pub fn try_lock_arc(self: &Arc<Self>) -> Option<MutexGuardArc<T>> {
        if self.state.compare_and_swap(0, 1, Ordering::Acquire) == 0 {
            Some(MutexGuardArc(self.clone()))
        } else {
            None
        }
    }
}

impl<T: fmt::Debug + ?Sized> fmt::Debug for Mutex<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        struct Locked;
        impl fmt::Debug for Locked {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                f.write_str("<locked>")
            }
        }

        match self.try_lock() {
            None => f.debug_struct("Mutex").field("data", &Locked).finish(),
            Some(guard) => f.debug_struct("Mutex").field("data", &&*guard).finish(),
        }
    }
}

impl<T> From<T> for Mutex<T> {
    fn from(val: T) -> Mutex<T> {
        Mutex::new(val)
    }
}

impl<T: Default + ?Sized> Default for Mutex<T> {
    fn default() -> Mutex<T> {
        Mutex::new(Default::default())
    }
}

/// A guard that releases the mutex when dropped.
pub struct MutexGuard<'a, T: ?Sized>(&'a Mutex<T>);

unsafe impl<T: Send + ?Sized> Send for MutexGuard<'_, T> {}
unsafe impl<T: Sync + ?Sized> Sync for MutexGuard<'_, T> {}

impl<'a, T: ?Sized> MutexGuard<'a, T> {
    /// Returns a reference to the mutex a guard came from.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_mutex::{Mutex, MutexGuard};
    ///
    /// let mutex = Mutex::new(10i32);
    /// let guard = mutex.lock().await;
    /// dbg!(MutexGuard::source(&guard));
    /// # })
    /// ```
    pub fn source(guard: &MutexGuard<'a, T>) -> &'a Mutex<T> {
        guard.0
    }
}

impl<T: ?Sized> Drop for MutexGuard<'_, T> {
    fn drop(&mut self) {
        // Remove the last bit and notify a waiting lock operation.
        self.0.state.fetch_sub(1, Ordering::Release);
        self.0.lock_ops.notify(1);
    }
}

impl<T: fmt::Debug + ?Sized> fmt::Debug for MutexGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&**self, f)
    }
}

impl<T: fmt::Display + ?Sized> fmt::Display for MutexGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

impl<T: ?Sized> Deref for MutexGuard<'_, T> {
    type Target = T;

    fn deref(&self) -> &T {
        unsafe { &*self.0.data.get() }
    }
}

impl<T: ?Sized> DerefMut for MutexGuard<'_, T> {
    fn deref_mut(&mut self) -> &mut T {
        unsafe { &mut *self.0.data.get() }
    }
}

/// An owned guard that releases the mutex when dropped.
pub struct MutexGuardArc<T: ?Sized>(Arc<Mutex<T>>);

unsafe impl<T: Send + ?Sized> Send for MutexGuardArc<T> {}
unsafe impl<T: Sync + ?Sized> Sync for MutexGuardArc<T> {}

impl<T: ?Sized> MutexGuardArc<T> {
    /// Returns a reference to the mutex a guard came from.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_mutex::{Mutex, MutexGuardArc};
    /// use std::sync::Arc;
    ///
    /// let mutex = Arc::new(Mutex::new(10i32));
    /// let guard = mutex.lock_arc().await;
    /// dbg!(MutexGuardArc::source(&guard));
    /// # })
    /// ```
    pub fn source(guard: &MutexGuardArc<T>) -> &Arc<Mutex<T>> {
        &guard.0
    }
}

impl<T: ?Sized> Drop for MutexGuardArc<T> {
    fn drop(&mut self) {
        // Remove the last bit and notify a waiting lock operation.
        self.0.state.fetch_sub(1, Ordering::Release);
        self.0.lock_ops.notify(1);
    }
}

impl<T: fmt::Debug + ?Sized> fmt::Debug for MutexGuardArc<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&**self, f)
    }
}

impl<T: fmt::Display + ?Sized> fmt::Display for MutexGuardArc<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

impl<T: ?Sized> Deref for MutexGuardArc<T> {
    type Target = T;

    fn deref(&self) -> &T {
        unsafe { &*self.0.data.get() }
    }
}

impl<T: ?Sized> DerefMut for MutexGuardArc<T> {
    fn deref_mut(&mut self) -> &mut T {
        unsafe { &mut *self.0.data.get() }
    }
}

/// Calls a function when dropped.
struct CallOnDrop<F: Fn()>(F);

impl<F: Fn()> Drop for CallOnDrop<F> {
    fn drop(&mut self) {
        (self.0)();
    }
}