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
use futures_core::future::{FusedFuture, Future};
use futures_core::task::{Context, Poll, Waker};
use slab::Slab;
use std::cell::UnsafeCell;
use std::marker::PhantomData;
use std::ops::{Deref, DerefMut};
use std::pin::Pin;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::sync::Mutex as StdMutex;
use std::{fmt, mem};

/// A futures-aware mutex.
///
/// # Fairness
///
/// This mutex provides no fairness guarantees. Tasks may not acquire the mutex
/// in the order that they requested the lock, and it's possible for a single task
/// which repeatedly takes the lock to starve other tasks, which may be left waiting
/// indefinitely.
pub struct Mutex<T: ?Sized> {
    state: AtomicUsize,
    waiters: StdMutex<Slab<Waiter>>,
    value: UnsafeCell<T>,
}

impl<T: ?Sized> fmt::Debug for Mutex<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let state = self.state.load(Ordering::SeqCst);
        f.debug_struct("Mutex")
            .field("is_locked", &((state & IS_LOCKED) != 0))
            .field("has_waiters", &((state & HAS_WAITERS) != 0))
            .finish()
    }
}

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

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

enum Waiter {
    Waiting(Waker),
    Woken,
}

impl Waiter {
    fn register(&mut self, waker: &Waker) {
        match self {
            Self::Waiting(w) if waker.will_wake(w) => {}
            _ => *self = Self::Waiting(waker.clone()),
        }
    }

    fn wake(&mut self) {
        match mem::replace(self, Self::Woken) {
            Self::Waiting(waker) => waker.wake(),
            Self::Woken => {}
        }
    }
}

const IS_LOCKED: usize = 1 << 0;
const HAS_WAITERS: usize = 1 << 1;

impl<T> Mutex<T> {
    /// Creates a new futures-aware mutex.
    pub fn new(t: T) -> Self {
        Self {
            state: AtomicUsize::new(0),
            waiters: StdMutex::new(Slab::new()),
            value: UnsafeCell::new(t),
        }
    }

    /// Consumes this mutex, returning the underlying data.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::lock::Mutex;
    ///
    /// let mutex = Mutex::new(0);
    /// assert_eq!(mutex.into_inner(), 0);
    /// ```
    pub fn into_inner(self) -> T {
        self.value.into_inner()
    }
}

impl<T: ?Sized> Mutex<T> {
    /// Attempt to acquire the lock immediately.
    ///
    /// If the lock is currently held, this will return `None`.
    pub fn try_lock(&self) -> Option<MutexGuard<'_, T>> {
        let old_state = self.state.fetch_or(IS_LOCKED, Ordering::Acquire);
        if (old_state & IS_LOCKED) == 0 {
            Some(MutexGuard { mutex: self })
        } else {
            None
        }
    }

    /// Acquire the lock asynchronously.
    ///
    /// This method returns a future that will resolve once the lock has been
    /// successfully acquired.
    pub fn lock(&self) -> MutexLockFuture<'_, T> {
        MutexLockFuture { mutex: Some(self), wait_key: WAIT_KEY_NONE }
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// Since this call borrows the `Mutex` mutably, no actual locking needs to
    /// take place -- the mutable borrow statically guarantees no locks exist.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures::executor::block_on(async {
    /// use futures::lock::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 {
        // We know statically that there are no other references to `self`, so
        // there's no need to lock the inner mutex.
        unsafe { &mut *self.value.get() }
    }

    fn remove_waker(&self, wait_key: usize, wake_another: bool) {
        if wait_key != WAIT_KEY_NONE {
            let mut waiters = self.waiters.lock().unwrap();
            match waiters.remove(wait_key) {
                Waiter::Waiting(_) => {}
                Waiter::Woken => {
                    // We were awoken, but then dropped before we could
                    // wake up to acquire the lock. Wake up another
                    // waiter.
                    if wake_another {
                        if let Some((_i, waiter)) = waiters.iter_mut().next() {
                            waiter.wake();
                        }
                    }
                }
            }
            if waiters.is_empty() {
                self.state.fetch_and(!HAS_WAITERS, Ordering::Relaxed); // released by mutex unlock
            }
        }
    }

    // Unlocks the mutex. Called by MutexGuard and MappedMutexGuard when they are
    // dropped.
    fn unlock(&self) {
        let old_state = self.state.fetch_and(!IS_LOCKED, Ordering::AcqRel);
        if (old_state & HAS_WAITERS) != 0 {
            let mut waiters = self.waiters.lock().unwrap();
            if let Some((_i, waiter)) = waiters.iter_mut().next() {
                waiter.wake();
            }
        }
    }
}

// Sentinel for when no slot in the `Slab` has been dedicated to this object.
const WAIT_KEY_NONE: usize = usize::max_value();

/// A future which resolves when the target mutex has been successfully acquired.
pub struct MutexLockFuture<'a, T: ?Sized> {
    // `None` indicates that the mutex was successfully acquired.
    mutex: Option<&'a Mutex<T>>,
    wait_key: usize,
}

impl<T: ?Sized> fmt::Debug for MutexLockFuture<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("MutexLockFuture")
            .field("was_acquired", &self.mutex.is_none())
            .field("mutex", &self.mutex)
            .field(
                "wait_key",
                &(if self.wait_key == WAIT_KEY_NONE { None } else { Some(self.wait_key) }),
            )
            .finish()
    }
}

impl<T: ?Sized> FusedFuture for MutexLockFuture<'_, T> {
    fn is_terminated(&self) -> bool {
        self.mutex.is_none()
    }
}

impl<'a, T: ?Sized> Future for MutexLockFuture<'a, T> {
    type Output = MutexGuard<'a, T>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let mutex = self.mutex.expect("polled MutexLockFuture after completion");

        if let Some(lock) = mutex.try_lock() {
            mutex.remove_waker(self.wait_key, false);
            self.mutex = None;
            return Poll::Ready(lock);
        }

        {
            let mut waiters = mutex.waiters.lock().unwrap();
            if self.wait_key == WAIT_KEY_NONE {
                self.wait_key = waiters.insert(Waiter::Waiting(cx.waker().clone()));
                if waiters.len() == 1 {
                    mutex.state.fetch_or(HAS_WAITERS, Ordering::Relaxed); // released by mutex unlock
                }
            } else {
                waiters[self.wait_key].register(cx.waker());
            }
        }

        // Ensure that we haven't raced `MutexGuard::drop`'s unlock path by
        // attempting to acquire the lock again.
        if let Some(lock) = mutex.try_lock() {
            mutex.remove_waker(self.wait_key, false);
            self.mutex = None;
            return Poll::Ready(lock);
        }

        Poll::Pending
    }
}

impl<T: ?Sized> Drop for MutexLockFuture<'_, T> {
    fn drop(&mut self) {
        if let Some(mutex) = self.mutex {
            // This future was dropped before it acquired the mutex.
            //
            // Remove ourselves from the map, waking up another waiter if we
            // had been awoken to acquire the lock.
            mutex.remove_waker(self.wait_key, true);
        }
    }
}

/// An RAII guard returned by the `lock` and `try_lock` methods.
/// When this structure is dropped (falls out of scope), the lock will be
/// unlocked.
pub struct MutexGuard<'a, T: ?Sized> {
    mutex: &'a Mutex<T>,
}

impl<'a, T: ?Sized> MutexGuard<'a, T> {
    /// Returns a locked view over a portion of the locked data.
    ///
    /// # Example
    ///
    /// ```
    /// # futures::executor::block_on(async {
    /// use futures::lock::{Mutex, MutexGuard};
    ///
    /// let data = Mutex::new(Some("value".to_string()));
    /// {
    ///     let locked_str = MutexGuard::map(data.lock().await, |opt| opt.as_mut().unwrap());
    ///     assert_eq!(&*locked_str, "value");
    /// }
    /// # });
    /// ```
    #[inline]
    pub fn map<U: ?Sized, F>(this: Self, f: F) -> MappedMutexGuard<'a, T, U>
    where
        F: FnOnce(&mut T) -> &mut U,
    {
        let mutex = this.mutex;
        let value = f(unsafe { &mut *this.mutex.value.get() });
        // Don't run the `drop` method for MutexGuard. The ownership of the underlying
        // locked state is being moved to the returned MappedMutexGuard.
        mem::forget(this);
        MappedMutexGuard { mutex, value, _marker: PhantomData }
    }
}

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

impl<T: ?Sized> Drop for MutexGuard<'_, T> {
    fn drop(&mut self) {
        self.mutex.unlock()
    }
}

impl<T: ?Sized> Deref for MutexGuard<'_, T> {
    type Target = T;
    fn deref(&self) -> &T {
        unsafe { &*self.mutex.value.get() }
    }
}

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

/// An RAII guard returned by the `MutexGuard::map` and `MappedMutexGuard::map` methods.
/// When this structure is dropped (falls out of scope), the lock will be unlocked.
pub struct MappedMutexGuard<'a, T: ?Sized, U: ?Sized> {
    mutex: &'a Mutex<T>,
    value: *mut U,
    _marker: PhantomData<&'a mut U>,
}

impl<'a, T: ?Sized, U: ?Sized> MappedMutexGuard<'a, T, U> {
    /// Returns a locked view over a portion of the locked data.
    ///
    /// # Example
    ///
    /// ```
    /// # futures::executor::block_on(async {
    /// use futures::lock::{MappedMutexGuard, Mutex, MutexGuard};
    ///
    /// let data = Mutex::new(Some("value".to_string()));
    /// {
    ///     let locked_str = MutexGuard::map(data.lock().await, |opt| opt.as_mut().unwrap());
    ///     let locked_char = MappedMutexGuard::map(locked_str, |s| s.get_mut(0..1).unwrap());
    ///     assert_eq!(&*locked_char, "v");
    /// }
    /// # });
    /// ```
    #[inline]
    pub fn map<V: ?Sized, F>(this: Self, f: F) -> MappedMutexGuard<'a, T, V>
    where
        F: FnOnce(&mut U) -> &mut V,
    {
        let mutex = this.mutex;
        let value = f(unsafe { &mut *this.value });
        // Don't run the `drop` method for MappedMutexGuard. The ownership of the underlying
        // locked state is being moved to the returned MappedMutexGuard.
        mem::forget(this);
        MappedMutexGuard { mutex, value, _marker: PhantomData }
    }
}

impl<T: ?Sized, U: ?Sized + fmt::Debug> fmt::Debug for MappedMutexGuard<'_, T, U> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("MappedMutexGuard")
            .field("value", &&**self)
            .field("mutex", &self.mutex)
            .finish()
    }
}

impl<T: ?Sized, U: ?Sized> Drop for MappedMutexGuard<'_, T, U> {
    fn drop(&mut self) {
        self.mutex.unlock()
    }
}

impl<T: ?Sized, U: ?Sized> Deref for MappedMutexGuard<'_, T, U> {
    type Target = U;
    fn deref(&self) -> &U {
        unsafe { &*self.value }
    }
}

impl<T: ?Sized, U: ?Sized> DerefMut for MappedMutexGuard<'_, T, U> {
    fn deref_mut(&mut self) -> &mut U {
        unsafe { &mut *self.value }
    }
}

// Mutexes can be moved freely between threads and acquired on any thread so long
// as the inner value can be safely sent between threads.
unsafe impl<T: ?Sized + Send> Send for Mutex<T> {}
unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}

// It's safe to switch which thread the acquire is being attempted on so long as
// `T` can be accessed on that thread.
unsafe impl<T: ?Sized + Send> Send for MutexLockFuture<'_, T> {}
// doesn't have any interesting `&self` methods (only Debug)
unsafe impl<T: ?Sized> Sync for MutexLockFuture<'_, T> {}

// Safe to send since we don't track any thread-specific details-- the inner
// lock is essentially spinlock-equivalent (attempt to flip an atomic bool)
unsafe impl<T: ?Sized + Send> Send for MutexGuard<'_, T> {}
unsafe impl<T: ?Sized + Sync> Sync for MutexGuard<'_, T> {}
unsafe impl<T: ?Sized + Send, U: ?Sized + Send> Send for MappedMutexGuard<'_, T, U> {}
unsafe impl<T: ?Sized + Sync, U: ?Sized + Sync> Sync for MappedMutexGuard<'_, T, U> {}

#[test]
fn test_mutex_guard_debug_not_recurse() {
    let mutex = Mutex::new(42);
    let guard = mutex.try_lock().unwrap();
    let _ = format!("{:?}", guard);
    let guard = MutexGuard::map(guard, |n| n);
    let _ = format!("{:?}", guard);
}