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
use core::fmt;
use core::marker::PhantomPinned;
use core::mem;
use core::pin::Pin;
use core::task::Poll;

use crate::sync::atomic::{AtomicUsize, Ordering};

use alloc::sync::Arc;

use event_listener::{Event, EventListener};
use event_listener_strategy::{easy_wrapper, EventListenerFuture, Strategy};

/// A counter for limiting the number of concurrent operations.
#[derive(Debug)]
pub struct Semaphore {
    count: AtomicUsize,
    event: Event,
}

impl Semaphore {
    const_fn! {
        const_if: #[cfg(not(loom))];
        /// Creates a new semaphore with a limit of `n` concurrent operations.
        ///
        /// # Examples
        ///
        /// ```
        /// use async_lock::Semaphore;
        ///
        /// let s = Semaphore::new(5);
        /// ```
        pub const fn new(n: usize) -> Semaphore {
            Semaphore {
                count: AtomicUsize::new(n),
                event: Event::new(),
            }
        }
    }

    /// Attempts to get a permit for a concurrent operation.
    ///
    /// If the permit 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_lock::Semaphore;
    ///
    /// let s = Semaphore::new(2);
    ///
    /// let g1 = s.try_acquire().unwrap();
    /// let g2 = s.try_acquire().unwrap();
    ///
    /// assert!(s.try_acquire().is_none());
    /// drop(g2);
    /// assert!(s.try_acquire().is_some());
    /// ```
    pub fn try_acquire(&self) -> Option<SemaphoreGuard<'_>> {
        let mut count = self.count.load(Ordering::Acquire);
        loop {
            if count == 0 {
                return None;
            }

            match self.count.compare_exchange_weak(
                count,
                count - 1,
                Ordering::AcqRel,
                Ordering::Acquire,
            ) {
                Ok(_) => return Some(SemaphoreGuard(self)),
                Err(c) => count = c,
            }
        }
    }

    /// Waits for a permit for a concurrent operation.
    ///
    /// Returns a guard that releases the permit when dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_lock::Semaphore;
    ///
    /// let s = Semaphore::new(2);
    /// let guard = s.acquire().await;
    /// # });
    /// ```
    pub fn acquire(&self) -> Acquire<'_> {
        Acquire::_new(AcquireInner {
            semaphore: self,
            listener: None,
            _pin: PhantomPinned,
        })
    }

    /// Waits for a permit for a concurrent operation.
    ///
    /// Returns a guard that releases the permit when dropped.
    ///
    /// # Blocking
    ///
    /// Rather than using asynchronous waiting, like the [`acquire`][Semaphore::acquire] method,
    /// this method will block the current thread until the permit is acquired.
    ///
    /// This method should not be used in an asynchronous context. It is intended to be
    /// used in a way that a semaphore can be used in both asynchronous and synchronous contexts.
    /// Calling this method in an asynchronous context may result in a deadlock.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_lock::Semaphore;
    ///
    /// let s = Semaphore::new(2);
    /// let guard = s.acquire_blocking();
    /// ```
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    #[inline]
    pub fn acquire_blocking(&self) -> SemaphoreGuard<'_> {
        self.acquire().wait()
    }

    /// Attempts to get an owned permit for a concurrent operation.
    ///
    /// If the permit 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_lock::Semaphore;
    /// use std::sync::Arc;
    ///
    /// let s = Arc::new(Semaphore::new(2));
    ///
    /// let g1 = s.try_acquire_arc().unwrap();
    /// let g2 = s.try_acquire_arc().unwrap();
    ///
    /// assert!(s.try_acquire_arc().is_none());
    /// drop(g2);
    /// assert!(s.try_acquire_arc().is_some());
    /// ```
    pub fn try_acquire_arc(self: &Arc<Self>) -> Option<SemaphoreGuardArc> {
        let mut count = self.count.load(Ordering::Acquire);
        loop {
            if count == 0 {
                return None;
            }

            match self.count.compare_exchange_weak(
                count,
                count - 1,
                Ordering::AcqRel,
                Ordering::Acquire,
            ) {
                Ok(_) => return Some(SemaphoreGuardArc(Some(self.clone()))),
                Err(c) => count = c,
            }
        }
    }

    /// Waits for an owned permit for a concurrent operation.
    ///
    /// Returns a guard that releases the permit when dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures_lite::future::block_on(async {
    /// use async_lock::Semaphore;
    /// use std::sync::Arc;
    ///
    /// let s = Arc::new(Semaphore::new(2));
    /// let guard = s.acquire_arc().await;
    /// # });
    /// ```
    pub fn acquire_arc(self: &Arc<Self>) -> AcquireArc {
        AcquireArc::_new(AcquireArcInner {
            semaphore: self.clone(),
            listener: None,
            _pin: PhantomPinned,
        })
    }

    /// Waits for an owned permit for a concurrent operation.
    ///
    /// Returns a guard that releases the permit when dropped.
    ///
    /// # Blocking
    ///
    /// Rather than using asynchronous waiting, like the [`acquire_arc`][Semaphore::acquire_arc] method,
    /// this method will block the current thread until the permit is acquired.
    ///
    /// This method should not be used in an asynchronous context. It is intended to be
    /// used in a way that a semaphore can be used in both asynchronous and synchronous contexts.
    /// Calling this method in an asynchronous context may result in a deadlock.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    /// use async_lock::Semaphore;
    ///
    /// let s = Arc::new(Semaphore::new(2));
    /// let guard = s.acquire_arc_blocking();
    /// ```
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    #[inline]
    pub fn acquire_arc_blocking(self: &Arc<Self>) -> SemaphoreGuardArc {
        self.acquire_arc().wait()
    }

    /// Adds `n` additional permits to the semaphore.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_lock::Semaphore;
    ///
    /// # futures_lite::future::block_on(async {
    /// let s = Semaphore::new(1);
    ///
    /// let _guard = s.acquire().await;
    /// assert!(s.try_acquire().is_none());
    ///
    /// s.add_permits(2);
    ///
    /// let _guard = s.acquire().await;
    /// let _guard = s.acquire().await;
    /// # });
    /// ```
    pub fn add_permits(&self, n: usize) {
        self.count.fetch_add(n, Ordering::AcqRel);
        self.event.notify(n);
    }
}

easy_wrapper! {
    /// The future returned by [`Semaphore::acquire`].
    pub struct Acquire<'a>(AcquireInner<'a> => SemaphoreGuard<'a>);
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    pub(crate) wait();
}

pin_project_lite::pin_project! {
    struct AcquireInner<'a> {
        // The semaphore being acquired.
        semaphore: &'a Semaphore,

        // The listener waiting on the semaphore.
        listener: Option<EventListener>,

        // Keeping this future `!Unpin` enables future optimizations.
        #[pin]
        _pin: PhantomPinned
    }
}

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

impl<'a> EventListenerFuture for AcquireInner<'a> {
    type Output = SemaphoreGuard<'a>;

    fn poll_with_strategy<'x, S: Strategy<'x>>(
        self: Pin<&mut Self>,
        strategy: &mut S,
        cx: &mut S::Context,
    ) -> Poll<Self::Output> {
        let this = self.project();

        loop {
            match this.semaphore.try_acquire() {
                Some(guard) => return Poll::Ready(guard),
                None => {
                    // Wait on the listener.
                    if this.listener.is_none() {
                        *this.listener = Some(this.semaphore.event.listen());
                    } else {
                        ready!(strategy.poll(this.listener, cx));
                    }
                }
            }
        }
    }
}

easy_wrapper! {
    /// The future returned by [`Semaphore::acquire_arc`].
    pub struct AcquireArc(AcquireArcInner => SemaphoreGuardArc);
    #[cfg(all(feature = "std", not(target_family = "wasm")))]
    pub(crate) wait();
}

pin_project_lite::pin_project! {
    struct AcquireArcInner {
        // The semaphore being acquired.
        semaphore: Arc<Semaphore>,

        // The listener waiting on the semaphore.
        listener: Option<EventListener>,

        // Keeping this future `!Unpin` enables future optimizations.
        #[pin]
        _pin: PhantomPinned
    }
}

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

impl EventListenerFuture for AcquireArcInner {
    type Output = SemaphoreGuardArc;

    fn poll_with_strategy<'x, S: Strategy<'x>>(
        self: Pin<&mut Self>,
        strategy: &mut S,
        cx: &mut S::Context,
    ) -> Poll<Self::Output> {
        let this = self.project();

        loop {
            match this.semaphore.try_acquire_arc() {
                Some(guard) => return Poll::Ready(guard),
                None => {
                    // Wait on the listener.
                    if this.listener.is_none() {
                        *this.listener = Some(this.semaphore.event.listen());
                    } else {
                        ready!(strategy.poll(this.listener, cx));
                    }
                }
            }
        }
    }
}

/// A guard that releases the acquired permit.
#[clippy::has_significant_drop]
#[derive(Debug)]
pub struct SemaphoreGuard<'a>(&'a Semaphore);

impl SemaphoreGuard<'_> {
    /// Drops the guard _without_ releasing the acquired permit.
    #[inline]
    pub fn forget(self) {
        mem::forget(self);
    }
}

impl Drop for SemaphoreGuard<'_> {
    fn drop(&mut self) {
        self.0.count.fetch_add(1, Ordering::AcqRel);
        self.0.event.notify(1);
    }
}

/// An owned guard that releases the acquired permit.
#[clippy::has_significant_drop]
#[derive(Debug)]
pub struct SemaphoreGuardArc(Option<Arc<Semaphore>>);

impl SemaphoreGuardArc {
    /// Drops the guard _without_ releasing the acquired permit.
    /// (Will still decrement the `Arc` reference count.)
    #[inline]
    pub fn forget(mut self) {
        // Drop the inner `Arc` in order to decrement the reference count.
        // FIXME: get rid of the `Option` once RFC 3466 or equivalent becomes available.
        drop(self.0.take());
        mem::forget(self);
    }
}

impl Drop for SemaphoreGuardArc {
    fn drop(&mut self) {
        let opt = self.0.take().unwrap();
        opt.count.fetch_add(1, Ordering::AcqRel);
        opt.event.notify(1);
    }
}