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
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
use super::batch_semaphore as ll; // low level implementation
use super::{AcquireError, TryAcquireError};
#[cfg(all(tokio_unstable, feature = "tracing"))]
use crate::util::trace;
use std::sync::Arc;

/// Counting semaphore performing asynchronous permit acquisition.
///
/// A semaphore maintains a set of permits. Permits are used to synchronize
/// access to a shared resource. A semaphore differs from a mutex in that it
/// can allow more than one concurrent caller to access the shared resource at a
/// time.
///
/// When `acquire` is called and the semaphore has remaining permits, the
/// function immediately returns a permit. However, if no remaining permits are
/// available, `acquire` (asynchronously) waits until an outstanding permit is
/// dropped. At this point, the freed permit is assigned to the caller.
///
/// This `Semaphore` is fair, which means that permits are given out in the order
/// they were requested. This fairness is also applied when `acquire_many` gets
/// involved, so if a call to `acquire_many` at the front of the queue requests
/// more permits than currently available, this can prevent a call to `acquire`
/// from completing, even if the semaphore has enough permits complete the call
/// to `acquire`.
///
/// To use the `Semaphore` in a poll function, you can use the [`PollSemaphore`]
/// utility.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use tokio::sync::{Semaphore, TryAcquireError};
///
/// #[tokio::main]
/// async fn main() {
///     let semaphore = Semaphore::new(3);
///
///     let a_permit = semaphore.acquire().await.unwrap();
///     let two_permits = semaphore.acquire_many(2).await.unwrap();
///
///     assert_eq!(semaphore.available_permits(), 0);
///
///     let permit_attempt = semaphore.try_acquire();
///     assert_eq!(permit_attempt.err(), Some(TryAcquireError::NoPermits));
/// }
/// ```
///
/// Use [`Semaphore::acquire_owned`] to move permits across tasks:
///
/// ```
/// use std::sync::Arc;
/// use tokio::sync::Semaphore;
///
/// #[tokio::main]
/// async fn main() {
///     let semaphore = Arc::new(Semaphore::new(3));
///     let mut join_handles = Vec::new();
///
///     for _ in 0..5 {
///         let permit = semaphore.clone().acquire_owned().await.unwrap();
///         join_handles.push(tokio::spawn(async move {
///             // perform task...
///             // explicitly own `permit` in the task
///             drop(permit);
///         }));
///     }
///
///     for handle in join_handles {
///         handle.await.unwrap();
///     }
/// }
/// ```
///
/// [`PollSemaphore`]: https://docs.rs/tokio-util/0.6/tokio_util/sync/struct.PollSemaphore.html
/// [`Semaphore::acquire_owned`]: crate::sync::Semaphore::acquire_owned
#[derive(Debug)]
pub struct Semaphore {
    /// The low level semaphore
    ll_sem: ll::Semaphore,
    #[cfg(all(tokio_unstable, feature = "tracing"))]
    resource_span: tracing::Span,
}

/// A permit from the semaphore.
///
/// This type is created by the [`acquire`] method.
///
/// [`acquire`]: crate::sync::Semaphore::acquire()
#[must_use]
#[derive(Debug)]
pub struct SemaphorePermit<'a> {
    sem: &'a Semaphore,
    permits: u32,
}

/// An owned permit from the semaphore.
///
/// This type is created by the [`acquire_owned`] method.
///
/// [`acquire_owned`]: crate::sync::Semaphore::acquire_owned()
#[must_use]
#[derive(Debug)]
pub struct OwnedSemaphorePermit {
    sem: Arc<Semaphore>,
    permits: u32,
}

#[test]
#[cfg(not(loom))]
fn bounds() {
    fn check_unpin<T: Unpin>() {}
    // This has to take a value, since the async fn's return type is unnameable.
    fn check_send_sync_val<T: Send + Sync>(_t: T) {}
    fn check_send_sync<T: Send + Sync>() {}
    check_unpin::<Semaphore>();
    check_unpin::<SemaphorePermit<'_>>();
    check_send_sync::<Semaphore>();

    let semaphore = Semaphore::new(0);
    check_send_sync_val(semaphore.acquire());
}

impl Semaphore {
    /// Creates a new semaphore with the initial number of permits.
    #[track_caller]
    pub fn new(permits: usize) -> Self {
        #[cfg(all(tokio_unstable, feature = "tracing"))]
        let resource_span = {
            let location = std::panic::Location::caller();

            tracing::trace_span!(
                "runtime.resource",
                concrete_type = "Semaphore",
                kind = "Sync",
                loc.file = location.file(),
                loc.line = location.line(),
                loc.col = location.column(),
                inherits_child_attrs = true,
            )
        };

        #[cfg(all(tokio_unstable, feature = "tracing"))]
        let ll_sem = resource_span.in_scope(|| ll::Semaphore::new(permits));

        #[cfg(any(not(tokio_unstable), not(feature = "tracing")))]
        let ll_sem = ll::Semaphore::new(permits);

        Self {
            ll_sem,
            #[cfg(all(tokio_unstable, feature = "tracing"))]
            resource_span,
        }
    }

    /// Creates a new semaphore with the initial number of permits.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::Semaphore;
    ///
    /// static SEM: Semaphore = Semaphore::const_new(10);
    /// ```
    ///
    #[cfg(all(feature = "parking_lot", not(all(loom, test))))]
    #[cfg_attr(docsrs, doc(cfg(feature = "parking_lot")))]
    pub const fn const_new(permits: usize) -> Self {
        #[cfg(all(tokio_unstable, feature = "tracing"))]
        return Self {
            ll_sem: ll::Semaphore::const_new(permits),
            resource_span: tracing::Span::none(),
        };

        #[cfg(any(not(tokio_unstable), not(feature = "tracing")))]
        return Self {
            ll_sem: ll::Semaphore::const_new(permits),
        };
    }

    /// Returns the current number of available permits.
    pub fn available_permits(&self) -> usize {
        self.ll_sem.available_permits()
    }

    /// Adds `n` new permits to the semaphore.
    ///
    /// The maximum number of permits is `usize::MAX >> 3`, and this function will panic if the limit is exceeded.
    pub fn add_permits(&self, n: usize) {
        self.ll_sem.release(n);
    }

    /// Acquires a permit from the semaphore.
    ///
    /// If the semaphore has been closed, this returns an [`AcquireError`].
    /// Otherwise, this returns a [`SemaphorePermit`] representing the
    /// acquired permit.
    ///
    /// # Cancel safety
    ///
    /// This method uses a queue to fairly distribute permits in the order they
    /// were requested. Cancelling a call to `acquire` makes you lose your place
    /// in the queue.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::Semaphore;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let semaphore = Semaphore::new(2);
    ///
    ///     let permit_1 = semaphore.acquire().await.unwrap();
    ///     assert_eq!(semaphore.available_permits(), 1);
    ///
    ///     let permit_2 = semaphore.acquire().await.unwrap();
    ///     assert_eq!(semaphore.available_permits(), 0);
    ///
    ///     drop(permit_1);
    ///     assert_eq!(semaphore.available_permits(), 1);
    /// }
    /// ```
    ///
    /// [`AcquireError`]: crate::sync::AcquireError
    /// [`SemaphorePermit`]: crate::sync::SemaphorePermit
    pub async fn acquire(&self) -> Result<SemaphorePermit<'_>, AcquireError> {
        #[cfg(all(tokio_unstable, feature = "tracing"))]
        let inner = trace::async_op(
            || self.ll_sem.acquire(1),
            self.resource_span.clone(),
            "Semaphore::acquire",
            "poll",
            true,
        );
        #[cfg(not(all(tokio_unstable, feature = "tracing")))]
        let inner = self.ll_sem.acquire(1);

        inner.await?;
        Ok(SemaphorePermit {
            sem: self,
            permits: 1,
        })
    }

    /// Acquires `n` permits from the semaphore.
    ///
    /// If the semaphore has been closed, this returns an [`AcquireError`].
    /// Otherwise, this returns a [`SemaphorePermit`] representing the
    /// acquired permits.
    ///
    /// # Cancel safety
    ///
    /// This method uses a queue to fairly distribute permits in the order they
    /// were requested. Cancelling a call to `acquire_many` makes you lose your
    /// place in the queue.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::Semaphore;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let semaphore = Semaphore::new(5);
    ///
    ///     let permit = semaphore.acquire_many(3).await.unwrap();
    ///     assert_eq!(semaphore.available_permits(), 2);
    /// }
    /// ```
    ///
    /// [`AcquireError`]: crate::sync::AcquireError
    /// [`SemaphorePermit`]: crate::sync::SemaphorePermit
    pub async fn acquire_many(&self, n: u32) -> Result<SemaphorePermit<'_>, AcquireError> {
        #[cfg(all(tokio_unstable, feature = "tracing"))]
        trace::async_op(
            || self.ll_sem.acquire(n),
            self.resource_span.clone(),
            "Semaphore::acquire_many",
            "poll",
            true,
        )
        .await?;

        #[cfg(not(all(tokio_unstable, feature = "tracing")))]
        self.ll_sem.acquire(n).await?;

        Ok(SemaphorePermit {
            sem: self,
            permits: n,
        })
    }

    /// Tries to acquire a permit from the semaphore.
    ///
    /// If the semaphore has been closed, this returns a [`TryAcquireError::Closed`]
    /// and a [`TryAcquireError::NoPermits`] if there are no permits left. Otherwise,
    /// this returns a [`SemaphorePermit`] representing the acquired permits.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::{Semaphore, TryAcquireError};
    ///
    /// # fn main() {
    /// let semaphore = Semaphore::new(2);
    ///
    /// let permit_1 = semaphore.try_acquire().unwrap();
    /// assert_eq!(semaphore.available_permits(), 1);
    ///
    /// let permit_2 = semaphore.try_acquire().unwrap();
    /// assert_eq!(semaphore.available_permits(), 0);
    ///
    /// let permit_3 = semaphore.try_acquire();
    /// assert_eq!(permit_3.err(), Some(TryAcquireError::NoPermits));
    /// # }
    /// ```
    ///
    /// [`TryAcquireError::Closed`]: crate::sync::TryAcquireError::Closed
    /// [`TryAcquireError::NoPermits`]: crate::sync::TryAcquireError::NoPermits
    /// [`SemaphorePermit`]: crate::sync::SemaphorePermit
    pub fn try_acquire(&self) -> Result<SemaphorePermit<'_>, TryAcquireError> {
        match self.ll_sem.try_acquire(1) {
            Ok(_) => Ok(SemaphorePermit {
                sem: self,
                permits: 1,
            }),
            Err(e) => Err(e),
        }
    }

    /// Tries to acquire `n` permits from the semaphore.
    ///
    /// If the semaphore has been closed, this returns a [`TryAcquireError::Closed`]
    /// and a [`TryAcquireError::NoPermits`] if there are not enough permits left.
    /// Otherwise, this returns a [`SemaphorePermit`] representing the acquired permits.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::{Semaphore, TryAcquireError};
    ///
    /// # fn main() {
    /// let semaphore = Semaphore::new(4);
    ///
    /// let permit_1 = semaphore.try_acquire_many(3).unwrap();
    /// assert_eq!(semaphore.available_permits(), 1);
    ///
    /// let permit_2 = semaphore.try_acquire_many(2);
    /// assert_eq!(permit_2.err(), Some(TryAcquireError::NoPermits));
    /// # }
    /// ```
    ///
    /// [`TryAcquireError::Closed`]: crate::sync::TryAcquireError::Closed
    /// [`TryAcquireError::NoPermits`]: crate::sync::TryAcquireError::NoPermits
    /// [`SemaphorePermit`]: crate::sync::SemaphorePermit
    pub fn try_acquire_many(&self, n: u32) -> Result<SemaphorePermit<'_>, TryAcquireError> {
        match self.ll_sem.try_acquire(n) {
            Ok(_) => Ok(SemaphorePermit {
                sem: self,
                permits: n,
            }),
            Err(e) => Err(e),
        }
    }

    /// Acquires a permit from the semaphore.
    ///
    /// The semaphore must be wrapped in an [`Arc`] to call this method.
    /// If the semaphore has been closed, this returns an [`AcquireError`].
    /// Otherwise, this returns a [`OwnedSemaphorePermit`] representing the
    /// acquired permit.
    ///
    /// # Cancel safety
    ///
    /// This method uses a queue to fairly distribute permits in the order they
    /// were requested. Cancelling a call to `acquire_owned` makes you lose your
    /// place in the queue.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    /// use tokio::sync::Semaphore;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let semaphore = Arc::new(Semaphore::new(3));
    ///     let mut join_handles = Vec::new();
    ///
    ///     for _ in 0..5 {
    ///         let permit = semaphore.clone().acquire_owned().await.unwrap();
    ///         join_handles.push(tokio::spawn(async move {
    ///             // perform task...
    ///             // explicitly own `permit` in the task
    ///             drop(permit);
    ///         }));
    ///     }
    ///
    ///     for handle in join_handles {
    ///         handle.await.unwrap();
    ///     }
    /// }
    /// ```
    ///
    /// [`Arc`]: std::sync::Arc
    /// [`AcquireError`]: crate::sync::AcquireError
    /// [`OwnedSemaphorePermit`]: crate::sync::OwnedSemaphorePermit
    pub async fn acquire_owned(self: Arc<Self>) -> Result<OwnedSemaphorePermit, AcquireError> {
        #[cfg(all(tokio_unstable, feature = "tracing"))]
        let inner = trace::async_op(
            || self.ll_sem.acquire(1),
            self.resource_span.clone(),
            "Semaphore::acquire_owned",
            "poll",
            true,
        );
        #[cfg(not(all(tokio_unstable, feature = "tracing")))]
        let inner = self.ll_sem.acquire(1);

        inner.await?;
        Ok(OwnedSemaphorePermit {
            sem: self,
            permits: 1,
        })
    }

    /// Acquires `n` permits from the semaphore.
    ///
    /// The semaphore must be wrapped in an [`Arc`] to call this method.
    /// If the semaphore has been closed, this returns an [`AcquireError`].
    /// Otherwise, this returns a [`OwnedSemaphorePermit`] representing the
    /// acquired permit.
    ///
    /// # Cancel safety
    ///
    /// This method uses a queue to fairly distribute permits in the order they
    /// were requested. Cancelling a call to `acquire_many_owned` makes you lose
    /// your place in the queue.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    /// use tokio::sync::Semaphore;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let semaphore = Arc::new(Semaphore::new(10));
    ///     let mut join_handles = Vec::new();
    ///
    ///     for _ in 0..5 {
    ///         let permit = semaphore.clone().acquire_many_owned(2).await.unwrap();
    ///         join_handles.push(tokio::spawn(async move {
    ///             // perform task...
    ///             // explicitly own `permit` in the task
    ///             drop(permit);
    ///         }));
    ///     }
    ///
    ///     for handle in join_handles {
    ///         handle.await.unwrap();
    ///     }
    /// }
    /// ```
    ///
    /// [`Arc`]: std::sync::Arc
    /// [`AcquireError`]: crate::sync::AcquireError
    /// [`OwnedSemaphorePermit`]: crate::sync::OwnedSemaphorePermit
    pub async fn acquire_many_owned(
        self: Arc<Self>,
        n: u32,
    ) -> Result<OwnedSemaphorePermit, AcquireError> {
        #[cfg(all(tokio_unstable, feature = "tracing"))]
        let inner = trace::async_op(
            || self.ll_sem.acquire(n),
            self.resource_span.clone(),
            "Semaphore::acquire_many_owned",
            "poll",
            true,
        );
        #[cfg(not(all(tokio_unstable, feature = "tracing")))]
        let inner = self.ll_sem.acquire(n);

        inner.await?;
        Ok(OwnedSemaphorePermit {
            sem: self,
            permits: n,
        })
    }

    /// Tries to acquire a permit from the semaphore.
    ///
    /// The semaphore must be wrapped in an [`Arc`] to call this method. If
    /// the semaphore has been closed, this returns a [`TryAcquireError::Closed`]
    /// and a [`TryAcquireError::NoPermits`] if there are no permits left.
    /// Otherwise, this returns a [`OwnedSemaphorePermit`] representing the
    /// acquired permit.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    /// use tokio::sync::{Semaphore, TryAcquireError};
    ///
    /// # fn main() {
    /// let semaphore = Arc::new(Semaphore::new(2));
    ///
    /// let permit_1 = Arc::clone(&semaphore).try_acquire_owned().unwrap();
    /// assert_eq!(semaphore.available_permits(), 1);
    ///
    /// let permit_2 = Arc::clone(&semaphore).try_acquire_owned().unwrap();
    /// assert_eq!(semaphore.available_permits(), 0);
    ///
    /// let permit_3 = semaphore.try_acquire_owned();
    /// assert_eq!(permit_3.err(), Some(TryAcquireError::NoPermits));
    /// # }
    /// ```
    ///
    /// [`Arc`]: std::sync::Arc
    /// [`TryAcquireError::Closed`]: crate::sync::TryAcquireError::Closed
    /// [`TryAcquireError::NoPermits`]: crate::sync::TryAcquireError::NoPermits
    /// [`OwnedSemaphorePermit`]: crate::sync::OwnedSemaphorePermit
    pub fn try_acquire_owned(self: Arc<Self>) -> Result<OwnedSemaphorePermit, TryAcquireError> {
        match self.ll_sem.try_acquire(1) {
            Ok(_) => Ok(OwnedSemaphorePermit {
                sem: self,
                permits: 1,
            }),
            Err(e) => Err(e),
        }
    }

    /// Tries to acquire `n` permits from the semaphore.
    ///
    /// The semaphore must be wrapped in an [`Arc`] to call this method. If
    /// the semaphore has been closed, this returns a [`TryAcquireError::Closed`]
    /// and a [`TryAcquireError::NoPermits`] if there are no permits left.
    /// Otherwise, this returns a [`OwnedSemaphorePermit`] representing the
    /// acquired permit.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    /// use tokio::sync::{Semaphore, TryAcquireError};
    ///
    /// # fn main() {
    /// let semaphore = Arc::new(Semaphore::new(4));
    ///
    /// let permit_1 = Arc::clone(&semaphore).try_acquire_many_owned(3).unwrap();
    /// assert_eq!(semaphore.available_permits(), 1);
    ///
    /// let permit_2 = semaphore.try_acquire_many_owned(2);
    /// assert_eq!(permit_2.err(), Some(TryAcquireError::NoPermits));
    /// # }
    /// ```
    ///
    /// [`Arc`]: std::sync::Arc
    /// [`TryAcquireError::Closed`]: crate::sync::TryAcquireError::Closed
    /// [`TryAcquireError::NoPermits`]: crate::sync::TryAcquireError::NoPermits
    /// [`OwnedSemaphorePermit`]: crate::sync::OwnedSemaphorePermit
    pub fn try_acquire_many_owned(
        self: Arc<Self>,
        n: u32,
    ) -> Result<OwnedSemaphorePermit, TryAcquireError> {
        match self.ll_sem.try_acquire(n) {
            Ok(_) => Ok(OwnedSemaphorePermit {
                sem: self,
                permits: n,
            }),
            Err(e) => Err(e),
        }
    }

    /// Closes the semaphore.
    ///
    /// This prevents the semaphore from issuing new permits and notifies all pending waiters.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::Semaphore;
    /// use std::sync::Arc;
    /// use tokio::sync::TryAcquireError;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let semaphore = Arc::new(Semaphore::new(1));
    ///     let semaphore2 = semaphore.clone();
    ///
    ///     tokio::spawn(async move {
    ///         let permit = semaphore.acquire_many(2).await;
    ///         assert!(permit.is_err());
    ///         println!("waiter received error");
    ///     });
    ///
    ///     println!("closing semaphore");
    ///     semaphore2.close();
    ///
    ///     // Cannot obtain more permits
    ///     assert_eq!(semaphore2.try_acquire().err(), Some(TryAcquireError::Closed))
    /// }
    /// ```
    pub fn close(&self) {
        self.ll_sem.close();
    }

    /// Returns true if the semaphore is closed
    pub fn is_closed(&self) -> bool {
        self.ll_sem.is_closed()
    }
}

impl<'a> SemaphorePermit<'a> {
    /// Forgets the permit **without** releasing it back to the semaphore.
    /// This can be used to reduce the amount of permits available from a
    /// semaphore.
    pub fn forget(mut self) {
        self.permits = 0;
    }
}

impl OwnedSemaphorePermit {
    /// Forgets the permit **without** releasing it back to the semaphore.
    /// This can be used to reduce the amount of permits available from a
    /// semaphore.
    pub fn forget(mut self) {
        self.permits = 0;
    }
}

impl<'a> Drop for SemaphorePermit<'_> {
    fn drop(&mut self) {
        self.sem.add_permits(self.permits as usize);
    }
}

impl Drop for OwnedSemaphorePermit {
    fn drop(&mut self) {
        self.sem.add_permits(self.permits as usize);
    }
}