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
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
use cell::UnsafeCell;
use fmt;
use mem;
use ops::{Deref, DerefMut};
use ptr;
use sys_common::poison::{self, LockResult, TryLockError, TryLockResult};
use sys_common::rwlock as sys;

/// A reader-writer lock
///
/// This type of lock allows a number of readers or at most one writer at any
/// point in time. The write portion of this lock typically allows modification
/// of the underlying data (exclusive access) and the read portion of this lock
/// typically allows for read-only access (shared access).
///
/// In comparison, a [`Mutex`] does not distinguish between readers or writers
/// that acquire the lock, therefore blocking any threads waiting for the lock to
/// become available. An `RwLock` will allow any number of readers to acquire the
/// lock as long as a writer is not holding the lock.
///
/// The priority policy of the lock is dependent on the underlying operating
/// system's implementation, and this type does not guarantee that any
/// particular policy will be used.
///
/// The type parameter `T` represents the data that this lock protects. It is
/// required that `T` satisfies [`Send`] to be shared across threads and
/// [`Sync`] to allow concurrent access through readers. The RAII guards
/// returned from the locking methods implement [`Deref`][] (and [`DerefMut`]
/// for the `write` methods) to allow access to the content of the lock.
///
/// # Poisoning
///
/// An `RwLock`, like [`Mutex`], will become poisoned on a panic. Note, however,
/// that an `RwLock` may only be poisoned if a panic occurs while it is locked
/// exclusively (write mode). If a panic occurs in any reader, then the lock
/// will not be poisoned.
///
/// # Examples
///
/// ```
/// use std::sync::RwLock;
///
/// let lock = RwLock::new(5);
///
/// // many reader locks can be held at once
/// {
///     let r1 = lock.read().unwrap();
///     let r2 = lock.read().unwrap();
///     assert_eq!(*r1, 5);
///     assert_eq!(*r2, 5);
/// } // read locks are dropped at this point
///
/// // only one write lock may be held, however
/// {
///     let mut w = lock.write().unwrap();
///     *w += 1;
///     assert_eq!(*w, 6);
/// } // write lock is dropped here
/// ```
///
/// [`Deref`]: ../../std/ops/trait.Deref.html
/// [`DerefMut`]: ../../std/ops/trait.DerefMut.html
/// [`Send`]: ../../std/marker/trait.Send.html
/// [`Sync`]: ../../std/marker/trait.Sync.html
/// [`Mutex`]: struct.Mutex.html
#[stable(feature = "rust1", since = "1.0.0")]
pub struct RwLock<T: ?Sized> {
    inner: Box<sys::RWLock>,
    poison: poison::Flag,
    data: UnsafeCell<T>,
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Send> Send for RwLock<T> {}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}

/// RAII structure used to release the shared read access of a lock when
/// dropped.
///
/// This structure is created by the [`read`] and [`try_read`] methods on
/// [`RwLock`].
///
/// [`read`]: struct.RwLock.html#method.read
/// [`try_read`]: struct.RwLock.html#method.try_read
/// [`RwLock`]: struct.RwLock.html
#[must_use = "if unused the RwLock will immediately unlock"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct RwLockReadGuard<'a, T: ?Sized + 'a> {
    __lock: &'a RwLock<T>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T: ?Sized> !Send for RwLockReadGuard<'a, T> {}

#[stable(feature = "rwlock_guard_sync", since = "1.23.0")]
unsafe impl<'a, T: ?Sized + Sync> Sync for RwLockReadGuard<'a, T> {}

/// RAII structure used to release the exclusive write access of a lock when
/// dropped.
///
/// This structure is created by the [`write`] and [`try_write`] methods
/// on [`RwLock`].
///
/// [`write`]: struct.RwLock.html#method.write
/// [`try_write`]: struct.RwLock.html#method.try_write
/// [`RwLock`]: struct.RwLock.html
#[must_use = "if unused the RwLock will immediately unlock"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct RwLockWriteGuard<'a, T: ?Sized + 'a> {
    __lock: &'a RwLock<T>,
    __poison: poison::Guard,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T: ?Sized> !Send for RwLockWriteGuard<'a, T> {}

#[stable(feature = "rwlock_guard_sync", since = "1.23.0")]
unsafe impl<'a, T: ?Sized + Sync> Sync for RwLockWriteGuard<'a, T> {}

impl<T> RwLock<T> {
    /// Creates a new instance of an `RwLock<T>` which is unlocked.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(5);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn new(t: T) -> RwLock<T> {
        RwLock {
            inner: box sys::RWLock::new(),
            poison: poison::Flag::new(),
            data: UnsafeCell::new(t),
        }
    }
}

impl<T: ?Sized> RwLock<T> {
    /// Locks this rwlock with shared read access, blocking the current thread
    /// until it can be acquired.
    ///
    /// The calling thread will be blocked until there are no more writers which
    /// hold the lock. There may be other readers currently inside the lock when
    /// this method returns. This method does not provide any guarantees with
    /// respect to the ordering of whether contentious readers or writers will
    /// acquire the lock first.
    ///
    /// Returns an RAII guard which will release this thread's shared access
    /// once it is dropped.
    ///
    /// # Errors
    ///
    /// This function will return an error if the RwLock is poisoned. An RwLock
    /// is poisoned whenever a writer panics while holding an exclusive lock.
    /// The failure will occur immediately after the lock has been acquired.
    ///
    /// # Panics
    ///
    /// This function might panic when called if the lock is already held by the current thread.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, RwLock};
    /// use std::thread;
    ///
    /// let lock = Arc::new(RwLock::new(1));
    /// let c_lock = lock.clone();
    ///
    /// let n = lock.read().unwrap();
    /// assert_eq!(*n, 1);
    ///
    /// thread::spawn(move || {
    ///     let r = c_lock.read();
    ///     assert!(r.is_ok());
    /// }).join().unwrap();
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn read(&self) -> LockResult<RwLockReadGuard<T>> {
        unsafe {
            self.inner.read();
            RwLockReadGuard::new(self)
        }
    }

    /// Attempts to acquire this rwlock with shared read access.
    ///
    /// If the access could not be granted at this time, then `Err` is returned.
    /// Otherwise, an RAII guard is returned which will release the shared access
    /// when it is dropped.
    ///
    /// This function does not block.
    ///
    /// This function does not provide any guarantees with respect to the ordering
    /// of whether contentious readers or writers will acquire the lock first.
    ///
    /// # Errors
    ///
    /// This function will return an error if the RwLock is poisoned. An RwLock
    /// is poisoned whenever a writer panics while holding an exclusive lock. An
    /// error will only be returned if the lock would have otherwise been
    /// acquired.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(1);
    ///
    /// match lock.try_read() {
    ///     Ok(n) => assert_eq!(*n, 1),
    ///     Err(_) => unreachable!(),
    /// };
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn try_read(&self) -> TryLockResult<RwLockReadGuard<T>> {
        unsafe {
            if self.inner.try_read() {
                Ok(RwLockReadGuard::new(self)?)
            } else {
                Err(TryLockError::WouldBlock)
            }
        }
    }

    /// Locks this rwlock with exclusive write access, blocking the current
    /// thread until it can be acquired.
    ///
    /// This function will not return while other writers or other readers
    /// currently have access to the lock.
    ///
    /// Returns an RAII guard which will drop the write access of this rwlock
    /// when dropped.
    ///
    /// # Errors
    ///
    /// This function will return an error if the RwLock is poisoned. An RwLock
    /// is poisoned whenever a writer panics while holding an exclusive lock.
    /// An error will be returned when the lock is acquired.
    ///
    /// # Panics
    ///
    /// This function might panic when called if the lock is already held by the current thread.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(1);
    ///
    /// let mut n = lock.write().unwrap();
    /// *n = 2;
    ///
    /// assert!(lock.try_read().is_err());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn write(&self) -> LockResult<RwLockWriteGuard<T>> {
        unsafe {
            self.inner.write();
            RwLockWriteGuard::new(self)
        }
    }

    /// Attempts to lock this rwlock with exclusive write access.
    ///
    /// If the lock could not be acquired at this time, then `Err` is returned.
    /// Otherwise, an RAII guard is returned which will release the lock when
    /// it is dropped.
    ///
    /// This function does not block.
    ///
    /// This function does not provide any guarantees with respect to the ordering
    /// of whether contentious readers or writers will acquire the lock first.
    ///
    /// # Errors
    ///
    /// This function will return an error if the RwLock is poisoned. An RwLock
    /// is poisoned whenever a writer panics while holding an exclusive lock. An
    /// error will only be returned if the lock would have otherwise been
    /// acquired.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(1);
    ///
    /// let n = lock.read().unwrap();
    /// assert_eq!(*n, 1);
    ///
    /// assert!(lock.try_write().is_err());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn try_write(&self) -> TryLockResult<RwLockWriteGuard<T>> {
        unsafe {
            if self.inner.try_write() {
                Ok(RwLockWriteGuard::new(self)?)
            } else {
                Err(TryLockError::WouldBlock)
            }
        }
    }

    /// Determines whether the lock is poisoned.
    ///
    /// If another thread is active, the lock can still become poisoned at any
    /// time.  You should not trust a `false` value for program correctness
    /// without additional synchronization.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, RwLock};
    /// use std::thread;
    ///
    /// let lock = Arc::new(RwLock::new(0));
    /// let c_lock = lock.clone();
    ///
    /// let _ = thread::spawn(move || {
    ///     let _lock = c_lock.write().unwrap();
    ///     panic!(); // the lock gets poisoned
    /// }).join();
    /// assert_eq!(lock.is_poisoned(), true);
    /// ```
    #[inline]
    #[stable(feature = "sync_poison", since = "1.2.0")]
    pub fn is_poisoned(&self) -> bool {
        self.poison.get()
    }

    /// Consumes this `RwLock`, returning the underlying data.
    ///
    /// # Errors
    ///
    /// This function will return an error if the RwLock is poisoned. An RwLock
    /// is poisoned whenever a writer panics while holding an exclusive lock. An
    /// error will only be returned if the lock would have otherwise been
    /// acquired.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(String::new());
    /// {
    ///     let mut s = lock.write().unwrap();
    ///     *s = "modified".to_owned();
    /// }
    /// assert_eq!(lock.into_inner().unwrap(), "modified");
    /// ```
    #[stable(feature = "rwlock_into_inner", since = "1.6.0")]
    pub fn into_inner(self) -> LockResult<T> where T: Sized {
        // We know statically that there are no outstanding references to
        // `self` so there's no need to lock the inner lock.
        //
        // To get the inner value, we'd like to call `data.into_inner()`,
        // but because `RwLock` impl-s `Drop`, we can't move out of it, so
        // we'll have to destructure it manually instead.
        unsafe {
            // Like `let RwLock { inner, poison, data } = self`.
            let (inner, poison, data) = {
                let RwLock { ref inner, ref poison, ref data } = self;
                (ptr::read(inner), ptr::read(poison), ptr::read(data))
            };
            mem::forget(self);
            inner.destroy(); // Keep in sync with the `Drop` impl.
            drop(inner);

            poison::map_result(poison.borrow(), |_| data.into_inner())
        }
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// Since this call borrows the `RwLock` mutably, no actual locking needs to
    /// take place---the mutable borrow statically guarantees no locks exist.
    ///
    /// # Errors
    ///
    /// This function will return an error if the RwLock is poisoned. An RwLock
    /// is poisoned whenever a writer panics while holding an exclusive lock. An
    /// error will only be returned if the lock would have otherwise been
    /// acquired.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let mut lock = RwLock::new(0);
    /// *lock.get_mut().unwrap() = 10;
    /// assert_eq!(*lock.read().unwrap(), 10);
    /// ```
    #[stable(feature = "rwlock_get_mut", since = "1.6.0")]
    pub fn get_mut(&mut self) -> LockResult<&mut T> {
        // We know statically that there are no other references to `self`, so
        // there's no need to lock the inner lock.
        let data = unsafe { &mut *self.data.get() };
        poison::map_result(self.poison.borrow(), |_| data)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<#[may_dangle] T: ?Sized> Drop for RwLock<T> {
    fn drop(&mut self) {
        // IMPORTANT: This code needs to be kept in sync with `RwLock::into_inner`.
        unsafe { self.inner.destroy() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self.try_read() {
            Ok(guard) => f.debug_struct("RwLock").field("data", &&*guard).finish(),
            Err(TryLockError::Poisoned(err)) => {
                f.debug_struct("RwLock").field("data", &&**err.get_ref()).finish()
            },
            Err(TryLockError::WouldBlock) => {
                struct LockedPlaceholder;
                impl fmt::Debug for LockedPlaceholder {
                    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.write_str("<locked>") }
                }

                f.debug_struct("RwLock").field("data", &LockedPlaceholder).finish()
            }
        }
    }
}

#[stable(feature = "rw_lock_default", since = "1.10.0")]
impl<T: Default> Default for RwLock<T> {
    /// Creates a new `RwLock<T>`, with the `Default` value for T.
    fn default() -> RwLock<T> {
        RwLock::new(Default::default())
    }
}

#[stable(feature = "rw_lock_from", since = "1.24.0")]
impl<T> From<T> for RwLock<T> {
    /// Creates a new instance of an `RwLock<T>` which is unlocked.
    /// This is equivalent to [`RwLock::new`].
    fn from(t: T) -> Self {
        RwLock::new(t)
    }
}

impl<'rwlock, T: ?Sized> RwLockReadGuard<'rwlock, T> {
    unsafe fn new(lock: &'rwlock RwLock<T>)
                  -> LockResult<RwLockReadGuard<'rwlock, T>> {
        poison::map_result(lock.poison.borrow(), |_| {
            RwLockReadGuard {
                __lock: lock,
            }
        })
    }
}

impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
    unsafe fn new(lock: &'rwlock RwLock<T>)
                  -> LockResult<RwLockWriteGuard<'rwlock, T>> {
        poison::map_result(lock.poison.borrow(), |guard| {
            RwLockWriteGuard {
                __lock: lock,
                __poison: guard,
            }
        })
    }
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<'a, T: fmt::Debug> fmt::Debug for RwLockReadGuard<'a, T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("RwLockReadGuard")
            .field("lock", &self.__lock)
            .finish()
    }
}

#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<'a, T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'a, T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        (**self).fmt(f)
    }
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<'a, T: fmt::Debug> fmt::Debug for RwLockWriteGuard<'a, T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("RwLockWriteGuard")
            .field("lock", &self.__lock)
            .finish()
    }
}

#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<'a, T: ?Sized + fmt::Display> fmt::Display for RwLockWriteGuard<'a, T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        (**self).fmt(f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'rwlock, T: ?Sized> Deref for RwLockReadGuard<'rwlock, T> {
    type Target = T;

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

#[stable(feature = "rust1", since = "1.0.0")]
impl<'rwlock, T: ?Sized> Deref for RwLockWriteGuard<'rwlock, T> {
    type Target = T;

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

#[stable(feature = "rust1", since = "1.0.0")]
impl<'rwlock, T: ?Sized> DerefMut for RwLockWriteGuard<'rwlock, T> {
    fn deref_mut(&mut self) -> &mut T {
        unsafe { &mut *self.__lock.data.get() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T: ?Sized> Drop for RwLockReadGuard<'a, T> {
    fn drop(&mut self) {
        unsafe { self.__lock.inner.read_unlock(); }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T: ?Sized> Drop for RwLockWriteGuard<'a, T> {
    fn drop(&mut self) {
        self.__lock.poison.done(&self.__poison);
        unsafe { self.__lock.inner.write_unlock(); }
    }
}

#[cfg(all(test, not(target_os = "emscripten")))]
mod tests {
    use rand::{self, Rng};
    use sync::mpsc::channel;
    use thread;
    use sync::{Arc, RwLock, TryLockError};
    use sync::atomic::{AtomicUsize, Ordering};

    #[derive(Eq, PartialEq, Debug)]
    struct NonCopy(i32);

    #[test]
    fn smoke() {
        let l = RwLock::new(());
        drop(l.read().unwrap());
        drop(l.write().unwrap());
        drop((l.read().unwrap(), l.read().unwrap()));
        drop(l.write().unwrap());
    }

    #[test]
    fn frob() {
        const N: u32 = 10;
        const M: usize = 1000;

        let r = Arc::new(RwLock::new(()));

        let (tx, rx) = channel::<()>();
        for _ in 0..N {
            let tx = tx.clone();
            let r = r.clone();
            thread::spawn(move || {
                let mut rng = rand::thread_rng();
                for _ in 0..M {
                    if rng.gen_bool(1.0 / (N as f64)) {
                        drop(r.write().unwrap());
                    } else {
                        drop(r.read().unwrap());
                    }
                }
                drop(tx);
            });
        }
        drop(tx);
        let _ = rx.recv();
    }

    #[test]
    fn test_rw_arc_poison_wr() {
        let arc = Arc::new(RwLock::new(1));
        let arc2 = arc.clone();
        let _: Result<(), _> = thread::spawn(move || {
            let _lock = arc2.write().unwrap();
            panic!();
        }).join();
        assert!(arc.read().is_err());
    }

    #[test]
    fn test_rw_arc_poison_ww() {
        let arc = Arc::new(RwLock::new(1));
        assert!(!arc.is_poisoned());
        let arc2 = arc.clone();
        let _: Result<(), _> = thread::spawn(move || {
            let _lock = arc2.write().unwrap();
            panic!();
        }).join();
        assert!(arc.write().is_err());
        assert!(arc.is_poisoned());
    }

    #[test]
    fn test_rw_arc_no_poison_rr() {
        let arc = Arc::new(RwLock::new(1));
        let arc2 = arc.clone();
        let _: Result<(), _> = thread::spawn(move || {
            let _lock = arc2.read().unwrap();
            panic!();
        }).join();
        let lock = arc.read().unwrap();
        assert_eq!(*lock, 1);
    }
    #[test]
    fn test_rw_arc_no_poison_rw() {
        let arc = Arc::new(RwLock::new(1));
        let arc2 = arc.clone();
        let _: Result<(), _> = thread::spawn(move || {
            let _lock = arc2.read().unwrap();
            panic!()
        }).join();
        let lock = arc.write().unwrap();
        assert_eq!(*lock, 1);
    }

    #[test]
    fn test_rw_arc() {
        let arc = Arc::new(RwLock::new(0));
        let arc2 = arc.clone();
        let (tx, rx) = channel();

        thread::spawn(move || {
            let mut lock = arc2.write().unwrap();
            for _ in 0..10 {
                let tmp = *lock;
                *lock = -1;
                thread::yield_now();
                *lock = tmp + 1;
            }
            tx.send(()).unwrap();
        });

        // Readers try to catch the writer in the act
        let mut children = Vec::new();
        for _ in 0..5 {
            let arc3 = arc.clone();
            children.push(thread::spawn(move || {
                let lock = arc3.read().unwrap();
                assert!(*lock >= 0);
            }));
        }

        // Wait for children to pass their asserts
        for r in children {
            assert!(r.join().is_ok());
        }

        // Wait for writer to finish
        rx.recv().unwrap();
        let lock = arc.read().unwrap();
        assert_eq!(*lock, 10);
    }

    #[test]
    fn test_rw_arc_access_in_unwind() {
        let arc = Arc::new(RwLock::new(1));
        let arc2 = arc.clone();
        let _ = thread::spawn(move || -> () {
            struct Unwinder {
                i: Arc<RwLock<isize>>,
            }
            impl Drop for Unwinder {
                fn drop(&mut self) {
                    let mut lock = self.i.write().unwrap();
                    *lock += 1;
                }
            }
            let _u = Unwinder { i: arc2 };
            panic!();
        }).join();
        let lock = arc.read().unwrap();
        assert_eq!(*lock, 2);
    }

    #[test]
    fn test_rwlock_unsized() {
        let rw: &RwLock<[i32]> = &RwLock::new([1, 2, 3]);
        {
            let b = &mut *rw.write().unwrap();
            b[0] = 4;
            b[2] = 5;
        }
        let comp: &[i32] = &[4, 2, 5];
        assert_eq!(&*rw.read().unwrap(), comp);
    }

    #[test]
    fn test_rwlock_try_write() {
        let lock = RwLock::new(0isize);
        let read_guard = lock.read().unwrap();

        let write_result = lock.try_write();
        match write_result {
            Err(TryLockError::WouldBlock) => (),
            Ok(_) => assert!(false, "try_write should not succeed while read_guard is in scope"),
            Err(_) => assert!(false, "unexpected error"),
        }

        drop(read_guard);
    }

    #[test]
    fn test_into_inner() {
        let m = RwLock::new(NonCopy(10));
        assert_eq!(m.into_inner().unwrap(), NonCopy(10));
    }

    #[test]
    fn test_into_inner_drop() {
        struct Foo(Arc<AtomicUsize>);
        impl Drop for Foo {
            fn drop(&mut self) {
                self.0.fetch_add(1, Ordering::SeqCst);
            }
        }
        let num_drops = Arc::new(AtomicUsize::new(0));
        let m = RwLock::new(Foo(num_drops.clone()));
        assert_eq!(num_drops.load(Ordering::SeqCst), 0);
        {
            let _inner = m.into_inner().unwrap();
            assert_eq!(num_drops.load(Ordering::SeqCst), 0);
        }
        assert_eq!(num_drops.load(Ordering::SeqCst), 1);
    }

    #[test]
    fn test_into_inner_poison() {
        let m = Arc::new(RwLock::new(NonCopy(10)));
        let m2 = m.clone();
        let _ = thread::spawn(move || {
            let _lock = m2.write().unwrap();
            panic!("test panic in inner thread to poison RwLock");
        }).join();

        assert!(m.is_poisoned());
        match Arc::try_unwrap(m).unwrap().into_inner() {
            Err(e) => assert_eq!(e.into_inner(), NonCopy(10)),
            Ok(x) => panic!("into_inner of poisoned RwLock is Ok: {:?}", x),
        }
    }

    #[test]
    fn test_get_mut() {
        let mut m = RwLock::new(NonCopy(10));
        *m.get_mut().unwrap() = NonCopy(20);
        assert_eq!(m.into_inner().unwrap(), NonCopy(20));
    }

    #[test]
    fn test_get_mut_poison() {
        let m = Arc::new(RwLock::new(NonCopy(10)));
        let m2 = m.clone();
        let _ = thread::spawn(move || {
            let _lock = m2.write().unwrap();
            panic!("test panic in inner thread to poison RwLock");
        }).join();

        assert!(m.is_poisoned());
        match Arc::try_unwrap(m).unwrap().get_mut() {
            Err(e) => assert_eq!(*e.into_inner(), NonCopy(10)),
            Ok(x) => panic!("get_mut of poisoned RwLock is Ok: {:?}", x),
        }
    }
}