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
// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use cell::UnsafeCell;
use fmt;
use mem;
use ops::{Deref, DerefMut};
use ptr;
use sys_common::mutex as sys;
use sys_common::poison::{self, TryLockError, TryLockResult, LockResult};

/// A mutual exclusion primitive useful for protecting shared data
///
/// This mutex will block threads waiting for the lock to become available. The
/// mutex can also be statically initialized or created via a [`new`]
/// constructor. Each mutex has a type parameter which represents the data that
/// it is protecting. The data can only be accessed through the RAII guards
/// returned from [`lock`] and [`try_lock`], which guarantees that the data is only
/// ever accessed when the mutex is locked.
///
/// # Poisoning
///
/// The mutexes in this module implement a strategy called "poisoning" where a
/// mutex is considered poisoned whenever a thread panics while holding the
/// mutex. Once a mutex is poisoned, all other threads are unable to access the
/// data by default as it is likely tainted (some invariant is not being
/// upheld).
///
/// For a mutex, this means that the [`lock`] and [`try_lock`] methods return a
/// [`Result`] which indicates whether a mutex has been poisoned or not. Most
/// usage of a mutex will simply [`unwrap()`] these results, propagating panics
/// among threads to ensure that a possibly invalid invariant is not witnessed.
///
/// A poisoned mutex, however, does not prevent all access to the underlying
/// data. The [`PoisonError`] type has an [`into_inner`] method which will return
/// the guard that would have otherwise been returned on a successful lock. This
/// allows access to the data, despite the lock being poisoned.
///
/// [`new`]: #method.new
/// [`lock`]: #method.lock
/// [`try_lock`]: #method.try_lock
/// [`Result`]: ../../std/result/enum.Result.html
/// [`unwrap()`]: ../../std/result/enum.Result.html#method.unwrap
/// [`PoisonError`]: ../../std/sync/struct.PoisonError.html
/// [`into_inner`]: ../../std/sync/struct.PoisonError.html#method.into_inner
///
/// # Examples
///
/// ```
/// use std::sync::{Arc, Mutex};
/// use std::thread;
/// use std::sync::mpsc::channel;
///
/// const N: usize = 10;
///
/// // Spawn a few threads to increment a shared variable (non-atomically), and
/// // let the main thread know once all increments are done.
/// //
/// // Here we're using an Arc to share memory among threads, and the data inside
/// // the Arc is protected with a mutex.
/// let data = Arc::new(Mutex::new(0));
///
/// let (tx, rx) = channel();
/// for _ in 0..N {
///     let (data, tx) = (data.clone(), tx.clone());
///     thread::spawn(move || {
///         // The shared state can only be accessed once the lock is held.
///         // Our non-atomic increment is safe because we're the only thread
///         // which can access the shared state when the lock is held.
///         //
///         // We unwrap() the return value to assert that we are not expecting
///         // threads to ever fail while holding the lock.
///         let mut data = data.lock().unwrap();
///         *data += 1;
///         if *data == N {
///             tx.send(()).unwrap();
///         }
///         // the lock is unlocked here when `data` goes out of scope.
///     });
/// }
///
/// rx.recv().unwrap();
/// ```
///
/// To recover from a poisoned mutex:
///
/// ```
/// use std::sync::{Arc, Mutex};
/// use std::thread;
///
/// let lock = Arc::new(Mutex::new(0_u32));
/// let lock2 = lock.clone();
///
/// let _ = thread::spawn(move || -> () {
///     // This thread will acquire the mutex first, unwrapping the result of
///     // `lock` because the lock has not been poisoned.
///     let _guard = lock2.lock().unwrap();
///
///     // This panic while holding the lock (`_guard` is in scope) will poison
///     // the mutex.
///     panic!();
/// }).join();
///
/// // The lock is poisoned by this point, but the returned result can be
/// // pattern matched on to return the underlying guard on both branches.
/// let mut guard = match lock.lock() {
///     Ok(guard) => guard,
///     Err(poisoned) => poisoned.into_inner(),
/// };
///
/// *guard += 1;
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Mutex<T: ?Sized> {
    // Note that this mutex is in a *box*, not inlined into the struct itself.
    // Once a native mutex has been used once, its address can never change (it
    // can't be moved). This mutex type can be safely moved at any time, so to
    // ensure that the native mutex is used correctly we box the inner mutex to
    // give it a constant address.
    inner: Box<sys::Mutex>,
    poison: poison::Flag,
    data: UnsafeCell<T>,
}

// these are the only places where `T: Send` matters; all other
// functionality works fine on a single thread.
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Send> Send for Mutex<T> { }
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Send> Sync for Mutex<T> { }

/// An RAII implementation of a "scoped lock" of a mutex. When this structure is
/// dropped (falls out of scope), the lock will be unlocked.
///
/// The data protected by the mutex can be accessed through this guard via its
/// [`Deref`] and [`DerefMut`] implementations.
///
/// This structure is created by the [`lock`] and [`try_lock`] methods on
/// [`Mutex`].
///
/// [`Deref`]: ../../std/ops/trait.Deref.html
/// [`DerefMut`]: ../../std/ops/trait.DerefMut.html
/// [`lock`]: struct.Mutex.html#method.lock
/// [`try_lock`]: struct.Mutex.html#method.try_lock
/// [`Mutex`]: struct.Mutex.html
#[must_use]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct MutexGuard<'a, T: ?Sized + 'a> {
    // funny underscores due to how Deref/DerefMut currently work (they
    // disregard field privacy).
    __lock: &'a Mutex<T>,
    __poison: poison::Guard,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T: ?Sized> !Send for MutexGuard<'a, T> { }
#[stable(feature = "mutexguard", since = "1.19.0")]
unsafe impl<'a, T: ?Sized + Sync> Sync for MutexGuard<'a, T> { }

impl<T> Mutex<T> {
    /// Creates a new mutex in an unlocked state ready for use.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Mutex;
    ///
    /// let mutex = Mutex::new(0);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn new(t: T) -> Mutex<T> {
        let mut m = Mutex {
            inner: box sys::Mutex::new(),
            poison: poison::Flag::new(),
            data: UnsafeCell::new(t),
        };
        unsafe {
            m.inner.init();
        }
        m
    }
}

impl<T: ?Sized> Mutex<T> {
    /// Acquires a mutex, blocking the current thread until it is able to do so.
    ///
    /// This function will block the local thread until it is available to acquire
    /// the mutex. Upon returning, the thread is the only thread with the lock
    /// held. An RAII guard is returned to allow scoped unlock of the lock. When
    /// the guard goes out of scope, the mutex will be unlocked.
    ///
    /// The exact behavior on locking a mutex in the thread which already holds
    /// the lock is left unspecified. However, this function will not return on
    /// the second call (it might panic or deadlock, for example).
    ///
    /// # Errors
    ///
    /// If another user of this mutex panicked while holding the mutex, then
    /// this call will return an error once the mutex is acquired.
    ///
    /// # Panics
    ///
    /// This function might panic when called if the lock is already held by
    /// the current thread.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, Mutex};
    /// use std::thread;
    ///
    /// let mutex = Arc::new(Mutex::new(0));
    /// let c_mutex = mutex.clone();
    ///
    /// thread::spawn(move || {
    ///     *c_mutex.lock().unwrap() = 10;
    /// }).join().expect("thread::spawn failed");
    /// assert_eq!(*mutex.lock().unwrap(), 10);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn lock(&self) -> LockResult<MutexGuard<T>> {
        unsafe {
            self.inner.lock();
            MutexGuard::new(self)
        }
    }

    /// Attempts to acquire this lock.
    ///
    /// If the lock could not be acquired at this time, then [`Err`] is returned.
    /// Otherwise, an RAII guard is returned. The lock will be unlocked when the
    /// guard is dropped.
    ///
    /// This function does not block.
    ///
    /// # Errors
    ///
    /// If another user of this mutex panicked while holding the mutex, then
    /// this call will return failure if the mutex would otherwise be
    /// acquired.
    ///
    /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, Mutex};
    /// use std::thread;
    ///
    /// let mutex = Arc::new(Mutex::new(0));
    /// let c_mutex = mutex.clone();
    ///
    /// thread::spawn(move || {
    ///     let mut lock = c_mutex.try_lock();
    ///     if let Ok(ref mut mutex) = lock {
    ///         **mutex = 10;
    ///     } else {
    ///         println!("try_lock failed");
    ///     }
    /// }).join().expect("thread::spawn failed");
    /// assert_eq!(*mutex.lock().unwrap(), 10);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn try_lock(&self) -> TryLockResult<MutexGuard<T>> {
        unsafe {
            if self.inner.try_lock() {
                Ok(MutexGuard::new(self)?)
            } else {
                Err(TryLockError::WouldBlock)
            }
        }
    }

    /// Determines whether the mutex is poisoned.
    ///
    /// If another thread is active, the mutex 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, Mutex};
    /// use std::thread;
    ///
    /// let mutex = Arc::new(Mutex::new(0));
    /// let c_mutex = mutex.clone();
    ///
    /// let _ = thread::spawn(move || {
    ///     let _lock = c_mutex.lock().unwrap();
    ///     panic!(); // the mutex gets poisoned
    /// }).join();
    /// assert_eq!(mutex.is_poisoned(), true);
    /// ```
    #[inline]
    #[stable(feature = "sync_poison", since = "1.2.0")]
    pub fn is_poisoned(&self) -> bool {
        self.poison.get()
    }

    /// Consumes this mutex, returning the underlying data.
    ///
    /// # Errors
    ///
    /// If another user of this mutex panicked while holding the mutex, then
    /// this call will return an error instead.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Mutex;
    ///
    /// let mutex = Mutex::new(0);
    /// assert_eq!(mutex.into_inner().unwrap(), 0);
    /// ```
    #[stable(feature = "mutex_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 mutex.
        //
        // To get the inner value, we'd like to call `data.into_inner()`,
        // but because `Mutex` impl-s `Drop`, we can't move out of it, so
        // we'll have to destructure it manually instead.
        unsafe {
            // Like `let Mutex { inner, poison, data } = self`.
            let (inner, poison, data) = {
                let Mutex { 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 `Mutex` mutably, no actual locking needs to
    /// take place---the mutable borrow statically guarantees no locks exist.
    ///
    /// # Errors
    ///
    /// If another user of this mutex panicked while holding the mutex, then
    /// this call will return an error instead.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Mutex;
    ///
    /// let mut mutex = Mutex::new(0);
    /// *mutex.get_mut().unwrap() = 10;
    /// assert_eq!(*mutex.lock().unwrap(), 10);
    /// ```
    #[stable(feature = "mutex_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 mutex.
        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 Mutex<T> {
    fn drop(&mut self) {
        // This is actually safe b/c we know that there is no further usage of
        // this mutex (it's up to the user to arrange for a mutex to get
        // dropped, that's not our job)
        //
        // IMPORTANT: This code must be kept in sync with `Mutex::into_inner`.
        unsafe { self.inner.destroy() }
    }
}

#[stable(feature = "mutex_from", since = "1.24.0")]
impl<T> From<T> for Mutex<T> {
    /// Creates a new mutex in an unlocked state ready for use.
    /// This is equivalent to [`Mutex::new`].
    ///
    /// [`Mutex::new`]: #method.new
    fn from(t: T) -> Self {
        Mutex::new(t)
    }
}

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

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

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

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

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

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

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

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

pub fn guard_lock<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a sys::Mutex {
    &guard.__lock.inner
}

pub fn guard_poison<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a poison::Flag {
    &guard.__lock.poison
}

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

    struct Packet<T>(Arc<(Mutex<T>, Condvar)>);

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

    #[test]
    fn smoke() {
        let m = Mutex::new(());
        drop(m.lock().unwrap());
        drop(m.lock().unwrap());
    }

    #[test]
    fn lots_and_lots() {
        const J: u32 = 1000;
        const K: u32 = 3;

        let m = Arc::new(Mutex::new(0));

        fn inc(m: &Mutex<u32>) {
            for _ in 0..J {
                *m.lock().unwrap() += 1;
            }
        }

        let (tx, rx) = channel();
        for _ in 0..K {
            let tx2 = tx.clone();
            let m2 = m.clone();
            thread::spawn(move|| { inc(&m2); tx2.send(()).unwrap(); });
            let tx2 = tx.clone();
            let m2 = m.clone();
            thread::spawn(move|| { inc(&m2); tx2.send(()).unwrap(); });
        }

        drop(tx);
        for _ in 0..2 * K {
            rx.recv().unwrap();
        }
        assert_eq!(*m.lock().unwrap(), J * K * 2);
    }

    #[test]
    fn try_lock() {
        let m = Mutex::new(());
        *m.try_lock().unwrap() = ();
    }

    #[test]
    fn test_into_inner() {
        let m = Mutex::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 = Mutex::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(Mutex::new(NonCopy(10)));
        let m2 = m.clone();
        let _ = thread::spawn(move || {
            let _lock = m2.lock().unwrap();
            panic!("test panic in inner thread to poison mutex");
        }).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 Mutex is Ok: {:?}", x),
        }
    }

    #[test]
    fn test_get_mut() {
        let mut m = Mutex::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(Mutex::new(NonCopy(10)));
        let m2 = m.clone();
        let _ = thread::spawn(move || {
            let _lock = m2.lock().unwrap();
            panic!("test panic in inner thread to poison mutex");
        }).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 Mutex is Ok: {:?}", x),
        }
    }

    #[test]
    fn test_mutex_arc_condvar() {
        let packet = Packet(Arc::new((Mutex::new(false), Condvar::new())));
        let packet2 = Packet(packet.0.clone());
        let (tx, rx) = channel();
        let _t = thread::spawn(move|| {
            // wait until parent gets in
            rx.recv().unwrap();
            let &(ref lock, ref cvar) = &*packet2.0;
            let mut lock = lock.lock().unwrap();
            *lock = true;
            cvar.notify_one();
        });

        let &(ref lock, ref cvar) = &*packet.0;
        let mut lock = lock.lock().unwrap();
        tx.send(()).unwrap();
        assert!(!*lock);
        while !*lock {
            lock = cvar.wait(lock).unwrap();
        }
    }

    #[test]
    fn test_arc_condvar_poison() {
        let packet = Packet(Arc::new((Mutex::new(1), Condvar::new())));
        let packet2 = Packet(packet.0.clone());
        let (tx, rx) = channel();

        let _t = thread::spawn(move || -> () {
            rx.recv().unwrap();
            let &(ref lock, ref cvar) = &*packet2.0;
            let _g = lock.lock().unwrap();
            cvar.notify_one();
            // Parent should fail when it wakes up.
            panic!();
        });

        let &(ref lock, ref cvar) = &*packet.0;
        let mut lock = lock.lock().unwrap();
        tx.send(()).unwrap();
        while *lock == 1 {
            match cvar.wait(lock) {
                Ok(l) => {
                    lock = l;
                    assert_eq!(*lock, 1);
                }
                Err(..) => break,
            }
        }
    }

    #[test]
    fn test_mutex_arc_poison() {
        let arc = Arc::new(Mutex::new(1));
        assert!(!arc.is_poisoned());
        let arc2 = arc.clone();
        let _ = thread::spawn(move|| {
            let lock = arc2.lock().unwrap();
            assert_eq!(*lock, 2);
        }).join();
        assert!(arc.lock().is_err());
        assert!(arc.is_poisoned());
    }

    #[test]
    fn test_mutex_arc_nested() {
        // Tests nested mutexes and access
        // to underlying data.
        let arc = Arc::new(Mutex::new(1));
        let arc2 = Arc::new(Mutex::new(arc));
        let (tx, rx) = channel();
        let _t = thread::spawn(move|| {
            let lock = arc2.lock().unwrap();
            let lock2 = lock.lock().unwrap();
            assert_eq!(*lock2, 1);
            tx.send(()).unwrap();
        });
        rx.recv().unwrap();
    }

    #[test]
    fn test_mutex_arc_access_in_unwind() {
        let arc = Arc::new(Mutex::new(1));
        let arc2 = arc.clone();
        let _ = thread::spawn(move|| -> () {
            struct Unwinder {
                i: Arc<Mutex<i32>>,
            }
            impl Drop for Unwinder {
                fn drop(&mut self) {
                    *self.i.lock().unwrap() += 1;
                }
            }
            let _u = Unwinder { i: arc2 };
            panic!();
        }).join();
        let lock = arc.lock().unwrap();
        assert_eq!(*lock, 2);
    }

    #[test]
    fn test_mutex_unsized() {
        let mutex: &Mutex<[i32]> = &Mutex::new([1, 2, 3]);
        {
            let b = &mut *mutex.lock().unwrap();
            b[0] = 4;
            b[2] = 5;
        }
        let comp: &[i32] = &[4, 2, 5];
        assert_eq!(&*mutex.lock().unwrap(), comp);
    }
}