src/libstd/sync/mutex.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use cell::UnsafeCell;
  12 use fmt;
  13 use mem;
  14 use ops::{Deref, DerefMut};
  15 use ptr;
  16 use sys_common::mutex as sys;
  17 use sys_common::poison::{self, TryLockError, TryLockResult, LockResult};
  18
  19 /// A mutual exclusion primitive useful for protecting shared data
  20 ///
  21 /// This mutex will block threads waiting for the lock to become available. The
  22 /// mutex can also be statically initialized or created via a `new`
  23 /// constructor. Each mutex has a type parameter which represents the data that
  24 /// it is protecting. The data can only be accessed through the RAII guards
  25 /// returned from `lock` and `try_lock`, which guarantees that the data is only
  26 /// ever accessed when the mutex is locked.
  27 ///
  28 /// # Poisoning
  29 ///
  30 /// The mutexes in this module implement a strategy called "poisoning" where a
  31 /// mutex is considered poisoned whenever a thread panics while holding the
  32 /// mutex. Once a mutex is poisoned, all other threads are unable to access the
  33 /// data by default as it is likely tainted (some invariant is not being
  34 /// upheld).
  35 ///
  36 /// For a mutex, this means that the `lock` and `try_lock` methods return a
  37 /// `Result` which indicates whether a mutex has been poisoned or not. Most
  38 /// usage of a mutex will simply `unwrap()` these results, propagating panics
  39 /// among threads to ensure that a possibly invalid invariant is not witnessed.
  40 ///
  41 /// A poisoned mutex, however, does not prevent all access to the underlying
  42 /// data. The `PoisonError` type has an `into_inner` method which will return
  43 /// the guard that would have otherwise been returned on a successful lock. This
  44 /// allows access to the data, despite the lock being poisoned.
  45 ///
  46 /// # Examples
  47 ///
  48 /// ```
  49 /// use std::sync::{Arc, Mutex};
  50 /// use std::thread;
  51 /// use std::sync::mpsc::channel;
  52 ///
  53 /// const N: usize = 10;
  54 ///
  55 /// // Spawn a few threads to increment a shared variable (non-atomically), and
  56 /// // let the main thread know once all increments are done.
  57 /// //
  58 /// // Here we're using an Arc to share memory among threads, and the data inside
  59 /// // the Arc is protected with a mutex.
  60 /// let data = Arc::new(Mutex::new(0));
  61 ///
  62 /// let (tx, rx) = channel();
  63 /// for _ in 0..N {
  64 ///     let (data, tx) = (data.clone(), tx.clone());
  65 ///     thread::spawn(move || {
  66 ///         // The shared state can only be accessed once the lock is held.
  67 ///         // Our non-atomic increment is safe because we're the only thread
  68 ///         // which can access the shared state when the lock is held.
  69 ///         //
  70 ///         // We unwrap() the return value to assert that we are not expecting
  71 ///         // threads to ever fail while holding the lock.
  72 ///         let mut data = data.lock().unwrap();
  73 ///         *data += 1;
  74 ///         if *data == N {
  75 ///             tx.send(()).unwrap();
  76 ///         }
  77 ///         // the lock is unlocked here when `data` goes out of scope.
  78 ///     });
  79 /// }
  80 ///
  81 /// rx.recv().unwrap();
  82 /// ```
  83 ///
  84 /// To recover from a poisoned mutex:
  85 ///
  86 /// ```
  87 /// use std::sync::{Arc, Mutex};
  88 /// use std::thread;
  89 ///
  90 /// let lock = Arc::new(Mutex::new(0_u32));
  91 /// let lock2 = lock.clone();
  92 ///
  93 /// let _ = thread::spawn(move || -> () {
  94 ///     // This thread will acquire the mutex first, unwrapping the result of
  95 ///     // `lock` because the lock has not been poisoned.
  96 ///     let _guard = lock2.lock().unwrap();
  97 ///
  98 ///     // This panic while holding the lock (`_guard` is in scope) will poison
  99 ///     // the mutex.
 100 ///     panic!();
 101 /// }).join();
 102 ///
 103 /// // The lock is poisoned by this point, but the returned result can be
 104 /// // pattern matched on to return the underlying guard on both branches.
 105 /// let mut guard = match lock.lock() {
 106 ///     Ok(guard) => guard,
 107 ///     Err(poisoned) => poisoned.into_inner(),
 108 /// };
 109 ///
 110 /// *guard += 1;
 111 /// ```
 112 #[stable(feature = "rust1", since = "1.0.0")]
 113 pub struct Mutex<T: ?Sized> {
 114     // Note that this mutex is in a *box*, not inlined into the struct itself.
 115     // Once a native mutex has been used once, its address can never change (it
 116     // can't be moved). This mutex type can be safely moved at any time, so to
 117     // ensure that the native mutex is used correctly we box the inner mutex to
 118     // give it a constant address.
 119     inner: Box<sys::Mutex>,
 120     poison: poison::Flag,
 121     data: UnsafeCell<T>,
 122 }
 123
 124 // these are the only places where `T: Send` matters; all other
 125 // functionality works fine on a single thread.
 126 #[stable(feature = "rust1", since = "1.0.0")]
 127 unsafe impl<T: ?Sized + Send> Send for Mutex<T> { }
 128 #[stable(feature = "rust1", since = "1.0.0")]
 129 unsafe impl<T: ?Sized + Send> Sync for Mutex<T> { }
 130
 131 /// An RAII implementation of a "scoped lock" of a mutex. When this structure is
 132 /// dropped (falls out of scope), the lock will be unlocked.
 133 ///
 134 /// The data protected by the mutex can be accessed through this guard via its
 135 /// [`Deref`] and [`DerefMut`] implementations.
 136 ///
 137 /// This structure is created by the [`lock`] and [`try_lock`] methods on
 138 /// [`Mutex`].
 139 ///
 140 /// [`Deref`]: ../../std/ops/trait.Deref.html
 141 /// [`DerefMut`]: ../../std/ops/trait.DerefMut.html
 142 /// [`lock`]: struct.Mutex.html#method.lock
 143 /// [`try_lock`]: struct.Mutex.html#method.try_lock
 144 /// [`Mutex`]: struct.Mutex.html
 145 #[must_use]
 146 #[stable(feature = "rust1", since = "1.0.0")]
 147 pub struct MutexGuard<'a, T: ?Sized + 'a> {
 148     // funny underscores due to how Deref/DerefMut currently work (they
 149     // disregard field privacy).
 150     __lock: &'a Mutex<T>,
 151     __poison: poison::Guard,
 152 }
 153
 154 #[stable(feature = "rust1", since = "1.0.0")]
 155 impl<'a, T: ?Sized> !Send for MutexGuard<'a, T> { }
 156 #[stable(feature = "mutexguard", since = "1.19.0")]
 157 unsafe impl<'a, T: ?Sized + Sync> Sync for MutexGuard<'a, T> { }
 158
 159 impl<T> Mutex<T> {
 160     /// Creates a new mutex in an unlocked state ready for use.
 161     ///
 162     /// # Examples
 163     ///
 164     /// ```
 165     /// use std::sync::Mutex;
 166     ///
 167     /// let mutex = Mutex::new(0);
 168     /// ```
 169     #[stable(feature = "rust1", since = "1.0.0")]
 170     pub fn new(t: T) -> Mutex<T> {
 171         let mut m = Mutex {
 172             inner: box sys::Mutex::new(),
 173             poison: poison::Flag::new(),
 174             data: UnsafeCell::new(t),
 175         };
 176         unsafe {
 177             m.inner.init();
 178         }
 179         m
 180     }
 181 }
 182
 183 impl<T: ?Sized> Mutex<T> {
 184     /// Acquires a mutex, blocking the current thread until it is able to do so.
 185     ///
 186     /// This function will block the local thread until it is available to acquire
 187     /// the mutex. Upon returning, the thread is the only thread with the lock
 188     /// held. An RAII guard is returned to allow scoped unlock of the lock. When
 189     /// the guard goes out of scope, the mutex will be unlocked.
 190     ///
 191     /// The exact behavior on locking a mutex in the thread which already holds
 192     /// the lock is left unspecified. However, this function will not return on
 193     /// the second call (it might panic or deadlock, for example).
 194     ///
 195     /// # Errors
 196     ///
 197     /// If another user of this mutex panicked while holding the mutex, then
 198     /// this call will return an error once the mutex is acquired.
 199     ///
 200     /// # Panics
 201     ///
 202     /// This function might panic when called if the lock is already held by
 203     /// the current thread.
 204     ///
 205     /// # Examples
 206     ///
 207     /// ```
 208     /// use std::sync::{Arc, Mutex};
 209     /// use std::thread;
 210     ///
 211     /// let mutex = Arc::new(Mutex::new(0));
 212     /// let c_mutex = mutex.clone();
 213     ///
 214     /// thread::spawn(move || {
 215     ///     *c_mutex.lock().unwrap() = 10;
 216     /// }).join().expect("thread::spawn failed");
 217     /// assert_eq!(*mutex.lock().unwrap(), 10);
 218     /// ```
 219     #[stable(feature = "rust1", since = "1.0.0")]
 220     pub fn lock(&self) -> LockResult<MutexGuard<T>> {
 221         unsafe {
 222             self.inner.lock();
 223             MutexGuard::new(self)
 224         }
 225     }
 226
 227     /// Attempts to acquire this lock.
 228     ///
 229     /// If the lock could not be acquired at this time, then `Err` is returned.
 230     /// Otherwise, an RAII guard is returned. The lock will be unlocked when the
 231     /// guard is dropped.
 232     ///
 233     /// This function does not block.
 234     ///
 235     /// # Errors
 236     ///
 237     /// If another user of this mutex panicked while holding the mutex, then
 238     /// this call will return failure if the mutex would otherwise be
 239     /// acquired.
 240     ///
 241     /// # Examples
 242     ///
 243     /// ```
 244     /// use std::sync::{Arc, Mutex};
 245     /// use std::thread;
 246     ///
 247     /// let mutex = Arc::new(Mutex::new(0));
 248     /// let c_mutex = mutex.clone();
 249     ///
 250     /// thread::spawn(move || {
 251     ///     let mut lock = c_mutex.try_lock();
 252     ///     if let Ok(ref mut mutex) = lock {
 253     ///         **mutex = 10;
 254     ///     } else {
 255     ///         println!("try_lock failed");
 256     ///     }
 257     /// }).join().expect("thread::spawn failed");
 258     /// assert_eq!(*mutex.lock().unwrap(), 10);
 259     /// ```
 260     #[stable(feature = "rust1", since = "1.0.0")]
 261     pub fn try_lock(&self) -> TryLockResult<MutexGuard<T>> {
 262         unsafe {
 263             if self.inner.try_lock() {
 264                 Ok(MutexGuard::new(self)?)
 265             } else {
 266                 Err(TryLockError::WouldBlock)
 267             }
 268         }
 269     }
 270
 271     /// Determines whether the mutex is poisoned.
 272     ///
 273     /// If another thread is active, the mutex can still become poisoned at any
 274     /// time. You should not trust a `false` value for program correctness
 275     /// without additional synchronization.
 276     ///
 277     /// # Examples
 278     ///
 279     /// ```
 280     /// use std::sync::{Arc, Mutex};
 281     /// use std::thread;
 282     ///
 283     /// let mutex = Arc::new(Mutex::new(0));
 284     /// let c_mutex = mutex.clone();
 285     ///
 286     /// let _ = thread::spawn(move || {
 287     ///     let _lock = c_mutex.lock().unwrap();
 288     ///     panic!(); // the mutex gets poisoned
 289     /// }).join();
 290     /// assert_eq!(mutex.is_poisoned(), true);
 291     /// ```
 292     #[inline]
 293     #[stable(feature = "sync_poison", since = "1.2.0")]
 294     pub fn is_poisoned(&self) -> bool {
 295         self.poison.get()
 296     }
 297
 298     /// Consumes this mutex, returning the underlying data.
 299     ///
 300     /// # Errors
 301     ///
 302     /// If another user of this mutex panicked while holding the mutex, then
 303     /// this call will return an error instead.
 304     ///
 305     /// # Examples
 306     ///
 307     /// ```
 308     /// use std::sync::Mutex;
 309     ///
 310     /// let mutex = Mutex::new(0);
 311     /// assert_eq!(mutex.into_inner().unwrap(), 0);
 312     /// ```
 313     #[stable(feature = "mutex_into_inner", since = "1.6.0")]
 314     pub fn into_inner(self) -> LockResult<T> where T: Sized {
 315         // We know statically that there are no outstanding references to
 316         // `self` so there's no need to lock the inner mutex.
 317         //
 318         // To get the inner value, we'd like to call `data.into_inner()`,
 319         // but because `Mutex` impl-s `Drop`, we can't move out of it, so
 320         // we'll have to destructure it manually instead.
 321         unsafe {
 322             // Like `let Mutex { inner, poison, data } = self`.
 323             let (inner, poison, data) = {
 324                 let Mutex { ref inner, ref poison, ref data } = self;
 325                 (ptr::read(inner), ptr::read(poison), ptr::read(data))
 326             };
 327             mem::forget(self);
 328             inner.destroy();  // Keep in sync with the `Drop` impl.
 329             drop(inner);
 330
 331             poison::map_result(poison.borrow(), |_| data.into_inner())
 332         }
 333     }
 334
 335     /// Returns a mutable reference to the underlying data.
 336     ///
 337     /// Since this call borrows the `Mutex` mutably, no actual locking needs to
 338     /// take place---the mutable borrow statically guarantees no locks exist.
 339     ///
 340     /// # Errors
 341     ///
 342     /// If another user of this mutex panicked while holding the mutex, then
 343     /// this call will return an error instead.
 344     ///
 345     /// # Examples
 346     ///
 347     /// ```
 348     /// use std::sync::Mutex;
 349     ///
 350     /// let mut mutex = Mutex::new(0);
 351     /// *mutex.get_mut().unwrap() = 10;
 352     /// assert_eq!(*mutex.lock().unwrap(), 10);
 353     /// ```
 354     #[stable(feature = "mutex_get_mut", since = "1.6.0")]
 355     pub fn get_mut(&mut self) -> LockResult<&mut T> {
 356         // We know statically that there are no other references to `self`, so
 357         // there's no need to lock the inner mutex.
 358         let data = unsafe { &mut *self.data.get() };
 359         poison::map_result(self.poison.borrow(), |_| data )
 360     }
 361 }
 362
 363 #[stable(feature = "rust1", since = "1.0.0")]
 364 unsafe impl<#[may_dangle] T: ?Sized> Drop for Mutex<T> {
 365     fn drop(&mut self) {
 366         // This is actually safe b/c we know that there is no further usage of
 367         // this mutex (it's up to the user to arrange for a mutex to get
 368         // dropped, that's not our job)
 369         //
 370         // IMPORTANT: This code must be kept in sync with `Mutex::into_inner`.
 371         unsafe { self.inner.destroy() }
 372     }
 373 }
 374
 375 #[stable(feature = "mutex_default", since = "1.10.0")]
 376 impl<T: ?Sized + Default> Default for Mutex<T> {
 377     /// Creates a `Mutex<T>`, with the `Default` value for T.
 378     fn default() -> Mutex<T> {
 379         Mutex::new(Default::default())
 380     }
 381 }
 382
 383 #[stable(feature = "rust1", since = "1.0.0")]
 384 impl<T: ?Sized + fmt::Debug> fmt::Debug for Mutex<T> {
 385     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 386         match self.try_lock() {
 387             Ok(guard) => write!(f, "Mutex {{ data: {:?} }}", &*guard),
 388             Err(TryLockError::Poisoned(err)) => {
 389                 write!(f, "Mutex {{ data: Poisoned({:?}) }}", &**err.get_ref())
 390             },
 391             Err(TryLockError::WouldBlock) => write!(f, "Mutex {{ <locked> }}")
 392         }
 393     }
 394 }
 395
 396 impl<'mutex, T: ?Sized> MutexGuard<'mutex, T> {
 397     unsafe fn new(lock: &'mutex Mutex<T>) -> LockResult<MutexGuard<'mutex, T>> {
 398         poison::map_result(lock.poison.borrow(), |guard| {
 399             MutexGuard {
 400                 __lock: lock,
 401                 __poison: guard,
 402             }
 403         })
 404     }
 405 }
 406
 407 #[stable(feature = "rust1", since = "1.0.0")]
 408 impl<'mutex, T: ?Sized> Deref for MutexGuard<'mutex, T> {
 409     type Target = T;
 410
 411     fn deref(&self) -> &T {
 412         unsafe { &*self.__lock.data.get() }
 413     }
 414 }
 415
 416 #[stable(feature = "rust1", since = "1.0.0")]
 417 impl<'mutex, T: ?Sized> DerefMut for MutexGuard<'mutex, T> {
 418     fn deref_mut(&mut self) -> &mut T {
 419         unsafe { &mut *self.__lock.data.get() }
 420     }
 421 }
 422
 423 #[stable(feature = "rust1", since = "1.0.0")]
 424 impl<'a, T: ?Sized> Drop for MutexGuard<'a, T> {
 425     #[inline]
 426     fn drop(&mut self) {
 427         unsafe {
 428             self.__lock.poison.done(&self.__poison);
 429             self.__lock.inner.unlock();
 430         }
 431     }
 432 }
 433
 434 #[stable(feature = "std_debug", since = "1.16.0")]
 435 impl<'a, T: ?Sized + fmt::Debug> fmt::Debug for MutexGuard<'a, T> {
 436     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 437         f.debug_struct("MutexGuard")
 438             .field("lock", &self.__lock)
 439             .finish()
 440     }
 441 }
 442
 443 #[stable(feature = "std_guard_impls", since = "1.20.0")]
 444 impl<'a, T: ?Sized + fmt::Display> fmt::Display for MutexGuard<'a, T> {
 445     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 446         (**self).fmt(f)
 447     }
 448 }
 449
 450 pub fn guard_lock<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a sys::Mutex {
 451     &guard.__lock.inner
 452 }
 453
 454 pub fn guard_poison<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a poison::Flag {
 455     &guard.__lock.poison
 456 }
 457
 458 #[cfg(all(test, not(target_os = "emscripten")))]
 459 mod tests {
 460     use sync::mpsc::channel;
 461     use sync::{Arc, Mutex, Condvar};
 462     use sync::atomic::{AtomicUsize, Ordering};
 463     use thread;
 464
 465     struct Packet<T>(Arc<(Mutex<T>, Condvar)>);
 466
 467     #[derive(Eq, PartialEq, Debug)]
 468     struct NonCopy(i32);
 469
 470     #[test]
 471     fn smoke() {
 472         let m = Mutex::new(());
 473         drop(m.lock().unwrap());
 474         drop(m.lock().unwrap());
 475     }
 476
 477     #[test]
 478     fn lots_and_lots() {
 479         const J: u32 = 1000;
 480         const K: u32 = 3;
 481
 482         let m = Arc::new(Mutex::new(0));
 483
 484         fn inc(m: &Mutex<u32>) {
 485             for _ in 0..J {
 486                 *m.lock().unwrap() += 1;
 487             }
 488         }
 489
 490         let (tx, rx) = channel();
 491         for _ in 0..K {
 492             let tx2 = tx.clone();
 493             let m2 = m.clone();
 494             thread::spawn(move|| { inc(&m2); tx2.send(()).unwrap(); });
 495             let tx2 = tx.clone();
 496             let m2 = m.clone();
 497             thread::spawn(move|| { inc(&m2); tx2.send(()).unwrap(); });
 498         }
 499
 500         drop(tx);
 501         for _ in 0..2 * K {
 502             rx.recv().unwrap();
 503         }
 504         assert_eq!(*m.lock().unwrap(), J * K * 2);
 505     }
 506
 507     #[test]
 508     fn try_lock() {
 509         let m = Mutex::new(());
 510         *m.try_lock().unwrap() = ();
 511     }
 512
 513     #[test]
 514     fn test_into_inner() {
 515         let m = Mutex::new(NonCopy(10));
 516         assert_eq!(m.into_inner().unwrap(), NonCopy(10));
 517     }
 518
 519     #[test]
 520     fn test_into_inner_drop() {
 521         struct Foo(Arc<AtomicUsize>);
 522         impl Drop for Foo {
 523             fn drop(&mut self) {
 524                 self.0.fetch_add(1, Ordering::SeqCst);
 525             }
 526         }
 527         let num_drops = Arc::new(AtomicUsize::new(0));
 528         let m = Mutex::new(Foo(num_drops.clone()));
 529         assert_eq!(num_drops.load(Ordering::SeqCst), 0);
 530         {
 531             let _inner = m.into_inner().unwrap();
 532             assert_eq!(num_drops.load(Ordering::SeqCst), 0);
 533         }
 534         assert_eq!(num_drops.load(Ordering::SeqCst), 1);
 535     }
 536
 537     #[test]
 538     fn test_into_inner_poison() {
 539         let m = Arc::new(Mutex::new(NonCopy(10)));
 540         let m2 = m.clone();
 541         let _ = thread::spawn(move || {
 542             let _lock = m2.lock().unwrap();
 543             panic!("test panic in inner thread to poison mutex");
 544         }).join();
 545
 546         assert!(m.is_poisoned());
 547         match Arc::try_unwrap(m).unwrap().into_inner() {
 548             Err(e) => assert_eq!(e.into_inner(), NonCopy(10)),
 549             Ok(x) => panic!("into_inner of poisoned Mutex is Ok: {:?}", x),
 550         }
 551     }
 552
 553     #[test]
 554     fn test_get_mut() {
 555         let mut m = Mutex::new(NonCopy(10));
 556         *m.get_mut().unwrap() = NonCopy(20);
 557         assert_eq!(m.into_inner().unwrap(), NonCopy(20));
 558     }
 559
 560     #[test]
 561     fn test_get_mut_poison() {
 562         let m = Arc::new(Mutex::new(NonCopy(10)));
 563         let m2 = m.clone();
 564         let _ = thread::spawn(move || {
 565             let _lock = m2.lock().unwrap();
 566             panic!("test panic in inner thread to poison mutex");
 567         }).join();
 568
 569         assert!(m.is_poisoned());
 570         match Arc::try_unwrap(m).unwrap().get_mut() {
 571             Err(e) => assert_eq!(*e.into_inner(), NonCopy(10)),
 572             Ok(x) => panic!("get_mut of poisoned Mutex is Ok: {:?}", x),
 573         }
 574     }
 575
 576     #[test]
 577     fn test_mutex_arc_condvar() {
 578         let packet = Packet(Arc::new((Mutex::new(false), Condvar::new())));
 579         let packet2 = Packet(packet.0.clone());
 580         let (tx, rx) = channel();
 581         let _t = thread::spawn(move|| {
 582             // wait until parent gets in
 583             rx.recv().unwrap();
 584             let &(ref lock, ref cvar) = &*packet2.0;
 585             let mut lock = lock.lock().unwrap();
 586             *lock = true;
 587             cvar.notify_one();
 588         });
 589
 590         let &(ref lock, ref cvar) = &*packet.0;
 591         let mut lock = lock.lock().unwrap();
 592         tx.send(()).unwrap();
 593         assert!(!*lock);
 594         while !*lock {
 595             lock = cvar.wait(lock).unwrap();
 596         }
 597     }
 598
 599     #[test]
 600     fn test_arc_condvar_poison() {
 601         let packet = Packet(Arc::new((Mutex::new(1), Condvar::new())));
 602         let packet2 = Packet(packet.0.clone());
 603         let (tx, rx) = channel();
 604
 605         let _t = thread::spawn(move || -> () {
 606             rx.recv().unwrap();
 607             let &(ref lock, ref cvar) = &*packet2.0;
 608             let _g = lock.lock().unwrap();
 609             cvar.notify_one();
 610             // Parent should fail when it wakes up.
 611             panic!();
 612         });
 613
 614         let &(ref lock, ref cvar) = &*packet.0;
 615         let mut lock = lock.lock().unwrap();
 616         tx.send(()).unwrap();
 617         while *lock == 1 {
 618             match cvar.wait(lock) {
 619                 Ok(l) => {
 620                     lock = l;
 621                     assert_eq!(*lock, 1);
 622                 }
 623                 Err(..) => break,
 624             }
 625         }
 626     }
 627
 628     #[test]
 629     fn test_mutex_arc_poison() {
 630         let arc = Arc::new(Mutex::new(1));
 631         assert!(!arc.is_poisoned());
 632         let arc2 = arc.clone();
 633         let _ = thread::spawn(move|| {
 634             let lock = arc2.lock().unwrap();
 635             assert_eq!(*lock, 2);
 636         }).join();
 637         assert!(arc.lock().is_err());
 638         assert!(arc.is_poisoned());
 639     }
 640
 641     #[test]
 642     fn test_mutex_arc_nested() {
 643         // Tests nested mutexes and access
 644         // to underlying data.
 645         let arc = Arc::new(Mutex::new(1));
 646         let arc2 = Arc::new(Mutex::new(arc));
 647         let (tx, rx) = channel();
 648         let _t = thread::spawn(move|| {
 649             let lock = arc2.lock().unwrap();
 650             let lock2 = lock.lock().unwrap();
 651             assert_eq!(*lock2, 1);
 652             tx.send(()).unwrap();
 653         });
 654         rx.recv().unwrap();
 655     }
 656
 657     #[test]
 658     fn test_mutex_arc_access_in_unwind() {
 659         let arc = Arc::new(Mutex::new(1));
 660         let arc2 = arc.clone();
 661         let _ = thread::spawn(move|| -> () {
 662             struct Unwinder {
 663                 i: Arc<Mutex<i32>>,
 664             }
 665             impl Drop for Unwinder {
 666                 fn drop(&mut self) {
 667                     *self.i.lock().unwrap() += 1;
 668                 }
 669             }
 670             let _u = Unwinder { i: arc2 };
 671             panic!();
 672         }).join();
 673         let lock = arc.lock().unwrap();
 674         assert_eq!(*lock, 2);
 675     }
 676
 677     #[test]
 678     fn test_mutex_unsized() {
 679         let mutex: &Mutex<[i32]> = &Mutex::new([1, 2, 3]);
 680         {
 681             let b = &mut *mutex.lock().unwrap();
 682             b[0] = 4;
 683             b[2] = 5;
 684         }
 685         let comp: &[i32] = &[4, 2, 5];
 686         assert_eq!(&*mutex.lock().unwrap(), comp);
 687     }
 688 }