src/libstd/sync/mutex.rs - third_party/rust - Git at Google

 // 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 prelude::v1::*;

 use cell::UnsafeCell;
 use fmt;
 use marker;
 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
 /// lock. 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.
 ///
 /// # 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..10 {
 ///     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 lock 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 access through this guard via its
 /// `Deref` and `DerefMut` implementations
 #[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> !marker::Send for MutexGuard<'a, T> {}

 impl<T> Mutex<T> {
     /// Creates a new mutex in an unlocked state ready for use.
     #[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 mutex
     /// 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.
     #[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.
     #[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 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.
     #[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.
     #[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 lock.
         //
         // 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.
     #[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 lock.
         let data = unsafe { &mut *self.data.get() };
         poison::map_result(self.poison.borrow(), |_| data )
     }
 }

 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> Drop for Mutex<T> {
     #[unsafe_destructor_blind_to_params]
     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_default", since = "1.9.0")]
 impl<T: ?Sized + Default> Default for Mutex<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) => write!(f, "Mutex {{ data: {:?} }}", &*guard),
             Err(TryLockError::Poisoned(err)) => {
                 write!(f, "Mutex {{ data: Poisoned({:?}) }}", &**err.get_ref())
             },
             Err(TryLockError::WouldBlock) => write!(f, "Mutex {{ <locked> }}")
         }
     }
 }

 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();
         }
     }
 }

 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(test)]
 mod tests {
     use prelude::v1::*;

     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);

     unsafe impl<T: Send> Send for Packet<T> {}
     unsafe impl<T> Sync for Packet<T> {}

     #[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);
     }
 }
	// 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 prelude::v1::*;

	use cell::UnsafeCell;
	use fmt;
	use marker;
	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
	/// lock. 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.
	///
	/// # 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..10 {
	/// 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 lock 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 access through this guard via its
	/// `Deref` and `DerefMut` implementations
	#[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> !marker::Send for MutexGuard<'a, T> {}

	impl<T> Mutex<T> {
	/// Creates a new mutex in an unlocked state ready for use.
	#[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 mutex
	/// 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.
	#[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.
	#[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 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.
	#[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.
	#[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 lock.
	//
	// 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.
	#[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 lock.
	let data = unsafe { &mut *self.data.get() };
	poison::map_result(self.poison.borrow(), \|_\| data )
	}
	}

	#[stable(feature = "rust1", since = "1.0.0")]
	impl<T: ?Sized> Drop for Mutex<T> {
	#[unsafe_destructor_blind_to_params]
	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_default", since = "1.9.0")]
	impl<T: ?Sized + Default> Default for Mutex<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) => write!(f, "Mutex {{ data: {:?} }}", &*guard),
	Err(TryLockError::Poisoned(err)) => {
	write!(f, "Mutex {{ data: Poisoned({:?}) }}", &**err.get_ref())
	},
	Err(TryLockError::WouldBlock) => write!(f, "Mutex {{ <locked> }}")
	}
	}
	}

	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();
	}
	}
	}

	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(test)]
	mod tests {
	use prelude::v1::*;

	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);

	unsafe impl<T: Send> Send for Packet<T> {}
	unsafe impl<T> Sync for Packet<T> {}

	#[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);
	}
	}