mutex.rs.html -- source
Description: Source to the Rust file `libstd/sync/mutex.rs`.
Keywords: rust, rustlang, rust-lang

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    // 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 = "1.22.0")]
    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);
        }
    }

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