New upstream version 1.46.0~beta.2+dfsg1

[rustc.git] / src / libstd / sync / mutex.rs

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

/// 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) = (Arc::clone(&data), 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;
/// ```
///
/// It is sometimes necessary to manually drop the mutex guard to unlock it
/// sooner than the end of the enclosing scope.
///
/// ```
/// use std::sync::{Arc, Mutex};
/// use std::thread;
///
/// const N: usize = 3;
///
/// let data_mutex = Arc::new(Mutex::new(vec![1, 2, 3, 4]));
/// let res_mutex = Arc::new(Mutex::new(0));
///
/// let mut threads = Vec::with_capacity(N);
/// (0..N).for_each(|_| {
///     let data_mutex_clone = Arc::clone(&data_mutex);
///     let res_mutex_clone = Arc::clone(&res_mutex);
///
///     threads.push(thread::spawn(move || {
///         let mut data = data_mutex_clone.lock().unwrap();
///         // This is the result of some important and long-ish work.
///         let result = data.iter().fold(0, |acc, x| acc + x * 2);
///         data.push(result);
///         drop(data);
///         *res_mutex_clone.lock().unwrap() += result;
///     }));
/// });
///
/// let mut data = data_mutex.lock().unwrap();
/// // This is the result of some important and long-ish work.
/// let result = data.iter().fold(0, |acc, x| acc + x * 2);
/// data.push(result);
/// // We drop the `data` explicitly because it's not necessary anymore and the
/// // thread still has work to do. This allow other threads to start working on
/// // the data immediately, without waiting for the rest of the unrelated work
/// // to be done here.
/// //
/// // It's even more important here than in the threads because we `.join` the
/// // threads after that. If we had not dropped the mutex guard, a thread could
/// // be waiting forever for it, causing a deadlock.
/// drop(data);
/// // Here the mutex guard is not assigned to a variable and so, even if the
/// // scope does not end after this line, the mutex is still released: there is
/// // no deadlock.
/// *res_mutex.lock().unwrap() += result;
///
/// threads.into_iter().for_each(|thread| {
///     thread
///         .join()
///         .expect("The thread creating or execution failed !")
/// });
///
/// assert_eq!(*res_mutex.lock().unwrap(), 800);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "mutex_type")]
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 = "if unused the Mutex will immediately unlock"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct MutexGuard<'a, T: ?Sized + 'a> {
    lock: &'a Mutex<T>,
    poison: poison::Guard,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Send for MutexGuard<'_, T> {}
#[stable(feature = "mutexguard", since = "1.19.0")]
unsafe impl<T: ?Sized + Sync> Sync for MutexGuard<'_, 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.raw_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`]: ../../std/sync/struct.Mutex.html#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, poison: guard })
    }
}

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

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

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

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Drop for MutexGuard<'_, T> {
    #[inline]
    fn drop(&mut self) {
        unsafe {
            self.lock.poison.done(&self.poison);
            self.lock.inner.raw_unlock();
        }
    }
}

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

#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<T: ?Sized + fmt::Display> fmt::Display for MutexGuard<'_, 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 crate::sync::atomic::{AtomicUsize, Ordering};
    use crate::sync::mpsc::channel;
    use crate::sync::{Arc, Condvar, Mutex};
    use crate::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);
    }
}