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

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    // Copyright 2015 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 fmt;
    use marker;
    use ops::Deref;
    use sys_common::poison::{self, TryLockError, TryLockResult, LockResult};
    use sys::mutex as sys;

    /// A re-entrant mutual exclusion
    ///
    /// This mutex will block *other* threads waiting for the lock to become
    /// available. The thread which has already locked the mutex can lock it
    /// multiple times without blocking, preventing a common source of deadlocks.
    pub struct ReentrantMutex<T> {
        inner: Box<sys::ReentrantMutex>,
        poison: poison::Flag,
        data: T,
    }

    unsafe impl<T: Send> Send for ReentrantMutex<T> {}
    unsafe impl<T: Send> Sync for ReentrantMutex<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 implementation.
    ///
    /// # Mutability
    ///
    /// Unlike `MutexGuard`, `ReentrantMutexGuard` does not implement `DerefMut`,
    /// because implementation of the trait would violate Rust’s reference aliasing
    /// rules. Use interior mutability (usually `RefCell`) in order to mutate the
    /// guarded data.
    #[must_use]
    pub struct ReentrantMutexGuard<'a, T: 'a> {
        // funny underscores due to how Deref currently works (it disregards field
        // privacy).
        __lock: &'a ReentrantMutex<T>,
        __poison: poison::Guard,
    }

    impl<'a, T> !marker::Send for ReentrantMutexGuard<'a, T> {}

    impl<T> ReentrantMutex<T> {
        /// Creates a new reentrant mutex in an unlocked state.
        pub fn new(t: T) -> ReentrantMutex<T> {
            unsafe {
                let mut mutex = ReentrantMutex {
                    inner: box sys::ReentrantMutex::uninitialized(),
                    poison: poison::Flag::new(),
                    data: t,
                };
                mutex.inner.init();
                mutex
            }
        }

        /// Acquires a mutex, blocking the current thread until it is able to do so.
        ///
        /// This function will block the caller until it is available to acquire the mutex.
        /// Upon returning, the thread is the only thread with the mutex held. When the thread
        /// calling this method already holds the lock, the call shall succeed without
        /// blocking.
        ///
        /// # 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.
        pub fn lock(&self) -> LockResult<ReentrantMutexGuard<T>> {
            unsafe { self.inner.lock() }
            ReentrantMutexGuard::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.
        ///
        /// 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.
        pub fn try_lock(&self) -> TryLockResult<ReentrantMutexGuard<T>> {
            if unsafe { self.inner.try_lock() } {
                Ok(ReentrantMutexGuard::new(&self)?)
            } else {
                Err(TryLockError::WouldBlock)
            }
        }
    }

    impl<T> Drop for ReentrantMutex<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)
            unsafe { self.inner.destroy() }
        }
    }

    impl<T: fmt::Debug + 'static> fmt::Debug for ReentrantMutex<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match self.try_lock() {
                Ok(guard) => f.debug_struct("ReentrantMutex").field("data", &*guard).finish(),
                Err(TryLockError::Poisoned(err)) => {
                    f.debug_struct("ReentrantMutex").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("ReentrantMutex").field("data", &LockedPlaceholder).finish()
                }
            }
        }
    }

    impl<'mutex, T> ReentrantMutexGuard<'mutex, T> {
        fn new(lock: &'mutex ReentrantMutex<T>)
                -> LockResult<ReentrantMutexGuard<'mutex, T>> {
            poison::map_result(lock.poison.borrow(), |guard| {
                ReentrantMutexGuard {
                    __lock: lock,
                    __poison: guard,
                }
            })
        }
    }

    impl<'mutex, T> Deref for ReentrantMutexGuard<'mutex, T> {
        type Target = T;

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

    impl<'a, T> Drop for ReentrantMutexGuard<'a, T> {
        #[inline]
        fn drop(&mut self) {
            unsafe {
                self.__lock.poison.done(&self.__poison);
                self.__lock.inner.unlock();
            }
        }
    }

    #[cfg(all(test, not(target_os = "emscripten")))]
    mod tests {
        use sys_common::remutex::{ReentrantMutex, ReentrantMutexGuard};
        use cell::RefCell;
        use sync::Arc;
        use thread;

        #[test]
        fn smoke() {
            let m = ReentrantMutex::new(());
            {
                let a = m.lock().unwrap();
                {
                    let b = m.lock().unwrap();
                    {
                        let c = m.lock().unwrap();
                        assert_eq!(*c, ());
                    }
                    assert_eq!(*b, ());
                }
                assert_eq!(*a, ());
            }
        }

        #[test]
        fn is_mutex() {
            let m = Arc::new(ReentrantMutex::new(RefCell::new(0)));
            let m2 = m.clone();
            let lock = m.lock().unwrap();
            let child = thread::spawn(move || {
                let lock = m2.lock().unwrap();
                assert_eq!(*lock.borrow(), 4950);
            });
            for i in 0..100 {
                let lock = m.lock().unwrap();
                *lock.borrow_mut() += i;
            }
            drop(lock);
            child.join().unwrap();
        }

        #[test]
        fn trylock_works() {
            let m = Arc::new(ReentrantMutex::new(()));
            let m2 = m.clone();
            let _lock = m.try_lock().unwrap();
            let _lock2 = m.try_lock().unwrap();
            thread::spawn(move || {
                let lock = m2.try_lock();
                assert!(lock.is_err());
            }).join().unwrap();
            let _lock3 = m.try_lock().unwrap();
        }

        pub struct Answer<'a>(pub ReentrantMutexGuard<'a, RefCell<u32>>);
        impl<'a> Drop for Answer<'a> {
            fn drop(&mut self) {
                *self.0.borrow_mut() = 42;
            }
        }

        #[test]
        fn poison_works() {
            let m = Arc::new(ReentrantMutex::new(RefCell::new(0)));
            let mc = m.clone();
            let result = thread::spawn(move ||{
                let lock = mc.lock().unwrap();
                *lock.borrow_mut() = 1;
                let lock2 = mc.lock().unwrap();
                *lock.borrow_mut() = 2;
                let _answer = Answer(lock2);
                panic!("What the answer to my lifetimes dilemma is?");
            }).join();
            assert!(result.is_err());
            let r = m.lock().err().unwrap().into_inner();
            assert_eq!(*r.borrow(), 42);
        }
    }

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