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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 marker;
    use mem;
    use ops::{Deref, DerefMut};
    use ptr;
    use sys_common::poison::{self, LockResult, TryLockError, TryLockResult};
    use sys_common::rwlock as sys;

    /// A reader-writer lock
    ///
    /// This type of lock allows a number of readers or at most one writer at any
    /// point in time. The write portion of this lock typically allows modification
    /// of the underlying data (exclusive access) and the read portion of this lock
    /// typically allows for read-only access (shared access).
    ///
    /// The priority policy of the lock is dependent on the underlying operating
    /// system's implementation, and this type does not guarantee that any
    /// particular policy will be used.
    ///
    /// The type parameter `T` represents the data that this lock protects. It is
    /// required that `T` satisfies `Send` to be shared across threads and `Sync` to
    /// allow concurrent access through readers. The RAII guards returned from the
    /// locking methods implement `Deref` (and `DerefMut` for the `write` methods)
    /// to allow access to the contained of the lock.
    ///
    /// # Poisoning
    ///
    /// An `RwLock`, like `Mutex`, will become poisoned on a panic. Note, however,
    /// that an `RwLock` may only be poisoned if a panic occurs while it is locked
    /// exclusively (write mode). If a panic occurs in any reader, then the lock
    /// will not be poisoned.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(5);
    ///
    /// // many reader locks can be held at once
    /// {
    ///     let r1 = lock.read().unwrap();
    ///     let r2 = lock.read().unwrap();
    ///     assert_eq!(*r1, 5);
    ///     assert_eq!(*r2, 5);
    /// } // read locks are dropped at this point
    ///
    /// // only one write lock may be held, however
    /// {
    ///     let mut w = lock.write().unwrap();
    ///     *w += 1;
    ///     assert_eq!(*w, 6);
    /// } // write lock is dropped here
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct RwLock<T: ?Sized> {
        inner: Box<sys::RWLock>,
        poison: poison::Flag,
        data: UnsafeCell<T>,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T: ?Sized + Send + Sync> Send for RwLock<T> {}
    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}

    /// RAII structure used to release the shared read access of a lock when
    /// dropped.
    ///
    /// This structure is created by the [`read`] and [`try_read`] methods on
    /// [`RwLock`].
    ///
    /// [`read`]: struct.RwLock.html#method.read
    /// [`try_read`]: struct.RwLock.html#method.try_read
    /// [`RwLock`]: struct.RwLock.html
    #[must_use]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct RwLockReadGuard<'a, T: ?Sized + 'a> {
        __lock: &'a RwLock<T>,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: ?Sized> !marker::Send for RwLockReadGuard<'a, T> {}

    /// RAII structure used to release the exclusive write access of a lock when
    /// dropped.
    ///
    /// This structure is created by the [`write`] and [`try_write`] methods
    /// on [`RwLock`].
    ///
    /// [`write`]: struct.RwLock.html#method.write
    /// [`try_write`]: struct.RwLock.html#method.try_write
    /// [`RwLock`]: struct.RwLock.html
    #[must_use]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct RwLockWriteGuard<'a, T: ?Sized + 'a> {
        __lock: &'a RwLock<T>,
        __poison: poison::Guard,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: ?Sized> !marker::Send for RwLockWriteGuard<'a, T> {}

    impl<T> RwLock<T> {
        /// Creates a new instance of an `RwLock<T>` which is unlocked.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::RwLock;
        ///
        /// let lock = RwLock::new(5);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new(t: T) -> RwLock<T> {
            RwLock {
                inner: box sys::RWLock::new(),
                poison: poison::Flag::new(),
                data: UnsafeCell::new(t),
            }
        }
    }

    impl<T: ?Sized> RwLock<T> {
        /// Locks this rwlock with shared read access, blocking the current thread
        /// until it can be acquired.
        ///
        /// The calling thread will be blocked until there are no more writers which
        /// hold the lock. There may be other readers currently inside the lock when
        /// this method returns. This method does not provide any guarantees with
        /// respect to the ordering of whether contentious readers or writers will
        /// acquire the lock first.
        ///
        /// Returns an RAII guard which will release this thread's shared access
        /// once it is dropped.
        ///
        /// # Errors
        ///
        /// This function will return an error if the RwLock is poisoned. An RwLock
        /// is poisoned whenever a writer panics while holding an exclusive lock.
        /// The failure will occur immediately after the lock has been acquired.
        ///
        /// # Panics
        ///
        /// This function might panic when called if the lock is already held by the current thread.
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn read(&self) -> LockResult<RwLockReadGuard<T>> {
            unsafe {
                self.inner.read();
                RwLockReadGuard::new(self)
            }
        }

        /// Attempts to acquire this rwlock with shared read access.
        ///
        /// If the access could not be granted at this time, then `Err` is returned.
        /// Otherwise, an RAII guard is returned which will release the shared access
        /// when it is dropped.
        ///
        /// This function does not block.
        ///
        /// This function does not provide any guarantees with respect to the ordering
        /// of whether contentious readers or writers will acquire the lock first.
        ///
        /// # Errors
        ///
        /// This function will return an error if the RwLock is poisoned. An RwLock
        /// is poisoned whenever a writer panics while holding an exclusive lock. An
        /// error will only be returned if the lock would have otherwise been
        /// acquired.
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn try_read(&self) -> TryLockResult<RwLockReadGuard<T>> {
            unsafe {
                if self.inner.try_read() {
                    Ok(RwLockReadGuard::new(self)?)
                } else {
                    Err(TryLockError::WouldBlock)
                }
            }
        }

        /// Locks this rwlock with exclusive write access, blocking the current
        /// thread until it can be acquired.
        ///
        /// This function will not return while other writers or other readers
        /// currently have access to the lock.
        ///
        /// Returns an RAII guard which will drop the write access of this rwlock
        /// when dropped.
        ///
        /// # Errors
        ///
        /// This function will return an error if the RwLock is poisoned. An RwLock
        /// is poisoned whenever a writer panics while holding an exclusive lock.
        /// An error will be returned when the lock is acquired.
        ///
        /// # Panics
        ///
        /// This function might panic when called if the lock is already held by the current thread.
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn write(&self) -> LockResult<RwLockWriteGuard<T>> {
            unsafe {
                self.inner.write();
                RwLockWriteGuard::new(self)
            }
        }

        /// Attempts to lock this rwlock with exclusive write access.
        ///
        /// If the lock could not be acquired at this time, then `Err` is returned.
        /// Otherwise, an RAII guard is returned which will release the lock when
        /// it is dropped.
        ///
        /// This function does not block.
        ///
        /// This function does not provide any guarantees with respect to the ordering
        /// of whether contentious readers or writers will acquire the lock first.
        ///
        /// # Errors
        ///
        /// This function will return an error if the RwLock is poisoned. An RwLock
        /// is poisoned whenever a writer panics while holding an exclusive lock. An
        /// error will only be returned if the lock would have otherwise been
        /// acquired.
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn try_write(&self) -> TryLockResult<RwLockWriteGuard<T>> {
            unsafe {
                if self.inner.try_write() {
                    Ok(RwLockWriteGuard::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 `RwLock`, returning the underlying data.
        ///
        /// # Errors
        ///
        /// This function will return an error if the RwLock is poisoned. An RwLock
        /// is poisoned whenever a writer panics while holding an exclusive lock. An
        /// error will only be returned if the lock would have otherwise been
        /// acquired.
        #[stable(feature = "rwlock_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 `RwLock` impl-s `Drop`, we can't move out of it, so
            // we'll have to destructure it manually instead.
            unsafe {
                // Like `let RwLock { inner, poison, data } = self`.
                let (inner, poison, data) = {
                    let RwLock { 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 `RwLock` mutably, no actual locking needs to
        /// take place---the mutable borrow statically guarantees no locks exist.
        ///
        /// # Errors
        ///
        /// This function will return an error if the RwLock is poisoned. An RwLock
        /// is poisoned whenever a writer panics while holding an exclusive lock. An
        /// error will only be returned if the lock would have otherwise been
        /// acquired.
        #[stable(feature = "rwlock_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")]
    unsafe impl<#[may_dangle] T: ?Sized> Drop for RwLock<T> {
        fn drop(&mut self) {
            // IMPORTANT: This code needs to be kept in sync with `RwLock::into_inner`.
            unsafe { self.inner.destroy() }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match self.try_read() {
                Ok(guard) => write!(f, "RwLock {{ data: {:?} }}", &*guard),
                Err(TryLockError::Poisoned(err)) => {
                    write!(f, "RwLock {{ data: Poisoned({:?}) }}", &**err.get_ref())
                },
                Err(TryLockError::WouldBlock) => write!(f, "RwLock {{ <locked> }}")
            }
        }
    }

    #[stable(feature = "rw_lock_default", since = "1.10.0")]
    impl<T: Default> Default for RwLock<T> {
        /// Creates a new `RwLock<T>`, with the `Default` value for T.
        fn default() -> RwLock<T> {
            RwLock::new(Default::default())
        }
    }

    impl<'rwlock, T: ?Sized> RwLockReadGuard<'rwlock, T> {
        unsafe fn new(lock: &'rwlock RwLock<T>)
                      -> LockResult<RwLockReadGuard<'rwlock, T>> {
            poison::map_result(lock.poison.borrow(), |_| {
                RwLockReadGuard {
                    __lock: lock,
                }
            })
        }
    }

    impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
        unsafe fn new(lock: &'rwlock RwLock<T>)
                      -> LockResult<RwLockWriteGuard<'rwlock, T>> {
            poison::map_result(lock.poison.borrow(), |guard| {
                RwLockWriteGuard {
                    __lock: lock,
                    __poison: guard,
                }
            })
        }
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl<'a, T: fmt::Debug> fmt::Debug for RwLockReadGuard<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("RwLockReadGuard")
                .field("lock", &self.__lock)
                .finish()
        }
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl<'a, T: fmt::Debug> fmt::Debug for RwLockWriteGuard<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("RwLockWriteGuard")
                .field("lock", &self.__lock)
                .finish()
        }
    }

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

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

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

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'rwlock, T: ?Sized> DerefMut for RwLockWriteGuard<'rwlock, 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 RwLockReadGuard<'a, T> {
        fn drop(&mut self) {
            unsafe { self.__lock.inner.read_unlock(); }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: ?Sized> Drop for RwLockWriteGuard<'a, T> {
        fn drop(&mut self) {
            self.__lock.poison.done(&self.__poison);
            unsafe { self.__lock.inner.write_unlock(); }
        }
    }

    #[cfg(all(test, not(target_os = "emscripten")))]
    mod tests {
        #![allow(deprecated)] // rand

        use rand::{self, Rng};
        use sync::mpsc::channel;
        use thread;
        use sync::{Arc, RwLock, TryLockError};
        use sync::atomic::{AtomicUsize, Ordering};

        #[derive(Eq, PartialEq, Debug)]
        struct NonCopy(i32);

        #[test]
        fn smoke() {
            let l = RwLock::new(());
            drop(l.read().unwrap());
            drop(l.write().unwrap());
            drop((l.read().unwrap(), l.read().unwrap()));
            drop(l.write().unwrap());
        }

        #[test]
        fn frob() {
            const N: usize = 10;
            const M: usize = 1000;

            let r = Arc::new(RwLock::new(()));

            let (tx, rx) = channel::<()>();
            for _ in 0..N {
                let tx = tx.clone();
                let r = r.clone();
                thread::spawn(move || {
                    let mut rng = rand::thread_rng();
                    for _ in 0..M {
                        if rng.gen_weighted_bool(N) {
                            drop(r.write().unwrap());
                        } else {
                            drop(r.read().unwrap());
                        }
                    }
                    drop(tx);
                });
            }
            drop(tx);
            let _ = rx.recv();
        }

        #[test]
        fn test_rw_arc_poison_wr() {
            let arc = Arc::new(RwLock::new(1));
            let arc2 = arc.clone();
            let _: Result<(), _> = thread::spawn(move|| {
                let _lock = arc2.write().unwrap();
                panic!();
            }).join();
            assert!(arc.read().is_err());
        }

        #[test]
        fn test_rw_arc_poison_ww() {
            let arc = Arc::new(RwLock::new(1));
            assert!(!arc.is_poisoned());
            let arc2 = arc.clone();
            let _: Result<(), _> = thread::spawn(move|| {
                let _lock = arc2.write().unwrap();
                panic!();
            }).join();
            assert!(arc.write().is_err());
            assert!(arc.is_poisoned());
        }

        #[test]
        fn test_rw_arc_no_poison_rr() {
            let arc = Arc::new(RwLock::new(1));
            let arc2 = arc.clone();
            let _: Result<(), _> = thread::spawn(move|| {
                let _lock = arc2.read().unwrap();
                panic!();
            }).join();
            let lock = arc.read().unwrap();
            assert_eq!(*lock, 1);
        }
        #[test]
        fn test_rw_arc_no_poison_rw() {
            let arc = Arc::new(RwLock::new(1));
            let arc2 = arc.clone();
            let _: Result<(), _> = thread::spawn(move|| {
                let _lock = arc2.read().unwrap();
                panic!()
            }).join();
            let lock = arc.write().unwrap();
            assert_eq!(*lock, 1);
        }

        #[test]
        fn test_rw_arc() {
            let arc = Arc::new(RwLock::new(0));
            let arc2 = arc.clone();
            let (tx, rx) = channel();

            thread::spawn(move|| {
                let mut lock = arc2.write().unwrap();
                for _ in 0..10 {
                    let tmp = *lock;
                    *lock = -1;
                    thread::yield_now();
                    *lock = tmp + 1;
                }
                tx.send(()).unwrap();
            });

            // Readers try to catch the writer in the act
            let mut children = Vec::new();
            for _ in 0..5 {
                let arc3 = arc.clone();
                children.push(thread::spawn(move|| {
                    let lock = arc3.read().unwrap();
                    assert!(*lock >= 0);
                }));
            }

            // Wait for children to pass their asserts
            for r in children {
                assert!(r.join().is_ok());
            }

            // Wait for writer to finish
            rx.recv().unwrap();
            let lock = arc.read().unwrap();
            assert_eq!(*lock, 10);
        }

        #[test]
        fn test_rw_arc_access_in_unwind() {
            let arc = Arc::new(RwLock::new(1));
            let arc2 = arc.clone();
            let _ = thread::spawn(move|| -> () {
                struct Unwinder {
                    i: Arc<RwLock<isize>>,
                }
                impl Drop for Unwinder {
                    fn drop(&mut self) {
                        let mut lock = self.i.write().unwrap();
                        *lock += 1;
                    }
                }
                let _u = Unwinder { i: arc2 };
                panic!();
            }).join();
            let lock = arc.read().unwrap();
            assert_eq!(*lock, 2);
        }

        #[test]
        fn test_rwlock_unsized() {
            let rw: &RwLock<[i32]> = &RwLock::new([1, 2, 3]);
            {
                let b = &mut *rw.write().unwrap();
                b[0] = 4;
                b[2] = 5;
            }
            let comp: &[i32] = &[4, 2, 5];
            assert_eq!(&*rw.read().unwrap(), comp);
        }

        #[test]
        fn test_rwlock_try_write() {
            let lock = RwLock::new(0isize);
            let read_guard = lock.read().unwrap();

            let write_result = lock.try_write();
            match write_result {
                Err(TryLockError::WouldBlock) => (),
                Ok(_) => assert!(false, "try_write should not succeed while read_guard is in scope"),
                Err(_) => assert!(false, "unexpected error"),
            }

            drop(read_guard);
        }

        #[test]
        fn test_into_inner() {
            let m = RwLock::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 = RwLock::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(RwLock::new(NonCopy(10)));
            let m2 = m.clone();
            let _ = thread::spawn(move || {
                let _lock = m2.write().unwrap();
                panic!("test panic in inner thread to poison RwLock");
            }).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 RwLock is Ok: {:?}", x),
            }
        }

        #[test]
        fn test_get_mut() {
            let mut m = RwLock::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(RwLock::new(NonCopy(10)));
            let m2 = m.clone();
            let _ = thread::spawn(move || {
                let _lock = m2.write().unwrap();
                panic!("test panic in inner thread to poison RwLock");
            }).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 RwLock is Ok: {:?}", x),
            }
        }
    }

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