parking_lot 0.2.2

// Copyright 2016 Amanieu d'Antras
//
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// http://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 std::cell::UnsafeCell;
use std::ops::{Deref, DerefMut};
use std::fmt;
use raw_rwlock::RawRwLock;

/// 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).
///
/// This lock will always prioritize writers over readers to avoid writer
/// starvation. This means that readers trying to acquire the lock will block
/// even if the lock is unlocked when there are writers waiting to acquire the
/// lock. Because of this, attempts to recursively acquire a read lock within a
/// single thread may result in a deadlock.
///
/// 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.
///
/// # Differences from the standard library `RwLock`
///
/// - Writer-preferred policy instead of an unspecified platform default.
/// - No poisoning, the lock is released normally on panic.
/// - Only requires 1 word of space, whereas the standard library boxes the
///   `RwLock` due to platform limitations.
/// - A lock guard can be sent to another thread and unlocked there.
/// - Can be statically constructed (requires the `const_fn` nightly feature).
/// - Does not require any drop glue when dropped.
/// - Inline fast path for the uncontended case.
/// - Efficient handling of micro-contention using adaptive spinning.
///
/// # Examples
///
/// ```
/// use parking_lot::RwLock;
///
/// let lock = RwLock::new(5);
///
/// // many reader locks can be held at once
/// {
///     let r1 = lock.read();
///     let r2 = lock.read();
///     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();
///     *w += 1;
///     assert_eq!(*w, 6);
/// } // write lock is dropped here
/// ```
pub struct RwLock<T: ?Sized> {
    rwlock: RawRwLock,
    data: UnsafeCell<T>,
}

unsafe impl<T: ?Sized + Send> Send for RwLock<T> {}
unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}

/// RAII structure used to release the shared read access of a lock when
/// dropped.
#[must_use]
pub struct RwLockReadGuard<'a, T: ?Sized + 'a> {
    rwlock: &'a RawRwLock,
    data: &'a T,
}

/// RAII structure used to release the exclusive write access of a lock when
/// dropped.
#[must_use]
pub struct RwLockWriteGuard<'a, T: ?Sized + 'a> {
    rwlock: &'a RawRwLock,
    data: &'a mut T,
}

impl<T> RwLock<T> {
    /// Creates a new instance of an `RwLock<T>` which is unlocked.
    ///
    /// # Examples
    ///
    /// ```
    /// use parking_lot::RwLock;
    ///
    /// let lock = RwLock::new(5);
    /// ```
    #[cfg(feature = "nightly")]
    #[inline]
    pub const fn new(val: T) -> RwLock<T> {
        RwLock {
            data: UnsafeCell::new(val),
            rwlock: RawRwLock::new(),
        }
    }

    /// Creates a new instance of an `RwLock<T>` which is unlocked.
    ///
    /// # Examples
    ///
    /// ```
    /// use parking_lot::RwLock;
    ///
    /// let lock = RwLock::new(5);
    /// ```
    #[cfg(not(feature = "nightly"))]
    #[inline]
    pub fn new(val: T) -> RwLock<T> {
        RwLock {
            data: UnsafeCell::new(val),
            rwlock: RawRwLock::new(),
        }
    }

    /// Consumes this `RwLock`, returning the underlying data.
    #[inline]
    pub fn into_inner(self) -> T {
        unsafe { self.data.into_inner() }
    }
}

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.
    ///
    /// Because `RwLock` prefers writers over readers, attempts to recursively
    /// acquire a read lock when the current already owns one may result in a
    /// deadlock.
    ///
    /// Returns an RAII guard which will release this thread's shared access
    /// once it is dropped.
    #[inline]
    pub fn read(&self) -> RwLockReadGuard<T> {
        self.rwlock.lock_shared();
        RwLockReadGuard {
            rwlock: &self.rwlock,
            data: unsafe { &*self.data.get() },
        }
    }

    /// Attempts to acquire this rwlock with shared read access.
    ///
    /// If the access could not be granted at this time, then `None` is returned.
    /// Otherwise, an RAII guard is returned which will release the shared access
    /// when it is dropped.
    ///
    /// This function does not block.
    #[inline]
    pub fn try_read(&self) -> Option<RwLockReadGuard<T>> {
        if self.rwlock.try_lock_shared() {
            Some(RwLockReadGuard {
                rwlock: &self.rwlock,
                data: unsafe { &*self.data.get() },
            })
        } else {
            None
        }
    }

    /// 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.
    #[inline]
    pub fn write(&self) -> RwLockWriteGuard<T> {
        self.rwlock.lock_exclusive();
        RwLockWriteGuard {
            rwlock: &self.rwlock,
            data: unsafe { &mut *self.data.get() },
        }
    }

    /// Attempts to lock this rwlock with exclusive write access.
    ///
    /// If the lock could not be acquired at this time, then `None` is returned.
    /// Otherwise, an RAII guard is returned which will release the lock when
    /// it is dropped.
    ///
    /// This function does not block.
    #[inline]
    pub fn try_write(&self) -> Option<RwLockWriteGuard<T>> {
        if self.rwlock.try_lock_exclusive() {
            Some(RwLockWriteGuard {
                rwlock: &self.rwlock,
                data: unsafe { &mut *self.data.get() },
            })
        } else {
            None
        }
    }

    /// 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.
    #[inline]
    pub fn get_mut(&mut self) -> &mut T {
        unsafe { &mut *self.data.get() }
    }
}

impl<T: ?Sized + Default> Default for RwLock<T> {
    #[inline]
    fn default() -> RwLock<T> {
        RwLock::new(Default::default())
    }
}

impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self.try_read() {
            Some(guard) => write!(f, "RwLock {{ data: {:?} }}", &*guard),
            None => write!(f, "RwLock {{ <locked> }}"),
        }
    }
}

impl<'a, T: ?Sized + 'a> Deref for RwLockReadGuard<'a, T> {
    type Target = T;
    #[inline]
    fn deref(&self) -> &T {
        self.data
    }
}

impl<'a, T: ?Sized + 'a> Drop for RwLockReadGuard<'a, T> {
    #[inline]
    fn drop(&mut self) {
        self.rwlock.unlock_shared();
    }
}

impl<'a, T: ?Sized + 'a> Deref for RwLockWriteGuard<'a, T> {
    type Target = T;
    #[inline]
    fn deref(&self) -> &T {
        self.data
    }
}

impl<'a, T: ?Sized + 'a> DerefMut for RwLockWriteGuard<'a, T> {
    #[inline]
    fn deref_mut(&mut self) -> &mut T {
        self.data
    }
}

impl<'a, T: ?Sized + 'a> Drop for RwLockWriteGuard<'a, T> {
    #[inline]
    fn drop(&mut self) {
        self.rwlock.unlock_exclusive();
    }
}

#[cfg(test)]
mod tests {
    extern crate rand;
    use self::rand::Rng;
    use std::sync::mpsc::channel;
    use std::thread;
    use std::sync::Arc;
    use std::sync::atomic::{AtomicUsize, Ordering};
    use RwLock;

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

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

    #[test]
    fn frob() {
        lazy_static! {
            static ref R: RwLock<()> = RwLock::new(());
        }
        const N: u32 = 10;
        const M: u32 = 1000;

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

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

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

    #[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();
                panic!();
            })
            .join();
        let lock = arc.read();
        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();
                panic!()
            })
            .join();
        let lock = arc.write();
        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();
            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();
                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();
        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();
                        *lock += 1;
                    }
                }
                let _u = Unwinder { i: arc2 };
                panic!();
            })
            .join();
        let lock = arc.read();
        assert_eq!(*lock, 2);
    }

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

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

        let write_result = lock.try_write();
        match write_result {
            None => (),
            Some(_) => {
                assert!(false,
                        "try_write should not succeed while read_guard is in scope")
            }
        }

        drop(read_guard);
    }

    #[test]
    fn test_into_inner() {
        let m = RwLock::new(NonCopy(10));
        assert_eq!(m.into_inner(), 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();
            assert_eq!(num_drops.load(Ordering::SeqCst), 0);
        }
        assert_eq!(num_drops.load(Ordering::SeqCst), 1);
    }

    #[test]
    fn test_get_mut() {
        let mut m = RwLock::new(NonCopy(10));
        *m.get_mut() = NonCopy(20);
        assert_eq!(m.into_inner(), NonCopy(20));
    }

    #[test]
    fn test_rwlockguard_send() {
        fn send<T: Send>(_: T) {}

        let rwlock = RwLock::new(());
        send(rwlock.read());
        send(rwlock.write());
    }
}