openethereum/crates/util/len-caching-lock/src/mutex.rs

// Copyright 2015-2020 Parity Technologies (UK) Ltd.
// This file is part of OpenEthereum.

// OpenEthereum is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// OpenEthereum is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with OpenEthereum.  If not, see <http://www.gnu.org/licenses/>.

use std::{
    ops::{Deref, DerefMut},
    sync::atomic::{AtomicUsize, Ordering},
};

use parking_lot::{Mutex, MutexGuard};

use Len;

/// Can be used in place of a [`Mutex`](../../lock_api/struct.Mutex.html) where reading `T`'s `len()` without
/// needing to lock, is advantageous.
/// When the Guard is released, `T`'s `len()` will be cached.
/// The cached `len()` may be at most 1 lock behind current state.
#[derive(Debug)]
pub struct LenCachingMutex<T: ?Sized> {
    len: AtomicUsize,
    data: Mutex<T>,
}

impl<T: Len + Default> Default for LenCachingMutex<T> {
    fn default() -> Self {
        LenCachingMutex::new(T::default())
    }
}

impl<T: Len> From<T> for LenCachingMutex<T> {
    fn from(data: T) -> Self {
        LenCachingMutex::new(data)
    }
}

impl<T: Len> LenCachingMutex<T> {
    /// Constructs a new LenCachingMutex
    pub fn new(data: T) -> Self {
        LenCachingMutex {
            len: AtomicUsize::new(data.len()),
            data: Mutex::new(data),
        }
    }
}

impl<T: Len + ?Sized> LenCachingMutex<T> {
    /// Load the cached value that was returned from your `T`'s `len()`
    /// subsequent to the most recent lock being released.
    pub fn load_len(&self) -> usize {
        self.len.load(Ordering::SeqCst)
    }

    /// Delegates to `parking_lot::Mutex`
    /// [`lock()`](../../lock_api/struct.Mutex.html#method.lock).
    pub fn lock(&self) -> CachingMutexGuard<T> {
        CachingMutexGuard {
            mutex_guard: self.data.lock(),
            len: &self.len,
        }
    }

    /// Delegates to `parking_lot::Mutex`
    /// [`try_lock()`](../../lock_api/struct.Mutex.html#method.try_lock).
    pub fn try_lock(&self) -> Option<CachingMutexGuard<T>> {
        Some(CachingMutexGuard {
            mutex_guard: self.data.try_lock()?,
            len: &self.len,
        })
    }
}

/// Guard comprising `MutexGuard` and `AtomicUsize` for cache
pub struct CachingMutexGuard<'a, T: Len + 'a + ?Sized> {
    mutex_guard: MutexGuard<'a, T>,
    len: &'a AtomicUsize,
}

impl<'a, T: Len + ?Sized> CachingMutexGuard<'a, T> {
    /// Returns a mutable reference to the contained
    /// [`MutexGuard`](../../parking_lot/mutex/type.MutexGuard.html)
    pub fn inner_mut(&mut self) -> &mut MutexGuard<'a, T> {
        &mut self.mutex_guard
    }

    /// Returns a non-mutable reference to the contained
    /// [`MutexGuard`](../../parking_lot/mutex/type.MutexGuard.html)
    pub fn inner(&self) -> &MutexGuard<'a, T> {
        &self.mutex_guard
    }
}

impl<'a, T: Len + ?Sized> Drop for CachingMutexGuard<'a, T> {
    fn drop(&mut self) {
        self.len.store(self.mutex_guard.len(), Ordering::SeqCst);
    }
}

impl<'a, T: Len + ?Sized> Deref for CachingMutexGuard<'a, T> {
    type Target = T;
    fn deref(&self) -> &T {
        self.mutex_guard.deref()
    }
}

impl<'a, T: Len + ?Sized> DerefMut for CachingMutexGuard<'a, T> {
    fn deref_mut(&mut self) -> &mut T {
        self.mutex_guard.deref_mut()
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::collections::VecDeque;

    #[test]
    fn caches_len() {
        let v = vec![1, 2, 3];
        let lcm = LenCachingMutex::new(v);
        assert_eq!(lcm.load_len(), 3);
        lcm.lock().push(4);
        assert_eq!(lcm.load_len(), 4);
    }

    #[test]
    fn works_with_vec() {
        let v: Vec<i32> = Vec::new();
        let lcm = LenCachingMutex::new(v);
        assert!(lcm.lock().is_empty());
    }

    #[test]
    fn works_with_vecdeque() {
        let v: VecDeque<i32> = VecDeque::new();
        let lcm = LenCachingMutex::new(v);
        lcm.lock().push_front(4);
        assert_eq!(lcm.load_len(), 1);
    }
}