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use crate::cell::UnsafeCell;
use crate::sync::atomic::{AtomicUsize, Ordering};

pub struct RWLock {
    inner: UnsafeCell<libc::pthread_rwlock_t>,
    write_locked: UnsafeCell<bool>, // guarded by the `inner` RwLock
    num_readers: AtomicUsize,
}

unsafe impl Send for RWLock {}
unsafe impl Sync for RWLock {}

impl RWLock {
    pub const fn new() -> RWLock {
        RWLock {
            inner: UnsafeCell::new(libc::PTHREAD_RWLOCK_INITIALIZER),
            write_locked: UnsafeCell::new(false),
            num_readers: AtomicUsize::new(0),
        }
    }
    #[inline]
    pub unsafe fn read(&self) {
        let r = libc::pthread_rwlock_rdlock(self.inner.get());

        // According to the pthread_rwlock_rdlock spec, this function **may**
        // fail with EDEADLK if a deadlock is detected. On the other hand
        // pthread mutexes will *never* return EDEADLK if they are initialized
        // as the "fast" kind (which ours always are). As a result, a deadlock
        // situation may actually return from the call to pthread_rwlock_rdlock
        // instead of blocking forever (as mutexes and Windows rwlocks do). Note
        // that not all unix implementations, however, will return EDEADLK for
        // their rwlocks.
        //
        // We roughly maintain the deadlocking behavior by panicking to ensure
        // that this lock acquisition does not succeed.
        //
        // We also check whether this lock is already write locked. This
        // is only possible if it was write locked by the current thread and
        // the implementation allows recursive locking. The POSIX standard
        // doesn't require recursively locking a rwlock to deadlock, but we can't
        // allow that because it could lead to aliasing issues.
        if r == libc::EAGAIN {
            panic!("rwlock maximum reader count exceeded");
        } else if r == libc::EDEADLK || (r == 0 && *self.write_locked.get()) {
            if r == 0 {
                self.raw_unlock();
            }
            panic!("rwlock read lock would result in deadlock");
        } else {
            assert_eq!(r, 0);
            self.num_readers.fetch_add(1, Ordering::Relaxed);
        }
    }
    #[inline]
    pub unsafe fn try_read(&self) -> bool {
        let r = libc::pthread_rwlock_tryrdlock(self.inner.get());
        if r == 0 {
            if *self.write_locked.get() {
                self.raw_unlock();
                false
            } else {
                self.num_readers.fetch_add(1, Ordering::Relaxed);
                true
            }
        } else {
            false
        }
    }
    #[inline]
    pub unsafe fn write(&self) {
        let r = libc::pthread_rwlock_wrlock(self.inner.get());
        // See comments above for why we check for EDEADLK and write_locked. We
        // also need to check that num_readers is 0.
        if r == libc::EDEADLK || *self.write_locked.get() ||
           self.num_readers.load(Ordering::Relaxed) != 0 {
            if r == 0 {
                self.raw_unlock();
            }
            panic!("rwlock write lock would result in deadlock");
        } else {
            debug_assert_eq!(r, 0);
        }
        *self.write_locked.get() = true;
    }
    #[inline]
    pub unsafe fn try_write(&self) -> bool {
        let r = libc::pthread_rwlock_trywrlock(self.inner.get());
        if r == 0 {
            if *self.write_locked.get() || self.num_readers.load(Ordering::Relaxed) != 0 {
                self.raw_unlock();
                false
            } else {
                *self.write_locked.get() = true;
                true
            }
        } else {
            false
        }
    }
    #[inline]
    unsafe fn raw_unlock(&self) {
        let r = libc::pthread_rwlock_unlock(self.inner.get());
        debug_assert_eq!(r, 0);
    }
    #[inline]
    pub unsafe fn read_unlock(&self) {
        debug_assert!(!*self.write_locked.get());
        self.num_readers.fetch_sub(1, Ordering::Relaxed);
        self.raw_unlock();
    }
    #[inline]
    pub unsafe fn write_unlock(&self) {
        debug_assert_eq!(self.num_readers.load(Ordering::Relaxed), 0);
        debug_assert!(*self.write_locked.get());
        *self.write_locked.get() = false;
        self.raw_unlock();
    }
    #[inline]
    pub unsafe fn destroy(&self) {
        let r = libc::pthread_rwlock_destroy(self.inner.get());
        // On DragonFly pthread_rwlock_destroy() returns EINVAL if called on a
        // rwlock that was just initialized with
        // libc::PTHREAD_RWLOCK_INITIALIZER. Once it is used (locked/unlocked)
        // or pthread_rwlock_init() is called, this behaviour no longer occurs.
        if cfg!(target_os = "dragonfly") {
            debug_assert!(r == 0 || r == libc::EINVAL);
        } else {
            debug_assert_eq!(r, 0);
        }
    }
}