//! A naïve spinning mutex.

//!

//! Waiting threads hammer an atomic variable until it becomes available. Best-case latency is low, but worst-case

//! latency is theoretically infinite.

use crate::{

    atomic::{AtomicBool, Ordering},

    RelaxStrategy, Spin,

};

use core::{

    cell::UnsafeCell,

    fmt,

    marker::PhantomData,

    mem::ManuallyDrop,

    ops::{Deref, DerefMut},

};

/// A [spin lock](https://en.m.wikipedia.org/wiki/Spinlock) providing mutually exclusive access to data.

///

/// # Example

///

/// ```

/// use spin;

///

/// let lock = spin::mutex::SpinMutex::<_>::new(0);

///

/// // Modify the data

/// *lock.lock() = 2;

///

/// // Read the data

/// let answer = *lock.lock();

/// assert_eq!(answer, 2);

/// ```

///

/// # Thread safety example

///

/// ```

/// use spin;

/// use std::sync::{Arc, Barrier};

///

/// let thread_count = 1000;

/// let spin_mutex = Arc::new(spin::mutex::SpinMutex::<_>::new(0));

///

/// // We use a barrier to ensure the readout happens after all writing

/// let barrier = Arc::new(Barrier::new(thread_count + 1));

///

/// # let mut ts = Vec::new();

/// for _ in (0..thread_count) {

///     let my_barrier = barrier.clone();

///     let my_lock = spin_mutex.clone();

/// # let t =

///     std::thread::spawn(move || {

///         let mut guard = my_lock.lock();

///         *guard += 1;

///

///         // Release the lock to prevent a deadlock

///         drop(guard);

///         my_barrier.wait();

///     });

/// # ts.push(t);

/// }

///

/// barrier.wait();

///

/// let answer = { *spin_mutex.lock() };

/// assert_eq!(answer, thread_count);

///

/// # for t in ts {

/// #     t.join().unwrap();

/// # }

/// ```

pub struct SpinMutex<T: ?Sized, R = Spin> {

    phantom: PhantomData<R>,

    pub(crate) lock: AtomicBool,

    data: UnsafeCell<T>,

}

/// A guard that provides mutable data access.

///

/// When the guard falls out of scope it will release the lock.

pub struct SpinMutexGuard<'a, T: ?Sized + 'a, R = Spin> {

    inner: &'a SpinMutex<T, R>,

}

// SAFETY: Same unsafe impls as `std::sync::Mutex`

unsafe impl<T: ?Sized + Send, R> Sync for SpinMutex<T, R> {}

unsafe impl<T: ?Sized + Send, R> Send for SpinMutex<T, R> {}

// SAFETY: Mutex guards can be thought of as mutable reference to the inner data. In fact, this

// would be their ideal representation if it were not for the need for the critical section to end

// *after* the reference is no longer live.

unsafe impl<T: ?Sized, R> Sync for SpinMutexGuard<'_, T, R> where for<'a> &'a mut T: Sync {}

unsafe impl<T: ?Sized, R> Send for SpinMutexGuard<'_, T, R> where for<'a> &'a mut T: Send {}

impl<T, R> SpinMutex<T, R> {

    /// Creates a new [`SpinMutex`] wrapping the supplied data.

    ///

    /// # Example

    ///

    /// ```

    /// use spin::mutex::SpinMutex;

    ///

    /// static MUTEX: SpinMutex<()> = SpinMutex::<_>::new(());

    ///

    /// fn demo() {

    ///     let lock = MUTEX.lock();

    ///     // do something with lock

    ///     drop(lock);

    /// }

    /// ```

    #[inline(always)]

    pub const fn new(data: T) -> Self {

        SpinMutex {

            lock: AtomicBool::new(false),

            data: UnsafeCell::new(data),

            phantom: PhantomData,

        }

    }

    /// Consumes this [`SpinMutex`] and unwraps the underlying data.

    ///

    /// # Example

    ///

    /// ```

    /// let lock = spin::mutex::SpinMutex::<_>::new(42);

    /// assert_eq!(42, lock.into_inner());

    /// ```

    #[inline(always)]

    pub fn into_inner(self) -> T {

        // We know statically that there are no outstanding references to

        // `self` so there's no need to lock.

        let SpinMutex { data, .. } = self;

        data.into_inner()

    }

    /// Returns a mutable pointer to the underlying data.

    ///

    /// This is mostly meant to be used for applications which require manual unlocking, but where

    /// storing both the lock and the pointer to the inner data gets inefficient.

    ///

    /// # Example

    /// ```

    /// let lock = spin::mutex::SpinMutex::<_>::new(42);

    ///

    /// unsafe {

    ///     core::mem::forget(lock.lock());

    ///

    ///     assert_eq!(lock.as_mut_ptr().read(), 42);

    ///     lock.as_mut_ptr().write(58);

    ///

    ///     lock.force_unlock();

    /// }

    ///

    /// assert_eq!(*lock.lock(), 58);

    ///

    /// ```

    #[inline(always)]

    pub fn as_mut_ptr(&self) -> *mut T {

        self.data.get()

    }

}

impl<T: ?Sized, R: RelaxStrategy> SpinMutex<T, R> {

    /// Locks the [`SpinMutex`] and returns a guard that permits access to the inner data.

    ///

    /// The returned value may be dereferenced for data access

    /// and the lock will be dropped when the guard falls out of scope.

    ///

    /// ```

    /// let lock = spin::mutex::SpinMutex::<_>::new(0);

    /// {

    ///     let mut data = lock.lock();

    ///     // The lock is now locked and the data can be accessed

    ///     *data += 1;

    ///     // The lock is implicitly dropped at the end of the scope

    /// }

    /// ```

    #[inline(always)]

    pub fn lock(&self) -> SpinMutexGuard<'_, T, R> {

        // Can fail to lock even if the spinlock is not locked. May be more efficient than `try_lock`

        // when called in a loop.

        loop {

            if let Some(guard) = self.try_lock_weak() {

                break guard;

            }

            while self.is_locked() {

                R::relax();

            }

        }

    }

}

impl<T: ?Sized, R> SpinMutex<T, R> {

    /// Returns `true` if the lock is currently held.

    ///

    /// # Safety

    ///

    /// This function provides no synchronization guarantees and so its result should be considered 'out of date'

    /// the instant it is called. Do not use it for synchronization purposes. However, it may be useful as a heuristic.

    #[inline(always)]

    pub fn is_locked(&self) -> bool {

        self.lock.load(Ordering::Relaxed)

    }

    /// Force unlock this [`SpinMutex`].

    ///

    /// # Safety

    ///

    /// This is *extremely* unsafe if the lock is not held by the current

    /// thread. However, this can be useful in some instances for exposing the

    /// lock to FFI that doesn't know how to deal with RAII.

    #[inline(always)]

    pub unsafe fn force_unlock(&self) {

        self.lock.store(false, Ordering::Release);

    }

    /// Try to lock this [`SpinMutex`], returning a lock guard if successful.

    ///

    /// # Example

    ///

    /// ```

    /// let lock = spin::mutex::SpinMutex::<_>::new(42);

    ///

    /// let maybe_guard = lock.try_lock();

    /// assert!(maybe_guard.is_some());

    ///

    /// // `maybe_guard` is still held, so the second call fails

    /// let maybe_guard2 = lock.try_lock();

    /// assert!(maybe_guard2.is_none());

    /// ```

    #[inline(always)]

    pub fn try_lock(&self) -> Option<SpinMutexGuard<'_, T, R>> {

        // The reason for using a strong compare_exchange is explained here:

        // https://github.com/Amanieu/parking_lot/pull/207#issuecomment-575869107

        //

        // See also the giant comment about Ordering::Acquire in try_lock_weak below.

        if self

            .lock

            .compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed)

            .is_ok()

        {

            Some(SpinMutexGuard { inner: self })

        } else {

            None

        }

    }

    /// Try to lock this [`SpinMutex`], returning a lock guard if successful.

    ///

    /// Unlike [`SpinMutex::try_lock`], this function is allowed to spuriously fail even when the mutex is unlocked,

    /// which can result in more efficient code on some platforms.

    #[inline(always)]

    pub fn try_lock_weak(&self) -> Option<SpinMutexGuard<'_, T, R>> {

        // There's some debate about whether Ordering::Acquire is sufficient here. In one

        // interpretation of the the C++ standard, a lock operation like this could be re-ordered

        // with a prior unlock of a different mutex. In other words this...

        //

        //     let a_guard = A.lock();

        //     // do stuff...

        //     drop(a_guard);

        //     let b_guard = B.lock();

        //     // do more stuff...

        //     drop(b_guard);

        //

        //  ... could be reordered by the compiler into this...

        //

        //     let a_guard = A.lock();

        //     let b_guard = B.lock();

        //     // do stuff...

        //     // do more stuff...

        //     drop(a_guard);

        //     drop(b_guard);

        //

        //  ...because both the store-release in `drop(a_guard)` and the load-acquire in `B.lock()`

        //  allow this code movement. (Using Ordering::AcqRel here instead would forbid this,

        //  because nothing can move down across a store-release, but it would also prevent valid

        //  optimizations.) The worry is that this could lead to deadlocks in arguably correct

        //  programs, for example one thread locking A-then-B while another thread locks B-then-A,

        //  even though a straight-line reading of the code says that can't happen.

        //

        //  However, there's another interpretation of the standard that says this reordering is

        //  illegal. The idea is that even though moving a (non-sequentially-consistent)

        //  store-release down across a load-acquire is ok, moving it down across an *unbounded

        //  loop* violates the requirement that atomic stores should be visible to other threads in

        //  a "finite period of time": https://eel.is/c++draft/basic#intro.progress-18. Concretely,

        //  `try_lock` or `try_lock_weak` could be reordered like this (not a problem?), but `lock`

        //  with its internal loop can't be. This seems to be how compilers currently behave, in

        //  any case. See also:

        //  - https://preshing.com/20170612/can-reordering-of-release-acquire-operations-introduce-deadlock

        //  - https://x.com/tvaneerd/status/1258426442649657346

        //  - https://youtu.be/A8eCGOqgvH4?t=2551

        if self

            .lock

            .compare_exchange_weak(false, true, Ordering::Acquire, Ordering::Relaxed)

            .is_ok()

        {

            Some(SpinMutexGuard { inner: self })

        } else {

            None

        }

    }

    /// Returns a mutable reference to the underlying data.

    ///

    /// Since this call borrows the [`SpinMutex`] mutably, and a mutable reference is guaranteed to be exclusive in

    /// Rust, no actual locking needs to take place -- the mutable borrow statically guarantees no locks exist. As

    /// such, this is a 'zero-cost' operation.

    ///

    /// # Example

    ///

    /// ```

    /// let mut lock = spin::mutex::SpinMutex::<_>::new(0);

    /// *lock.get_mut() = 10;

    /// assert_eq!(*lock.lock(), 10);

    /// ```

    #[inline(always)]

    pub fn get_mut(&mut self) -> &mut T {

        self.data.get_mut()

    }

}

impl<T: ?Sized + fmt::Debug, R> fmt::Debug for SpinMutex<T, R> {

    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

        match self.try_lock() {

            Some(guard) => write!(f, "Mutex {{ data: ")

                .and_then(|()| (*guard).fmt(f))

                .and_then(|()| write!(f, " }}")),

            None => write!(f, "Mutex {{ <locked> }}"),

        }

    }

}

impl<T: Default, R> Default for SpinMutex<T, R> {

    fn default() -> Self {

        Self::new(Default::default())

    }

}

impl<T, R> From<T> for SpinMutex<T, R> {

    fn from(data: T) -> Self {

        Self::new(data)

    }

}

impl<'a, T: ?Sized, R> SpinMutexGuard<'a, T, R> {

    /// Leak the lock guard, yielding a mutable reference to the underlying data.

    ///

    /// Note that this function will permanently lock the original [`SpinMutex`].

    ///

    /// ```

    /// let mylock = spin::mutex::SpinMutex::<_>::new(0);

    ///

    /// let data: &mut i32 = spin::mutex::SpinMutexGuard::leak(mylock.lock());

    ///

    /// *data = 1;

    /// assert_eq!(*data, 1);

    /// ```

    #[inline(always)]

    pub fn leak(this: Self) -> &'a mut T {

        // Use ManuallyDrop to avoid stacked-borrow invalidation

        let this = ManuallyDrop::new(this);

        // We know statically that only we are referencing data

        unsafe { &mut *this.inner.data.get() }

    }

}

impl<'a, T: ?Sized + fmt::Debug, R> fmt::Debug for SpinMutexGuard<'a, T, R> {

    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

        fmt::Debug::fmt(&**self, f)

    }

}

impl<'a, T: ?Sized + fmt::Display, R> fmt::Display for SpinMutexGuard<'a, T, R> {

    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

        fmt::Display::fmt(&**self, f)

    }

}

impl<'a, T: ?Sized, R> Deref for SpinMutexGuard<'a, T, R> {

    type Target = T;

    fn deref(&self) -> &T {

        // We know statically that only we are referencing data

        unsafe { &*self.inner.data.get() }

    }

}

impl<'a, T: ?Sized, R> DerefMut for SpinMutexGuard<'a, T, R> {

    fn deref_mut(&mut self) -> &mut T {

        // We know statically that only we are referencing data

        unsafe { &mut *self.inner.data.get() }

    }

}

impl<'a, T: ?Sized, R> Drop for SpinMutexGuard<'a, T, R> {

    /// The dropping of the MutexGuard will release the lock it was created from.

    fn drop(&mut self) {

        self.inner.lock.store(false, Ordering::Release);

    }

}

#[cfg(feature = "lock_api")]

unsafe impl<R: RelaxStrategy> lock_api_crate::RawMutex for SpinMutex<(), R> {

    type GuardMarker = lock_api_crate::GuardSend;

    const INIT: Self = Self::new(());

    fn lock(&self) {

        // Prevent guard destructor running

        core::mem::forget(Self::lock(self));

    }

    fn try_lock(&self) -> bool {

        // Prevent guard destructor running

        Self::try_lock(self).map(core::mem::forget).is_some()

    }

    unsafe fn unlock(&self) {

        self.force_unlock();

    }

    fn is_locked(&self) -> bool {

        Self::is_locked(self)

    }

}

#[cfg(test)]

mod tests {

    use std::prelude::v1::*;

    use std::sync::atomic::{AtomicUsize, Ordering};

    use std::sync::mpsc::channel;

    use std::sync::Arc;

    use std::thread;

    type SpinMutex<T> = super::SpinMutex<T>;

    #[derive(Eq, PartialEq, Debug)]

    struct NonCopy(i32);

    #[test]

    fn smoke() {

        let m = SpinMutex::<_>::new(());

        drop(m.lock());

        drop(m.lock());

    }

    #[test]

    fn lots_and_lots() {

        static M: SpinMutex<()> = SpinMutex::<_>::new(());

        static mut CNT: u32 = 0;

        const J: u32 = 1000;

        const K: u32 = 3;

        fn inc() {

            for _ in 0..J {

                unsafe {

                    let _g = M.lock();

                    CNT += 1;

                }

            }

        }

        let (tx, rx) = channel();

        let mut ts = Vec::new();

        for _ in 0..K {

            let tx2 = tx.clone();

            ts.push(thread::spawn(move || {

                inc();

                tx2.send(()).unwrap();

            }));

            let tx2 = tx.clone();

            ts.push(thread::spawn(move || {

                inc();

                tx2.send(()).unwrap();

            }));

        }

        drop(tx);

        for _ in 0..2 * K {

            rx.recv().unwrap();

        }

        assert_eq!(unsafe { CNT }, J * K * 2);

        for t in ts {

            t.join().unwrap();

        }

    }

    #[test]

    fn try_lock() {

        let mutex = SpinMutex::<_>::new(42);

        // First lock succeeds

        let a = mutex.try_lock();

        assert_eq!(a.as_ref().map(|r| **r), Some(42));

        // Additional lock fails

        let b = mutex.try_lock();

        assert!(b.is_none());

        // After dropping lock, it succeeds again

        ::core::mem::drop(a);

        let c = mutex.try_lock();

        assert_eq!(c.as_ref().map(|r| **r), Some(42));

    }

    #[test]

    fn test_into_inner() {

        let m = SpinMutex::<_>::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 = SpinMutex::<_>::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_mutex_arc_nested() {

        // Tests nested mutexes and access

        // to underlying data.

        let arc = Arc::new(SpinMutex::<_>::new(1));

        let arc2 = Arc::new(SpinMutex::<_>::new(arc));

        let (tx, rx) = channel();

        let t = thread::spawn(move || {

            let lock = arc2.lock();

            let lock2 = lock.lock();

            assert_eq!(*lock2, 1);

            tx.send(()).unwrap();

        });

        rx.recv().unwrap();

        t.join().unwrap();

    }

    #[test]

    fn test_mutex_arc_access_in_unwind() {

        let arc = Arc::new(SpinMutex::<_>::new(1));

        let arc2 = arc.clone();

        let _ = thread::spawn(move || -> () {

            struct Unwinder {

                i: Arc<SpinMutex<i32>>,

            }

            impl Drop for Unwinder {

                fn drop(&mut self) {

                    *self.i.lock() += 1;

                }

            }

            let _u = Unwinder { i: arc2 };

            panic!();

        })

        .join();

        let lock = arc.lock();

        assert_eq!(*lock, 2);

    }

    #[test]

    fn test_mutex_unsized() {

        let mutex: &SpinMutex<[i32]> = &SpinMutex::<_>::new([1, 2, 3]);

        {

            let b = &mut *mutex.lock();

            b[0] = 4;

            b[2] = 5;

        }

        let comp: &[i32] = &[4, 2, 5];

        assert_eq!(&*mutex.lock(), comp);

    }

    #[test]

    fn test_mutex_force_lock() {

        let lock = SpinMutex::<_>::new(());

        ::std::mem::forget(lock.lock());

        unsafe {

            lock.force_unlock();

        }

        assert!(lock.try_lock().is_some());

    }

}