werkzeug/src/sync.rs

use core::{
    mem,
    sync::atomic::{AtomicU32, Ordering},
};

/// A simple lock implementation using an atomic u32.
#[repr(transparent)]
pub struct Lock {
    inner: AtomicU32,
}

impl Lock {
    const LOCKED: u32 = 0b001;
    const EMPTY: u32 = 0;

    /// Creates a new lock in the unlocked state.
    pub const fn new() -> Self {
        Self {
            inner: AtomicU32::new(0),
        }
    }

    pub fn as_ptr(&self) -> *mut u32 {
        self.inner.as_ptr()
    }

    pub unsafe fn from_ptr<'a>(ptr: *mut u32) -> &'a Self {
        // SAFETY: The caller must ensure that `ptr` is not aliased, and lasts
        // for the lifetime of the `Lock`.
        unsafe { mem::transmute(AtomicU32::from_ptr(ptr)) }
    }

    /// Acquires the lock, blocking until it is available.
    pub fn lock(&self) {
        // attempt acquiring the lock with no contention.
        if self
            .inner
            .compare_exchange_weak(
                Self::EMPTY,
                Self::LOCKED,
                Ordering::Acquire,
                Ordering::Relaxed,
            )
            .is_ok()
        {
            // We successfully acquired the lock.
            return;
        } else {
            self.lock_slow();
        }
    }

    pub fn unlock(&self) {
        // use release semantics to ensure that all previous writes are
        // available to other threads.
        self.inner.fetch_and(!Self::LOCKED, Ordering::Release);
    }

    fn lock_slow(&self) {
        // The lock is either locked, or someone is waiting for it:

        let mut spin_wait = SpinWait::new();
        let mut state = self.inner.load(Ordering::Acquire);
        loop {
            // If the lock isn't locked, we can try to acquire it.
            if state & Self::LOCKED == 0 {
                // Try to acquire the lock.
                match self.inner.compare_exchange_weak(
                    state,
                    state | Self::LOCKED,
                    Ordering::Acquire,
                    Ordering::Relaxed,
                ) {
                    Ok(_) => {
                        // We successfully acquired the lock.
                        return;
                    }
                    Err(new_state) => {
                        // The state changed, we need to check again.
                        state = new_state;
                        continue;
                    }
                }
            }

            if {
                let spun: bool;
                #[cfg(feature = "std")]
                {
                    spun = spin_wait.spin_yield();
                }
                #[cfg(not(feature = "std"))]
                {
                    spun = spin_wait.spin();
                }

                spun
            } {
                // We can spin for a little while and see if it becomes available.
                state = self.inner.load(Ordering::Relaxed);
                continue;
            }

            // If we reach here, we need to park the thread.
            atomic_wait::wait(&self.inner, Self::LOCKED);

            if self
                .inner
                .compare_exchange_weak(
                    state,
                    state | Self::LOCKED,
                    Ordering::Acquire,
                    Ordering::Relaxed,
                )
                .is_ok()
            {
                // We successfully acquired the lock after being woken up.
                return;
            }

            spin_wait.reset();
            state = self.inner.load(Ordering::Relaxed);
        }
    }

    pub fn wait(&self) {
        let state = self.inner.load(Ordering::Acquire);
        atomic_wait::wait(&self.inner, state);
    }

    pub fn wake_one(&self) {
        // Notify one thread waiting on this lock.
        atomic_wait::wake_one(&self.inner);
    }
}

// from parking_lot_core
pub struct SpinWait {
    counter: u32,
}

impl SpinWait {
    /// Creates a new `SpinWait` with an initial counter value.
    pub const fn new() -> Self {
        Self { counter: 0 }
    }

    /// Resets the counter to zero.
    pub fn reset(&mut self) {
        self.counter = 0;
    }

    pub fn spin(&mut self) -> bool {
        if self.counter >= 10 {
            // If the counter is too high, we signal the caller to potentially park.
            return false;
        }
        self.counter += 1;

        // spin for a small number of iterations based on the counter value.
        for _ in 0..(1 << self.counter) {
            core::hint::spin_loop();
        }

        true
    }

    #[cfg(feature = "std")]
    pub fn spin_yield(&mut self) -> bool {
        if self.counter >= 10 {
            // If the counter is too high, we signal the caller to potentially park.
            return false;
        }
        self.counter += 1;

        if self.counter >= 3 {
            // spin for a small number of iterations based on the counter value.
            for _ in 0..(1 << self.counter) {
                core::hint::spin_loop();
            }
        } else {
            // yield the thread and wait for the OS to reschedule us.
            std::thread::yield_now();
        }

        true
    }
}

// taken from `std`
#[derive(Debug)]
#[repr(transparent)]
pub struct Parker {
    mutex: AtomicU32,
}

impl Parker {
    const PARKED: u32 = u32::MAX;
    const EMPTY: u32 = 0;
    const NOTIFIED: u32 = 1;
    pub fn new() -> Self {
        Self {
            mutex: AtomicU32::new(Self::EMPTY),
        }
    }

    pub fn as_ptr(&self) -> *mut u32 {
        self.mutex.as_ptr()
    }

    pub unsafe fn from_ptr<'a>(ptr: *mut u32) -> &'a Self {
        // SAFETY: The caller must ensure that `ptr` is not aliased, and lasts
        // for the lifetime of the `Parker`.
        unsafe { mem::transmute(AtomicU32::from_ptr(ptr)) }
    }

    pub fn is_parked(&self) -> bool {
        self.mutex.load(Ordering::Acquire) == Self::PARKED
    }

    pub fn park(&self) {
        self.park_inner(|| ());
    }

    pub fn park_with_callback<F>(&self, before_sleep: F)
    where
        F: FnOnce(),
    {
        // This function is called when the thread is about to park.
        self.park_inner(before_sleep);
    }

    // If the caller wants to park the thread on a mutex'd condition, it is
    // possible for a deadlock to occur when the mutex is dropped just before
    // this parker atomically changes it's state to `PARKED`. For that reason,
    // we use a callback to allow the caller to perform any necessary actions
    // before parking, but after the parker is set to `PARKED`. If another
    // thread then checks the condition and checks if this thread should be
    // woken, we will be immediately notified.
    // Thusly, it is important that any caller synchronise any conditionals with
    // a mutex externally, and then unlock that mutex in `before_sleep`.
    fn park_inner<F>(&self, before_sleep: F)
    where
        F: FnOnce(),
    {
        if self.mutex.fetch_sub(1, Ordering::Acquire) == Self::NOTIFIED {
            before_sleep();
            // The thread was notified, so we can return immediately.
            return;
        }

        before_sleep();

        loop {
            atomic_wait::wait(&self.mutex, Self::PARKED);

            // We check whether we were notified or woke up spuriously with
            // acquire ordering in order to make-visible any writes made by the
            // thread that notified us.
            if self.mutex.swap(Self::EMPTY, Ordering::Acquire) == Self::NOTIFIED {
                // The thread was notified, so we can return immediately.
                return;
            } else {
                // spurious wakeup, so we need to re-park.
                continue;
            }
        }
    }

    pub fn unpark(&self) {
        // write with Release ordering to ensure that any writes made by this
        // thread are made-available to the unparked thread.
        if self.mutex.swap(Self::NOTIFIED, Ordering::Release) == Self::PARKED {
            // The thread was parked, so we need to notify it.
            atomic_wait::wake_one(&self.mutex);
        } else {
            // The thread was not parked, so we don't need to do anything.
        }
    }
}

#[cfg(feature = "alloc")]
pub mod channel {
    use alloc::sync::Arc;
    use core::{
        cell::{Cell, UnsafeCell},
        marker::PhantomData,
        mem::MaybeUninit,
        sync::atomic::{AtomicU32, Ordering},
    };

    #[repr(C)]
    #[derive(Debug)]
    struct Channel<T = ()> {
        state: AtomicU32,
        val: UnsafeCell<MaybeUninit<T>>,
    }

    unsafe impl<T: Send> Send for Channel<T> {}
    unsafe impl<T: Send> Sync for Channel<T> {}

    impl<T> Channel<T> {
        const OCCUPIED_BIT: u32 = 0b01;
        const WAITING_BIT: u32 = 0b10;

        fn new() -> Self {
            Self {
                state: AtomicU32::new(0),
                val: UnsafeCell::new(MaybeUninit::uninit()),
            }
        }
    }

    pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
        let channel = Arc::new(Channel::<T>::new());
        let receiver = Receiver(channel.clone(), PhantomData);
        let sender = Sender(channel);
        (sender, receiver)
    }

    #[derive(Debug)]
    #[repr(transparent)]
    // `PhantomData<Cell<()>>` is used to ensure that `Receiver` is `!Sync` but `Send`.
    pub struct Receiver<T = ()>(Arc<Channel<T>>, PhantomData<Cell<()>>);

    #[derive(Debug)]
    #[repr(transparent)]
    pub struct Sender<T = ()>(Arc<Channel<T>>);

    impl<T> Receiver<T> {
        pub fn is_empty(&self) -> bool {
            self.0.state.load(Ordering::Acquire) & Channel::<T>::OCCUPIED_BIT == 0
        }

        pub fn as_sender(self) -> Sender<T> {
            Sender(self.0.clone())
        }

        fn wait(&mut self) {
            loop {
                let state = self
                    .0
                    .state
                    .fetch_or(Channel::<T>::WAITING_BIT, Ordering::Acquire);
                if state & Channel::<T>::OCCUPIED_BIT == 0 {
                    // The channel is empty, so we need to wait for a value to be sent.
                    // We will block until the sender wakes us up.
                    atomic_wait::wait(&self.0.state, Channel::<T>::WAITING_BIT);
                } else {
                    // The channel is occupied, so we can return.
                    self.0
                        .state
                        .fetch_and(!Channel::<T>::WAITING_BIT, Ordering::Release);
                    break;
                }
            }
        }

        /// Takes the value from the channel, if it is present.
        /// this function must only ever return `Some` once.
        pub unsafe fn take(&mut self) -> Option<T> {
            // unset the OCCUPIED_BIT to indicate that we are taking the value, if any is present.
            if self
                .0
                .state
                .fetch_and(!Channel::<T>::OCCUPIED_BIT, Ordering::Acquire)
                & Channel::<T>::OCCUPIED_BIT
                == 0
            {
                // The channel was empty, so we return None.
                None
            } else {
                // SAFETY: we only ever access this field by pointer
                // the OCCUPIED_BIT was set, so we can safely read the value.
                // this function is only called once, within `recv`,
                // guaranteeing that the value will only be dropped once.
                unsafe { Some(self.0.val.get().read().assume_init_read()) }
            }
        }

        pub fn recv(mut self) -> T {
            loop {
                // SAFETY: recv can only be called once, since it takes ownership of `self`.
                // if `take` returns a value, it will never be called again.
                if let Some(t) = unsafe { self.take() } {
                    return t;
                }

                self.wait();
            }
        }
    }

    impl<T> Sender<T> {
        pub fn send(self, value: T) {
            unsafe {
                self.0.val.get().write(MaybeUninit::new(value));
            }

            // Set the OCCUPIED_BIT to indicate that a value is present.
            let state = self
                .0
                .state
                .fetch_or(Channel::<T>::OCCUPIED_BIT, Ordering::Release);
            assert!(
                state & Channel::<T>::OCCUPIED_BIT == 0,
                "Channel is already occupied"
            );

            // If there are any receivers waiting, we need to wake them up.
            if state & Channel::<T>::WAITING_BIT != 0 {
                // There are receivers waiting, so we need to wake them up.
                atomic_wait::wake_all(&self.0.state);
            }
        }
    }
}

#[cfg(feature = "alloc")]
pub mod queue {
    //! A Queue with multiple receivers and multiple producers, where a producer can send a message to one of any of the receivers (any-cast), or one of the receivers (uni-cast).
    //! After being woken up from waiting on a message, the receiver will look up the index of the message in the queue and return it.

    use alloc::{boxed::Box, sync::Arc, vec::Vec};
    use core::{
        cell::UnsafeCell,
        marker::{PhantomData, PhantomPinned},
        mem::{self, MaybeUninit},
        pin::Pin,
        ptr::{self, NonNull},
        sync::atomic::{AtomicU32, Ordering},
    };

    use hashbrown::HashMap;

    use crate::{CachePadded, ptr::TaggedAtomicPtr};

    use super::Parker;

    struct QueueInner<T> {
        receivers: HashMap<ReceiverToken, CachePadded<(Slot<T>, bool)>>,
        messages: Vec<T>,
        _phantom: core::marker::PhantomData<T>,
    }

    pub struct Queue<T> {
        inner: UnsafeCell<QueueInner<T>>,
        lock: AtomicU32,
    }

    unsafe impl<T> Send for Queue<T> {}
    unsafe impl<T> Sync for Queue<T> where T: Send {}

    pub struct Receiver<T> {
        queue: Arc<Queue<T>>,
        lock: Pin<Box<(Parker, PhantomPinned)>>,
    }

    #[repr(transparent)]
    pub struct Sender<T> {
        queue: Arc<Queue<T>>,
    }

    // TODO: make this a linked list of slots so we can queue multiple messages for
    // a single receiver
    const SLOT_ALIGN: u8 = core::mem::align_of::<usize>().ilog2() as u8;
    struct Slot<T> {
        value: UnsafeCell<MaybeUninit<T>>,
        next_and_state: TaggedAtomicPtr<Self, SLOT_ALIGN>,
        _phantom: PhantomData<Self>,
    }

    impl<T> Slot<T> {
        fn new() -> Self {
            Self {
                value: UnsafeCell::new(MaybeUninit::uninit()),
                next_and_state: TaggedAtomicPtr::new(ptr::null_mut(), 0), // 0 means empty
                _phantom: PhantomData,
            }
        }

        fn is_set(&self) -> bool {
            self.next_and_state.tag(Ordering::Acquire) == 1
        }

        unsafe fn pop(&self) -> Option<T> {
            NonNull::new(self.next_and_state.ptr(Ordering::Acquire))
                .and_then(|next| {
                    // SAFETY: The next slot is a valid pointer to a Slot<T> that was allocated by us.
                    unsafe { next.as_ref().pop() }
                })
                .or_else(|| {
                    if self
                        .next_and_state
                        .swap_tag(0, Ordering::AcqRel, Ordering::Relaxed)
                        == 1
                    {
                        // SAFETY: The value is only initialized when the state is set to 1.
                        Some(unsafe { (&mut *self.value.get()).assume_init_read() })
                    } else {
                        None
                    }
                })
        }

        /// this operation isn't atomic.
        #[allow(dead_code)]
        unsafe fn pop_front(&self) -> Option<T> {
            // swap the slot at `next` with self, and return the value of self.

            // get next ptr, if it is non-null.
            if let Some(next) = NonNull::new(self.next_and_state.ptr(Ordering::Acquire)) {
                unsafe {
                    // copy the next slot's next_and_state into self's next_and_state
                    let (_, old) = self.next_and_state.copy_from(
                        &next.as_ref().next_and_state,
                        Ordering::Acquire,
                        Ordering::Release,
                    );

                    // copy the next slot's value into self's value
                    mem::swap(&mut *self.value.get(), &mut *next.as_ref().value.get());

                    if old == 1 {
                        // SAFETY: The value is only initialized when the state is set to 1.
                        Some(next.as_ref().value.get().read().assume_init())
                    } else {
                        // next was empty, so we return None.
                        None
                    }
                }
            } else {
                // next is null, so popping from the back or front is the same.
                unsafe { self.pop() }
            }
        }

        /// the caller must ensure that they have exclusive access to the slot
        unsafe fn push(&self, value: T) {
            if self.is_set() {
                let next = self.next_ptr();
                unsafe {
                    (next.as_ref()).push(value);
                }
            } else {
                // SAFETY: The value is only initialized when the state is set to 1.
                unsafe { (&mut *self.value.get()).write(value) };
                self.next_and_state
                    .set_tag(1, Ordering::Release, Ordering::Relaxed);
            }
        }

        fn next_ptr(&self) -> NonNull<Slot<T>> {
            if let Some(next) = NonNull::new(self.next_and_state.ptr(Ordering::Acquire)) {
                next.cast()
            } else {
                self.alloc_next()
            }
        }

        fn alloc_next(&self) -> NonNull<Slot<T>> {
            let next = Box::into_raw(Box::new(Slot::new()));

            let next = loop {
                match self.next_and_state.compare_exchange_weak_ptr(
                    ptr::null_mut(),
                    next,
                    Ordering::Release,
                    Ordering::Acquire,
                ) {
                    Ok(_) => break next,
                    Err(other) => {
                        if other.is_null() {
                            continue;
                        }
                        // next was allocated under us, so we need to drop the slot we just allocated again.
                        _ = unsafe { Box::from_raw(next) };
                        break other;
                    }
                }
            };

            unsafe {
                // SAFETY: The next slot is a valid pointer to a Slot<T> that was allocated by us.
                NonNull::new_unchecked(next)
            }
        }
    }

    impl<T> Drop for Slot<T> {
        fn drop(&mut self) {
            // drop next chain
            if let Some(next) = NonNull::new(self.next_and_state.swap_ptr(
                ptr::null_mut(),
                Ordering::Release,
                Ordering::Relaxed,
            )) {
                // SAFETY: The next slot is a valid pointer to a Slot<T> that was allocated by us.
                // We drop this in place because idk..
                unsafe {
                    next.drop_in_place();
                    _ = Box::<mem::ManuallyDrop<Self>>::from_raw(next.cast().as_ptr());
                }
            }

            // SAFETY: The value is only initialized when the state is set to 1.
            if mem::needs_drop::<T>() && self.next_and_state.tag(Ordering::Acquire) == 1 {
                unsafe { (&mut *self.value.get()).assume_init_drop() };
            }
        }
    }

    // const BLOCK_SIZE: usize = 8;
    // struct Block<T> {
    //     next: AtomicPtr<Block<T>>,
    //     slots: [CachePadded<Slot<T>>; BLOCK_SIZE],
    // }

    /// A token that can be used to identify a specific receiver in a queue.
    #[repr(transparent)]
    #[derive(Debug, Clone, Copy, Hash, PartialEq, Eq)]
    pub struct ReceiverToken(crate::util::Send<NonNull<u32>>);

    impl ReceiverToken {
        pub fn as_ptr(&self) -> *mut u32 {
            self.0.into_inner().as_ptr()
        }

        pub unsafe fn as_parker(&self) -> &Parker {
            // SAFETY: The pointer is guaranteed to be valid and aligned, as it comes from a pinned Parker.
            unsafe { Parker::from_ptr(self.as_ptr()) }
        }

        pub unsafe fn from_parker(parker: &Parker) -> Self {
            // SAFETY: The pointer is guaranteed to be valid and aligned, as it comes from a pinned Parker.
            let ptr = NonNull::from(parker).cast::<u32>();
            ReceiverToken(crate::util::Send(ptr))
        }
    }

    impl<T> Queue<T> {
        pub fn new() -> Arc<Self> {
            Arc::new(Self {
                inner: UnsafeCell::new(QueueInner {
                    messages: Vec::new(),
                    receivers: HashMap::new(),
                    _phantom: PhantomData,
                }),
                lock: AtomicU32::new(0),
            })
        }

        pub fn new_sender(self: &Arc<Self>) -> Sender<T> {
            Sender {
                queue: self.clone(),
            }
        }

        pub fn num_receivers(self: &Arc<Self>) -> usize {
            let _guard = self.lock();
            self.inner().receivers.len()
        }

        pub fn as_sender(self: &Arc<Self>) -> &Sender<T> {
            unsafe { mem::transmute::<&Arc<Self>, &Sender<T>>(self) }
        }

        pub fn new_receiver(self: &Arc<Self>) -> Receiver<T> {
            let recv = Receiver {
                queue: self.clone(),
                lock: Box::pin((Parker::new(), PhantomPinned)),
            };

            // allocate slot for the receiver
            let token = recv.get_token();
            let _guard = recv.queue.lock();
            recv.queue
                .inner()
                .receivers
                .insert(token, CachePadded::new((Slot::<T>::new(), false)));

            drop(_guard);
            recv
        }

        fn lock(&self) -> impl Drop {
            unsafe {
                let lock = crate::sync::Lock::from_ptr(&self.lock as *const _ as _);
                lock.lock();
                crate::drop_guard::DropGuard::new(|| lock.unlock())
            }
        }

        fn inner(&self) -> &mut QueueInner<T> {
            // SAFETY: The inner is only accessed while the queue is locked.
            unsafe { &mut *self.inner.get() }
        }
    }

    impl<T> QueueInner<T> {
        fn poll(&mut self, token: ReceiverToken) -> Option<T> {
            // check if someone has sent a message to this receiver
            let CachePadded((slot, _)) = self.receivers.get(&token)?;

            unsafe { slot.pop() }.or_else(|| {
                // if the slot is empty, we can check the indexed messages

                self.messages.pop()
            })
        }
    }

    impl<T> Receiver<T> {
        pub fn get_token(&self) -> ReceiverToken {
            // the token is just the pointer to the lock of this receiver.
            // the lock is pinned, so it's address is stable across calls to `receive`.

            ReceiverToken(crate::util::Send(NonNull::from(&self.lock.0).cast()))
        }
    }

    impl<T> Drop for Receiver<T> {
        fn drop(&mut self) {
            if mem::needs_drop::<T>() {
                // lock the queue
                let _guard = self.queue.lock();
                let queue = self.queue.inner();

                // remove the receiver from the queue
                _ = queue.receivers.remove(&self.get_token());
            }
        }
    }

    impl<T: Send> Receiver<T> {
        pub fn recv(&self) -> T {
            let token = self.get_token();

            loop {
                // lock the queue
                let _guard = self.queue.lock();
                let queue = self.queue.inner();

                // check if someone has sent a message to this receiver
                if let Some(t) = queue.poll(token) {
                    queue.receivers.get_mut(&token).unwrap().1 = false; // mark the slot as not parked
                    return t;
                }

                // there was no message for this receiver, so we need to park it
                queue.receivers.get_mut(&token).unwrap().1 = true; // mark the slot as parked

                self.lock.0.park_with_callback(move || {
                    // drop the lock guard after having set the lock state to waiting.
                    // this avoids a deadlock if the sender tries to send a message
                    // while the receiver is in the process of parking (I think..)
                    drop(_guard);
                });
            }
        }

        pub fn try_recv(&self) -> Option<T> {
            let token = self.get_token();

            // lock the queue
            let _guard = self.queue.lock();
            let queue = self.queue.inner();

            // check if someone has sent a message to this receiver
            queue.poll(token)
        }
    }

    impl<T: Send> Sender<T> {
        /// Sends a message to one of the receivers in the queue, or makes it
        /// available to any receiver that will park in the future.
        pub fn anycast(&self, value: T) {
            let _guard = self.queue.lock();

            // SAFETY: The queue is locked, so we can safely access the inner queue.
            match unsafe { self.try_anycast_inner(value) } {
                Ok(_) => {}
                Err(value) => {
                    // no parked receiver found, so we want to add the message to the indexed slots
                    let queue = self.queue.inner();
                    queue.messages.push(value);

                    // waking up a parked receiver is not necessary here, as any
                    // receivers that don't have a free slot are currently waking up.
                }
            }
        }

        pub fn try_anycast(&self, value: T) -> Result<(), T> {
            // lock the queue
            let _guard = self.queue.lock();

            // SAFETY: The queue is locked, so we can safely access the inner queue.
            unsafe { self.try_anycast_inner(value) }
        }

        /// The caller must hold the lock on the queue for the duration of this function.
        unsafe fn try_anycast_inner(&self, value: T) -> Result<(), T> {
            // look for a receiver that is parked
            let queue = self.queue.inner();
            if let Some((token, slot)) =
                queue
                    .receivers
                    .iter()
                    .find_map(|(token, CachePadded((slot, is_parked)))| {
                        // ensure the slot is available
                        if *is_parked && !slot.is_set() {
                            Some((*token, slot))
                        } else {
                            None
                        }
                    })
            {
                // we found a receiver that is parked, so we can send the message to it
                unsafe {
                    (&mut *slot.value.get()).write(value);
                    slot.next_and_state
                        .set_tag(1, Ordering::Release, Ordering::Relaxed);
                    Parker::from_ptr(token.0.into_inner().as_ptr()).unpark();
                }

                return Ok(());
            } else {
                return Err(value);
            }
        }

        /// Sends a message to a specific receiver, waking it if it is parked.
        pub fn unicast(&self, value: T, receiver: ReceiverToken) -> Result<(), T> {
            // lock the queue
            let _guard = self.queue.lock();
            let queue = self.queue.inner();

            let Some(CachePadded((slot, _))) = queue.receivers.get_mut(&receiver) else {
                return Err(value);
            };

            unsafe {
                slot.push(value);
            }

            // wake the receiver
            unsafe {
                Parker::from_ptr(receiver.0.into_inner().as_ptr()).unpark();
            }

            Ok(())
        }

        pub fn broadcast(&self, value: T)
        where
            T: Clone,
        {
            // lock the queue
            let _guard = self.queue.lock();
            let queue = self.queue.inner();

            // send the message to all receivers
            for (token, CachePadded((slot, _))) in queue.receivers.iter() {
                // SAFETY: The slot is owned by this receiver.

                unsafe { slot.push(value.clone()) };

                // wake the receiver
                unsafe {
                    Parker::from_ptr(token.0.into_inner().as_ptr()).unpark();
                }
            }
        }

        pub fn broadcast_with<F>(&self, mut f: F)
        where
            F: FnMut() -> T,
        {
            // lock the queue
            let _guard = self.queue.lock();
            let queue = self.queue.inner();

            // send the message to all receivers
            for (token, CachePadded((slot, _))) in queue.receivers.iter() {
                // SAFETY: The slot is owned by this receiver.

                unsafe { slot.push(f()) };

                // check if the receiver is parked
                // wake the receiver
                unsafe {
                    Parker::from_ptr(token.0.into_inner().as_ptr()).unpark();
                }
            }
        }
    }

    #[cfg(test)]
    mod tests {
        use std::println;

        use super::*;

        #[test]
        fn test_queue() {
            let queue = Queue::<i32>::new();

            let sender = queue.new_sender();
            let receiver1 = queue.new_receiver();
            let receiver2 = queue.new_receiver();

            let token2 = receiver2.get_token();

            sender.anycast(42);

            assert_eq!(receiver1.recv(), 42);

            sender.unicast(100, token2).unwrap();
            assert_eq!(receiver1.try_recv(), None);
            assert_eq!(receiver2.recv(), 100);
        }

        #[test]
        fn queue_broadcast() {
            let queue = Queue::<i32>::new();

            let sender = queue.new_sender();
            let receiver1 = queue.new_receiver();
            let receiver2 = queue.new_receiver();

            sender.broadcast(42);

            assert_eq!(receiver1.recv(), 42);
            assert_eq!(receiver2.recv(), 42);
        }

        #[test]
        fn queue_multiple_messages() {
            let queue = Queue::<i32>::new();

            let sender = queue.new_sender();
            let receiver = queue.new_receiver();

            sender.anycast(1);
            sender.unicast(2, receiver.get_token()).unwrap();

            assert_eq!(receiver.recv(), 2);
            assert_eq!(receiver.recv(), 1);
        }

        #[test]
        fn queue_threaded() {
            #[derive(Debug, Clone, Copy)]
            enum Message {
                Send(i32),
                Exit,
            }

            let queue = Queue::<Message>::new();

            let sender = queue.new_sender();

            let threads = (0..5)
                .map(|_| {
                    let queue_clone = queue.clone();
                    let receiver = queue_clone.new_receiver();

                    std::thread::spawn(move || {
                        loop {
                            match receiver.recv() {
                                Message::Send(value) => {
                                    println!(
                                        "Receiver {:?} Received: {}",
                                        receiver.get_token(),
                                        value
                                    );
                                }
                                Message::Exit => {
                                    println!("Exiting thread");
                                    break;
                                }
                            }
                        }
                    })
                })
                .collect::<Vec<_>>();

            // Send messages to the receivers
            for i in 0..10 {
                sender.anycast(Message::Send(i));
            }

            // Send exit messages to all receivers
            sender.broadcast(Message::Exit);
            for thread in threads {
                thread.join().unwrap();
            }
            println!("All threads have exited.");
        }

        #[test]
        fn drop_slot() {
            // Test that dropping a slot does not cause a double free or panic
            let slot = Slot::<i32>::new();
            unsafe {
                slot.push(42);
                drop(slot);
            }
        }

        #[test]
        fn drop_slot_chain() {
            struct DropCheck<'a>(&'a AtomicU32);
            impl Drop for DropCheck<'_> {
                fn drop(&mut self) {
                    self.0.fetch_sub(1, Ordering::SeqCst);
                }
            }

            impl<'a> DropCheck<'a> {
                fn new(counter: &'a AtomicU32) -> Self {
                    counter.fetch_add(1, Ordering::SeqCst);
                    Self(counter)
                }
            }
            let counter = AtomicU32::new(0);
            let slot = Slot::<DropCheck>::new();
            for _ in 0..10 {
                unsafe {
                    slot.push(DropCheck::new(&counter));
                }
            }
            assert_eq!(counter.load(Ordering::SeqCst), 10);
            drop(slot);
            assert_eq!(
                counter.load(Ordering::SeqCst),
                0,
                "All DropCheck instances should have been dropped"
            );
        }

        #[test]
        fn send_self() {
            // Test that sending a message to self works
            let queue = Queue::<i32>::new();
            let sender = queue.new_sender();
            let receiver = queue.new_receiver();

            sender.unicast(42, receiver.get_token()).unwrap();
            assert_eq!(receiver.recv(), 42);
        }

        #[test]
        fn send_self_many() {
            // Test that sending multiple messages to self works
            let queue = Queue::<i32>::new();
            let sender = queue.new_sender();
            let receiver = queue.new_receiver();

            for i in 0..10 {
                sender.unicast(i, receiver.get_token()).unwrap();
            }

            for i in (0..10).rev() {
                assert_eq!(receiver.recv(), i);
            }
        }

        #[test]
        fn slot_pop_front() {
            // Test that popping from the front of a slot works correctly
            let slot = Slot::<i32>::new();
            unsafe {
                slot.push(1);
                slot.push(2);
                slot.push(3);
            }

            assert_eq!(unsafe { slot.pop_front() }, Some(1));
            assert_eq!(unsafe { slot.pop_front() }, Some(2));
            assert_eq!(unsafe { slot.pop_front() }, Some(3));
            assert_eq!(unsafe { slot.pop_front() }, None);
        }
    }
}