path: root/embassy-sync/src/rwlock.rs

//! Async read-write lock.
//!
//! This module provides a read-write lock that can be used to synchronize data between asynchronous tasks.
use core::cell::{RefCell, UnsafeCell};
use core::fmt;
use core::future::{Future, poll_fn};
use core::ops::{Deref, DerefMut};
use core::task::Poll;

use crate::blocking_mutex::Mutex as BlockingMutex;
use crate::blocking_mutex::raw::RawMutex;
use crate::waitqueue::WakerRegistration;

/// Error returned by [`RwLock::try_read`] and [`RwLock::try_write`] when the lock is already held.
#[derive(PartialEq, Eq, Clone, Copy, Debug)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
pub struct TryLockError;

#[derive(Debug)]
struct State {
    readers: usize,
    writer: bool,
    waker: WakerRegistration,
}

/// Async read-write lock.
///
/// The read-write lock is generic over the raw mutex implementation `M` and the data `T` it protects.
/// The raw read-write lock is used to guard access to the internal state. It
/// is held for very short periods only, while locking and unlocking. It is *not* held
/// for the entire time the async RwLock is locked.
///
/// Which implementation you select depends on the context in which you're using the read-write lock.
///
/// Use [`CriticalSectionRawMutex`](crate::blocking_mutex::raw::CriticalSectionRawMutex) when data can be shared between threads and interrupts.
///
/// Use [`NoopRawMutex`](crate::blocking_mutex::raw::NoopRawMutex) when data is only shared between tasks running on the same executor.
///
/// Use [`ThreadModeRawMutex`](crate::blocking_mutex::raw::ThreadModeRawMutex) when data is shared between tasks running on the same executor but you want a singleton.
pub struct RwLock<M, T>
where
    M: RawMutex,
    T: ?Sized,
{
    state: BlockingMutex<M, RefCell<State>>,
    inner: UnsafeCell<T>,
}

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

/// Async read-write lock.
impl<M, T> RwLock<M, T>
where
    M: RawMutex,
{
    /// Create a new read-write lock with the given value.
    pub const fn new(value: T) -> Self {
        Self {
            inner: UnsafeCell::new(value),
            state: BlockingMutex::new(RefCell::new(State {
                readers: 0,
                writer: false,
                waker: WakerRegistration::new(),
            })),
        }
    }
}

impl<M, T> RwLock<M, T>
where
    M: RawMutex,
    T: ?Sized,
{
    /// Lock the read-write lock for reading.
    ///
    /// This will wait for the lock to be available if it's already locked for writing.
    pub fn read(&self) -> impl Future<Output = RwLockReadGuard<'_, M, T>> {
        poll_fn(|cx| {
            let ready = self.state.lock(|s| {
                let mut s = s.borrow_mut();
                if s.writer {
                    s.waker.register(cx.waker());
                    false
                } else {
                    s.readers += 1;
                    true
                }
            });

            if ready {
                Poll::Ready(RwLockReadGuard { rwlock: self })
            } else {
                Poll::Pending
            }
        })
    }

    /// Lock the read-write lock for writing.
    ///
    /// This will wait for the lock to be available if it's already locked for reading or writing.
    pub fn write(&self) -> impl Future<Output = RwLockWriteGuard<'_, M, T>> {
        poll_fn(|cx| {
            let ready = self.state.lock(|s| {
                let mut s = s.borrow_mut();
                if s.writer || s.readers > 0 {
                    s.waker.register(cx.waker());
                    false
                } else {
                    s.writer = true;
                    true
                }
            });

            if ready {
                Poll::Ready(RwLockWriteGuard { rwlock: self })
            } else {
                Poll::Pending
            }
        })
    }

    /// Attempt to immediately lock the rwlock.
    ///
    /// If the rwlock is already locked, this will return an error instead of waiting.
    pub fn try_read(&self) -> Result<RwLockReadGuard<'_, M, T>, TryLockError> {
        self.state
            .lock(|s| {
                let mut s = s.borrow_mut();
                if s.writer {
                    return Err(());
                }
                s.readers += 1;
                Ok(())
            })
            .map_err(|_| TryLockError)?;

        Ok(RwLockReadGuard { rwlock: self })
    }

    /// Attempt to immediately lock the rwlock.
    ///
    /// If the rwlock is already locked, this will return an error instead of waiting.
    pub fn try_write(&self) -> Result<RwLockWriteGuard<'_, M, T>, TryLockError> {
        self.state
            .lock(|s| {
                let mut s = s.borrow_mut();
                if s.writer || s.readers > 0 {
                    return Err(());
                }
                s.writer = true;
                Ok(())
            })
            .map_err(|_| TryLockError)?;

        Ok(RwLockWriteGuard { rwlock: self })
    }

    /// Consumes this read-write lock, returning the underlying data.
    pub fn into_inner(self) -> T
    where
        T: Sized,
    {
        self.inner.into_inner()
    }

    /// 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.
    pub fn get_mut(&mut self) -> &mut T {
        self.inner.get_mut()
    }
}

impl<M: RawMutex, T> From<T> for RwLock<M, T> {
    fn from(from: T) -> Self {
        Self::new(from)
    }
}

impl<M, T> Default for RwLock<M, T>
where
    M: RawMutex,
    T: Default,
{
    fn default() -> Self {
        Self::new(Default::default())
    }
}

impl<M, T> fmt::Debug for RwLock<M, T>
where
    M: RawMutex,
    T: ?Sized + fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut d = f.debug_struct("RwLock");
        match self.try_read() {
            Ok(guard) => d.field("inner", &&*guard),
            Err(TryLockError) => d.field("inner", &"Locked"),
        }
        .finish_non_exhaustive()
    }
}

/// Async read lock guard.
///
/// Owning an instance of this type indicates having
/// successfully locked the read-write lock for reading, and grants access to the contents.
///
/// Dropping it unlocks the read-write lock.
#[clippy::has_significant_drop]
#[must_use = "if unused the RwLock will immediately unlock"]
pub struct RwLockReadGuard<'a, R, T>
where
    R: RawMutex,
    T: ?Sized,
{
    rwlock: &'a RwLock<R, T>,
}

impl<'a, M, T> Drop for RwLockReadGuard<'a, M, T>
where
    M: RawMutex,
    T: ?Sized,
{
    fn drop(&mut self) {
        self.rwlock.state.lock(|s| {
            let mut s = unwrap!(s.try_borrow_mut());
            s.readers -= 1;
            if s.readers == 0 {
                s.waker.wake();
            }
        })
    }
}

impl<'a, M, T> Deref for RwLockReadGuard<'a, M, T>
where
    M: RawMutex,
    T: ?Sized,
{
    type Target = T;
    fn deref(&self) -> &Self::Target {
        // Safety: the RwLockReadGuard represents shared access to the contents
        // of the read-write lock, so it's OK to get it.
        unsafe { &*(self.rwlock.inner.get() as *const T) }
    }
}

impl<'a, M, T> fmt::Debug for RwLockReadGuard<'a, M, T>
where
    M: RawMutex,
    T: ?Sized + fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&**self, f)
    }
}

impl<'a, M, T> fmt::Display for RwLockReadGuard<'a, M, T>
where
    M: RawMutex,
    T: ?Sized + fmt::Display,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&**self, f)
    }
}

/// Async write lock guard.
///
/// Owning an instance of this type indicates having
/// successfully locked the read-write lock for writing, and grants access to the contents.
///
/// Dropping it unlocks the read-write lock.
#[clippy::has_significant_drop]
#[must_use = "if unused the RwLock will immediately unlock"]
pub struct RwLockWriteGuard<'a, R, T>
where
    R: RawMutex,
    T: ?Sized,
{
    rwlock: &'a RwLock<R, T>,
}

impl<'a, R, T> Drop for RwLockWriteGuard<'a, R, T>
where
    R: RawMutex,
    T: ?Sized,
{
    fn drop(&mut self) {
        self.rwlock.state.lock(|s| {
            let mut s = unwrap!(s.try_borrow_mut());
            s.writer = false;
            s.waker.wake();
        })
    }
}

impl<'a, R, T> Deref for RwLockWriteGuard<'a, R, T>
where
    R: RawMutex,
    T: ?Sized,
{
    type Target = T;
    fn deref(&self) -> &Self::Target {
        // Safety: the RwLockWriteGuard represents exclusive access to the contents
        // of the read-write lock, so it's OK to get it.
        unsafe { &*(self.rwlock.inner.get() as *mut T) }
    }
}

impl<'a, R, T> DerefMut for RwLockWriteGuard<'a, R, T>
where
    R: RawMutex,
    T: ?Sized,
{
    fn deref_mut(&mut self) -> &mut Self::Target {
        // Safety: the RwLockWriteGuard represents exclusive access to the contents
        // of the read-write lock, so it's OK to get it.
        unsafe { &mut *(self.rwlock.inner.get()) }
    }
}

impl<'a, R, T> fmt::Debug for RwLockWriteGuard<'a, R, T>
where
    R: RawMutex,
    T: ?Sized + fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&**self, f)
    }
}

impl<'a, R, T> fmt::Display for RwLockWriteGuard<'a, R, T>
where
    R: RawMutex,
    T: ?Sized + fmt::Display,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&**self, f)
    }
}

#[cfg(test)]
mod tests {
    use crate::blocking_mutex::raw::NoopRawMutex;
    use crate::rwlock::RwLock;

    #[futures_test::test]
    async fn read_guard_releases_lock_when_dropped() {
        let rwlock: RwLock<NoopRawMutex, [i32; 2]> = RwLock::new([0, 1]);

        {
            let guard = rwlock.read().await;
            assert_eq!(*guard, [0, 1]);
        }

        {
            let guard = rwlock.read().await;
            assert_eq!(*guard, [0, 1]);
        }

        assert_eq!(*rwlock.read().await, [0, 1]);
    }

    #[futures_test::test]
    async fn write_guard_releases_lock_when_dropped() {
        let rwlock: RwLock<NoopRawMutex, [i32; 2]> = RwLock::new([0, 1]);

        {
            let mut guard = rwlock.write().await;
            assert_eq!(*guard, [0, 1]);
            guard[1] = 2;
        }

        {
            let guard = rwlock.read().await;
            assert_eq!(*guard, [0, 2]);
        }

        assert_eq!(*rwlock.read().await, [0, 2]);
    }
}