1 files changed, 387 insertions, 0 deletions

diff --git a/embassy-sync/src/rwlock.rs b/embassy-sync/src/rwlock.rs
new file mode 100644
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+++ b/embassy-sync/src/rwlock.rs
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+//! 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::{poll_fn, Future};
+use core::ops::{Deref, DerefMut};
+use core::task::Poll;
+
+use crate::blocking_mutex::raw::RawMutex;
+use crate::blocking_mutex::Mutex as BlockingMutex;
+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;
+
+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]);
+    }
+}