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use super::*;
use crate::semaphore;
use alloc::sync::Arc;
/// Owned [RAII] structure used to release the shared read access of a
/// [`RwLock`] when dropped.
///
/// This type is similar to the [`RwLockReadGuard`] type, but it is only
/// returned by an [`RwLock`] that is wrapped in an an [`Arc`]. Instead
/// of borrowing the [`RwLock`], this guard holds an [`Arc`] clone of
/// the [`RwLock`], incrementing its reference count. Therefore, this
/// type can outlive the [`RwLock`] that created it, and it is valid for
/// the `'static` lifetime. Beyond this, it is identical to the
/// [`RwLockReadGuard`] type.
///
/// The data protected by the [`RwLock`] can be accessed through this
/// guard via its [`Deref`](#impl-Deref) implementation.
///
/// This guard can be held across any `.await` point, as it implements
/// [`Send`].
///
/// This structure is created by the [`read_owned`] and
/// [`try_read_owned`] methods on [`Arc`]`<`[`RwLock`]`>`.
///
/// [RAII]: https://rust-unofficial.github.io/patterns/patterns/behavioural/RAII.html
/// [`read_owned`]: RwLock::read_owned
/// [`try_read_owned`]: RwLock::try_read_owned
#[must_use = "if unused, the `RwLock` will immediately unlock"]
pub struct OwnedRwLockReadGuard<T: ?Sized> {
/// /!\ WARNING: semi-load-bearing drop order /!\
///
/// This struct's field ordering is important for Loom tests; the `ConstPtr`
/// must be dropped before the semaphore permit is released back to the
/// semaphore, as this may wake another task that wants to mutably access
/// the cell. However, Loom will still consider the data to be "immutably
/// accessed" until the ConstPtr` is dropped, so we must drop the `ConstPtr`
/// first.
///
/// This isn't actually a bug in "real life", because we're not going to
/// actually *read* the data through the `ConstPtr` in the guard's `Drop`
/// impl, but Loom considers us to be "accessing" it as long as the
/// `ConstPtr` exists.
data: cell::ConstPtr<T>,
_lock: AddPermits<1, T>,
}
/// Owned [RAII] structure used to release the exclusive write access of a
/// [`RwLock`] when dropped.
///
/// This type is similar to the [`RwLockWriteGuard`] type, but it is
/// only returned by an [`RwLock`] that is wrapped in an an [`Arc`].
/// Instead of borrowing the [`RwLock`], this guard holds an [`Arc`]
/// clone of the [`RwLock`], incrementing its reference count.
/// Therefore, this type can outlive the [`RwLock`] that created it, and
/// it is valid for the `'static` lifetime. Beyond this, is identical to
/// the [`RwLockWriteGuard`] type.
///
/// The data protected by the [`RwLock`] can be accessed through this
/// guard via its [`Deref`](#impl-Deref) and [`DerefMut`](#impl-Deref)
/// implementations.
///
/// This guard can be held across any `.await` point, as it implements
/// [`Send`].
///
/// This structure is created by the [`read_owned`] and
/// [`try_read_owned`] methods on [`Arc`]`<`[`RwLock`]`>`.
///
/// [RAII]: https://rust-unofficial.github.io/patterns/patterns/behavioural/RAII.html
/// [`read_owned`]: RwLock::read_owned
/// [`try_read_owned`]: RwLock::try_read_owned
#[must_use = "if unused, the `RwLock` will immediately unlock"]
pub struct OwnedRwLockWriteGuard<T: ?Sized> {
/// /!\ WARNING: semi-load-bearing drop order /!\
///
/// This struct's field ordering is important for Loom tests; the `MutPtr`
/// must be dropped before the semaphore permits are released back to the
/// semaphore, as this may wake another task that wants to access the cell.
/// However, Loom will still consider the data to be "mutably accessed"
/// until the `MutPtr` is dropped, so we must drop the `MutPtr` first.
///
/// This isn't actually a bug in "real life", because we're not going to
/// actually read or write the data through the `MutPtr` in the guard's
/// `Drop` impl, but Loom considers us to be "accessing" it as long as the
/// `MutPtr` exists.
data: cell::MutPtr<T>,
_lock: AddPermits<{ semaphore::MAX_PERMITS }, T>,
}
/// A wrapper around an `RwLock` `Arc` clone that releases a fixed number of
/// permits when it's dropped.
///
/// This is factored out to a separate type to ensure that it's dropped *after*
/// the `MutPtr`/`ConstPtr`s are dropped, to placate `loom`.
struct AddPermits<const PERMITS: usize, T: ?Sized>(Arc<RwLock<T>>);
// === impl RwLock ===
impl<T: ?Sized> RwLock<T> {
/// Locks this `RwLock` with shared read access, returning an [owned RAII
/// guard][guard].
///
/// This method is identical to [`RwLock::read`], execept that it requires
/// the `RwLock` to be wrapped in an [`Arc`], and returns an
/// [`OwnedRwLockReadGuard`] that clones the [`Arc`] rather than
/// borrowing the lock. Therefore, the returned guard is valid for the
/// `'static` lifetime.
///
/// If the lock is locked for write access, the calling task will yield and
/// wait until there are no writers which hold the lock. There may be other
/// readers inside the lock when the task resumes.
///
/// Note that under the [priority policy] of [`RwLock`], read locks are not
/// granted until prior write locks, to prevent starvation. Therefore
/// deadlock may occur if a read lock is held by the current task, a write
/// lock attempt is made, and then a subsequent read lock attempt is made
/// by the current task.
///
/// Returns [an RAII guard][guard] which will release this read access of the
/// `RwLock` when dropped.
///
/// # Cancellation
///
/// This method [uses a queue to fairly distribute locks][priority policy]
/// in the order they were requested. Cancelling a call to `read` results
/// in the calling task losing its place in the queue.
///
/// # Examples
///
/// ```
/// # use tokio::task;
/// # #[tokio::main(flavor = "current_thread")]
/// # async fn test() {
/// # // since we are targeting no-std, it makes more sense to use `alloc`
/// # // in these examples, rather than `std`...but i don't want to make
/// # // the tests actually `#![no_std]`...
/// # use std as alloc;
/// use maitake_sync::RwLock;
/// use alloc::sync::Arc;
///
/// let lock = Arc::new(RwLock::new(1));
/// // hold the lock for reading in `main`.
/// let n = lock
/// .try_read()
/// .expect("read lock must be acquired, as the lock is unlocked");
/// assert_eq!(*n, 1);
///
/// # let task =
/// task::spawn({
/// let lock = lock.clone();
/// async move {
/// // While main has an active read lock, this task can acquire
/// // one too.
/// let n = lock.read_owned().await;
/// assert_eq!(*n, 1);
/// }
/// });
/// # task.await.unwrap();
/// # }
/// # test();
/// ```
///
/// [priority policy]: Self#priority-policy
/// [guard]: OwnedRwLockReadGuard
pub async fn read_owned(self: &Arc<Self>) -> OwnedRwLockReadGuard<T> {
let guard = self.read().await;
OwnedRwLockReadGuard::from_borrowed(self.clone(), guard)
}
/// Locks this `RwLock` with exclusive write access,returning an [owned RAII
/// guard][guard].
///
/// This method is identical to [`RwLock::write`], execept that it requires
/// the `RwLock` to be wrapped in an [`Arc`], and returns an
/// [`OwnedRwLockWriteGuard`] that clones the [`Arc`] rather than
/// borrowing the lock. Therefore, the returned guard is valid for the
/// `'static` lifetime.
///
/// # Returns
///
/// If other tasks are holding a read or write lock, the calling task will
/// wait until the write lock or all read locks are released.
///
/// Returns [an RAII guard][guard] which will release the write access of this
/// `RwLock` when dropped.
///
/// # Cancellation
///
/// This method [uses a queue to fairly distribute
/// locks](Self#priority-policy) in the order they were requested.
/// Cancelling a call to `write` results in the calling task losing its place
/// in the queue.
///
/// # Examples
///
/// ```
/// # use tokio::task;
/// # #[tokio::main(flavor = "current_thread")]
/// # async fn test() {
/// # // since we are targeting no-std, it makes more sense to use `alloc`
/// # // in these examples, rather than `std`...but i don't want to make
/// # // the tests actually `#![no_std]`...
/// # use std as alloc;
/// use maitake_sync::RwLock;
/// use alloc::sync::Arc;
///
/// let lock = Arc::new(RwLock::new(1));
///
/// # let task =
/// task::spawn(async move {
/// let mut guard = lock.write_owned().await;
/// *guard += 1;
/// });
/// # task.await.unwrap();
/// # }
/// # test();
/// ```
///
/// [guard]: OwnedRwLockWriteGuard
pub async fn write_owned(self: &Arc<Self>) -> OwnedRwLockWriteGuard<T> {
let guard = self.write().await;
OwnedRwLockWriteGuard::from_borrowed(self.clone(), guard)
}
/// Attempts to acquire this `RwLock` for shared read access, without
/// waiting, and returning an [owned RAII guard][guard].
///
/// This method is identical to [`RwLock::try_read`], execept that it requires
/// the `RwLock` to be wrapped in an [`Arc`], and returns an
/// [`OwnedRwLockReadGuard`] that clones the [`Arc`] rather than
/// borrowing the lock. Therefore, the returned guard is valid for the
/// `'static` lifetime.
///
/// # Returns
///
/// If the access couldn't be acquired immediately, this method returns
/// [`None`] rather than waiting.
///
/// Otherwise, [an RAII guard][guard] is returned, which allows read access to the
/// protected data and will release that access when dropped.
///
/// # Examples
///
/// ```
/// # fn main() {
/// # // since we are targeting no-std, it makes more sense to use `alloc`
/// # // in these examples, rather than `std`...but i don't want to make
/// # // the tests actually `#![no_std]`...
/// # use std as alloc;
/// use maitake_sync::RwLock;
/// use alloc::sync::Arc;
///
/// let lock = Arc::new(RwLock::new(1));
///
/// let mut write_guard = lock
/// .try_write()
/// .expect("lock is unlocked, so write access should be acquired");
/// *write_guard += 1;
///
/// // because a write guard is held, we cannot acquire the read lock, so
/// // this will return `None`.
/// assert!(lock.try_read_owned().is_none());
/// # }
/// ```
///
/// [guard]: OwnedRwLockReadGuard
pub fn try_read_owned(self: &Arc<Self>) -> Option<OwnedRwLockReadGuard<T>> {
self.try_read()
.map(|guard| OwnedRwLockReadGuard::from_borrowed(self.clone(), guard))
}
/// Attempts to acquire this `RwLock` for exclusive write access, without
/// waiting, and returning an [owned RAII guard][guard].
///
/// This method is identical to [`RwLock::try_write`], execept that it requires
/// the `RwLock` to be wrapped in an [`Arc`], and returns an
/// [`OwnedRwLockWriteGuard`] that clones the [`Arc`] rather than
/// borrowing the lock. Therefore, the returned guard is valid for the
/// `'static` lifetime.
///
/// # Returns
///
/// If the access couldn't be acquired immediately, this method returns
/// [`None`] rather than waiting.
///
/// Otherwise, [an RAII guard][guard] is returned, which allows write access to the
/// protected data and will release that access when dropped.
///
/// # Examples
///
/// ```
/// # fn main() {
/// # // since we are targeting no-std, it makes more sense to use `alloc`
/// # // in these examples, rather than `std`...but i don't want to make
/// # // the tests actually `#![no_std]`...
/// # use std as alloc;
/// use maitake_sync::RwLock;
/// use alloc::sync::Arc;
///
/// let lock = Arc::new(RwLock::new(1));
///
/// let read_guard = lock
/// .try_read()
/// .expect("lock is unlocked, so read access should be acquired");
/// assert_eq!(*read_guard, 1);
///
/// // because a read guard is held, we cannot acquire the write lock, so
/// // this will return `None`.
/// assert!(lock.try_write_owned().is_none());
/// # }
/// ```
///
/// [guard]: OwnedRwLockWriteGuard
pub fn try_write_owned(self: &Arc<Self>) -> Option<OwnedRwLockWriteGuard<T>> {
self.try_write()
.map(|guard| OwnedRwLockWriteGuard::from_borrowed(self.clone(), guard))
}
}
// === impl OwnedRwLockReadGuard ===
impl<T: ?Sized> OwnedRwLockReadGuard<T> {
fn from_borrowed(
lock: Arc<RwLock<T>>,
RwLockReadGuard { data, _permit }: RwLockReadGuard<'_, T>,
) -> Self {
// forget the semaphore permit, as it has a lifetime tied to the
// borrowed semaphore. we'll manually release the permit in
// `OwnedRwLockReadGuard`'s `Drop` impl.
_permit.forget();
Self {
_lock: AddPermits(lock),
data,
}
}
}
impl<T: ?Sized> Deref for OwnedRwLockReadGuard<T> {
type Target = T;
#[inline]
fn deref(&self) -> &Self::Target {
unsafe {
// safety: we are holding the semaphore permit that ensures the lock
// cannot be accessed mutably.
self.data.deref()
}
}
}
impl<T: ?Sized + fmt::Debug> fmt::Debug for OwnedRwLockReadGuard<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
self.deref().fmt(f)
}
}
// Safety: A read guard can be shared or sent between threads as long as `T` is
// `Sync`. It can implement `Send` even if `T` does not implement `Send`, as
// long as `T` is `Sync`, because the read guard only permits borrowing the `T`.
unsafe impl<T> Send for OwnedRwLockReadGuard<T> where T: ?Sized + Sync {}
unsafe impl<T> Sync for OwnedRwLockReadGuard<T> where T: ?Sized + Send + Sync {}
// === impl OwnedRwLockWriteGuard ===
impl<T: ?Sized> OwnedRwLockWriteGuard<T> {
fn from_borrowed(
lock: Arc<RwLock<T>>,
RwLockWriteGuard { data, _permit }: RwLockWriteGuard<'_, T>,
) -> Self {
// forget the semaphore permit, as it has a lifetime tied to the
// borrowed semaphore. we'll manually release the permit in
// `OwnedRwLockWriteGuard`'s `Drop` impl.
_permit.forget();
Self {
_lock: AddPermits(lock),
data,
}
}
}
impl<T: ?Sized> Deref for OwnedRwLockWriteGuard<T> {
type Target = T;
#[inline]
fn deref(&self) -> &Self::Target {
unsafe {
// safety: we are holding all the semaphore permits, so the data
// inside the lock cannot be accessed by another thread.
self.data.deref()
}
}
}
impl<T: ?Sized> DerefMut for OwnedRwLockWriteGuard<T> {
#[inline]
fn deref_mut(&mut self) -> &mut Self::Target {
unsafe {
// safety: we are holding all the semaphore permits, so the data
// inside the lock cannot be accessed by another thread.
self.data.deref()
}
}
}
impl<T: ?Sized + fmt::Debug> fmt::Debug for OwnedRwLockWriteGuard<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
self.deref().fmt(f)
}
}
// Safety: Unlike the read guard, `T` must be both `Send` and `Sync` for the
// write guard to be `Send`, because the mutable access provided by the write
// guard can be used to `mem::replace` or `mem::take` the value, transferring
// ownership of it across threads.
unsafe impl<T> Send for OwnedRwLockWriteGuard<T> where T: ?Sized + Send + Sync {}
unsafe impl<T> Sync for OwnedRwLockWriteGuard<T> where T: ?Sized + Send + Sync {}
// === impl AddPermits ===
impl<const PERMITS: usize, T: ?Sized> Drop for AddPermits<PERMITS, T> {
fn drop(&mut self) {
self.0.sem.add_permits(PERMITS);
}
}