use std::cell::UnsafeCell;
use std::fmt;
use std::isize;
use std::ops::{Deref, DerefMut};
use std::pin::Pin;
use std::process;
use std::future::Future;
use std::sync::atomic::{AtomicUsize, Ordering};
use crate::sync::WakerSet;
use crate::task::{Context, Poll};
/// Set if a write lock is held.
#[allow(clippy::identity_op)]
const WRITE_LOCK: usize = 1 << 0;
/// The value of a single blocked read contributing to the read count.
const ONE_READ: usize = 1 << 1;
/// The bits in which the read count is stored.
const READ_COUNT_MASK: usize = !(ONE_READ - 1);
/// A reader-writer lock for protecting shared data.
///
/// This type is an async version of [`std::sync::RwLock`].
///
/// [`std::sync::RwLock`]: https://doc.rust-lang.org/std/sync/struct.RwLock.html
///
/// # Examples
///
/// ```
/// # async_std::task::block_on(async {
/// #
/// use async_std::sync::RwLock;
///
/// let lock = RwLock::new(5);
///
/// // Multiple read locks can be held at a time.
/// let r1 = lock.read().await;
/// let r2 = lock.read().await;
/// assert_eq!(*r1, 5);
/// assert_eq!(*r2, 5);
/// drop((r1, r2));
///
/// // Only one write locks can be held at a time.
/// let mut w = lock.write().await;
/// *w += 1;
/// assert_eq!(*w, 6);
/// #
/// # })
/// ```
pub struct RwLock<T: ?Sized> {
state: AtomicUsize,
read_wakers: WakerSet,
write_wakers: WakerSet,
value: UnsafeCell<T>,
}
unsafe impl<T: ?Sized + Send> Send for RwLock<T> {}
unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}
impl<T> RwLock<T> {
/// Creates a new reader-writer lock.
///
/// # Examples
///
/// ```
/// use async_std::sync::RwLock;
///
/// let lock = RwLock::new(0);
/// ```
pub fn new(t: T) -> RwLock<T> {
RwLock {
state: AtomicUsize::new(0),
read_wakers: WakerSet::new(),
write_wakers: WakerSet::new(),
value: UnsafeCell::new(t),
}
}
}
impl<T: ?Sized> RwLock<T> {
/// Acquires a read lock.
///
/// Returns a guard that releases the lock when dropped.
///
/// # Examples
///
/// ```
/// # async_std::task::block_on(async {
/// #
/// use async_std::sync::RwLock;
///
/// let lock = RwLock::new(1);
///
/// let n = lock.read().await;
/// assert_eq!(*n, 1);
///
/// assert!(lock.try_read().is_some());
/// #
/// # })
/// ```
pub async fn read(&self) -> RwLockReadGuard<'_, T> {
pub struct ReadFuture<'a, T: ?Sized> {
lock: &'a RwLock<T>,
opt_key: Option<usize>,
}
impl<'a, T: ?Sized> Future for ReadFuture<'a, T> {
type Output = RwLockReadGuard<'a, T>;
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
loop {
// If the current task is in the set, remove it.
if let Some(key) = self.opt_key.take() {
self.lock.read_wakers.remove(key);
}
// Try acquiring a read lock.
match self.lock.try_read() {
Some(guard) => return Poll::Ready(guard),
None => {
// Insert this lock operation.
self.opt_key = Some(self.lock.read_wakers.insert(cx));
// If the lock is still acquired for writing, return.
if self.lock.state.load(Ordering::SeqCst) & WRITE_LOCK != 0 {
return Poll::Pending;
}
}
}
}
}
}
impl<T: ?Sized> Drop for ReadFuture<'_, T> {
fn drop(&mut self) {
// If the current task is still in the set, that means it is being cancelled now.
if let Some(key) = self.opt_key {
self.lock.read_wakers.cancel(key);
// If there are no active readers, notify a blocked writer if none were
// notified already.
if self.lock.state.load(Ordering::SeqCst) & READ_COUNT_MASK == 0 {
self.lock.write_wakers.notify_any();
}
}
}
}
ReadFuture {
lock: self,
opt_key: None,
}
.await
}
/// Attempts to acquire a read lock.
///
/// If a read lock could not be acquired at this time, then [`None`] is returned. Otherwise, a
/// guard is returned that releases the lock when dropped.
///
/// [`None`]: https://doc.rust-lang.org/std/option/enum.Option.html#variant.None
///
/// # Examples
///
/// ```
/// # async_std::task::block_on(async {
/// #
/// use async_std::sync::RwLock;
///
/// let lock = RwLock::new(1);
///
/// let n = lock.read().await;
/// assert_eq!(*n, 1);
///
/// assert!(lock.try_read().is_some());
/// #
/// # })
/// ```
pub fn try_read(&self) -> Option<RwLockReadGuard<'_, T>> {
let mut state = self.state.load(Ordering::SeqCst);
loop {
// If a write lock is currently held, then a read lock cannot be acquired.
if state & WRITE_LOCK != 0 {
return None;
}
// Make sure the number of readers doesn't overflow.
if state > isize::MAX as usize {
process::abort();
}
// Increment the number of active reads.
match self.state.compare_exchange_weak(
state,
state + ONE_READ,
Ordering::SeqCst,
Ordering::SeqCst,
) {
Ok(_) => return Some(RwLockReadGuard(self)),
Err(s) => state = s,
}
}
}
/// Acquires a write lock.
///
/// Returns a guard that releases the lock when dropped.
///
/// # Examples
///
/// ```
/// # async_std::task::block_on(async {
/// #
/// use async_std::sync::RwLock;
///
/// let lock = RwLock::new(1);
///
/// let mut n = lock.write().await;
/// *n = 2;
///
/// assert!(lock.try_read().is_none());
/// #
/// # })
/// ```
pub async fn write(&self) -> RwLockWriteGuard<'_, T> {
pub struct WriteFuture<'a, T: ?Sized> {
lock: &'a RwLock<T>,
opt_key: Option<usize>,
}
impl<'a, T: ?Sized> Future for WriteFuture<'a, T> {
type Output = RwLockWriteGuard<'a, T>;
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
loop {
// If the current task is in the set, remove it.
if let Some(key) = self.opt_key.take() {
self.lock.write_wakers.remove(key);
}
// Try acquiring a write lock.
match self.lock.try_write() {
Some(guard) => return Poll::Ready(guard),
None => {
// Insert this lock operation.
self.opt_key = Some(self.lock.write_wakers.insert(cx));
// If the lock is still acquired for reading or writing, return.
if self.lock.state.load(Ordering::SeqCst) != 0 {
return Poll::Pending;
}
}
}
}
}
}
impl<T: ?Sized> Drop for WriteFuture<'_, T> {
fn drop(&mut self) {
// If the current task is still in the set, that means it is being cancelled now.
if let Some(key) = self.opt_key {
if !self.lock.write_wakers.cancel(key) {
// If no other blocked reader was notified, notify all readers.
self.lock.read_wakers.notify_all();
}
}
}
}
WriteFuture {
lock: self,
opt_key: None,
}
.await
}
/// Attempts to acquire a write lock.
///
/// If a write lock could not be acquired at this time, then [`None`] is returned. Otherwise, a
/// guard is returned that releases the lock when dropped.
///
/// [`None`]: https://doc.rust-lang.org/std/option/enum.Option.html#variant.None
///
/// # Examples
///
/// ```
/// # async_std::task::block_on(async {
/// #
/// use async_std::sync::RwLock;
///
/// let lock = RwLock::new(1);
///
/// let n = lock.read().await;
/// assert_eq!(*n, 1);
///
/// assert!(lock.try_write().is_none());
/// #
/// # })
/// ```
pub fn try_write(&self) -> Option<RwLockWriteGuard<'_, T>> {
if self.state.compare_and_swap(0, WRITE_LOCK, Ordering::SeqCst) == 0 {
Some(RwLockWriteGuard(self))
} else {
None
}
}
/// Consumes the lock, returning the underlying data.
///
/// # Examples
///
/// ```
/// use async_std::sync::RwLock;
///
/// let lock = RwLock::new(10);
/// assert_eq!(lock.into_inner(), 10);
/// ```
pub fn into_inner(self) -> T where T: Sized {
self.value.into_inner()
}
/// Returns a mutable reference to the underlying data.
///
/// Since this call borrows the lock mutably, no actual locking takes place -- the mutable
/// borrow statically guarantees no locks exist.
///
/// # Examples
///
/// ```
/// # async_std::task::block_on(async {
/// #
/// use async_std::sync::RwLock;
///
/// let mut lock = RwLock::new(0);
/// *lock.get_mut() = 10;
/// assert_eq!(*lock.write().await, 10);
/// #
/// # })
/// ```
pub fn get_mut(&mut self) -> &mut T {
unsafe { &mut *self.value.get() }
}
}
impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
struct Locked;
impl fmt::Debug for Locked {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.write_str("<locked>")
}
}
match self.try_read() {
None => f.debug_struct("RwLock").field("data", &Locked).finish(),
Some(guard) => f.debug_struct("RwLock").field("data", &&*guard).finish(),
}
}
}
impl<T> From<T> for RwLock<T> {
fn from(val: T) -> RwLock<T> {
RwLock::new(val)
}
}
impl<T: ?Sized + Default> Default for RwLock<T> {
fn default() -> RwLock<T> {
RwLock::new(Default::default())
}
}
/// A guard that releases the read lock when dropped.
pub struct RwLockReadGuard<'a, T: ?Sized>(&'a RwLock<T>);
unsafe impl<T: ?Sized + Send> Send for RwLockReadGuard<'_, T> {}
unsafe impl<T: ?Sized + Sync> Sync for RwLockReadGuard<'_, T> {}
impl<T: ?Sized> Drop for RwLockReadGuard<'_, T> {
fn drop(&mut self) {
let state = self.0.state.fetch_sub(ONE_READ, Ordering::SeqCst);
// If this was the last reader, notify a blocked writer if none were notified already.
if state & READ_COUNT_MASK == ONE_READ {
self.0.write_wakers.notify_any();
}
}
}
impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockReadGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
impl<T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(**self).fmt(f)
}
}
impl<T: ?Sized> Deref for RwLockReadGuard<'_, T> {
type Target = T;
fn deref(&self) -> &T {
unsafe { &*self.0.value.get() }
}
}
/// A guard that releases the write lock when dropped.
pub struct RwLockWriteGuard<'a, T: ?Sized>(&'a RwLock<T>);
unsafe impl<T: ?Sized + Send> Send for RwLockWriteGuard<'_, T> {}
unsafe impl<T: ?Sized + Sync> Sync for RwLockWriteGuard<'_, T> {}
impl<T: ?Sized> Drop for RwLockWriteGuard<'_, T> {
fn drop(&mut self) {
self.0.state.store(0, Ordering::SeqCst);
// Notify all blocked readers.
if !self.0.read_wakers.notify_all() {
// If there were no blocked readers, notify a blocked writer if none were notified
// already.
self.0.write_wakers.notify_any();
}
}
}
impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockWriteGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
impl<T: ?Sized + fmt::Display> fmt::Display for RwLockWriteGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(**self).fmt(f)
}
}
impl<T: ?Sized> Deref for RwLockWriteGuard<'_, T> {
type Target = T;
fn deref(&self) -> &T {
unsafe { &*self.0.value.get() }
}
}
impl<T: ?Sized> DerefMut for RwLockWriteGuard<'_, T> {
fn deref_mut(&mut self) -> &mut T {
unsafe { &mut *self.0.value.get() }
}
}