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//! An async mutex.
//!
//! The locking mechanism uses eventual fairness to ensure locking will be fair on average without
//! sacrificing performance. This is done by forcing a fair lock whenever a lock operation is
//! starved for longer than 0.5 milliseconds.
//!
//! # Examples
//!
//! ```
//! # futures_lite::future::block_on(async {
//! use async_mutex::Mutex;
//!
//! let m = Mutex::new(1);
//!
//! let mut guard = m.lock().await;
//! *guard = 2;
//!
//! assert!(m.try_lock().is_none());
//! drop(guard);
//! assert_eq!(*m.try_lock().unwrap(), 2);
//! # })
//! ```
#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
use std::cell::UnsafeCell;
use std::fmt;
use std::ops::{Deref, DerefMut};
use std::process;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::sync::Arc;
use std::time::{Duration, Instant};
use std::usize;
use event_listener::Event;
/// An async mutex.
pub struct Mutex<T: ?Sized> {
/// Current state of the mutex.
///
/// The least significant bit is set to 1 if the mutex is locked.
/// The other bits hold the number of starved lock operations.
state: AtomicUsize,
/// Lock operations waiting for the mutex to be released.
lock_ops: Event,
/// The value inside the mutex.
data: UnsafeCell<T>,
}
unsafe impl<T: Send + ?Sized> Send for Mutex<T> {}
unsafe impl<T: Send + ?Sized> Sync for Mutex<T> {}
impl<T> Mutex<T> {
/// Creates a new async mutex.
///
/// # Examples
///
/// ```
/// use async_mutex::Mutex;
///
/// let mutex = Mutex::new(0);
/// ```
pub const fn new(data: T) -> Mutex<T> {
Mutex {
state: AtomicUsize::new(0),
lock_ops: Event::new(),
data: UnsafeCell::new(data),
}
}
/// Consumes the mutex, returning the underlying data.
///
/// # Examples
///
/// ```
/// use async_mutex::Mutex;
///
/// let mutex = Mutex::new(10);
/// assert_eq!(mutex.into_inner(), 10);
/// ```
pub fn into_inner(self) -> T {
self.data.into_inner()
}
}
impl<T: ?Sized> Mutex<T> {
/// Acquires the mutex.
///
/// Returns a guard that releases the mutex when dropped.
///
/// # Examples
///
/// ```
/// # futures_lite::future::block_on(async {
/// use async_mutex::Mutex;
///
/// let mutex = Mutex::new(10);
/// let guard = mutex.lock().await;
/// assert_eq!(*guard, 10);
/// # })
/// ```
#[inline]
pub async fn lock(&self) -> MutexGuard<'_, T> {
if let Some(guard) = self.try_lock() {
return guard;
}
self.acquire_slow().await;
MutexGuard(self)
}
/// Slow path for acquiring the mutex.
#[cold]
async fn acquire_slow(&self) {
// Get the current time.
let start = Instant::now();
loop {
// Start listening for events.
let listener = self.lock_ops.listen();
// Try locking if nobody is being starved.
match self.state.compare_and_swap(0, 1, Ordering::Acquire) {
// Lock acquired!
0 => return,
// Lock is held and nobody is starved.
1 => {}
// Somebody is starved.
_ => break,
}
// Wait for a notification.
listener.await;
// Try locking if nobody is being starved.
match self.state.compare_and_swap(0, 1, Ordering::Acquire) {
// Lock acquired!
0 => return,
// Lock is held and nobody is starved.
1 => {}
// Somebody is starved.
_ => {
// Notify the first listener in line because we probably received a
// notification that was meant for a starved task.
self.lock_ops.notify(1);
break;
}
}
// If waiting for too long, fall back to a fairer locking strategy that will prevent
// newer lock operations from starving us forever.
if start.elapsed() > Duration::from_micros(500) {
break;
}
}
// Increment the number of starved lock operations.
if self.state.fetch_add(2, Ordering::Release) > usize::MAX / 2 {
// In case of potential overflow, abort.
process::abort();
}
// Decrement the counter when exiting this function.
let _call = CallOnDrop(|| {
self.state.fetch_sub(2, Ordering::Release);
});
loop {
// Start listening for events.
let listener = self.lock_ops.listen();
// Try locking if nobody else is being starved.
match self.state.compare_and_swap(2, 2 | 1, Ordering::Acquire) {
// Lock acquired!
2 => return,
// Lock is held by someone.
s if s % 2 == 1 => {}
// Lock is available.
_ => {
// Be fair: notify the first listener and then go wait in line.
self.lock_ops.notify(1);
}
}
// Wait for a notification.
listener.await;
// Try acquiring the lock without waiting for others.
if self.state.fetch_or(1, Ordering::Acquire) % 2 == 0 {
return;
}
}
}
/// Attempts to acquire the mutex.
///
/// If the mutex could not be acquired at this time, then [`None`] is returned. Otherwise, a
/// guard is returned that releases the mutex when dropped.
///
/// # Examples
///
/// ```
/// use async_mutex::Mutex;
///
/// let mutex = Mutex::new(10);
/// if let Some(guard) = mutex.try_lock() {
/// assert_eq!(*guard, 10);
/// }
/// # ;
/// ```
#[inline]
pub fn try_lock(&self) -> Option<MutexGuard<'_, T>> {
if self.state.compare_and_swap(0, 1, Ordering::Acquire) == 0 {
Some(MutexGuard(self))
} else {
None
}
}
/// Returns a mutable reference to the underlying data.
///
/// Since this call borrows the mutex mutably, no actual locking takes place -- the mutable
/// borrow statically guarantees the mutex is not already acquired.
///
/// # Examples
///
/// ```
/// # futures_lite::future::block_on(async {
/// use async_mutex::Mutex;
///
/// let mut mutex = Mutex::new(0);
/// *mutex.get_mut() = 10;
/// assert_eq!(*mutex.lock().await, 10);
/// # })
/// ```
pub fn get_mut(&mut self) -> &mut T {
unsafe { &mut *self.data.get() }
}
}
impl<T: ?Sized> Mutex<T> {
/// Acquires the mutex and clones a reference to it.
///
/// Returns an owned guard that releases the mutex when dropped.
///
/// # Examples
///
/// ```
/// # futures_lite::future::block_on(async {
/// use async_mutex::Mutex;
/// use std::sync::Arc;
///
/// let mutex = Arc::new(Mutex::new(10));
/// let guard = mutex.lock_arc().await;
/// assert_eq!(*guard, 10);
/// # })
/// ```
#[inline]
pub async fn lock_arc(self: &Arc<Self>) -> MutexGuardArc<T> {
if let Some(guard) = self.try_lock_arc() {
return guard;
}
self.acquire_slow().await;
MutexGuardArc(self.clone())
}
/// Attempts to acquire the mutex and clone a reference to it.
///
/// If the mutex could not be acquired at this time, then [`None`] is returned. Otherwise, an
/// owned guard is returned that releases the mutex when dropped.
///
/// # Examples
///
/// ```
/// use async_mutex::Mutex;
/// use std::sync::Arc;
///
/// let mutex = Arc::new(Mutex::new(10));
/// if let Some(guard) = mutex.try_lock() {
/// assert_eq!(*guard, 10);
/// }
/// # ;
/// ```
#[inline]
pub fn try_lock_arc(self: &Arc<Self>) -> Option<MutexGuardArc<T>> {
if self.state.compare_and_swap(0, 1, Ordering::Acquire) == 0 {
Some(MutexGuardArc(self.clone()))
} else {
None
}
}
}
impl<T: fmt::Debug + ?Sized> fmt::Debug for Mutex<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_lock() {
None => f.debug_struct("Mutex").field("data", &Locked).finish(),
Some(guard) => f.debug_struct("Mutex").field("data", &&*guard).finish(),
}
}
}
impl<T> From<T> for Mutex<T> {
fn from(val: T) -> Mutex<T> {
Mutex::new(val)
}
}
impl<T: Default + ?Sized> Default for Mutex<T> {
fn default() -> Mutex<T> {
Mutex::new(Default::default())
}
}
/// A guard that releases the mutex when dropped.
pub struct MutexGuard<'a, T: ?Sized>(&'a Mutex<T>);
unsafe impl<T: Send + ?Sized> Send for MutexGuard<'_, T> {}
unsafe impl<T: Sync + ?Sized> Sync for MutexGuard<'_, T> {}
impl<'a, T: ?Sized> MutexGuard<'a, T> {
/// Returns a reference to the mutex a guard came from.
///
/// # Examples
///
/// ```
/// # futures_lite::future::block_on(async {
/// use async_mutex::{Mutex, MutexGuard};
///
/// let mutex = Mutex::new(10i32);
/// let guard = mutex.lock().await;
/// dbg!(MutexGuard::source(&guard));
/// # })
/// ```
pub fn source(guard: &MutexGuard<'a, T>) -> &'a Mutex<T> {
guard.0
}
}
impl<T: ?Sized> Drop for MutexGuard<'_, T> {
fn drop(&mut self) {
// Remove the last bit and notify a waiting lock operation.
self.0.state.fetch_sub(1, Ordering::Release);
self.0.lock_ops.notify(1);
}
}
impl<T: fmt::Debug + ?Sized> fmt::Debug for MutexGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
impl<T: fmt::Display + ?Sized> fmt::Display for MutexGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(**self).fmt(f)
}
}
impl<T: ?Sized> Deref for MutexGuard<'_, T> {
type Target = T;
fn deref(&self) -> &T {
unsafe { &*self.0.data.get() }
}
}
impl<T: ?Sized> DerefMut for MutexGuard<'_, T> {
fn deref_mut(&mut self) -> &mut T {
unsafe { &mut *self.0.data.get() }
}
}
/// An owned guard that releases the mutex when dropped.
pub struct MutexGuardArc<T: ?Sized>(Arc<Mutex<T>>);
unsafe impl<T: Send + ?Sized> Send for MutexGuardArc<T> {}
unsafe impl<T: Sync + ?Sized> Sync for MutexGuardArc<T> {}
impl<T: ?Sized> MutexGuardArc<T> {
/// Returns a reference to the mutex a guard came from.
///
/// # Examples
///
/// ```
/// # futures_lite::future::block_on(async {
/// use async_mutex::{Mutex, MutexGuardArc};
/// use std::sync::Arc;
///
/// let mutex = Arc::new(Mutex::new(10i32));
/// let guard = mutex.lock_arc().await;
/// dbg!(MutexGuardArc::source(&guard));
/// # })
/// ```
pub fn source(guard: &MutexGuardArc<T>) -> &Arc<Mutex<T>> {
&guard.0
}
}
impl<T: ?Sized> Drop for MutexGuardArc<T> {
fn drop(&mut self) {
// Remove the last bit and notify a waiting lock operation.
self.0.state.fetch_sub(1, Ordering::Release);
self.0.lock_ops.notify(1);
}
}
impl<T: fmt::Debug + ?Sized> fmt::Debug for MutexGuardArc<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
impl<T: fmt::Display + ?Sized> fmt::Display for MutexGuardArc<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(**self).fmt(f)
}
}
impl<T: ?Sized> Deref for MutexGuardArc<T> {
type Target = T;
fn deref(&self) -> &T {
unsafe { &*self.0.data.get() }
}
}
impl<T: ?Sized> DerefMut for MutexGuardArc<T> {
fn deref_mut(&mut self) -> &mut T {
unsafe { &mut *self.0.data.get() }
}
}
/// Calls a function when dropped.
struct CallOnDrop<F: Fn()>(F);
impl<F: Fn()> Drop for CallOnDrop<F> {
fn drop(&mut self) {
(self.0)();
}
}