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#![deny(missing_docs)]
#![deny(missing_debug_implementations)]
#![deny(warnings)]
#![cfg_attr(not(test), no_std)]
//! A light-weight lock guarded by an atomic boolean.
//!
//! Most efficient when contention is low, acquiring the lock is a single
//! atomic swap, and releasing it just 1 more atomic swap.
//!
//! # Example
//!
//! ```
//! use std::sync::Arc;
//! use try_lock::TryLock;
//!
//! // a thing we want to share
//! struct Widget {
//! name: String,
//! }
//!
//! // lock it up!
//! let widget1 = Arc::new(TryLock::new(Widget {
//! name: "Spanner".into(),
//! }));
//!
//! let widget2 = widget1.clone();
//!
//!
//! // mutate the widget
//! let mut locked = widget1.try_lock().expect("example isn't locked yet");
//! locked.name.push_str(" Bundle");
//!
//! // hands off, buddy
//! let not_locked = widget2.try_lock();
//! assert!(not_locked.is_none(), "widget1 has the lock");
//!
//! // ok, you can have it
//! drop(locked);
//!
//! let locked2 = widget2.try_lock().expect("widget1 lock is released");
//!
//! assert_eq!(locked2.name, "Spanner Bundle");
//! ```
#[cfg(test)]
extern crate core;
use core::cell::UnsafeCell;
use core::fmt;
use core::ops::{Deref, DerefMut};
use core::sync::atomic::{AtomicBool, Ordering};
use core::marker::PhantomData;
/// A light-weight lock guarded by an atomic boolean.
///
/// Most efficient when contention is low, acquiring the lock is a single
/// atomic swap, and releasing it just 1 more atomic swap.
///
/// It is only possible to try to acquire the lock, it is not possible to
/// wait for the lock to become ready, like with a `Mutex`.
#[derive(Default)]
pub struct TryLock<T> {
is_locked: AtomicBool,
value: UnsafeCell<T>,
}
impl<T> TryLock<T> {
/// Create a `TryLock` around the value.
#[inline]
pub fn new(val: T) -> TryLock<T> {
TryLock {
is_locked: AtomicBool::new(false),
value: UnsafeCell::new(val),
}
}
/// Try to acquire the lock of this value.
///
/// If the lock is already acquired by someone else, this returns
/// `None`. You can try to acquire again whenever you want, perhaps
/// by spinning a few times, or by using some other means of
/// notification.
///
/// # Note
///
/// The default memory ordering is to use `Acquire` to lock, and `Release`
/// to unlock. If different ordering is required, use
/// [`try_lock_explicit`](TryLock::try_lock_explicit) or
/// [`try_lock_explicit_unchecked`](TryLock::try_lock_explicit_unchecked).
#[inline]
pub fn try_lock(&self) -> Option<Locked<T>> {
unsafe {
self.try_lock_explicit_unchecked(Ordering::Acquire, Ordering::Release)
}
}
/// Try to acquire the lock of this value using the lock and unlock orderings.
///
/// If the lock is already acquired by someone else, this returns
/// `None`. You can try to acquire again whenever you want, perhaps
/// by spinning a few times, or by using some other means of
/// notification.
#[inline]
#[deprecated(
since = "0.2.3",
note = "This method is actually unsafe because it unsafely allows \
the use of weaker memory ordering. Please use try_lock_explicit instead"
)]
pub fn try_lock_order(&self, lock_order: Ordering, unlock_order: Ordering) -> Option<Locked<T>> {
unsafe {
self.try_lock_explicit_unchecked(lock_order, unlock_order)
}
}
/// Try to acquire the lock of this value using the specified lock and
/// unlock orderings.
///
/// If the lock is already acquired by someone else, this returns
/// `None`. You can try to acquire again whenever you want, perhaps
/// by spinning a few times, or by using some other means of
/// notification.
///
/// # Panic
///
/// This method panics if `lock_order` is not any of `Acquire`, `AcqRel`,
/// and `SeqCst`, or `unlock_order` is not any of `Release` and `SeqCst`.
#[inline]
pub fn try_lock_explicit(&self, lock_order: Ordering, unlock_order: Ordering) -> Option<Locked<T>> {
match lock_order {
Ordering::Acquire |
Ordering::AcqRel |
Ordering::SeqCst => {}
_ => panic!("lock ordering must be `Acquire`, `AcqRel`, or `SeqCst`"),
}
match unlock_order {
Ordering::Release |
Ordering::SeqCst => {}
_ => panic!("unlock ordering must be `Release` or `SeqCst`"),
}
unsafe {
self.try_lock_explicit_unchecked(lock_order, unlock_order)
}
}
/// Try to acquire the lock of this value using the specified lock and
/// unlock orderings without checking that the specified orderings are
/// strong enough to be safe.
///
/// If the lock is already acquired by someone else, this returns
/// `None`. You can try to acquire again whenever you want, perhaps
/// by spinning a few times, or by using some other means of
/// notification.
///
/// # Safety
///
/// Unlike [`try_lock_explicit`], this method is unsafe because it does not
/// check that the given memory orderings are strong enough to prevent data
/// race.
///
/// [`try_lock_explicit`]: Self::try_lock_explicit
#[inline]
pub unsafe fn try_lock_explicit_unchecked(&self, lock_order: Ordering, unlock_order: Ordering) -> Option<Locked<T>> {
if !self.is_locked.swap(true, lock_order) {
Some(Locked {
lock: self,
order: unlock_order,
_p: PhantomData,
})
} else {
None
}
}
/// Take the value back out of the lock when this is the sole owner.
#[inline]
pub fn into_inner(self) -> T {
debug_assert!(!self.is_locked.load(Ordering::Relaxed), "TryLock was mem::forgotten");
// Since the compiler can statically determine this is the only owner,
// it's safe to take the value out. In fact, in newer versions of Rust,
// `UnsafeCell::into_inner` has been marked safe.
//
// To support older version (1.21), the unsafe block is still here.
#[allow(unused_unsafe)]
unsafe {
self.value.into_inner()
}
}
}
unsafe impl<T: Send> Send for TryLock<T> {}
unsafe impl<T: Send> Sync for TryLock<T> {}
impl<T: fmt::Debug> fmt::Debug for TryLock<T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
// Used if the TryLock cannot acquire the lock.
struct LockedPlaceholder;
impl fmt::Debug for LockedPlaceholder {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.write_str("<locked>")
}
}
let mut builder = f.debug_struct("TryLock");
if let Some(locked) = self.try_lock() {
builder.field("value", &*locked);
} else {
builder.field("value", &LockedPlaceholder);
}
builder.finish()
}
}
/// A locked value acquired from a `TryLock`.
///
/// The type represents an exclusive view at the underlying value. The lock is
/// released when this type is dropped.
///
/// This type derefs to the underlying value.
#[must_use = "TryLock will immediately unlock if not used"]
pub struct Locked<'a, T: 'a> {
lock: &'a TryLock<T>,
order: Ordering,
/// Suppresses Send and Sync autotraits for `struct Locked`.
_p: PhantomData<*mut T>,
}
impl<'a, T> Deref for Locked<'a, T> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
unsafe { &*self.lock.value.get() }
}
}
impl<'a, T> DerefMut for Locked<'a, T> {
#[inline]
fn deref_mut(&mut self) -> &mut T {
unsafe { &mut *self.lock.value.get() }
}
}
impl<'a, T> Drop for Locked<'a, T> {
#[inline]
fn drop(&mut self) {
self.lock.is_locked.store(false, self.order);
}
}
impl<'a, T: fmt::Debug> fmt::Debug for Locked<'a, T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
#[cfg(test)]
mod tests {
use super::TryLock;
#[test]
fn fmt_debug() {
let lock = TryLock::new(5);
assert_eq!(format!("{:?}", lock), "TryLock { value: 5 }");
let locked = lock.try_lock().unwrap();
assert_eq!(format!("{:?}", locked), "5");
assert_eq!(format!("{:?}", lock), "TryLock { value: <locked> }");
}
}