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//! A safe, strongly typed (generic) atomic pointer abstraction to
//! build datastructures, and lock-free algorithms on top of. Only
//! uses `libstd`.
//!
//! The standard library contains an `AtomicPtr` type, which by itself
//! isn't very ergonomic to use, because it deals with raw pointers.
//! This library assumes that types can always be heap allocated,
//! wrapping them in a `Box<T>`, and provides a nicer (and safe!)
//! abstraction for `std::sync::atomic::AtomicPtr`. Using this crate
//! is fairely self-explanatory:
//!
//! ```rust
//! use atomptr::AtomPtr;
//!
//! struct MyData { name: String }
//! let data = MyData { name: "Kookie".into() };
//!
//! let a = AtomPtr::new(data);
//! println!("Name is: {}", a.get_ref().name);
//!
//! let old_ref = a.swap(MyData { name: "Bob".into() });
//! println!("Name now is: {}, was {}", a.get_ref().name, old_ref.name);
//! ```
//!
//! Note that the type that is returned by `get_ref` and `swap` is
//! `Ref<T>`, which means that the old data is not de-allocated after
//! a swap, before this last reference goes out of scope. You can of
//! course always manually call `drop()` on it.
use std::sync::{
atomic::{AtomicPtr, Ordering},
Arc,
};
use std::{cmp::PartialEq, ops::Deref};
/// An alias for a referenced pointer
pub struct Ref<T> {
inner: Box<Arc<T>>,
ptr: *const Arc<T>,
}
impl<T> Ref<T> {
/// Consume this Ref wrapper to yield the underlying `Arc<T>`
///
/// If you want to take ownership of the underlying type data, and
/// you can prove that only one strong-reference Arc exists to
/// this type, you can use `std::arc::Arc::try_unwrap()` to peel
/// the reference counter and take exclusive ownership.
pub fn consume(self) -> Arc<T> {
*self.inner
}
fn as_ptr(&self) -> *const Arc<T> {
self.ptr
}
}
impl<T: Copy> Ref<Option<T>> {
/// Filter out references that internally contain `None`
pub fn is_some(&self) -> bool {
self.inner.is_some()
}
/// Unwrap the inner Option, copying out the data
pub fn copy_unwrap(&self) -> T {
self.inner.as_ref().unwrap()
}
}
impl<T> Deref for Ref<T> {
type Target = Arc<T>;
fn deref(&self) -> &Self::Target {
&self.inner
}
}
/// Result from a CAS operation
pub enum CasResult<T> {
Success(Ref<T>),
Failure(Ref<T>),
}
impl<T> CasResult<T> {
pub fn success(&self) -> bool {
match self {
Self::Success(_) => true,
Self::Failure(_) => false,
}
}
pub fn inner(self) -> Ref<T> {
match self {
Self::Success(r) => r,
Self::Failure(r) => r,
}
}
}
/// A safe atomic pointer wrapper
#[derive(Clone, Debug)]
pub struct AtomPtr<T> {
inner: Arc<AtomicPtr<Arc<T>>>,
}
// Implement Default for all T that implement default
impl<T: Default> Default for AtomPtr<T> {
fn default() -> Self {
Self::new(T::default())
}
}
impl<T> PartialEq for AtomPtr<T> {
fn eq(&self, other: &Self) -> bool {
Arc::ptr_eq(&self.get_ref().inner, &other.get_ref().inner)
}
}
impl<T> Drop for AtomPtr<T> {
fn drop(&mut self) {
if Arc::strong_count(&self.inner) == 1 {
let ptr = self.inner.load(Ordering::Acquire);
let _b = unsafe { Box::from_raw(ptr) };
// Let _b go out of scope to clean up memory
}
}
}
impl<T> AtomPtr<T> {
fn make_raw_ptr(t: T) -> *mut Arc<T> {
Box::into_raw(Box::new(Arc::new(t)))
}
/// Create a new atomic pointer for a type
pub fn new(t: T) -> Self {
let ptr = Self::make_raw_ptr(t);
let inner = Arc::new(AtomicPtr::from(ptr));
Self { inner }
}
/// Get an immutable reference to the current value
pub fn get_ref(&self) -> Ref<T> {
let ptr = self.inner.load(Ordering::Relaxed);
let b = unsafe { Box::from_raw(ptr) };
let arc = Arc::clone(&*b);
std::mem::forget(b);
Ref {
inner: Box::new(arc),
ptr,
}
}
/// Swap the data entry with a new value, returning the old
pub fn swap(&self, new: T) -> Ref<T> {
let new = Self::make_raw_ptr(new);
let prev = self.inner.swap(new, Ordering::AcqRel);
let inner = unsafe { Box::from_raw(prev) };
Ref { inner, ptr: prev }
}
/// Compare and swap this pointer for a new one
///
/// This API takes two pieces of data: a previously handed out
/// data reference, and a new piece of data. The reference data
/// will be compared to the existing value, and if they match, the
/// new data is inserted into the pointer.
///
/// This API follows the
/// [`AtomicPtr::compare_exchange`](std::sync::atomic::AtomicPtr::compare_exchange)
/// API. The deprecated `compare_and_swap` API is not
/// implemented.
pub fn compare_exchange(&self, prev: Ref<T>, new: T) -> CasResult<T> {
let new = Self::make_raw_ptr(new);
let prev: *const Arc<T> = prev.as_ptr();
let prev_mut = prev as *mut Arc<T>;
match self
.inner
.compare_exchange(prev_mut, new, Ordering::SeqCst, Ordering::Acquire)
{
Ok(t) => CasResult::Success(Ref {
inner: unsafe { Box::from_raw(t) },
ptr: t,
}),
Err(t) => CasResult::Failure(Ref {
inner: unsafe { Box::from_raw(t) },
ptr: t,
}),
}
}
/// Compare and swap this pointer for a new one
///
/// Use this variant when called in a loop
pub fn compare_exchange_weak(&self, prev: Ref<T>, new: T) -> CasResult<T> {
let new = Self::make_raw_ptr(new);
let prev: *const Arc<T> = prev.as_ptr();
let prev_mut = prev as *mut Arc<T>;
match self
.inner
.compare_exchange_weak(prev_mut, new, Ordering::SeqCst, Ordering::Acquire)
{
Ok(t) => CasResult::Success(Ref {
inner: unsafe { Box::from_raw(t) },
ptr: t,
}),
Err(t) => CasResult::Failure(Ref {
inner: unsafe { Box::from_raw(t) },
ptr: t,
}),
}
}
}
#[cfg(test)]
#[derive(Clone, Debug, PartialEq)]
struct TestStruct {
name: String,
}
#[test]
fn cloned() {
let ts = TestStruct {
name: "Hello".into(),
};
let ptr1 = AtomPtr::new(ts);
let ptr2 = ptr1.clone();
assert_eq!(ptr1, ptr2);
}
#[test]
fn swap() {
let ts1 = TestStruct {
name: "Hello 1".into(),
};
let ts2 = TestStruct {
name: "Hello 2".into(),
};
// Make an AtomPtr with some data
let ptr = AtomPtr::new(ts1.clone());
assert_eq!(ptr.get_ref().name, "Hello 1".to_string());
// Swap the data
let still_ts1 = ptr.swap(ts2);
assert_eq!(ptr.get_ref().name, "Hello 2".to_string());
// But the old ref is still valid
assert_eq!(ts1, *still_ts1.as_ref());
}
#[test]
fn compare_exchange() {
let ts1 = TestStruct {
name: "Hello 1".into(),
};
let ts2 = TestStruct {
name: "Hello 2".into(),
};
let ts3 = TestStruct {
name: "Hello 3".into(),
};
// Make an AtomPtr with some data
let ptr = AtomPtr::new(ts1.clone());
assert_eq!(ptr.get_ref().name, "Hello 1".to_string());
// Swap the data
let still_ts1 = ptr.compare_exchange(ptr.get_ref(), ts2.clone()).inner();
assert_eq!(ptr.get_ref().name, "Hello 2".to_string());
let still_ts2 = ptr.compare_exchange(ptr.get_ref(), ts3).inner();
assert_eq!(ptr.get_ref().name, "Hello 3".to_string());
// But the old ref is still valid
assert_eq!(ts1, *still_ts1.as_ref());
assert_eq!(ts2, *still_ts2.as_ref());
}
#[test]
fn take_from_swap() {
let ts1 = TestStruct {
name: "Hello 1".into(),
};
let ts2 = TestStruct {
name: "Hello 2".into(),
};
// Make an AtomPtr with some data
let ptr = AtomPtr::new(ts1.clone());
assert_eq!(ptr.get_ref().name, "Hello 1".to_string());
// Swap the data
let still_ts1 = ptr.swap(ts2);
assert_eq!(ptr.get_ref().name, "Hello 2".to_string());
assert_eq!(Arc::strong_count(&still_ts1), 1);
// We can now also take ownership of the Arc
let ts1_again = Arc::try_unwrap(still_ts1.consume()).unwrap();
assert_eq!(ts1_again, ts1);
}
#[test]
fn release() {
let ts1 = TestStruct {
name: "Hello world!".into(),
};
let ts2 = TestStruct {
name: "Isn't it lovely outside?".into(),
};
let ptr = AtomPtr::new(ts1);
let first = ptr.get_ref();
println!("Pointer: {:?}", *first);
let prev = ptr.compare_exchange(first, ts2);
if prev.success() {
println!("Successfully swapped pointer values!");
let second = ptr.get_ref();
println!("First: {:?}, Second: {:?}", *prev.inner(), *second);
}
}