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use super::underlying::{Link, Underlying};
use super::{Barrier, Ptr};
use std::ops::Deref;
use std::ptr::NonNull;
/// [`Arc`] is a reference-counted handle to an instance.
#[derive(Debug)]
pub struct Arc<T: 'static> {
instance_ptr: NonNull<Underlying<T>>,
}
impl<T: 'static> Arc<T> {
/// Creates a new instance of [`Arc`].
///
/// # Examples
///
/// ```
/// use scc::ebr::Arc;
///
/// let arc: Arc<usize> = Arc::new(31);
/// ```
#[inline]
pub fn new(t: T) -> Arc<T> {
let boxed = Box::new(Underlying::new(t));
Arc {
instance_ptr: unsafe { NonNull::new_unchecked(Box::into_raw(boxed)) },
}
}
/// Generates a [`Ptr`] out of the [`Arc`].
///
/// # Examples
///
/// ```
/// use scc::ebr::{Arc, Barrier};
///
/// let arc: Arc<usize> = Arc::new(37);
/// let barrier = Barrier::new();
/// let ptr = arc.ptr(&barrier);
/// assert_eq!(*ptr.as_ref().unwrap(), 37);
/// ```
#[must_use]
#[inline]
pub fn ptr<'b>(&self, _barrier: &'b Barrier) -> Ptr<'b, T> {
Ptr::from(self.instance_ptr.as_ptr())
}
/// Returns a mutable reference to the underlying instance if the instance is exclusively
/// owned.
///
/// # Examples
///
/// ```
/// use scc::ebr::Arc;
///
/// let mut arc: Arc<usize> = Arc::new(38);
/// *arc.get_mut().unwrap() += 1;
/// assert_eq!(*arc, 39);
/// ```
#[inline]
pub fn get_mut(&mut self) -> Option<&mut T> {
unsafe { self.instance_ptr.as_mut().get_mut() }
}
/// Drops the underlying instance if the last reference is dropped.
///
/// The instance is not passed to the garbage collector when the last reference is dropped,
/// instead the method drops the instance immediately. The semantics is the same as that of
/// [`std::sync::Arc`].
///
/// # Safety
///
/// The caller must ensure that there is no [`Ptr`] pointing to the instance.
///
/// # Examples
///
/// ```
/// use scc::ebr::Arc;
/// use std::sync::atomic::AtomicBool;
/// use std::sync::atomic::Ordering::Relaxed;
///
/// static DROPPED: AtomicBool = AtomicBool::new(false);
/// struct T(&'static AtomicBool);
/// impl Drop for T {
/// fn drop(&mut self) {
/// self.0.store(true, Relaxed);
/// }
/// }
///
/// let arc: Arc<T> = Arc::new(T(&DROPPED));
///
/// unsafe {
/// arc.drop_in_place();
/// }
/// assert!(DROPPED.load(Relaxed));
/// ```
pub unsafe fn drop_in_place(mut self) {
if self.underlying().drop_ref() {
self.instance_ptr.as_mut().free();
std::mem::forget(self);
}
}
/// Creates a new [`Arc`] from the given pointer.
pub(super) fn from(ptr: NonNull<Underlying<T>>) -> Arc<T> {
debug_assert_ne!(
unsafe {
ptr.as_ref()
.ref_cnt()
.load(std::sync::atomic::Ordering::Relaxed)
},
0
);
Arc { instance_ptr: ptr }
}
/// Returns its underlying pointer.
pub(super) fn raw_ptr(&self) -> *mut Underlying<T> {
self.instance_ptr.as_ptr()
}
/// Drops the reference, and returns the underlying pointer if the last reference was
/// dropped.
pub(super) fn drop_ref(&self) -> Option<*mut Underlying<T>> {
if self.underlying().drop_ref() {
Some(self.instance_ptr.as_ptr())
} else {
None
}
}
/// Returns a reference to the underlying instance.
fn underlying(&self) -> &Underlying<T> {
unsafe { self.instance_ptr.as_ref() }
}
}
impl<T: 'static> Clone for Arc<T> {
#[inline]
fn clone(&self) -> Self {
debug_assert_ne!(
self.underlying()
.ref_cnt()
.load(std::sync::atomic::Ordering::Relaxed),
0
);
self.underlying().add_ref();
Self {
instance_ptr: self.instance_ptr,
}
}
}
impl<T: 'static> Deref for Arc<T> {
type Target = T;
#[inline]
fn deref(&self) -> &Self::Target {
&**self.underlying()
}
}
impl<T: 'static> Drop for Arc<T> {
#[inline]
fn drop(&mut self) {
if self.underlying().drop_ref() {
let barrier = Barrier::new();
barrier.reclaim_underlying(self.instance_ptr.as_ptr());
}
}
}
unsafe impl<T: 'static + Send> Send for Arc<T> {}
unsafe impl<T: 'static + Sync> Sync for Arc<T> {}
#[cfg(test)]
mod test {
use super::*;
use std::sync::atomic::Ordering::Relaxed;
use std::sync::atomic::{AtomicBool, AtomicUsize};
struct A(AtomicUsize, usize, &'static AtomicBool);
impl Drop for A {
fn drop(&mut self) {
self.2.swap(true, Relaxed);
}
}
#[test]
fn arc() {
static DESTROYED: AtomicBool = AtomicBool::new(false);
let mut arc = Arc::new(A(AtomicUsize::new(10), 10, &DESTROYED));
if let Some(mut_ref) = arc.get_mut() {
mut_ref.1 += 1;
}
arc.0.fetch_add(1, Relaxed);
assert_eq!(arc.deref().0.load(Relaxed), 11);
assert_eq!(arc.deref().1, 11);
let mut arc_cloned = arc.clone();
assert!(arc_cloned.get_mut().is_none());
arc_cloned.0.fetch_add(1, Relaxed);
assert_eq!(arc_cloned.deref().0.load(Relaxed), 12);
assert_eq!(arc_cloned.deref().1, 11);
let mut arc_cloned_again = arc_cloned.clone();
assert!(arc_cloned_again.get_mut().is_none());
assert_eq!(arc_cloned_again.deref().0.load(Relaxed), 12);
assert_eq!(arc_cloned_again.deref().1, 11);
drop(arc);
assert!(!DESTROYED.load(Relaxed));
assert!(arc_cloned_again.get_mut().is_none());
drop(arc_cloned);
assert!(!DESTROYED.load(Relaxed));
assert!(arc_cloned_again.get_mut().is_some());
drop(arc_cloned_again);
while !DESTROYED.load(Relaxed) {
drop(Barrier::new());
}
}
#[test]
fn arc_send() {
static DESTROYED: AtomicBool = AtomicBool::new(false);
let arc = Arc::new(A(AtomicUsize::new(14), 14, &DESTROYED));
let arc_cloned = arc.clone();
let thread = std::thread::spawn(move || {
assert_eq!(arc_cloned.0.load(Relaxed), arc_cloned.1);
});
assert!(thread.join().is_ok());
assert_eq!(arc.0.load(Relaxed), arc.1);
}
#[test]
fn arc_arc_send() {
static DESTROYED: AtomicBool = AtomicBool::new(false);
let arc_arc = Arc::new(A(AtomicUsize::new(14), 14, &DESTROYED));
let arc_arc_cloned = arc_arc.clone();
let thread = std::thread::spawn(move || {
assert_eq!(arc_arc_cloned.0.load(Relaxed), 14);
});
assert!(thread.join().is_ok());
assert_eq!(arc_arc.0.load(Relaxed), 14);
unsafe {
arc_arc.drop_in_place();
}
assert!(DESTROYED.load(Relaxed));
}
#[test]
fn arc_nested() {
static DESTROYED: AtomicBool = AtomicBool::new(false);
struct Nest(Arc<A>);
let nested_arc = Arc::new(Nest(Arc::new(A(AtomicUsize::new(10), 10, &DESTROYED))));
assert!(!DESTROYED.load(Relaxed));
drop(nested_arc);
while !DESTROYED.load(Relaxed) {
drop(Barrier::new());
}
}
}