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use super::ref_counted::RefCounted;
use super::{Guard, Ptr};
use std::mem::forget;
use std::ops::Deref;
use std::panic::UnwindSafe;
use std::ptr::{addr_of, NonNull};
/// [`Shared`] is a reference-counted handle to an instance.
///
/// The instance is passed to the EBR garbage collector when the last strong reference is dropped.
#[derive(Debug)]
pub struct Shared<T> {
instance_ptr: *const RefCounted<T>,
}
impl<T: 'static> Shared<T> {
/// Creates a new [`Shared`].
///
/// The type of the instance must be determined at compile-time, must not contain non-static
/// references, and must not be a non-static reference since the instance can, theoretically,
/// survive the process. For instance, `struct Disallowed<'l, T>(&'l T)` is not allowed,
/// because an instance of the type cannot outlive `'l` whereas the garbage collector does not
/// guarantee that the instance is dropped within `'l`.
///
/// # Examples
///
/// ```
/// use sdd::Shared;
///
/// let shared: Shared<usize> = Shared::new(31);
/// ```
#[inline]
pub fn new(t: T) -> Self {
Self {
instance_ptr: RefCounted::new_shared(t),
}
}
}
impl<T> Shared<T> {
/// Creates a new [`Shared`] without checking the lifetime of `T`.
///
/// # Safety
///
/// `T::drop` can be run after the last strong reference is dropped, therefore it is safe only
/// if `T::drop` does not access short-lived data or [`std::mem::needs_drop`] is `false` for
/// `T`. Otherwise, the instance must be manually dropped by invoking [`Self::drop_in_place`]
/// before the lifetime of `T` is reached.
///
/// # Examples
///
/// ```
/// use sdd::Shared;
///
/// let hello = String::from("hello");
/// let shared: Shared<&str> = unsafe { Shared::new_unchecked(hello.as_str()) };
///
/// assert!(unsafe { shared.drop_in_place() });
/// ```
#[inline]
pub unsafe fn new_unchecked(t: T) -> Self {
Self {
instance_ptr: RefCounted::new_shared(t),
}
}
/// Returns a [`Ptr`] to the instance that may live as long as the supplied [`Guard`].
///
/// # Examples
///
/// ```
/// use sdd::{Guard, Shared};
///
/// let shared: Shared<usize> = Shared::new(37);
/// let guard = Guard::new();
/// let ptr = shared.get_guarded_ptr(&guard);
/// drop(shared);
///
/// assert_eq!(*ptr.as_ref().unwrap(), 37);
/// ```
#[inline]
#[must_use]
pub fn get_guarded_ptr<'g>(&self, _guard: &'g Guard) -> Ptr<'g, T> {
Ptr::from(self.instance_ptr)
}
/// Returns a reference to the instance that may live as long as the supplied [`Guard`].
///
/// # Examples
///
/// ```
/// use sdd::{Guard, Shared};
///
/// let shared: Shared<usize> = Shared::new(37);
/// let guard = Guard::new();
/// let ref_b = shared.get_guarded_ref(&guard);
/// drop(shared);
///
/// assert_eq!(*ref_b, 37);
/// ```
#[inline]
#[must_use]
pub fn get_guarded_ref<'g>(&self, _guard: &'g Guard) -> &'g T {
unsafe { std::mem::transmute::<&T, _>(&**self) }
}
/// Returns a mutable reference to the instance if the [`Shared`] is holding the only strong
/// reference.
///
/// # Safety
///
/// The method is `unsafe` since there can be a [`Ptr`] to the instance without holding a
/// strong reference.
///
/// # Examples
///
/// ```
/// use sdd::Shared;
///
/// let mut shared: Shared<usize> = Shared::new(38);
/// unsafe {
/// *shared.get_mut().unwrap() += 1;
/// }
/// assert_eq!(*shared, 39);
/// ```
#[inline]
pub unsafe fn get_mut(&mut self) -> Option<&mut T> {
self.instance_ptr
.cast_mut()
.as_mut()
.and_then(|r| r.get_mut_shared())
}
/// Provides a raw pointer to the instance.
///
/// # Examples
///
/// ```
/// use sdd::Shared;
/// use std::sync::atomic::AtomicBool;
/// use std::sync::atomic::Ordering::Relaxed;
///
/// let shared: Shared<usize> = Shared::new(10);
/// let shared_clone: Shared<usize> = shared.clone();
///
/// assert_eq!(shared.as_ptr(), shared_clone.as_ptr());
/// assert_eq!(unsafe { *shared.as_ptr() }, unsafe { *shared_clone.as_ptr() });
/// ```
#[inline]
#[must_use]
pub fn as_ptr(&self) -> *const T {
addr_of!(**self)
}
/// Releases the strong reference by passing `self` to the given [`Guard`].
///
/// Returns `true` if the last reference was released.
///
/// # Examples
///
/// ```
/// use sdd::Shared;
///
/// let shared: Shared<usize> = Shared::new(47);
/// let shared_clone = shared.clone();
/// assert!(!shared.release());
/// assert!(shared_clone.release());
/// ```
#[inline]
#[must_use]
pub fn release(self) -> bool {
let released = if unsafe { (*self.instance_ptr).drop_ref() } {
RefCounted::pass_to_collector(self.instance_ptr.cast_mut());
true
} else {
false
};
forget(self);
released
}
/// Drops the instance immediately if it has held the last reference to the instance.
///
/// Returns `true` if the instance was dropped.
///
/// # Safety
///
/// The caller must ensure that there is no [`Ptr`] pointing to the instance.
///
/// # Examples
///
/// ```
/// use sdd::Shared;
/// 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 shared: Shared<T> = Shared::new(T(&DROPPED));
/// let shared_clone = shared.clone();
///
/// unsafe {
/// assert!(!shared.drop_in_place());
/// assert!(!DROPPED.load(Relaxed));
/// assert!(shared_clone.drop_in_place());
/// assert!(DROPPED.load(Relaxed));
/// }
/// ```
#[inline]
#[must_use]
pub unsafe fn drop_in_place(self) -> bool {
let dropped = if (*self.instance_ptr).drop_ref() {
drop(Box::from_raw(self.instance_ptr.cast_mut()));
true
} else {
false
};
forget(self);
dropped
}
/// Creates a new [`Shared`] from the given pointer.
#[inline]
pub(super) fn from(ptr: NonNull<RefCounted<T>>) -> Self {
Self {
instance_ptr: ptr.as_ptr(),
}
}
/// Returns a pointer to the [`RefCounted`].
#[inline]
pub(super) const fn underlying_ptr(&self) -> *const RefCounted<T> {
self.instance_ptr
}
}
impl<T> AsRef<T> for Shared<T> {
#[inline]
fn as_ref(&self) -> &T {
unsafe { &*self.instance_ptr }
}
}
impl<T> Clone for Shared<T> {
#[inline]
fn clone(&self) -> Self {
unsafe { (*self.instance_ptr).add_ref() }
Self {
instance_ptr: self.instance_ptr,
}
}
}
impl<T> Deref for Shared<T> {
type Target = T;
#[inline]
fn deref(&self) -> &Self::Target {
self.as_ref()
}
}
impl<T> Drop for Shared<T> {
#[inline]
fn drop(&mut self) {
if unsafe { (*self.instance_ptr).drop_ref() } {
RefCounted::pass_to_collector(self.instance_ptr.cast_mut());
}
}
}
impl<'g, T> TryFrom<Ptr<'g, T>> for Shared<T> {
type Error = Ptr<'g, T>;
#[inline]
fn try_from(ptr: Ptr<'g, T>) -> Result<Self, Self::Error> {
if let Some(shared) = ptr.get_shared() {
Ok(shared)
} else {
Err(ptr)
}
}
}
unsafe impl<T: Send> Send for Shared<T> {}
unsafe impl<T: Sync> Sync for Shared<T> {}
impl<T: UnwindSafe> UnwindSafe for Shared<T> {}