sdd/

shared.rs

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> {}