Add RawRc type

Author: EFanZh

library/alloc/src/raw_rc/mod.rs

@@ -66,6 +66,7 @@ use core::cell::UnsafeCell;
 use core::mem;
 use core::sync::atomic::Atomic;
 
+mod raw_rc;
 mod raw_weak;
 mod rc_alloc;
 mod rc_layout;

library/alloc/src/raw_rc/raw_rc.rs

@@ -0,0 +1,357 @@
+use core::alloc::Allocator;
+use core::cell::UnsafeCell;
+#[cfg(not(no_global_oom_handling))]
+use core::clone::CloneToUninit;
+use core::marker::PhantomData;
+#[cfg(not(no_global_oom_handling))]
+use core::mem::{self, DropGuard};
+#[cfg(not(no_global_oom_handling))]
+use core::ops::DerefMut;
+#[cfg(not(no_global_oom_handling))]
+use core::ptr;
+use core::ptr::NonNull;
+
+#[cfg(not(no_global_oom_handling))]
+use crate::raw_rc::MakeMutStrategy;
+use crate::raw_rc::RefCounter;
+#[cfg(not(no_global_oom_handling))]
+use crate::raw_rc::raw_weak;
+use crate::raw_rc::raw_weak::RawWeak;
+#[cfg(not(no_global_oom_handling))]
+use crate::raw_rc::rc_alloc;
+#[cfg(not(no_global_oom_handling))]
+use crate::raw_rc::rc_layout::RcLayout;
+use crate::raw_rc::rc_value_pointer::RcValuePointer;
+
+/// Decrements strong reference count in a reference-counted allocation with a value object that is
+/// pointed to by `value_ptr`.
+#[inline]
+unsafe fn decrement_strong_ref_count<R>(value_ptr: RcValuePointer) -> bool
+where
+    R: RefCounter,
+{
+    unsafe { R::from_raw_counter(value_ptr.strong_count_ptr().as_ref()).decrement() }
+}
+
+/// Increments strong reference count in a reference-counted allocation with a value object that is
+/// pointed to by `value_ptr`.
+#[inline]
+unsafe fn increment_strong_ref_count<R>(value_ptr: RcValuePointer)
+where
+    R: RefCounter,
+{
+    unsafe { R::from_raw_counter(value_ptr.strong_count_ptr().as_ref()).increment() };
+}
+
+#[inline]
+unsafe fn is_unique<R>(value_ptr: RcValuePointer) -> bool
+where
+    R: RefCounter,
+{
+    let ref_counts = unsafe { value_ptr.ref_counts_ptr().as_ref() };
+
+    unsafe {
+        R::is_unique(R::from_raw_counter(&ref_counts.strong), R::from_raw_counter(&ref_counts.weak))
+    }
+}
+
+/// Base implementation of a strong pointer. `RawRc` does not implement `Drop`, user should call
+/// `RawRc::drop` manually to drop this object.
+#[repr(transparent)]
+pub(crate) struct RawRc<T, A>
+where
+    T: ?Sized,
+{
+    /// A `RawRc` is just a non-dangling `RawWeak` that has a strong reference count that is owned
+    /// by the `RawRc` object. The weak pointer is always non-dangling.
+    weak: RawWeak<T, A>,
+
+    // Defines the ownership of `T` for drop-check.
+    _phantom_data: PhantomData<T>,
+}
+
+impl<T, A> RawRc<T, A>
+where
+    T: ?Sized,
+{
+    /// # Safety
+    ///
+    /// - `ptr` points to a value inside a reference-counted allocation.
+    /// - The allocation can be freed by `A::default()`.
+    pub(crate) unsafe fn from_raw(ptr: NonNull<T>) -> Self
+    where
+        A: Default,
+    {
+        unsafe { Self::from_raw_parts(ptr, A::default()) }
+    }
+
+    /// # Safety
+    ///
+    /// - `ptr` points to a value inside a reference-counted allocation.
+    /// - The allocation can be freed by `alloc`.
+    pub(crate) unsafe fn from_raw_parts(ptr: NonNull<T>, alloc: A) -> Self {
+        unsafe { Self::from_weak(RawWeak::from_raw_parts(ptr, alloc)) }
+    }
+
+    /// # Safety
+    ///
+    /// `weak` must have at least one unowned strong reference count. The newly created `RawRc` will
+    /// take the ownership of exactly one strong reference count.
+    pub(super) unsafe fn from_weak(weak: RawWeak<T, A>) -> Self {
+        Self { weak, _phantom_data: PhantomData }
+    }
+
+    pub(crate) fn allocator(&self) -> &A {
+        &self.weak.allocator()
+    }
+
+    pub(crate) fn as_ptr(&self) -> NonNull<T> {
+        self.weak.as_ptr()
+    }
+
+    pub(crate) unsafe fn cast<U>(self) -> RawRc<U, A> {
+        unsafe { RawRc::from_weak(self.weak.cast()) }
+    }
+
+    #[inline]
+    pub(crate) unsafe fn cast_with<U, F>(self, f: F) -> RawRc<U, A>
+    where
+        U: ?Sized,
+        F: FnOnce(NonNull<T>) -> NonNull<U>,
+    {
+        unsafe { RawRc::from_weak(self.weak.cast_with(f)) }
+    }
+
+    #[inline]
+    pub(crate) unsafe fn clone<R>(&self) -> Self
+    where
+        A: Clone,
+        R: RefCounter,
+    {
+        unsafe {
+            increment_strong_ref_count::<R>(self.value_ptr());
+
+            Self::from_raw_parts(self.weak.as_ptr(), self.allocator().clone())
+        }
+    }
+
+    pub(crate) unsafe fn decrement_strong_count<R: RefCounter>(ptr: NonNull<T>)
+    where
+        A: Allocator + Default,
+    {
+        unsafe { Self::decrement_strong_count_in::<R>(ptr, A::default()) };
+    }
+
+    pub(crate) unsafe fn decrement_strong_count_in<R: RefCounter>(ptr: NonNull<T>, alloc: A)
+    where
+        A: Allocator,
+    {
+        unsafe { RawRc::from_raw_parts(ptr, alloc).drop::<R>() };
+    }
+
+    pub(crate) unsafe fn increment_strong_count<R: RefCounter>(ptr: NonNull<T>) {
+        unsafe { increment_strong_ref_count::<R>(RcValuePointer::from_value_ptr(ptr.cast())) };
+    }
+
+    pub(crate) unsafe fn downgrade<R>(&self) -> RawWeak<T, A>
+    where
+        A: Clone,
+        R: RefCounter,
+    {
+        unsafe fn inner<R>(value_ptr: RcValuePointer)
+        where
+            R: RefCounter,
+        {
+            unsafe {
+                R::from_raw_counter(value_ptr.weak_count_ptr().as_ref()).downgrade_increment_weak();
+            }
+        }
+
+        unsafe {
+            inner::<R>(self.value_ptr());
+
+            RawWeak::from_raw_parts(self.weak.as_ptr(), self.allocator().clone())
+        }
+    }
+
+    #[inline]
+    pub(crate) unsafe fn drop<R>(&mut self)
+    where
+        A: Allocator,
+        R: RefCounter,
+    {
+        let is_last_strong_ref = unsafe { decrement_strong_ref_count::<R>(self.value_ptr()) };
+
+        if is_last_strong_ref {
+            unsafe { self.weak.assume_init_drop::<R>() }
+        }
+    }
+
+    pub(crate) unsafe fn get_mut<R>(&mut self) -> Option<&mut T>
+    where
+        R: RefCounter,
+    {
+        unsafe fn inner<R>(value_ptr: RcValuePointer) -> Option<RcValuePointer>
+        where
+            R: RefCounter,
+        {
+            unsafe { is_unique::<R>(value_ptr) }.then_some(value_ptr)
+        }
+
+        let (ptr, metadata) = self.weak.as_ptr().to_raw_parts();
+
+        unsafe { inner::<R>(RcValuePointer::from_value_ptr(ptr)) }
+            .map(|ptr| unsafe { NonNull::from_raw_parts(ptr.as_ptr(), metadata).as_mut() })
+    }
+
+    /// Returns a mutable reference to the contained value.
+    ///
+    /// # Safety
+    ///
+    /// No other active references to the contained value should exist, and no new references to the
+    /// contained value will be acquired for the duration of the returned borrow.
+    pub(crate) unsafe fn get_mut_unchecked(&mut self) -> &mut T {
+        // SAFETY: The caller guarantees that we can access the contained value exclusively. Note
+        // that we can't create mutable references that have access to reference counters, because
+        // the caller only guarantee exclusive access to the contained value, not the reference
+        // counters.
+        unsafe { self.weak.as_ptr().as_mut() }
+    }
+
+    pub(crate) fn into_raw(self) -> NonNull<T> {
+        self.weak.into_raw()
+    }
+
+    pub(crate) fn into_raw_parts(self) -> (NonNull<T>, A) {
+        self.weak.into_raw_parts()
+    }
+
+    #[cfg(not(no_global_oom_handling))]
+    pub(crate) unsafe fn make_mut<R>(&mut self) -> &mut T
+    where
+        T: CloneToUninit,
+        A: Allocator + Clone,
+        R: RefCounter,
+    {
+        /// Returns a drop guard that sets the pointer in `rc` to `ptr` on drop.
+        ///
+        /// # Safety
+        ///
+        /// - `ptr` must point to a valid reference counted value that can be deallocated with the
+        ///   allocator associated with `rc`.
+        /// - The value pointed to by `ptr` must have an unowned strong reference count that can be
+        ///   taken ownership by `rc`.
+        unsafe fn set_rc_ptr_on_drop<'a, T, A>(
+            rc: &'a mut RawRc<T, A>,
+            ptr: NonNull<T>,
+        ) -> impl DerefMut<Target = &'a mut RawRc<T, A>>
+        where
+            T: ?Sized,
+        {
+            DropGuard::new(rc, move |rc| unsafe { rc.weak.set_ptr(ptr) })
+        }
+
+        unsafe {
+            let ref_counts = self.ref_counts();
+
+            if let Some(strategy) = R::make_mut(
+                R::from_raw_counter(&ref_counts.strong),
+                R::from_raw_counter(&ref_counts.weak),
+            ) {
+                let rc_layout = RcLayout::from_value_ptr_unchecked(self.weak.as_ptr());
+
+                match strategy {
+                    MakeMutStrategy::Move => {
+                        // `R::make_mut` has made strong reference count to zero, so the `RawRc`
+                        // object is essentially a `RawWeak` object but has its value initialized.
+                        // This means we are the only owner of the value and we can safely move the
+                        // value into a new allocation.
+
+                        // This guarantees to drop old `RawRc` object even if the allocation
+                        // panics.
+
+                        let guard = raw_weak::new_weak_guard::<T, A, R>(&mut self.weak);
+
+                        let new_ptr = rc_alloc::allocate_with_bytes_in::<A, 1>(
+                            guard.as_ptr().cast(),
+                            &guard.allocator(),
+                            rc_layout,
+                        );
+
+                        // No panic happens, defuse the guard.
+                        mem::forget(guard);
+
+                        let new_ptr = NonNull::from_raw_parts(
+                            new_ptr.as_ptr(),
+                            ptr::metadata(self.weak.as_ptr().as_ptr()),
+                        );
+
+                        // Ensure the value pointer in `self` is updated to `new_ptr`.
+                        let mut update_ptr_on_drop = set_rc_ptr_on_drop(self, new_ptr);
+
+                        // `MakeMutStrategy::Move` guarantees that the strong count is zero, also we
+                        // have copied the value to a new allocation, so we can pretend the original
+                        // `RawRc` is now essentially an `RawWeak` object, we can call the `RawWeak`
+                        // destructor to finish the cleanup.
+                        update_ptr_on_drop.weak.drop_unchecked::<R>();
+                    }
+                    MakeMutStrategy::Clone => {
+                        // There are multiple owners of the value, we need to clone the value into a
+                        // new allocation.
+
+                        let new_ptr = rc_alloc::allocate_with_in::<A, _, 1>(
+                            &self.allocator(),
+                            rc_layout,
+                            |dst_ptr| {
+                                T::clone_to_uninit(
+                                    self.as_ptr().as_ref(),
+                                    dst_ptr.as_ptr().as_ptr().cast(),
+                                )
+                            },
+                        );
+
+                        let new_ptr = NonNull::from_raw_parts(
+                            new_ptr.as_ptr(),
+                            ptr::metadata(self.weak.as_ptr().as_ptr()),
+                        );
+
+                        // Ensure the value pointer in `self` is updated to `new_ptr`.
+                        let mut update_ptr_on_drop = set_rc_ptr_on_drop(self, new_ptr);
+
+                        // Manually drop old `RawRc`.
+                        update_ptr_on_drop.drop::<R>();
+                    }
+                }
+            }
+
+            self.get_mut_unchecked()
+        }
+    }
+
+    pub(crate) fn ptr_eq(&self, other: &Self) -> bool {
+        RawWeak::ptr_eq(&self.weak, &other.weak)
+    }
+
+    pub(crate) fn ptr_ne(&self, other: &Self) -> bool {
+        RawWeak::ptr_ne(&self.weak, &other.weak)
+    }
+
+    #[cfg(not(no_global_oom_handling))]
+    pub(crate) fn ref_counts(&self) -> &crate::raw_rc::RefCounts {
+        unsafe { self.weak.ref_counts_unchecked() }
+    }
+
+    pub(crate) fn strong_count(&self) -> &UnsafeCell<usize> {
+        unsafe { self.weak.strong_count_unchecked() }
+    }
+
+    pub(crate) fn weak_count(&self) -> &UnsafeCell<usize> {
+        unsafe { self.weak.weak_count_unchecked() }
+    }
+
+    #[inline]
+    fn value_ptr(&self) -> RcValuePointer {
+        // SAFETY: `self.weak` is guaranteed to be non-dangling.
+        unsafe { self.weak.value_ptr_unchecked() }
+    }
+}