diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs index 88c7a12db23ca..b9a92749aaea8 100644 --- a/library/alloc/src/rc.rs +++ b/library/alloc/src/rc.rs @@ -460,42 +460,7 @@ impl Rc { where F: FnOnce(&Weak) -> T, { - // Construct the inner in the "uninitialized" state with a single - // weak reference. - let uninit_ptr: NonNull<_> = Box::leak(Box::new(RcBox { - strong: Cell::new(0), - weak: Cell::new(1), - value: mem::MaybeUninit::::uninit(), - })) - .into(); - - let init_ptr: NonNull> = uninit_ptr.cast(); - - let weak = Weak { ptr: init_ptr, alloc: Global }; - - // It's important we don't give up ownership of the weak pointer, or - // else the memory might be freed by the time `data_fn` returns. If - // we really wanted to pass ownership, we could create an additional - // weak pointer for ourselves, but this would result in additional - // updates to the weak reference count which might not be necessary - // otherwise. - let data = data_fn(&weak); - - let strong = unsafe { - let inner = init_ptr.as_ptr(); - ptr::write(ptr::addr_of_mut!((*inner).value), data); - - let prev_value = (*inner).strong.get(); - debug_assert_eq!(prev_value, 0, "No prior strong references should exist"); - (*inner).strong.set(1); - - Rc::from_inner(init_ptr) - }; - - // Strong references should collectively own a shared weak reference, - // so don't run the destructor for our old weak reference. - mem::forget(weak); - strong + Self::new_cyclic_in(data_fn, Global) } /// Constructs a new `Rc` with uninitialized contents. @@ -762,6 +727,84 @@ impl Rc { } } + /// Constructs a new `Rc` in the given allocator while giving you a `Weak` to the allocation, + /// to allow you to construct a `T` which holds a weak pointer to itself. + /// + /// Generally, a structure circularly referencing itself, either directly or + /// indirectly, should not hold a strong reference to itself to prevent a memory leak. + /// Using this function, you get access to the weak pointer during the + /// initialization of `T`, before the `Rc` is created, such that you can + /// clone and store it inside the `T`. + /// + /// `new_cyclic_in` first allocates the managed allocation for the `Rc`, + /// then calls your closure, giving it a `Weak` to this allocation, + /// and only afterwards completes the construction of the `Rc` by placing + /// the `T` returned from your closure into the allocation. + /// + /// Since the new `Rc` is not fully-constructed until `Rc::new_cyclic_in` + /// returns, calling [`upgrade`] on the weak reference inside your closure will + /// fail and result in a `None` value. + /// + /// # Panics + /// + /// If `data_fn` panics, the panic is propagated to the caller, and the + /// temporary [`Weak`] is dropped normally. + /// + /// # Examples + /// + /// See [`new_cyclic`]. + /// + /// [`new_cyclic`]: Rc::new_cyclic + /// [`upgrade`]: Weak::upgrade + #[cfg(not(no_global_oom_handling))] + #[unstable(feature = "allocator_api", issue = "32838")] + pub fn new_cyclic_in(data_fn: F, alloc: A) -> Rc + where + F: FnOnce(&Weak) -> T, + { + // Construct the inner in the "uninitialized" state with a single + // weak reference. + let (uninit_raw_ptr, alloc) = Box::into_raw_with_allocator(Box::new_in( + RcBox { + strong: Cell::new(0), + weak: Cell::new(1), + value: mem::MaybeUninit::::uninit(), + }, + alloc, + )); + let uninit_ptr: NonNull<_> = (unsafe { &mut *uninit_raw_ptr }).into(); + let init_ptr: NonNull> = uninit_ptr.cast(); + + let weak = Weak { ptr: init_ptr, alloc: alloc }; + + // It's important we don't give up ownership of the weak pointer, or + // else the memory might be freed by the time `data_fn` returns. If + // we really wanted to pass ownership, we could create an additional + // weak pointer for ourselves, but this would result in additional + // updates to the weak reference count which might not be necessary + // otherwise. + let data = data_fn(&weak); + + let strong = unsafe { + let inner = init_ptr.as_ptr(); + ptr::write(ptr::addr_of_mut!((*inner).value), data); + + let prev_value = (*inner).strong.get(); + debug_assert_eq!(prev_value, 0, "No prior strong references should exist"); + (*inner).strong.set(1); + + // Strong references should collectively own a shared weak reference, + // so don't run the destructor for our old weak reference. + // Calling into_raw_with_allocator has the double effect of giving us back the allocator, + // and forgetting the weak reference. + let alloc = weak.into_raw_with_allocator().1; + + Rc::from_inner_in(init_ptr, alloc) + }; + + strong + } + /// Constructs a new `Rc` in the provided allocator, returning an error if the allocation /// fails /// diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs index 43684f31cb723..4d4e84bfda587 100644 --- a/library/alloc/src/sync.rs +++ b/library/alloc/src/sync.rs @@ -450,54 +450,7 @@ impl Arc { where F: FnOnce(&Weak) -> T, { - // Construct the inner in the "uninitialized" state with a single - // weak reference. - let uninit_ptr: NonNull<_> = Box::leak(Box::new(ArcInner { - strong: atomic::AtomicUsize::new(0), - weak: atomic::AtomicUsize::new(1), - data: mem::MaybeUninit::::uninit(), - })) - .into(); - let init_ptr: NonNull> = uninit_ptr.cast(); - - let weak = Weak { ptr: init_ptr, alloc: Global }; - - // It's important we don't give up ownership of the weak pointer, or - // else the memory might be freed by the time `data_fn` returns. If - // we really wanted to pass ownership, we could create an additional - // weak pointer for ourselves, but this would result in additional - // updates to the weak reference count which might not be necessary - // otherwise. - let data = data_fn(&weak); - - // Now we can properly initialize the inner value and turn our weak - // reference into a strong reference. - let strong = unsafe { - let inner = init_ptr.as_ptr(); - ptr::write(ptr::addr_of_mut!((*inner).data), data); - - // The above write to the data field must be visible to any threads which - // observe a non-zero strong count. Therefore we need at least "Release" ordering - // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`. - // - // "Acquire" ordering is not required. When considering the possible behaviours - // of `data_fn` we only need to look at what it could do with a reference to a - // non-upgradeable `Weak`: - // - It can *clone* the `Weak`, increasing the weak reference count. - // - It can drop those clones, decreasing the weak reference count (but never to zero). - // - // These side effects do not impact us in any way, and no other side effects are - // possible with safe code alone. - let prev_value = (*inner).strong.fetch_add(1, Release); - debug_assert_eq!(prev_value, 0, "No prior strong references should exist"); - - Arc::from_inner(init_ptr) - }; - - // Strong references should collectively own a shared weak reference, - // so don't run the destructor for our old weak reference. - mem::forget(weak); - strong + Self::new_cyclic_in(data_fn, Global) } /// Constructs a new `Arc` with uninitialized contents. @@ -781,6 +734,98 @@ impl Arc { } } + /// Constructs a new `Arc` in the given allocator while giving you a `Weak` to the allocation, + /// to allow you to construct a `T` which holds a weak pointer to itself. + /// + /// Generally, a structure circularly referencing itself, either directly or + /// indirectly, should not hold a strong reference to itself to prevent a memory leak. + /// Using this function, you get access to the weak pointer during the + /// initialization of `T`, before the `Arc` is created, such that you can + /// clone and store it inside the `T`. + /// + /// `new_cyclic_in` first allocates the managed allocation for the `Arc`, + /// then calls your closure, giving it a `Weak` to this allocation, + /// and only afterwards completes the construction of the `Arc` by placing + /// the `T` returned from your closure into the allocation. + /// + /// Since the new `Arc` is not fully-constructed until `Arc::new_cyclic_in` + /// returns, calling [`upgrade`] on the weak reference inside your closure will + /// fail and result in a `None` value. + /// + /// # Panics + /// + /// If `data_fn` panics, the panic is propagated to the caller, and the + /// temporary [`Weak`] is dropped normally. + /// + /// # Example + /// + /// See [`new_cyclic`] + /// + /// [`new_cyclic`]: Arc::new_cyclic + /// [`upgrade`]: Weak::upgrade + #[cfg(not(no_global_oom_handling))] + #[inline] + #[unstable(feature = "allocator_api", issue = "32838")] + pub fn new_cyclic_in(data_fn: F, alloc: A) -> Arc + where + F: FnOnce(&Weak) -> T, + { + // Construct the inner in the "uninitialized" state with a single + // weak reference. + let (uninit_raw_ptr, alloc) = Box::into_raw_with_allocator(Box::new_in( + ArcInner { + strong: atomic::AtomicUsize::new(0), + weak: atomic::AtomicUsize::new(1), + data: mem::MaybeUninit::::uninit(), + }, + alloc, + )); + let uninit_ptr: NonNull<_> = (unsafe { &mut *uninit_raw_ptr }).into(); + let init_ptr: NonNull> = uninit_ptr.cast(); + + let weak = Weak { ptr: init_ptr, alloc: alloc }; + + // It's important we don't give up ownership of the weak pointer, or + // else the memory might be freed by the time `data_fn` returns. If + // we really wanted to pass ownership, we could create an additional + // weak pointer for ourselves, but this would result in additional + // updates to the weak reference count which might not be necessary + // otherwise. + let data = data_fn(&weak); + + // Now we can properly initialize the inner value and turn our weak + // reference into a strong reference. + let strong = unsafe { + let inner = init_ptr.as_ptr(); + ptr::write(ptr::addr_of_mut!((*inner).data), data); + + // The above write to the data field must be visible to any threads which + // observe a non-zero strong count. Therefore we need at least "Release" ordering + // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`. + // + // "Acquire" ordering is not required. When considering the possible behaviours + // of `data_fn` we only need to look at what it could do with a reference to a + // non-upgradeable `Weak`: + // - It can *clone* the `Weak`, increasing the weak reference count. + // - It can drop those clones, decreasing the weak reference count (but never to zero). + // + // These side effects do not impact us in any way, and no other side effects are + // possible with safe code alone. + let prev_value = (*inner).strong.fetch_add(1, Release); + debug_assert_eq!(prev_value, 0, "No prior strong references should exist"); + + // Strong references should collectively own a shared weak reference, + // so don't run the destructor for our old weak reference. + // Calling into_raw_with_allocator has the double effect of giving us back the allocator, + // and forgetting the weak reference. + let alloc = weak.into_raw_with_allocator().1; + + Arc::from_inner_in(init_ptr, alloc) + }; + + strong + } + /// Constructs a new `Pin>` in the provided allocator. If `T` does not implement `Unpin`, /// then `data` will be pinned in memory and unable to be moved. #[cfg(not(no_global_oom_handling))]