From 88c3db57e4c3cda4e446999202171d9a459069a9 Mon Sep 17 00:00:00 2001 From: Kevin Reid Date: Sun, 24 Sep 2023 10:57:54 -0700 Subject: [PATCH] Generalize `{Rc,Arc}::make_mut()` to unsized types. This requires introducing a new internal type `RcUninit` (and `ArcUninit`), which can own an `RcBox` without requiring it to be initialized, sized, or a slice. This is similar to `UniqueRc`, but `UniqueRc` doesn't support the allocator parameter, and there is no `UniqueArc`. --- library/alloc/src/rc.rs | 112 ++++++++++++++++++++++++++++++---- library/alloc/src/rc/tests.rs | 18 ++++++ library/alloc/src/sync.rs | 107 ++++++++++++++++++++++++++++---- library/alloc/tests/arc.rs | 18 ++++++ 4 files changed, 229 insertions(+), 26 deletions(-) diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs index f3a036989035a..3745ecb48c18e 100644 --- a/library/alloc/src/rc.rs +++ b/library/alloc/src/rc.rs @@ -1749,7 +1749,8 @@ impl Rc { } } -impl Rc { +#[cfg(not(no_global_oom_handling))] +impl Rc { /// Makes a mutable reference into the given `Rc`. /// /// If there are other `Rc` pointers to the same allocation, then `make_mut` will @@ -1800,31 +1801,52 @@ impl Rc { /// assert!(76 == *data); /// assert!(weak.upgrade().is_none()); /// ``` - #[cfg(not(no_global_oom_handling))] #[inline] #[stable(feature = "rc_unique", since = "1.4.0")] pub fn make_mut(this: &mut Self) -> &mut T { + let size_of_val = size_of_val::(&**this); + if Rc::strong_count(this) != 1 { // Gotta clone the data, there are other Rcs. - // Pre-allocate memory to allow writing the cloned value directly. - let mut rc = Self::new_uninit_in(this.alloc.clone()); - unsafe { - let data = Rc::get_mut_unchecked(&mut rc); - (**this).clone_to_uninit(data.as_mut_ptr()); - *this = rc.assume_init(); - } + + let this_data_ref: &T = &**this; + // `in_progress` drops the allocation if we panic before finishing initializing it. + let mut in_progress: UniqueRcUninit = + UniqueRcUninit::new(this_data_ref, this.alloc.clone()); + + // Initialize with clone of this. + let initialized_clone = unsafe { + // Clone. If the clone panics, `in_progress` will be dropped and clean up. + this_data_ref.clone_to_uninit(in_progress.data_ptr()); + // Cast type of pointer, now that it is initialized. + in_progress.into_rc() + }; + + // Replace `this` with newly constructed Rc. + *this = initialized_clone; } else if Rc::weak_count(this) != 0 { // Can just steal the data, all that's left is Weaks - let mut rc = Self::new_uninit_in(this.alloc.clone()); + + // We don't need panic-protection like the above branch does, but we might as well + // use the same mechanism. + let mut in_progress: UniqueRcUninit = + UniqueRcUninit::new(&**this, this.alloc.clone()); unsafe { - let data = Rc::get_mut_unchecked(&mut rc); - data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1); + // Initialize `in_progress` with move of **this. + // We have to express this in terms of bytes because `T: ?Sized`; there is no + // operation that just copies a value based on its `size_of_val()`. + ptr::copy_nonoverlapping( + ptr::from_ref(&**this).cast::(), + in_progress.data_ptr().cast::(), + size_of_val, + ); this.inner().dec_strong(); // Remove implicit strong-weak ref (no need to craft a fake // Weak here -- we know other Weaks can clean up for us) this.inner().dec_weak(); - ptr::write(this, rc.assume_init()); + // Replace `this` with newly constructed Rc that has the moved data. + ptr::write(this, in_progress.into_rc()); } } // This unsafety is ok because we're guaranteed that the pointer @@ -3686,3 +3708,67 @@ unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for UniqueRc { } } } + +/// A unique owning pointer to a [`RcBox`] **that does not imply the contents are initialized,** +/// but will deallocate it (without dropping the value) when dropped. +/// +/// This is a helper for [`Rc::make_mut()`] to ensure correct cleanup on panic. +/// It is nearly a duplicate of `UniqueRc, A>` except that it allows `T: !Sized`, +/// which `MaybeUninit` does not. +#[cfg(not(no_global_oom_handling))] +struct UniqueRcUninit { + ptr: NonNull>, + layout_for_value: Layout, + alloc: Option, +} + +#[cfg(not(no_global_oom_handling))] +impl UniqueRcUninit { + /// Allocate a RcBox with layout suitable to contain `for_value` or a clone of it. + fn new(for_value: &T, alloc: A) -> UniqueRcUninit { + let layout = Layout::for_value(for_value); + let ptr = unsafe { + Rc::allocate_for_layout( + layout, + |layout_for_rcbox| alloc.allocate(layout_for_rcbox), + |mem| mem.with_metadata_of(ptr::from_ref(for_value) as *const RcBox), + ) + }; + Self { ptr: NonNull::new(ptr).unwrap(), layout_for_value: layout, alloc: Some(alloc) } + } + + /// Returns the pointer to be written into to initialize the [`Rc`]. + fn data_ptr(&mut self) -> *mut T { + let offset = data_offset_align(self.layout_for_value.align()); + unsafe { self.ptr.as_ptr().byte_add(offset) as *mut T } + } + + /// Upgrade this into a normal [`Rc`]. + /// + /// # Safety + /// + /// The data must have been initialized (by writing to [`Self::data_ptr()`]). + unsafe fn into_rc(mut self) -> Rc { + let ptr = self.ptr; + let alloc = self.alloc.take().unwrap(); + mem::forget(self); + // SAFETY: The pointer is valid as per `UniqueRcUninit::new`, and the caller is responsible + // for having initialized the data. + unsafe { Rc::from_ptr_in(ptr.as_ptr(), alloc) } + } +} + +#[cfg(not(no_global_oom_handling))] +impl Drop for UniqueRcUninit { + fn drop(&mut self) { + // SAFETY: + // * new() produced a pointer safe to deallocate. + // * We own the pointer unless into_rc() was called, which forgets us. + unsafe { + self.alloc + .take() + .unwrap() + .deallocate(self.ptr.cast(), rcbox_layout_for_value_layout(self.layout_for_value)); + } + } +} diff --git a/library/alloc/src/rc/tests.rs b/library/alloc/src/rc/tests.rs index 0f09be7721fa9..5e2e4beb94a2b 100644 --- a/library/alloc/src/rc/tests.rs +++ b/library/alloc/src/rc/tests.rs @@ -316,6 +316,24 @@ fn test_cowrc_clone_weak() { assert!(cow1_weak.upgrade().is_none()); } +/// This is similar to the doc-test for `Rc::make_mut()`, but on an unsized type (slice). +#[test] +fn test_cowrc_unsized() { + use std::rc::Rc; + + let mut data: Rc<[i32]> = Rc::new([10, 20, 30]); + + Rc::make_mut(&mut data)[0] += 1; // Won't clone anything + let mut other_data = Rc::clone(&data); // Won't clone inner data + Rc::make_mut(&mut data)[1] += 1; // Clones inner data + Rc::make_mut(&mut data)[2] += 1; // Won't clone anything + Rc::make_mut(&mut other_data)[0] *= 10; // Won't clone anything + + // Now `data` and `other_data` point to different allocations. + assert_eq!(*data, [11, 21, 31]); + assert_eq!(*other_data, [110, 20, 30]); +} + #[test] fn test_show() { let foo = Rc::new(75); diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs index 6570fab42be1c..90672164cb932 100644 --- a/library/alloc/src/sync.rs +++ b/library/alloc/src/sync.rs @@ -2150,7 +2150,8 @@ unsafe impl DerefPure for Arc {} #[unstable(feature = "receiver_trait", issue = "none")] impl Receiver for Arc {} -impl Arc { +#[cfg(not(no_global_oom_handling))] +impl Arc { /// Makes a mutable reference into the given `Arc`. /// /// If there are other `Arc` pointers to the same allocation, then `make_mut` will @@ -2201,10 +2202,11 @@ impl Arc { /// assert!(76 == *data); /// assert!(weak.upgrade().is_none()); /// ``` - #[cfg(not(no_global_oom_handling))] #[inline] #[stable(feature = "arc_unique", since = "1.4.0")] pub fn make_mut(this: &mut Self) -> &mut T { + let size_of_val = mem::size_of_val::(&**this); + // Note that we hold both a strong reference and a weak reference. // Thus, releasing our strong reference only will not, by itself, cause // the memory to be deallocated. @@ -2215,13 +2217,19 @@ impl Arc { // deallocated. if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() { // Another strong pointer exists, so we must clone. - // Pre-allocate memory to allow writing the cloned value directly. - let mut arc = Self::new_uninit_in(this.alloc.clone()); - unsafe { - let data = Arc::get_mut_unchecked(&mut arc); - (**this).clone_to_uninit(data.as_mut_ptr()); - *this = arc.assume_init(); - } + + let this_data_ref: &T = &**this; + // `in_progress` drops the allocation if we panic before finishing initializing it. + let mut in_progress: UniqueArcUninit = + UniqueArcUninit::new(this_data_ref, this.alloc.clone()); + + let initialized_clone = unsafe { + // Clone. If the clone panics, `in_progress` will be dropped and clean up. + this_data_ref.clone_to_uninit(in_progress.data_ptr()); + // Cast type of pointer, now that it is initialized. + in_progress.into_arc() + }; + *this = initialized_clone; } else if this.inner().weak.load(Relaxed) != 1 { // Relaxed suffices in the above because this is fundamentally an // optimization: we are always racing with weak pointers being @@ -2240,11 +2248,22 @@ impl Arc { let _weak = Weak { ptr: this.ptr, alloc: this.alloc.clone() }; // Can just steal the data, all that's left is Weaks - let mut arc = Self::new_uninit_in(this.alloc.clone()); + // + // We don't need panic-protection like the above branch does, but we might as well + // use the same mechanism. + let mut in_progress: UniqueArcUninit = + UniqueArcUninit::new(&**this, this.alloc.clone()); unsafe { - let data = Arc::get_mut_unchecked(&mut arc); - data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1); - ptr::write(this, arc.assume_init()); + // Initialize `in_progress` with move of **this. + // We have to express this in terms of bytes because `T: ?Sized`; there is no + // operation that just copies a value based on its `size_of_val()`. + ptr::copy_nonoverlapping( + ptr::from_ref(&**this).cast::(), + in_progress.data_ptr().cast::(), + size_of_val, + ); + + ptr::write(this, in_progress.into_arc()); } } else { // We were the sole reference of either kind; bump back up the @@ -3809,6 +3828,68 @@ fn data_offset_align(align: usize) -> usize { layout.size() + layout.padding_needed_for(align) } +/// A unique owning pointer to a [`ArcInner`] **that does not imply the contents are initialized,** +/// but will deallocate it (without dropping the value) when dropped. +/// +/// This is a helper for [`Arc::make_mut()`] to ensure correct cleanup on panic. +#[cfg(not(no_global_oom_handling))] +struct UniqueArcUninit { + ptr: NonNull>, + layout_for_value: Layout, + alloc: Option, +} + +#[cfg(not(no_global_oom_handling))] +impl UniqueArcUninit { + /// Allocate a ArcInner with layout suitable to contain `for_value` or a clone of it. + fn new(for_value: &T, alloc: A) -> UniqueArcUninit { + let layout = Layout::for_value(for_value); + let ptr = unsafe { + Arc::allocate_for_layout( + layout, + |layout_for_arcinner| alloc.allocate(layout_for_arcinner), + |mem| mem.with_metadata_of(ptr::from_ref(for_value) as *const ArcInner), + ) + }; + Self { ptr: NonNull::new(ptr).unwrap(), layout_for_value: layout, alloc: Some(alloc) } + } + + /// Returns the pointer to be written into to initialize the [`Arc`]. + fn data_ptr(&mut self) -> *mut T { + let offset = data_offset_align(self.layout_for_value.align()); + unsafe { self.ptr.as_ptr().byte_add(offset) as *mut T } + } + + /// Upgrade this into a normal [`Arc`]. + /// + /// # Safety + /// + /// The data must have been initialized (by writing to [`Self::data_ptr()`]). + unsafe fn into_arc(mut self) -> Arc { + let ptr = self.ptr; + let alloc = self.alloc.take().unwrap(); + mem::forget(self); + // SAFETY: The pointer is valid as per `UniqueArcUninit::new`, and the caller is responsible + // for having initialized the data. + unsafe { Arc::from_ptr_in(ptr.as_ptr(), alloc) } + } +} + +#[cfg(not(no_global_oom_handling))] +impl Drop for UniqueArcUninit { + fn drop(&mut self) { + // SAFETY: + // * new() produced a pointer safe to deallocate. + // * We own the pointer unless into_arc() was called, which forgets us. + unsafe { + self.alloc.take().unwrap().deallocate( + self.ptr.cast(), + arcinner_layout_for_value_layout(self.layout_for_value), + ); + } + } +} + #[stable(feature = "arc_error", since = "1.52.0")] impl core::error::Error for Arc { #[allow(deprecated, deprecated_in_future)] diff --git a/library/alloc/tests/arc.rs b/library/alloc/tests/arc.rs index d564a30b10394..c37a80dca95c8 100644 --- a/library/alloc/tests/arc.rs +++ b/library/alloc/tests/arc.rs @@ -209,3 +209,21 @@ fn weak_may_dangle() { // `val` dropped here while still borrowed // borrow might be used here, when `val` is dropped and runs the `Drop` code for type `std::sync::Weak` } + +/// This is similar to the doc-test for `Arc::make_mut()`, but on an unsized type (slice). +#[test] +fn make_mut_unsized() { + use alloc::sync::Arc; + + let mut data: Arc<[i32]> = Arc::new([10, 20, 30]); + + Arc::make_mut(&mut data)[0] += 1; // Won't clone anything + let mut other_data = Arc::clone(&data); // Won't clone inner data + Arc::make_mut(&mut data)[1] += 1; // Clones inner data + Arc::make_mut(&mut data)[2] += 1; // Won't clone anything + Arc::make_mut(&mut other_data)[0] *= 10; // Won't clone anything + + // Now `data` and `other_data` point to different allocations. + assert_eq!(*data, [11, 21, 31]); + assert_eq!(*other_data, [110, 20, 30]); +}