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Generalize {Rc,Arc}::make_mut() to unsized types.

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This requires introducing a new internal type `RcUninit` (and
`ArcUninit`), which can own an `RcBox<T>` 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`.
This commit is contained in:
Kevin Reid 2023-09-24 10:57:54 -07:00
parent a9a4830d25
commit 88c3db57e4
4 changed files with 229 additions and 26 deletions
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112
library/alloc/src/rc.rs
View file

@ -1749,7 +1749,8 @@ pub fn ptr_eq(this: &Self, other: &Self) -> bool {
}
}
impl<T: Clone, A: Allocator + Clone> Rc<T, A> {
#[cfg(not(no_global_oom_handling))]
impl<T: ?Sized + CloneToUninit, A: Allocator + Clone> Rc<T, A> {
/// 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<T: Clone, A: Allocator + Clone> Rc<T, A> {
/// 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::<T>(&**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<T, A> =
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<T, A> =
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::<u8>(),
in_progress.data_ptr().cast::<u8>(),
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 @@ fn drop(&mut self) {
}
}
}
/// 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<MaybeUninit<T>, A>` except that it allows `T: !Sized`,
/// which `MaybeUninit` does not.
#[cfg(not(no_global_oom_handling))]
struct UniqueRcUninit<T: ?Sized, A: Allocator> {
ptr: NonNull<RcBox<T>>,
layout_for_value: Layout,
alloc: Option<A>,
}
#[cfg(not(no_global_oom_handling))]
impl<T: ?Sized, A: Allocator> UniqueRcUninit<T, A> {
/// Allocate a RcBox with layout suitable to contain `for_value` or a clone of it.
fn new(for_value: &T, alloc: A) -> UniqueRcUninit<T, A> {
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<T>),
)
};
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<T, A> {
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<T: ?Sized, A: Allocator> Drop for UniqueRcUninit<T, A> {
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));
}
}
}

18
library/alloc/src/rc/tests.rs
View file

@ -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);

107
library/alloc/src/sync.rs
View file

@ -2150,7 +2150,8 @@ unsafe impl<T: ?Sized, A: Allocator> DerefPure for Arc<T, A> {}
#[unstable(feature = "receiver_trait", issue = "none")]
impl<T: ?Sized> Receiver for Arc<T> {}
impl<T: Clone, A: Allocator + Clone> Arc<T, A> {
#[cfg(not(no_global_oom_handling))]
impl<T: ?Sized + CloneToUninit, A: Allocator + Clone> Arc<T, A> {
/// 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<T: Clone, A: Allocator + Clone> Arc<T, A> {
/// 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::<T>(&**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 @@ pub fn make_mut(this: &mut Self) -> &mut T {
// 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<T, A> =
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 @@ pub fn make_mut(this: &mut Self) -> &mut T {
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<T, A> =
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::<u8>(),
in_progress.data_ptr().cast::<u8>(),
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<T: ?Sized, A: Allocator> {
ptr: NonNull<ArcInner<T>>,
layout_for_value: Layout,
alloc: Option<A>,
}
#[cfg(not(no_global_oom_handling))]
impl<T: ?Sized, A: Allocator> UniqueArcUninit<T, A> {
/// Allocate a ArcInner with layout suitable to contain `for_value` or a clone of it.
fn new(for_value: &T, alloc: A) -> UniqueArcUninit<T, A> {
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<T>),
)
};
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<T, A> {
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<T: ?Sized, A: Allocator> Drop for UniqueArcUninit<T, A> {
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<T: core::error::Error + ?Sized> core::error::Error for Arc<T> {
#[allow(deprecated, deprecated_in_future)]

18
library/alloc/tests/arc.rs
View file

@ -209,3 +209,21 @@ fn hmm<'a>(val: &'a mut Weak<&'a str>) -> Weak<&'a str> {
// `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]);
}