Rollup merge of #72533 - Diggsey:db-fix-arc-ub2, r=dtolnay
Resolve UB in Arc/Weak interaction (2) Use raw pointers to avoid making any assertions about the data field. Follow up from #72479, see that PR for more detail on the motivation. @RalfJung I was able to avoid a lot of the changes to `Weak`, by making a helper type (`WeakInner`) - because of auto-deref and because the fields have the same name, the rest of the code continues to compile.
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8f95dc8d4e
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@ -867,12 +867,10 @@ impl<T: ?Sized> Arc<T> {
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unsafe fn drop_slow(&mut self) {
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unsafe fn drop_slow(&mut self) {
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// Destroy the data at this time, even though we may not free the box
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// Destroy the data at this time, even though we may not free the box
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// allocation itself (there may still be weak pointers lying around).
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// allocation itself (there may still be weak pointers lying around).
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ptr::drop_in_place(&mut self.ptr.as_mut().data);
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ptr::drop_in_place(Self::get_mut_unchecked(self));
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if self.inner().weak.fetch_sub(1, Release) == 1 {
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// Drop the weak ref collectively held by all strong references
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acquire!(self.inner().weak);
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drop(Weak { ptr: self.ptr });
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Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
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}
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}
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}
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#[inline]
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#[inline]
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@ -1204,7 +1202,7 @@ impl<T: Clone> Arc<T> {
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// As with `get_mut()`, the unsafety is ok because our reference was
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// As with `get_mut()`, the unsafety is ok because our reference was
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// either unique to begin with, or became one upon cloning the contents.
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// either unique to begin with, or became one upon cloning the contents.
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unsafe { &mut this.ptr.as_mut().data }
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unsafe { Self::get_mut_unchecked(this) }
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}
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}
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}
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}
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@ -1280,7 +1278,9 @@ impl<T: ?Sized> Arc<T> {
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#[inline]
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#[inline]
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#[unstable(feature = "get_mut_unchecked", issue = "63292")]
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#[unstable(feature = "get_mut_unchecked", issue = "63292")]
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pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
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pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
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&mut this.ptr.as_mut().data
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// We are careful to *not* create a reference covering the "count" fields, as
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// this would alias with concurrent access to the reference counts (e.g. by `Weak`).
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&mut (*this.ptr.as_ptr()).data
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}
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}
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/// Determine whether this is the unique reference (including weak refs) to
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/// Determine whether this is the unique reference (including weak refs) to
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@ -1571,6 +1571,13 @@ impl<T> Weak<T> {
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}
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}
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}
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}
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/// Helper type to allow accessing the reference counts without
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/// making any assertions about the data field.
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struct WeakInner<'a> {
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weak: &'a atomic::AtomicUsize,
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strong: &'a atomic::AtomicUsize,
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}
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impl<T: ?Sized> Weak<T> {
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impl<T: ?Sized> Weak<T> {
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/// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
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/// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
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/// dropping of the inner value if successful.
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/// dropping of the inner value if successful.
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@ -1678,8 +1685,18 @@ impl<T: ?Sized> Weak<T> {
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/// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
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/// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
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/// (i.e., when this `Weak` was created by `Weak::new`).
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/// (i.e., when this `Weak` was created by `Weak::new`).
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#[inline]
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#[inline]
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fn inner(&self) -> Option<&ArcInner<T>> {
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fn inner(&self) -> Option<WeakInner<'_>> {
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if is_dangling(self.ptr) { None } else { Some(unsafe { self.ptr.as_ref() }) }
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if is_dangling(self.ptr) {
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None
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} else {
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// We are careful to *not* create a reference covering the "data" field, as
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// the field may be mutated concurrently (for example, if the last `Arc`
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// is dropped, the data field will be dropped in-place).
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Some(unsafe {
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let ptr = self.ptr.as_ptr();
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WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
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})
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}
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}
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}
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/// Returns `true` if the two `Weak`s point to the same allocation (similar to
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/// Returns `true` if the two `Weak`s point to the same allocation (similar to
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