385 lines
12 KiB
Rust
385 lines
12 KiB
Rust
// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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/*!
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* Concurrency-enabled mechanisms for sharing mutable and/or immutable state
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* between tasks.
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*/
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use std::cast;
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use std::ptr;
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use std::rt::global_heap;
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use std::sync::atomics;
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/// An atomically reference counted wrapper for shared state.
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///
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/// # Example
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///
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/// In this example, a large vector of floats is shared between several tasks.
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/// With simple pipes, without `Arc`, a copy would have to be made for each
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/// task.
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///
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/// ```rust
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/// use sync::Arc;
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///
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/// fn main() {
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/// let numbers = Vec::from_fn(100, |i| i as f32);
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/// let shared_numbers = Arc::new(numbers);
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///
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/// for _ in range(0, 10) {
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/// let child_numbers = shared_numbers.clone();
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///
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/// spawn(proc() {
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/// let local_numbers = child_numbers.as_slice();
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///
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/// // Work with the local numbers
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/// });
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/// }
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/// }
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/// ```
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#[unsafe_no_drop_flag]
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pub struct Arc<T> {
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x: *mut ArcInner<T>,
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}
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/// A weak pointer to an `Arc`.
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///
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/// Weak pointers will not keep the data inside of the `Arc` alive, and can be
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/// used to break cycles between `Arc` pointers.
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#[unsafe_no_drop_flag]
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pub struct Weak<T> {
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x: *mut ArcInner<T>,
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}
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struct ArcInner<T> {
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strong: atomics::AtomicUint,
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weak: atomics::AtomicUint,
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data: T,
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}
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impl<T: Share + Send> Arc<T> {
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/// Create an atomically reference counted wrapper.
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#[inline]
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pub fn new(data: T) -> Arc<T> {
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// Start the weak pointer count as 1 which is the weak pointer that's
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// held by all the strong pointers (kinda), see std/rc.rs for more info
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let x = ~ArcInner {
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strong: atomics::AtomicUint::new(1),
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weak: atomics::AtomicUint::new(1),
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data: data,
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};
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Arc { x: unsafe { cast::transmute(x) } }
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}
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#[inline]
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fn inner<'a>(&'a self) -> &'a ArcInner<T> {
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// This unsafety is ok because while this arc is alive we're guaranteed
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// that the inner pointer is valid. Furthermore, we know that the
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// `ArcInner` structure itself is `Share` because the inner data is
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// `Share` as well, so we're ok loaning out an immutable pointer to
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// these contents.
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unsafe { &*self.x }
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}
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/// Downgrades a strong pointer to a weak pointer
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///
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/// Weak pointers will not keep the data alive. Once all strong references
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/// to the underlying data have been dropped, the data itself will be
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/// destroyed.
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pub fn downgrade(&self) -> Weak<T> {
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// See the clone() impl for why this is relaxed
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self.inner().weak.fetch_add(1, atomics::Relaxed);
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Weak { x: self.x }
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}
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}
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impl<T: Share + Send> Clone for Arc<T> {
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/// Duplicate an atomically reference counted wrapper.
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///
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/// The resulting two `Arc` objects will point to the same underlying data
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/// object. However, one of the `Arc` objects can be sent to another task,
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/// allowing them to share the underlying data.
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#[inline]
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fn clone(&self) -> Arc<T> {
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// Using a relaxed ordering is alright here, as knowledge of the
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// original reference prevents other threads from erroneously deleting
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// the object.
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//
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// As explained in the [Boost documentation][1], Increasing the
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// reference counter can always be done with memory_order_relaxed: New
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// references to an object can only be formed from an existing
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// reference, and passing an existing reference from one thread to
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// another must already provide any required synchronization.
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//
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// [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
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self.inner().strong.fetch_add(1, atomics::Relaxed);
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Arc { x: self.x }
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}
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}
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impl<T: Send + Share> Deref<T> for Arc<T> {
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#[inline]
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fn deref<'a>(&'a self) -> &'a T {
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&self.inner().data
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}
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}
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impl<T: Send + Share + Clone> Arc<T> {
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/// Acquires a mutable pointer to the inner contents by guaranteeing that
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/// the reference count is one (no sharing is possible).
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///
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/// This is also referred to as a copy-on-write operation because the inner
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/// data is cloned if the reference count is greater than one.
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#[inline]
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#[experimental]
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pub fn make_unique<'a>(&'a mut self) -> &'a mut T {
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if self.inner().strong.load(atomics::SeqCst) != 1 {
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*self = Arc::new(self.deref().clone())
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}
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// This unsafety is ok because we're guaranteed that the pointer
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// returned is the *only* pointer that will ever be returned to T. Our
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// reference count is guaranteed to be 1 at this point, and we required
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// the Arc itself to be `mut`, so we're returning the only possible
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// reference to the inner data.
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unsafe { cast::transmute_mut(self.deref()) }
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}
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}
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#[unsafe_destructor]
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impl<T: Share + Send> Drop for Arc<T> {
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fn drop(&mut self) {
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// This structure has #[unsafe_no_drop_flag], so this drop glue may run
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// more than once (but it is guaranteed to be zeroed after the first if
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// it's run more than once)
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if self.x.is_null() { return }
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// Because `fetch_sub` is already atomic, we do not need to synchronize
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// with other threads unless we are going to delete the object. This
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// same logic applies to the below `fetch_sub` to the `weak` count.
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if self.inner().strong.fetch_sub(1, atomics::Release) != 1 { return }
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// This fence is needed to prevent reordering of use of the data and
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// deletion of the data. Because it is marked `Release`, the
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// decreasing of the reference count sychronizes with this `Acquire`
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// fence. This means that use of the data happens before decreasing
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// the refernce count, which happens before this fence, which
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// happens before the deletion of the data.
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//
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// As explained in the [Boost documentation][1],
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//
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// It is important to enforce any possible access to the object in
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// one thread (through an existing reference) to *happen before*
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// deleting the object in a different thread. This is achieved by a
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// "release" operation after dropping a reference (any access to the
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// object through this reference must obviously happened before),
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// and an "acquire" operation before deleting the object.
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//
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// [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
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atomics::fence(atomics::Acquire);
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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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unsafe { drop(ptr::read(&self.inner().data)); }
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if self.inner().weak.fetch_sub(1, atomics::Release) == 1 {
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atomics::fence(atomics::Acquire);
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unsafe { global_heap::exchange_free(self.x as *u8) }
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}
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}
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}
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impl<T: Share + Send> Weak<T> {
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/// Attempts to upgrade this weak reference to a strong reference.
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///
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/// This method will fail to upgrade this reference if the strong reference
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/// count has already reached 0, but if there are still other active strong
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/// references this function will return a new strong reference to the data
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pub fn upgrade(&self) -> Option<Arc<T>> {
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// We use a CAS loop to increment the strong count instead of a
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// fetch_add because once the count hits 0 is must never be above 0.
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let inner = self.inner();
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loop {
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let n = inner.strong.load(atomics::SeqCst);
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if n == 0 { return None }
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let old = inner.strong.compare_and_swap(n, n + 1, atomics::SeqCst);
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if old == n { return Some(Arc { x: self.x }) }
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}
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}
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#[inline]
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fn inner<'a>(&'a self) -> &'a ArcInner<T> {
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// See comments above for why this is "safe"
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unsafe { &*self.x }
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}
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}
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impl<T: Share + Send> Clone for Weak<T> {
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#[inline]
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fn clone(&self) -> Weak<T> {
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// See comments in Arc::clone() for why this is relaxed
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self.inner().weak.fetch_add(1, atomics::Relaxed);
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Weak { x: self.x }
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}
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}
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#[unsafe_destructor]
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impl<T: Share + Send> Drop for Weak<T> {
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fn drop(&mut self) {
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// see comments above for why this check is here
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if self.x.is_null() { return }
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// If we find out that we were the last weak pointer, then its time to
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// deallocate the data entirely. See the discussion in Arc::drop() about
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// the memory orderings
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if self.inner().weak.fetch_sub(1, atomics::Release) == 1 {
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atomics::fence(atomics::Acquire);
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unsafe { global_heap::exchange_free(self.x as *u8) }
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}
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}
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}
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#[cfg(test)]
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#[allow(experimental)]
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mod tests {
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use super::{Arc, Weak};
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use std::sync::atomics;
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use std::task;
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use Mutex;
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struct Canary(*mut atomics::AtomicUint);
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impl Drop for Canary
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{
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fn drop(&mut self) {
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unsafe {
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match *self {
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Canary(c) => {
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(*c).fetch_add(1, atomics::SeqCst);
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}
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}
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}
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}
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}
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#[test]
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fn manually_share_arc() {
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let v = vec!(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
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let arc_v = Arc::new(v);
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let (tx, rx) = channel();
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task::spawn(proc() {
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let arc_v: Arc<Vec<int>> = rx.recv();
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assert_eq!(*arc_v.get(3), 4);
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});
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tx.send(arc_v.clone());
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assert_eq!(*arc_v.get(2), 3);
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assert_eq!(*arc_v.get(4), 5);
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info!("{:?}", arc_v);
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}
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#[test]
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fn test_cowarc_clone_make_unique() {
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let mut cow0 = Arc::new(75u);
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let mut cow1 = cow0.clone();
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let mut cow2 = cow1.clone();
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assert!(75 == *cow0.make_unique());
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assert!(75 == *cow1.make_unique());
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assert!(75 == *cow2.make_unique());
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*cow0.make_unique() += 1;
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*cow1.make_unique() += 2;
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*cow2.make_unique() += 3;
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assert!(76 == *cow0);
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assert!(77 == *cow1);
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assert!(78 == *cow2);
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// none should point to the same backing memory
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assert!(*cow0 != *cow1);
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assert!(*cow0 != *cow2);
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assert!(*cow1 != *cow2);
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}
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#[test]
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fn test_cowarc_clone_unique2() {
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let mut cow0 = Arc::new(75u);
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let cow1 = cow0.clone();
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let cow2 = cow1.clone();
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assert!(75 == *cow0);
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assert!(75 == *cow1);
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assert!(75 == *cow2);
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*cow0.make_unique() += 1;
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assert!(76 == *cow0);
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assert!(75 == *cow1);
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assert!(75 == *cow2);
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// cow1 and cow2 should share the same contents
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// cow0 should have a unique reference
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assert!(*cow0 != *cow1);
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assert!(*cow0 != *cow2);
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assert!(*cow1 == *cow2);
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}
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#[test]
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fn test_live() {
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let x = Arc::new(5);
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let y = x.downgrade();
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assert!(y.upgrade().is_some());
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}
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#[test]
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fn test_dead() {
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let x = Arc::new(5);
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let y = x.downgrade();
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drop(x);
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assert!(y.upgrade().is_none());
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}
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#[test]
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fn weak_self_cyclic() {
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struct Cycle {
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x: Mutex<Option<Weak<Cycle>>>
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}
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let a = Arc::new(Cycle { x: Mutex::new(None) });
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let b = a.clone().downgrade();
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*a.deref().x.lock().deref_mut() = Some(b);
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// hopefully we don't double-free (or leak)...
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}
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#[test]
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fn drop_arc() {
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let mut canary = atomics::AtomicUint::new(0);
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let x = Arc::new(Canary(&mut canary as *mut atomics::AtomicUint));
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drop(x);
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assert!(canary.load(atomics::Acquire) == 1);
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}
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#[test]
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fn drop_arc_weak() {
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let mut canary = atomics::AtomicUint::new(0);
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let arc = Arc::new(Canary(&mut canary as *mut atomics::AtomicUint));
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let arc_weak = arc.downgrade();
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assert!(canary.load(atomics::Acquire) == 0);
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drop(arc);
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assert!(canary.load(atomics::Acquire) == 1);
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drop(arc_weak);
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}
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}
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