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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
//! Reference-counted pointers to flows.
//!
//! Eventually, with dynamically sized types in Rust, much of this code will
//! be superfluous. This design is largely duplicating logic of Arc<T> and
//! Weak<T>; please see comments there for details.
#![allow(unsafe_code)]
use flow;
use flow::{Flow, BaseFlow};
use std::mem;
use std::ops::Deref;
use std::ptr;
use std::raw;
use std::rt::heap;
use std::sync::atomic::{self, Ordering};
#[unsafe_no_drop_flag]
pub struct FlowRef {
object: raw::TraitObject,
}
unsafe impl Send for FlowRef {}
unsafe impl Sync for FlowRef {}
#[unsafe_no_drop_flag]
pub struct WeakFlowRef {
object: raw::TraitObject,
}
unsafe impl Send for WeakFlowRef {}
unsafe impl Sync for WeakFlowRef {}
impl FlowRef {
pub fn new(mut flow: Box<Flow>) -> FlowRef {
unsafe {
let result = {
let flow_ref: &mut Flow = &mut *flow;
let object = mem::transmute::<&mut Flow, raw::TraitObject>(flow_ref);
FlowRef { object: object }
};
mem::forget(flow);
result
}
}
/// Downgrades the FlowRef to a WeakFlowRef.
pub fn downgrade(&self) -> WeakFlowRef {
unsafe {
flow::base(&**self).weak_ref_count().fetch_add(1, Ordering::Relaxed);
}
WeakFlowRef { object: self.object }
}
}
impl<'a> Deref for FlowRef {
type Target = Flow + 'a;
fn deref(&self) -> &(Flow + 'a) {
unsafe {
mem::transmute_copy::<raw::TraitObject, &(Flow + 'a)>(&self.object)
}
}
}
// FIXME(https://github.com/servo/servo/issues/6503) This introduces unsound mutable aliasing.
// Try to replace it with Arc::get_mut (which checks that the reference count is 1).
pub unsafe fn deref_mut<'a>(flow_ref: &mut FlowRef) -> &mut (Flow + 'a) {
mem::transmute_copy::<raw::TraitObject, &mut (Flow + 'a)>(&flow_ref.object)
}
impl Drop for FlowRef {
fn drop(&mut self) {
unsafe {
if self.object.vtable.is_null() ||
self.object.vtable as usize == mem::POST_DROP_USIZE {
return
}
if flow::base(&**self).strong_ref_count().fetch_sub(1, Ordering::Release) != 1 {
return
}
atomic::fence(Ordering::Acquire);
// Normally we'd call the underlying Drop logic but not free the
// allocation, but this is almost impossible without DST in
// Rust. Instead we make a fake trait object to run the drop logic
// on.
let flow_ref: FlowRef = mem::replace(self, FlowRef {
object: raw::TraitObject {
vtable: ptr::null_mut(),
data: ptr::null_mut(),
}
});
let vtable: &[usize; 3] = mem::transmute::<*mut (), &[usize; 3]>(flow_ref.object.vtable);
let object_size = vtable[1];
let object_align = vtable[2];
let fake_data = heap::allocate(object_size, object_align);
ptr::copy(flow_ref.object.data as *const u8, fake_data, object_size);
let fake_box = raw::TraitObject { vtable: flow_ref.object.vtable, data: fake_data as *mut () };
let fake_flow = mem::transmute::<raw::TraitObject, Box<Flow>>(fake_box);
drop(fake_flow);
if flow::base(&*flow_ref).weak_ref_count().fetch_sub(1, Ordering::Release) == 1 {
atomic::fence(Ordering::Acquire);
heap::deallocate(flow_ref.object.data as *mut u8, object_size, object_align);
}
mem::forget(flow_ref);
}
}
}
impl Clone for FlowRef {
fn clone(&self) -> FlowRef {
unsafe {
let _ = flow::base(&**self).strong_ref_count().fetch_add(1, Ordering::Relaxed);
FlowRef {
object: raw::TraitObject {
vtable: self.object.vtable,
data: self.object.data,
}
}
}
}
}
fn base<'a>(r: &WeakFlowRef) -> &'a BaseFlow {
let data = r.object.data;
debug_assert!(!data.is_null());
unsafe {
mem::transmute::<*mut (), &'a BaseFlow>(data)
}
}
impl WeakFlowRef {
/// Upgrades a WeakFlowRef to a FlowRef.
pub fn upgrade(&self) -> Option<FlowRef> {
unsafe {
let object = base(self);
// We use a CAS loop to increment the strong count instead of a
// fetch_add because once the count hits 0 is must never be above
// 0.
loop {
let n = object.strong_ref_count().load(Ordering::SeqCst);
if n == 0 { return None }
let old = object.strong_ref_count().compare_and_swap(n, n + 1, Ordering::SeqCst);
if old == n {
return Some(FlowRef { object: self.object })
}
}
}
}
}
impl Clone for WeakFlowRef {
fn clone(&self) -> WeakFlowRef {
unsafe {
base(self).weak_ref_count().fetch_add(1, Ordering::Relaxed);
}
WeakFlowRef { object: self. object }
}
}
impl Drop for WeakFlowRef {
fn drop(&mut self) {
unsafe {
if self.object.vtable.is_null() ||
self.object.vtable as usize == mem::POST_DROP_USIZE {
return
}
if base(self).weak_ref_count().fetch_sub(1, Ordering::Release) == 1 {
atomic::fence(Ordering::Acquire);
// This dance deallocates the Box<Flow> without running its
// drop glue. The drop glue is run when the last strong
// reference is released.
let weak_ref: WeakFlowRef = mem::replace(self, WeakFlowRef {
object: raw::TraitObject {
vtable: ptr::null_mut(),
data: ptr::null_mut(),
}
});
let vtable: &[usize; 3] = mem::transmute::<*mut (), &[usize; 3]>(weak_ref.object.vtable);
let object_size = vtable[1];
let object_align = vtable[2];
heap::deallocate(weak_ref.object.data as *mut u8, object_size, object_align);
mem::forget(weak_ref);
}
}
}
}
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