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203 lines
8 KiB
Rust
203 lines
8 KiB
Rust
/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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//! A generic, safe mechanism by which DOM objects can be pinned and transferred
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//! between tasks (or intra-task for asynchronous events). Akin to Gecko's
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//! nsMainThreadPtrHandle, this uses thread-safe reference counting and ensures
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//! that the actual SpiderMonkey GC integration occurs on the script task via
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//! message passing. Ownership of a `Trusted<T>` object means the DOM object of
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//! type T to which it points remains alive. Any other behaviour is undefined.
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//! To guarantee the lifetime of a DOM object when performing asynchronous operations,
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//! obtain a `Trusted<T>` from that object and pass it along with each operation.
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//! A usable pointer to the original DOM object can be obtained on the script task
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//! from a `Trusted<T>` via the `to_temporary` method.
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//!
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//! The implementation of Trusted<T> is as follows:
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//! A hashtable resides in the script task, keyed on the pointer to the Rust DOM object.
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//! The values in this hashtable are atomic reference counts. When a Trusted<T> object is
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//! created or cloned, this count is increased. When a Trusted<T> is dropped, the count
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//! decreases. If the count hits zero, a message is dispatched to the script task to remove
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//! the entry from the hashmap if the count is still zero. The JS reflector for the DOM object
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//! is rooted when a hashmap entry is first created, and unrooted when the hashmap entry
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//! is removed.
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use dom::bindings::js::Root;
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use dom::bindings::utils::{Reflector, Reflectable};
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use dom::bindings::trace::trace_reflector;
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use script_task::{ScriptMsg, ScriptChan};
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use js::jsapi::{JSContext, JSTracer};
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use libc;
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use std::cell::RefCell;
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use std::collections::hash_map::HashMap;
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use std::collections::hash_map::Entry::{Vacant, Occupied};
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use std::marker::PhantomData;
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use std::rc::Rc;
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use std::sync::{Arc, Mutex};
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use core::nonzero::NonZero;
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thread_local!(pub static LIVE_REFERENCES: Rc<RefCell<Option<LiveDOMReferences>>> = Rc::new(RefCell::new(None)));
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/// A pointer to a Rust DOM object that needs to be destroyed.
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pub struct TrustedReference(*const libc::c_void);
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unsafe impl Send for TrustedReference {}
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/// A safe wrapper around a raw pointer to a DOM object that can be
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/// shared among tasks for use in asynchronous operations. The underlying
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/// DOM object is guaranteed to live at least as long as the last outstanding
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/// `Trusted<T>` instance.
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pub struct Trusted<T: Reflectable> {
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/// A pointer to the Rust DOM object of type T, but void to allow
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/// sending `Trusted<T>` between tasks, regardless of T's sendability.
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ptr: *const libc::c_void,
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refcount: Arc<Mutex<usize>>,
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script_chan: Box<ScriptChan + Send>,
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owner_thread: *const libc::c_void,
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phantom: PhantomData<T>,
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}
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unsafe impl<T: Reflectable> Send for Trusted<T> {}
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impl<T: Reflectable> Trusted<T> {
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/// Create a new `Trusted<T>` instance from an existing DOM pointer. The DOM object will
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/// be prevented from being GCed for the duration of the resulting `Trusted<T>` object's
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/// lifetime.
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pub fn new(_cx: *mut JSContext, ptr: &T, script_chan: Box<ScriptChan + Send>) -> Trusted<T> {
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LIVE_REFERENCES.with(|ref r| {
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let r = r.borrow();
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let live_references = r.as_ref().unwrap();
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let refcount = live_references.addref(&*ptr as *const T);
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Trusted {
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ptr: &*ptr as *const T as *const libc::c_void,
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refcount: refcount,
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script_chan: script_chan.clone(),
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owner_thread: (&*live_references) as *const _ as *const libc::c_void,
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phantom: PhantomData,
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}
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})
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}
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/// Obtain a usable DOM pointer from a pinned `Trusted<T>` value. Fails if used on
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/// a different thread than the original value from which this `Trusted<T>` was
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/// obtained.
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pub fn root(&self) -> Root<T> {
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assert!(LIVE_REFERENCES.with(|ref r| {
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let r = r.borrow();
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let live_references = r.as_ref().unwrap();
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self.owner_thread == (&*live_references) as *const _ as *const libc::c_void
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}));
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unsafe {
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Root::new(NonZero::new(self.ptr as *const T))
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}
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}
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}
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impl<T: Reflectable> Clone for Trusted<T> {
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fn clone(&self) -> Trusted<T> {
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{
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let mut refcount = self.refcount.lock().unwrap();
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*refcount += 1;
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}
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Trusted {
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ptr: self.ptr,
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refcount: self.refcount.clone(),
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script_chan: self.script_chan.clone(),
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owner_thread: self.owner_thread,
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phantom: PhantomData,
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}
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}
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}
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impl<T: Reflectable> Drop for Trusted<T> {
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fn drop(&mut self) {
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let mut refcount = self.refcount.lock().unwrap();
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assert!(*refcount > 0);
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*refcount -= 1;
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if *refcount == 0 {
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// It's possible this send will fail if the script task
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// has already exited. There's not much we can do at this
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// point though.
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let msg = ScriptMsg::RefcountCleanup(TrustedReference(self.ptr));
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let _ = self.script_chan.send(msg);
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}
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}
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}
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/// The set of live, pinned DOM objects that are currently prevented
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/// from being garbage collected due to outstanding references.
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pub struct LiveDOMReferences {
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// keyed on pointer to Rust DOM object
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table: RefCell<HashMap<*const libc::c_void, Arc<Mutex<usize>>>>
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}
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impl LiveDOMReferences {
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/// Set up the task-local data required for storing the outstanding DOM references.
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pub fn initialize() {
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LIVE_REFERENCES.with(|ref r| {
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*r.borrow_mut() = Some(LiveDOMReferences {
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table: RefCell::new(HashMap::new()),
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})
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});
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}
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fn addref<T: Reflectable>(&self, ptr: *const T) -> Arc<Mutex<usize>> {
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let mut table = self.table.borrow_mut();
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match table.entry(ptr as *const libc::c_void) {
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Occupied(mut entry) => {
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let refcount = entry.get_mut();
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*refcount.lock().unwrap() += 1;
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refcount.clone()
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}
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Vacant(entry) => {
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let refcount = Arc::new(Mutex::new(1));
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entry.insert(refcount.clone());
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refcount
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}
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}
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}
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/// Unpin the given DOM object if its refcount is 0.
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pub fn cleanup(raw_reflectable: TrustedReference) {
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let TrustedReference(raw_reflectable) = raw_reflectable;
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LIVE_REFERENCES.with(|ref r| {
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let r = r.borrow();
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let live_references = r.as_ref().unwrap();
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let mut table = live_references.table.borrow_mut();
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match table.entry(raw_reflectable) {
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Occupied(entry) => {
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if *entry.get().lock().unwrap() != 0 {
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// there could have been a new reference taken since
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// this message was dispatched.
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return;
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}
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let _ = entry.remove();
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}
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Vacant(_) => {
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// there could be a cleanup message dispatched, then a new
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// pinned reference obtained and released before the message
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// is processed, at which point there would be no matching
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// hashtable entry.
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info!("attempt to cleanup an unrecognized reflector");
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}
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}
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})
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}
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}
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/// A JSTraceDataOp for tracing reflectors held in LIVE_REFERENCES
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pub unsafe extern fn trace_refcounted_objects(tracer: *mut JSTracer, _data: *mut libc::c_void) {
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LIVE_REFERENCES.with(|ref r| {
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let r = r.borrow();
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let live_references = r.as_ref().unwrap();
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let table = live_references.table.borrow();
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for obj in table.keys() {
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let reflectable = &*(*obj as *const Reflector);
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trace_reflector(tracer, "LIVE_REFERENCES", reflectable);
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}
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});
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}
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