script/dom/bindings/
refcounted.rs

1/* This Source Code Form is subject to the terms of the Mozilla Public
2 * License, v. 2.0. If a copy of the MPL was not distributed with this
3 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
4
5//! A generic, safe mechanism by which DOM objects can be pinned and transferred
6//! between threads (or intra-thread for asynchronous events). Akin to Gecko's
7//! nsMainThreadPtrHandle, this uses thread-safe reference counting and ensures
8//! that the actual SpiderMonkey GC integration occurs on the script thread via
9//! weak refcounts. Ownership of a `Trusted<T>` object means the DOM object of
10//! type T to which it points remains alive. Any other behaviour is undefined.
11//! To guarantee the lifetime of a DOM object when performing asynchronous operations,
12//! obtain a `Trusted<T>` from that object and pass it along with each operation.
13//! A usable pointer to the original DOM object can be obtained on the script thread
14//! from a `Trusted<T>` via the `root` method.
15//!
16//! The implementation of `Trusted<T>` is as follows:
17//! The `Trusted<T>` object contains an atomic reference counted pointer to the Rust DOM object.
18//! A hashtable resides in the script thread, keyed on the pointer.
19//! The values in this hashtable are weak reference counts. When a `Trusted<T>` object is
20//! created or cloned, the reference count is increased. When a `Trusted<T>` is dropped, the count
21//! decreases. If the count hits zero, the weak reference is emptied, and is removed from
22//! its hash table during the next GC. During GC, the entries of the hash table are counted
23//! as JS roots.
24
25use std::cell::RefCell;
26use std::collections::hash_map::Entry::{Occupied, Vacant};
27use std::hash::Hash;
28use std::marker::PhantomData;
29use std::rc::Rc;
30use std::sync::{Arc, Weak};
31
32use js::jsapi::JSTracer;
33use rustc_hash::FxHashMap;
34use script_bindings::script_runtime::CanGc;
35
36use crate::dom::bindings::conversions::ToJSValConvertible;
37use crate::dom::bindings::error::Error;
38use crate::dom::bindings::reflector::{DomObject, Reflector};
39use crate::dom::bindings::root::DomRoot;
40use crate::dom::bindings::trace::trace_reflector;
41use crate::dom::promise::Promise;
42use crate::task::TaskOnce;
43
44mod dummy {
45    // Attributes don’t apply through the macro.
46    use std::cell::RefCell;
47    use std::rc::Rc;
48
49    use rustc_hash::FxHashMap;
50
51    use super::LiveDOMReferences;
52    thread_local!(pub(crate) static LIVE_REFERENCES: Rc<RefCell<LiveDOMReferences>> =
53        Rc::new(RefCell::new(
54        LiveDOMReferences {
55            reflectable_table: RefCell::new(FxHashMap::default()),
56            promise_table: RefCell::new(FxHashMap::default()),
57        }
58    )));
59}
60pub(crate) use self::dummy::LIVE_REFERENCES;
61
62/// A pointer to a Rust DOM object that needs to be destroyed.
63#[derive(MallocSizeOf)]
64struct TrustedReference(
65    #[ignore_malloc_size_of = "This is a shared reference."] *const libc::c_void,
66);
67unsafe impl Send for TrustedReference {}
68
69impl TrustedReference {
70    /// Creates a new TrustedReference from a pointer to a value that impements DOMObject.
71    /// This is not enforced by the type system to reduce duplicated generic code,
72    /// which is acceptable since this method is internal to this module.
73    unsafe fn new(ptr: *const libc::c_void) -> TrustedReference {
74        TrustedReference(ptr)
75    }
76}
77
78/// A safe wrapper around a DOM Promise object that can be shared among threads for use
79/// in asynchronous operations. The underlying DOM object is guaranteed to live at least
80/// as long as the last outstanding `TrustedPromise` instance. These values cannot be cloned,
81/// only created from existing `Rc<Promise>` values.
82pub struct TrustedPromise {
83    dom_object: *const Promise,
84    owner_thread: *const libc::c_void,
85}
86
87unsafe impl Send for TrustedPromise {}
88
89impl TrustedPromise {
90    /// Create a new `TrustedPromise` instance from an existing DOM object. The object will
91    /// be prevented from being GCed for the duration of the resulting `TrustedPromise` object's
92    /// lifetime.
93    pub(crate) fn new(promise: Rc<Promise>) -> TrustedPromise {
94        LIVE_REFERENCES.with(|r| {
95            let live_references = &*r.borrow();
96            let ptr = &raw const *promise;
97            live_references.addref_promise(promise);
98            TrustedPromise {
99                dom_object: ptr,
100                owner_thread: (live_references) as *const _ as *const libc::c_void,
101            }
102        })
103    }
104
105    /// Obtain a usable DOM Promise from a pinned `TrustedPromise` value. Fails if used on
106    /// a different thread than the original value from which this `TrustedPromise` was
107    /// obtained.
108    pub(crate) fn root(self) -> Rc<Promise> {
109        LIVE_REFERENCES.with(|r| {
110            let live_references = &*r.borrow();
111            assert_eq!(
112                self.owner_thread,
113                live_references as *const _ as *const libc::c_void
114            );
115            match live_references
116                .promise_table
117                .borrow_mut()
118                .entry(self.dom_object)
119            {
120                Occupied(mut entry) => {
121                    let promise = {
122                        let promises = entry.get_mut();
123                        promises
124                            .pop()
125                            .expect("rooted promise list unexpectedly empty")
126                    };
127                    if entry.get().is_empty() {
128                        entry.remove();
129                    }
130                    promise
131                },
132                Vacant(_) => unreachable!(),
133            }
134        })
135    }
136
137    /// A task which will reject the promise.
138    pub(crate) fn reject_task(self, error: Error) -> impl TaskOnce {
139        let this = self;
140        task!(reject_promise: move || {
141            debug!("Rejecting promise.");
142            this.root().reject_error(error, CanGc::note());
143        })
144    }
145
146    /// A task which will resolve the promise.
147    pub(crate) fn resolve_task<T>(self, value: T) -> impl TaskOnce
148    where
149        T: ToJSValConvertible + Send,
150    {
151        let this = self;
152        task!(resolve_promise: move || {
153            debug!("Resolving promise.");
154            this.root().resolve_native(&value, CanGc::note());
155        })
156    }
157}
158
159/// A safe wrapper around a raw pointer to a DOM object that can be
160/// shared among threads for use in asynchronous operations. The underlying
161/// DOM object is guaranteed to live at least as long as the last outstanding
162/// `Trusted<T>` instance.
163#[cfg_attr(crown, crown::unrooted_must_root_lint::allow_unrooted_interior)]
164#[derive(MallocSizeOf)]
165pub(crate) struct Trusted<T: DomObject> {
166    /// A pointer to the Rust DOM object of type T, but void to allow
167    /// sending `Trusted<T>` between threads, regardless of T's sendability.
168    #[conditional_malloc_size_of]
169    refcount: Arc<TrustedReference>,
170    #[ignore_malloc_size_of = "These are shared by all `Trusted` types."]
171    owner_thread: *const LiveDOMReferences,
172    phantom: PhantomData<T>,
173}
174
175impl<T: DomObject> std::fmt::Debug for Trusted<T> {
176    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
177        f.write_str("...")
178    }
179}
180
181unsafe impl<T: DomObject> Send for Trusted<T> {}
182
183impl<T: DomObject> Trusted<T> {
184    /// Create a new `Trusted<T>` instance from an existing DOM pointer. The DOM object will
185    /// be prevented from being GCed for the duration of the resulting `Trusted<T>` object's
186    /// lifetime.
187    pub(crate) fn new(ptr: &T) -> Trusted<T> {
188        fn add_live_reference(
189            ptr: *const libc::c_void,
190        ) -> (Arc<TrustedReference>, *const LiveDOMReferences) {
191            LIVE_REFERENCES.with(|r| {
192                let live_references = &*r.borrow();
193                let refcount = unsafe { live_references.addref(ptr) };
194                (refcount, live_references as *const _)
195            })
196        }
197
198        let (refcount, owner_thread) = add_live_reference(ptr as *const T as *const _);
199        Trusted {
200            refcount,
201            owner_thread,
202            phantom: PhantomData,
203        }
204    }
205
206    /// Obtain a usable DOM pointer from a pinned `Trusted<T>` value. Fails if used on
207    /// a different thread than the original value from which this `Trusted<T>` was
208    /// obtained.
209    pub(crate) fn root(&self) -> DomRoot<T> {
210        fn validate(owner_thread: *const LiveDOMReferences) {
211            assert!(LIVE_REFERENCES.with(|r| {
212                let r = r.borrow();
213                let live_references = &*r;
214                owner_thread == live_references
215            }));
216        }
217        validate(self.owner_thread);
218        unsafe { DomRoot::from_ref(&*(self.refcount.0 as *const T)) }
219    }
220}
221
222impl<T: DomObject> Clone for Trusted<T> {
223    fn clone(&self) -> Trusted<T> {
224        Trusted {
225            refcount: self.refcount.clone(),
226            owner_thread: self.owner_thread,
227            phantom: PhantomData,
228        }
229    }
230}
231
232/// The set of live, pinned DOM objects that are currently prevented
233/// from being garbage collected due to outstanding references.
234pub(crate) struct LiveDOMReferences {
235    // keyed on pointer to Rust DOM object
236    reflectable_table: RefCell<FxHashMap<*const libc::c_void, Weak<TrustedReference>>>,
237    promise_table: RefCell<FxHashMap<*const Promise, Vec<Rc<Promise>>>>,
238}
239
240impl LiveDOMReferences {
241    pub(crate) fn destruct() {
242        LIVE_REFERENCES.with(|r| {
243            let live_references = r.borrow_mut();
244            let _ = live_references.promise_table.take();
245            let _ = live_references.reflectable_table.take();
246        });
247    }
248
249    fn addref_promise(&self, promise: Rc<Promise>) {
250        let mut table = self.promise_table.borrow_mut();
251        table.entry(&*promise).or_default().push(promise)
252    }
253
254    /// ptr must be a pointer to a type that implements DOMObject.
255    /// This is not enforced by the type system to reduce duplicated generic code,
256    /// which is acceptable since this method is internal to this module.
257    #[expect(clippy::arc_with_non_send_sync)]
258    unsafe fn addref(&self, ptr: *const libc::c_void) -> Arc<TrustedReference> {
259        let mut table = self.reflectable_table.borrow_mut();
260        let capacity = table.capacity();
261        let len = table.len();
262        if (0 < capacity) && (capacity <= len) {
263            trace!("growing refcounted references by {}", len);
264            remove_nulls(&mut table);
265            table.reserve(len);
266        }
267        match table.entry(ptr) {
268            Occupied(mut entry) => match entry.get().upgrade() {
269                Some(refcount) => refcount,
270                None => {
271                    let refcount = Arc::new(unsafe { TrustedReference::new(ptr) });
272                    entry.insert(Arc::downgrade(&refcount));
273                    refcount
274                },
275            },
276            Vacant(entry) => {
277                let refcount = Arc::new(unsafe { TrustedReference::new(ptr) });
278                entry.insert(Arc::downgrade(&refcount));
279                refcount
280            },
281        }
282    }
283}
284
285/// Remove null entries from the live references table
286fn remove_nulls<K: Eq + Hash + Clone, V>(table: &mut FxHashMap<K, Weak<V>>) {
287    let to_remove: Vec<K> = table
288        .iter()
289        .filter(|&(_, value)| Weak::upgrade(value).is_none())
290        .map(|(key, _)| key.clone())
291        .collect();
292    trace!("removing {} refcounted references", to_remove.len());
293    for key in to_remove {
294        table.remove(&key);
295    }
296}
297
298/// A JSTraceDataOp for tracing reflectors held in LIVE_REFERENCES
299pub(crate) unsafe fn trace_refcounted_objects(tracer: *mut JSTracer) {
300    trace!("tracing live refcounted references");
301    LIVE_REFERENCES.with(|r| {
302        let live_references = &*r.borrow();
303        {
304            let mut table = live_references.reflectable_table.borrow_mut();
305            remove_nulls(&mut table);
306            for obj in table.keys() {
307                unsafe {
308                    trace_reflector(tracer, "refcounted", &*(*obj as *const Reflector));
309                }
310            }
311        }
312
313        {
314            let table = live_references.promise_table.borrow_mut();
315            for promise in table.keys() {
316                unsafe {
317                    trace_reflector(tracer, "refcounted", (**promise).reflector());
318                }
319            }
320        }
321    });
322}
script/dom/bindings/refcounted.rs

script/dom/bindings/
refcounted.rs
