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
44#[allow(missing_docs)] // FIXME
45mod dummy {
46    // Attributes don’t apply through the macro.
47    use std::cell::RefCell;
48    use std::rc::Rc;
49
50    use rustc_hash::FxHashMap;
51
52    use super::LiveDOMReferences;
53    thread_local!(pub(crate) static LIVE_REFERENCES: Rc<RefCell<LiveDOMReferences>> =
54        Rc::new(RefCell::new(
55        LiveDOMReferences {
56            reflectable_table: RefCell::new(FxHashMap::default()),
57            promise_table: RefCell::new(FxHashMap::default()),
58        }
59    )));
60}
61pub(crate) use self::dummy::LIVE_REFERENCES;
62
63/// A pointer to a Rust DOM object that needs to be destroyed.
64#[derive(MallocSizeOf)]
65struct TrustedReference(
66    #[ignore_malloc_size_of = "This is a shared reference."] *const libc::c_void,
67);
68unsafe impl Send for TrustedReference {}
69
70impl TrustedReference {
71    /// Creates a new TrustedReference from a pointer to a value that impements DOMObject.
72    /// This is not enforced by the type system to reduce duplicated generic code,
73    /// which is acceptable since this method is internal to this module.
74    unsafe fn new(ptr: *const libc::c_void) -> TrustedReference {
75        TrustedReference(ptr)
76    }
77}
78
79/// A safe wrapper around a DOM Promise object that can be shared among threads for use
80/// in asynchronous operations. The underlying DOM object is guaranteed to live at least
81/// as long as the last outstanding `TrustedPromise` instance. These values cannot be cloned,
82/// only created from existing `Rc<Promise>` values.
83pub struct TrustedPromise {
84    dom_object: *const Promise,
85    owner_thread: *const libc::c_void,
86}
87
88unsafe impl Send for TrustedPromise {}
89
90impl TrustedPromise {
91    /// Create a new `TrustedPromise` instance from an existing DOM object. The object will
92    /// be prevented from being GCed for the duration of the resulting `TrustedPromise` object's
93    /// lifetime.
94    #[cfg_attr(crown, allow(crown::unrooted_must_root))]
95    pub(crate) fn new(promise: Rc<Promise>) -> TrustedPromise {
96        LIVE_REFERENCES.with(|r| {
97            let live_references = &*r.borrow();
98            let ptr = &raw const *promise;
99            live_references.addref_promise(promise);
100            TrustedPromise {
101                dom_object: ptr,
102                owner_thread: (live_references) as *const _ as *const libc::c_void,
103            }
104        })
105    }
106
107    /// Obtain a usable DOM Promise from a pinned `TrustedPromise` value. Fails if used on
108    /// a different thread than the original value from which this `TrustedPromise` was
109    /// obtained.
110    pub(crate) fn root(self) -> Rc<Promise> {
111        LIVE_REFERENCES.with(|r| {
112            let live_references = &*r.borrow();
113            assert_eq!(
114                self.owner_thread,
115                live_references as *const _ as *const libc::c_void
116            );
117            match live_references
118                .promise_table
119                .borrow_mut()
120                .entry(self.dom_object)
121            {
122                Occupied(mut entry) => {
123                    let promise = {
124                        let promises = entry.get_mut();
125                        promises
126                            .pop()
127                            .expect("rooted promise list unexpectedly empty")
128                    };
129                    if entry.get().is_empty() {
130                        entry.remove();
131                    }
132                    promise
133                },
134                Vacant(_) => unreachable!(),
135            }
136        })
137    }
138
139    /// A task which will reject the promise.
140    #[cfg_attr(crown, allow(crown::unrooted_must_root))]
141    pub(crate) fn reject_task(self, error: Error) -> impl TaskOnce {
142        let this = self;
143        task!(reject_promise: move || {
144            debug!("Rejecting promise.");
145            this.root().reject_error(error, CanGc::note());
146        })
147    }
148
149    /// A task which will resolve the promise.
150    #[cfg_attr(crown, allow(crown::unrooted_must_root))]
151    pub(crate) fn resolve_task<T>(self, value: T) -> impl TaskOnce
152    where
153        T: ToJSValConvertible + Send,
154    {
155        let this = self;
156        task!(resolve_promise: move || {
157            debug!("Resolving promise.");
158            this.root().resolve_native(&value, CanGc::note());
159        })
160    }
161}
162
163/// A safe wrapper around a raw pointer to a DOM object that can be
164/// shared among threads for use in asynchronous operations. The underlying
165/// DOM object is guaranteed to live at least as long as the last outstanding
166/// `Trusted<T>` instance.
167#[cfg_attr(crown, crown::unrooted_must_root_lint::allow_unrooted_interior)]
168#[derive(MallocSizeOf)]
169pub(crate) struct Trusted<T: DomObject> {
170    /// A pointer to the Rust DOM object of type T, but void to allow
171    /// sending `Trusted<T>` between threads, regardless of T's sendability.
172    #[conditional_malloc_size_of]
173    refcount: Arc<TrustedReference>,
174    #[ignore_malloc_size_of = "These are shared by all `Trusted` types."]
175    owner_thread: *const LiveDOMReferences,
176    phantom: PhantomData<T>,
177}
178
179unsafe impl<T: DomObject> Send for Trusted<T> {}
180
181impl<T: DomObject> Trusted<T> {
182    /// Create a new `Trusted<T>` instance from an existing DOM pointer. The DOM object will
183    /// be prevented from being GCed for the duration of the resulting `Trusted<T>` object's
184    /// lifetime.
185    pub(crate) fn new(ptr: &T) -> Trusted<T> {
186        fn add_live_reference(
187            ptr: *const libc::c_void,
188        ) -> (Arc<TrustedReference>, *const LiveDOMReferences) {
189            LIVE_REFERENCES.with(|r| {
190                let live_references = &*r.borrow();
191                let refcount = unsafe { live_references.addref(ptr) };
192                (refcount, live_references as *const _)
193            })
194        }
195
196        let (refcount, owner_thread) = add_live_reference(ptr as *const T as *const _);
197        Trusted {
198            refcount,
199            owner_thread,
200            phantom: PhantomData,
201        }
202    }
203
204    /// Obtain a usable DOM pointer from a pinned `Trusted<T>` value. Fails if used on
205    /// a different thread than the original value from which this `Trusted<T>` was
206    /// obtained.
207    pub(crate) fn root(&self) -> DomRoot<T> {
208        fn validate(owner_thread: *const LiveDOMReferences) {
209            assert!(LIVE_REFERENCES.with(|r| {
210                let r = r.borrow();
211                let live_references = &*r;
212                owner_thread == live_references
213            }));
214        }
215        validate(self.owner_thread);
216        unsafe { DomRoot::from_ref(&*(self.refcount.0 as *const T)) }
217    }
218}
219
220impl<T: DomObject> Clone for Trusted<T> {
221    fn clone(&self) -> Trusted<T> {
222        Trusted {
223            refcount: self.refcount.clone(),
224            owner_thread: self.owner_thread,
225            phantom: PhantomData,
226        }
227    }
228}
229
230/// The set of live, pinned DOM objects that are currently prevented
231/// from being garbage collected due to outstanding references.
232#[cfg_attr(crown, allow(crown::unrooted_must_root))]
233pub(crate) struct LiveDOMReferences {
234    // keyed on pointer to Rust DOM object
235    reflectable_table: RefCell<FxHashMap<*const libc::c_void, Weak<TrustedReference>>>,
236    promise_table: RefCell<FxHashMap<*const Promise, Vec<Rc<Promise>>>>,
237}
238
239impl LiveDOMReferences {
240    pub(crate) fn destruct() {
241        LIVE_REFERENCES.with(|r| {
242            let live_references = r.borrow_mut();
243            let _ = live_references.promise_table.take();
244            let _ = live_references.reflectable_table.take();
245        });
246    }
247
248    #[cfg_attr(crown, allow(crown::unrooted_must_root))]
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    #[allow(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
299#[cfg_attr(crown, allow(crown::unrooted_must_root))]
300pub(crate) unsafe fn trace_refcounted_objects(tracer: *mut JSTracer) {
301    trace!("tracing live refcounted references");
302    LIVE_REFERENCES.with(|r| {
303        let live_references = &*r.borrow();
304        {
305            let mut table = live_references.reflectable_table.borrow_mut();
306            remove_nulls(&mut table);
307            for obj in table.keys() {
308                unsafe {
309                    trace_reflector(tracer, "refcounted", &*(*obj as *const Reflector));
310                }
311            }
312        }
313
314        {
315            let table = live_references.promise_table.borrow_mut();
316            for promise in table.keys() {
317                unsafe {
318                    trace_reflector(tracer, "refcounted", (**promise).reflector());
319                }
320            }
321        }
322    });
323}
script/dom/bindings/refcounted.rs

script/dom/bindings/
refcounted.rs
