servo/components/script/dom/bindings/trace.rs

/* 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 https://mozilla.org/MPL/2.0/. */

//! Utilities for tracing JS-managed values.
//!
//! The lifetime of DOM objects is managed by the SpiderMonkey Garbage
//! Collector. A rooted DOM object implementing the interface `Foo` is traced
//! as follows:
//!
//! 1. The GC calls `_trace` defined in `FooBinding` during the marking
//!    phase. (This happens through `JSClass.trace` for non-proxy bindings, and
//!    through `ProxyTraps.trace` otherwise.)
//! 2. `_trace` calls `Foo::trace()` (an implementation of `JSTraceable`).
//!    This is typically derived via a `#[dom_struct]`
//!    (implies `#[derive(JSTraceable)]`) annotation.
//!    Non-JS-managed types have an empty inline `trace()` method,
//!    achieved via `unsafe_no_jsmanaged_fields!` or similar.
//! 3. For all fields, `Foo::trace()`
//!    calls `trace()` on the field.
//!    For example, for fields of type `Dom<T>`, `Dom<T>::trace()` calls
//!    `trace_reflector()`.
//! 4. `trace_reflector()` calls `Dom::TraceEdge()` with a
//!    pointer to the `JSObject` for the reflector. This notifies the GC, which
//!    will add the object to the graph, and will trace that object as well.
//! 5. When the GC finishes tracing, it [`finalizes`](../index.html#destruction)
//!    any reflectors that were not reachable.
//!
//! The `unsafe_no_jsmanaged_fields!()` macro adds an empty implementation of
//! `JSTraceable` to a datatype.

use std::cell::OnceCell;
use std::collections::hash_map::RandomState;
use std::collections::HashMap;
use std::fmt::Display;
use std::hash::{BuildHasher, Hash};
use std::mem;
use std::ops::{Deref, DerefMut};

use crossbeam_channel::Sender;
use indexmap::IndexMap;
/// A trait to allow tracing (only) DOM objects.
pub(crate) use js::gc::Traceable as JSTraceable;
pub(crate) use js::gc::{RootableVec, RootedVec};
use js::glue::{CallObjectTracer, CallScriptTracer, CallStringTracer, CallValueTracer};
use js::jsapi::{GCTraceKindToAscii, Heap, JSObject, JSScript, JSString, JSTracer, TraceKind};
use js::jsval::JSVal;
use js::rust::{GCMethods, Handle};
use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
use parking_lot::RwLock;
use servo_arc::Arc as ServoArc;
use smallvec::SmallVec;
use style::author_styles::AuthorStyles;
use style::stylesheet_set::{AuthorStylesheetSet, DocumentStylesheetSet};
use tendril::fmt::UTF8;
use tendril::stream::LossyDecoder;
use tendril::TendrilSink;
#[cfg(feature = "webxr")]
use webxr_api::{Finger, Hand};

use crate::dom::bindings::cell::DomRefCell;
use crate::dom::bindings::error::Error;
use crate::dom::bindings::refcounted::{Trusted, TrustedPromise};
use crate::dom::bindings::reflector::{DomObject, Reflector};
use crate::dom::htmlimageelement::SourceSet;
use crate::dom::htmlmediaelement::HTMLMediaElementFetchContext;
use crate::dom::windowproxy::WindowProxyHandler;
use crate::script_runtime::StreamConsumer;
use crate::script_thread::IncompleteParserContexts;
use crate::task::TaskBox;

/// A trait to allow tracing only DOM sub-objects.
///
/// # Safety
///
/// This trait is unsafe; if it is implemented incorrectly, the GC may end up collecting objects
/// that are still reachable.
pub(crate) unsafe trait CustomTraceable {
    /// Trace `self`.
    ///
    /// # Safety
    ///
    /// The `JSTracer` argument must point to a valid `JSTracer` in memory. In addition,
    /// implementors of this method must ensure that all active objects are properly traced
    /// or else the garbage collector may end up collecting objects that are still reachable.
    unsafe fn trace(&self, trc: *mut JSTracer);
}

unsafe impl<T: CustomTraceable> CustomTraceable for Box<T> {
    #[inline]
    unsafe fn trace(&self, trc: *mut JSTracer) {
        (**self).trace(trc);
    }
}

unsafe impl<T: CustomTraceable> CustomTraceable for DomRefCell<T> {
    unsafe fn trace(&self, trc: *mut JSTracer) {
        (*self).borrow().trace(trc)
    }
}

unsafe impl<T: JSTraceable> CustomTraceable for OnceCell<T> {
    unsafe fn trace(&self, tracer: *mut JSTracer) {
        if let Some(value) = self.get() {
            value.trace(tracer)
        }
    }
}

unsafe impl<T> CustomTraceable for Sender<T> {
    unsafe fn trace(&self, _: *mut JSTracer) {}
}

/// Wrapper type for nop traceble
///
/// SAFETY: Inner type must not impl JSTraceable
#[derive(Clone, Copy, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
#[cfg_attr(crown, crown::trace_in_no_trace_lint::must_not_have_traceable)]
pub(crate) struct NoTrace<T>(pub(crate) T);

impl<T: Display> Display for NoTrace<T> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        self.0.fmt(f)
    }
}

impl<T> From<T> for NoTrace<T> {
    fn from(item: T) -> Self {
        Self(item)
    }
}

#[allow(unsafe_code)]
unsafe impl<T> JSTraceable for NoTrace<T> {
    #[inline]
    unsafe fn trace(&self, _: *mut ::js::jsapi::JSTracer) {}
}

impl<T: MallocSizeOf> MallocSizeOf for NoTrace<T> {
    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
        self.0.size_of(ops)
    }
}

/// HashMap wrapper, that has non-jsmanaged keys
///
/// Not all methods are reexposed, but you can access inner type via .0
#[cfg_attr(crown, crown::trace_in_no_trace_lint::must_not_have_traceable(0))]
#[derive(Clone, Debug)]
pub(crate) struct HashMapTracedValues<K, V, S = RandomState>(pub(crate) HashMap<K, V, S>);

impl<K, V, S: Default> Default for HashMapTracedValues<K, V, S> {
    fn default() -> Self {
        Self(Default::default())
    }
}

impl<K, V> HashMapTracedValues<K, V, RandomState> {
    /// Wrapper for HashMap::new()
    #[inline]
    #[must_use]
    pub(crate) fn new() -> HashMapTracedValues<K, V, RandomState> {
        Self(HashMap::new())
    }
}

impl<K, V, S> HashMapTracedValues<K, V, S> {
    #[inline]
    pub(crate) fn iter(&self) -> std::collections::hash_map::Iter<'_, K, V> {
        self.0.iter()
    }

    #[inline]
    pub(crate) fn drain(&mut self) -> std::collections::hash_map::Drain<'_, K, V> {
        self.0.drain()
    }

    #[inline]
    pub(crate) fn is_empty(&self) -> bool {
        self.0.is_empty()
    }
}

impl<K, V, S> HashMapTracedValues<K, V, S>
where
    K: Eq + Hash,
    S: BuildHasher,
{
    #[inline]
    pub(crate) fn insert(&mut self, k: K, v: V) -> Option<V> {
        self.0.insert(k, v)
    }

    #[inline]
    pub(crate) fn get<Q>(&self, k: &Q) -> Option<&V>
    where
        K: std::borrow::Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        self.0.get(k)
    }

    #[inline]
    pub(crate) fn get_mut<Q: Hash + Eq + ?Sized>(&mut self, k: &Q) -> Option<&mut V>
    where
        K: std::borrow::Borrow<Q>,
    {
        self.0.get_mut(k)
    }

    #[inline]
    pub(crate) fn contains_key<Q: Hash + Eq + ?Sized>(&self, k: &Q) -> bool
    where
        K: std::borrow::Borrow<Q>,
    {
        self.0.contains_key(k)
    }

    #[inline]
    pub(crate) fn remove<Q: Hash + Eq + ?Sized>(&mut self, k: &Q) -> Option<V>
    where
        K: std::borrow::Borrow<Q>,
    {
        self.0.remove(k)
    }

    #[inline]
    pub(crate) fn entry(&mut self, key: K) -> std::collections::hash_map::Entry<'_, K, V> {
        self.0.entry(key)
    }
}

impl<K, V, S> MallocSizeOf for HashMapTracedValues<K, V, S>
where
    K: Eq + Hash + MallocSizeOf,
    V: MallocSizeOf,
    S: BuildHasher,
{
    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
        self.0.size_of(ops)
    }
}

#[allow(unsafe_code)]
unsafe impl<K, V: JSTraceable, S> JSTraceable for HashMapTracedValues<K, V, S> {
    #[inline]
    unsafe fn trace(&self, trc: *mut ::js::jsapi::JSTracer) {
        for v in self.0.values() {
            v.trace(trc);
        }
    }
}

unsafe_no_jsmanaged_fields!(Box<dyn TaskBox>);

unsafe_no_jsmanaged_fields!(IncompleteParserContexts);

#[allow(dead_code)]
/// Trace a `JSScript`.
pub(crate) fn trace_script(tracer: *mut JSTracer, description: &str, script: &Heap<*mut JSScript>) {
    unsafe {
        trace!("tracing {}", description);
        CallScriptTracer(
            tracer,
            script.ptr.get() as *mut _,
            GCTraceKindToAscii(TraceKind::Script),
        );
    }
}

#[allow(dead_code)]
/// Trace a `JSVal`.
pub(crate) fn trace_jsval(tracer: *mut JSTracer, description: &str, val: &Heap<JSVal>) {
    unsafe {
        if !val.get().is_markable() {
            return;
        }

        trace!("tracing value {}", description);
        CallValueTracer(
            tracer,
            val.ptr.get() as *mut _,
            GCTraceKindToAscii(val.get().trace_kind()),
        );
    }
}

/// Trace the `JSObject` held by `reflector`.
#[cfg_attr(crown, allow(crown::unrooted_must_root))]
pub(crate) fn trace_reflector(tracer: *mut JSTracer, description: &str, reflector: &Reflector) {
    trace!("tracing reflector {}", description);
    trace_object(tracer, description, reflector.rootable())
}

/// Trace a `JSObject`.
pub(crate) fn trace_object(tracer: *mut JSTracer, description: &str, obj: &Heap<*mut JSObject>) {
    unsafe {
        trace!("tracing {}", description);
        CallObjectTracer(
            tracer,
            obj.ptr.get() as *mut _,
            GCTraceKindToAscii(TraceKind::Object),
        );
    }
}

#[allow(dead_code)]
/// Trace a `JSString`.
pub(crate) fn trace_string(tracer: *mut JSTracer, description: &str, s: &Heap<*mut JSString>) {
    unsafe {
        trace!("tracing {}", description);
        CallStringTracer(
            tracer,
            s.ptr.get() as *mut _,
            GCTraceKindToAscii(TraceKind::String),
        );
    }
}

unsafe impl<T: JSTraceable> CustomTraceable for ServoArc<T> {
    unsafe fn trace(&self, trc: *mut JSTracer) {
        (**self).trace(trc)
    }
}

unsafe impl<T: JSTraceable> CustomTraceable for RwLock<T> {
    unsafe fn trace(&self, trc: *mut JSTracer) {
        self.read().trace(trc)
    }
}

unsafe impl<T: JSTraceable> JSTraceable for DomRefCell<T> {
    unsafe fn trace(&self, trc: *mut JSTracer) {
        (*self).borrow().trace(trc)
    }
}

unsafe impl<T: JSTraceable + Eq + Hash> CustomTraceable for indexmap::IndexSet<T> {
    #[inline]
    unsafe fn trace(&self, trc: *mut JSTracer) {
        for e in self.iter() {
            e.trace(trc);
        }
    }
}

// XXXManishearth Check if the following three are optimized to no-ops
// if e.trace() is a no-op (e.g it is an unsafe_no_jsmanaged_fields type)
unsafe impl<T: JSTraceable + 'static> CustomTraceable for SmallVec<[T; 1]> {
    #[inline]
    unsafe fn trace(&self, trc: *mut JSTracer) {
        for e in self.iter() {
            e.trace(trc);
        }
    }
}

unsafe impl<K, V, S> CustomTraceable for IndexMap<K, V, S>
where
    K: Hash + Eq + JSTraceable,
    V: JSTraceable,
    S: BuildHasher,
{
    #[inline]
    unsafe fn trace(&self, trc: *mut JSTracer) {
        for (k, v) in self {
            k.trace(trc);
            v.trace(trc);
        }
    }
}

unsafe_no_jsmanaged_fields!(Error);
unsafe_no_jsmanaged_fields!(TrustedPromise);

unsafe_no_jsmanaged_fields!(WindowProxyHandler);
unsafe_no_jsmanaged_fields!(SourceSet);
unsafe_no_jsmanaged_fields!(HTMLMediaElementFetchContext);
unsafe_no_jsmanaged_fields!(StreamConsumer);

unsafe impl<T: DomObject> JSTraceable for Trusted<T> {
    #[inline]
    unsafe fn trace(&self, _: *mut JSTracer) {
        // Do nothing
    }
}

unsafe impl<S> CustomTraceable for DocumentStylesheetSet<S>
where
    S: JSTraceable + ::style::stylesheets::StylesheetInDocument + PartialEq + 'static,
{
    unsafe fn trace(&self, tracer: *mut JSTracer) {
        for (s, _origin) in self.iter() {
            s.trace(tracer)
        }
    }
}

unsafe impl<S> CustomTraceable for AuthorStylesheetSet<S>
where
    S: JSTraceable + ::style::stylesheets::StylesheetInDocument + PartialEq + 'static,
{
    unsafe fn trace(&self, tracer: *mut JSTracer) {
        for s in self.iter() {
            s.trace(tracer)
        }
    }
}

unsafe impl<S> CustomTraceable for AuthorStyles<S>
where
    S: JSTraceable + ::style::stylesheets::StylesheetInDocument + PartialEq + 'static,
{
    unsafe fn trace(&self, tracer: *mut JSTracer) {
        self.stylesheets.trace(tracer)
    }
}

unsafe impl<Sink> CustomTraceable for LossyDecoder<Sink>
where
    Sink: JSTraceable + TendrilSink<UTF8>,
{
    unsafe fn trace(&self, tracer: *mut JSTracer) {
        self.inner_sink().trace(tracer);
    }
}

#[cfg(feature = "webxr")]
unsafe impl<J> CustomTraceable for Hand<J>
where
    J: JSTraceable,
{
    #[inline]
    unsafe fn trace(&self, trc: *mut JSTracer) {
        // exhaustive match so we don't miss new fields
        let Hand {
            ref wrist,
            ref thumb_metacarpal,
            ref thumb_phalanx_proximal,
            ref thumb_phalanx_distal,
            ref thumb_phalanx_tip,
            ref index,
            ref middle,
            ref ring,
            ref little,
        } = *self;
        wrist.trace(trc);
        thumb_metacarpal.trace(trc);
        thumb_phalanx_proximal.trace(trc);
        thumb_phalanx_distal.trace(trc);
        thumb_phalanx_tip.trace(trc);
        index.trace(trc);
        middle.trace(trc);
        ring.trace(trc);
        little.trace(trc);
    }
}

#[cfg(feature = "webxr")]
unsafe impl<J> CustomTraceable for Finger<J>
where
    J: JSTraceable,
{
    #[inline]
    unsafe fn trace(&self, trc: *mut JSTracer) {
        // exhaustive match so we don't miss new fields
        let Finger {
            ref metacarpal,
            ref phalanx_proximal,
            ref phalanx_intermediate,
            ref phalanx_distal,
            ref phalanx_tip,
        } = *self;
        metacarpal.trace(trc);
        phalanx_proximal.trace(trc);
        phalanx_intermediate.trace(trc);
        phalanx_distal.trace(trc);
        phalanx_tip.trace(trc);
    }
}

/// Roots any JSTraceable thing
///
/// If you have a valid DomObject, use DomRoot.
/// If you have GC things like *mut JSObject or JSVal, use rooted!.
/// If you have an arbitrary number of DomObjects to root, use rooted_vec!.
/// If you know what you're doing, use this.
#[cfg_attr(crown, crown::unrooted_must_root_lint::allow_unrooted_interior)]
pub(crate) struct RootedTraceableBox<T: JSTraceable + 'static>(js::gc::RootedTraceableBox<T>);

unsafe impl<T: JSTraceable + 'static> JSTraceable for RootedTraceableBox<T> {
    unsafe fn trace(&self, tracer: *mut JSTracer) {
        self.0.trace(tracer);
    }
}

impl<T: JSTraceable + 'static> RootedTraceableBox<T> {
    /// DomRoot a JSTraceable thing for the life of this RootedTraceableBox
    pub(crate) fn new(traceable: T) -> RootedTraceableBox<T> {
        Self(js::gc::RootedTraceableBox::new(traceable))
    }

    /// Consumes a boxed JSTraceable and roots it for the life of this RootedTraceableBox.
    pub(crate) fn from_box(boxed_traceable: Box<T>) -> RootedTraceableBox<T> {
        Self(js::gc::RootedTraceableBox::from_box(boxed_traceable))
    }
}

impl<T> RootedTraceableBox<Heap<T>>
where
    Heap<T>: JSTraceable + 'static,
    T: GCMethods + Copy,
{
    pub(crate) fn handle(&self) -> Handle<T> {
        self.0.handle()
    }
}

impl<T: JSTraceable + MallocSizeOf> MallocSizeOf for RootedTraceableBox<T> {
    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
        // Briefly resurrect the real Box value so we can rely on the existing calculations.
        // Then immediately forget about it again to avoid dropping the box.
        let inner = unsafe { Box::from_raw(self.0.ptr()) };
        let size = inner.size_of(ops);
        mem::forget(inner);
        size
    }
}

impl<T: JSTraceable + Default> Default for RootedTraceableBox<T> {
    fn default() -> RootedTraceableBox<T> {
        RootedTraceableBox::new(T::default())
    }
}

impl<T: JSTraceable> Deref for RootedTraceableBox<T> {
    type Target = T;
    fn deref(&self) -> &T {
        self.0.deref()
    }
}

impl<T: JSTraceable> DerefMut for RootedTraceableBox<T> {
    fn deref_mut(&mut self) -> &mut T {
        self.0.deref_mut()
    }
}