servo: Merge #16014 - Per-process lock for CSSOM objects (from servo:style-ref); r=emilio

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Before this PR, every object reflected in CSSOM is in `Arc<RwLock<_>>` to enable safe (synchronized) mutable aliasing. Acquiring all these locks has significant cost during selector matching:

* https://bugzilla.mozilla.org/show_bug.cgi?id=1311469
* https://bugzilla.mozilla.org/show_bug.cgi?id=1335941
* https://bugzilla.mozilla.org/show_bug.cgi?id=1339703

This PR introduce a mechanism to protect many objects with the same `RwLock` that only needs to be acquired once.

In Stylo, there is one such lock per process (in a `lazy_static`), used for everything.

I non-Stylo Servo, I originally intended to have one such lock per document (for author-origin stylesheets, and one per process for user-agent and user sytlesheets since they’re shared across documents, and never mutated anyway). However I failed to have the same document-specific (or pipeline-specific) `Arc` reachable from both `Document` nodes and `LayoutThread`. Recursively following callers lead me to include this `Arc` in `UnprivilegedPipelineContent`, but that needs to be serializable. So there is a second process-wide lock.

This was previously #15998, closed accidentally.

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- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
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Source-Repo: https://github.com/servo/servo
Source-Revision: bb54f0a429de0e8b8861f8071b6cf82f73622664

servo/components/script/dom/csskeyframerule.rs

@@ -12,21 +12,20 @@ use dom::cssstyledeclaration::{CSSModificationAccess, CSSStyleDeclaration, CSSSt
 use dom::cssstylesheet::CSSStyleSheet;
 use dom::window::Window;
 use dom_struct::dom_struct;
-use parking_lot::RwLock;
 use std::sync::Arc;
 use style::keyframes::Keyframe;
-use style_traits::ToCss;
+use style::shared_lock::{Locked, ToCssWithGuard};
 
 #[dom_struct]
 pub struct CSSKeyframeRule {
     cssrule: CSSRule,
     #[ignore_heap_size_of = "Arc"]
-    keyframerule: Arc<RwLock<Keyframe>>,
+    keyframerule: Arc<Locked<Keyframe>>,
     style_decl: MutNullableJS<CSSStyleDeclaration>,
 }
 
 impl CSSKeyframeRule {
-    fn new_inherited(parent_stylesheet: &CSSStyleSheet, keyframerule: Arc<RwLock<Keyframe>>)
+    fn new_inherited(parent_stylesheet: &CSSStyleSheet, keyframerule: Arc<Locked<Keyframe>>)
                      -> CSSKeyframeRule {
         CSSKeyframeRule {
             cssrule: CSSRule::new_inherited(parent_stylesheet),
@@ -37,7 +36,7 @@ impl CSSKeyframeRule {
 
     #[allow(unrooted_must_root)]
     pub fn new(window: &Window, parent_stylesheet: &CSSStyleSheet,
-               keyframerule: Arc<RwLock<Keyframe>>) -> Root<CSSKeyframeRule> {
+               keyframerule: Arc<Locked<Keyframe>>) -> Root<CSSKeyframeRule> {
         reflect_dom_object(box CSSKeyframeRule::new_inherited(parent_stylesheet, keyframerule),
                            window,
                            CSSKeyframeRuleBinding::Wrap)
@@ -48,11 +47,16 @@ impl CSSKeyframeRuleMethods for CSSKeyframeRule {
     // https://drafts.csswg.org/css-animations/#dom-csskeyframerule-style
     fn Style(&self) -> Root<CSSStyleDeclaration> {
         self.style_decl.or_init(|| {
-            CSSStyleDeclaration::new(self.global().as_window(),
-                                     CSSStyleOwner::CSSRule(JS::from_ref(self.upcast()),
-                                                            self.keyframerule.read().block.clone()),
-                                     None,
-                                     CSSModificationAccess::ReadWrite)
+            let guard = self.cssrule.shared_lock().read();
+            CSSStyleDeclaration::new(
+                self.global().as_window(),
+                CSSStyleOwner::CSSRule(
+                    JS::from_ref(self.upcast()),
+                    self.keyframerule.read_with(&guard).block.clone(),
+                ),
+                None,
+                CSSModificationAccess::ReadWrite,
+            )
         })
     }
 }
@@ -64,6 +68,7 @@ impl SpecificCSSRule for CSSKeyframeRule {
     }
 
     fn get_css(&self) -> DOMString {
-        self.keyframerule.read().to_css_string().into()
+        let guard = self.cssrule.shared_lock().read();
+        self.keyframerule.read_with(&guard).to_css_string(&guard).into()
     }
 }