Auto merge of #16014 - servo:style-ref, r=emilio

Per-process lock for CSSOM objects

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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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- [ ] These changes do not require tests because _____

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components/layout/block.rs

@@ -446,7 +446,7 @@ fn translate_including_floats(cur_b: &mut Au, delta: Au, floats: &mut Floats) {
 ///
 /// Note that flows with position 'fixed' just form a flat list as they all
 /// have the Root flow as their CB.
-pub struct AbsoluteAssignBSizesTraversal<'a>(pub &'a SharedStyleContext);
+pub struct AbsoluteAssignBSizesTraversal<'a>(pub &'a SharedStyleContext<'a>);
 
 impl<'a> PreorderFlowTraversal for AbsoluteAssignBSizesTraversal<'a> {
     #[inline]

components/layout/construct.rs

@@ -311,7 +311,7 @@ impl InlineFragmentsAccumulator {
 /// An object that knows how to create flows.
 pub struct FlowConstructor<'a, N: ThreadSafeLayoutNode> {
     /// The layout context.
-    pub layout_context: &'a LayoutContext,
+    pub layout_context: &'a LayoutContext<'a>,
     /// Satisfy the compiler about the unused parameters, which we use to improve the ergonomics of
     /// the ensuing impl {} by removing the need to parameterize all the methods individually.
     phantom2: PhantomData<N>,
@@ -320,7 +320,7 @@ pub struct FlowConstructor<'a, N: ThreadSafeLayoutNode> {
 impl<'a, ConcreteThreadSafeLayoutNode: ThreadSafeLayoutNode>
   FlowConstructor<'a, ConcreteThreadSafeLayoutNode> {
     /// Creates a new flow constructor.
-    pub fn new(layout_context: &'a LayoutContext) -> Self {
+    pub fn new(layout_context: &'a LayoutContext<'a>) -> Self {
         FlowConstructor {
             layout_context: layout_context,
             phantom2: PhantomData,
@@ -660,10 +660,9 @@ impl<'a, ConcreteThreadSafeLayoutNode: ThreadSafeLayoutNode>
 
             let mut style = node.style(self.style_context());
             if node_is_input_or_text_area {
-                style = self.style_context()
-                            .stylist
-                            .style_for_anonymous_box(&PseudoElement::ServoInputText,
-                                                     &style)
+                let context = self.style_context();
+                style = context.stylist.style_for_anonymous_box(
+                    &context.guards, &PseudoElement::ServoInputText, &style)
             }
 
             self.create_fragments_for_node_text_content(&mut initial_fragments, node, &style)
@@ -1096,11 +1095,14 @@ impl<'a, ConcreteThreadSafeLayoutNode: ThreadSafeLayoutNode>
                             -> ConstructionResult {
         let mut legalizer = Legalizer::new();
 
-        let table_style = node.style(self.style_context());
-        let wrapper_style = self.style_context()
-                                .stylist
-                                .style_for_anonymous_box(&PseudoElement::ServoTableWrapper,
-                                                         &table_style);
+        let table_style;
+        let wrapper_style;
+        {
+            let context = self.style_context();
+            table_style = node.style(context);
+            wrapper_style = context.stylist.style_for_anonymous_box(
+                &context.guards, &PseudoElement::ServoTableWrapper, &table_style);
+        }
         let wrapper_fragment =
             Fragment::from_opaque_node_and_style(node.opaque(),
                                                  PseudoElementType::Normal,
@@ -2080,8 +2082,7 @@ impl Legalizer {
         let reference_block = reference.as_block();
         let mut new_style = reference_block.fragment.style.clone();
         for pseudo in pseudos {
-            new_style = context.stylist.style_for_anonymous_box(pseudo,
-                                                                &new_style)
+            new_style = context.stylist.style_for_anonymous_box(&context.guards, pseudo, &new_style)
         }
         let fragment = reference_block.fragment
                                       .create_similar_anonymous_fragment(new_style,

components/layout/context.rs

@@ -75,9 +75,9 @@ pub fn heap_size_of_persistent_local_context() -> usize {
 }
 
 /// Layout information shared among all workers. This must be thread-safe.
-pub struct LayoutContext {
+pub struct LayoutContext<'a> {
     /// Bits shared by the layout and style system.
-    pub style_context: SharedStyleContext,
+    pub style_context: SharedStyleContext<'a>,
 
     /// The shared image cache thread.
     pub image_cache_thread: Mutex<ImageCacheThread>,
@@ -95,7 +95,7 @@ pub struct LayoutContext {
     pub pending_images: Option<Mutex<Vec<PendingImage>>>
 }
 
-impl Drop for LayoutContext {
+impl<'a> Drop for LayoutContext<'a> {
     fn drop(&mut self) {
         if !thread::panicking() {
             if let Some(ref pending_images) = self.pending_images {
@@ -105,7 +105,7 @@ impl Drop for LayoutContext {
     }
 }
 
-impl LayoutContext {
+impl<'a> LayoutContext<'a> {
     #[inline(always)]
     pub fn shared_context(&self) -> &SharedStyleContext {
         &self.style_context

components/layout/display_list_builder.rs

@@ -121,7 +121,7 @@ fn get_cyclic<T>(arr: &[T], index: usize) -> &T {
 }
 
 pub struct DisplayListBuildState<'a> {
-    pub layout_context: &'a LayoutContext,
+    pub layout_context: &'a LayoutContext<'a>,
     pub root_stacking_context: StackingContext,
     pub items: HashMap<StackingContextId, Vec<DisplayItem>>,
     pub stacking_context_children: HashMap<StackingContextId, Vec<StackingContext>>,

components/layout/generated_content.rs

@@ -97,7 +97,7 @@ static KATAKANA_IROHA: [char; 47] = [
 /// The generated content resolution traversal.
 pub struct ResolveGeneratedContent<'a> {
     /// The layout context.
-    layout_context: &'a LayoutContext,
+    layout_context: &'a LayoutContext<'a>,
     /// The counter representing an ordered list item.
     list_item: Counter,
     /// Named CSS counters.

components/layout/traversal.rs

@@ -22,20 +22,20 @@ use style::traversal::PerLevelTraversalData;
 use wrapper::{GetRawData, LayoutNodeHelpers, LayoutNodeLayoutData};
 use wrapper::ThreadSafeLayoutNodeHelpers;
 
-pub struct RecalcStyleAndConstructFlows {
-    context: LayoutContext,
+pub struct RecalcStyleAndConstructFlows<'a> {
+    context: LayoutContext<'a>,
     driver: TraversalDriver,
 }
 
-impl RecalcStyleAndConstructFlows {
-    pub fn layout_context(&self) -> &LayoutContext {
+impl<'a> RecalcStyleAndConstructFlows<'a> {
+    pub fn layout_context(&self) -> &LayoutContext<'a> {
         &self.context
     }
 }
 
-impl RecalcStyleAndConstructFlows {
+impl<'a> RecalcStyleAndConstructFlows<'a> {
     /// Creates a traversal context, taking ownership of the shared layout context.
-    pub fn new(context: LayoutContext, driver: TraversalDriver) -> Self {
+    pub fn new(context: LayoutContext<'a>, driver: TraversalDriver) -> Self {
         RecalcStyleAndConstructFlows {
             context: context,
             driver: driver,
@@ -44,13 +44,13 @@ impl RecalcStyleAndConstructFlows {
 
     /// Consumes this traversal context, returning ownership of the shared layout
     /// context to the caller.
-    pub fn destroy(self) -> LayoutContext {
+    pub fn destroy(self) -> LayoutContext<'a> {
         self.context
     }
 }
 
 #[allow(unsafe_code)]
-impl<E> DomTraversal<E> for RecalcStyleAndConstructFlows
+impl<'a, E> DomTraversal<E> for RecalcStyleAndConstructFlows<'a>
     where E: TElement,
           E::ConcreteNode: LayoutNode,
 {
@@ -152,7 +152,7 @@ fn construct_flows_at<N>(context: &LayoutContext,
 /// The bubble-inline-sizes traversal, the first part of layout computation. This computes
 /// preferred and intrinsic inline-sizes and bubbles them up the tree.
 pub struct BubbleISizes<'a> {
-    pub layout_context: &'a LayoutContext,
+    pub layout_context: &'a LayoutContext<'a>,
 }
 
 impl<'a> PostorderFlowTraversal for BubbleISizes<'a> {
@@ -171,7 +171,7 @@ impl<'a> PostorderFlowTraversal for BubbleISizes<'a> {
 /// The assign-inline-sizes traversal. In Gecko this corresponds to `Reflow`.
 #[derive(Copy, Clone)]
 pub struct AssignISizes<'a> {
-    pub layout_context: &'a LayoutContext,
+    pub layout_context: &'a LayoutContext<'a>,
 }
 
 impl<'a> PreorderFlowTraversal for AssignISizes<'a> {
@@ -191,7 +191,7 @@ impl<'a> PreorderFlowTraversal for AssignISizes<'a> {
 /// positions. In Gecko this corresponds to `Reflow`.
 #[derive(Copy, Clone)]
 pub struct AssignBSizes<'a> {
-    pub layout_context: &'a LayoutContext,
+    pub layout_context: &'a LayoutContext<'a>,
 }
 
 impl<'a> PostorderFlowTraversal for AssignBSizes<'a> {
@@ -220,7 +220,7 @@ impl<'a> PostorderFlowTraversal for AssignBSizes<'a> {
 
 #[derive(Copy, Clone)]
 pub struct ComputeAbsolutePositions<'a> {
-    pub layout_context: &'a LayoutContext,
+    pub layout_context: &'a LayoutContext<'a>,
 }
 
 impl<'a> PreorderFlowTraversal for ComputeAbsolutePositions<'a> {