Auto merge of #26220 - servo:layout-2020-rule-tree-new-on-write, r=emilio

Always upgrade existing weak child references in the rule tree

Just because we didn't find a child when read-locking a node children list
doesn't mean it still won't exist while we wait to upgrade the read lock
into a write lock to create the child.

components/style/rule_tree/core.rs

@@ -402,18 +402,31 @@ impl StrongRuleNode {
             return child.upgrade();
         }
         let mut children = RwLockUpgradableReadGuard::upgrade(children);
-        let weak = children.get_or_insert_with(
+        let mut is_new = false;
+        let weak = {
+            let is_new = &mut is_new;
+            children.get_or_insert_with(
                 key,
                 |node| node.p.key(),
                 move || {
+                    *is_new = true;
                     let root = unsafe { root.downgrade() };
-                let strong =
+                    let strong = StrongRuleNode::new(Box::new(RuleNode::new(
-                    StrongRuleNode::new(Box::new(RuleNode::new(root, self.clone(), source, level)));
+                        root,
+                        self.clone(),
+                        source,
+                        level,
+                    )));
                     let weak = unsafe { strong.downgrade() };
                     mem::forget(strong);
                     weak
                 },
-        );
+            )
+        };
+
+        if !is_new {
+            return weak.upgrade();
+        }
 
         unsafe { StrongRuleNode::from_unsafe_box(UnsafeBox::clone(&weak.p)) }
     }