fix: meta referrer updating to follow spec (servo#36390)

Previously the referrer policy used tree order but the spec only cares
about the most-recently-updated or most-recently-added meta referrer.

Testing: change has existing WPT tests

---------

Signed-off-by: Sebastian C <sebsebmc@gmail.com>

Author: sebsebmc

components/script/dom/htmlheadelement.rs

@@ -10,7 +10,7 @@ use js::rust::HandleObject;
 use crate::dom::bindings::codegen::Bindings::DocumentBinding::DocumentMethods;
 use crate::dom::bindings::inheritance::Castable;
 use crate::dom::bindings::root::DomRoot;
-use crate::dom::document::{Document, determine_policy_for_token};
+use crate::dom::document::Document;
 use crate::dom::element::Element;
 use crate::dom::htmlelement::HTMLElement;
 use crate::dom::htmlmetaelement::HTMLMetaElement;
@@ -54,37 +54,6 @@ impl HTMLHeadElement {
         n
     }
 
-    /// <https://html.spec.whatwg.org/multipage/#meta-referrer>
-    pub(crate) fn set_document_referrer(&self) {
-        let doc = self.owner_document();
-
-        if doc.GetHead().as_deref() != Some(self) {
-            return;
-        }
-
-        let node = self.upcast::<Node>();
-        let candidates = node
-            .traverse_preorder(ShadowIncluding::No)
-            .filter_map(DomRoot::downcast::<Element>)
-            .filter(|elem| elem.is::<HTMLMetaElement>())
-            .filter(|elem| elem.get_name() == Some(atom!("referrer")))
-            .filter(|elem| {
-                elem.get_attribute(&ns!(), &local_name!("content"))
-                    .is_some()
-            });
-
-        for meta in candidates {
-            if let Some(ref content) = meta.get_attribute(&ns!(), &local_name!("content")) {
-                let content = content.value();
-                let content_val = content.trim();
-                if !content_val.is_empty() {
-                    doc.set_referrer_policy(determine_policy_for_token(content_val));
-                    return;
-                }
-            }
-        }
-    }
-
     /// <https://html.spec.whatwg.org/multipage/#attr-meta-http-equiv-content-security-policy>
     pub(crate) fn set_content_security_policy(&self) {
         let doc = self.owner_document();

components/script/dom/htmlmetaelement.rs

@@ -8,7 +8,7 @@ use std::time::Duration;
 
 use constellation_traits::NavigationHistoryBehavior;
 use dom_struct::dom_struct;
-use html5ever::{LocalName, Prefix};
+use html5ever::{LocalName, Prefix, local_name, namespace_url, ns};
 use js::rust::HandleObject;
 use regex::bytes::Regex;
 use servo_url::ServoUrl;
@@ -21,7 +21,7 @@ use crate::dom::bindings::codegen::Bindings::WindowBinding::WindowMethods;
 use crate::dom::bindings::inheritance::Castable;
 use crate::dom::bindings::root::DomRoot;
 use crate::dom::bindings::str::DOMString;
-use crate::dom::document::{DeclarativeRefresh, Document};
+use crate::dom::document::{DeclarativeRefresh, Document, determine_policy_for_token};
 use crate::dom::element::{AttributeMutation, Element};
 use crate::dom::htmlelement::HTMLElement;
 use crate::dom::htmlheadelement::HTMLHeadElement;
@@ -115,9 +115,31 @@ impl HTMLMetaElement {
 
     /// <https://html.spec.whatwg.org/multipage/#meta-referrer>
     fn apply_referrer(&self) {
-        if let Some(parent) = self.upcast::<Node>().GetParentElement() {
-            if let Some(head) = parent.downcast::<HTMLHeadElement>() {
-                head.set_document_referrer();
+        let doc = self.owner_document();
+        // From spec: For historical reasons, unlike other standard metadata names, the processing model for referrer
+        // is not responsive to element removals, and does not use tree order. Only the most-recently-inserted or
+        // most-recently-modified meta element in this state has an effect.
+        // 1. If element is not in a document tree, then return.
+        let meta_node = self.upcast::<Node>();
+        if !meta_node.is_in_a_document_tree() {
+            return;
+        }
+
+        // 2. If element does not have a name attribute whose value is an ASCII case-insensitive match for "referrer",
+        // then return.
+        if self.upcast::<Element>().get_name() != Some(atom!("referrer")) {
+            return;
+        }
+
+        // 3. If element does not have a content attribute, or that attribute's value is the empty string, then return.
+        let content = self
+            .upcast::<Element>()
+            .get_attribute(&ns!(), &local_name!("content"));
+        if let Some(attr) = content {
+            let attr = attr.value();
+            let attr_val = attr.trim();
+            if !attr_val.is_empty() {
+                doc.set_referrer_policy(determine_policy_for_token(attr_val));
             }
         }
     }