Add preference observer API for runtime webxr preference changes (#38649)

Adds a global preference observer that is notified whenever any
preference value is updated. This is used to support runtime
configuration of WebXR automated testing, which is a prerequisite for
running multiple WPT tests in a single browser session.

Testing: Ran `./mach test-wpt /webxr --product=servodriver`
Fixes: #38647

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/config/macro/lib.rs

@@ -34,6 +34,12 @@ fn servo_preferences_derive(input: synstructure::Structure) -> TokenStream {
         set_match_cases.extend(quote!(stringify!(#name) => self.#name = value.try_into().unwrap(),))
     }
 
+    let mut comparisons = quote!();
+    for field in named_fields.named.iter() {
+        let name = field.ident.as_ref().unwrap();
+        comparisons.extend(quote!(if self.#name != other.#name { changes.push((stringify!(#name), self.#name.clone().into(),)) }))
+    }
+
     let structure_name = &ast.ident;
     quote! {
         impl #structure_name {
@@ -50,6 +56,12 @@ fn servo_preferences_derive(input: synstructure::Structure) -> TokenStream {
                     _ => { panic!("Unknown preference: {:?}", name); }
                 }
             }
+
+            pub fn diff(&self, other: &Self) -> Vec<(&'static str, PrefValue)> {
+                let mut changes = vec![];
+                #comparisons
+                changes
+            }
         }
     }
 }

components/config/prefs.rs

@@ -11,12 +11,22 @@ pub use crate::pref_util::PrefValue;
 
 static PREFERENCES: RwLock<Preferences> = RwLock::new(Preferences::const_default());
 
+pub trait Observer: Send + Sync {
+    fn prefs_changed(&self, _changes: Vec<(&'static str, PrefValue)>) {}
+}
+
+static OBSERVER: RwLock<Option<Box<dyn Observer>>> = RwLock::new(None);
+
 #[inline]
 /// Get the current set of global preferences for Servo.
 pub fn get() -> RwLockReadGuard<'static, Preferences> {
     PREFERENCES.read().unwrap()
 }
 
+pub fn set_observer(observer: Box<dyn Observer>) {
+    *OBSERVER.write().unwrap() = Some(observer);
+}
+
 pub fn set(preferences: Preferences) {
     // Map between Stylo preference names and Servo preference names as the This should be
     // kept in sync with components/script/dom/bindings/codegen/run.py which generates the
@@ -39,7 +49,13 @@ pub fn set(preferences: Preferences) {
         preferences.layout_container_queries_enabled,
     );
 
+    let changed = preferences.diff(&PREFERENCES.read().unwrap());
+
     *PREFERENCES.write().unwrap() = preferences;
+
+    if let Some(observer) = OBSERVER.read().unwrap().as_deref() {
+        observer.prefs_changed(changed);
+    }
 }
 
 /// A convenience macro for accessing a preference value using its static path.