constellation: Broadcast preference changes to all content processes (#38716)

Building on the preference observer work from #38649, we now
automatically install an observer when multiprocess mode is enabled.
This observer notifies the constellation of updated preferences, which
in turn notifies each content process so the changes will be reflected
into script/layout as expected. There's a unit test that verifies this
works correctly by checking a preference-gated WebIDL property before
and after the preference is toggled.

Testing: New unit test added.
Fixes: #35966

Depends on #38649.

---------

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

components/constellation/constellation.rs

@@ -160,6 +160,7 @@ use script_traits::{
     ScriptThreadMessage, UpdatePipelineIdReason,
 };
 use serde::{Deserialize, Serialize};
+use servo_config::prefs::{self, PrefValue};
 use servo_config::{opts, pref};
 use servo_rand::{Rng, ServoRng, SliceRandom, random};
 use servo_url::{Host, ImmutableOrigin, ServoUrl};
@@ -254,6 +255,18 @@ struct BrowsingContextGroup {
     webgpus: HashMap<Host, WebGPU>,
 }
 
+struct PreferenceForwarder(Sender<EmbedderToConstellationMessage>);
+
+impl prefs::Observer for PreferenceForwarder {
+    fn prefs_changed(&self, changes: &[(&'static str, PrefValue)]) {
+        let _ = self
+            .0
+            .send(EmbedderToConstellationMessage::PreferencesUpdated(
+                changes.to_owned(),
+            ));
+    }
+}
+
 /// The `Constellation` itself. In the servo browser, there is one
 /// constellation, which maintains all of the browser global data.
 /// In embedded applications, there may be more than one constellation,
@@ -583,6 +596,7 @@ where
         hard_fail: bool,
     ) -> Sender<EmbedderToConstellationMessage> {
         let (compositor_sender, compositor_receiver) = unbounded();
+        let compositor_sender_self = compositor_sender.clone();
 
         // service worker manager to communicate with constellation
         let (swmanager_ipc_sender, swmanager_ipc_receiver) =
@@ -654,6 +668,10 @@ where
 
                 let rippy_data = resources::read_bytes(Resource::RippyPNG);
 
+                if opts::get().multiprocess {
+                    prefs::add_observer(Box::new(PreferenceForwarder(compositor_sender_self)));
+                }
+
                 let mut constellation: Constellation<STF, SWF> = Constellation {
                     namespace_receiver,
                     namespace_ipc_sender,
@@ -1507,6 +1525,20 @@ where
             EmbedderToConstellationMessage::SetWebDriverResponseSender(sender) => {
                 self.webdriver_input_command_reponse_sender = Some(sender);
             },
+            EmbedderToConstellationMessage::PreferencesUpdated(updates) => {
+                let event_loops = self
+                    .pipelines
+                    .values()
+                    .map(|pipeline| pipeline.event_loop.clone());
+                for event_loop in event_loops {
+                    let _ = event_loop.send(ScriptThreadMessage::PreferencesUpdated(
+                        updates
+                            .iter()
+                            .map(|(name, value)| (String::from(*name), value.clone()))
+                            .collect(),
+                    ));
+                }
+            },
         }
     }
 

components/constellation/tracing.rs

@@ -78,6 +78,7 @@ mod from_compositor {
                 Self::CreateMemoryReport(..) => target!("CreateMemoryReport"),
                 Self::SendImageKeysForPipeline(..) => target!("SendImageKeysForPipeline"),
                 Self::SetWebDriverResponseSender(..) => target!("SetWebDriverResponseSender"),
+                Self::PreferencesUpdated(..) => target!("PreferencesUpdated"),
             }
         }
     }