libservo: Don't bounce ready-to-present frame notifications to the Constellation (#35369)

Instead of telling the Constellation to tell the embedder that new
frames are ready, have the compositor tell the embedder directly. This
should reduce frame latency. Now, after processing compositor
updates, run any pending `WebView::new_frame_ready` delegate methods.

This change also removes the `refresh` call from the Java interface as
that was the only other place that the compositor was rendering the
WebRender scene outside of event looping spinning. This `refresh` call
was completely unused.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/compositing/compositor.rs

@@ -3,6 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use std::cell::OnceCell;
+use std::collections::hash_set::Iter;
 use std::collections::HashMap;
 use std::env;
 use std::fs::{create_dir_all, File};
@@ -212,8 +213,9 @@ pub struct IOCompositor {
     /// The number of frames pending to receive from WebRender.
     pending_frames: usize,
 
-    /// Waiting for external code to call present.
-    waiting_on_present: bool,
+    /// A list of [`WebViewId`]s of WebViews that have new frames ready that are waiting to
+    /// be presented.
+    webviews_waiting_on_present: FnvHashSet<WebViewId>,
 
     /// The [`Instant`] of the last animation tick, used to avoid flooding the Constellation and
     /// ScriptThread with a deluge of animation ticks.
@@ -403,7 +405,7 @@ impl IOCompositor {
             exit_after_load,
             convert_mouse_to_touch,
             pending_frames: 0,
-            waiting_on_present: false,
+            webviews_waiting_on_present: Default::default(),
             last_animation_tick: Instant::now(),
             version_string,
         };
@@ -424,6 +426,10 @@ impl IOCompositor {
         }
     }
 
+    pub fn webviews_waiting_on_present(&self) -> Iter<'_, WebViewId> {
+        self.webviews_waiting_on_present.iter()
+    }
+
     fn update_cursor(&mut self, result: CompositorHitTestResult) {
         let cursor = match result.cursor {
             Some(cursor) if cursor != self.cursor => cursor,
@@ -1976,7 +1982,7 @@ impl IOCompositor {
         target: CompositeTarget,
         page_rect: Option<Rect<f32, CSSPixel>>,
     ) -> Result<Option<Image>, UnableToComposite> {
-        if self.waiting_on_present {
+        if !self.webviews_waiting_on_present.is_empty() {
             debug!("tried to composite while waiting on present");
             return Err(UnableToComposite::NotReadyToPaintImage(
                 NotReadyToPaint::WaitingOnConstellation,
@@ -2146,15 +2152,11 @@ impl IOCompositor {
         let _span =
             tracing::trace_span!("ConstellationMsg::ReadyToPresent", servo_profiling = true)
                 .entered();
+
         // Notify embedder that servo is ready to present.
         // Embedder should call `present` to tell compositor to continue rendering.
-        self.waiting_on_present = true;
-        let webview_ids = self.webviews.painting_order().map(|(&id, _)| id);
-        let msg = ConstellationMsg::ReadyToPresent(webview_ids.collect());
-        if let Err(e) = self.constellation_chan.send(msg) {
-            warn!("Sending event to constellation failed ({:?}).", e);
-        }
-
+        self.webviews_waiting_on_present
+            .extend(self.webviews.painting_order().map(|(&id, _)| id));
         self.composition_request = CompositionRequest::NoCompositingNecessary;
 
         self.process_animations(true);
@@ -2245,7 +2247,7 @@ impl IOCompositor {
         let _span =
             tracing::trace_span!("Compositor Present Surface", servo_profiling = true).entered();
         self.rendering_context.present();
-        self.waiting_on_present = false;
+        self.webviews_waiting_on_present.clear();
     }
 
     fn composite_if_necessary(&mut self, reason: CompositingReason) {

components/constellation/constellation.rs

@@ -1459,16 +1459,6 @@ where
             FromCompositorMsg::SetWebViewThrottled(webview_id, throttled) => {
                 self.set_webview_throttled(webview_id, throttled);
             },
-            FromCompositorMsg::ReadyToPresent(webview_ids) => {
-                #[cfg(feature = "tracing")]
-                let _span = tracing::trace_span!(
-                    "FromCompositorMsg::ReadyToPresent",
-                    servo_profiling = true,
-                )
-                .entered();
-                self.embedder_proxy
-                    .send(EmbedderMsg::ReadyToPresent(webview_ids));
-            },
             FromCompositorMsg::Gamepad(gamepad_event) => {
                 self.handle_gamepad_msg(gamepad_event);
             },

components/constellation/tracing.rs

@@ -90,7 +90,6 @@ mod from_compositor {
                 Self::MediaSessionAction(_) => target!("MediaSessionAction"),
                 Self::SetWebViewThrottled(_, _) => target!("SetWebViewThrottled"),
                 Self::IMEDismissed => target!("IMEDismissed"),
-                Self::ReadyToPresent(..) => target!("ReadyToPresent"),
                 Self::Gamepad(..) => target!("Gamepad"),
                 Self::Clipboard(..) => target!("Clipboard"),
             }
@@ -248,7 +247,6 @@ mod from_script {
                 Self::MediaSessionEvent(..) => target_variant!("MediaSessionEvent"),
                 Self::OnDevtoolsStarted(..) => target_variant!("OnDevtoolsStarted"),
                 Self::RequestDevtoolsConnection(..) => target_variant!("RequestDevtoolsConnection"),
-                Self::ReadyToPresent(..) => target_variant!("ReadyToPresent"),
                 Self::EventDelivered(..) => target_variant!("EventDelivered"),
                 Self::PlayGamepadHapticEffect(..) => target_variant!("PlayGamepadHapticEffect"),
                 Self::StopGamepadHapticEffect(..) => target_variant!("StopGamepadHapticEffect"),

components/servo/lib.rs

@@ -643,6 +643,7 @@ impl Servo {
         }
 
         self.compositor.borrow_mut().perform_updates();
+        self.send_new_frame_ready_messages();
 
         if self.compositor.borrow().shutdown_state == ShutdownState::FinishedShuttingDown {
             return false;
@@ -651,6 +652,17 @@ impl Servo {
         true
     }
 
+    fn send_new_frame_ready_messages(&self) {
+        for webview in self
+            .compositor
+            .borrow()
+            .webviews_waiting_on_present()
+            .filter_map(|id| self.get_webview_handle(*id))
+        {
+            webview.delegate().notify_new_frame_ready(webview);
+        }
+    }
+
     pub fn pinch_zoom_level(&self) -> f32 {
         self.compositor.borrow_mut().pinch_zoom_level().get()
     }
@@ -987,13 +999,6 @@ impl Servo {
                     },
                 );
             },
-            EmbedderMsg::ReadyToPresent(webview_ids) => {
-                for webview_id in webview_ids {
-                    if let Some(webview) = self.get_webview_handle(webview_id) {
-                        webview.delegate().notify_new_frame_ready(webview);
-                    }
-                }
-            },
             EmbedderMsg::EventDelivered(webview_id, event) => {
                 if let Some(webview) = self.get_webview_handle(webview_id) {
                     webview.delegate().notify_event_delivered(webview, event);

components/servo/webview.rs

@@ -441,10 +441,6 @@ impl WebView {
         self.inner().compositor.borrow_mut().capture_webrender();
     }
 
-    pub fn composite(&self) {
-        self.inner().compositor.borrow_mut().composite();
-    }
-
     pub fn toggle_sampling_profiler(&self, rate: Duration, max_duration: Duration) {
         self.inner()
             .constellation_proxy

components/shared/compositing/constellation_msg.rs

@@ -84,8 +84,6 @@ pub enum ConstellationMsg {
     SetWebViewThrottled(TopLevelBrowsingContextId, bool),
     /// Virtual keyboard was dismissed
     IMEDismissed,
-    /// Notify the embedder that it needs to present a new frame.
-    ReadyToPresent(Vec<WebViewId>),
     /// Gamepad state has changed
     Gamepad(GamepadEvent),
     /// Inform the constellation of a clipboard event.
@@ -133,7 +131,6 @@ impl ConstellationMsg {
             SetWebViewThrottled(..) => "SetWebViewThrottled",
             IMEDismissed => "IMEDismissed",
             ClearCache => "ClearCache",
-            ReadyToPresent(..) => "ReadyToPresent",
             Gamepad(..) => "Gamepad",
             Clipboard(..) => "Clipboard",
         }

components/shared/embedder/lib.rs

@@ -250,8 +250,6 @@ pub enum EmbedderMsg {
     OnDevtoolsStarted(Result<u16, ()>, String),
     /// Ask the user to allow a devtools client to connect.
     RequestDevtoolsConnection(IpcSender<AllowOrDeny>),
-    /// Notify the embedder that it needs to present a new frame.
-    ReadyToPresent(Vec<WebViewId>),
     /// The given event was delivered to a pipeline in the given browser.
     EventDelivered(WebViewId, CompositorEventVariant),
     /// Request to play a haptic effect on a connected gamepad.
@@ -314,7 +312,6 @@ impl Debug for EmbedderMsg {
             EmbedderMsg::OnDevtoolsStarted(..) => write!(f, "OnDevtoolsStarted"),
             EmbedderMsg::RequestDevtoolsConnection(..) => write!(f, "RequestDevtoolsConnection"),
             EmbedderMsg::ShowContextMenu(..) => write!(f, "ShowContextMenu"),
-            EmbedderMsg::ReadyToPresent(..) => write!(f, "ReadyToPresent"),
             EmbedderMsg::EventDelivered(..) => write!(f, "HitTestedEvent"),
             EmbedderMsg::PlayGamepadHapticEffect(..) => write!(f, "PlayGamepadHapticEffect"),
             EmbedderMsg::StopGamepadHapticEffect(..) => write!(f, "StopGamepadHapticEffect"),

ports/servoshell/egl/android.rs

@@ -222,15 +222,6 @@ pub extern "C" fn Java_org_servo_servoview_JNIServo_stop<'local>(
     call(&mut env, |s| s.stop());
 }
 
-#[no_mangle]
-pub extern "C" fn Java_org_servo_servoview_JNIServo_refresh<'local>(
-    mut env: JNIEnv<'local>,
-    _class: JClass<'local>,
-) {
-    debug!("refresh");
-    call(&mut env, |s| s.refresh());
-}
-
 #[no_mangle]
 pub extern "C" fn Java_org_servo_servoview_JNIServo_goBack<'local>(
     mut env: JNIEnv<'local>,

ports/servoshell/egl/app_state.rs

@@ -401,13 +401,6 @@ impl RunningAppState {
         self.perform_updates();
     }
 
-    /// Redraw the page.
-    pub fn refresh(&self) {
-        info!("refresh");
-        self.active_webview().composite();
-        self.perform_updates();
-    }
-
     /// Stop loading the page.
     pub fn stop(&self) {
         warn!("TODO can't stop won't stop");

support/android/apk/servoview/src/main/java/org/servo/servoview/JNIServo.java

@@ -35,8 +35,6 @@ public class JNIServo {
 
     public native void stop();
 
-    public native void refresh();
-
     public native void goBack();
 
     public native void goForward();

support/android/apk/servoview/src/main/java/org/servo/servoview/Servo.java

@@ -87,10 +87,6 @@ public class Servo {
         mRunCallback.inGLThread(() -> mJNI.resize(coords));
     }
 
-    public void refresh() {
-        mRunCallback.inGLThread(() -> mJNI.refresh());
-    }
-
     public void reload() {
         mRunCallback.inGLThread(() -> mJNI.reload());
     }