layout: Ensure that the Epoch stored in layout is accurate (#39568)

The first epoch is 0 as that is the one used in the initial transaction,
but the code was setting the first `Epoch` to `Epoch(1)`. This means
that
when layout advanced the epoch, the `Epoch` of the first produced
display list was `Epoch(2)`.

This change makes the value reflected in `current_epoch` actually match
the index of the display list produced. In addition, we always store
this epoch in `PipelineDetails` in the renderer. This will be important
when adding the `WebView::take_screenshot` API.

Testing: This should not change behavior, so is covered by existing
tests which
rely on proper `Epoch` advancement.

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

components/compositing/compositor.rs

@@ -221,6 +221,10 @@ pub(crate) struct PipelineDetails {
     /// Which parts of Servo have reported that this `Pipeline` has exited. Only when all
     /// have done so will it be discarded.
     pub exited: PipelineExitSource,
+
+    /// The [`Epoch`] of the latest display list received for this `Pipeline` or `None` if no
+    /// display list has been received.
+    pub display_list_epoch: Option<Epoch>,
 }
 
 impl PipelineDetails {
@@ -246,6 +250,7 @@ impl PipelineDetails {
             first_paint_metric: PaintMetricState::Waiting,
             first_contentful_paint_metric: PaintMetricState::Waiting,
             exited: PipelineExitSource::empty(),
+            display_list_epoch: None,
         }
     }
 
@@ -537,9 +542,17 @@ impl IOCompositor {
                 }
             },
 
-            CompositorMsg::SendInitialTransaction(pipeline) => {
+            CompositorMsg::SendInitialTransaction(webview_id, pipeline_id) => {
+                let Some(webview_renderer) = self.webview_renderers.get_mut(webview_id) else {
+                    return warn!("Could not find WebView for incoming display list");
+                };
+
+                let starting_epoch = Epoch(0);
+                let details = webview_renderer.ensure_pipeline_details(pipeline_id.into());
+                details.display_list_epoch = Some(starting_epoch);
+
                 let mut txn = Transaction::new();
-                txn.set_display_list(WebRenderEpoch(0), (pipeline, Default::default()));
+                txn.set_display_list(starting_epoch.into(), (pipeline_id, Default::default()));
                 self.generate_frame(&mut txn, RenderReasons::SCENE);
                 self.global.borrow_mut().send_transaction(txn);
             },
@@ -653,6 +666,8 @@ impl IOCompositor {
                     Some(display_list_info.viewport_details.hidpi_scale_factor);
 
                 let epoch = display_list_info.epoch;
+                details.display_list_epoch = Some(Epoch(epoch.0));
+
                 let first_reflow = display_list_info.first_reflow;
                 if details.first_paint_metric == PaintMetricState::Waiting {
                     details.first_paint_metric = PaintMetricState::Seen(epoch, first_reflow);

components/layout/layout_impl.rs

@@ -175,8 +175,10 @@ pub struct LayoutThread {
     /// The [`StackingContextTree`] cached from previous layouts.
     stacking_context_tree: RefCell<Option<StackingContextTree>>,
 
-    /// A counter for epoch messages
-    epoch: Cell<Epoch>,
+    /// The epoch of the current display list that's been sent to [`WebRender`]. Every
+    /// layout sends an initial empty display list, so this always starts at `0. The
+    /// next display list will have the value of this epoch plus `1`.
+    current_epoch: Cell<Epoch>,
 
     // A cache that maps image resources specified in CSS (e.g as the `url()` value
     // for `background-image` or `content` properties) to either the final resolved
@@ -223,7 +225,7 @@ impl Layout for LayoutThread {
     }
 
     fn current_epoch(&self) -> Epoch {
-        self.epoch.get()
+        self.current_epoch.get()
     }
 
     fn load_web_fonts_from_stylesheet(&self, stylesheet: ServoArc<Stylesheet>) {
@@ -646,7 +648,7 @@ impl LayoutThread {
         // Let webrender know about this pipeline by sending an empty display list.
         config
             .compositor_api
-            .send_initial_transaction(config.id.into());
+            .send_initial_transaction(config.webview_id, config.id.into());
 
         let mut font = Font::initial_values();
         let default_font_size = pref!(fonts_default_size);
@@ -685,8 +687,7 @@ impl LayoutThread {
             box_tree: Default::default(),
             fragment_tree: Default::default(),
             stacking_context_tree: Default::default(),
-            // Epoch starts at 1 because of the initial display list for epoch 0 that we send to WR
-            epoch: Cell::new(Epoch(1)),
+            current_epoch: Cell::new(Epoch(0)),
             compositor_api: config.compositor_api,
             stylist: Stylist::new(device, QuirksMode::NoQuirks),
             resolved_images_cache: Default::default(),
@@ -1232,10 +1233,8 @@ impl LayoutThread {
             return false;
         }
 
-        let mut epoch = self.epoch.get();
-        epoch.next();
-        self.epoch.set(epoch);
-        stacking_context_tree.compositor_info.epoch = epoch.into();
+        self.current_epoch.set(self.current_epoch().next());
+        stacking_context_tree.compositor_info.epoch = self.current_epoch.get().into();
 
         let built_display_list = DisplayListBuilder::build(
             stacking_context_tree,

components/script/dom/document.rs

@@ -569,8 +569,7 @@ pub(crate) struct Document {
     /// uploaded to the renderer after a rendering update. Until those images are uploaded
     /// this `Document` will not perform any more rendering updates.
     #[no_trace]
-    current_canvas_epoch: RefCell<Epoch>,
-
+    current_canvas_epoch: Cell<Epoch>,
     /// The global custom element reaction stack for this script thread.
     #[conditional_malloc_size_of]
     custom_element_reaction_stack: Rc<CustomElementReactionStack>,
@@ -2742,8 +2741,9 @@ impl Document {
         }
 
         // All dirty canvases are flushed before updating the rendering.
-        self.current_canvas_epoch.borrow_mut().next();
-        let canvas_epoch = *self.current_canvas_epoch.borrow();
+        self.current_canvas_epoch
+            .set(self.current_canvas_epoch.get().next());
+        let canvas_epoch = self.current_canvas_epoch.get();
         let mut image_keys = Vec::new();
 
         #[cfg(feature = "webgpu")]
@@ -3499,7 +3499,7 @@ impl Document {
             pending_scroll_event_targets: Default::default(),
             resize_observer_started_observing_target: Cell::new(false),
             waiting_on_canvas_image_updates: Cell::new(false),
-            current_canvas_epoch: RefCell::new(Epoch(0)),
+            current_canvas_epoch: Cell::new(Epoch(0)),
             custom_element_reaction_stack,
             active_sandboxing_flag_set: Cell::new(SandboxingFlagSet::empty()),
             favicon: RefCell::new(None),

components/shared/base/lib.rs

@@ -77,8 +77,8 @@ where
 pub struct Epoch(pub u32);
 
 impl Epoch {
-    pub fn next(&mut self) {
-        self.0 += 1;
+    pub fn next(&self) -> Self {
+        Self(self.0 + 1)
     }
 }
 

components/shared/compositing/lib.rs

@@ -103,7 +103,7 @@ pub enum CompositorMsg {
     /// The load of a page has completed
     LoadComplete(WebViewId),
     /// Inform WebRender of the existence of this pipeline.
-    SendInitialTransaction(WebRenderPipelineId),
+    SendInitialTransaction(WebViewId, WebRenderPipelineId),
     /// Perform a scroll operation.
     SendScrollNode(
         WebViewId,
@@ -204,8 +204,11 @@ impl CrossProcessCompositorApi {
     }
 
     /// Inform WebRender of the existence of this pipeline.
-    pub fn send_initial_transaction(&self, pipeline: WebRenderPipelineId) {
-        if let Err(e) = self.0.send(CompositorMsg::SendInitialTransaction(pipeline)) {
+    pub fn send_initial_transaction(&self, webview_id: WebViewId, pipeline: WebRenderPipelineId) {
+        if let Err(e) = self
+            .0
+            .send(CompositorMsg::SendInitialTransaction(webview_id, pipeline))
+        {
             warn!("Error sending initial transaction: {}", e);
         }
     }