Lazily initialize canvas paint thread in constellation (#37765)

This PR moves canvas paint thread initialization to constellation. This
allows us to lazily initialize it on first create canvas request (like
we do for webgpu). If we didn't started canvas paint thread we also do
not need to wait for it's teardown.

Per https://chromestatus.com/metrics/webfeature/timeline/popularity/201
~30% of websites still use 2d canvas.

Testing: Existing WPT tests

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

Cargo.lock

@@ -1454,6 +1454,7 @@ dependencies = [
  "backtrace",
  "base",
  "bluetooth_traits",
+ "canvas",
  "canvas_traits",
  "compositing_traits",
  "constellation_traits",
@@ -4846,7 +4847,6 @@ dependencies = [
  "bincode",
  "bluetooth",
  "bluetooth_traits",
- "canvas",
  "canvas_traits",
  "compositing",
  "compositing_traits",

components/constellation/Cargo.toml

@@ -23,6 +23,7 @@ background_hang_monitor_api = { workspace = true }
 backtrace = { workspace = true }
 base = { workspace = true }
 bluetooth_traits = { workspace = true, optional = true }
+canvas = { path = "../canvas" }
 canvas_traits = { workspace = true }
 compositing_traits = { workspace = true }
 constellation_traits = { workspace = true }

components/constellation/constellation.rs

@@ -85,6 +85,7 @@
 //! See <https://github.com/servo/servo/issues/14704>
 
 use std::borrow::ToOwned;
+use std::cell::OnceCell;
 use std::collections::hash_map::Entry;
 use std::collections::{HashMap, HashSet, VecDeque};
 use std::marker::PhantomData;
@@ -105,6 +106,7 @@ use base::id::{
 };
 #[cfg(feature = "bluetooth")]
 use bluetooth_traits::BluetoothRequest;
+use canvas::canvas_paint_thread::CanvasPaintThread;
 use canvas_traits::ConstellationCanvasMsg;
 use canvas_traits::canvas::{CanvasId, CanvasMsg};
 use canvas_traits::webgl::WebGLThreads;
@@ -425,10 +427,8 @@ pub struct Constellation<STF, SWF> {
     /// The XR device registry
     webxr_registry: Option<webxr_api::Registry>,
 
-    /// A channel through which messages can be sent to the canvas paint thread.
-    canvas_sender: Sender<ConstellationCanvasMsg>,
-
-    canvas_ipc_sender: IpcSender<CanvasMsg>,
+    /// Lazily initialized channels for canvas paint thread.
+    canvas: OnceCell<(Sender<ConstellationCanvasMsg>, IpcSender<CanvasMsg>)>,
 
     /// Navigation requests from script awaiting approval from the embedder.
     pending_approval_navigations: PendingApprovalNavigations,
@@ -584,8 +584,6 @@ where
         random_pipeline_closure_probability: Option<f32>,
         random_pipeline_closure_seed: Option<usize>,
         hard_fail: bool,
-        canvas_create_sender: Sender<ConstellationCanvasMsg>,
-        canvas_ipc_sender: IpcSender<CanvasMsg>,
     ) -> Sender<EmbedderToConstellationMessage> {
         let (compositor_sender, compositor_receiver) = unbounded();
 
@@ -707,8 +705,7 @@ where
                     }),
                     webgl_threads: state.webgl_threads,
                     webxr_registry: state.webxr_registry,
-                    canvas_sender: canvas_create_sender,
-                    canvas_ipc_sender,
+                    canvas: OnceCell::new(),
                     pending_approval_navigations: HashMap::new(),
                     pressed_mouse_buttons: 0,
                     active_keyboard_modifiers: Modifiers::empty(),
@@ -2665,14 +2662,16 @@ where
             }
         }
 
-        debug!("Exiting Canvas Paint thread.");
-        let (canvas_exit_sender, canvas_exit_receiver) = unbounded();
-        if let Err(e) = self
-            .canvas_sender
-            .send(ConstellationCanvasMsg::Exit(canvas_exit_sender))
-        {
-            warn!("Exit Canvas Paint thread failed ({})", e);
-        }
+        let canvas_exit_receiver = if let Some((canvas_sender, _)) = self.canvas.get() {
+            debug!("Exiting Canvas Paint thread.");
+            let (canvas_exit_sender, canvas_exit_receiver) = unbounded();
+            if let Err(e) = canvas_sender.send(ConstellationCanvasMsg::Exit(canvas_exit_sender)) {
+                warn!("Exit Canvas Paint thread failed ({})", e);
+            }
+            Some(canvas_exit_receiver)
+        } else {
+            None
+        };
 
         debug!("Exiting WebGPU threads.");
         #[cfg(feature = "webgpu")]
@@ -2714,7 +2713,9 @@ where
 
         // Wait for the canvas thread to exit before shutting down the font service, as
         // canvas might still be using the system font service before shutting down.
-        let _ = canvas_exit_receiver.recv();
+        if let Some(canvas_exit_receiver) = canvas_exit_receiver {
+            let _ = canvas_exit_receiver.recv();
+        }
 
         debug!("Exiting the system font service thread.");
         self.system_font_service.exit();
@@ -4493,8 +4494,11 @@ where
         response_sender: IpcSender<(IpcSender<CanvasMsg>, CanvasId, ImageKey)>,
     ) {
         let (canvas_data_sender, canvas_data_receiver) = unbounded();
+        let (canvas_sender, canvas_ipc_sender) = self
+            .canvas
+            .get_or_init(|| self.create_canvas_paint_thread());
 
-        if let Err(e) = self.canvas_sender.send(ConstellationCanvasMsg::Create {
+        if let Err(e) = canvas_sender.send(ConstellationCanvasMsg::Create {
             sender: canvas_data_sender,
             size,
         }) {
@@ -4504,8 +4508,7 @@ where
             Ok(canvas_data) => canvas_data,
             Err(e) => return warn!("Create canvas paint thread id response failed ({})", e),
         };
-        if let Err(e) = response_sender.send((self.canvas_ipc_sender.clone(), canvas_id, image_key))
-        {
+        if let Err(e) = response_sender.send((canvas_ipc_sender.clone(), canvas_id, image_key)) {
             warn!("Create canvas paint thread response failed ({})", e);
         }
     }
@@ -5745,4 +5748,12 @@ where
             warn!("Could not sent paint metric event to pipeline: {pipeline_id:?}: {error:?}");
         }
     }
+
+    fn create_canvas_paint_thread(&self) -> (Sender<ConstellationCanvasMsg>, IpcSender<CanvasMsg>) {
+        CanvasPaintThread::start(
+            self.compositor_proxy.cross_process_compositor_api.clone(),
+            self.system_font_service.clone(),
+            self.public_resource_threads.clone(),
+        )
+    }
 }

components/servo/Cargo.toml

@@ -69,7 +69,6 @@ base = { workspace = true }
 bincode = { workspace = true }
 bluetooth = { path = "../bluetooth", optional = true }
 bluetooth_traits = { workspace = true, optional = true }
-canvas = { path = "../canvas" }
 webgl = { path = "../webgl", default-features = false }
 canvas_traits = { workspace = true }
 compositing = { path = "../compositing" }

components/servo/lib.rs

@@ -39,7 +39,6 @@ use base::id::{PipelineNamespace, PipelineNamespaceId};
 use bluetooth::BluetoothThreadFactory;
 #[cfg(feature = "bluetooth")]
 use bluetooth_traits::BluetoothRequest;
-use canvas::canvas_paint_thread::CanvasPaintThread;
 use canvas_traits::webgl::{GlType, WebGLThreads};
 use clipboard_delegate::StringRequest;
 pub use compositing::WebRenderDebugOption;
@@ -115,7 +114,7 @@ use webview::WebViewInner;
 #[cfg(feature = "webxr")]
 pub use webxr;
 pub use {
-    background_hang_monitor, base, canvas, canvas_traits, devtools, devtools_traits, euclid, fonts,
+    background_hang_monitor, base, canvas_traits, devtools, devtools_traits, euclid, fonts,
     ipc_channel, layout_api, media, net, net_traits, profile, profile_traits, script,
     script_traits, servo_config as config, servo_config, servo_geometry, servo_url, style,
     style_traits, webrender_api,
@@ -1095,12 +1094,6 @@ fn create_constellation(
         .to_proxy(),
     );
 
-    let (canvas_create_sender, canvas_ipc_sender) = CanvasPaintThread::start(
-        compositor_proxy.cross_process_compositor_api.clone(),
-        system_font_service.clone(),
-        public_resource_threads.clone(),
-    );
-
     let initial_state = InitialConstellationState {
         compositor_proxy,
         embedder_proxy,
@@ -1133,8 +1126,6 @@ fn create_constellation(
         opts.random_pipeline_closure_probability,
         opts.random_pipeline_closure_seed,
         opts.hard_fail,
-        canvas_create_sender,
-        canvas_ipc_sender,
     )
 }