Add direct script to embedder channel (#39039)

This PR **removes** `ScriptToConstellationMessage::ForwardToEmbedder`,
and replaces it with an explicit `ScriptToEmbedderChannel`. This new
channel is based on `GenericCallback` and in single-process mode will
directly send the message the to the embedder and wake it. In
multi-process mode, the message is routed via the ROUTER, since waking
is only possible from the same process currently. This means in
multi-process mode there are likely no direct perf benefits, since we
still need to hop the message over the ROUTER (instead of over the
constellation).
In single-process mode we can directly send the message to the embedder,
which should provide a noticable latency improvement in all cases where
script is blocked waiting on the embedder to reply.

This does not change the way the embedder receives messages - the
receiving end is unchanged.

## How was sending messages to the embedder working before?

1. Script wraps it's message to the embedder in
`ScriptToConstellationMessage::ForwardToEmbedder` and sends it to
constellation.
2. The [constellation event loop] receives the message in
[handle_request]
3. If deserialization fails, [an error is logged and the message is
ignored]
4. Since our message came from script, it is handle in
[handle_request_from_script]
5. The message is logged with trace log level
6. If the pipeline is closed, [a warning is logged and the message
ignored]
7. The wrapped `EmbedderMsg` [is forwarded to the embedder]. Sending the
message also invokes `wake()` on the embedder eventloop waker.

[constellation event loop]:
2e1b2e7260/components/constellation/constellation.rs (L755)

[handle request]:
2e1b2e7260/components/constellation/constellation.rs (L1182)

[an error is logged and the message is ignored]:
2e1b2e7260/components/constellation/constellation.rs (L1252)

[handle_request_from_script]:
https://github.com/servo/servo/blob/main/components/constellation/constellation.rs#L1590
 
[a warning is logged and the message ignored]:
2e1b2e7260/components/constellation/constellation.rs (L1599)

[is forwarded to the embedder]:
2e1b2e7260/components/constellation/constellation.rs (L1701)

Testing: Communication between Script and Embedder is extensive, so this
should be covered by existing tests.

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

components/script/dom/debuggerglobalscope.rs

@@ -8,8 +8,8 @@ use base::id::{Index, PipelineId, PipelineNamespaceId};
 use constellation_traits::ScriptToConstellationChan;
 use devtools_traits::{DevtoolScriptControlMsg, ScriptToDevtoolsControlMsg, SourceInfo, WorkerId};
 use dom_struct::dom_struct;
-use embedder_traits::JavaScriptEvaluationError;
 use embedder_traits::resources::{self, Resource};
+use embedder_traits::{JavaScriptEvaluationError, ScriptToEmbedderChan};
 use ipc_channel::ipc::IpcSender;
 use js::jsval::UndefinedValue;
 use js::rust::wrappers::JS_DefineDebuggerObject;
@@ -66,6 +66,7 @@ impl DebuggerGlobalScope {
         mem_profiler_chan: mem::ProfilerChan,
         time_profiler_chan: time::ProfilerChan,
         script_to_constellation_chan: ScriptToConstellationChan,
+        script_to_embedder_chan: ScriptToEmbedderChan,
         resource_threads: ResourceThreads,
         #[cfg(feature = "webgpu")] gpu_id_hub: std::sync::Arc<IdentityHub>,
         can_gc: CanGc,
@@ -77,6 +78,7 @@ impl DebuggerGlobalScope {
                 mem_profiler_chan,
                 time_profiler_chan,
                 script_to_constellation_chan,
+                script_to_embedder_chan,
                 resource_threads,
                 MutableOrigin::new(ImmutableOrigin::new_opaque()),
                 ServoUrl::parse_with_base(None, "about:internal/debugger")

components/script/dom/dedicatedworkerglobalscope.rs

@@ -438,6 +438,7 @@ impl DedicatedWorkerGlobalScope {
                     init.mem_profiler_chan.clone(),
                     init.time_profiler_chan.clone(),
                     init.script_to_constellation_chan.clone(),
+                    init.script_to_embedder_chan.clone(),
                     init.resource_threads.clone(),
                     #[cfg(feature = "webgpu")]
                     gpu_id_hub.clone(),

components/script/dom/dissimilaroriginwindow.rs

@@ -58,6 +58,7 @@ impl DissimilarOriginWindow {
                 global_to_clone_from.mem_profiler_chan().clone(),
                 global_to_clone_from.time_profiler_chan().clone(),
                 global_to_clone_from.script_to_constellation_chan().clone(),
+                global_to_clone_from.script_to_embedder_chan().clone(),
                 global_to_clone_from.resource_threads().clone(),
                 global_to_clone_from.origin().clone(),
                 global_to_clone_from.creation_url().clone(),

components/script/dom/document_event_handler.rs

@@ -1345,10 +1345,10 @@ impl DocumentEventHandler {
             },
             ClipboardEventType::Paste => {
                 let (sender, receiver) = ipc::channel().unwrap();
-                self.window
-                    .send_to_constellation(ScriptToConstellationMessage::ForwardToEmbedder(
-                        EmbedderMsg::GetClipboardText(self.window.webview_id(), sender),
-                    ));
+                self.window.send_to_embedder(EmbedderMsg::GetClipboardText(
+                    self.window.webview_id(),
+                    sender,
+                ));
                 let text_contents = receiver
                     .recv()
                     .map(Result::unwrap_or_default)

components/script/dom/globalscope.rs

@@ -26,7 +26,7 @@ use content_security_policy::CspList;
 use crossbeam_channel::Sender;
 use devtools_traits::{PageError, ScriptToDevtoolsControlMsg};
 use dom_struct::dom_struct;
-use embedder_traits::{EmbedderMsg, JavaScriptEvaluationError};
+use embedder_traits::{EmbedderMsg, JavaScriptEvaluationError, ScriptToEmbedderChan};
 use fonts::FontContext;
 use ipc_channel::ipc::{self, IpcSender};
 use ipc_channel::router::ROUTER;
@@ -254,6 +254,11 @@ pub(crate) struct GlobalScope {
     #[no_trace]
     script_to_constellation_chan: ScriptToConstellationChan,
 
+    /// A handle for communicating messages to the Embedder.
+    #[ignore_malloc_size_of = "channels are hard"]
+    #[no_trace]
+    script_to_embedder_chan: ScriptToEmbedderChan,
+
     /// <https://html.spec.whatwg.org/multipage/#in-error-reporting-mode>
     in_error_reporting_mode: Cell<bool>,
 
@@ -741,6 +746,7 @@ impl GlobalScope {
         mem_profiler_chan: profile_mem::ProfilerChan,
         time_profiler_chan: profile_time::ProfilerChan,
         script_to_constellation_chan: ScriptToConstellationChan,
+        script_to_embedder_chan: ScriptToEmbedderChan,
         resource_threads: ResourceThreads,
         origin: MutableOrigin,
         creation_url: ServoUrl,
@@ -770,6 +776,7 @@ impl GlobalScope {
             mem_profiler_chan,
             time_profiler_chan,
             script_to_constellation_chan,
+            script_to_embedder_chan,
             in_error_reporting_mode: Default::default(),
             resource_threads,
             timers: OnceCell::default(),
@@ -2477,8 +2484,12 @@ impl GlobalScope {
         &self.script_to_constellation_chan
     }
 
+    pub(crate) fn script_to_embedder_chan(&self) -> &ScriptToEmbedderChan {
+        &self.script_to_embedder_chan
+    }
+
     pub(crate) fn send_to_embedder(&self, msg: EmbedderMsg) {
-        self.send_to_constellation(ScriptToConstellationMessage::ForwardToEmbedder(msg));
+        self.script_to_embedder_chan().send(msg).unwrap();
     }
 
     pub(crate) fn send_to_constellation(&self, msg: ScriptToConstellationMessage) {

components/script/dom/html/htmlinputelement.rs

@@ -456,10 +456,10 @@ impl HTMLInputElement {
         prefix: Option<Prefix>,
         document: &Document,
     ) -> HTMLInputElement {
-        let constellation_sender = document
+        let embedder_sender = document
             .window()
             .as_global_scope()
-            .script_to_constellation_chan()
+            .script_to_embedder_chan()
             .clone();
         HTMLInputElement {
             htmlelement: HTMLElement::new_inherited_with_state(
@@ -478,7 +478,7 @@ impl HTMLInputElement {
                 Single,
                 DOMString::new(),
                 EmbedderClipboardProvider {
-                    constellation_sender,
+                    embedder_sender,
                     webview_id: document.webview_id(),
                 },
                 None,

components/script/dom/html/htmltextareaelement.rs

@@ -141,10 +141,10 @@ impl HTMLTextAreaElement {
         prefix: Option<Prefix>,
         document: &Document,
     ) -> HTMLTextAreaElement {
-        let constellation_sender = document
+        let embedder_sender = document
             .window()
             .as_global_scope()
-            .script_to_constellation_chan()
+            .script_to_embedder_chan()
             .clone();
         HTMLTextAreaElement {
             htmlelement: HTMLElement::new_inherited_with_state(
@@ -158,7 +158,7 @@ impl HTMLTextAreaElement {
                 Lines::Multiple,
                 DOMString::new(),
                 EmbedderClipboardProvider {
-                    constellation_sender,
+                    embedder_sender,
                     webview_id: document.webview_id(),
                 },
                 None,

components/script/dom/window.rs

@@ -35,9 +35,9 @@ use devtools_traits::{ScriptToDevtoolsControlMsg, TimelineMarker, TimelineMarker
 use dom_struct::dom_struct;
 use embedder_traits::user_content_manager::{UserContentManager, UserScript};
 use embedder_traits::{
-    AlertResponse, ConfirmResponse, EmbedderMsg, PromptResponse, SimpleDialog, Theme,
-    UntrustedNodeAddress, ViewportDetails, WebDriverJSError, WebDriverJSResult,
-    WebDriverLoadStatus,
+    AlertResponse, ConfirmResponse, EmbedderMsg, PromptResponse, ScriptToEmbedderChan,
+    SimpleDialog, Theme, UntrustedNodeAddress, ViewportDetails, WebDriverJSError,
+    WebDriverJSResult, WebDriverLoadStatus,
 };
 use euclid::default::{Point2D as UntypedPoint2D, Rect as UntypedRect, Size2D as UntypedSize2D};
 use euclid::{Point2D, Scale, Size2D, Vector2D};
@@ -2995,7 +2995,10 @@ impl Window {
     }
 
     pub(crate) fn send_to_embedder(&self, msg: EmbedderMsg) {
-        self.send_to_constellation(ScriptToConstellationMessage::ForwardToEmbedder(msg));
+        self.as_global_scope()
+            .script_to_embedder_chan()
+            .send(msg)
+            .unwrap();
     }
 
     pub(crate) fn send_to_constellation(&self, msg: ScriptToConstellationMessage) {
@@ -3100,6 +3103,7 @@ impl Window {
         time_profiler_chan: TimeProfilerChan,
         devtools_chan: Option<IpcSender<ScriptToDevtoolsControlMsg>>,
         constellation_chan: ScriptToConstellationChan,
+        embedder_chan: ScriptToEmbedderChan,
         control_chan: GenericSender<ScriptThreadMessage>,
         pipeline_id: PipelineId,
         parent_info: Option<PipelineId>,
@@ -3139,6 +3143,7 @@ impl Window {
                 mem_profiler_chan,
                 time_profiler_chan,
                 constellation_chan,
+                embedder_chan,
                 resource_threads,
                 origin,
                 creation_url,

components/script/dom/workerglobalscope.rs

@@ -90,6 +90,7 @@ pub(crate) fn prepare_workerscope_init(
         time_profiler_chan: global.time_profiler_chan().clone(),
         from_devtools_sender: devtools_sender,
         script_to_constellation_chan: global.script_to_constellation_chan().clone(),
+        script_to_embedder_chan: global.script_to_embedder_chan().clone(),
         worker_id: worker_id.unwrap_or_else(|| WorkerId(Uuid::new_v4())),
         pipeline_id: global.pipeline_id(),
         origin: global.origin().immutable().clone(),
@@ -185,6 +186,7 @@ impl WorkerGlobalScope {
                 init.mem_profiler_chan,
                 init.time_profiler_chan,
                 init.script_to_constellation_chan,
+                init.script_to_embedder_chan,
                 init.resource_threads,
                 MutableOrigin::new(init.origin),
                 init.creation_url,

components/script/dom/workletglobalscope.rs

@@ -10,7 +10,7 @@ use constellation_traits::{ScriptToConstellationChan, ScriptToConstellationMessa
 use crossbeam_channel::Sender;
 use devtools_traits::ScriptToDevtoolsControlMsg;
 use dom_struct::dom_struct;
-use embedder_traits::JavaScriptEvaluationError;
+use embedder_traits::{JavaScriptEvaluationError, ScriptToEmbedderChan};
 use ipc_channel::ipc::IpcSender;
 use js::jsval::UndefinedValue;
 use net_traits::ResourceThreads;
@@ -100,6 +100,7 @@ impl WorkletGlobalScope {
                 init.mem_profiler_chan.clone(),
                 init.time_profiler_chan.clone(),
                 script_to_constellation_chan,
+                init.to_embedder_sender.clone(),
                 init.resource_threads.clone(),
                 MutableOrigin::new(ImmutableOrigin::new_opaque()),
                 base_url.clone(),
@@ -198,6 +199,8 @@ pub(crate) struct WorkletGlobalScopeInit {
     pub(crate) devtools_chan: Option<IpcSender<ScriptToDevtoolsControlMsg>>,
     /// Messages to send to constellation
     pub(crate) to_constellation_sender: GenericSender<(PipelineId, ScriptToConstellationMessage)>,
+    /// Messages to send to the Embedder
+    pub(crate) to_embedder_sender: ScriptToEmbedderChan,
     /// The image cache
     pub(crate) image_cache: Arc<dyn ImageCache>,
     /// Identity manager for WebGPU resources