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Add direct script to embedder channel (#39039)

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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>
This commit is contained in:
Jonathan Schwender 2025-09-02 08:33:44 +02:00 • committed by GitHub
parent 97c8c83cbb
commit f4dd2960b8
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21 changed files with 120 additions and 113 deletions
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13
components/script/dom/window.rs
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@ -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,