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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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1
components/shared/script/Cargo.toml
View file

@ -44,3 +44,4 @@ stylo_traits = { workspace = true }
webgpu_traits = { workspace = true, optional = true }
webrender_api = { workspace = true }
webxr-api = { workspace = true, features = ["ipc"] }
log = "0.4.27"

5
components/shared/script/lib.rs
View file

@ -30,7 +30,7 @@ use devtools_traits::ScriptToDevtoolsControlMsg;
use embedder_traits::user_content_manager::UserContentManager;
use embedder_traits::{
CompositorHitTestResult, FocusSequenceNumber, InputEvent, JavaScriptEvaluationId,
MediaSessionActionType, Theme, ViewportDetails, WebDriverScriptCommand,
MediaSessionActionType, ScriptToEmbedderChan, Theme, ViewportDetails, WebDriverScriptCommand,
};
use euclid::{Rect, Scale, Size2D, UnknownUnit};
use ipc_channel::ipc::{IpcReceiver, IpcSender};
@ -321,6 +321,9 @@ pub struct InitialScriptState {
pub constellation_receiver: GenericReceiver<ScriptThreadMessage>,
/// A channel on which messages can be sent to the constellation from script.
pub pipeline_to_constellation_sender: ScriptToConstellationChan,
/// A channel which allows script to send messages directly to the Embedder
/// This will pump the embedder event loop.
pub pipeline_to_embedder_sender: ScriptToEmbedderChan,
/// A handle to register script-(and associated layout-)threads for hang monitoring.
pub background_hang_monitor_register: Box<dyn BackgroundHangMonitorRegister>,
/// A channel to the resource manager thread.