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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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5
components/script/messaging.rs
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@ -16,6 +16,7 @@ use bluetooth_traits::BluetoothRequest;
use constellation_traits::ScriptToConstellationMessage;
use crossbeam_channel::{Receiver, SendError, Sender, select};
use devtools_traits::{DevtoolScriptControlMsg, ScriptToDevtoolsControlMsg};
use embedder_traits::ScriptToEmbedderChan;
use ipc_channel::ipc::IpcSender;
use net_traits::FetchResponseMsg;
use net_traits::image_cache::ImageCacheResponseMessage;
@ -344,6 +345,10 @@ pub(crate) struct ScriptThreadSenders {
pub(crate) pipeline_to_constellation_sender:
GenericSender<(PipelineId, ScriptToConstellationMessage)>,
/// A channel to send messages to the Embedder.
#[no_trace]
pub(crate) pipeline_to_embedder_sender: ScriptToEmbedderChan,
/// The shared [`IpcSender`] which is sent to the `ImageCache` when requesting an image. The
/// messages on this channel are routed to crossbeam [`Sender`] on the router thread, which
/// in turn sends messages to [`ScriptThreadReceivers::image_cache_receiver`].