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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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4
components/script/dom/debuggerglobalscope.rs
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@ -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")