Use GenericChannel for script_chan (#38645)

Motivation: 
Using our GenericChannel abstraction allows us to optimize IPC in
single-process mode to just use cross-beam channel.
To keep the diff low, and get early feedback, this PR only tackles a
single channel, but the intention is to port all ipc channels to the
generic channel, which allows us to skip serializing and deserializing
messages in single process mode.

Based on: 
- https://github.com/servo/servo/pull/38638
- https://github.com/servo/servo/pull/38636

Testing: Covered by existing tests

---------

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

components/shared/base/Cargo.toml

@@ -20,6 +20,7 @@ malloc_size_of = { workspace = true }
 malloc_size_of_derive = { workspace = true }
 parking_lot = { workspace = true }
 serde = { workspace = true }
+servo_config = { path = "../../config" }
 time = { workspace = true }
 webrender_api = { workspace = true }
 

components/shared/base/generic_channel.rs

@@ -7,6 +7,7 @@
 use std::fmt;
 
 use ipc_channel::router::ROUTER;
+use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
 static GENERIC_CHANNEL_USAGE_ERROR_PANIC_MSG: &str = "May not send a crossbeam channel over an IPC channel. \
@@ -65,6 +66,12 @@ impl<T: Serialize> GenericSender<T> {
     }
 }
 
+impl<T: Serialize> MallocSizeOf for GenericSender<T> {
+    fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
+        0
+    }
+}
+
 #[derive(Debug)]
 pub struct SendError;
 pub type SendResult = Result<(), SendError>;
@@ -145,11 +152,11 @@ where
 /// Creates a Servo channel that can select different channel implementations based on multiprocess
 /// mode or not. If the scenario doesn't require message to pass process boundary, a simple
 /// crossbeam channel is preferred.
-pub fn channel<T>(multiprocess: bool) -> Option<(GenericSender<T>, GenericReceiver<T>)>
+pub fn channel<T>() -> Option<(GenericSender<T>, GenericReceiver<T>)>
 where
     T: for<'de> Deserialize<'de> + Serialize,
 {
-    if multiprocess {
+    if servo_config::opts::get().multiprocess {
         ipc_channel::ipc::channel()
             .map(|(tx, rx)| (GenericSender::Ipc(tx), GenericReceiver::Ipc(rx)))
             .ok()

components/shared/canvas/webgl.rs

@@ -33,7 +33,7 @@ pub fn webgl_channel<T>() -> Option<(WebGLSender<T>, WebGLReceiver<T>)>
 where
     T: for<'de> Deserialize<'de> + Serialize,
 {
-    base::generic_channel::channel(servo_config::opts::get().multiprocess)
+    base::generic_channel::channel()
 }
 
 /// Entry point channel type used for sending WebGLMsg messages to the WebGL renderer.

components/shared/layout/lib.rs

@@ -20,6 +20,7 @@ use std::thread::JoinHandle;
 use app_units::Au;
 use atomic_refcell::AtomicRefCell;
 use base::Epoch;
+use base::generic_channel::GenericSender;
 use base::id::{BrowsingContextId, PipelineId, WebViewId};
 use bitflags::bitflags;
 use compositing_traits::CrossProcessCompositorApi;
@@ -30,7 +31,6 @@ use euclid::default::{Point2D as UntypedPoint2D, Rect};
 use fnv::FnvHashMap;
 use fonts::{FontContext, SystemFontServiceProxy};
 use fxhash::FxHashMap;
-use ipc_channel::ipc::IpcSender;
 pub use layout_damage::LayoutDamage;
 use libc::c_void;
 use malloc_size_of::{MallocSizeOf as MallocSizeOfTrait, MallocSizeOfOps, malloc_size_of_is_0};
@@ -197,7 +197,7 @@ pub struct LayoutConfig {
     pub webview_id: WebViewId,
     pub url: ServoUrl,
     pub is_iframe: bool,
-    pub script_chan: IpcSender<ScriptThreadMessage>,
+    pub script_chan: GenericSender<ScriptThreadMessage>,
     pub image_cache: Arc<dyn ImageCache>,
     pub font_context: Arc<FontContext>,
     pub time_profiler_chan: time::ProfilerChan,

components/shared/script/lib.rs

@@ -15,6 +15,7 @@ use std::sync::Arc;
 
 use background_hang_monitor_api::BackgroundHangMonitorRegister;
 use base::cross_process_instant::CrossProcessInstant;
+use base::generic_channel::{GenericReceiver, GenericSender};
 use base::id::{BrowsingContextId, HistoryStateId, PipelineId, PipelineNamespaceId, WebViewId};
 #[cfg(feature = "bluetooth")]
 use bluetooth_traits::BluetoothRequest;
@@ -307,9 +308,9 @@ pub struct InitialScriptState {
     /// Loading into a Secure Context
     pub inherited_secure_context: Option<bool>,
     /// A channel with which messages can be sent to us (the script thread).
-    pub constellation_sender: IpcSender<ScriptThreadMessage>,
+    pub constellation_sender: GenericSender<ScriptThreadMessage>,
     /// A port on which messages sent by the constellation to script can be received.
-    pub constellation_receiver: IpcReceiver<ScriptThreadMessage>,
+    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 handle to register script-(and associated layout-)threads for hang monitoring.