constellation: Use one image cache thread pool in the single-process mode (#38783)

Previously, each ScriptThread was creating a new image cache with a
separate thread pool. These changes add an image cache to the
constellation and create new image caches by calling the
`create_new_image_cache` method. It reuses the original image cache's
thread pool and reduces the number of spawned threads in the
single-process mode.

Testing: Tested manually, using `ps -M` to see the number of spawned
threads with multiple tabs open in servoshell before and after these
changes.
Fixes: #37770

Signed-off-by: Rodion Borovyk <rodion.borovyk@gmail.com>

components/constellation/constellation.rs

@@ -147,6 +147,8 @@ use keyboard_types::{Key, KeyState, Modifiers, NamedKey};
 use layout_api::{LayoutFactory, ScriptThreadFactory};
 use log::{debug, error, info, trace, warn};
 use media::WindowGLContext;
+use net::image_cache::ImageCacheImpl;
+use net_traits::image_cache::ImageCache;
 use net_traits::pub_domains::reg_host;
 use net_traits::request::Referrer;
 use net_traits::storage_thread::{StorageThreadMsg, StorageType};
@@ -486,6 +488,9 @@ pub struct Constellation<STF, SWF> {
 
     /// A list of URLs that can access privileged internal APIs.
     privileged_urls: Vec<ServoUrl>,
+
+    /// The image cache for the single-process mode
+    image_cache: Box<dyn ImageCache>,
 }
 
 /// State needed to construct a constellation.
@@ -684,7 +689,7 @@ where
                     compositor_receiver,
                     layout_factory,
                     embedder_proxy: state.embedder_proxy,
-                    compositor_proxy: state.compositor_proxy,
+                    compositor_proxy: state.compositor_proxy.clone(),
                     webviews: WebViewManager::default(),
                     devtools_sender: state.devtools_sender,
                     #[cfg(feature = "bluetooth")]
@@ -731,12 +736,16 @@ where
                     active_keyboard_modifiers: Modifiers::empty(),
                     hard_fail,
                     active_media_session: None,
-                    rippy_data,
+                    rippy_data: rippy_data.clone(),
                     user_content_manager: state.user_content_manager,
                     process_manager: ProcessManager::new(state.mem_profiler_chan),
                     async_runtime: state.async_runtime,
                     script_join_handles: Default::default(),
                     privileged_urls: state.privileged_urls,
+                    image_cache: Box::new(ImageCacheImpl::new(
+                        state.compositor_proxy.cross_process_compositor_api,
+                        rippy_data,
+                    )),
                 };
 
                 constellation.run();
@@ -1014,6 +1023,10 @@ where
             rippy_data: self.rippy_data.clone(),
             user_content_manager: self.user_content_manager.clone(),
             privileged_urls: self.privileged_urls.clone(),
+            image_cache: self.image_cache.create_new_image_cache(
+                Some(pipeline_id),
+                self.compositor_proxy.cross_process_compositor_api.clone(),
+            ),
         });
 
         let pipeline = match result {

components/constellation/pipeline.rs

@@ -212,6 +212,9 @@ pub struct InitialPipelineState {
 
     /// A list of URLs that can access privileged internal APIs.
     pub privileged_urls: Vec<ServoUrl>,
+
+    /// The image cache for the single-process mode
+    pub image_cache: Arc<dyn ImageCache>,
 }
 
 pub struct NewPipeline {
@@ -338,6 +341,7 @@ impl Pipeline {
                         false,
                         state.layout_factory,
                         register,
+                        Some(state.image_cache),
                     );
                     (None, None, Some(join_handle))
                 };
@@ -517,15 +521,20 @@ impl UnprivilegedPipelineContent {
         wait_for_completion: bool,
         layout_factory: Arc<dyn LayoutFactory>,
         background_hang_monitor_register: Box<dyn BackgroundHangMonitorRegister>,
+        image_cache: Option<Arc<dyn ImageCache>>,
     ) -> JoinHandle<()> {
         // Setup pipeline-namespace-installing for all threads in this process.
         // Idempotent in single-process mode.
         PipelineNamespace::set_installer_sender(self.namespace_request_sender);
 
-        let image_cache = Arc::new(ImageCacheImpl::new(
+        let image_cache = image_cache.unwrap_or_else(|| {
+            // Create a new image cache (in the multiprocess case)
+            Arc::new(ImageCacheImpl::new(
                 self.cross_process_compositor_api.clone(),
                 self.rippy_data,
-        ));
+            ))
+        });
+
         let (content_process_shutdown_chan, content_process_shutdown_port) = unbounded();
         let join_handle = STF::create(
             InitialScriptState {
@@ -542,7 +551,7 @@ impl UnprivilegedPipelineContent {
                 #[cfg(feature = "bluetooth")]
                 bluetooth_sender: self.bluetooth_thread,
                 resource_threads: self.resource_threads,
-                image_cache: image_cache.clone(),
+                image_cache,
                 time_profiler_sender: self.time_profiler_chan.clone(),
                 memory_profiler_sender: self.mem_profiler_chan.clone(),
                 devtools_server_sender: self.devtools_ipc_sender,

components/servo/lib.rs

@@ -1289,6 +1289,7 @@ pub fn run_content_process(token: String) {
                 true,
                 layout_factory,
                 background_hang_monitor_register,
+                None,
             );
 
             // Since wait_for_completion is true,