layout: Shut down style thread-pool when exiting a content process / shutting down the Constellation (#38924)

To ensure a clean-shutdown of Servo, we need to shutdown our internal
threading before deinit. This shuts down the style thread pool either
using the constellation(in single-process mode), or when a content
process exits.

Testing: Manual testing by opening a window, opening and closing
multiple tabs, and closing the window, with a sleep added before deinit
and a sample taken to ensure the loose threads previously noticed are
gone.
Fixes: part of https://github.com/servo/servo/issues/30849

---------

Signed-off-by: gterzian <2792687+gterzian@users.noreply.github.com>

Cargo.lock

@@ -1514,6 +1514,7 @@ dependencies = [
  "servo_config",
  "servo_rand",
  "servo_url",
+ "stylo",
  "stylo_traits",
  "tracing",
  "webgpu",

components/constellation/Cargo.toml

@@ -55,6 +55,7 @@ servo_config = { path = "../config" }
 servo_rand = { path = "../rand" }
 servo_url = { path = "../url" }
 stylo_traits = { workspace = true }
+stylo = { workspace = true }
 tracing = { workspace = true, optional = true }
 webgpu = { path = "../webgpu" }
 webgpu_traits = { workspace = true }

components/constellation/constellation.rs

@@ -165,6 +165,7 @@ use servo_config::prefs::{self, PrefValue};
 use servo_config::{opts, pref};
 use servo_rand::{Rng, ServoRng, SliceRandom, random};
 use servo_url::{Host, ImmutableOrigin, ServoUrl};
+use style::global_style_data::StyleThreadPool;
 #[cfg(feature = "webgpu")]
 use webgpu::swapchain::WGPUImageMap;
 #[cfg(feature = "webgpu")]
@@ -755,6 +756,10 @@ where
         }
         self.handle_shutdown();
 
+        if !opts::get().multiprocess {
+            StyleThreadPool::shutdown();
+        }
+
         // Shut down the fetch thread started above.
         exit_fetch_thread();
         join_handle

components/servo/lib.rs

@@ -107,6 +107,7 @@ use servo_geometry::{
 use servo_media::ServoMedia;
 use servo_media::player::context::GlContext;
 use servo_url::ServoUrl;
+use style::global_style_data::StyleThreadPool;
 use webgl::WebGLComm;
 #[cfg(feature = "webgpu")]
 pub use webgpu;
@@ -1281,6 +1282,8 @@ pub fn run_content_process(token: String) {
                 .join()
                 .expect("Failed to join on the BHM background thread.");
 
+            StyleThreadPool::shutdown();
+
             // Shut down the fetch thread started above.
             exit_fetch_thread();
             fetch_thread_join_handle