register stylo threads with the gecko profiler

Now that we require rayon 0.7, we can register start and stop handlers
that take care of dealing with the Gecko profiler for rayon threads in
the style thread pool.

components/style/gecko/global_style_data.rs

@@ -5,11 +5,13 @@
 //! Global style data
 
 use context::StyleSystemOptions;
+use gecko_bindings::bindings::{Gecko_RegisterProfilerThread, Gecko_UnregisterProfilerThread};
 use num_cpus;
 use rayon;
 use shared_lock::SharedRwLock;
 use std::cmp;
 use std::env;
+use std::ffi::CString;
 
 /// Global style data
 pub struct GlobalStyleData {
@@ -26,6 +28,25 @@ pub struct GlobalStyleData {
     pub options: StyleSystemOptions,
 }
 
+fn thread_name(index: usize) -> String {
+    format!("StyleThread#{}", index)
+}
+
+fn thread_startup(index: usize) {
+    let name = thread_name(index);
+    let name = CString::new(name).unwrap();
+    unsafe {
+        // Gecko_RegisterProfilerThread copies the passed name here.
+        Gecko_RegisterProfilerThread(name.as_ptr());
+    }
+}
+
+fn thread_shutdown(_: usize) {
+    unsafe {
+        Gecko_UnregisterProfilerThread();
+    }
+}
+
 lazy_static! {
     /// Global style data
     pub static ref GLOBAL_STYLE_DATA: GlobalStyleData = {
@@ -39,8 +60,11 @@ lazy_static! {
         let pool = if num_threads <= 1 {
             None
         } else {
-            let configuration =
-                rayon::Configuration::new().num_threads(num_threads);
+            let configuration = rayon::Configuration::new()
+                .num_threads(num_threads)
+                .thread_name(thread_name)
+                .start_handler(thread_startup)
+                .exit_handler(thread_shutdown);
             let pool = rayon::ThreadPool::new(configuration).ok();
             pool
         };