Auto merge of #24862 - jdm:js-init-destroy-fit-the-second, r=asajeffrey

Ensure SpiderMonkey shuts down cleanly

This is the alternate solution that I described in #24845. Given how much simpler the resulting code is, I'm now much more in favour of this design. Depends on https://github.com/servo/rust-mozjs/pull/487.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #21696 and fix #22276
- [x] There are tests for these changes

components/script/init.rs

@@ -4,6 +4,7 @@
 
 use crate::dom::bindings::codegen::RegisterBindings;
 use crate::dom::bindings::proxyhandler;
+use crate::script_runtime::JSEngineSetup;
 use crate::serviceworker_manager::ServiceWorkerManager;
 use script_traits::SWManagerSenders;
 
@@ -56,7 +57,7 @@ pub fn init_service_workers(sw_senders: SWManagerSenders) {
 }
 
 #[allow(unsafe_code)]
-pub fn init() {
+pub fn init() -> JSEngineSetup {
     unsafe {
         proxyhandler::init();
 
@@ -66,4 +67,6 @@ pub fn init() {
     }
 
     perform_platform_specific_initialization();
+
+    JSEngineSetup::new()
 }

components/script/lib.rs

@@ -113,6 +113,7 @@ mod unpremultiplytable;
 mod webdriver_handlers;
 
 pub use init::{init, init_service_workers};
+pub use script_runtime::JSEngineSetup;
 
 /// A module with everything layout can use from script.
 ///

components/script/script_runtime.rs

@@ -61,9 +61,9 @@ use js::rust::wrappers::{GetPromiseIsHandled, JS_GetPromiseResult};
 use js::rust::Handle;
 use js::rust::HandleObject as RustHandleObject;
 use js::rust::IntoHandle;
-use js::rust::JSEngine;
 use js::rust::ParentRuntime;
 use js::rust::Runtime as RustRuntime;
+use js::rust::{JSEngine, JSEngineHandle};
 use malloc_size_of::MallocSizeOfOps;
 use msg::constellation_msg::PipelineId;
 use profile_traits::mem::{Report, ReportKind, ReportsChan};
@@ -79,7 +79,9 @@ use std::os::raw::c_void;
 use std::panic::AssertUnwindSafe;
 use std::ptr;
 use std::rc::Rc;
-use std::sync::Arc;
+use std::sync::Mutex;
+use std::thread;
+use std::time::Duration;
 use style::thread_state::{self, ThreadState};
 use time::{now, Tm};
 
@@ -395,8 +397,28 @@ impl Deref for Runtime {
     }
 }
 
+pub struct JSEngineSetup(JSEngine);
+
+impl JSEngineSetup {
+    pub fn new() -> Self {
+        let engine = JSEngine::init().unwrap();
+        *JS_ENGINE.lock().unwrap() = Some(engine.handle());
+        Self(engine)
+    }
+}
+
+impl Drop for JSEngineSetup {
+    fn drop(&mut self) {
+        *JS_ENGINE.lock().unwrap() = None;
+
+        while !self.0.can_shutdown() {
+            thread::sleep(Duration::from_millis(50));
+        }
+    }
+}
+
 lazy_static! {
-    static ref JS_ENGINE: Arc<JSEngine> = JSEngine::init().unwrap();
+    static ref JS_ENGINE: Mutex<Option<JSEngineHandle>> = Mutex::new(None);
 }
 
 #[allow(unsafe_code)]
@@ -421,7 +443,7 @@ unsafe fn new_rt_and_cx_with_parent(
     let runtime = if let Some(parent) = parent {
         RustRuntime::create_with_parent(parent)
     } else {
-        RustRuntime::new(JS_ENGINE.clone())
+        RustRuntime::new(JS_ENGINE.lock().unwrap().as_ref().unwrap().clone())
     };
     let cx = runtime.cx();
 
@@ -794,7 +816,7 @@ unsafe fn set_gc_zeal_options(cx: *mut RawJSContext) {
     use js::jsapi::{JS_SetGCZeal, JS_DEFAULT_ZEAL_FREQ};
 
     let level = match pref!(js.mem.gc.zeal.level) {
-        level @ 0...14 => level as u8,
+        level @ 0..=14 => level as u8,
         _ => return,
     };
     let frequency = match pref!(js.mem.gc.zeal.frequency) {