Auto merge of #28536 - yvt:fix-one-compartment, r=jdm

Create only one compartment for each script thread (agent)

Documents in the same [agent][1] can share and exchange JS and DOM objects freely, so putting them in separate compartments would require almost every instance of `Dom` to be capable of handling cross-compartment references.

[1]: https://html.spec.whatwg.org/multipage/webappapis.html#integration-with-the-javascript-agent-formalism

---
- [x] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

---
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because I think there's already a wide test coverage for same-origin-domain JS object passing, albeit this requires Servo to be built with `--debug-mozjs` for the errors to be (reliably) observable

2 files changed, 47 insertions, 7 deletions

diff --git a/components/script/dom/bindings/interface.rs b/components/script/dom/bindings/interface.rs
index 1085a137428..82303f800ba 100644
--- a/components/script/dom/bindings/interface.rs
+++ b/components/script/dom/bindings/interface.rs
@@ -16,6 +16,10 @@ use js::glue::UncheckedUnwrapObject;
 use js::jsapi::GetWellKnownSymbol;
 use js::jsapi::HandleObject as RawHandleObject;
 use js::jsapi::{jsid, JSClass, JSClassOps};
+use js::jsapi::{
+    Compartment, CompartmentSpecifier, IsSharableCompartment, IsSystemCompartment,
+    JS_IterateCompartments, JS::CompartmentIterResult,
+};
 use js::jsapi::{JSAutoRealm, JSContext, JSFunctionSpec, JSObject, JSFUN_CONSTRUCTOR};
 use js::jsapi::{JSPropertySpec, JSString, JSTracer, JS_AtomizeAndPinString};
 use js::jsapi::{JS_GetFunctionObject, JS_NewFunction, JS_NewGlobalObject};
@@ -139,6 +143,7 @@ pub unsafe fn create_global_object(
     options.creationOptions_.traceGlobal_ = Some(trace);
     options.creationOptions_.sharedMemoryAndAtomics_ = false;
     options.creationOptions_.streams_ = true;
+    select_compartment(cx, &mut options);
 
     rval.set(JS_NewGlobalObject(
         *cx,
@@ -162,6 +167,43 @@ pub unsafe fn create_global_object(
     JS_FireOnNewGlobalObject(*cx, rval.handle());
 }
 
+/// Choose the compartment to create a new global object in.
+fn select_compartment(cx: SafeJSContext, options: &mut RealmOptions) {
+    type Data = *mut Compartment;
+    unsafe extern "C" fn callback(
+        _cx: *mut JSContext,
+        data: *mut libc::c_void,
+        compartment: *mut Compartment,
+    ) -> CompartmentIterResult {
+        let data = data as *mut Data;
+
+        if !IsSharableCompartment(compartment) || IsSystemCompartment(compartment) {
+            return CompartmentIterResult::KeepGoing;
+        }
+
+        // Choose any sharable, non-system compartment in this context to allow
+        // same-agent documents to share JS and DOM objects.
+        *data = compartment;
+        CompartmentIterResult::Stop
+    }
+
+    let mut compartment: Data = ptr::null_mut();
+    unsafe {
+        JS_IterateCompartments(
+            *cx,
+            (&mut compartment) as *mut Data as *mut libc::c_void,
+            Some(callback),
+        );
+    }
+
+    if compartment.is_null() {
+        options.creationOptions_.compSpec_ = CompartmentSpecifier::NewCompartmentAndZone;
+    } else {
+        options.creationOptions_.compSpec_ = CompartmentSpecifier::ExistingCompartment;
+        options.creationOptions_.__bindgen_anon_1.comp_ = compartment;
+    }
+}
+
 /// Create and define the interface object of a callback interface.
 pub fn create_callback_interface_object(
     cx: SafeJSContext,
diff --git a/components/script/dom/windowproxy.rs b/components/script/dom/windowproxy.rs
index e37e561af06..0f92e4b3367 100644
--- a/components/script/dom/windowproxy.rs
+++ b/components/script/dom/windowproxy.rs
@@ -637,13 +637,11 @@ impl WindowProxy {
             // The old window proxy no longer owns this browsing context.
             SetProxyReservedSlot(old_js_proxy.get(), 0, &PrivateValue(ptr::null_mut()));
 
-            // Brain transpant the window proxy.
-            // We need to do this, because the Window and WindowProxy
-            // objects need to be in the same realm.
-            // JS_TransplantObject does this by copying the contents
-            // of the old window proxy to the new window proxy, then
-            // making the old window proxy a cross-realm wrapper
-            // pointing to the new window proxy.
+            // Brain transpant the window proxy. Brain transplantation is
+            // usually done to move a window proxy between compartments, but
+            // that's not what we are doing here. We need to do this just
+            // because we want to replace the wrapper's `ProxyTraps`, but we
+            // don't want to update its identity.
             rooted!(in(*cx) let new_js_proxy = NewWindowProxy(*cx, window_jsobject, handler));
             debug!(
                 "Transplanting proxy from {:p} to {:p}.",