Detach port on closure and check for detached during transfer

components/script/dom/bindings/structuredclone.rs

@@ -217,6 +217,10 @@ unsafe extern "C" fn write_transfer_callback(
     extra_data: *mut u64,
 ) -> bool {
     if let Ok(port) = root_from_handleobject::<MessagePort>(Handle::from_raw(obj)) {
+        if let Some(true) = port.detached() {
+            return false;
+        }
+
         *tag = StructuredCloneTags::MessagePort as u32;
         *ownership = TransferableOwnership::SCTAG_TMO_CUSTOM;
         if port.transfer(closure, content, extra_data) {

components/script/dom/messageport.rs

@@ -364,6 +364,10 @@ impl MessagePortMethods for MessagePort {
 
     /// <https://html.spec.whatwg.org/multipage/#dom-messageport-close>
     fn Close(&self) {
+        // Step 1
+        self.detached.set(true);
+
+        // Step 2
         let maybe_port = {
             let internal = self.message_port_internal.lock().unwrap();
             let mut maybe_port = internal.entangled_port.borrow_mut();