Bug 1622219 - Stop returning any value from ThreadActor.attach. r=jdescottes

We weren't actually returning any value.
The only case when we return something is when the RDP method throws and return
an object response with an "error" attribute.
I imagine this has been refactored incorectly from old style actor to protocol.js.
We should throw in case of errors if we want to transfer a RDP message with "error" attribute.

Differential Revision: https://phabricator.services.mozilla.com/D66132

--HG--
extra : moz-landing-system : lando

devtools/server/actors/thread.js

@@ -4,6 +4,10 @@
 
 "use strict";
 
+// protocol.js uses objects as exceptions in order to define
+// error packets.
+/* eslint-disable no-throw-literal */
+
 const DebuggerNotificationObserver = require("DebuggerNotificationObserver");
 const Services = require("Services");
 const { Cr, Ci } = require("chrome");
@@ -341,14 +345,14 @@ const ThreadActor = ActorClassWithSpec(threadSpec, {
   // Request handlers
   onAttach: function({ options }) {
     if (this.state === "exited") {
-      return {
+      throw {
         error: "exited",
         message: "threadActor has exited",
       };
     }
 
     if (this.state !== "detached") {
-      return {
+      throw {
         error: "wrongState",
         message: "Current state is " + this.state,
       };
@@ -395,7 +399,7 @@ const ThreadActor = ActorClassWithSpec(threadSpec, {
       // Put ourselves in the paused state.
       const packet = this._paused();
       if (!packet) {
-        return {
+        throw {
           error: "notAttached",
           message: "cannot attach, could not create pause packet",
         };
@@ -411,12 +415,12 @@ const ThreadActor = ActorClassWithSpec(threadSpec, {
       // Start a nested event loop.
       this._pushThreadPause();
 
-      // We already sent a response to this request, don't send one
-      // now
-      return null;
+      // We already sent a response to this request via this.conn.send(),
+      // don't send one now. But protocol.js probably still emits a second
+      // empty packet.
     } catch (e) {
       reportException("DBG-SERVER", e);
-      return {
+      throw {
         error: "notAttached",
         message: e.toString(),
       };

devtools/shared/fronts/thread.js

@@ -208,15 +208,9 @@ class ThreadFront extends FrontClassWithSpec(threadSpec) {
    * attach to the thread actor.
    */
   async attach(options) {
-    let response;
-    try {
-      const onPaused = this.once("paused");
-      response = await super.attach(options);
-      await onPaused;
-    } catch (e) {
-      throw new Error(e);
-    }
-    return response;
+    const onPaused = this.once("paused");
+    await super.attach(options);
+    await onPaused;
   }
 
   /**

devtools/shared/specs/thread.js

@@ -73,7 +73,7 @@ const threadSpec = generateActorSpec({
       request: {
         options: Arg(0, "json"),
       },
-      response: RetVal("nullable:json"),
+      response: {},
     },
     detach: {
       request: {},