Bug 1337537 - Avoid SystemGroup assertion during WebAudio test (r=ehsan)

MozReview-Commit-ID: 9GbXnkF4fvZ

dom/media/webaudio/test/test_bug1027864.html

@@ -29,10 +29,10 @@ function id(node) {
   return SpecialPowers.getPrivilegedProps(node, "id");
 }
 
-SpecialPowers.addObserver(observer, "webaudio-node-demise", false);
+SpecialPowers.addAsyncObserver(observer, "webaudio-node-demise", false);
 
 SimpleTest.registerCleanupFunction(function() {
-  SpecialPowers.removeObserver(observer, "webaudio-node-demise");
+  SpecialPowers.removeAsyncObserver(observer, "webaudio-node-demise");
 });
 
 var ac = new AudioContext();

testing/specialpowers/content/specialpowersAPI.js

@@ -1213,6 +1213,38 @@ SpecialPowersAPI.prototype = {
     Services.obs.notifyObservers(subject, topic, data);
   },
 
+  /**
+   * An async observer is useful if you're listening for a
+   * notification that normally is only used by C++ code or chrome
+   * code (so it runs in the SystemGroup), but we need to know about
+   * it for a test (which runs as web content). If we used
+   * addObserver, we would assert when trying to enter web content
+   * from a runnabled labeled by the SystemGroup. An async observer
+   * avoids this problem.
+   */
+  _asyncObservers: new WeakMap(),
+  addAsyncObserver: function(obs, notification, weak) {
+    obs = Cu.waiveXrays(obs);
+    if (typeof obs == 'object' && obs.observe.name != 'SpecialPowersCallbackWrapper') {
+      obs.observe = wrapCallback(obs.observe);
+    }
+    let asyncObs = (...args) => {
+      Services.tm.mainThread.dispatch(() => {
+        if (typeof obs == 'function') {
+          obs.call(undefined, ...args);
+        } else {
+          obs.observe.call(undefined, ...args);
+        }
+      }, Ci.nsIThread.DISPATCH_NORMAL);
+    };
+    this._asyncObservers.set(obs, asyncObs);
+    Services.obs.addObserver(asyncObs, notification, weak);
+  },
+  removeAsyncObserver: function(obs, notification) {
+    let asyncObs = this._asyncObservers.get(Cu.waiveXrays(obs));
+    Services.obs.removeObserver(asyncObs, notification);
+  },
+
   can_QI: function(obj) {
     return obj.QueryInterface !== undefined;
   },