Bug 1318226. P1 - preserve the order of regular tasks. r=bholley

MozReview-Commit-ID: FYjbJWYDyd0

--HG--
extra : rebase_source : 1baeeb8d4e5904094bd258893d5cb59ec9ac8944
extra : intermediate-source : 224087f24319d562484be55f3b97a21f9d50577b
extra : source : b6e940be26be76ae7620037b93c98e42504ed979

xpcom/threads/TaskDispatcher.h

@@ -124,7 +124,15 @@ public:
                already_AddRefed<nsIRunnable> aRunnable,
                AbstractThread::DispatchFailureHandling aFailureHandling) override
   {
-    PerThreadTaskGroup& group = EnsureTaskGroup(aThread);
+    // To preserve the event order, we need to append a new group if the last
+    // group is not targeted for |aThread|.
+    // See https://bugzilla.mozilla.org/show_bug.cgi?id=1318226&mark=0-3#c0
+    // for the details of the issue.
+    if (mTaskGroups.Length() == 0 || mTaskGroups.LastElement()->mThread != aThread) {
+      mTaskGroups.AppendElement(new PerThreadTaskGroup(aThread));
+    }
+
+    PerThreadTaskGroup& group = *mTaskGroups.LastElement();
     group.mRegularTasks.AppendElement(aRunnable);
 
     // The task group needs to assert dispatch success if any of the runnables
@@ -142,11 +150,11 @@ public:
 
   void DispatchTasksFor(AbstractThread* aThread) override
   {
+    // Dispatch all groups that match |aThread|.
     for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
       if (mTaskGroups[i]->mThread == aThread) {
         DispatchTaskGroup(Move(mTaskGroups[i]));
-        mTaskGroups.RemoveElementAt(i);
-        return;
+        mTaskGroups.RemoveElementAt(i--);
       }
     }
   }