Hook interfaces and implementation performs unnecessary protobuf copying.

Labels HookManager::masterLaunchTaskLabelDecorator(
    const TaskInfo& taskInfo,
    const FrameworkInfo& frameworkInfo,
    const SlaveInfo& slaveInfo)
{
  synchronized (mutex) {
    // We need a mutable copy of the task info and set the new
    // labels after each hook invocation. Otherwise, the last hook
    // will be the only effective hook setting the labels.
    TaskInfo taskInfo_ = taskInfo;

    foreachpair (const string& name, Hook* hook, availableHooks) {
      const Result<Labels> result =
        hook->masterLaunchTaskLabelDecorator(
            taskInfo_,
            frameworkInfo,
            slaveInfo);

      // NOTE: If the hook returns None(), the task labels won't be
      // changed.
      if (result.isSome()) {
        taskInfo_.mutable_labels()->CopyFrom(result.get());
      } else if (result.isError()) {
        LOG(WARNING) << "Master label decorator hook failed for module '"
                    << name << "': " << result.error();
      }
    }

    return taskInfo_.labels();
  }
}

        foreach (
            TaskInfo& task, *message.mutable_task_group()->mutable_tasks()) {
          taskIds.insert(task.task_id());
          totalResources += task.resources();

          addTask(task, framework, slave);

          if (HookManager::hooksAvailable()) {
            // Set labels retrieved from label-decorator hooks.
            task.mutable_labels()->CopyFrom(
                HookManager::masterLaunchTaskLabelDecorator(
                    task,
                    framework->info,
                    slave->info));
          }
        }