<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def invalidate(self):
 """Mark the data invalidated. Clients will refetch the volume."""

        with self._mesh_generator_lock:
            self._mesh_generator_pending = None
            self._mesh_generator = None
        self._dispatch_changed_callbacks()