code: func (m *Manager) CloseDB() error {
  	m.dbMtx.Lock()
  	defer m.dbMtx.Unlock()
  	if m.db == nil {
  		return nil
  	}
  	if err := m.db.Close(); err != nil {
  		return err
  	}
  	m.db = nil
  	return nil
  }
docs: // CloseDB closes the persistence DB.
  //
  // The DB mutex is locked to protect m.db, but also prevents closing the
  // DB when it could still be needed to service API requests (see LockDB).