code: def _ssh_master_cmd(addr, user, command, local_key=None):
      
      ssh_call = ['ssh', '-qNfL%d:127.0.0.1:12042' % find_port(addr, user),
                  '-o', 'ControlPath=~/.ssh/unixpipe_%%r@%%h_%d' %
                         find_port(addr, user),
                  '-O', command, '%s@%s' % (user, addr,)]
  
      if local_key:
          ssh_call.insert(1, local_key)
          ssh_call.insert(1, '-i')
  
      return subprocess.call(ssh_call)
docs: Exit or check ssh mux