How to correctly shutdown a rosnode?

edit

I test a simple code snippet:

import rospy
import time
import sys
if __name__=="__main__":
  rospy.init_node("a node")
  rospy.signal_shutdown("an exception")

this code will cause /rosout create a file descriptor leak, check with:

lsof -n | grep rosout

which shows a line "rosout 111215 13u sock 0,8 0t0 797756 can't identify protocol".

Each time the code is executed, one more file descriptor leak will be created.

How to prevent this?