python flask server port deforced by ntpd

Question

I have a rest server implemented by python with flask. And implement an api to restart ntpd.
The code test_flask.py:

from flask import Flask
import subprocess
import logging
import sys

app = Flask(__name__)


def run_shell_cmd(cmd):
    logging.info("run cmd: %s", cmd)
    try:
        rc = subprocess.call(cmd, shell=True)
        if rc != 0:
            logging.error("Fail to run %s , rc: %s" % (cmd, rc))
    except OSError as e:
        logging.error("Fail to run cmd: %s" % e)
    return rc

@app.route("/restart_ntpd")
def restart():
    run_shell_cmd("service ntpd restart")
    return "Success!"

if __name__ == "__main__":
    LOG_FORMAT = '%(asctime)s, %(levelname)s, %(filename)s:%(lineno)d, %(message)s'
    logging.basicConfig(
        format=LOG_FORMAT,
        level=logging.INFO,
        stream=sys.stdout,
    )
    app.run()

Then I operated as follow:

1. start flask server: python test_flask.py
2. curl "http://localhost:5000/restart_ntpd. Then ntpd restart & return "success"
3. stop flask server: just use Ctrl+c to stop
4. start flask server again, it will raise a exception:

   socket.error: [Errno 98] Address already in use.

5. use sh $ netstat -ntlp | grep 5000, the port was deforced by ntpd

I think the ntpd will use port 123 in default. In my scene, why the port 5000 is deforced by ntpd? Is it the problem of flask?

Answer 1

ntpd is not listening on TCP port 5000 itself, it's the environment where it is running - the process.

And that process is a child of your Flask server process, which opens a socket listening on TCP port 5000.

This socket is inherited in the child process, and since the ntpd process is a long-running one, it continues running with the socket it inherits from you, occupying the port 5000.

Check how to deal with Python BaseHTTPServer killed,but the port is still be occupied? on how to prevent children processes from inheriting the socket.

Of course, first you have to find a way to customize the way Flask start the server.