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I have a flask app which I need to start and then shut down within the same process and repeat this multiple times. I am using gevents in my application so I'm using gevents.pywsgi as my WSGI server. Now I'm trying to gracefully shutdown the server so I can restart it in the same process after I do a few other things.

So the following code creates a basic flask app, which when receives a POST request with any valid data on /hit endpoint populates the data field. A greenlet is running in parallel with this app and when it sees that the data field is populated, it shuts down the server. 

    def func():
    global data
    data = None

    app = Flask(__name__)

    @app.route('/hit', methods=['POST'])
    def hit():
        global data
        data = request.json

        if data is not None:
            return "Input Recieved, Server closed "
        else:
            return "Invalid Input, Try again"

    def shutdown_server(_server):
        global data
        while data is None:
            sleep(0.5)
        _server.stop()
        _server.close()

    server = WSGIServer(('0.0.0.0',5100), app)

    start = spawn(server.start)
    stop = spawn(shutdown_server, request, server)

    joinall([start, stop])

    return True

Now this code is running fine if I run the server once, but if I try to run the server again within the same process it throws the following error:

Traceback (most recent call last):
  File "src/gevent/greenlet.py", line 766, in gevent._greenlet.Greenlet.run
  File "/home/batman/Documents/genisys/lib/python3.6/site-packages/gevent/baseserver.py", line 308, in start
    self.start_accepting()
  File "/home/batman/Documents/genisys/lib/python3.6/site-packages/gevent/baseserver.py", line 160, in start_accepting
    self._watcher = self.loop.io(self.socket.fileno(), 1)
AttributeError: 'WSGIServer' object has no attribute 'socket'

I'm not sure if this is even possible, or if I have to run the server in a separate process if I want to run it multiple times. Can anybody tell me why I'm facing this error and is there a better and cleaner way to shutdown the server so I don't face this error?

EDIT : I give IP and port as arguments to func, so I have tried call func with a different port multiple times and with the same port multiple times, I still get the same error.

Batman
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1 Answers1

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I think problem is you already running the server on port 5100 so again you can't run process on same port , so you have to dynamically change the port numbers when you want start the process

Sai Kumar
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  • I tried that, I actually give the ip and port as arguments to func. I should probably mention that in the question, sorry. – Batman Feb 18 '20 at 12:52
  • i think this one help to you [ https://stackoverflow.com/questions/15562446/how-to-stop-flask-application-without-using-ctrl-c] , please check it – Sai Kumar Feb 18 '20 at 13:12
  • Well this was actually my initial design, but then I realized I need my server to be compatible with gevents, so now I use a WSGI server from gevents.pywsgi to run my app instead of Flask's default werkzeug server. Hence I cannot shut it down this way. Thanks for the suggestion though!! – Batman Feb 18 '20 at 17:36