Exception gevent.hub.LoopExit: LoopExit('This operation would block forever',)

Question

I am always getting this error when running my Flask App with Websockets. I have tried to follow this guide - http://blog.miguelgrinberg.com/post/easy-websockets-with-flask-and-gevent

I have a flask app that provides the GUI interface for my network sniffer. The sniffer is inside a thread as shown below : ( l is the thread for my sniffer; isRunning is a boolean to check if the thread is already running)

try:
    if l.isRunning == False:  # if the thread has been shut down
        l.isRunning = True  # change it to true, so it could loop again
        running = True
        l.start()  # starts the forever loop / declared from the top to be a global variable
        print str(running)
    else:
        running = True
        print str(running)
        l.start()
except Exception, e:
    raise e
return flask.render_template('test.html', running=running)  #goes to the test.html page

The sniffer runs fine without the socketio and i am able to sniff the network while traversing my gui.However, when I included the socketio in the code, I first see the socketio working in my index page and i am able to receive the messages from the server to the page. I could also traverse fine to the other static paages in my GUI ;however, activating my threaded network sniffer would leave my browser hangup. I always get the Exception gevent.hub.LoopExit: LoopExit('This operation would block forever',) error and when I rerun my program, the console would say that the address is already in use. It seems to me that I may not be closing my sockets correctly as this is happening. I also think that some operation is blocking based from the error. The code in my python flask app is shown below

def background_thread():
    """Example of how to send server generated events to clients."""
    count = 0
    while True:
        time.sleep(10)
        count += 1
        socketio.emit('my response',{'data': 'Server generated event', 'count': count},namespace='/test')
if socketflag is None:
    thread = Thread(target=background_thread)
    thread.start()


@socketio.on('my event', namespace='/test')
def test_message(message):
    emit('my response', {'data': message['data']})

@socketio.on('my broadcast event', namespace='/test')
def test_message(message):
    emit('my response', {'data': message['data']}, broadcast=True)

@socketio.on('connect', namespace='/test')
def test_connect():
    emit('my response', {'data': 'Connected'})

@socketio.on('disconnect', namespace='/test')
def test_disconnect():
    print('Client disconnected')

Here is the code present in my index page.

<script type="text/javascript" charset="utf-8">
        $(document).ready(function() {
            namespace = '/test'; // change to an empty string to use the global namespace

            // the socket.io documentation recommends sending an explicit package upon connection
            // this is specially important when using the global namespace
            var socket = io.connect('http://' + document.domain + ':' + location.port + namespace);
            socket.on('connect', function () {
                socket.emit('my event', {data: 'I\'m connected!'});
            });

            // event handler for server sent data
            // the data is displayed in the "Received" section of the page
            socket.on('my response', function (msg) {
                $('#log').append('<br>Received #' + msg.count + ': ' + msg.data);
            });
        });
  </script>

Answer 1

I ran into this problem a while back and it was caused by not monkey-patching the threading module correctly.

At the top of your app, apply the gevent patches:

from gevent import monkey, sleep
monkey.patch_all()

Answer 2

you should avoid using time.sleep() with gevent . You should use gevent.sleep() instead. time.sleep() will block the gevent loop

Answer 3

From https://github.com/gevent/gevent/issues/585#issuecomment-122406552

Ensure that your GEvent Pool size is more than 1. If it is set to 1, GEvent will not have any workers to switch to when processing events and will thus complain about it.