only monkey patch required modules/functions (fix errors such as 'module' object has no attribute 'epoll')

I am working on a project that uses Flask-SocketIO and runs the select.epoll() function in a websocket callback. When debug=False, there is no issue and everything works using both eventlet and gevent. Also, when debug=True and mode=eventlet there is no issue.

However, when debug=True and mode=gevent I get the exception (at the bottom) from the following psuedo code. I’m not sure if this is a Flask-SocketIO issue or a gevent issue, so feel free to redirect me if I’m in the wrong place.

from flask import Flask
from flask_socketio import SocketIO
import select

app = Flask('name')
socketio = SocketIO(async_mode='gevent')
socketio.run(app, debug=True)

@socketio.on('connect')
def client_connected():
  select.epoll()

The actual code in question is here for the flask code and here for the select.epoll() call.

  File "/home/.virtualenvs/p3/lib/python3.4/site-packages/gevent/pywsgi.py", line 936, in handle_one_response
    self.run_application()
  File "/home/.virtualenvs/p3/lib/python3.4/site-packages/gevent/pywsgi.py", line 909, in run_application
    self.result = self.application(self.environ, self.start_response)
  File "/home/.virtualenvs/p3/lib/python3.4/site-packages/flask/app.py", line 1994, in __call__
    return self.wsgi_app(environ, start_response)
  File "/home/.virtualenvs/p3/lib/python3.4/site-packages/flask_socketio/__init__.py", line 42, in __call__
    start_response)
  File "/home/.virtualenvs/p3/lib/python3.4/site-packages/engineio/middleware.py", line 47, in __call__
    return self.engineio_app.handle_request(environ, start_response)
  File "/home/.virtualenvs/p3/lib/python3.4/site-packages/socketio/server.py", line 353, in handle_request
    return self.eio.handle_request(environ, start_response)
  File "/home/.virtualenvs/p3/lib/python3.4/site-packages/engineio/server.py", line 278, in handle_request
    socket.handle_post_request(environ)
  File "/home/.virtualenvs/p3/lib/python3.4/site-packages/engineio/socket.py", line 103, in handle_post_request
    self.receive(pkt)
  File "/home/.virtualenvs/p3/lib/python3.4/site-packages/engineio/socket.py", line 55, in receive
    async=self.server.async_handlers)
  File "/home/.virtualenvs/p3/lib/python3.4/site-packages/engineio/server.py", line 378, in _trigger_event
    return self.handlers[event](*args)
  File "/home/.virtualenvs/p3/lib/python3.4/site-packages/socketio/server.py", line 509, in _handle_eio_message
    self._handle_connect(sid, pkt.namespace)
  File "/home/.virtualenvs/p3/lib/python3.4/site-packages/socketio/server.py", line 414, in _handle_connect
    self.environ[sid]) is False:
  File "/home/.virtualenvs/p3/lib/python3.4/site-packages/socketio/server.py", line 481, in _trigger_event
    return self.handlers[namespace][event](*args)
  File "/home/.virtualenvs/p3/lib/python3.4/site-packages/flask_socketio/__init__.py", line 224, in _handler
    *args)
  File "/home/.virtualenvs/p3/lib/python3.4/site-packages/flask_socketio/__init__.py", line 589, in _handle_event
    ret = handler()
  File "gdbgui/backend.py", line 64, in client_connected
    _gdb[request.sid] = GdbController(gdb_path=GDB_PATH, gdb_args=['-nx', '--interpreter=mi2'])
  File "/home/git/pygdbmi/pygdbmi/gdbcontroller.py", line 33, in __init__
    self.epoll = select.epoll()
AttributeError: 'module' object has no attribute 'epoll'

[miguelgrinberg]

My understanding is that select.epoll() is not a greenlet friendly function at this time. Neither gevent nor eventlet implement it. In the cases that you don’t get the crash, you are simply using Python’s original epoll function, which is blocking, so it won’t allow other greenlets in the system to run concurrently.

The case in which you get the error is caused by the monkey patching of gevent. In that case, gevent replaces the select package with its own greenlet aware version, and the replacement lacks the epoll function.

[miguelgrinberg]

Fixed in https://github.com/miguelgrinberg/Flask-SocketIO/commit/55d02d97708bd91b7d8f761ab57aba8d946039ff.