Allow setting SocketIO async mode from envvar or config.py (#773)

* Allow setting SocketIO async mode from envvar or config.py

CTFd/__init__.py

@@ -134,6 +134,7 @@ def create_app(config='CTFd.config.Config'):
         # If you have multiple workers you must have a shared cache
         socketio.init_app(
             app,
+            async_mode=app.config.get('SOCKETIO_ASYNC_MODE'),
             message_queue=app.config.get('CACHE_REDIS_URL')
         )
 

CTFd/config.py

@@ -184,12 +184,19 @@ class Config(object):
     APPLICATION_ROOT:
         Specifies what path CTFd is mounted under. It can be used to run CTFd in a subdirectory.
         Example: /ctfd
+
+    SOCKETIO_ASYNC_MODE:
+        Specifies what async mode SocketIO should use. The simplest but least performant option is 'threading'.
+        Switching to a different async mode is not recommended without the appropriate load balancing mechanisms
+        in place and proper understanding of how websockets are supported by Flask.
+        https://flask-socketio.readthedocs.io/en/latest/#deployment
     '''
     REVERSE_PROXY = os.getenv("REVERSE_PROXY") or False
     TEMPLATES_AUTO_RELOAD = (not os.getenv("TEMPLATES_AUTO_RELOAD"))  # Defaults True
     SQLALCHEMY_TRACK_MODIFICATIONS = (not os.getenv("SQLALCHEMY_TRACK_MODIFICATIONS"))  # Defaults True
     UPDATE_CHECK = (not os.getenv("UPDATE_CHECK"))  # Defaults True
     APPLICATION_ROOT = os.getenv('APPLICATION_ROOT') or '/'
+    SOCKETIO_ASYNC_MODE = os.getenv('SOCKETIO_ASYNC_MODE') or 'threading'
 
     '''
     === OAUTH ===

CTFd/utils/events/__init__.py

@@ -2,4 +2,4 @@ from flask_socketio import SocketIO
 
 # The choice to use threading is intentional to simplify deployment.
 # At the moment it is not recommended to build full-duplex systems inside CTFd.
-socketio = SocketIO(async_mode='threading')
+socketio = SocketIO()