prevent race conditions with thread start

Fixes #493

example/app.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python
+from threading import Lock
 from flask import Flask, render_template, session, request
 from flask_socketio import SocketIO, emit, join_room, leave_room, \
     close_room, rooms, disconnect
@@ -12,6 +13,7 @@
 app.config['SECRET_KEY'] = 'secret!'
 socketio = SocketIO(app, async_mode=async_mode)
 thread = None
+thread_lock = Lock()
 
 
 def background_thread():
@@ -96,8 +98,9 @@ def ping_pong():
 @socketio.on('connect', namespace='/test')
 def test_connect():
     global thread
-    if thread is None:
-        thread = socketio.start_background_task(target=background_thread)
+    with thread_lock:
+        if thread is None:
+            thread = socketio.start_background_task(target=background_thread)
     emit('my_response', {'data': 'Connected', 'count': 0})
 
 

example/app_namespace.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python
+from threading import Lock
 from flask import Flask, render_template, session, request
 from flask_socketio import SocketIO, Namespace, emit, join_room, leave_room, \
     close_room, rooms, disconnect
@@ -12,6 +13,7 @@
 app.config['SECRET_KEY'] = 'secret!'
 socketio = SocketIO(app, async_mode=async_mode)
 thread = None
+thread_lock = Lock()
 
 
 def background_thread():
@@ -80,8 +82,10 @@ def on_my_ping(self):
 
     def on_connect(self):
         global thread
-        if thread is None:
-            thread = socketio.start_background_task(target=background_thread)
+        with thread_lock:
+            if thread is None:
+                thread = socketio.start_background_task(
+                    target=background_thread)
         emit('my_response', {'data': 'Connected', 'count': 0})
 
     def on_disconnect(self):