Merge pull request #497 from polymorphm/master

tornado.ioloop.IOLoop.instance() should be threadsafe

tornado/ioloop.py

@@ -104,6 +104,9 @@ class IOLoop(object):
     WRITE = _EPOLLOUT
     ERROR = _EPOLLERR | _EPOLLHUP
 
+    # Global lock for creating global IOLoop instance
+    _instance_lock = threading.Lock()
+
     def __init__(self, impl=None):
         self._impl = impl or _poll()
         if hasattr(self._impl, 'fileno'):
@@ -142,7 +145,10 @@ class IOLoop(object):
                     self.io_loop = io_loop or IOLoop.instance()
         """
         if not hasattr(IOLoop, "_instance"):
-            IOLoop._instance = IOLoop()
+            with IOLoop._instance_lock:
+                if not hasattr(IOLoop, "_instance"):
+                    # New instance after double check
+                    IOLoop._instance = IOLoop()
         return IOLoop._instance
 
     @staticmethod