From: Ben Darnell <ben@bendarnell.com>
Date: Mon, 22 Jan 2018 01:12:06 +0000 (-0500)
Subject: concurrent: Fix importability of tornado.wsgi on app engine
X-Git-Tag: v5.0.0~13^2~3
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=6e8a137e6824716531d749bce8a18a1f229ed873;p=thirdparty%2Ftornado.git

concurrent: Fix importability of tornado.wsgi on app engine
---

diff --git a/tornado/concurrent.py b/tornado/concurrent.py
index cca6468c4..65da0ac83 100644
--- a/tornado/concurrent.py
+++ b/tornado/concurrent.py
@@ -337,11 +337,12 @@ class Future(object):
 
     def _set_done(self):
         self._done = True
-        from tornado.ioloop import IOLoop
-        loop = IOLoop.current()
-        for cb in self._callbacks:
-            loop.add_callback(cb, self)
-        self._callbacks = None
+        if self._callbacks:
+            from tornado.ioloop import IOLoop
+            loop = IOLoop.current()
+            for cb in self._callbacks:
+                loop.add_callback(cb, self)
+            self._callbacks = None
 
     # On Python 3.3 or older, objects with a destructor part of a reference
     # cycle are never destroyed. It's no longer the case on Python 3.4 thanks to