concurrent: Fix importability of tornado.wsgi on app engine

tornadoweb · Jan 26, 2018 · 6e8a137 · 6e8a137
1 parent 606f0b5
commit 6e8a137
Showing 1 changed file with 6 additions and 5 deletions.
diff --git a/tornado/concurrent.py b/tornado/concurrent.py
@@ -337,11 +337,12 @@ def _check_done(self):
 
     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