Remove weak reference from WaitIterator.

This reference is the only thing holding the asynchronous "stack"
together in some cases, so making it weak could allow coroutines
to be garbage collected while running. This was mainly a problem
on Python 3.4, which is much better at garbage collecting generators
than older versions.

Fixes #1429.

diff --git a/tornado/gen.py b/tornado/gen.py
index 4cc578ebb268e3257550992a6e7fb117a50b6ce6..9145768951dbe035044bf442241222d308b79b75 100644 (file)
--- a/tornado/gen.py
+++ b/tornado/gen.py
@@ -336,12 +336,8 @@ class WaitIterator(object):
         self.current_index = self.current_future = None
         self._running_future = None
 
-        # Use a weak reference to self to avoid cycles that may delay
-        # garbage collection.
-        self_ref = weakref.ref(self)
         for future in futures:
-            future.add_done_callback(functools.partial(
-                self._done_callback, self_ref))
+            future.add_done_callback(self._done_callback)
 
     def done(self):
         """Returns True if this iterator has no more results."""
@@ -358,26 +354,17 @@ class WaitIterator(object):
         the inputs.
         """
         self._running_future = TracebackFuture()
-        # As long as there is an active _running_future, we must
-        # ensure that the WaitIterator is not GC'd (due to the
-        # use of weak references in __init__). Add a callback that
-        # references self so there is a hard reference that will be
-        # cleared automatically when this Future finishes.
-        self._running_future.add_done_callback(lambda f: self)
 
         if self._finished:
             self._return_result(self._finished.popleft())
 
         return self._running_future
 
-    @staticmethod
-    def _done_callback(self_ref, done):
-        self = self_ref()
-        if self is not None:
-            if self._running_future and not self._running_future.done():
-                self._return_result(done)
-            else:
-                self._finished.append(done)
+    def _done_callback(self, done):
+        if self._running_future and not self._running_future.done():
+            self._return_result(done)
+        else:
+            self._finished.append(done)
 
     def _return_result(self, done):
         """Called set the returned future's state that of the future