From: Ben Darnell <ben@bendarnell.com>
Date: Sat, 30 Aug 2014 19:27:31 +0000 (-0400)
Subject: Fix a subtle reference cycle that can lead to increased memory consumption.
X-Git-Tag: v4.0.2~4
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=c1a123a8cd0b5de360e280ea34b8e97c078f2291;p=thirdparty%2Ftornado.git

Fix a subtle reference cycle that can lead to increased memory consumption.

Closes #1165.
---

diff --git a/tornado/gen.py b/tornado/gen.py
index 06f2715d6..4bb82d422 100644
--- a/tornado/gen.py
+++ b/tornado/gen.py
@@ -185,7 +185,18 @@ def _make_coroutine_wrapper(func, replace_callback):
                     future.set_exc_info(sys.exc_info())
                 else:
                     Runner(result, future, yielded)
-                return future
+                try:
+                    return future
+                finally:
+                    # Subtle memory optimization: if next() raised an exception,
+                    # the future's exc_info contains a traceback which
+                    # includes this stack frame.  This creates a cycle,
+                    # which will be collected at the next full GC but has
+                    # been shown to greatly increase memory usage of
+                    # benchmarks (relative to the refcount-based scheme
+                    # used in the absence of cycles).  We can avoid the
+                    # cycle by clearing the local variable after we return it.
+                    future = None
         future.set_result(result)
         return future
     return wrapper