Fix memory leak with gen.engine, RequestHandler.flush, and closed con…

…nections.

This was unfortunately difficult to reproduce in a unit test, even though
the test program attached to bug 660 easily reproduces it.

Closes #660.

tornado/httpserver.py

@@ -186,6 +186,18 @@ def __init__(self, stream, address, request_callback, no_keep_alive=False,
         self._header_callback = stack_context.wrap(self._on_headers)
         self.stream.read_until(b"\r\n\r\n", self._header_callback)
         self._write_callback = None
+        self._close_callback = None
+
+    def set_close_callback(self, callback):
+        self._close_callback = stack_context.wrap(callback)
+        self.stream.set_close_callback(self._on_connection_close)
+
+    def _on_connection_close(self):
+        callback = self._close_callback
+        self._close_callback = None
+        callback()
+        # Delete any unfinished callbacks to break up reference cycles.
+        self._write_callback = None
 
     def close(self):
         self.stream.close()

tornado/iostream.py

@@ -242,6 +242,8 @@ def _maybe_run_close_callback(self):
             cb = self._close_callback
             self._close_callback = None
             self._run_callback(cb)
+            # Delete any unfinished callbacks to break up reference cycles.
+            self._read_callback = self._write_callback = None
 
     def reading(self):
         """Returns true if we are currently reading from the stream."""

tornado/web.py

@@ -141,7 +141,7 @@ def __init__(self, application, request, **kwargs):
         self.clear()
         # Check since connection is not available in WSGI
         if getattr(self.request, "connection", None):
-            self.request.connection.stream.set_close_callback(
+            self.request.connection.set_close_callback(
                 self.on_connection_close)
         self.initialize(**kwargs)
 

website/sphinx/releases/next.rst

@@ -210,3 +210,8 @@ In progress
 * Python 2.5 is no longer supported.
 * Installation under Python 3 no longer uses ``2to3``.
 * `tornado.util.b` (which was only intended for internal use) is gone.
+* Fixed a memory leak involving ``gen.engine``, `RequestHandler.flush`,
+  and clients closing connections while output is being written.
+* `tornado.httpserver.HTTPConnection` now has a `set_close_callback`
+  method that should be used instead of reaching into its ``stream``
+  attribute.