Fix a memory leak in HTTPServer with very large file uploads.

This is not quite a true leak since the garbage collector will
reclaim everything when it runs, but it might as well be a leak
because CPython's garbage collector uses heuristics based on the
number of allocated objects to decide when to run.  Since this
scenario resulted in a small number of very large objects, the process
could consume a large amount of memory.

The actual change is to ensure that HTTPConnection always sets a
close_callback on the IOStream instead of waiting for the Application
to set one.  I also nulled out a few more references to break up cycles.

Closes #740.
Closes #747.
Closes #760.

tornado/httpserver.py

@@ -180,23 +180,23 @@ class HTTPConnection(object):
         self.no_keep_alive = no_keep_alive
         self.xheaders = xheaders
         self.protocol = protocol
-        self._request = None
+        self._clear_request_state()
-        self._request_finished = False
-        self._write_callback = None
-        self._close_callback = None
         # Save stack context here, outside of any request.  This keeps
         # contexts from one request from leaking into the next.
         self._header_callback = stack_context.wrap(self._on_headers)
+        self.stream.set_close_callback(self._on_connection_close)
         self.stream.read_until(b"\r\n\r\n", self._header_callback)
 
-    def _clear_callbacks(self):
+    def _clear_request_state(self):
-        """Clears the per-request callbacks.
+        """Clears the per-request state.
 
         This is run in between requests to allow the previous handler
         to be garbage collected (and prevent spurious close callbacks),
         and when the connection is closed (to break up cycles and
         facilitate garbage collection in cpython).
         """
+        self._request = None
+        self._request_finished = False
         self._write_callback = None
         self._close_callback = None
 
@@ -209,7 +209,6 @@ class HTTPConnection(object):
         recommended approach prior to Tornado 3.0).
         """
         self._close_callback = stack_context.wrap(callback)
-        self.stream.set_close_callback(self._on_connection_close)
 
     def _on_connection_close(self):
         if self._close_callback is not None:
@@ -218,25 +217,23 @@ class HTTPConnection(object):
             callback()
         # Delete any unfinished callbacks to break up reference cycles.
         self._header_callback = None
-        self._clear_callbacks()
+        self._clear_request_state()
 
     def close(self):
         self.stream.close()
         # Remove this reference to self, which would otherwise cause a
         # cycle and delay garbage collection of this connection.
         self._header_callback = None
-        self._clear_callbacks()
+        self._clear_request_state()
 
     def write(self, chunk, callback=None):
         """Writes a chunk of output to the stream."""
-        assert self._request, "Request closed"
         if not self.stream.closed():
             self._write_callback = stack_context.wrap(callback)
             self.stream.write(chunk, self._on_write_complete)
 
     def finish(self):
         """Finishes the request."""
-        assert self._request, "Request closed"
         self._request_finished = True
         if not self.stream.writing():
             self._finish_request()
@@ -257,7 +254,7 @@ class HTTPConnection(object):
             self._finish_request()
 
     def _finish_request(self):
-        if self.no_keep_alive:
+        if self.no_keep_alive or self._request is None:
             disconnect = True
         else:
             connection_header = self._request.headers.get("Connection")
@@ -270,9 +267,7 @@ class HTTPConnection(object):
                 disconnect = connection_header != "keep-alive"
             else:
                 disconnect = True
-        self._request = None
+        self._clear_request_state()
-        self._request_finished = False
-        self._clear_callbacks()
         if disconnect:
             self.close()
             return