Access with server-sent events only logged after connection is closed

I have extended tornado.web.RequestHandler to support server-sent events:

class EventSource(RequestHandler):
    """Base handler for server-sent events."""
    def initialize(self, source):
        """The ``source`` parameter is a string that is updated with
        new data. The :class:`EventSource` instance will continuously
        check if it is updated and publish to clients when it is.

        """
        assert isinstance(source, stores.DataStore)
        self.source = source
        self._last = None
        self.finished = False
        self.set_header('content-type', 'text/event-stream')
        self.set_header('cache-control', 'no-cache')

    @gen.coroutine
    def publish(self, data):
        """Pushes data to a listener."""
        try:
            self.write('data: {}\n\n'.format(data))
            yield self.flush()
        except StreamClosedError:
            self.finished = True

    @gen.coroutine
    def get(self):
        while not self.finished:
            if self.source.data != self._last:
                yield self.publish(self.source.data)
                self._last = self.source.data
            else:
                yield gen.sleep(0.01)
        self.finish()

When connecting to an implementation of EventSource, I correctly receive the streamed data. However, accessing the stream is not logged until after the connection is closed. This results in access logs that indicate very long times:

[I 150829 14:30:20 web:1908] 200 GET /stream/all (::1) 7856.83ms
[I 150829 14:30:22 web:1908] 200 GET /stream/all (::1) 25523.45ms
[I 150829 14:30:23 web:1908] 200 GET /stream/all (::1) 43150.50ms

It would be more useful for streams like this to log when the initial connection is made.

Start at tornado.web.RequestHandler and the access-log path, then reproduce the behavior with the server-sent EventSource example in the issue. Determine where the request is currently logged and what connection-time event is available. Done means streaming requests produce an access log when the initial connection is made rather than only after it closes.