@gen.coroutine
def wait_for_http_server(url, timeout=10):
    """Wait for an HTTP Server to respond at url
    
    Any non-5XX response code will do, even 404.
    """
    loop = ioloop.IOLoop.current()
    tic = loop.time()
    client = AsyncHTTPClient()
    while loop.time() - tic < timeout:
        try:
            r = yield client.fetch(url, follow_redirects=False)
        except __HOLE__ as e:
            if e.code >= 500:
                # failed to respond properly, wait and try again
                if e.code != 599:
                    # we expect 599 for no connection,
                    # but 502 or other proxy error is conceivable
                    app_log.warning("Server at %s responded with error: %s", url, e.code)
                yield gen.sleep(0.1)
            else:
                app_log.debug("Server at %s responded with %s", url, e.code)
                return
        except (OSError, socket.error) as e:
            if e.errno not in {errno.ECONNABORTED, errno.ECONNREFUSED, errno.ECONNRESET}:
                app_log.warning("Failed to connect to %s (%s)", url, e)
            yield gen.sleep(0.1)
        else:
            return
    
    raise TimeoutError("Server at {url} didn't respond in {timeout} seconds".format(
        **locals()
    ))


# Decorators for authenticated Handlers
dataset/ETHPy150Open jupyterhub/jupyterhub/jupyterhub/utils.py/wait_for_http_server