Thanks Maxim for the documentation link.

I am running into a similar issue. But it is for a 504 server time out.

I'm doing a long-poll, but it is a cross domain long poll GET request. The client implementation is trying to use CORS.
It all works fine with when the GET requests returns something.
But when the server times out (HTTP 504) there is no CORS header information on the reply and the client code treats it as:
'No 'Access-Control-Allow-Origin' header is present on the requested resource'

Just extending the keepalive_timeout indefinitely is also not a good idea.

So what is the recommended way to handle a 504. It seems I don't get this in the client side XmlHttpRequest. The exceptions occurs before.