Pre-process request via upstream then forward to proxy.

[AndrewH]

As part of processing a request, I need to first send and receive a
response to/from an upstream to authenticate some credentials in the
request.  Once this has been done, the original request needs to be
proxied to another upstream.

According to Evan Miller's tutorial, the only valid location for
spawning sub-requests is in an output filter ie. to post-process a
response.  So it appears that this approach is useful for massaging a
reponse but not valid for actually performing pre-processing on a
request.

Does anyone know what the sanctioned way of going about this in NGINX
is?  Or which existing modules/examples implement similar
functionality?

- Andrew

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,195702,195702#msg-195702