Skipping the proxy cache based on a cookie?

[ehudros2]

I've dug a bit deeper into the documentation and it seems like
proxy_no_cache and proxy_cache_bypass should somehow do the trick, but I
can get things to work with my exact scenario.
Here's the flow I'd like to create:
1. when a new user reaches the site, it is forwarded to the backend and
receives a few cookies back. 
A "new user" is identified by a [b]lack[/b] of a certain cookie, which
is something I couldn't figure out how to implement (seems like it only
supports bypassing/no caching if a cookie exists, but not if it
doesn't).

This request should not be taken from the cache, but the response can be
stored in cache as long as further requests will not be sent the cookies
this request produced.

2. Returning visitors can be served from the cache, unless they are
logged in (in which case they should be sent to the backend and the
response should not be stored). This is simple enough to do with the
regular bypass/no_cache combination based on a cookie that is created
when a user logs in.

----

It doesn't seem like a complicated scenario, but I can't get it to work
just right.
Any help would be appreciated :)

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,161969,162238#msg-162238