$upstream_http_* variables exist but do not seem to be readable

[shrikeh]

Here is what I'm trying to achieve:

1. Backend sends response header saying "this page requires filtering".
2. Lua script generates a token, and populates the appropriate tag in the
body copy with the token.
3. Token is pushed into user's cookies and also stored in Redis.

>From what I can gather, to do this, I need to:

1. Add conditional logic within body_filter_by_lua based on the value of
ngx.var.sent_http_csrf 
2. Generate the token
3. Filter the body.
4. Make a subrequest with the token to a separate internal location block so
that Redis can save the token (as redis will be disabled within the context
of body_filter_by_lua)

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,235808,235881#msg-235881