proxy_cache_valid, how to get cache valid time by status in source code

[demi]

hi,
I have sevral questions about source code.
1.about the directive proxy_cache_valid 200 5m, how does nginx get cache
valid time by its status? it seems function ngx_http_file_cache_valid()
in file  ngx_http_file_cache.c isnt used. I've checked other functions,
but nothing found.
2.now, I need to set cache valid time by extensions, not status. there
are parameters status_n, status_line in u->headers_in,
(ngx_http_upstream_t *u), and other parameters in u->headers_in have
there own fuctions. what am I supposed to do? there's no parameter for
me to use in u->headers_in, need I add new parameter in u->headers_in?
but it's a complex work.
I would appreciate if anyone can give a hint.
Regards.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,212508,212508#msg-212508