Upstream: cache revalidation with conditional requests. The following new directives are introduced: proxy_cache_revalidate, fastcgi_cache_revalidate, scgi_cache_revalidate, uwsgi_cache_revalidate. Default is off. When set to on, they enable cache revalidation using conditional requests with If-Modified-Since for expired cache items. As of now, no attempts are made to merge headers given in a 304 response during cache revalidation with headers previously stored in a cache item. Headers in a 304 response are only used to calculate new validity time of a cache item.

--- a/src/http/ngx_http_upstream.h
+++ b/src/http/ngx_http_upstream.h
@@ -178,6 +178,8 @@ typedef struct {
     ngx_flag_t                       cache_lock;
     ngx_msec_t                       cache_lock_timeout;
 
+    ngx_flag_t                       cache_revalidate;
+
     ngx_array_t                     *cache_valid;
     ngx_array_t                     *cache_bypass;
     ngx_array_t                     *no_cache;