[PATCH] Use the upstream Age header to adjust our cache duration

[Florent Le Coz]

Hello,

Here's the second part of a revised patch, trying to address the issue 
of the Age headers (in upstream’s responses) being ignored by nginx.

It relies on the sec_valid_prio value (see previous patch) to determine 
if the cache duration as been set by a Cache-Control header, and if 
that’s the case it subtracts this from valid_sec.
If the result would be an age in the past, the response is 
non-cacheable. If the result is "now", the response is also 
non-cacheable, as it is already done in the case of a max-age=0 in 
Cache-Control.
If age as an invalid value (negative), it is ignored.

Note: I use

+    { ngx_string("Age"),
+                 ngx_http_upstream_process_header_line,
+                 offsetof(ngx_http_upstream_headers_in_t, age),
+                 ngx_http_upstream_copy_header_line, 0, 0 },
+

to copy this header in a special pointer, to access it easily in 
process_headers(), where the valid_sec modification based on age is done.
Is this the correct way to do? I could of course remove this 
ngx_http_upstream_headers_in_t.age variable and look for a Age header in 
the headers list when I need it, but I think this would be suboptimal. 
Please correct me if I’m wrong.

Regards,

-- 
Florent Le Coz
Smartjog
-------------- next part --------------
# HG changeset patch
# User Florent Le Coz <florent.lecoz at smartjog.com>
# Date 1384532950 0
# Node ID 721fec3e72d3b273146df60dcb5c45fdc2dbf97a
# Parent  c26efc188ac84a8b8f2bb4478efb795635a3498d
Use the upstream Age header when calculating our caching duration

diff -r c26efc188ac8 -r 721fec3e72d3 src/http/ngx_http_upstream.c
--- a/src/http/ngx_http_upstream.c	Fri Nov 15 15:05:55 2013 +0000
+++ b/src/http/ngx_http_upstream.c	Fri Nov 15 16:29:10 2013 +0000
@@ -229,6 +229,11 @@ ngx_http_upstream_header_t  ngx_http_ups
                  ngx_http_upstream_copy_header_line,
                  offsetof(ngx_http_headers_out_t, expires), 1 },
 
+    { ngx_string("Age"),
+                 ngx_http_upstream_process_header_line,
+                 offsetof(ngx_http_upstream_headers_in_t, age),
+                 ngx_http_upstream_copy_header_line, 0, 0 },
+
     { ngx_string("Accept-Ranges"),
                  ngx_http_upstream_process_header_line,
                  offsetof(ngx_http_upstream_headers_in_t, accept_ranges),
@@ -1939,6 +1944,8 @@ ngx_http_upstream_process_headers(ngx_ht
 {
     ngx_str_t                      *uri, args;
     ngx_uint_t                      i, flags;
+    time_t                          now;
+    ngx_int_t                       age;
     ngx_list_part_t                *part;
     ngx_table_elt_t                *h;
     ngx_http_upstream_header_t     *hh;
@@ -1946,6 +1953,20 @@ ngx_http_upstream_process_headers(ngx_ht
 
     umcf = ngx_http_get_module_main_conf(r, ngx_http_upstream_module);
 
+    if (u->cacheable && u->headers_in.age &&
+        u->headers_in.valid_sec_prio == NGX_HTTP_UPSTREAM_CACHE_CONTROL_H_P)
+    {
+        age = ngx_atoi(u->headers_in.age->value.data,
+                       u->headers_in.age->value.len);
+        if (age >= 0 && age != NGX_ERROR)
+        {
+            now = ngx_time();
+            if (r->cache->valid_sec - age > now)
+                r->cache->valid_sec -= age;
+            else
+                u->cacheable = 0;
+        }
+    }
     if (u->headers_in.x_accel_redirect
         && !(u->conf->ignore_headers & NGX_HTTP_UPSTREAM_IGN_XA_REDIRECT))
     {
diff -r c26efc188ac8 -r 721fec3e72d3 src/http/ngx_http_upstream.h
--- a/src/http/ngx_http_upstream.h	Fri Nov 15 15:05:55 2013 +0000
+++ b/src/http/ngx_http_upstream.h	Fri Nov 15 16:29:10 2013 +0000
@@ -225,6 +225,7 @@ typedef struct {
     ngx_table_elt_t                 *connection;
 
     ngx_table_elt_t                 *expires;
+    ngx_table_elt_t                 *age;
     ngx_table_elt_t                 *etag;
     ngx_table_elt_t                 *x_accel_expires;
     ngx_table_elt_t                 *x_accel_redirect;