Upstream: cache revalidation using If-None-Match.

src/http/modules/ngx_http_fastcgi_module.c

diff --git a/src/http/modules/ngx_http_fastcgi_module.c b/src/http/modules/ngx_http_fastcgi_module.c
index 80cd1a2..8016f5b 100644
--- a/src/http/modules/ngx_http_fastcgi_module.c
+++ b/src/http/modules/ngx_http_fastcgi_module.c
@@ -573,7 +573,7 @@
     { ngx_string("HTTP_IF_MODIFIED_SINCE"),
       ngx_string("$upstream_cache_last_modified") },
     { ngx_string("HTTP_IF_UNMODIFIED_SINCE"), ngx_string("") },
-    { ngx_string("HTTP_IF_NONE_MATCH"), ngx_string("") },
+    { ngx_string("HTTP_IF_NONE_MATCH"), ngx_string("$upstream_cache_etag") },
     { ngx_string("HTTP_IF_MATCH"), ngx_string("") },
     { ngx_string("HTTP_RANGE"), ngx_string("") },
     { ngx_string("HTTP_IF_RANGE"), ngx_string("") },

src/http/modules/ngx_http_proxy_module.c

diff --git a/src/http/modules/ngx_http_proxy_module.c b/src/http/modules/ngx_http_proxy_module.c
index 61af3df..52c63e1 100644
--- a/src/http/modules/ngx_http_proxy_module.c
+++ b/src/http/modules/ngx_http_proxy_module.c
@@ -677,7 +677,7 @@
     { ngx_string("If-Modified-Since"),
       ngx_string("$upstream_cache_last_modified") },
     { ngx_string("If-Unmodified-Since"), ngx_string("") },
-    { ngx_string("If-None-Match"), ngx_string("") },
+    { ngx_string("If-None-Match"), ngx_string("$upstream_cache_etag") },
     { ngx_string("If-Match"), ngx_string("") },
     { ngx_string("Range"), ngx_string("") },
     { ngx_string("If-Range"), ngx_string("") },

src/http/modules/ngx_http_scgi_module.c

diff --git a/src/http/modules/ngx_http_scgi_module.c b/src/http/modules/ngx_http_scgi_module.c
index 884cb50..d3a8f11 100644
--- a/src/http/modules/ngx_http_scgi_module.c
+++ b/src/http/modules/ngx_http_scgi_module.c
@@ -379,7 +379,7 @@
     { ngx_string("HTTP_IF_MODIFIED_SINCE"),
       ngx_string("$upstream_cache_last_modified") },
     { ngx_string("HTTP_IF_UNMODIFIED_SINCE"), ngx_string("") },
-    { ngx_string("HTTP_IF_NONE_MATCH"), ngx_string("") },
+    { ngx_string("HTTP_IF_NONE_MATCH"), ngx_string("$upstream_cache_etag") },
     { ngx_string("HTTP_IF_MATCH"), ngx_string("") },
     { ngx_string("HTTP_RANGE"), ngx_string("") },
     { ngx_string("HTTP_IF_RANGE"), ngx_string("") },

src/http/modules/ngx_http_uwsgi_module.c

diff --git a/src/http/modules/ngx_http_uwsgi_module.c b/src/http/modules/ngx_http_uwsgi_module.c
index 1d515cd..588b135 100644
--- a/src/http/modules/ngx_http_uwsgi_module.c
+++ b/src/http/modules/ngx_http_uwsgi_module.c
@@ -507,7 +507,7 @@
     { ngx_string("HTTP_IF_MODIFIED_SINCE"),
       ngx_string("$upstream_cache_last_modified") },
     { ngx_string("HTTP_IF_UNMODIFIED_SINCE"), ngx_string("") },
-    { ngx_string("HTTP_IF_NONE_MATCH"), ngx_string("") },
+    { ngx_string("HTTP_IF_NONE_MATCH"), ngx_string("$upstream_cache_etag") },
     { ngx_string("HTTP_IF_MATCH"), ngx_string("") },
     { ngx_string("HTTP_RANGE"), ngx_string("") },
     { ngx_string("HTTP_IF_RANGE"), ngx_string("") },

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 262466b..42b8ba8 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -19,6 +19,8 @@
     ngx_http_variable_value_t *v, uintptr_t data);
 static ngx_int_t ngx_http_upstream_cache_last_modified(ngx_http_request_t *r,
     ngx_http_variable_value_t *v, uintptr_t data);
+static ngx_int_t ngx_http_upstream_cache_etag(ngx_http_request_t *r,
+    ngx_http_variable_value_t *v, uintptr_t data);
 #endif
 
 static void ngx_http_upstream_init_request(ngx_http_request_t *r);
@@ -367,6 +369,10 @@
       ngx_http_upstream_cache_last_modified, 0,
       NGX_HTTP_VAR_NOCACHEABLE|NGX_HTTP_VAR_NOHASH, 0 },
 
+    { ngx_string("upstream_cache_etag"), NULL,
+      ngx_http_upstream_cache_etag, 0,
+      NGX_HTTP_VAR_NOCACHEABLE|NGX_HTTP_VAR_NOHASH, 0 },
+
 #endif
 
     { ngx_null_string, NULL, NULL, 0, 0, 0 }
@@ -4792,6 +4798,29 @@
     return NGX_OK;
 }
 
+
+static ngx_int_t
+ngx_http_upstream_cache_etag(ngx_http_request_t *r,
+    ngx_http_variable_value_t *v, uintptr_t data)
+{
+    if (r->upstream == NULL
+        || !r->upstream->conf->cache_revalidate
+        || r->upstream->cache_status != NGX_HTTP_CACHE_EXPIRED
+        || r->cache->etag.len == 0)
+    {
+        v->not_found = 1;
+        return NGX_OK;
+    }
+
+    v->valid = 1;
+    v->no_cacheable = 0;
+    v->not_found = 0;
+    v->len = r->cache->etag.len;
+    v->data = r->cache->etag.data;
+
+    return NGX_OK;
+}
+
 #endif