[PATCH] Upstream: Cache stale responses if they may be revalidated.

[Thorvaldur Thorvaldsson]

# HG changeset patch
# User Thorvaldur Thorvaldsson <thorvaldur.thorvaldsson at gmail.com>
# Date 1450190015 -3600
#      Tue Dec 15 15:33:35 2015 +0100
# Node ID 2c1f00c7f857c12587f0ac47323f04c6a881843a
# Parent  def9c9c9ae05cfa7467b0ec96e76afa180c23dfb
Upstream: Cache stale responses if they may be revalidated.

Previously, the proxy cache would never store stale responses, e.g.,
when the "Cache-Control" header contained "max-age=0", even if the
"proxy_cache_revalidate" directive was "on" and the response included
both an "ETag" and a "Last-Modified" header. This came as a surprise.

Now, a header like "Cache-Control: max-age=0, must-revalidate" can be
used along with an ETag/Last-Modified header to make nginx cache
responses that always require revalidation, e.g., when authorization is
required (and cheap).

diff -r def9c9c9ae05 -r 2c1f00c7f857 src/http/ngx_http_file_cache.c
--- a/src/http/ngx_http_file_cache.c Sat Dec 12 10:32:58 2015 +0300
+++ b/src/http/ngx_http_file_cache.c Tue Dec 15 15:33:35 2015 +0100
@@ -628,7 +628,7 @@

     now = ngx_time();

-    if (c->valid_sec < now) {
+    if (c->valid_sec <= now) {

         ngx_shmtx_lock(&cache->shpool->mutex);

@@ -831,7 +831,7 @@

         if (fcn->error) {

-            if (fcn->valid_sec < ngx_time()) {
+            if (fcn->valid_sec <= ngx_time()) {
                 goto renew;
             }

diff -r def9c9c9ae05 -r 2c1f00c7f857 src/http/ngx_http_upstream.c
--- a/src/http/ngx_http_upstream.c Sat Dec 12 10:32:58 2015 +0300
+++ b/src/http/ngx_http_upstream.c Tue Dec 15 15:33:35 2015 +0100
@@ -2815,11 +2815,16 @@
             valid = ngx_http_file_cache_valid(u->conf->cache_valid,
                                               u->headers_in.status_n);
             if (valid) {
-                r->cache->valid_sec = now + valid;
+                valid += now;
+                r->cache->valid_sec = valid;
             }
         }

-        if (valid) {
+        if (valid > now
+            || (r->upstream->conf->cache_revalidate
+                && (u->headers_in.etag
+                    || u->headers_in.last_modified_time != -1)))
+        {
             r->cache->date = now;
             r->cache->body_start = (u_short) (u->buffer.pos - u->buffer.start);

@@ -4272,11 +4277,6 @@
         return NGX_OK;
     }

-    if (n == 0) {
-        u->cacheable = 0;
-        return NGX_OK;
-    }
-
     r->cache->valid_sec = ngx_time() + n;
     }
 #endif
@@ -4312,7 +4312,7 @@

     expires = ngx_parse_http_time(h->value.data, h->value.len);

-    if (expires == NGX_ERROR || expires < ngx_time()) {
+    if (expires == NGX_ERROR) {
         u->cacheable = 0;
         return NGX_OK;
     }
@@ -4354,15 +4354,9 @@
     if (p[0] != '@') {
         n = ngx_atoi(p, len);

-        switch (n) {
-        case 0:
-            u->cacheable = 0;
-            /* fall through */
-
-        case NGX_ERROR:
+        if (n == NGX_ERROR) {
             return NGX_OK;
-
-        default:
+        } else {
             r->cache->valid_sec = ngx_time() + n;
             return NGX_OK;
         }