BUG/MINOR: cache: reduce the max-age lifetime by the response's Age

A response received with "Cache-Control: max-age=3" and "Age: 2" is kept
fresh in the cache for 3 seconds, and delivered with an Age reaching 4 or
5 seconds, while it only had one second of freshness left. In general,
responses coming through upstream caches are kept fresh for as long as
their Age in excess, at most the cache's max-age, which delivers stale
contents to clients, against RFC 9111#4.2. Clients may see outdated
versions of resources for longer than what the origin server specified.

The expiration date of an entry is computed as the reception date plus
the lifetime returned by http_calc_maxage(), capped by the cache's
max-age. The Age header is parsed afterwards, only to refuse responses
whose Age already exceeds their lifetime, and to compute the Age header of
the copies. But a lifetime given by max-age or s-maxage counts from the
generation of the response at the origin, so the time already spent in
upstream caches, given by Age, must be deducted from it. A lifetime given
by Expires is an absolute date and must not be reduced.

http_calc_maxage() now also reports through a new argument whether the
lifetime comes from s-maxage or max-age, and in this case the effective
lifetime is reduced to what remains after deducing the Age.

This patch may be backported to all supported versions.

diff --git a/src/cache.c b/src/cache.c
index 2ab066a83510da117de8b43e1ee4e5647596e2f2..8d3bc03a13fe80594d8728d58b33af53e94fd7dc 100644 (file)
--- a/src/cache.c
+++ b/src/cache.c
@@ -1040,7 +1040,10 @@ char *directive_value(const char *sample, int slen, const char *word, int wlen)
  * The returned value will always take the cache's configuration into account
  * (cache->maxage) but the actual max age of the response will be set in the
  * true_maxage parameter. It will be used to determine if a response is already
- * stale or not.
+ * stale or not. If <age_relative> is not NULL, it is set to 1 when the true max
+ * age comes from a s-maxage or max-age directive, which is relative to the
+ * generation of the response and must thus be reduced by its Age, or 0 when it
+ * comes from an absolute date or the cache's configuration.
  * Compute the maxage using either:
  *  - the assigned max-age of the cache
  *  - the s-maxage directive
@@ -1049,7 +1052,7 @@ char *directive_value(const char *sample, int slen, const char *word, int wlen)
  *  - the default-max-age of the cache
  *
  */
-int http_calc_maxage(struct stream *s, struct cache *cache, int *true_maxage)
+int http_calc_maxage(struct stream *s, struct cache *cache, int *true_maxage, int *age_relative)
 {
        struct htx *htx = htxbuf(&s->res.buf);
        struct http_hdr_ctx ctx = { .blk = NULL };
@@ -1117,6 +1120,9 @@ int http_calc_maxage(struct stream *s, struct cache *cache, int *true_maxage)
        }
 
 
+       if (age_relative)
+               *age_relative = (smaxage > 0 || maxage > 0);
+
        if (smaxage > 0) {
                if (true_maxage)
                        *true_maxage = smaxage;
@@ -1641,6 +1647,7 @@ enum act_return http_action_store_cache(struct act_rule *rule, struct proxy *px,
 {
        int effective_maxage = 0;
        int true_maxage = 0;
+       int age_relative = 0;
        struct http_txn *txn = s->txn.http;
        struct http_msg *msg = &txn->rsp;
        struct filter *filter;
@@ -1809,7 +1816,7 @@ enum act_return http_action_store_cache(struct act_rule *rule, struct proxy *px,
        /* Determine the entry's maximum age (taking into account the cache's
         * configuration) as well as the response's explicit max age (extracted
         * from cache-control directives or the expires header). */
-       effective_maxage = http_calc_maxage(s, cache, &true_maxage);
+       effective_maxage = http_calc_maxage(s, cache, &true_maxage, &age_relative);
 
        ctx.blk = NULL;
        if (http_find_header(htx, ist("Age"), &ctx, 0)) {
@@ -1822,6 +1829,13 @@ enum act_return http_action_store_cache(struct act_rule *rule, struct proxy *px,
                        object->age = hdr_age;
                        if (unlikely(object->age > true_maxage))
                                goto out;
+
+                       /* The response already spent <age> seconds of a
+                        * lifetime given by max-age or s-maxage in caches
+                        * before reaching us.
+                        */
+                       if (age_relative && effective_maxage > true_maxage - (int)object->age)
+                               effective_maxage = true_maxage - object->age;
                }
                else
                        goto out;