stale-while-revalidate experimental implementation.

net/http/http_response_headers.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // The rules for header parsing were borrowed from Firefox:
 // http://lxr.mozilla.org/seamonkey/source/netwerk/protocol/http/src/nsHttpResponseHead.cpp
 // The rules for parsing content-types were also borrowed from Firefox:
 // http://lxr.mozilla.org/mozilla/source/netwerk/base/src/nsURLHelper.cpp#834
 
 #include "net/http/http_response_headers.h"
@@ skipping 938 matching lines @@
 
 // From RFC 2616 section 13.2.4:
 //
 // The calculation to determine if a response has expired is quite simple:
 //
 //   response_is_fresh = (freshness_lifetime > current_age)
 //
 // Of course, there are other factors that can force a response to always be
 // validated or re-fetched.
 //
-bool HttpResponseHeaders::RequiresValidation(const Time& request_time,
-                                             const Time& response_time,
-                                             const Time& current_time) const {
-  TimeDelta lifetime =
-      GetFreshnessLifetime(response_time);
-  if (lifetime == TimeDelta())
-    return true;
+// From RFC 5861 section 3, a stale response may be used while revalidation is
+// performed in the background if
+//
+//   freshness_lifetime + stale_while_revalidate > current_age
+//
+ValidationType HttpResponseHeaders::RequiresValidation(
+    const Time& request_time,
+    const Time& response_time,
+    const Time& current_time) const {
+  FreshnessLifetimes lifetimes = GetFreshnessLifetimes(response_time);
+  if (lifetimes.fresh == TimeDelta() && lifetimes.stale == TimeDelta())
+    return VALIDATION_SYNCHRONOUS;
 
-  return lifetime <= GetCurrentAge(request_time, response_time, current_time);
+  TimeDelta age = GetCurrentAge(request_time, response_time, current_time);
+
+  if (lifetimes.fresh > age)
+    return VALIDATION_NONE;
+
+  if (lifetimes.fresh + lifetimes.stale > age)
+    return VALIDATION_ASYNCHRONOUS;
+
+  return VALIDATION_SYNCHRONOUS;
 }
 
 // From RFC 2616 section 13.2.4:
 //
 // The max-age directive takes priority over Expires, so if max-age is present
 // in a response, the calculation is simply:
 //
 //   freshness_lifetime = max_age_value
 //
 // Otherwise, if Expires is present in the response, the calculation is:
 //
 //   freshness_lifetime = expires_value - date_value
 //
 // Note that neither of these calculations is vulnerable to clock skew, since
 // all of the information comes from the origin server.
 //
 // Also, if the response does have a Last-Modified time, the heuristic
 // expiration value SHOULD be no more than some fraction of the interval since
 // that time. A typical setting of this fraction might be 10%:
 //
 //   freshness_lifetime = (date_value - last_modified_value) * 0.10
 //
-TimeDelta HttpResponseHeaders::GetFreshnessLifetime(
-    const Time& response_time) const {
+// If the stale-while-revalidate directive is present, then it is used to set
+// the |stale| time, unless it overridden by another directive.
+//
+HttpResponseHeaders::FreshnessLifetimes
+HttpResponseHeaders::GetFreshnessLifetimes(const Time& response_time) const {
+  FreshnessLifetimes lifetimes;
   // Check for headers that force a response to never be fresh.  For backwards
   // compat, we treat "Pragma: no-cache" as a synonym for "Cache-Control:
   // no-cache" even though RFC 2616 does not specify it.
   if (HasHeaderValue("cache-control", "no-cache") ||
       HasHeaderValue("cache-control", "no-store") ||
       HasHeaderValue("pragma", "no-cache") ||
-      HasHeaderValue("vary", "*"))  // see RFC 2616 section 13.6
-    return TimeDelta();  // not fresh
+      // Vary: * is never usable: see RFC 2616 section 13.6.
+      HasHeaderValue("vary", "*")) {
+    return lifetimes;
+  }
+
+  // Cache-Control directive must_revalidate overrides stale-while-revalidate.
+  bool must_revalidate = HasHeaderValue("cache-control", "must-revalidate");
+
+  if (must_revalidate || !GetStaleWhileRevalidateValue(&lifetimes.stale))
+    DCHECK(lifetimes.stale == TimeDelta());
 
   // NOTE: "Cache-Control: max-age" overrides Expires, so we only check the
-  // Expires header after checking for max-age in GetFreshnessLifetime.  This
+  // Expires header after checking for max-age in GetFreshnessLifetimes.  This
   // is important since "Expires: <date in the past>" means not fresh, but
   // it should not trump a max-age value.
-
-  TimeDelta max_age_value;
-  if (GetMaxAgeValue(&max_age_value))
-    return max_age_value;
+  if (GetMaxAgeValue(&lifetimes.fresh))
+    return lifetimes;
 
   // If there is no Date header, then assume that the server response was
   // generated at the time when we received the response.
   Time date_value;
   if (!GetDateValue(&date_value))
     date_value = response_time;
 
   Time expires_value;
   if (GetExpiresValue(&expires_value)) {
     // The expires value can be a date in the past!
-    if (expires_value > date_value)
-      return expires_value - date_value;
+    if (expires_value > date_value) {
+      lifetimes.fresh = expires_value - date_value;
+      return lifetimes;
+    }
 
-    return TimeDelta();  // not fresh
+    DCHECK(lifetimes.fresh == TimeDelta());
+    return lifetimes;
   }
 
   // From RFC 2616 section 13.4:
   //
   //   A response received with a status code of 200, 203, 206, 300, 301 or 410
   //   MAY be stored by a cache and used in reply to a subsequent request,
   //   subject to the expiration mechanism, unless a cache-control directive
   //   prohibits caching.
   //   ...
   //   A response received with any other status code (e.g. status codes 302
   //   and 307) MUST NOT be returned in a reply to a subsequent request unless
   //   there are cache-control directives or another header(s) that explicitly
   //   allow it.
   //
   // From RFC 2616 section 14.9.4:
   //
   //   When the must-revalidate directive is present in a response received by
   //   a cache, that cache MUST NOT use the entry after it becomes stale to
   //   respond to a subsequent request without first revalidating it with the
   //   origin server. (I.e., the cache MUST do an end-to-end revalidation every
   //   time, if, based solely on the origin server's Expires or max-age value,
   //   the cached response is stale.)
   //
   // https://datatracker.ietf.org/doc/draft-reschke-http-status-308/ is an
   // experimental RFC that adds 308 permanent redirect as well, for which "any
   // future references ... SHOULD use one of the returned URIs."
   if ((response_code_ == 200 || response_code_ == 203 ||
        response_code_ == 206) &&
-      !HasHeaderValue("cache-control", "must-revalidate")) {
+      !must_revalidate) {

[rvargas (doing something else), 2014/09/17 04:32:20]

tiny nit: I think it's fine to move this to the previous line.

[Adam Rice, 2014/09/30 13:44:50]

Done.

     // TODO(darin): Implement a smarter heuristic.
     Time last_modified_value;
     if (GetLastModifiedValue(&last_modified_value)) {
-      // The last-modified value can be a date in the past!
-      if (last_modified_value <= date_value)
-        return (date_value - last_modified_value) / 10;
+      // The last-modified value can be a date in the future!
+      if (last_modified_value <= date_value) {
+        lifetimes.fresh = (date_value - last_modified_value) / 10;
+        return lifetimes;
+      }
     }
   }
 
   // These responses are implicitly fresh (unless otherwise overruled):
   if (response_code_ == 300 || response_code_ == 301 || response_code_ == 308 ||
       response_code_ == 410) {
-    return TimeDelta::Max();
+    lifetimes.fresh = TimeDelta::Max();
+    lifetimes.stale = TimeDelta();  // It should never be stale.
+    return lifetimes;
   }
 
-  return TimeDelta();  // not fresh
+  // Our heuristic freshness estimate for this resource is 0 seconds, in
+  // accordance with common browser behaviour. However, stale-while-revalidate
+  // may still apply.
+  DCHECK(lifetimes.fresh == TimeDelta());
+  return lifetimes;
 }
 
 // From RFC 2616 section 13.2.3:
 //
 // Summary of age calculation algorithm, when a cache receives a response:
 //
 //   /*
 //    * age_value
 //    *      is the value of Age: header received by the cache with
 //    *              this response.
@@ skipping 319 matching lines @@
   return true;
 }
 
 bool HttpResponseHeaders::IsChunkEncoded() const {
   // Ignore spurious chunked responses from HTTP/1.0 servers and proxies.
   return GetHttpVersion() >= HttpVersion(1, 1) &&
       HasHeaderValue("Transfer-Encoding", "chunked");
 }
 
 }  // namespace net