Heuristic long expiration warning

index.js

@@ -221,7 +221,14 @@ module.exports = class CachePolicy {
 
     responseHeaders() {
         const headers = this._copyWithoutHopByHopHeaders(this._resHeaders);
-        headers.age = `${Math.round(this.age())}`;
+        const age = this.age();
+
+        // A cache SHOULD generate 113 warning if it heuristically chose a freshness
+        // lifetime greater than 24 hours and the response's age is greater than 24 hours.
+        if (age > 3600*24 && !this._hasExplicitExpiration() && this.maxAge() > 3600*24) {
+            headers.warning = (headers.warning ? `${headers.warning}, ` : '') + '113 - "rfc7234 5.5.4"';
+        }
+        headers.age = `${Math.round(age)}`;
         return headers;
     }
 

test/revalidatetest.js

@@ -106,6 +106,7 @@ describe('Can be revalidated?', function() {
         const headers = cache.revalidationHeaders(simpleRequest);
         assertHeadersPassed(headers);
         assert.equal(headers['if-modified-since'], 'Tue, 15 Nov 1994 12:45:26 GMT');
+        assert(!/113/.test(headers.warning));
     });
 
     it('not without validators', function() {
@@ -113,6 +114,20 @@ describe('Can be revalidated?', function() {
         const headers = cache.revalidationHeaders(simpleRequest);
         assertHeadersPassed(headers);
         assertNoValidators(headers);
+        assert(!/113/.test(headers.warning));
+    })
+
+    it('113 added', function() {
+        const veryOldResponse = {
+            headers: {
+                age: 3600*72,
+                'last-modified': 'Tue, 15 Nov 1994 12:45:26 GMT',
+            },
+        };
+
+        const cache = new CachePolicy(simpleRequest, veryOldResponse);
+        const headers = cache.responseHeaders(simpleRequest);
+        assert(/113/.test(headers.warning));
     })
 
 });