Apply stale-while-revalidate also for responses without a validator

Due to its name, the stale-while-revalidate Cache-Control extension
might suggest that it only applies to revalidation requests. However,
looking at https://tools.ietf.org/html/rfc5861#section-3...

> The stale-while-revalidate Cache-Control Extension
> When present in an HTTP response, the stale-while-revalidate
> Cache-Control extension indicates that caches MAY serve the
> response in which it appears after it becomes stale, up to the
> indicated number of seconds.
> stale-while-revalidate = "stale-while-revalidate" "=" delta-seconds
> If a cached response is served stale due to the presence of this
> extension, the cache SHOULD attempt to revalidate it while still
> serving stale responses (i.e., without blocking).

There is no reason why a cache should not also return a stale response
while a complete re-fetch happens in the background (just what would
happen if validation fails).

This makes a difference if, for example, a resource has
`Cache-Control: public, max-age=30, stale-while-revalidate=30` set and
no additional Last-Modified or ETag headers.

Co-Authored-By: Philip Hofstetter <phofstetter@sensational.ch

Author: mpdude

src/CacheMiddleware.php

@@ -164,25 +164,28 @@ function (ResponseInterface $response) use ($request) {
                     return new FulfilledPromise(
                         $cacheEntry->getResponse()->withHeader(static::HEADER_CACHE_INFO, static::HEADER_CACHE_HIT)
                     );
-                } elseif ($staleResponse
-                    || ($maxStaleCache !== null && $cacheEntry->getStaleAge() <= $maxStaleCache)
+                } elseif ($cacheEntry->staleWhileValidate()
+                    && ($maxStaleCache === null || $cacheEntry->getStaleAge() <= $maxStaleCache)
                 ) {
-                    // Staled cache!
+                    /*
+                     * The cached response indicated that it may be served stale while background revalidation (or fetch)
+                     * occurs, and the client did not limit maximum staleness. (https://tools.ietf.org/html/rfc5861#section-3)
+                     *
+                     * Return the cached, stale response; initiate deferred revalidation/re-fetch.
+                     */
+                    static::addReValidationRequest(
+                        static::getRequestWithReValidationHeader($request, $cacheEntry),
+                        $this->cacheStorage,
+                        $cacheEntry
+                    );
+
                     return new FulfilledPromise(
-                        $cacheEntry->getResponse()->withHeader(static::HEADER_CACHE_INFO, static::HEADER_CACHE_HIT)
+                        $cacheEntry->getResponse()
+                            ->withHeader(self::HEADER_CACHE_INFO, self::HEADER_CACHE_STALE)
                     );
                 } elseif ($cacheEntry->hasValidationInformation() && !$onlyFromCache) {
                     // Re-validation header
                     $request = static::getRequestWithReValidationHeader($request, $cacheEntry);
-
-                    if ($cacheEntry->staleWhileValidate()) {
-                        static::addReValidationRequest($request, $this->cacheStorage, $cacheEntry);
-
-                        return new FulfilledPromise(
-                            $cacheEntry->getResponse()
-                                ->withHeader(static::HEADER_CACHE_INFO, static::HEADER_CACHE_STALE)
-                        );
-                    }
                 }
             } else {
                 $cacheEntry = null;

tests/ResponseCacheControlTest.php

@@ -30,7 +30,7 @@ protected function setUp(): void
                 case '/2s-complex':
                     return new FulfilledPromise(
                         (new Response())
-                            ->withAddedHeader('Cache-Control', 'invalid-token="yes", max-age=2, stale-while-revalidate=60')
+                            ->withAddedHeader('Cache-Control', 'invalid-token="yes", max-age=2, stale-while-revalidate=3')
                     );
                 case '/no-store':
                     return new FulfilledPromise(
@@ -91,8 +91,14 @@ public function testMaxAgeComplexHeader()
 
         sleep(3);
 
+        $response = $this->client->get('http://test.com/2s-complex');
+        $this->assertEquals(CacheMiddleware::HEADER_CACHE_STALE, $response->getHeaderLine(CacheMiddleware::HEADER_CACHE_INFO));
+
+        sleep(4);
+
         $response = $this->client->get('http://test.com/2s-complex');
         $this->assertEquals(CacheMiddleware::HEADER_CACHE_MISS, $response->getHeaderLine(CacheMiddleware::HEADER_CACHE_INFO));
+
     }
 
     public function testNoStoreHeader()