[HttpKernel] enabling cache-reloading when cache file is rebuilt

This allows the cache file to be deleted and thus rebuilt between Container builds in WebTestCase scenario, to enable testing of multiple configurations of the same application through WebTestCase.

src/Symfony/Component/HttpKernel/Kernel.php

@@ -482,12 +482,17 @@ protected function initializeContainer()
         if (!$cache->isFresh()) {
             $container = $this->buildContainer();
             $container->compile();
+            $class = $this->getNextClassVersion($class);
             $this->dumpContainer($cache, $container, $class, $this->getContainerBaseClass());
 
             $fresh = false;
+        } else {
+            $class = $this->getLatestClassVersion($class);
         }
 
-        require_once $cache->getPath();
+        if(!class_exists($class)) {
+            require $cache->getPath();
+        }
 
         $this->container = new $class();
         $this->container->set('kernel', $this);
@@ -497,6 +502,34 @@ protected function initializeContainer()
         }
     }
 
+    /**
+     * Returns the first version of the given class name that doesn't yet exist.
+     *
+     * eg. 'MyClass' is the first version, 'MyClass1' the second, 'MyClass2' the second, etc.
+     *
+     * @param string $class
+     * @return string
+     */
+    protected function getNextClassVersion($class)
+    {
+        for ($classVer = 0; class_exists($class . ($classVer ?: '')); $classVer++);
+
+        return $class . ($classVer ?: '');
+    }
+
+    /**
+     * Returns the last version of the given class name that does exist
+     *
+     * @param string $class
+     * @return string
+     */
+    protected function getLatestClassVersion($class)
+    {
+        for ($classVer = 0; class_exists($class . ($classVer ?: '')); $classVer++);
+
+        return $class . ($classVer > 1 ? $classVer - 1 : '');
+    }
+
     /**
      * Returns the kernel parameters.
      *