minor #16941 [FrameworkBundle] Compute the kernel root hash only one time (dunglas)

This PR was merged into the 2.7 branch.

Discussion
----------

[FrameworkBundle] Compute the kernel root hash only one time

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Commits
-------

23431e9 [FrameworkBundle] Compute the kernel root hash only one time

Author: fabpot

DependencyInjection/FrameworkExtension.php

@@ -38,6 +38,11 @@ class FrameworkExtension extends Extension
     private $translationConfigEnabled = false;
     private $sessionConfigEnabled = false;
 
+    /**
+     * @var string|null
+     */
+    private $kernelRootHash;
+
     /**
      * Responds to the app.config configuration parameter.
      *
@@ -779,7 +784,7 @@ private function registerValidationConfiguration(array $config, ContainerBuilder
         if (isset($config['cache'])) {
             $container->setParameter(
                 'validator.mapping.cache.prefix',
-                'validator_'.hash('sha256', $container->getParameter('kernel.root_dir'))
+                'validator_'.$this->getKernelRootHash($container)
             );
 
             $validatorBuilder->addMethodCall('setMetadataCache', array(new Reference($config['cache'])));
@@ -961,7 +966,7 @@ private function registerSerializerConfiguration(array $config, ContainerBuilder
         if (isset($config['cache']) && $config['cache']) {
             $container->setParameter(
                 'serializer.mapping.cache.prefix',
-                'serializer_'.hash('sha256', $container->getParameter('kernel.root_dir'))
+                'serializer_'.$this->getKernelRootHash($container)
             );
 
             $container->getDefinition('serializer.mapping.class_metadata_factory')->replaceArgument(
@@ -970,6 +975,22 @@ private function registerSerializerConfiguration(array $config, ContainerBuilder
         }
     }
 
+    /**
+     * Gets a hash of the kernel root directory.
+     *
+     * @param ContainerBuilder $container
+     *
+     * @return string
+     */
+    private function getKernelRootHash(ContainerBuilder $container)
+    {
+        if (!$this->kernelRootHash) {
+            $this->kernelRootHash = hash('sha256', $container->getParameter('kernel.root_dir'));
+        }
+
+        return $this->kernelRootHash;
+    }
+
     /**
      * Returns the base path for the XSD files.
      *