[FrameworkBundle] cache ClassMetadataFactory in debug

We already track modification in serialization/validator
config directory so we just need to clear the cache at
warmup.

Idea taken from apip:
https://github.com/api-platform/core/blob/master/src/Bridge/Symfony/Bundle/CacheWarmer/CachePoolClearerCacheWarmer.php

src/Symfony/Bundle/FrameworkBundle/CacheWarmer/CachePoolClearerCacheWarmer.php

@@ -0,0 +1,57 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Bundle\FrameworkBundle\CacheWarmer;
+
+use Symfony\Component\HttpKernel\CacheClearer\Psr6CacheClearer;
+use Symfony\Component\HttpKernel\CacheWarmer\CacheWarmerInterface;
+
+/**
+ * Clears the cache pools when warming up the cache.
+ *
+ * Do not use in production!
+ *
+ * @author Kévin Dunglas <dunglas@gmail.com>
+ *
+ * @internal
+ */
+final class CachePoolClearerCacheWarmer implements CacheWarmerInterface
+{
+    private $poolClearer;
+    private $pools;
+
+    public function __construct(Psr6CacheClearer $poolClearer, array $pools = [])
+    {
+        $this->poolClearer = $poolClearer;
+        $this->pools = $pools;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function warmUp($cacheDirectory): void
+    {
+        foreach ($this->pools as $pool) {
+            if ($this->poolClearer->hasPool($pool)) {
+                $this->poolClearer->clearPool($pool);
+            }
+        }
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function isOptional(): bool
+    {
+        // optional cache warmers are not run when handling the request
+        return false;
+    }
+}

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php

@@ -1457,10 +1457,6 @@ class FrameworkExtension extends Extension
         $chainLoader->replaceArgument(0, $serializerLoaders);
         $container->getDefinition('serializer.mapping.cache_warmer')->replaceArgument(0, $serializerLoaders);
 
-        if ($container->getParameter('kernel.debug')) {
-            $container->removeDefinition('serializer.mapping.cache_class_metadata_factory');
-        }
-
         if (isset($config['name_converter']) && $config['name_converter']) {
             $container->getDefinition('serializer.name_converter.metadata_aware')->setArgument(1, new Reference($config['name_converter']));
         }

src/Symfony/Bundle/FrameworkBundle/Resources/config/cache_debug.xml

@@ -11,5 +11,15 @@
         <service id="data_collector.cache" class="Symfony\Component\Cache\DataCollector\CacheDataCollector" public="true">
             <tag name="data_collector" template="@WebProfiler/Collector/cache.html.twig" id="cache" priority="275" />
         </service>
+
+        <!-- CacheWarmer used in dev to clear cache pool -->
+        <service id="cache_pool_clearer.cache_warmer" class="Symfony\Bundle\FrameworkBundle\CacheWarmer\CachePoolClearerCacheWarmer" public="false">
+            <argument type="service" id="cache.system_clearer" />
+            <argument type="collection">
+                <argument>cache.validator</argument>
+                <argument>cache.serializer</argument>
+            </argument>
+            <tag name="kernel.cache_warmer" priority="64" />
+        </service>
     </services>
 </container>

src/Symfony/Bundle/FrameworkBundle/Tests/DependencyInjection/FrameworkExtensionTest.php

@@ -1102,10 +1102,10 @@ abstract class FrameworkExtensionTest extends TestCase
         $this->assertEquals(new Reference('serializer.mapping.cache.symfony'), $cache);
     }
 
-    public function testSerializerCacheDisabled()
+    public function testSerializerCacheActivatedDebug()
     {
         $container = $this->createContainerFromFile('serializer_enabled', ['kernel.debug' => true, 'kernel.container_class' => __CLASS__]);
-        $this->assertFalse($container->hasDefinition('serializer.mapping.cache_class_metadata_factory'));
+        $this->assertTrue($container->hasDefinition('serializer.mapping.cache_class_metadata_factory'));
     }
 
     public function testSerializerMapping()