[DoctrineBundle] split Registry::getEntityManager() in two methods

Resetting an entity manager has a lot of consequences and the developer should
be aware of that. So, instead of implicitly reset an entity manager when it is
closed, the developer should reset it by hand if he thinks that this is
possible:

    $em = $this->get('registry')->getEntityManager();
    $em->getConnection()->beginTransaction(); // suspend auto-commit
    try {
        //... do some work
    } catch (Exception $e) {
        $em->getConnection()->rollback();
        $em->close();

        $this->get('registry')->resetEntityManager();
    }

    // you will get a new one
    $em = $this->get('registry')->getEntityManager();

src/Symfony/Bundle/DoctrineBundle/Registry.php

@@ -87,7 +87,7 @@ class Registry
     }
 
     /**
-     * Gets the named entity manager.
+     * Gets a named entity manager.
      *
      * @param string $name The entity manager name (null for the default one)
      *
@@ -103,16 +103,39 @@ class Registry
             throw new \InvalidArgumentException(sprintf('Doctrine EntityManager named "%s" does not exist.', $name));
         }
 
-        $em = $this->container->get($this->entityManagers[$name]);
+        return $this->container->get($this->entityManagers[$name]);
+    }
+
+    /**
+     * Resets a named entity manager.
+     *
+     * This method is useful when an entity manager has been closed
+     * because of a rollbacked transaction AND when you think that
+     * it makes sense to get a new one to replace the closed one.
+     *
+     * Be warned that you will get a brand new entity manager as
+     * the existing one is not useable anymore. This means that any
+     * other object with a dependency on this entity manager will
+     * hold an obsolete reference. You can inject the registry instead
+     * to avoid this problem.
+     *
+     * @param string $name The entity manager name (null for the default one)
+     *
+     * @return EntityManager
+     */
+    public function resetEntityManager($name = null)
+    {
+        if (null === $name) {
+            $name = $this->defaultEntityManager;
+        }
 
-        if (!$em->isOpen()) {
-            // force the creation of a new entity manager
-            // if the current one is closed
-            $this->container->set($this->entityManagers[$name], null);
-            $em = $this->container->get($this->entityManagers[$name]);
+        if (!isset($this->entityManagers[$name])) {
+            throw new \InvalidArgumentException(sprintf('Doctrine EntityManager named "%s" does not exist.', $name));
         }
 
-        return $em;
+        // force the creation of a new entity manager
+        // if the current one is closed
+        $this->container->set($this->entityManagers[$name], null);
     }
 
     /**