exception when registering bags for started sessions

Author: xabbuh

src/Symfony/Component/HttpFoundation/Session/Storage/NativeSessionStorage.php

@@ -260,6 +260,10 @@ public function clear()
      */
     public function registerBag(SessionBagInterface $bag)
     {
+        if ($this->started) {
+            throw new \LogicException('Cannot register a bag when the session is already started.');
+        }
+
         $this->bags[$bag->getName()] = $bag;
     }
 

src/Symfony/Component/HttpFoundation/Tests/Session/Storage/NativeSessionStorageTest.php

@@ -83,6 +83,16 @@ public function testRegisterBagException()
         $storage->getBag('non_existing');
     }
 
+    /**
+     * @expectedException \LogicException
+     */
+    public function testRegisterBagForAStartedSessionThrowsException()
+    {
+        $storage = $this->getStorage();
+        $storage->start();
+        $storage->registerBag(new AttributeBag());
+    }
+
     public function testGetId()
     {
         $storage = $this->getStorage();