Hi there,

the exception is thrown by this line:
https://github.com/symfony/symfony/blob/2.5/src/Symfony/Component/HttpFoundation/Session/Storage/MockArraySessionStorage.php#L134

My setup works completely fine within a valid PHP environment (without TestSessionListener) - data getting stored in the session before the TestSessionListener::onKernelRequest has been executed (using MockFileSessionStorage).

What's the intention of this exception? I can't get my head around it. The TestSessionListener is explicitly doing this (setId) and if there is another object accessing the session before the TestSessionListener sets the id, an id is automatically generated. The listener is not correctly mimicking the session creation of PHP and this exception makes it (for some parts) unusable.

If there is no actual use on this exception, it may be removed, no?