Some explanations on how it works now:
* The Session is an optional dependency of the Request. If you create the
Request yourself (which is mandatory now in the front controller) and if
you don't inject a Session yourself (which is recommended if you want the
session to be configured via dependency injection), the Symfony2 Kernel
will associate the Session configured in the Container with the Request
automatically.
* When duplicating a request, the session is shared between the parent and
the child (that's because duplicated requests are sub-requests of the main
one most of the time.) Notice that when you use ::create(), the behavior is
the same as for the constructor; no session is attached to the Request.
* Symfony2 tries hard to not create a session cookie when it is not needed
but a Session object is always available (the cookie is only created when
"something" is stored in the session.)
* Symfony2 only starts a session when:
* A session already exists in the request ($_COOKIE[session_name()] is
defined -- this is done by RequestListener);
* There is something written in the session object (the cookie will be sent
to the Client).
* Notice that reading from the session does not start the session anymore (as
we don't need to start a new session to get the default values, and because
if a session exists, it has already been started by RequestListener.)
Because that's just not possible (have a look at the unit tests to see all possibilities
-- as you will notice, there is no way we can determine the context and whether the
data are already escaped or not).
So, we always escape data, which means that sometimes, we will try to escape already
escaped data. This is not a problem for everything except strings. That's because
strings are not wrapped with an object like everything else (for performance reason).
This means that all escapers must be able to avoid double-escaping (that's the case
for the default escapers as both htmlspecialchars() and htmlentities() have a flag
that does just this).
Because
* it's better for performance (no need to init form templates if there is no forms)
* right now, it crashes for all renderer except HTML (because the form templates obviously only exist for the HTML renderer)
The only other possible fix would be to force those resources to always use the HTML renderer
