Symfony: Get logged user in 404 Error Page - twig

Question

I'm trying to get the username of logged user in the 404 error page, like the symfony docs says;
{# app/Resources/TwigBundle/views/Exception/error404.html.twig #}
{% extends 'base.html.twig' %}
{% block body %}
<h1>Page not found</h1>
{# example security usage, see below #}
{% if is_granted('IS_AUTHENTICATED_FULLY') %}
IS_AUTHENTICATED_FULLY: {{ app.user.username }}
{% endif %}
{% if is_granted('IS_AUTHENTICATED_REMEMBERED') %}
IS_AUTHENTICATED_REMEMBERED: {{ app.user.username }}
{% endif %}
{% if app.user %}
app.user: {{ app.user.username }}
{% endif %}
<p>
The requested page couldn't be located. Checkout for any URL
misspelling or return to the homepage.
</p>
{% endblock %}
Everything works in dev, but in production the user is always not logged.
Every if condition fails.
In this discussion I found this:
The cause of this problem is that routing is done before security. If
a 404 error occurs, the security layer isn't loaded and thus
So... can be possible to get the logged user in twig exception page?
Update
It looks like it is a desired behavior: https://github.com/symfony/symfony/issues/8414#issuecomment-23661839
This hack can "solve" but it is really ugly...

Answer 1

The only solution I found so far is this:
routing.yml
(this must be the last rule)
pageNotFound:
pattern: /{path}
defaults: { _controller: MyContentBundle:PageNotFound:pageNotFound, path: '' }
requirements:
path: .*
the controller action
public function pageNotFoundAction()
{
throw new NotFoundHttpException();
}
Reference
I hope to find a better solution

Answer 2

EDIT: Unfortunately this works for all error pages except 404, so this is not correct answer. Leaving this only as reference.
Original answer: This works for me:
{% if app.user %}
{{ app.user.username }}
{% endif %}
is_granted('IS_AUTHENTICATED_FULLY') will be true only when you logged in this session:
IS_AUTHENTICATED_FULLY is actually stronger. You only have this if you’ve actually logged in during this session. If you’re logged in because of a remember me cookie, you won’t have this;
https://knpuniversity.com/screencast/symfony2-ep2/twig-security-is-authenticated