Today I deployed an upgrade of my biggest app from symfony 3.3 to 3.4.1. So far so good but unfortunately there seems to be a BC break regarding some Cache-Control headers that are being sent now for some reason once a session is started.

This seems to be caused by this line https://github.com/symfony/symfony/pull/24878/files#diff-d29419a0d7043fce54a98005c26a82aaR36

I have some responses that are cached publicly in Varnish using some cache control headers being added to the response in the controller action:

    "cache-control" => array:1 [
      0 => "public, s-maxage=3600"
    ]

Those headers are now unfortunately useless and the final headers being received from the backend by Varnish are:

Cache-Control: max-age=0, private, must-revalidate, public, s-maxage=3600

cc @nicolas-grekas