I have Apache2 + PHP-FPM setup and after update to 3.4.0-BETA3 (from BETA1) I encountered a problem with Apache returning 304 for all consequent requests on a single URL. Even if response body sent from PHP changed.

This is expected behaviour in case when client sends If-Modified-Since and PHP response with Last-Modified header. The problem is that the Last-Modified header is sent even if I don't set the header by myself.

I was able to trace down the problem to the commit 95d0b72. It removes explicit check for a started session in RequestDataCollector::onKernelResponse and then by calling $event->getRequest()->getSession()->has('sf_redirect') it triggers session_start() which directly sends Last-Modified header.

Here is dump of headers_sent() at end of request for BETA2 and BETA3:

# BETA2
array(4) {
  [0]=>
  string(32) "Cache-Control: no-cache, private"
  [1]=>
  string(35) "Date: Tue, 07 Nov 2017 07:23:35 GMT"
  [2]=>
  string(30) "Content-Type: application/json"
  [3]=>
  string(21) "X-Debug-Token: 1366e0"
}

# BETA3
array(6) {
  [0]=>
  string(33) "Cache-Control: private, max-age=0"
  [1]=>
  string(44) "Last-Modified: Tue, 07 Nov 2017 07:25:09 GMT"
  [2]=>
  string(32) "Cache-Control: no-cache, private"
  [3]=>
  string(35) "Date: Tue, 07 Nov 2017 07:28:31 GMT"
  [4]=>
  string(30) "Content-Type: application/json"
  [5]=>
  string(21) "X-Debug-Token: ad9ad3"
}