Fix Apache's mod_expires Cache-Control issue

[pbowyer]

Apaches's mod_expires is a widely used module to set HTTP caching headers. It allows you to set a default cache lifetime as well as lifetimes by mime_type.

When an application server has set a Cache-Control header, mod_expires ignores this and sets its own, resulting in duplicate Cache-Control headers and conflicting information. It does this unless the application server sets an Expires header, in which case mod_expires does nothing. This is documented on the link above:

When the Expires header is already part of the response generated by the server, for example when generated by a CGI script or proxied from an origin server, this module does not change or add an Expires or Cache-Control header.

Symfony automatically sets a Cache-Control header if a session exists. This patch ensures it's respected by mod_expires.

Example 1

With the following Apache config:

<IfModule mod_expires.c>
    ExpiresActive on
    ExpiresDefault                                      "access plus 1 month"
</IfModule>

The HTTP response headers are:

Without the patch

HTTP/1.1 200 OK
Date: Fri, 06 Sep 2019 08:02:02 GMT
Server: Apache/2.4.37 (Ubuntu)
Cache-Control: max-age=0, must-revalidate, private
Cache-Control: max-age=2592000
Expires: Sun, 06 Oct 2019 08:02:00 GMT
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Length: 13099
Connection: close
Content-Type: text/html; charset=UTF-8

With the patch

HTTP/1.1 200 OK
Date: Fri, 06 Sep 2019 08:21:34 GMT
Server: Apache/2.4.37 (Ubuntu)
Cache-Control: max-age=0, must-revalidate, private
Expires: Fri, 06 Sep 2019 08:21:34 GMT
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Length: 13098
Connection: close
Content-Type: text/html; charset=UTF-8

Example 2

With the following Apache config:

<IfModule mod_expires.c>
    ExpiresActive on
    ExpiresDefault                                      "access plus 1 month"
    ExpiresByType text/html                             "access plus 0 seconds"
</IfModule>

Without the patch

HTTP/1.1 200 OK
Date: Fri, 06 Sep 2019 08:18:40 GMT
Server: Apache/2.4.37 (Ubuntu)
Cache-Control: max-age=0, must-revalidate, private
Cache-Control: max-age=0
Expires: Fri, 06 Sep 2019 08:18:39 GMT
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Length: 13099
Connection: close
Content-Type: text/html; charset=UTF-8

With the patch

HTTP/1.1 200 OK
Date: Fri, 06 Sep 2019 08:20:40 GMT
Server: Apache/2.4.37 (Ubuntu)
Cache-Control: max-age=0, must-revalidate, private
Expires: Fri, 06 Sep 2019 08:20:40 GMT
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Length: 13100
Connection: close
Content-Type: text/html; charset=UTF-8

[symfony-bot]

Thanks for your pull request! We love contributions.

However, this repository is what we call a "subtree split": a read-only copy of one directory of the main Symfony repository. It is used by Composer to allow developers to depend on specific Symfony components.

If you want to contribute, you should instead open a pull request on the main repository:

https://github.com/symfony/symfony

Thank you for your contribution!

PS: if you haven't already, please add tests, and beware that bug fixes should be submitted on the lowest maintained branch where they apply; only features should be submitted against the master branch.