Improve JavaScriptServlet client side caching strategy

[jayblanc]

This PR aims to improve client caching strategy for javascript csrf client.

Context :
In our usage, we inject the script on pages using a ServletFilter that build the <script> and add it in the outgoing html. At that time, we have session information about user which query the page and we can decide a particular client side caching strategy for the csrf javascript client code using a watermark (tag) in the URL. That tag is based on the MD5 of the JS generated for that particular client and can then be cached during the session.
If anything changes in the JS (including CSRF master token) the link won't be the same and not retrieved from browser cache.

The JavaScript servlet has been modified to take into consideration that query param (tag) and include a specific client cache header for that purpose.
When param is not used, the servlet now include a ETag with a must revalidate strategy that will compare existing client side ETag avoiding, in case of same hash, to send again the content but a 304 response status code.

Modifications are mainly :

• Add Etag in JS Servlet to avoid network bandwidth usage when file is the same (using md5)
• Externalize JavaScript Etag to allow custom client side strategy using the Etag in a query param for that script.
• Add a specific configuration to override cache-control when using tag query param to retrieve the clientside JS script.

[jayblanc]

Hi @forgedhallpass, do you think it is possible to have a feedback on that PR ? Is it something that could be merged ?

[forgedhallpass]

Hello @jayblanc,

Thank you for your contributions. Sorry for the late response, I've been quite busy lately. I'll try to review everything next week and, if all goes well, create a release with the changes.

[jayblanc]

Hello @forgedhallpass ,
Thanks for your message and for starting the review ; If I can help in any way providing explanation or anything else do not hesitate to send me a message. As csrf guard is included in Jahia and because you mentioned a new release ; we would really appreciate to upgrade the dependency taking benefit of it in a stable way.
Best regards.