Add "no-store" to Cache-Control header to prevent history caching of admin resources

[soulseekah]

The current implementation of wp_get_nocache_headers does not take into account history caching, which results in a browser serving a cached copy of pages from history (by pressing the Back button) even if the user has long logged out.

​RFC 2616 14.9.2 no-store describes this cache directive.

To repoduce: login to dashboard, logout, press the back button.
Expected: the login screen.
Reality: a copy of the previous page.

By adding the "no-store" directive to all non-cachable resources the behavior was mitigated successfully in Chrome 21, Firefox 15. Fails on Opera 12 (they chose to disregard "no-store" when applied to history, RFC allows this).

"no-store" please

Replying to toscho:

That needs good tests. For example the ​behavior after POST requests can be quite annoying. Performance might be an issue too.

Agreed, under certain circumstances this could be a nuisance, especially if users are logged in and the admin bar is shown. As for performance, it seems that all of twitter is served with "no-store" and it does appear (to me) to be quite jagged navigation-wise.

Without a proper step-by-step testing plan in mind, I guess I'll use the patch in production for a bit to see if it causes any unexpected issues in general.

The other issue is sometimes you want the back button to be where you were last, without a complete reload. How many times have you lost a comment or some other content by clicking away (in WP or not) and scrambled to recover it?