Prevent page caches from storing content-negotiation redirect

[TBarregren]

Summary

• Full-page caches (LiteSpeed Cache, WP Super Cache, W3TC, etc.) ignore the Vary: Accept header and store the 303 redirect from handle_accept_negotiation() keyed by URL alone
• All subsequent visitors get redirected to the .md URL instead of seeing the HTML page
• This adds three cache-prevention layers before the redirect to cover the entire WordPress caching ecosystem

The Fix

Three complementary layers added before the redirect in handle_accept_negotiation():

Layer	Target	Coverage
DONOTCACHEPAGE constant	WP Super Cache, W3TC, LiteSpeed Cache, and most WP page cache plugins	Universal WordPress convention
Cache-Control: private, no-store	CDNs, reverse proxies (Varnish, Nginx), browser cache	HTTP standard
do_action('litespeed_control_set_nocache')	LiteSpeed Cache specifically	LSCWP's own API; safe no-op if inactive

The .md URLs themselves remain fully cacheable — only the content-negotiation redirect (which depends on the Accept header) is excluded from caching.

Test Plan

• Normal HTML request returns 200 (no redirect):

  curl -sI https://example.com/some-page/

• Accept: text/markdown returns 303 with Cache-Control: private, no-store:

  curl -sI -H "Accept: text/markdown" https://example.com/some-page/

• .md URL returns 200 with text/markdown content type:

  curl -sI https://example.com/some-page.md

Fixes #30
Relates to PITFALLS.md Pitfall #5 ("Content Negotiation Interferes with Caching")

🤖 Generated with Claude Code

[jdevalk]

Hmm this solution would prevent caching everywhere, including in solutions that do understand they should... vary cache, by Vary.

[TBarregren]

Thanks for the feedback @jdevalk — fair point. I've reworked the fix:

What changed:

• Cache-Control changed from private, no-store to just private — only prevents shared caches from storing the redirect; browser caching is unaffected
• DONOTCACHEPAGE and the LiteSpeed action are now behind a filter, so sites with Vary-aware caching (e.g. a properly configured Varnish) can opt out:

add_filter( 'markdown_alternate_disable_page_cache_on_redirect', '__return_false' );

The idea is that Cache-Control: private is always safe (it just tells shared caches to stay out of responses that vary by request header), while the WP-level prevention is a safety net for the many page-cache plugins that cache at the PHP level without considering Vary. But if your stack handles Vary: Accept correctly, the filter lets you skip that layer entirely.