This is proposal for a cross-team focus timebox in a future quarter to better understand, harden the integrity of, and improve, the HTTP cache handling of MediaWiki core.

Examples of tickets to consider including:

• Unresolved incident prevention and follow-up work from T256395/T264369:
  • T256956: Clean up getCacheVaryCookies()/haveCacheVaryCookies()
  • T257703: Clean up Cache-Control handling in MediaWiki
  • {T260098}
  • T264794: SessionManager should not emit Set-Cookies on session renewal
• From MediaWiki-Core-HTTP-Cache
  • Harden logic, reduce maintenance cost, improve/ease understanding:
    • T50835: Separate Cache-Control header for proxy and client
    • T33639: MediaWiki should use ETags instead of Last-Modified and the Logged out cookie hack
  • Fix bugs:
    • T34364: Logged in status incorrectly displayed after a PHP session timeout
• Purging correctness/performance/UX:
  • Improve latency and infra load (need more strategic purging):
    • T250205: Reduce rate of purges emitted by MediaWiki
  • Fix defects or confusions (need more purging):
    • T26575: Purge Category and File description pages from HTTP/File cache when members/usage changes via LinkUpdate
    • T104711: File description page should be purged after deleting file
    • T109214: File upload should clear parser and file cache on usage pages
    • T238426: Creating a file description page with a redirect to an existing image, doesn't purge pages using the recently created redirect
    • T250511: Re-evaluate caching and purging of language variants (e.g. "/zh-hans/Page_name")

/cc @tstarling @Tgr @aaron @Krinkle