I need to investigate an issue around full page cache (fpc) in M1, because we sometimes receive 404 errors on product pages when the cache is invalidated - and no 404 pages anymore after the cache has been refreshed.
When I refresh the cache via the controls at the top of the same page, everything is fine; no "INVALIDATED" anymore, and no 404 pages. But I want to automate this. For automation, I need to be able to test it. I can only test it, if I can reproduce it.
Therefore, my first question around this topic is:
How can I manually make the fpc "INVALIDATED"?
And, as an advanced question: How can I manually cause the fpc to be marked as "INVALIDATED", and a specific product page to return a 404, which will be no 404 any more once the fpc is refreshed?