PageCache determines which responses to cache by inspecting the Expires header. (This should be updated to use the Cache-Control header instead, but that's out of scope here.)
Responses that have Expires: Sun, 19 Nov 1978 05:00:00 GMT are meant to not be cached, because it's an expiration date in the past. Yet PageCache caches them permanently:
$expire = ($date > $request_time) ? $date : Cache::PERMANENT;
This has been the case for a very long time, even predating #2527126: Only send X-Drupal-Cache-Tags and -Contexts headers when developer explicitly enables them → . Quite possibly the rationale was .
Now, to make matters worse, even when you configure a non-zero "max age" at /admin/config/development/performance, \Drupal\Core\EventSubscriber\FinishResponseSubscriber::setResponseCacheable() will still cause that same Expires header to be set? Why? Because when \Drupal\Core\EventSubscriber\FinishResponseSubscriber::onRespond() calls \Drupal\Core\EventSubscriber\FinishResponseSubscriber::setResponseCacheable(), the latter contains this code:
// HTTP/1.0 proxies do not support the Vary header, so prevent any caching
// by sending an Expires date in the past. HTTP/1.1 clients ignore the
// Expires header if a Cache-Control: max-age directive is specified (see
// RFC 2616, section 14.9.3).
if (!$response->headers->has('Expires')) {
$this->setExpiresNoCache($response);
}
In other words:
Make \Drupal\page_cache\StackMiddleware\PageCache a HTTP/1.1 proxy: make it inspect Cache-Control rather than Expires
TBD
TBD
None.
None.
None.
Needs work
11.0 🔥
Worse Than Failure. Approximates the unpleasant remark made by Drupal developers when they first encounter a particular (mis)feature.
Used to track the progress of issues reviewed by the Drupal Needs Review Queue Initiative.
The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
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#13 had many failures and was not ready for review.
Which show this will need it's own test case and work for BC.