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.
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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It's likely this issue predates Contrib.social: some issue and comment data are missing.
#13 had many failures and was not ready for review.
Which show this will need it's own test case and work for BC.