Major Description
If you set proxy.config.http.cache.open_write_fail_action=2 there is no change of behaviour.
If you just do a GET on a stale object, the root cause of this is that OverridableHttpConfigParams::cache_open_write_fail_action is never copied to HttpTransact::State::cache_open_write_fail_action when this is checked. If you fix this, then it always returns a stale object without revalidating.
If you cause the object to be locked in the cache by doing a GET that takes a long time to return, a second lookup goes direct to origin and the cache_open_write_fail_action is never checked.