Determine the expiration time of the cache type, or NULL if no expire.
Plugins must override this to implement expiration.
$type: The cache type, either 'query', 'result' or 'output'.
Overrides CachePluginBase::cache_expire
function cache_expire($type) {
$lifespan = $this
->get_lifespan($type);
if ($lifespan) {
$cutoff = REQUEST_TIME - $lifespan;
return $cutoff;
}
else {
return FALSE;
}
}