resp.stale
BOOL, can be read and set, but not unset.
Available inall subroutines.
Whether or not the response is a stale cache hit.
Stale hits are caused by asynchronous revalidation, backend failure, or an explicit return(deliver_stale).
To learn more, refer to staleness and revalidation, resp.stale.is_error, and resp.stale.is_revalidating.