Function block_for_cache_ready 

fn block_for_cache_ready(
    context: &FetchContext,
    http_request: &mut Request,
    done_chan: &mut DoneChannel,
    revalidating_flag: &mut bool,
    response: &mut Option<Response>,
)

If the cache is not ready to construct a response, wait.

The cache is not ready if a previous fetch checked the cache, found nothing, and moved on to a network fetch, and hasn’t updated the cache yet with a pending resource.

Note that this is a different workflow from the one involving wait_for_cached_response. That one happens when a fetch gets a cache hit, and the resource is pending completion from the network.