In delete action, the function get_serial() return nonzero number when 'recursion' is on

backend: bind9

1.Create a zone named "zepp.com.".
2.Query the serial number from my nameservers with get_serial()(designate/worker/utils.py).
zepp.com. 3600 IN SOA ns3.zepp.com. admin.zepp.com. 1541648152 3546 600 86400 3600
3.Delete the zone.
4.Query the serial number from my nameservers with get_serial()(designate/worker/utils.py).
zepp.com. 794 IN SOA ns-805.awsdns-36.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
5.Expect 0 but serial number 1 was found, so func parse_query_results() says the zone "zepp.com." is still alive and I cannot delete the zone from designate's database.

code from master branch:

get_serial():

def get_serial(zone_name, host, port=53):
    """
    Possibly raises dns.exception.Timeout or dns.query.BadResponse.
    Possibly returns 0 if, e.g., the answer section is empty.
    """
    resp = dig(zone_name, host, dns.rdatatype.SOA, port=port)
    if not resp.answer:
        return 0
    rdataset = resp.answer[0].to_rdataset()
    if not rdataset:
        return 0
    return rdataset[0].serial

parse_query_results():

def parse_query_results(results, zone):
    """
    results is a [serial/None, ...]
    """
    delete = zone.action == 'DELETE'
    positives = 0
    no_zones = 0
    low_serial = 0

    for serial in results:
        if serial is None:
            # Intentionally don't handle None
            continue
        if delete:
            if serial == 0:
                no_zones += 1
                positives += 1
        else:
            if serial >= zone.serial:
                positives += 1

                # Update the lowest valid serial aka the consensus
                # serial
                if low_serial == 0 or serial < low_serial:
                    low_serial = serial
            else:
                if serial == 0:
                    no_zones += 1

    result = DNSQueryResult(positives, no_zones, low_serial, results)
    LOG.debug('Results for polling %(zone)s-%(serial)d: %(tup)s',
              {'zone': zone.name, 'serial': zone.serial, 'tup': result})
    return result

I suggest to use some attributes to distinguish the zones from different nameservers when 'recursion' is on.