'Check collection for results, return list if they exist.
Args:
flow_urn: URN of the flow
Returns:
The collection contents as a list
Raises:
ErrorEmptyCollection: if the collection has no results after
self.RESULTS_SLA_SECONDS'

def CheckResultCollectionNotEmptyWithRetry(self, flow_urn):

    coll = flow.GRRFlow.ResultCollectionForFID(flow_urn, token=self.token)
    coll_list = list(coll)
    if (not coll_list):
        for _ in range(self.RESULTS_SLA_SECONDS):
            time.sleep(1)
            coll_list = list(coll)
            if coll_list:
                return coll_list
        raise ErrorEmptyCollection(('No    values    in    %s    after    SLA:    %s    seconds' % (flow_urn, self.RESULTS_SLA_SECONDS)))
    return coll_list
