Diego Fronza authored Dec 11, 2020 and Matthijs Mekking committed Jan 29, 2021

The general logic behind the addition of this new feature works as
folows:

When a client query arrives, the basic path (query.c / ns_query_recurse)
was to create a fetch, waiting for completion in fetch_callback.

With the introduction of stale-answer-client-timeout, a new event of
type DNS_EVENT_TRYSTALE may invoke fetch_callback, whenever stale
answers are enabled and the fetch took longer than
stale-answer-client-timeout to complete.

When an event of type DNS_EVENT_TRYSTALE triggers fetch_callback, we
must ensure that the folowing happens:

1. Setup a new query context with the sole purpose of looking up for
   stale RRset only data, for that matters a new flag was added
   'DNS_DBFIND_STALEONLY' used in database lookups.

    . If a stale RRset is found, mark the original client query as
      answered (with a new query attribute named NS_QUERYATTR_ANSWERED),
      so when the fetch completion event is received later, we avoid
      answering the client twice.

    . If a stale RRset is not found, cleanup and wait for the normal
      fetch completion event.

2. In ns_query_done, we must change this part:
	/*
	 * If we're recursing then just return; the query will
	 * resume when recursion ends.
	 */
	if (RECURSING(qctx->client)) {
		return (qctx->result);
	}

   To this:

	if (RECURSING(qctx->client) && !QUERY_STALEONLY(qctx->client)) {
		return (qctx->result);
	}

   Otherwise we would not proceed to answer the client if it happened
   that a stale answer was found when looking up for stale only data.

When an event of type DNS_EVENT_FETCHDONE triggers fetch_callback, we
proceed as before, resuming query, updating stats, etc, but a few
exceptions had to be added, most important of which are two:

1. Before answering the client (ns_client_send), check if the query
   wasn't already answered before.

2. Before detaching a client, e.g.
   isc_nmhandle_detach(&client->reqhandle), ensure that this is the
   fetch completion event, and not the one triggered due to
   stale-answer-client-timeout, so a correct call would be:
   if (!QUERY_STALEONLY(client)) {
        isc_nmhandle_detach(&client->reqhandle);
   }

Other than these notes, comments were added in code in attempt to make
these updates easier to follow.

(cherry picked from commit 171a5b75)