protocols/kad/behaviour: Return peers independent of record existence (…

…libp2p#1544)

A node receiving a `GetRecord` request first checks whether it has the
given record. If it does have the record it does not return closer
nodes.

A node that knows the record for the given key is likely within a
neighborhood of nodes that know the record as well. In addition the node
likely knows its neighboorhood well.

When querying for a key with a quorum of 1 the above behavior of only
returning the record but not any close peers is fine. Once one queries
with a higher quorum having a node respond with the record as well as
close nodes is likely going to speed up the query, given that the
returned peers probably know the record as well.

protocols/kad/src/behaviour.rs

@@ -1278,13 +1278,7 @@ where
                     None => None
                 };
 
-                // If no record is found, at least report known closer peers.
-                let closer_peers =
-                    if record.is_none() {
-                        self.find_closest(&kbucket::Key::new(key), &source)
-                    } else {
-                        Vec::new()
-                    };
+                let closer_peers = self.find_closest(&kbucket::Key::new(key), &source);
 
                 self.queued_events.push_back(NetworkBehaviourAction::NotifyHandler {
                     peer_id: source,