Kademlia: Speed-up the record fetching

Before libp2p 0.50.0 we used a quorum of one to fetch records from the DHT. In the pr that upgraded
to libp2p 0.50.0 we accidentally changed this behavior. This pr brings back the old behavior of
using a qorum of one and thus, a faster discovery. After finding the first value, we directly finish
the query. There was also another behavior change in libp2p, they stopped automatic caching on
remote nodes. This pr also brings back the remote caching on nodes that are nearest to the key from
our point of view of the network.

The pr that changed the behavior in libp2p: libp2p/rust-libp2p#2712

Author: bkchr

client/network/src/discovery.rs

@@ -249,7 +249,7 @@ impl DiscoveryConfig {
 				NonZeroUsize::new(MAX_KNOWN_EXTERNAL_ADDRESSES)
 					.expect("value is a constant; constant is non-zero; qed."),
 			),
-			outbound_query_records: Vec::new(),
+			records_to_publish: Default::default(),
 		}
 	}
 }
@@ -287,8 +287,12 @@ pub struct DiscoveryBehaviour {
 	allow_non_globals_in_dht: bool,
 	/// A cache of discovered external addresses. Only used for logging purposes.
 	known_external_addresses: LruHashSet<Multiaddr>,
-	/// A cache of outbound query records.
-	outbound_query_records: Vec<(record::Key, Vec<u8>)>,
+	/// Records to publish per QueryId.
+	///
+	/// After finishing a Kademlia query, libp2p will return us a list of the closest peers that
+	/// did not return the record(in `FinishedWithNoAdditionalRecord`). We will then put the record
+	/// to these peers.
+	records_to_publish: HashMap<QueryId, Record>,
 }
 
 impl DiscoveryBehaviour {
@@ -692,33 +696,54 @@ impl NetworkBehaviour for DiscoveryBehaviour {
 					KademliaEvent::OutboundQueryProgressed {
 						result: QueryResult::GetRecord(res),
 						stats,
-						step,
+						id,
 						..
 					} => {
 						let ev = match res {
-							Ok(ok) =>
-								if let GetRecordOk::FoundRecord(r) = ok {
-									self.outbound_query_records
-										.push((r.record.key, r.record.value));
+							Ok(GetRecordOk::FoundRecord(r)) => {
+								debug!(
+									target: "sub-libp2p",
+									"Libp2p => Found record ({:?}) with value: {:?}",
+									r.record.key,
+									r.record.value,
+								);
+
+								// Let's directly finish the query, as we are only interested in a
+								// quorum of 1.
+								if let Some(kad) = self.kademlia.as_mut() {
+									if let Some(mut query) = kad.query_mut(&id) {
+										query.finish();
+									}
+								}
+
+								self.records_to_publish.insert(id, r.record.clone());
+
+								DiscoveryOut::ValueFound(
+									vec![(r.record.key, r.record.value)],
+									stats.duration().unwrap_or_default(),
+								)
+							},
+							Ok(GetRecordOk::FinishedWithNoAdditionalRecord {
+								cache_candidates,
+							}) => {
+								if cache_candidates.is_empty() {
 									continue
-								} else {
-									debug!(
-										target: "sub-libp2p",
-										"Libp2p => Query progressed to {:?} step (last: {:?})",
-										step.count,
-										step.last,
-									);
-									if step.last {
-										let records =
-											self.outbound_query_records.drain(..).collect();
-										DiscoveryOut::ValueFound(
-											records,
-											stats.duration().unwrap_or_default(),
-										)
-									} else {
-										continue
+								}
+
+								// Put the record to the `cache_candidates` that are nearest to the
+								// record key from our point of view of the network.
+								if let Some(record) = self.records_to_publish.remove(&id) {
+									if let Some(kad) = self.kademlia.as_mut() {
+										kad.put_record_to(
+											record,
+											cache_candidates.into_iter().map(|v| v.1),
+											Quorum::One,
+										);
 									}
-								},
+								}
+
+								continue
+							},
 							Err(e @ libp2p::kad::GetRecordError::NotFound { .. }) => {
 								trace!(
 									target: "sub-libp2p",