fix: always set Kademlia mode to server (#14703)

otherwise, connectivity drops significantly.

substrate/client/network/src/discovery.rs

@@ -55,6 +55,7 @@ use ip_network::IpNetwork;
 use libp2p::{
 	core::{Endpoint, Multiaddr},
 	kad::{
+		self,
 		record::store::{MemoryStore, RecordStore},
 		GetClosestPeersError, GetRecordOk, Kademlia, KademliaBucketInserts, KademliaConfig,
 		KademliaEvent, QueryId, QueryResult, Quorum, Record, RecordKey,
@@ -222,6 +223,9 @@ impl DiscoveryConfig {
 
 			let store = MemoryStore::new(local_peer_id);
 			let mut kad = Kademlia::with_config(local_peer_id, store, config);
+			// Always set the mode to server, so that any node can accept incoming Kademlia
+			// requests. Otherwise, the connectivity degrades significantly.
+			kad.set_mode(Some(kad::Mode::Server));
 
 			for (peer_id, addr) in &permanent_addresses {
 				kad.add_address(peer_id, addr.clone());
@@ -983,7 +987,7 @@ mod tests {
 			upgrade,
 		},
 		identity::Keypair,
-		kad, noise,
+		noise,
 		swarm::{Executor, Swarm, SwarmBuilder, SwarmEvent},
 		yamux, Multiaddr,
 	};
@@ -1038,17 +1042,6 @@ mod tests {
 				)
 				.build();
 
-				// Set the Kademlia mode to server so that it can accept incoming requests.
-				//
-				// Note: the server mode is set automatically when the node learns its external
-				// address, but that does not happen in tests => hence we set it manually.
-				swarm
-					.behaviour_mut()
-					.kademlia
-					.as_mut()
-					.unwrap()
-					.set_mode(Some(kad::Mode::Server));
-
 				let listen_addr: Multiaddr =
 					format!("/memory/{}", rand::random::<u64>()).parse().unwrap();