expose setting kademlia replication factor through node CLI (#14391)

* expose kademlia replication factor through node CLI

* set default CLI flag value for kademlia_replication_factor

Co-authored-by: Bastian Köcher <git@kchr.de>

* wrap CLI value as Option

* make kademlia replication factor non-optional

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

substrate/client/network/src/discovery.rs

@@ -87,6 +87,10 @@ use std::{
 /// a given address.
 const MAX_KNOWN_EXTERNAL_ADDRESSES: usize = 32;
 
+/// Default value for Kademlia replication factor which  determines to how many closest peers a
+/// record is replicated to.
+pub const DEFAULT_KADEMLIA_REPLICATION_FACTOR: usize = 20;
+
 /// `DiscoveryBehaviour` configuration.
 ///
 /// Note: In order to discover nodes or load and store values via Kademlia one has to add
@@ -101,6 +105,7 @@ pub struct DiscoveryConfig {
 	enable_mdns: bool,
 	kademlia_disjoint_query_paths: bool,
 	kademlia_protocols: Vec<Vec<u8>>,
+	kademlia_replication_factor: NonZeroUsize,
 }
 
 impl DiscoveryConfig {
@@ -116,6 +121,8 @@ impl DiscoveryConfig {
 			enable_mdns: false,
 			kademlia_disjoint_query_paths: false,
 			kademlia_protocols: Vec::new(),
+			kademlia_replication_factor: NonZeroUsize::new(DEFAULT_KADEMLIA_REPLICATION_FACTOR)
+				.expect("value is a constant; constant is non-zero; qed."),
 		}
 	}
 
@@ -182,6 +189,12 @@ impl DiscoveryConfig {
 		self
 	}
 
+	/// Sets Kademlia replication factor.
+	pub fn with_kademlia_replication_factor(&mut self, value: NonZeroUsize) -> &mut Self {
+		self.kademlia_replication_factor = value;
+		self
+	}
+
 	/// Create a `DiscoveryBehaviour` from this config.
 	pub fn finish(self) -> DiscoveryBehaviour {
 		let Self {
@@ -194,10 +207,13 @@ impl DiscoveryConfig {
 			enable_mdns,
 			kademlia_disjoint_query_paths,
 			kademlia_protocols,
+			kademlia_replication_factor,
 		} = self;
 
 		let kademlia = if !kademlia_protocols.is_empty() {
 			let mut config = KademliaConfig::default();
+
+			config.set_replication_factor(kademlia_replication_factor);
 			config.set_protocol_names(kademlia_protocols.into_iter().map(Into::into).collect());
 			// By default Kademlia attempts to insert all peers into its routing table once a
 			// dialing attempt succeeds. In order to control which peer is added, disable the