feat: allow setting Kad K_VALUE using feat flag

Introduces new feature flags to change the `K_VALUE` for Kademlia. The new feat flags are: `k-4`, `k-8`, `k-12` and `k-16`.

Author: mickvandijke

libp2p/Cargo.toml

@@ -64,6 +64,10 @@ floodsub = ["dep:libp2p-floodsub"]
 gossipsub = ["dep:libp2p-gossipsub", "libp2p-metrics?/gossipsub"]
 identify = ["dep:libp2p-identify", "libp2p-metrics?/identify"]
 json = ["libp2p-request-response?/json"]
+k-4 = ["kad"]
+k-8 = ["kad"]
+k-12 = ["kad"]
+k-16 = ["kad"]
 kad = ["dep:libp2p-kad", "libp2p-metrics?/kad"]
 macros = ["libp2p-swarm/macros"]
 mdns = ["dep:libp2p-mdns"]

protocols/kad/Cargo.toml

@@ -45,6 +45,10 @@ tracing-subscriber = { workspace = true, features = ["env-filter"] }
 
 [features]
 serde = ["dep:serde", "bytes/serde"]
+k-4 = []
+k-8 = []
+k-12 = []
+k-16 = []
 
 # Passing arguments to the docsrs builder in order to properly document cfg's.
 # More information: https://docs.rs/about/builds#cross-compiling

protocols/kad/src/lib.rs

@@ -74,6 +74,23 @@ pub use protocol::{ConnectionType, KadPeer};
 pub use query::QueryId;
 pub use record::{store, Key as RecordKey, ProviderRecord, Record};
 
+#[cfg(all(feature = "k-4", not(any(feature = "k-8", feature = "k-12", feature = "k-16"))))]
+const K: usize = 4;
+#[cfg(all(feature = "k-8", not(any(feature = "k-4", feature = "k-12", feature = "k-16"))))]
+const K: usize = 8;
+#[cfg(all(feature = "k-12", not(any(feature = "k-4", feature = "k-8", feature = "k-16"))))]
+const K: usize = 12;
+#[cfg(all(feature = "k-16", not(any(feature = "k-4", feature = "k-8", feature = "k-12"))))]
+const K: usize = 16;
+// Enable if none of the k-n features are enabled or if all the k-n features are enabled.
+#[cfg(any(not(any(feature = "k-4", feature = "k-8", feature = "k-12", feature = "k-16")), all(
+    feature = "k-4",
+    feature = "k-8",
+    feature = "k-12",
+    feature = "k-16"
+)))]
+const K: usize = 20;
+
 /// The `k` parameter of the Kademlia specification.
 ///
 /// This parameter determines:
@@ -86,9 +103,7 @@ pub use record::{store, Key as RecordKey, ProviderRecord, Record};
 /// The choice of (1) is fixed to this constant. The replication factor is configurable
 /// but should generally be no greater than `K_VALUE`. All nodes in a Kademlia
 /// DHT should agree on the choices made for (1) and (2).
-///
-/// The current value is `20`.
-pub const K_VALUE: NonZeroUsize = NonZeroUsize::new(20).unwrap();
+pub const K_VALUE: NonZeroUsize = NonZeroUsize::new(K).unwrap();
 
 /// The `α` parameter of the Kademlia specification.
 ///