feat: Add neighbours_max_ipv4 test (#836)

crates/net/discv4/src/lib.rs

@@ -93,6 +93,7 @@ const MAX_NODES_PING: usize = 2 * MAX_NODES_PER_BUCKET;
 /// fit in one datagram. The safe number of nodes that always fit in a datagram is 12, with worst
 /// case all of them being IPv6 nodes. This is calculated by `(MAX_PACKET_SIZE - (header + expire +
 /// rlp overhead) / size(rlp(Node_IPv6))`
+/// Even in the best case where all nodes are IPv4, only 14 nodes fit into one packet.
 const SAFE_MAX_DATAGRAM_NEIGHBOUR_RECORDS: usize = (MAX_PACKET_SIZE - 109) / 91;
 
 /// The timeout used to identify expired nodes, 24h

crates/net/discv4/src/proto.rs

@@ -563,6 +563,20 @@ mod tests {
         }
     }
 
+    #[test]
+    fn neighbours_max_ipv4() {
+        let mut rng = thread_rng();
+        let msg = Message::Neighbours(Neighbours {
+            nodes: std::iter::repeat_with(|| rng_ipv4_record(&mut rng)).take(16).collect(),
+            expire: rng.gen(),
+        });
+        let (secret_key, _) = SECP256K1.generate_keypair(&mut rng);
+
+        let (encoded, _) = msg.encode(&secret_key);
+        // Assret that 16 nodes never fit into one packet
+        assert!(encoded.len() > MAX_PACKET_SIZE, "{} {msg:?}", encoded.len());
+    }
+
     #[test]
     fn neighbours_max_nodes() {
         let mut rng = thread_rng();