optimize findnode message processing logic

[xxo1shine]

Background

When the neighbor discovery protocol is running, a findnode message will be sent to the selected node. If the node does not reply to the neighbor's message, no processing will be done. If the node is offline, it will not reply to the neighbor's message, which will affect the efficiency of neighbor discovery.

Rationale

Nodes that do not reply to the findnode message may have gone offline. If there are enough K-bucket nodes (greater than or equal to 4), they can be removed from the K-bucket to improve the efficiency of neighbor discovery.

[jakamobiii]

If the node comes back online, can it be added to the k-bucket again?

[xxo1shine]

@jakamobiii Good question. After the node is removed from the K bucket, the node's state needs to be set to DEAD. After the node is online, re-run the state machine and the node can be added to the K bucket again.

public void handlePing(PingMessage msg) {
    if (!kadService.getTable().getNode().equals(node)) {
      sendPong();
    }
    node.setP2pVersion(msg.getNetworkId());
    if (!node.isConnectible(Parameter.p2pConfig.getNetworkId())) {
      changeState(State.DEAD);
    } else if (state.equals(State.DEAD)) {
      changeState(State.DISCOVERED);
    }
  }