AntiDonkeyDupe.java

package com.christ.AntiDonkeyDupe;

import io.netty.channel.*;
import net.minecraft.server.v1_12_R1.PacketPlayInSteerVehicle;
import org.bukkit.Bukkit;
import org.bukkit.ChatColor;
import org.bukkit.craftbukkit.v1_12_R1.entity.CraftPlayer;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.player.PlayerJoinEvent;
import org.bukkit.event.player.PlayerQuitEvent;
import org.bukkit.plugin.java.JavaPlugin;
import net.minecraft.util.io.netty.channel.Channel;
import net.minecraft.util.io.netty.channel.ChannelDuplexHandler;
import net.minecraft.util.io.netty.channel.ChannelHandlerContext;
import net.minecraft.util.io.netty.channel.ChannelPromise;

public class AntiDonkeyDupe extends JavaPlugin implements Listener {

    @Override
    public void onEnable() {
        Bukkit.getPluginManager().registerEvents(this, this);
        System.out.println("[AntiDonkeyDupe] Plugin v1.2 was successfully enabled.");
    }

    @EventHandler
    public void onjoin(PlayerJoinEvent event){
        injectPlayer(event.getPlayer());
    }

    @EventHandler
    public void onleave(PlayerQuitEvent event){
        removePlayer(event.getPlayer());
    }
    private void removePlayer(Player player) {
        Channel channel = ((CraftPlayer) player).getHandle().playerConnection.networkManager.channel;
        channel.eventLoop().submit(() -> {
            channel.pipeline().remove(player.getName());
            return null;
        });
    }

    private void injectPlayer(Player player) {
        ChannelDuplexHandler channelDuplexHandler = new ChannelDuplexHandler() {

            @Override
            public void channelRead(ChannelHandlerContext channelHandlerContext, Object packet) throws Exception {

                if(player.isInsideVehicle()) {
                    if(packet instanceof PacketPlayInSteerVehicle){
                        PacketPlayInSteerVehicle PacketPlayInSteerVehicle = (PacketPlayInSteerVehicle) packet;
                        return;
                    }
                }
                super.channelRead(channelHandlerContext, packet);
            }
        };

        ChannelPipeline pipeline = ((CraftPlayer) player).getHandle().playerConnection.networkManager.channel.pipeline();
        pipeline.addBefore("packet_handler", player.getName(), channelDuplexHandler);

    }
}