PlayerConnection.java

//Part of CraftBukkit
    public void a(PacketPlayInUseEntity packetplayinuseentity) {
        PlayerConnectionUtils.ensureMainThread(packetplayinuseentity, this, this.player.x());
        if (this.player.isFrozen()) return; // CraftBukkit
        WorldServer worldserver = this.minecraftServer.getWorldServer(this.player.dimension);
        Entity entity = packetplayinuseentity.a((World) worldserver);

        this.player.resetIdleTimer();
        if (entity != null) {
            boolean flag = this.player.hasLineOfSight(entity);
            double d0 = 36.0D;

            if (!flag) {
                d0 = 9.0D;
            }

            if (this.player.h(entity) < d0) {
                EnumHand enumhand;

                ItemStack itemInHand = this.player.b(packetplayinuseentity.b() == null ? EnumHand.MAIN_HAND : packetplayinuseentity.b()); // CraftBukkit

                if (packetplayinuseentity.a() == PacketPlayInUseEntity.EnumEntityUseAction.INTERACT
                        || packetplayinuseentity.a() == PacketPlayInUseEntity.EnumEntityUseAction.INTERACT_AT) {
                    // CraftBukkit start
                    boolean triggerLeashUpdate = itemInHand != null && itemInHand.getItem() == Items.LEAD && entity instanceof EntityInsentient;
                    Item origItem = this.player.inventory.getItemInHand() == null ? null : this.player.inventory.getItemInHand().getItem();
                    PlayerInteractEntityEvent event;
                    if (packetplayinuseentity.a() == PacketPlayInUseEntity.EnumEntityUseAction.INTERACT) {
                        event = new PlayerInteractEntityEvent((Player) this.getPlayer(), entity.getBukkitEntity(), (packetplayinuseentity.b() == EnumHand.OFF_HAND) ? EquipmentSlot.OFF_HAND : EquipmentSlot.HAND);
                    } else {
                        Vec3D target = packetplayinuseentity.c();
                        event = new PlayerInteractAtEntityEvent((Player) this.getPlayer(), entity.getBukkitEntity(), new org.bukkit.util.Vector(target.x, target.y, target.z), (packetplayinuseentity.b() == EnumHand.OFF_HAND) ? EquipmentSlot.OFF_HAND : EquipmentSlot.HAND);
                    }
                    this.server.getPluginManager().callEvent(event);

                    if (triggerLeashUpdate && (event.isCancelled() || this.player.inventory.getItemInHand() == null || this.player.inventory.getItemInHand().getItem() != Items.LEAD)) {
                        // Refresh the current leash state
                        this.sendPacket(new PacketPlayOutAttachEntity(entity, ((EntityInsentient) entity).getLeashHolder()));
                    }

                    if (event.isCancelled() || this.player.inventory.getItemInHand() == null || this.player.inventory.getItemInHand().getItem() != origItem) {
                        // Refresh the current entity metadata
                        this.sendPacket(new PacketPlayOutEntityMetadata(entity.getId(), entity.datawatcher, true));
                    }

                    if (event.isCancelled()) {
                        return;
                    }
                    // CraftBukkit end
                }

                if (packetplayinuseentity.a() == PacketPlayInUseEntity.EnumEntityUseAction.INTERACT) {
                    enumhand = packetplayinuseentity.b();
                    this.player.a(entity, enumhand);
                    // CraftBukkit start
                    if (!itemInHand.isEmpty() && itemInHand.getCount() <= -1) {
                        this.player.updateInventory(this.player.activeContainer);
                    }
                    // CraftBukkit end
                } else if (packetplayinuseentity.a() == PacketPlayInUseEntity.EnumEntityUseAction.INTERACT_AT) {
                    enumhand = packetplayinuseentity.b();
                    entity.a(this.player, packetplayinuseentity.c(), enumhand);
                    // CraftBukkit start
                    if (!itemInHand.isEmpty() && itemInHand.getCount() <= -1) {
                        this.player.updateInventory(this.player.activeContainer);
                    }
                    // CraftBukkit end
                } else if (packetplayinuseentity.a() == PacketPlayInUseEntity.EnumEntityUseAction.ATTACK) {
                    if (entity instanceof EntityItem || entity instanceof EntityExperienceOrb || entity instanceof EntityArrow || (entity == this.player && !player.isSpectator())) { // CraftBukkit
                        this.disconnect(new ChatMessage("multiplayer.disconnect.invalid_entity_attacked", new Object[0]));
                        this.minecraftServer.warning("Player " + this.player.getName() + " tried to attack an invalid entity");
                        return;
                    }

                    this.player.attack(entity);

                    // CraftBukkit start
                    if (!itemInHand.isEmpty() && itemInHand.getCount() <= -1) {
                        this.player.updateInventory(this.player.activeContainer);
                    }
                    // CraftBukkit end
                }
            }
        }

    }