Packet reordering can cause a wrong client-side map

[HybridDog]

Minetest version

Minetest 5.9.0-dev-e8a8525bc (Linux)
Using LuaJIT 2.1.0-beta3
Built by GCC 13.2
Running on Linux/6.8.1-arch1-1 x86_64
BUILD_TYPE=Release
RUN_IN_PLACE=0
USE_CURL=1
USE_GETTEXT=1
USE_SOUND=1
STATIC_SHAREDIR="/usr/local/share/minetest"
STATIC_LOCALEDIR="/usr/local/share/locale"

Irrlicht device

X11

Operating system and version

Arch Linux

CPU model

dual core Intel Core i5-6200U [MT MCP]

GPU model

No response

Active renderer

No response

Summary

Minetest detects and fixes network packet reordering if the packets are in the same channel (as far as I know).
TOCLIENT_BLOCKDATA, TOCLIENT_ADDNODE and TOCLIENT_REMOVENODE packets all modify the client-side map but TOCLIENT_BLOCKDATA is in a different channel than the other two:

minetest/src/network/serveropcodes.cpp

Lines 161 to 163 in e79587c

	{ "TOCLIENT_BLOCKDATA", 2, true }, // 0x20
	{ "TOCLIENT_ADDNODE", 0, true }, // 0x21
	{ "TOCLIENT_REMOVENODE", 0, true }, // 0x22

If TOCLIENT_BLOCKDATA and TOCLIENT_ADDNODE (or TOCLIENT_REMOVENODE) packets arrive in the wrong order, the client-side map can contain different nodes than the server-side map although the server has sent all map updates (if I understand the code correctly).

Steps to reproduce

• Use minetest_game and add this chatcommand:

local nplace_state = false
minetest.register_chatcommand("nplace", {
	description = "set_node and voxel_manip in the same mapblock",
	privs = {server = true},
	func = function(name)
		local node_vm = "default:wood"
		local node_set = "default:stone"
		if nplace_state then
			node_vm, node_set = node_set, node_vm
		end
		nplace_state = not nplace_state
		local pos = vector.new(0, 511, 0)

		minetest.set_node(pos, {name = node_set})

		local vm = minetest.get_voxel_manip()
		local area_p1, area_p2 = vm:read_from_map(pos, pos)
		local data = vm:get_data()
		local node_vm_id = minetest.get_content_id(node_vm)
		for i = 1, #data do
			data[i] = node_vm_id
		end
		vm:set_data(data)
		vm:write_to_map()

		return true, ("Nodes changed: Set %s at %s and afterwards filled " ..
			"the mapblock with %s"):format(
			node_set, minetest.pos_to_string(pos), node_vm)
	end
})

• Configure the operating system to apply random delays to network packets on localhost. On Linux, this can be done with, for example, the traffic control command:

sudo tc qdisc add dev lo root handle 1: htb default 12
sudo tc qdisc add dev lo root netem delay 700ms 500ms

This adds 700 ms delay to packets with 500 ms jitter, which often leads to packet reordering.
To undo the traffic control manipulation later, execute sudo tc qdisc del dev lo root.

• Join a world in singleplayer, teleport to (0, 511, 0) and execute the nplace chat command. Without packet reordering, the client-side map and the server-side one are the same. Otherwise, the TOCLIENT_ADDNODE packet from minetest.set_node and the TOCLIENT_BLOCKDATA packet from voxel manipuation arrive in the wrong order, so the client map has a wrong node in the corner.

Here's a video of the problem:

vm_setnode_packet_reordering.mp4

[sfan5]

touched by this proposal: #14431 (comment)