minetest_modding_book/_en/quality/common_mistakes.md

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Common Mistakes	default	../..	8.1	/en/chapters/common_mistakes.html

Introduction

This chapter details common mistakes, and how to avoid them.

• Be Careful When Storing ObjectRefs (ie: players or entities)
• Don't Trust Formspec Submissions
• Set ItemStacks After Changing Them

Be Careful When Storing ObjectRefs (ie: players or entities)

An ObjectRef is invalidated when the player or entity it represents leaves the game. This may happen when the player goes offline, or the entity is unloaded or removed.

The methods of ObjectRefs will always return nil when invalid, since Minetest 5.2. Any call will essentially be ignored.

You should avoid storing ObjectRefs where possible. If you do to store an ObjectRef, you should make sure you check it before use, like so:

-- This only works in Minetest 5.2+
if obj:get_pos() then
	-- is valid!
end

Don't Trust Formspec Submissions

Malicious clients can submit formspecs whenever they like, with whatever content they like.

For example, the following code has a vulnerability which allows players to give themselves moderator privileges:

local function show_formspec(name)
    if not minetest.check_player_privs(name, { privs = true }) then
        return false
    end

    minetest.show_formspec(name, "modman:modman", [[
        size[3,2]
        field[0,0;3,1;target;Name;]
        button_exit[0,1;3,1;sub;Promote]
    ]])
    return true
})

minetest.register_on_player_receive_fields(function(player,
        formname, fields)
    -- BAD! Missing privilege check here!

    local privs = minetest.get_player_privs(fields.target)
    privs.kick  = true
    privs.ban   = true
    minetest.set_player_privs(fields.target, privs)
    return true
end)

Add a privilege check to solve this:

minetest.register_on_player_receive_fields(function(player,
        formname, fields)
    if not minetest.check_player_privs(name, { privs = true }) then
        return false
    end

    -- code
end)

Set ItemStacks After Changing Them

Have you noticed that it's simply called an ItemStack in the API, not an ItemStackRef, similar to InvRef? This is because an ItemStack isn't a reference - it's a copy. Stacks work on a copy of the data rather than the stack in the inventory. This means that modifying a stack won't actually modify that stack in the inventory.

For example, don't do this:

local inv = player:get_inventory()
local stack = inv:get_stack("main", 1)
stack:get_meta():set_string("description", "Partially eaten")
-- BAD! Modification will be lost

Do this instead:

local inv = player:get_inventory()
local stack = inv:get_stack("main", 1)
stack:get_meta():set_string("description", "Partially eaten")
inv:set_stack("main", 1, stack)
-- Correct! Item stack is set

The behaviour of callbacks is slightly more complicated. Modifying an ItemStack you are given will change it for the caller too, and any subsequent callbacks. However, it will only be saved in the engine if the callback caller sets it.

minetest.register_on_item_eat(function(hp_change, replace_with_item,
        itemstack, user, pointed_thing)
    itemstack:get_meta():set_string("description", "Partially eaten")
    -- Almost correct! Data will be lost if another
    -- callback cancels the behaviour
end)

If no callbacks cancel this, the stack will be set and the description will be updated, but if a callback does cancel this, then the update may be lost.

It's better to do this instead:

minetest.register_on_item_eat(function(hp_change, replace_with_item,
        itemstack, user, pointed_thing)
    itemstack:get_meta():set_string("description", "Partially eaten")
    user:get_inventory():set_stack("main", user:get_wield_index(),
            itemstack)
    -- Correct, description will always be set!
end)

If the callbacks cancel or the callback runner doesn't set the stack, then the update will still be set. If the callbacks or the callback runner set the stack, then the use of set_stack doesn't matter.