Common Mistakes

Introduction

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

Never Store ObjectRefs (ie: players or entities)

If the object an ObjectRef represents is deleted - for example, if the player goes offline or the entity is unloaded - then calling methods on that object will result in a crash.

For example, don’t do this:

minetest.register_on_joinplayer(function(player)
    local function func()
        local pos = player:get_pos() -- BAD!
        -- `player` is stored then accessed later.
        -- If the player leaves in that second, the server *will* crash.
    end

    minetest.after(1, func)

    foobar[player:get_player_name()] = player
    -- RISKY
    -- It's not recommended to do this.
    -- Use minetest.get_connected_players() and
    -- minetest.get_player_by_name() instead.
end)

Do this instead:

minetest.register_on_joinplayer(function(player)
    local function func(name)
        -- Attempt to get the ref again
        local player = minetest.get_player_by_name(name)

        -- Check that the player is still online
        if player then
            -- Yay! This is fine
            local pos = player:get_pos()
        end
    end

    -- Pass the name into the function
    minetest.after(1, func, player:get_player_name())
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.

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