nalc_game/game_api.txt

Minetest Game API
=================
GitHub Repo: https://github.com/minetest/minetest_game

Introduction
------------

The Minetest Game subgame offers multiple new possibilities in addition to the Minetest engine's built-in API,
allowing you to add new plants to farming mod, buckets for new liquids, new stairs and custom panes.
For information on the Minetest API, visit https://github.com/minetest/minetest/blob/master/doc/lua_api.txt
Please note:

 * [XYZ] refers to a section the Minetest API
 * [#ABC] refers to a section in this document
 * [pos] refers to a position table `{x = -5, y = 0, z = 200}`

Bucket API
----------

The bucket API allows registering new types of buckets for non-default liquids.

	bucket.register_liquid(
		"default:lava_source",   -- name of the source node
		"default:lava_flowing",  -- name of the flowing node
		"bucket:bucket_lava",    -- name of the new bucket item (or nil if liquid is not takeable)
		"bucket_lava.png",       -- texture of the new bucket item (ignored if itemname == nil)
		"Lava Bucket",           -- text description of the bucket item
		{lava_bucket = 1},       -- groups of the bucket item, OPTIONAL
		false                    -- force-renew, OPTIONAL. Force the liquid source to renew if it has
		                         -- a source neighbour, even if defined as 'liquid_renewable = false'.
		                         -- Needed to avoid creating holes in sloping rivers.
	)

The filled bucket item is returned to the player that uses an empty bucket pointing to the given liquid source.
When punching with an empty bucket pointing to an entity or a non-liquid node, the on_punch of the entity or node will be triggered.

Beds API
--------

	beds.register_bed(
		"beds:bed",    -- Bed name
		def            -- See [#Bed definition]
	)

 * `beds.read_spawns() `   Returns a table containing players respawn positions
 * `beds.kick_players()`  Forces all players to leave bed
 * `beds.skip_night()`   Sets world time to morning and saves respawn position of all players currently sleeping

### Bed definition

	{
		description = "Simple Bed",
		inventory_image = "beds_bed.png",
		wield_image = "beds_bed.png",
		tiles = {
			bottom = {'Tile definition'}, -- the tiles of the bottom part of the bed.
			top = {Tile definition} -- the tiles of the bottom part of the bed.
		},
		nodebox = {
			bottom = 'regular nodebox',     -- bottom part of bed (see [Node boxes])
			top = 'regular nodebox',        -- top part of bed (see [Node boxes])
		},
		selectionbox = 'regular nodebox',  -- for both nodeboxes (see [Node boxes])
		recipe = {                                         -- Craft recipe
			{"group:wool", "group:wool", "group:wool"},
			{"group:wood", "group:wood", "group:wood"}
		}
	}

Creative API
------------

Use `creative.register_tab(name, title, items)` to add a tab with filtered items.
For example,

	creative.register_tab("tools", "Tools", minetest.registered_tools)

is used to show all tools. Name is used in the sfinv page name, title is the
human readable title.

`is_enabled_for` is used to check whether a player is in creative mode:

    creative.is_enabled_for(name)

Override this to allow per-player game modes.

The contents of `creative.formspec_add` is appended to every creative inventory
page. Mods can use it to add additional formspec elements onto the default
creative inventory formspec to be drawn after each update.

Doors API
---------

The doors mod allows modders to register custom doors and trapdoors.

`doors.register_door(name, def)`

 * Registers new door
 * `name` Name for door
 * `def`  See [#Door definition]

`doors.register_trapdoor(name, def)`

 * Registers new trapdoor
 * `name` Name for trapdoor
 * `def`  See [#Trapdoor definition]

`doors.register_fencegate(name, def)`

 * Registers new fence gate
 * `name` Name for fence gate
 * `def`  See [#Fence gate definition]

`doors.get(pos)`

 * `pos` A position as a table, e.g `{x = 1, y = 1, z = 1}`
 * Returns an ObjectRef to a door, or nil if the position does not contain a door

    ### Methods

        :open(player)   -- Open the door object, returns if door was opened
        :close(player)  -- Close the door object, returns if door was closed
        :toggle(player) -- Toggle the door state, returns if state was toggled
        :state()        -- returns the door state, true = open, false = closed

    the "player" parameter can be omitted in all methods. If passed then
    the usual permission checks will be performed to make sure the player
    has the permissions needed to open this door. If omitted then no
    permission checks are performed.

### Door definition

	description = "Door description",
	inventory_image = "mod_door_inv.png",
	groups = {choppy = 2},
	tiles = {"mod_door.png"}, -- UV map.
	recipe = craftrecipe,
	sounds = default.node_sound_wood_defaults(), -- optional
	sound_open = sound play for open door, -- optional
	sound_close = sound play for close door, -- optional
	protected = false, -- If true, only placer can open the door (locked for others)

### Trapdoor definition

	description = "Trapdoor description",
	inventory_image = "mod_trapdoor_inv.png",
	groups = {choppy = 2},
	tile_front = "doors_trapdoor.png", -- the texture for the front and back of the trapdoor
	tile_side = "doors_trapdoor_side.png", -- the tiles of the four side parts of the trapdoor
	sounds = default.node_sound_wood_defaults(), -- optional
	sound_open = sound play for open door, -- optional
	sound_close = sound play for close door, -- optional
	protected = false, -- If true, only placer can open the door (locked for others)

### Fence gate definition

	description = "Wooden Fence Gate",
	texture = "default_wood.png",
	material = "default:wood",
	groups = {choppy = 2, oddly_breakable_by_hand = 2, flammable = 2},
	sounds = default.node_sound_wood_defaults(), -- optional

Fence API
---------

Allows creation of new fences with "fencelike" drawtype.

`default.register_fence(name, item definition)`

 Registers a new fence. Custom fields texture and material are required, as
 are name and description. The rest is optional. You can pass most normal
 nodedef fields here except drawtype. The fence group will always be added
 for this node.

### fence definition

	name = "default:fence_wood",
	description = "Wooden Fence",
	texture = "default_wood.png",
	material = "default:wood",
	groups = {choppy = 2, oddly_breakable_by_hand = 2, flammable = 2},
	sounds = default.node_sound_wood_defaults(),

Walls API
---------

The walls API allows easy addition of stone auto-connecting wall nodes.

walls.register(name, desc, texture, mat, sounds)
^ name = "walls:stone_wall". Node name.
^ desc = "A Stone wall"
^ texture = "default_stone.png"
^ mat = "default:stone". Used to auto-generate crafting recipe.
^ sounds = sounds: see [#Default sounds]

Farming API
-----------

The farming API allows you to easily register plants and hoes.

`farming.register_hoe(name, hoe definition)`
 * Register a new hoe, see [#hoe definition]

`farming.register_plant(name, Plant definition)`
 * Register a new growing plant, see [#Plant definition]

`farming.registered_plants[name] = definition`
 * Table of registered plants, indexed by plant name

### Hoe Definition


	{
		description = "",                      -- Description for tooltip
		inventory_image = "unknown_item.png",  -- Image to be used as wield- and inventory image
		max_uses = 30,                         -- Uses until destroyed
		material = "",                         -- Material for recipes
		recipe = {                             -- Craft recipe, if material isn't used
			{"air", "air", "air"},
			{"", "group:stick"},
			{"", "group:stick"},
		}
	}

### Plant definition

	{
		description = "",                      -- Description of seed item
		inventory_image = "unknown_item.png",  -- Image to be used as seed's wield- and inventory image
		steps = 8,                             -- How many steps the plant has to grow, until it can be harvested
		-- ^ Always provide a plant texture for each step, format: modname_plantname_i.png (i = stepnumber)
		minlight = 13,                         -- Minimum light to grow
		maxlight = default.LIGHT_MAX           -- Maximum light to grow
	}

Fire API
--------

New node def property:

`on_burn(pos)`

 * Called when fire attempts to remove a burning node.
 * `pos` Position of the burning node.

 `on_ignite(pos, igniter)`

  * Called when Flint and steel (or a mod defined ignitor) is used on a node.
    Defining it may prevent the default action (spawning flames) from triggering.
  * `pos` Position of the ignited node.
  * `igniter` Player that used the tool, when available.


Give Initial Stuff API
----------------------

`give_initial_stuff.give(player)`

^ Give initial stuff to "player"

`give_initial_stuff.add(stack)`

^ Add item to the initial stuff
^ Stack can be an ItemStack or a item name eg: "default:dirt 99"
^ Can be called after the game has loaded

`give_initial_stuff.clear()`

^ Removes all items from the initial stuff
^ Can be called after the game has loaded

`give_initial_stuff.get_list()`

^ returns list of item stacks

`give_initial_stuff.set_list(list)`

^ List of initial items with numeric indices.

`give_initial_stuff.add_from_csv(str)`

^ str is a comma separated list of initial stuff
^ Adds items to the list of items to be given

Nyancat API
-----------

`nyancat.place(pos, facedir, length)`

^ Place a cat at `pos` facing `facedir` with tail length `length`
  Only accepts facedir 0-3, if facedir > 3 then it will be interpreted as facedir = 0

`nyancat.generate(minp, maxp, seed)`

^ Called by `minetest.register_on_generated`. To disable nyancat generation,
  you can redefine nyancat.generate() to be an empty function

TNT API
----------

`tnt.register_tnt(definition)`

^ Register a new type of tnt.

 * `name` The name of the node. If no prefix is given `tnt` is used.
 * `description` A description for your TNT.
 * `radius` The radius within which the TNT can destroy nodes. The default is 3.
 * `damage_radius` The radius within which the TNT can damage players and mobs. By default it is twice the `radius`.
 * `disable_drops` Disable drops. By default it is set to false.
 * `ignore_protection` Don't check `minetest.is_protected` before removing a node.
 * `ignore_on_blast` Don't call `on_blast` even if a node has one.
 * `tiles` Textures for node
  * `side`  Side tiles. By default the name of the tnt with a suffix of `_side.png`.
  * `top`  Top tile. By default the name of the tnt with a suffix of `_top.png`.
  * `bottom` Bottom tile. By default the name of the tnt with a suffix of `_bottom.png`.
  * `burning` Top tile when lit. By default the name of the tnt with a suffix of `_top_burning_animated.png".

`tnt.boom(position, definition)`

^ Create an explosion.

* `position` The center of explosion.
* `definition` The TNT definion as passed to `tnt.register`

`tnt.burn(position, [nodename])`

^ Ignite TNT at position, nodename isn't required unless already known.


To make dropping items from node inventories easier, you can use the
following helper function from 'default':

default.get_inventory_drops(pos, inventory, drops)

^ Return drops from node inventory "inventory" in drops.

* `pos` - the node position
* `inventory` - the name of the inventory (string)
* `drops` - an initialized list

The function returns no values. The drops are returned in the `drops`
parameter, and drops is not reinitialized so you can call it several
times in a row to add more inventory items to it.


`on_blast` callbacks:

Both nodedefs and entitydefs can provide an `on_blast()` callback

`nodedef.on_blast(pos, intensity)`
^ Allow drop and node removal overriding
* `pos` - node position
* `intensity` - TNT explosion measure. larger or equal to 1.0
^ Should return a list of drops (e.g. {"default:stone"})
^ Should perform node removal itself. If callback exists in the nodedef
^ then the TNT code will not destroy this node.

`entitydef.on_blast(luaobj, damage)`
^ Allow TNT effects on entities to be overridden
* `luaobj` - LuaEntityRef of the entity
* `damage` - suggested HP damage value
^ Should return a list of (bool do_damage, bool do_knockback, table drops)
* `do_damage` - if true then TNT mod wil damage the entity
* `do_knockback` - if true then TNT mod will knock the entity away
* `drops` - a list of drops, e.g. {"wool:red"}


Screwdriver API
---------------

The screwdriver API allows you to control a node's behaviour when a screwdriver is used on it.
To use it, add the `on_screwdriver` function to the node definition.

`on_rotate(pos, node, user, mode, new_param2)`

 * `pos` Position of the node that the screwdriver is being used on
 * `node` that node
 * `user` The player who used the screwdriver
 * `mode` screwdriver.ROTATE_FACE or screwdriver.ROTATE_AXIS
 * `new_param2` the new value of param2 that would have been set if on_rotate wasn't there
 * return value: false to disallow rotation, nil to keep default behaviour, true to allow
 	it but to indicate that changed have already been made (so the screwdriver will wear out)
 * use `on_rotate = false` to always disallow rotation
 * use `on_rotate = screwdriver.rotate_simple` to allow only face rotation


Sethome API
-----------

The sethome API adds three global functions to allow mods to read a players home position,
set a players home position and teleport a player to home position.

`sethome.get(name)`

 * `name` Player who's home position you wish to get
 * return value: false if no player home coords exist, position table if true

`sethome.set(name, pos)`

 * `name` Player who's home position you wish to set
 * `pos` Position table containing coords of home position
 * return value: false if unable to set and save new home position, otherwise true

`sethome.go(name)`

 * `name` Player you wish to teleport to their home position
 * return value: false if player cannot be sent home, otherwise true


Sfinv API
---------

### sfinv Methods

* sfinv.set_player_inventory_formspec(player, context) - builds page formspec
            and calls set_inventory_formspec().
            If context is nil, it is either found or created.
* sfinv.get_formspec(player, context) - builds current page's formspec
* sfinv.get_nav_fs(player, context, nav, current_idx) - see above
* sfinv.get_homepage_name(player) - get the page name of the first page to show to a player
* sfinv.make_formspec(player, context, content, show_inv, size) - adds a theme to a formspec
    * show_inv, defaults to false. Whether to show the player's main inventory
    * size, defaults to `size[8,8.6]` if not specified
* sfinv.register_page(name, def) - register a page, see section below
* sfinv.override_page(name, def) - overrides fields of an page registered with register_page.
    * Note: Page must already be defined, (opt)depend on the mod defining it.

### sfinv Members

* pages - table of pages[pagename] = def
* pages_unordered - array table of pages in order of addition (used to build navigation tabs).
* contexts - contexts[playername] = player_context
* enabled - set to false to disable. Good for inventory rehaul mods like unified inventory

### Context

A table with these keys:

* page - current page name
* nav - a list of page names
* nav_titles - a list of page titles
* nav_idx - current nav index (in nav and nav_titles)
* any thing you want to store
    * sfinv will clear the stored data on log out / log in

### sfinv.register_page

sfinv.register_page(name, def)

def is a table containing:

* `title` - human readable page name (required)
* `get(self, player, context)` - returns a formspec string. See formspec variables. (required)
* `is_in_nav(self, player, context)` - return true to show in the navigation (the tab header, by default)
* `on_player_receive_fields(self, player, context, fields)` - on formspec submit.
* `on_enter(self, player, context)` - called when the player changes pages, usually using the tabs.
* `on_leave(self, player, context)` - when leaving this page to go to another, called before other's on_enter

### get formspec

Use sfinv.make_formspec to apply a layout:

	return sfinv.make_formspec(player, context, [[
		list[current_player;craft;1.75,0.5;3,3;]
		list[current_player;craftpreview;5.75,1.5;1,1;]
		image[4.75,1.5;1,1;gui_furnace_arrow_bg.png^[transformR270]
		listring[current_player;main]
		listring[current_player;craft]
		image[0,4.25;1,1;gui_hb_bg.png]
		image[1,4.25;1,1;gui_hb_bg.png]
		image[2,4.25;1,1;gui_hb_bg.png]
		image[3,4.25;1,1;gui_hb_bg.png]
		image[4,4.25;1,1;gui_hb_bg.png]
		image[5,4.25;1,1;gui_hb_bg.png]
		image[6,4.25;1,1;gui_hb_bg.png]
		image[7,4.25;1,1;gui_hb_bg.png]
	]], true)

See above (methods section) for more options.

### Customising themes

Simply override this function to change the navigation:

	function sfinv.get_nav_fs(player, context, nav, current_idx)
		return "navformspec"
	end

And override this function to change the layout:

	function sfinv.make_formspec(player, context, content, show_inv, size)
		local tmp = {
			size or "size[8,8.6]",
			theme_main,
			sfinv.get_nav_fs(player, context, context.nav_titles, context.nav_idx),
			content
		}
		if show_inv then
			tmp[4] = theme_inv
		end
		return table.concat(tmp, "")
	end

Stairs API
----------

The stairs API lets you register stairs and slabs and ensures that they are registered the same way as those
delivered with Minetest Game, to keep them compatible with other mods.

`stairs.register_stair(subname, recipeitem, groups, images, description, sounds)`

 * Registers a stair.
 * `subname`: Basically the material name (e.g. cobble) used for the stair name. Nodename pattern: "stairs:stair_subname"
 * `recipeitem`: Item used in the craft recipe, e.g. "default:cobble", may be `nil`
 * `groups`: see [Known damage and digging time defining groups]
 * `images`: see [Tile definition]
 * `description`: used for the description field in the stair's definition
 * `sounds`: see [#Default sounds]

`stairs.register_slab(subname, recipeitem, groups, images, description, sounds)`

 * Registers a slabs
 * `subname`: Basically the material name (e.g. cobble) used for the stair name. Nodename pattern: "stairs:stair_subname"
 * `recipeitem`: Item used in the craft recipe, e.g. "default:cobble"
 * `groups`: see [Known damage and digging time defining groups]
 * `images`: see [Tile definition]
 * `description`: used for the description field in the stair's definition
 * `sounds`: see [#Default sounds]

`stairs.register_stair_and_slab(subname, recipeitem, groups, images, desc_stair, desc_slab, sounds)`

 * A wrapper for stairs.register_stair and stairs.register_slab
 * Uses almost the same arguments as stairs.register_stair
 * `desc_stair`: Description for stair node
 * `desc_slab`: Description for slab node

Xpanes API
----------

Creates panes that automatically connect to each other

`xpanes.register_pane(subname, def)`

 * `subname`: used for nodename. Result: "xpanes:subname" and "xpanes:subname_{2..15}"
 * `def`: See [#Pane definition]

### Pane definition

	{
		textures = {"texture for sides", (unused), "texture for top and bottom"}, -- More tiles aren't supported
		groups = {group = rating}, -- Uses the known node groups, see [Known damage and digging time defining groups]
		sounds = SoundSpec,        -- See [#Default sounds]
		recipe = {{"","","","","","","","",""}}, -- Recipe field only
	}

Raillike definitions
--------------------

The following nodes use the group `connect_to_raillike` and will only connect to
raillike nodes within this group and the same group value.
Use `minetest.raillike_group(<Name>)` to get the group value.

| Node type             | Raillike group name
|-----------------------|---------------------
| default:rail          | "rail"
| tnt:gunpowder         | "gunpowder"
| tnt:gunpowder_burning	| "gunpowder"

Example:
If you want to add a new rail type and want it to connect with default:rail,
add `connect_to_raillike=minetest.raillike_group("rail")` into the `groups` table
of your node.


Default sounds
--------------

Sounds inside the default table can be used within the sounds field of node definitions.

 * `default.node_sound_defaults()`
 * `default.node_sound_stone_defaults()`
 * `default.node_sound_dirt_defaults()`
 * `default.node_sound_sand_defaults()`
 * `default.node_sound_wood_defaults()`
 * `default.node_sound_leaves_defaults()`
 * `default.node_sound_glass_defaults()`
 * `default.node_sound_metal_defaults()`

Default constants
-----------------

`default.LIGHT_MAX`  The maximum light level (see [Node definition] light_source)

Player API
----------

The player API can register player models and update the player's appearence

`default.player_register_model(name, def)`

 * Register a new model to be used by players.
 * name: model filename such as "character.x", "foo.b3d", etc.
 * def: See [#Model definition]

`default.registered_player_models[name]`

 * Get a model's definition
 * see [#Model definition]

`default.player_set_model(player, model_name)`

 * Change a player's model
 * `player`: PlayerRef
 * `model_name`: model registered with player_register_model()

`default.player_set_animation(player, anim_name [, speed])`

 * Applies an animation to a player
 * anim_name: name of the animation.
 * speed: frames per second. If nil, default from the model is used

`default.player_set_textures(player, textures)`

 * Sets player textures
 * `player`: PlayerRef
 * `textures`: array of textures, If `textures` is nil, the default textures from the model def are used

default.player_get_animation(player)

 * Returns a table containing fields `model`, `textures` and `animation`.
 * Any of the fields of the returned table may be nil.
 * player: PlayerRef

### Model Definition

	{
		animation_speed = 30,            -- Default animation speed, in FPS.
		textures = {"character.png", },  -- Default array of textures.
		visual_size = {x = 1, y = 1},    -- Used to scale the model.
		animations = {
			-- <anim_name> = {x = <start_frame>, y = <end_frame>},
			foo = {x = 0, y = 19},
			bar = {x = 20, y = 39},
		-- ...
		},
	}

Leafdecay
---------

To enable leaf decay for a node, add it to the `leafdecay` group.

The rating of the group determines how far from a node in the group `tree`
the node can be without decaying.

If `param2` of the node is ~= 0, the node will always be preserved. Thus, if
the player places a node of that kind, you will want to set `param2 = 1` or so.

The function `default.after_place_leaves` can be set as `after_place_node of a node`
to set param2 to 1 if the player places the node (should not be used for nodes
that use param2 otherwise (e.g. facedir)).

If the node is in the `leafdecay_drop` group then it will always be dropped as an
item.

Dyes
----

To make recipes that will work with any dye ever made by anybody, define
them based on groups. You can select any group of groups, based on your need for
amount of colors.

### Color groups

Base color groups:

 * `basecolor_white`
 * `basecolor_grey`
 * `basecolor_black`
 * `basecolor_red`
 * `basecolor_yellow`
 * `basecolor_green`
 * `basecolor_cyan`
 * `basecolor_blue`
 * `basecolor_magenta`

Extended color groups ( * means also base color )

 * `excolor_white` *
 * `excolor_lightgrey`
 * `excolor_grey` *
 * `excolor_darkgrey`
 * `excolor_black` *
 * `excolor_red` *
 * `excolor_orange`
 * `excolor_yellow` *
 * `excolor_lime`
 * `excolor_green` *
 * `excolor_aqua`
 * `excolor_cyan` *
 * `excolor_sky_blue`
 * `excolor_blue` *
 * `excolor_violet`
 * `excolor_magenta` *
 * `excolor_red_violet`

The whole unifieddyes palette as groups:

 * `unicolor_<excolor>`

For the following, no white/grey/black is allowed:

 * `unicolor_medium_<excolor>`
 * `unicolor_dark_<excolor>`
 * `unicolor_light_<excolor>`
 * `unicolor_<excolor>_s50`
 * `unicolor_medium_<excolor>_s50`
 * `unicolor_dark_<excolor>_s50`

Example of one shapeless recipe using a color group:

	minetest.register_craft({
		type = "shapeless",
		output = '<mod>:item_yellow',
		recipe = {'<mod>:item_no_color', 'group:basecolor_yellow'},
	})

### Color lists

 * `dye.basecolors` are an array containing the names of available base colors

 * `dye.excolors` are an array containing the names of the available extended colors

Trees
-----

 * `default.grow_tree(pos, is_apple_tree)`
  * Grows a mgv6 tree or apple tree at pos

 * `default.grow_jungle_tree(pos)`
  * Grows a mgv6 jungletree at pos

 * `default.grow_pine_tree(pos)`
  * Grows a mgv6 pinetree at pos

 * `default.grow_new_apple_tree(pos)`
  * Grows a new design apple tree at pos

 * `default.grow_new_jungle_tree(pos)`
  * Grows a new design jungle tree at pos

 * `default.grow_new_pine_tree(pos)`
  * Grows a new design pine tree at pos

 * `default.grow_new_acacia_tree(pos)`
  * Grows a new design acacia tree at pos

 * `default.grow_new_aspen_tree(pos)`
  * Grows a new design aspen tree at pos

 * `default.grow_new_snowy_pine_tree(pos)`
  * Grows a new design snowy pine tree at pos

Carts
-----

	carts.register_rail(
		"mycarts:myrail", -- Rail name
		nodedef,          -- standard nodedef
		railparams        -- rail parameter struct (optional)
	)

	railparams = {
		on_step(obj, dtime), -- Event handler called when
		                     -- cart is on rail
		acceleration, -- integer acceleration factor (negative
		              -- values to brake)
	}

	The event handler is called after all default calculations
	are made, so the custom on_step handler can override things
	like speed, acceleration, player attachment. The handler will
	likely be called many times per second, so the function needs
	to make sure that the event is handled properly.

Key API
-------

The key API allows mods to add key functionality to nodes that have
ownership or specific permissions. Using the API will make it so
that a node owner can use skeleton keys on their nodes to create keys
for that node in that location, and give that key to other players,
allowing them some sort of access that they otherwise would not have
due to node protection.

To make your new nodes work with the key API, you need to register
two callback functions in each nodedef:


`on_key_use(pos, player)`
 * Is called when a player right-clicks (uses) a normal key on your
 * node.
 * `pos` - position of the node
 * `player` - PlayerRef
 * return value: none, ignored

The `on_key_use` callback should validate that the player is wielding
a key item with the right key meta secret. If needed the code should
deny access to the node functionality.

If formspecs are used, the formspec callbacks should duplicate these
checks in the metadata callback functions.


`on_skeleton_key_use(pos, player, newsecret)`

 * Is called when a player right-clicks (uses) a skeleton key on your
 * node.
 * `pos` - position of the node
 * `player` - PlayerRef
 * `newsecret` - a secret value(string)
 * return values:
 * `secret` - `nil` or the secret value that unlocks the door
 * `name` - a string description of the node ("a locked chest")
 * `owner` - name of the node owner

The `on_skeleton_key_use` function should validate that the player has
the right permissions to make a new key for the item. The newsecret
value is useful if the node has no secret value. The function should
store this secret value somewhere so that in the future it may compare
key secrets and match them to allow access. If a node already has a
secret value, the function should return that secret value instead
of the newsecret value. The secret value stored for the node should
not be overwritten, as this would invalidate existing keys.

Aside from the secret value, the function should retun a descriptive
name for the node and the owner name. The return values are all
encoded in the key that will be given to the player in replacement
for the wielded skeleton key.

if `nil` is returned, it is assumed that the wielder did not have
permissions to create a key for this node, and no key is created.