Add luacontroller, a microcontroller that you can code in lua.

It still misses some functionality such as a persistent memory and a
timer, but that is subject to change. The code runs in a sandbox.
Speaking long term this will hopefully replace the old controller.

mesecons_luacontroller/depends.txt

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+mesecons

mesecons_luacontroller/init.lua

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+-- Reference
+-- ports = get_real_portstates(pos): gets if inputs are powered from outside
+-- newport = merge_portstates(state1, state2): just does result = state1 or state2 for every port
+-- action_setports(pos, ports, vports): activates/deactivates the mesecons according to the portstates (helper for action)
+-- action(pos, ports): Applies new portstates to a luacontroller at pos
+-- update(pos): updates the controller at pos by executing the code
+-- reset_meta (pos, code, errmsg): performs a software-reset, installs new code and prints error messages
+-- reset (pos): performs a hardware reset, turns off all ports
+--
+-- The Sandbox
+-- The whole code of the controller runs in a sandbox,
+-- a very restricted environment.
+-- However, as this does not prevent you from using e.g. loops,
+-- we need to check for these prohibited commands first.
+-- Actually the only way to damage the server is to
+-- use too much memory from the sandbox.
+-- You can add more functions to the environment
+-- (see where local env is defined)
+-- Something nice to play is is appending minetest.env to it.
+
+local BASENAME = "mesecons_luacontroller:luacontroller"
+
+local rules = {}
+rules.a = {x = -1, y = 0, z =  0}
+rules.b = {x =  0, y = 0, z =  1}
+rules.c = {x =  1, y = 0, z =  0}
+rules.d = {x =  0, y = 0, z = -1}
+
+local get_real_portstates = function(pos) -- determine if ports are powered (by itself or from outside)
+	ports = {
+		a = mesecon:is_power_on(mesecon:addPosRule(pos, rules.a))
+			and mesecon:rules_link(mesecon:addPosRule(pos, rules.a), pos),
+		b = mesecon:is_power_on(mesecon:addPosRule(pos, rules.b))
+			and mesecon:rules_link(mesecon:addPosRule(pos, rules.b), pos),
+		c = mesecon:is_power_on(mesecon:addPosRule(pos, rules.c))
+			and mesecon:rules_link(mesecon:addPosRule(pos, rules.c), pos),
+		d = mesecon:is_power_on(mesecon:addPosRule(pos, rules.d))
+			and mesecon:rules_link(mesecon:addPosRule(pos, rules.d), pos),
+	}
+	return ports
+end
+
+local merge_portstates = function (ports, vports)
+	local npo = {a=false, b=false, c=false, d=false}
+	npo.a = vports.a or ports.a
+	npo.b = vports.b or ports.b
+	npo.c = vports.c or ports.c
+	npo.d = vports.d or ports.d
+	return npo
+end
+
+local action_setports = function (pos, ports, vports)
+	if vports.a ~= ports.a then
+		if ports.a then mesecon:receptor_on(pos, {rules.a})
+		else mesecon:receptor_off(pos, {rules.a}) end
+	end
+	if vports.b ~= ports.b then
+		if ports.b then mesecon:receptor_on(pos, {rules.b})
+		else mesecon:receptor_off(pos, {rules.b}) end
+	end
+	if vports.c ~= ports.c then
+		if ports.c then mesecon:receptor_on(pos, {rules.c})
+		else mesecon:receptor_off(pos, {rules.c}) end
+	end
+	if vports.d ~= ports.d then
+		if ports.d then mesecon:receptor_on(pos, {rules.d})
+		else mesecon:receptor_off(pos, {rules.d}) end
+	end
+end
+
+local action = function (pos, ports)
+	local vports = minetest.registered_nodes[minetest.env:get_node(pos).name].virtual_portstates;
+	local name = BASENAME
+		..tonumber(ports.d and 1 or 0)
+		..tonumber(ports.c and 1 or 0)
+		..tonumber(ports.b and 1 or 0)
+		..tonumber(ports.a and 1 or 0)
+	mesecon:swap_node(pos, name)
+
+	action_setports (pos, ports, vports)
+end
+
+-- Overheat Stuff
+local heat = function (meta) -- warm up
+	h = meta:get_int("heat")
+	if h ~= nil then
+		meta:set_int("heat", h + 1)
+	end
+end
+
+local cool = function (meta) -- cool down after a while
+	h = meta:get_int("heat")
+	if h ~= nil then
+		meta:set_int("heat", h - 1)
+	end
+end
+
+local overheat = function (meta) -- determine if too hot
+	h = meta:get_int("heat")
+	if h == nil then return true end -- if nil then overheat
+	if h > 30 then 
+		return true
+	else 
+		return false 
+	end
+end
+
+local overheat_off = function(pos)
+	rules = mesecon:get_rules("mesecons_microcontroller:microcontroller1111")
+	mesecon:receptor_off(pos, rules)
+end
+
+local update = function (pos)
+	local meta = minetest.env:get_meta(pos)
+	code = meta:get_string("code")
+
+	-- Clean code
+	local prohibited = {"while", "for", "repeat", "until"}
+	for _, p in ipairs(prohibited) do
+		if string.find(code, p) then
+			return "Prohibited command: "..p
+		end
+	end
+
+	-- Gather variables for the environment
+	local vports = minetest.registered_nodes[minetest.env:get_node(pos).name].virtual_portstates
+	vports = {a = vports.a, b = vports.b, c = vports.c, d = vports.d}
+	local rports = get_real_portstates(pos)
+
+	local env = {	print = print,
+				selfpos = pos,
+				dump = dump,
+				pin = merge_portstates(vports, rports),
+				port = vports}
+
+	-- Create Sandbox
+	if code:byte(1) == 27 then
+		return "You Hacker You! Don't use binary code!"
+	end
+	local f, msg = loadstring(code)
+	if not f then return msg end
+	setfenv(f, env)
+	success, msg = pcall(f)
+
+	-- Overheat protection
+	heat(meta)
+	minetest.after(0.5, cool, meta)
+	if overheat(meta) then
+		minetest.env:remove_node(pos)
+		minetest.after(0.2, overheat_off, pos) -- wait for pending operations
+		minetest.env:add_item(pos, BASENAME.."0000")
+		return
+	end
+
+	-- Actually set the ports
+	if not success then
+		return msg
+	else
+		action(pos, env.port)
+	end
+end
+
+local reset_meta = function(pos, code, errmsg)
+	local meta = minetest.env:get_meta(pos)
+	code = code or "";
+	errmsg = errmsg or "";
+	errmsg = string.gsub(errmsg, "%[", "(") -- would otherwise
+	errmsg = string.gsub(errmsg, "%]", ")") -- corrupt formspec
+	meta:set_string("code", code)
+	meta:set_string("formspec", "size[10,8]"..
+		"textarea[0.2,0.4;10.2,5;code;Code:;"..code.."]"..
+		"button[3.5,7.5;2,0;program;Program]"..
+		"image_button_exit[9.62,-0.35;0.7,0.7;jeija_close_window.png;exit;]"..
+		"label[0.1,4.5;"..errmsg.."]")
+	meta:set_int("heat", 0)
+end
+
+local reset = function (pos)
+	action(pos, {a=false, b=false, c=false, d=false})
+end
+
+--        ______
+--       |
+-- |   | |
+-- |___| |        __       ___  _   __         _  _
+-- |     |       |  | |\ |  |  |_| |  | |  |  |_ |_|
+-- |     |______ |__| | \|  |  | \ |__| |_ |_ |_ |\
+--
+-- Actually register all the stuff:
+
+for a = 0, 1 do
+for b = 0, 1 do
+for c = 0, 1 do
+for d = 0, 1 do
+local nodename = BASENAME..tostring(d)..tostring(c)..tostring(b)..tostring(a)
+local top = "jeija_luacontroller_top.png"
+if a == 1 then
+	top = top.."^jeija_luacontroller_LED_A.png"
+end
+if b == 1 then
+	top = top.."^jeija_luacontroller_LED_B.png"
+end
+if c == 1 then
+	top = top.."^jeija_luacontroller_LED_C.png"
+end
+if d == 1 then
+	top = top.."^jeija_luacontroller_LED_D.png"
+end
+
+if a + b + c + d ~= 0 then
+	groups = {dig_immediate=2, not_in_creative_inventory=1}
+else
+	groups = {dig_immediate=2}
+end
+
+local output_rules={}
+if (a == 1) then table.insert(output_rules, rules.a) end
+if (b == 1) then table.insert(output_rules, rules.b) end
+if (c == 1) then table.insert(output_rules, rules.c) end
+if (d == 1) then table.insert(output_rules, rules.d) end
+
+local input_rules={}
+if (a == 0) then table.insert(input_rules, rules.a) end
+if (b == 0) then table.insert(input_rules, rules.b) end
+if (c == 0) then table.insert(input_rules, rules.c) end
+if (d == 0) then table.insert(input_rules, rules.d) end
+
+local mesecons = {effector =
+{
+	rules = input_rules,
+	action_change = function (pos)
+		update(pos)
+	end
+}}
+if nodename ~= BASENAME.."0000" then
+	mesecons.receptor = {
+		state = mesecon.state.on,
+		rules = output_rules
+	}
+end
+
+local nodebox = {
+		type = "fixed",
+		fixed = {
+			{ -8/16, -8/16, -8/16, 8/16, -7/16, 8/16 }, -- bottom slab
+			{ -5/16, -7/16, -5/16, 5/16, -6/16, 5/16 }, -- circuit board
+			{ -3/16, -6/16, -3/16, 3/16, -5/16, 3/16 }, -- IC
+		}
+	}
+
+local selectionbox = {
+		type = "fixed",
+		fixed = { -8/16, -8/16, -8/16, 8/16, -5/16, 8/16 },
+	}
+
+minetest.register_node(nodename, {
+	description = "Luacontroller",
+	drawtype = "nodebox",
+	tiles = {
+		top,
+		"jeija_microcontroller_bottom.png",
+		"jeija_microcontroller_sides.png",
+		"jeija_microcontroller_sides.png",
+		"jeija_microcontroller_sides.png",
+		"jeija_microcontroller_sides.png"
+		},
+
+	paramtype = "light",
+	groups = groups,
+	drop = BASENAME.."0000",
+	sunlight_propagates = true,
+	selection_box = selectionbox,
+	node_box = nodebox,
+	on_construct = reset_meta,
+	on_receive_fields = function(pos, formname, fields)
+		reset(pos)
+		reset_meta(pos, fields.code)
+		local err = update(pos)
+		if err then print(err) end
+		reset_meta(pos, fields.code, err)
+	end,
+	mesecons = mesecons,
+	virtual_portstates = {	a = a == 1, -- virtual portstates are
+					b = b == 1, -- the ports the the
+					c = c == 1, -- controller powers itself
+					d = d == 1},-- so those that light up
+	after_dig_node = function (pos, node)
+		mesecon:receptor_off(pos, output_rules)
+	end,
+})
+end
+end
+end
+end
+
+minetest.register_craft({
+	output = BASENAME.."0000 2",
+	recipe = {
+		{'mesecons_materials:silicon', 'mesecons_materials:silicon', 'group:mesecon_conductor_craftable'},
+		{'mesecons_materials:silicon', 'mesecons_materials:silicon', 'group:mesecon_conductor_craftable'},
+		{'group:mesecon_conductor_craftable', 'group:mesecon_conductor_craftable', ''},
+	}
+})