-- protect the current environment so that accessing undefined variables raises an error

-- cache all globals so that the metamethods don't recursively trigger themselves
local getinfo = debug.getinfo
local rawget = rawget
local _G = _G

local mt = getmetatable(_ENV)
if mt == nil then
	mt = {}
	setmetatable(_ENV, mt)
end

local function what()
	local d = getinfo(3, "S")
	return d and d.what or "C"
end

mt.__newindex = function(t, key, value)
	-- allow native code to set values to the environment, otherwise we cannot load anything
	if what() ~= "C" then
		error("attempt to set undefined global variable '" .. tostring(key) .. "'", 2)
	end
	rawset(t, key, value)
end
  
mt.__index = function(t, key)
	-- reading undefined value from _ENV goes through to _G
	local value = rawget(_G, key)
	if value then return value end
	error("undefined global variable '" .. tostring(key) .. "'", 2)
end