require modules which polutes globals

local function require_external(modname)
    if type(modname) ~= 'string' then
        error("bad argument #1 to 'require_external' (string expected, got "..type(modname)..")", 2)
    end
    if package.loaded[modname] ~= nil then
        if package.loaded[modname] == false then
            error("loop or previous error loading module '".. modname .. "'", 2)
        end
        return package.loaded[modname]
    end

    package.loaded[modname] = false

    -- search file on FS (Tarantool only)
    local file = assert(package.search(modname), "module "..modname.." not found")

    -- Create separate Global
    local mod = setmetatable({}, { __index = _G })

    -- Run loader (syntax linting)
    local loader = assert(loadfile(file))

    -- Change global environment (to exclude polution of app's global)
    setfenv(loader, mod)

    -- Load module
    local result = loader(modname)

    -- Drop metatable of module for good
    setmetatable(mod, nil)

    -- Check if module returns something
    if result ~= true then
        result = result or mod
    else
        -- If module returns nothing (true by default) return mod
        result = mod
    end

    package.loaded[mod] = result

    return result
end