I've contacted Jow. This is your answer:
The index procedure of a controller file is cached as bytecode and executed from cache on subsequent page invocations. This means you cannot access local functions.
Make your get_username_session function non-local and in your index() procedure do the following:
function index()
local ctrl = require 'luci.controller.admin.test' -- must correspond to the controller path
local foo = ctrl.get_username_session()
-- ...
end
Now I've fixed file test.lua and this is the link --> https://drive.google.com/open?id=1-B9zgZBjnAzyCJnM0qMKmxlG17gpXzjc
replace this file and test it!
Bye