#+TITLE: Run a Lua Demo
#+DATE: <2023-07-25 二>
#+OPTIONS: \n:nil

Lua is everywhere.
Neovim uses lua, Vis uses lua; when I visited [[./internet_collections.org::#lantian][Lantian]]'s website[fn:1], it's running [[https://openresty.org/][OpenResty]];
when I'm [[https://bookfere.com/post/970.html][jailbreaking]] Kindle, it uses a lua script and [[https://github.com/koreader/koreader][Koreader]] uses lua;
[[https://www.huijiwiki.com/wiki/帮助:模块][MediaWiki Modules]] are powered by lua.

So I want to learn a little lua, just enough to handle daily tasks.

I visited lua.org and found the [[https://www.lua.org/cgi-bin/demo][demo]]. The idea of knowing how it works immediately came to my mind.
By searching on DDG[fn:2] I fount it have a hidden demo program called demo,
it's just the script for running demo (talk about that later why I'm sure about it).

I copied it and added the essential html in the print block,
put it on the server, use [[https://man.openbsd.org/slowcgi.8][slowcgi(8)]] to serve it with [[https://man.openbsd.org/httpd.8][httpd(8)]].
Then I got "No such file or directory".
I carefully checked the path and permission, but the error still exists

Finally StackOverflow taught me that the execution environment should also
be located in the chroot environment. For security, chroot is needed.
Well, just [[https://blog.syndcat.com/?p=181][statically]] link lua[fn:3].
#+BEGIN_SRC makefile
MYLDFLAGS= -static
#+END_SRC

It runs well, but the =-- continue HTML began in shell script= line
indicates it's not the only file in the CGI application.
I found the rest of them at [[https://web.tecgraf.puc-rio.br/~lhf/ftp/lua/#demo]] from [[http://lua-users.org/lists/lua-l/][lua-l]] mailing list.

It contains 3 files
#+BEGIN_EXAMPLE
        demo            the CGI script
        demo.html       the HTML page
        demo.lua        the Lua program that runs user programs
#+END_EXAMPLE
The shell script takes =QUERY_STRING= and replace textarea with =$QUERY_STRING.lua=.
So demo?demo just prints the demo.lua itself in the same directory.

I thought my single-file solution is better because it don't need to fork twice and perform file IO,
and it can be even better.

I added resource limit according to [[https://www.lua.org/pil/][PIL]] chapter 25 "SandBoxing"
#+BEGIN_SRC lua
local steplimit = 100
local memlimit  = 100

local count = 0
local function step ()
	count = count + 1
	if collectgarbage("count") > memlimit then
		error("DoSer uses too much memory")
	end
	if count > steplimit then
		error("DoSer uses too much CPU")
	end
end

debug.sethook(step, "", 100)
#+END_SRC

source code: https://github.com/dongdigua/demo.lua


Tsoding made a stream yesterday [[https://www.twitch.tv/videos/1879677165][twitch: New Apartment]] trying to compile lua to wasm using clang.
He failed, and said "Lua is not a real language, it's a toy language." The stream is really fun, though.
I found some solution:
- [[https://github.com/vvanders/wasm_lua][wasm_lua]]
- [[https://github.com/daurnimator/lua.vm.js][lua.vm.js]]
- [[https://fengari.io][Fengari]]


* Footnotes
[fn:1] I found his page all empty so I sent him an email, he quickly fixed it.
[fn:2] DuckDuckGo, not DongDiGua
[fn:3] If =make freebsd= don't work, try =make generic=