Overwritten global Lua variables "attempt to index a NN value"

This is not a concrete answer to the question, but I'd like to write a generic answer anyway.

attempt to index a boolean value (global 'debug')

is a Lua error message which tells you that you are attempting to use a member of a Lua table, but what you treat as a table is not a table (anymore).

In my case I tried to call the function getinfo() from the table debug (which is built into Lua and should be available anywhere), but when invoking it Lua found a boolean value (true or false) instead.

It is very likely that this observation indicates that the builtin table debug has at some point been overwritten with a boolean, either in your preamble or - as a bug - in a package.

To find the culprit it is useful to insert

directlua{print(debug)}
directlua{debug.go}

in your preamble as suggested in this comment. The first line will print the table address - as long as there still is a table - the second line will cause an error and stop the compilation. You can iterate through every step of your preamble to find exactly where the problem starts.

Underlying problem

The reason for this issue is a typical Lua programming error, in this case in a package. In order to create a status flag the package contained the line

debug = false

setting the debug variable to false as its default value, which is then used in a function outputting debug information.

The problem is that in Lua variables are by default in the global namespace (opposite behaviour from e.g. Python) - which means that the line above overwrites the global variable and effectively "kills" the whole builtin Lua table.

This is a very good example for the advice never to forget using local variables in Lua wherever possible:

local debug = false

This way debug is local to the current module and does not affect the global table with the same name.

When trying to find out who is responsible for changing global variables in Lua, it is sometimes useful to temporarily assign a metatable to the global table. (I strongly recommend never to do this outside of debugging and to let the metatable work as close to a normal tableas possible, otherwise you get all kinds of weird behavior.)

So, in order to detect changes to the global debug through metamethods, you have to delete the debug table. You can virtually put it back though the __index metamethod. Then you can use __newindex to detect attempts create a new global debug in which case you can print a warning, using the original debug table to get information about the responsible Lua function. In isn't that helpful though if it was reset e.g. in directlua, but you can use LuaTeX's status table to additionally provide the current TeX file and line.

Of yourse you have to do this before loading anything which might be responsble, do it's best to put it even before documentclass:

directlua{
  local debug = debug
  local traceback, getinfo = debug.traceback, debug.getinfo
  _G.debug = nil
  setmetatable(_G, {
    __index = {debug = debug},
    __newindex = function(t, k, v)
      if k == "debug" then
        local guilty = getinfo(2)
        texio.write_nl("We found the naughty party: " .. (guilty.name or "nameless devil") .. " in " .. guilty.source .. " at line " .. guilty.currentline)
        texio.write_nl("TeX is currently reading " .. status.filename .. " at line " .. status.linenumber)
      end
      return rawset(t, k, v)
    end,
  })
}
documentclass{article}
usepackage{grandmother}
begin{document}
end{document}

which would e.g. provide the output

...
(/usr/local/texlive/2020/texmf-dist/tex/latex/base/size10.clo))
(./grandmother.sty
We found the naughty party: rose_tylor in @./bad_wolf.lua at line 2
TeX is currently reading ./grandmother.sty at line 4)
(/usr/local/texlive/2020/texmf-dist/tex/latex/l3backend/l3backend-luatex.def)
...

indicating that a function rose_tylor in the file ./bad_wolf.lua is changing debug, the change happens in line 2 of the file and it is loaded in grandmother.sty on line 4.