Devanagari UTF-8 text extraction code produces lua error while using HarfBuzz renderer

I don't know too much about how HarfBuzz fonts are handled by Luaotfload, but I was able to find the way how to get the tounicode fields, thanks to table.serialize. So my original code adapted for Harfbuzz looks like this:

documentclass{article}
usepackage[lmargin=0.5in,tmargin=0.5in,rmargin=0.5in,bmargin=0.5in]{geometry}
usepackage{fontspec}
usepackage{microtype}
usepackage{luacode}

%newfontscript{Devanagari}{deva,dev2}
newfontfamily{devanagarifam}{Noto Sans Devanagari}[Script=Devanagari, Scale=1, Renderer=HarfBuzz]
newfontfamily{arabicfam}{Amiri}[Script=Arabic, Scale=1, Renderer=HarfBuzz]

begin{document}

setbox0=hbox{Příliš žluťoučký textit{kůň} úpěl hbox{ďábelské} ódy difference diffierence. devanagarifam एक गांव -- में मोहन नाम का लड़का रहता था। उसके पिताजी एक मामूली मजदूर थे।}
setbox1=hbox{arabicfam textdir TRT  هذه المقالة عن براغ. لتصفح عناوين مشابهة، انظر براغ (توضيح).}

begin{luacode*}
  -- local fontstyles = require "l4fontstyles"
  local char = unicode.utf8.char
  local glyph_id = node.id("glyph")
  local glue_id  = node.id("glue")
  local hlist_id = node.id("hlist")
  local vlist_id = node.id("vlist")
  local disc_id  = node.id("disc")
  local minglue  = tex.sp("0.2em")
  local usedcharacters = {}
  local identifiers = fonts.hashes.identifiers
  local fontcache = {}

  local function to_unicode_chars(uchar)
    local uchar = uchar or ""
    -- put characters into a table
    local current = {}
    -- each codepoint is 4 bytes long, we loop over tounicode entry and cut it into 4 bytes chunks
    for i= 1, string.len(uchar), 4 do
      local cchar = string.sub(uchar, i, i + 3)
      -- codepoint is hex string, we need to convert it to number ad then to UTF8 char
      table.insert(current,char(tonumber(cchar,16)))
    end
    return current
  end
  -- cache character lookup, to speed up things
  local function get_character_from_cache(xchar, font_id)
    local current_font = fontcache[font_id] or {characters = {}}
    fontcache[font_id] = current_font -- initialize font cache for the current font if it doesn't exist
    return current_font.characters[xchar]
  end

  -- save characters to cache for faster lookup
  local function save_character_to_cache(xchar, font_id, replace)
    fontcache[font_id][xchar] = replace
    -- return value
    return replace
  end

  local function initialize_harfbuzz_cache(font_id, hb)
    -- save some harfbuzz tables for faster lookup
    local current_font = fontcache[font_id]
    -- the unicode data can be in two places
    -- 1. hb.shared.glyphs[glyphid].backmap
    current_font.glyphs = current_font.glyphs or hb.shared.glyphs
    -- 2. hb.shared.unicodes 
    -- it contains mapping between Unicode and glyph id
    -- we must create new table that contains reverse mapping
    if not current_font.backmap then 
      current_font.backmap = {} 
      for k,v in pairs(hb.shared.unicodes) do
        current_font.backmap[v] = k
      end
    end
    -- save it back to the font cache
    fontcache[font_id] = current_font
    return current_font.glyphs, current_font.backmap
  end

  local function get_unicode(xchar,font_id)
    -- try to load character from cache first
    local current_char = get_character_from_cache(xchar, font_id) 
    if current_char then return current_char end
    -- get tounicode for non HarfBuzz fonts
    local characters = identifiers[font_id].characters
    local uchar = characters[xchar].tounicode
    -- stop processing if tounicode exists
    if uchar then return save_character_to_cache(xchar, font_id, to_unicode_chars(uchar)) end
    -- detect if font is processed by Harfbuzz
    local hb = identifiers[font_id].hb
    -- try HarfBuzz data
    if not uchar and hb then 
      -- get glyph index of the character
      local index = characters[xchar].index
      -- load HarfBuzz tables from cache
      local glyphs, backmap = initialize_harfbuzz_cache(font_id, hb)
      -- get tounicode field from HarfBuzz glyph info
      local tounicode = glyphs[index].tounicode
      if tounicode then
        return save_character_to_cache(xchar, font_id, to_unicode_chars(tounicode))
      end
      -- if this fails, try backmap, which contains mapping between glyph index and Unicode
      local backuni = backmap[index]
      if backuni then 
        return save_character_to_cache(xchar, font_id, {char(backuni)})
      end
      -- if this fails too, discard this character
      return save_character_to_cache(xchar, font_id, {})
    end
    -- return just the original char if everything else fails
    return save_character_to_cache(xchar, font_id, {char(xchar)})
  end

  local function nodeText(n)
    -- output buffer
    local t =  {}
    for x in node.traverse(n) do
      -- glyph node
      if x.id == glyph_id then
        -- get table with characters for current node.char
        local chars = get_unicode(x.char,x.font)
        for _, current_char in ipairs(chars) do
          -- save characters to the output buffer
          table.insert(t,current_char)
        end
      -- glue node
      elseif x.id == glue_id and  node.getglue(x) > minglue then
        table.insert(t," ")
      -- discretionaries
      elseif x.id == disc_id then
        table.insert(t, nodeText(x.replace))
      -- recursivelly process hlist and vlist nodes
      elseif x.id == hlist_id or x.id == vlist_id then
        table.insert(t,nodeText(x.head))
      end
    end
    return table.concat(t)
  end
  local n = tex.getbox(0)
  local n1 = tex.getbox(1)
  print(nodeText(n.head))
  local f = io.open("hello.txt","w")
  f:write(nodeText(n.head))
  f:write(nodeText(n1.head))
  f:close()
end{luacode*}

box0

box1
end{document}

I've added also an Arabic sample from Wikipedia. Here is the content of hello.txt:

Příliš žluťoučký kůň úpěl ďábelské ódy difference diffierence. एक गांव -- में मोहन नाम का लड़का रहता था। उसके पताजी एक मामूली मजदूर थे।هذه المقالة عن براغ. لتصفح عناوين مشابهة، انظر براغ (توضيح).

Two important functions are these

  local function to_unicode_chars(uchar)
    local uchar = uchar or ""
    local current = {}
    for i= 1, string.len(uchar), 4 do
      local cchar = string.sub(uchar, i, i + 3)
      table.insert(current,char(tonumber(cchar,16)))
    end
    return current
  end

to_unicode_chars function splits to_unicode entries to four byte chunks, which are then converted to UTF 8 characters. It can also handle glyphs without tounicode entries, in this case it just returns empty string.

  local function get_unicode(xchar,font_id)
    -- try to load character from cache first
    local current_char = get_character_from_cache(xchar, font_id) 
    if current_char then return current_char end
    -- get tounicode for non HarfBuzz fonts
    local characters = identifiers[font_id].characters
    local uchar = characters[xchar].tounicode
    -- stop processing if tounicode exists
    if uchar then return save_character_to_cache(xchar, font_id, to_unicode_chars(uchar)) end
    -- detect if font is processed by Harfbuzz
    local hb = identifiers[font_id].hb
    -- try HarfBuzz data
    if not uchar and hb then 
      -- get glyph index of the character
      local index = characters[xchar].index
      -- load HarfBuzz tables from cache
      local glyphs, backmap = initialize_harfbuzz_cache(font_id, hb)
      -- get tounicode field from HarfBuzz glyph info
      local tounicode = glyphs[index].tounicode
      if tounicode then
        return save_character_to_cache(xchar, font_id, to_unicode_chars(tounicode))
      end
      -- if this fails, try backmap, which contains mapping between glyph index and Unicode
      local backuni = backmap[index]
      if backuni then 
        return save_character_to_cache(xchar, font_id, {char(backuni)})
      end
      -- if this fails too, discard this character
      return save_character_to_cache(xchar, font_id, {})
    end
    -- return just the original char if everything else fails
    return save_character_to_cache(xchar, font_id, {char(xchar)})
  end

This function first tries to load Uniocode data from current font info. If it fails, it tries to lookup in Harfbuzz tables. Most of characters have tounicode mapping in the glyphs table. If it isn't available, it tries the unicodes table, which contains mapping between glyph indices and Unicode. If even this fails, then we discard this character.