[luatex] inserting user-defined whatsit node

[Stephan Hennig]

Hi,

in the attached example, the hyphenate callback is used to scan for
glyph nodes corresponding to the character 'x'.  If such nodes are
found, a copy of that node is inserted before the node in question
(effectively doubling all 'x').  This works fine.

It breaks, however, if a user-defined whatsit node is inserted instead
of a copy of a glyph node.  Error message is

  ! Unidentified user defined whatsit.

The LuaTeX manual has this about user-defined whatsits:

  The LuaTeX engine will simply step over such whatsits without
  ever looking at the contents.

Why does the example break?

Best regards,
Stephan Hennig


% -*- coding: utf-8 -*-
\directlua{
% Declare constants.
  local GLYPH = node.id('glyph')
  local WHATSIT = node.id('whatsit')
  local USER_DEFINED = node.subtype('user_defined')
  local MYCHARCODE = string.byte('x')
  local Ncopy = node.copy
  local Nnew = node.new
  local Ninsert_before = node.insert_before
  local Ntraverse = node.traverse
% Create user-defined whatsit.
  local what = Nnew(WHATSIT, USER_DEFINED)
  what.user_id = 20141117
% Register callback.
  callback.register('hyphenate',
    function (head, tail)
      for n in Ntraverse(head) do
        if n.id == GLYPH and n.char == MYCHARCODE then
%          Ninsert_before(head, n, Ncopy(n))
          Ninsert_before(head, n, Ncopy(what))
        end
      end
      lang.hyphenate(head, tail)
    end
  )
}
text
\bye


> This is LuaTeX, Version beta-0.79.1 (TeX Live 2014/W32TeX) (rev 4971)  (format=luatex 2014.7.9)  17 NOV 2014 14:20
>  restricted \write18 enabled.
> **udwhatsit.tex
> (./udwhatsit.tex [1{c:/texlive/2014/texmf-var/fonts/map/pdftex/updmap/pdftex.map
> }]
> ! Unidentified user defined whatsit.
> \plainoutput ...headline \pagebody \makefootline }
>                                                   \advancepageno \ifnum \out...
> <output> {\plainoutput 
>               }
> \supereject {-\par \penalty -\@MM 
>                                   
> \bye {-\par \vfill \supereject 
>                                \end 
> l.29 \bye
>        
> ? 
> )<<c:/texlive/2014/texmf-dist/fonts/type1/public/amsfonts/cm/cmr10.pfb>
> Output written on udwhatsit.pdf (1 page, 10206 bytes).
> 
> PDF statistics: 12 PDF objects out of 1000 (max. 8388607)
>  7 compressed objects within 1 object stream
>  0 named destinations out of 1000 (max. 131072)
>  1 words of extra memory for PDF output out of 10000 (max. 10000000)
>