[luatex] Assignment to tex.skip shows an unexpected behaviour

[Takayuki YATO]

Hello,

I found a strange behavior about tex.skip array.
The LuaTeX manual says that assignment to TeX registers
through tex.count, tex.skip, etc. is local (perhaps unless
\globaldefs > 0).

Thus when I compile the following plain TeX source:

\newskip\skipA
\skipA=50pt
\begingroup
%\skipA=60pt  %(*)
\message{\the\skipA}
\directlua{
  local gsp = node.new(node.id("glue_spec"))
  gsp.width = 100 * 0x10000
  tex.skip.skipA = gsp
}
\message{\the\skipA}
\endgroup
\message{\the\skipA}
\bye

the expected terminal output will be:

50.0pt 100.0pt 50.0pt

But the actual output is:

50.0pt 100.0pt 100.0pt

that is, the assignment through tex.skip turns to be global.

The more strange fact is that uncommenting the line (*)
(thus once doing local assignment in TeX) causes the Lua
assignment local, leading to the output:

60.0pt 100.0pt 50.0pt

And finally I make the nesting one-level deeper:

\newskip\skipA
\skipA=25pt
\begingroup
    \skipA=50pt
    \begingroup
        \directlua{
          local gsp = node.new(node.id("glue_spec"))
          gsp.width = 100 * 0x10000
          tex.skip.skipA = gsp
        }
        \message{\the\skipA}
    \endgroup
    \message{\the\skipA} % here \skipA becomes zero!
\endgroup
\message{\the\skipA}
\bye

the resulting output is totally unreasonable:

100.0pt 0.0pt 25.0pt

I inspected the source code and found that Lua assignments
are handled by the following function:

int set_tex_skip_register(int j, halfword v)
{
    //......
    if (type(v) != glue_spec_node)
        return 1;
    word_define(j + skip_base, v);   //(**)
    return 0;
}

If Lua assignment should be done in the same way as in
TeX, then the line (**) should be:

    define(j + skip_base, glue_ref_cmd, v);

Happy (Lua)TeXing,
  Takayuki YATO