spurious enddir node

[Udi-Fogiel]

This is not a Babel bug, but a LuaTeX one. In some cases using \textdirection/\linedirection in a paragraph may produce a spurious enddir node.

The simplest example is

\textdirection=0
\leavevmode\textdirection=0
\bye

Usually this goes undetectable, but in some cases it can confuse Lua code which inspect direction nodes (such as babel-bidi-basic.lua), or even worse, corrupt LuaTeX's coordinate system during the pdf creation phase. Here is an example using babel with a noticeable effect (note the last "run" on the bottom left corner):

\documentclass{article}
\usepackage[hebrew,english,bidi=basic]{babel}
\begin{document}
run \selectlanguage{hebrew}run \selectlanguage{english}run

run {\selectlanguage{hebrew}run} run \selectlanguage{hebrew}run
\end{document}

It also might affects other packages if babel is used (lua-visual-debug for example).

I'm not sure when it will be fixed, so maybe a workaround should be added? A possible workaround:

\documentclass{article}
\usepackage[hebrew,english,bidi=basic]{babel}
\directlua{
luatexbase.add_to_callback("pre_linebreak_filter",
function(head)
  local stack = 0
  for n, sb in node.traverse_id(node.id('dir'),head) do
    stack = stack + 1-2*sb
    if stack == -1 then
      head, n = node.remove(head,n)
      return head
    end
  end
  return head
end, "Babel.remove_spurious_enddir",0)
}
\begin{document}
run \selectlanguage{hebrew}run \selectlanguage{english}run

run {\selectlanguage{hebrew}run} run \selectlanguage{hebrew}run
\end{document}

This function should be called early.