Disabled/restricted shell access does not produce human-readable error message

[Witiko]

I tried compiling the following document with pdfTeX:

% (file name: test.tex)
\documentclass{article}
\usepackage{markdown}
\begin{document}

Using inline markdown:

\begin{markdown}

Hello World.

\end{markdown}
\end{document}

The result was an emergency stop and the error message displayed below. It compiles just fine with LuaTeX.

System information: I am using TeX Live 2023 (installed yesterday, October 22) on Windows 10. I got the same error message with TeX Live 2022, which was why I installed the 2023 version yesterday.

This is pdfTeX, Version 3.141592653-2.6-1.40.25 (TeX Live 2023) (preloaded format=pdflatex 2023.10.22)  23 OCT 2023 10:31
entering extended mode
 restricted \write18 enabled.

...

! I can't find file `"|texlua ./test.markdown.lua"'.
<to be read again> 
                   \scan_stop: 
l.13 \end{markdown}

...

[...] perhaps the error message could be made more clear?

—originally posted by @sorsted in Witiko/markdown#365.

It definitely should, thanks for bringing this to our attention. Currently, the execution of Lua code and all error handling is delegated to the lt3luabridge package, which uses the \sys_get_shell:nnNTF expl3 function in pdfTeX to determine whether the shell is available and to either use the shell or report an error:

\msg_new:nnnn
  { luabridge }
  { level-disabled }
  {
    Shell~escape~seems~to~be~disabled
  }
  {
    You~may~need~to~run~TeX~with~the~--shell-escape~or~the~
    --enable-write18~flag,~or~write~shell_escape=t~in~the~texmf.cnf~file.
  }

However, from your report it seems that the function \sys_get_shell:nnNTF fails to properly handle the case where the shell escape is disabled/restricted and does not produce a human-readable error.

—originally posted by @Witiko in Witiko/markdown#365.

[Witiko]

I opened a ticket upstream in latex3/latex3#1339. This is likely not a bug but the intended behavior, because expl3 cannot detect the case, where shell access is restricted and the shell command is not allowed. Furthermore, this seems to affect LuaTeX as well, so i am removing the "in pdfTeX" bit from the title of the ticket.