\sys_get_shell:nnNTF produces unhandled errors when the shell command is not allowed

[Witiko]

The documentation of the conditional function \sys_get_shell:nnNTF says:

The \sys_get_shell:nnNTF conditional inserts the ⟨true code⟩ if the shell is available and no quote is detected, and the ⟨false code⟩ otherwise.

Here is how I understand this description: The conditional function \sys_get_shell:nnNTF enters the true branch when the shell access is restricted or unrestricted, the shell command contains no ASCII quote characters, and the TeX engine made an actual attempt to execute the shell command (regardless of the exit code).

However, as shown by the following example document example.tex, the conditional function \sys_get_shell:nnNTF does not seem to handle the last case, where TeX did not make an attempt to execute the shell command, because the shell access was restricted:

\input expl3-generic
\ExplSyntaxOn
\sys_get_shell:nnNTF { echo~1 + 2 | bc~-l } { } \l_tmpa_tl { $1 + 2 = \l_tmpa_tl$ } { Failure }
\bye

Running pdftex example.tex produces the following output:

This is pdfTeX, Version 3.141592653-2.6-1.40.25 (TeX Live 2023) (preloaded format=pdftex)
 restricted \write18 enabled.
entering extended mode
(./example.tex
(/usr/local/texlive/2023/texmf-dist/tex/latex/l3kernel/expl3-generic.tex
(/usr/local/texlive/2023/texmf-dist/tex/latex/l3kernel/expl3-code.tex)
(/usr/local/texlive/2023/texmf-dist/tex/latex/l3backend/l3backend-pdftex.def))
runpopen command not allowed: echo1+2

! I can't find file `"|echo 1+2|bc -l"'.
<to be read again> 
                   \scan_stop: 
l.3 ...t_shell:nnNTF { echo~1 + 2 | bc~-l } { } \l_tmpa_tl
                                                   { $1 + 2 = \l_tmpa_tl$ } ...

(Press Enter to retry, or Control-D to exit)
Please type another input file name: 

I would expect either a document with the text Failure to be produced or a sentence in the documentation of the conditional function \sys_get_shell:nnNTF that clarifies that the consitional function does not handle cases, where shell access is restricted and the shell command is not allowed, and that in such cases, the TeX engine will produce unhandled errors.

[josephwright]

I'm not sure what we can do here: the shell is available, but we can't tell from the TeX side what will be allowed by restricted shell escape.

[Witiko]

It may be useful to highlight this in the documentation.