On Thu, Apr 7, 2022 at 12:10 PM Jérôme LAURENS <jerome.laurens@u-bourgogne.fr> wrote:
New CLI options --busy related to typesetting

luatex --busy=0
    default behavior, nothing new
luatex --busy=1
    Instead of writing the pdf output to jobname.pdf, write to jobname.pdf(busy)
    At the end remove any jobname.pdf and move jobname.pdf(busy) to jobname.pdf
luatex --busy=2 foo.tex
    If there is a jobname.pdf(busy) bail out with a diagnostic message « typesetting already in progress »
    Otherwise do like --busy=1

Actually, the pdf is removed at the start of the typesetting process such that it is no longer available during that whole time.
When the typesetting is a bit long, it is not practical at all. For continuous typesetting neither.
Considering that we work in general on the end of a document, the first  pages will not change a lot but are no longer available.

synctex already supports this « busy » busyness such that the previous .synctex file is available during next typesetting process.

hm, perhaps  you can process the command line as below 

\directlua{print(); print("===========================");
for k,v in pairs(arg) do print(k,v) end;

and use the  wrapup_run callback .