[BUG] oddity tangling src_blk with :var [9.7-pre (release_9.6.26-1368-g1ae978 @ /home/minshall/.emacs.d/straight/build/org/)]

[Greg Minshall]

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hi.  the following org block describes a problem i noticed.  possibly
the fix is simply issuing an appropriate error message (and aborting the
process of tangling.

thanks for looking at this when you have a chance.  and, apologies if
this is a FAQ.

cheers, Greg

#+begin_src org
  ,** introduction

  this is to show an oddity -- a bug, or a security feature? -- in
  tangling a source code block that has a variable (=:var=) header
  argument.

  if the variable =org-babel-load-languages= includes the source type of
  the code block (e.g., "shell", "python", etc.), the variable will be
  set in the tangled file.  if =org-babel-load-languages= does *not*
  include that source type, the variable will be unset in the tangled
  file.  without any error message.

  (note that in a running emacs, *removing* elements from
  =org-babel-load-languages= doesn't seem to cause this problem to
  appear.  maybe this is because the relevant "ob-..." files have
  already been loaded?)

  to see this, first *tangle* this file, to get works.el, fails.el, and
  variable.sh, and then evaluate this code block:

  ,#+name: DemonstrateBehavior
  ,#+begin_src sh :results output
    ./works.el
    echo -n "works.el: "
    ./variable.sh
    ./fails.el
    echo -n "fails.el: "
    ./variable.sh
  ,#+end_src

  ,#+RESULTS: DemonstrateBehavior
  : works.el: 2.4
  : fails.el: 

  notice that after running fails.el, the 2.4 disappears.


  ,** this is the test code and blocks

  here is the variable to be included.
  ,#+name: variable
  | 2.4 |

  and, the code block that includes that variable
  ,#+begin_src sh :var variable=variable :shebang #!/bin/sh :tangle variable.sh
    echo "${variable}"
  ,#+end_src

  tangling with this works
  ,#+begin_src elisp :tangle works.el :shebang "#!/usr/bin/env -S emacs 
--script"
    (org-babel-do-load-languages
     'org-babel-load-languages
     '((emacs-lisp . t)
       (shell . t)))
    (org-babel-tangle-file "maybe.org")
  ,#+end_src

  tangling with this fails (notice the absence of =(shell . t)=)
  ,#+begin_src elisp :tangle fails.el :shebang "#!/usr/bin/env -S emacs 
--script"
    (org-babel-do-load-languages
     'org-babel-load-languages
     '((emacs-lisp . t)))
    (org-babel-tangle-file "maybe.org")
  ,#+end_src
#+end_src


Emacs  : GNU Emacs 30.0.50 (build 14, x86_64-pc-linux-gnu, GTK+ Version 
3.24.38, cairo version 1.18.0)
 of 2023-11-20
Package: Org mode version 9.7-pre (release_9.6.26-1368-g1ae978 @ 
/home/minshall/.emacs.d/straight/build/org/)