markdown-get-lang-mode would accidentally enable treesit major modes globally

[blahgeek]

In emacs 29, when markdown-get-lang-mode selects a python-ts-mode for python, it would also enable it globally for all *.py files.

Steps to produce:

• Use emacs 29 with treesit support and python treesit library available
• Open some markdown file with python code block
• Open another xxx.py file

Expected behavior: The xxx.py file is opened with python-mode major mode (which is the default)

Actual behavior: The xxx.py file is opened with python-ts-mode.

The reason is that, in emacs 29, running python-ts-mode even for single buffer would modify auto-mode-alist globally: https://github.com/emacs-mirror/emacs/blob/ba2c76fa2bc3aabfda7d1d09cc5148f3f9d8e08e/lisp/progmodes/python.el#L6751

This is also true for almost all other treesit based major modes.

(I think this is a bug in the emacs core. I would file a bug report there too.)

[blahgeek]

Update: I reported this to emacs and got a reply: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=63463 so apparently that behavior is intended and markdown-mode should not call treesit-based major modes.

[syohex]

How about f40808f ?
Load the language mode only if user sets it in auto-mode-alist.

[blahgeek]

Look good, thank you for your work!