bug#64439: 28.2; auto-fill-mode gets turned on all over the place

[Jim Porter <jporterbugs@gmail.com>]

On 7/3/2023 11:29 AM, Eli Zaretskii wrote:
> I guess tracking down this problem will be harder, then.  I hope I at
> least gave you some ideas with which you will be able to construct a
> suitable trap and find the culprit.  Good luck!

I got a backtrace when this happened to me. Outside of Emacs, I checked 
out a different Git branch, and then went back to Emacs and started 
typing into "etc/NEWS" (the specific file probably doesn't matter much), 
which triggered the supersession prompt as you'd expect. This was all 
over Tramp/SSH (again, not sure this is relevant).

Here's the debug code I added to trace this:

----------------------------------------

(use-package text-mode
   :preface
   (defun user/turn-on-auto-fill ()
     "Turn on auto-fill, but log a warning if it's not local to a buffer."
     (turn-on-auto-fill)
     (when (default-value 'auto-fill-function)
       (display-warning 'auto-fill :error
                        "Default value of `auto-fill-function' was set!")
       (backtrace)))

   :hook (text-mode . user/turn-on-auto-fill))

----------------------------------------

And here's the backtrace (plus a couple lines immediately preceding):

----------------------------------------

NEWS changed on disk; really edit the buffer? (y, n, r or C-h) r
Tramp: Inserting ‘/sshx:jim@remote:/home/jim/src/emacs/etc/NEWS’...done
   backtrace()
   (progn (display-warning 'auto-fill :error "Default value of 
`auto-fill-function' was set!") (backtrace))
   (if (default-value 'auto-fill-function) (progn (display-warning 
'auto-fill :error "Default value of `auto-fill-function' was set!") 
(backtrace)))
   (when (default-value 'auto-fill-function) (display-warning 'auto-fill 
:error "Default value of `auto-fill-function' was set!") (backtrace))
   user/turn-on-auto-fill()
   run-hooks(change-major-mode-after-body-hook text-mode-hook 
outline-mode-hook)
   apply(run-hooks (change-major-mode-after-body-hook text-mode-hook 
outline-mode-hook))
   run-mode-hooks(outline-mode-hook)
   outline-mode()
   set-auto-mode-0(outline-mode nil)
   set-auto-mode()
   normal-mode(t)
   after-find-file(nil nil t nil nil)
   revert-buffer--default(nil t)
   revert-buffer(nil t)
 
ask-user-about-supersession-threat("/sshx:jim@remote:~/src/emacs/etc/NEWS")
   tramp-handle-lock-file("/sshx:jim@remote:~/src/emacs/etc/NEWS")
   apply(tramp-handle-lock-file "/sshx:jim@remote:~/src/emacs/etc/NEWS")
   tramp-sh-file-name-handler(lock-file 
"/sshx:jim@remote:~/src/emacs/etc/NEWS")
   apply(tramp-sh-file-name-handler lock-file 
"/sshx:jim@remote:~/src/emacs/etc/NEWS")
   tramp-file-name-handler(lock-file 
"/sshx:jim@remote:~/src/emacs/etc/NEWS")
   newline(nil 1)
   funcall-interactively(newline nil 1)
   command-execute(newline)

----------------------------------------

This might not be the most useful backtrace in the world, but hopefully 
it at least confirms one way this can happen. It's rare enough though 
that I can't reproduce it on command. It does lead me to believe that 
there's a bug with automatically-buffer-local variables, since 
'turn-on-auto-fill' should Just Work here (and it typically does).