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

[Eli Zaretskii]

> Date: Sat, 8 Jul 2023 22:57:12 -0700
> Cc: 64439@debbugs.gnu.org
> From: 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).

I must be missing something: why is the above deemed to be a bug?
AFAIU, you asked any text-mode derivative mode to turn on auto-fill,
and this is what happened here: normal-mode called outline-mode, which
turned on auto-fill.  What am I missing?