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

[Jim Porter]

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).