bug#59201: `fill-paragraph' works differently depending on whether Emacs is run in batch mode or not

[Eli Zaretskii]

tags 59201 notabug
thanks

> From: Paul Pogonyshev <pogonyshev@gmail.com>
> Date: Fri, 11 Nov 2022 20:40:25 +0100
> 
>     $ emacs --batch --eval "(princ (with-temp-buffer (insert \"Lorem ipsum 
> dolor sit amet, consectetur
> adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna 
> aliqua...\n\") (set-mark 1)
> (fill-paragraph nil t) (buffer-string)) 'external-debugging-output)"
> 
> Prints the text in one line, i.e. `fill-paragraph' appears to have done 
> nothing.
> 
> If you replace `--batch' in the command line with `-Q' without changing 
> anything else:
> 
>     $ emacs -Q --eval "(princ (with-temp-buffer (insert \"Lorem ipsum dolor 
> sit amet, consectetur adipiscing
> elit, sed do eiusmod tempor incididunt ut labore et dolore magna 
> aliqua...\n\") (set-mark 1) (fill-paragraph nil
> t) (buffer-string)) 'external-debugging-output)"
> 
> the output in the console becomes formatted in two lines, i.e. 
> `fill-paragraph' does its work.
> 
> In both cases the code is exactly the same and the function is called 
> non-interactively. The only difference,
> as far as I see, is the batch mode.

Your program has a bug: it doesn't make sure transient-mark-mode is
turned ON, and it doesn't move point after inserting the text into the
inserted text.  Thus, point is left at EOB, where there's no paragraph
that fill-paragraph can work on.  The doc string of fill-paragraph
says:

  Fill paragraph at or after point.
  [...]
  The REGION argument is non-nil if called interactively; in that
  case, if Transient Mark mode is enabled and the mark is active,
  call ‘fill-region’ to fill each of the paragraphs in the active
  region, instead of just filling the current paragraph.

If you modify the program as follows:

  $ emacs --batch --eval "(princ (with-temp-buffer (insert \"Lorem ipsum dolor 
sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut 
labore et dolore magna aliqua...\n\") (goto-char (point-min)) (set-mark 1) 
(fill-paragraph nil t) (buffer-string)) 'external-debugging-output)"

then the text will be filled in --batch mode as well.

Transient Mark mode is turned on by default only in interactive
sessions, and if you meant fill-region to work on the active region,
then the solution is to modify the program like this:

  $ emacs --batch --eval "(princ (with-temp-buffer (insert \"Lorem ipsum dolor 
sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut 
labore et dolore magna aliqua...\n\") (transient-mark-mode 1) (set-mark 1) 
(fill-paragraph nil t) (buffer-string)) 'external-debugging-output)"