bug#23999: 25.0.95; Emacs hangs in rectangle-mark-mode with numeric prefix argument to `C-f'

[Stephen Berman <stephen.berman@gmx.net>]

Ping!  (This was made a blocking bug for 25.1.)

On Sat, 16 Jul 2016 12:24:41 +0200 Stephen Berman <stephen.berman@gmx.net> wrote:

> On Fri, 15 Jul 2016 19:25:52 -0300 nljlistbox2@gmail.com (N. Jackson) wrote:
>
>> emacs -Q
>> C-<home>	; Go to top left corner of scratch buffer.
>> C-x <SPC>	; Start a rectangular selection.
>> C-3 C-f		; Extend the selection to the right by 3 characters.
>>
>> Expected result:  Emacs creates a rectangular selection one character
>> high and three characters wide.
>>
>> Actual result:  Emacs hangs. (C-g quits.)
>>
>> With debug-on-quit on, I then get:
>>
>>     Debugger entered--Lisp error: (quit)
>>       rectangle--*-char(forward-char 3 backward-char)
>>       rectangle-forward-char(3)
>>       funcall-interactively(rectangle-forward-char 3)
>>       call-interactively(rectangle-forward-char nil nil)
>>       command-execute(rectangle-forward-char)
>>
>> On the other hand, the following recipe works as expected:
>>
>> emacs -Q
>> C-<home>	; Go to top left corner of scratch buffer.
>> C-x <SPC>	; Start a rectangular selection.
>> C-f C-f C-f	; Extend the selection to the right by 3 characters.
>>
>> Note 1:  The exact position in the scratch buffer seems to be unimportant.
>>
>> Note 2:  `C-n' with a numeric prefix argument seems to work fine to extend
>> the rectangular region downwards.
>>
>> Note 3: I see this with the Emacs 25.0.95 pre-test tarball [and with the
>> 25.0.90 pre-test for that matter]. The problem is still present in the
>> Emacs 25 branch as of earlier this afternoon. I do not see the problem
>> with Emacs 24.5.
>
> This is due to using save-excursion in rectangle--*-char.  Replacing
> that with progn makes it work with a numeric prefix argument AFAICT, and
> I have not seen any resulting problems, but maybe Stefan Monnier, who
> made the changes in commit 7e74ad0 that caused this regression, had a
> particular case in mind that the save-excursion was intended to deal
> with.  If not, I think this change should go into emacs-25 (since it is
> a regression from 24.5).
>
> diff --git a/lisp/rect.el b/lisp/rect.el
> index 43621d9..9dd5db4 100644
> --- a/lisp/rect.el
> +++ b/lisp/rect.el
> @@ -644,7 +644,7 @@ rectangle--*-char
>                 (curcol (current-column))
>                 (nextcol
>                  (condition-case nil
> -                    (save-excursion
> +                    (progn
>                        (funcall cmd 1)
>                        (cond
>                         ((> bol (point)) (- curcol 1))
>
> Steve Berman