Re: Edebug corrupting point in buffers.

[Alan Mackenzie]

Hello, Eli.

On Tue, Nov 01, 2022 at 08:45:42 +0200, Eli Zaretskii wrote:
> > Date: Mon, 31 Oct 2022 21:25:08 +0000
> > From: Alan Mackenzie <acm@muc.de>

> > +(defcustom edebug-save-buffer-points nil
> > +  "If non-nil save and restore the buffer points in some buffers.
> > +
> > +Saving and restoring the buffer point in a buffer is needed if you
> > +are debugging code which sets point in that buffer, particularly if
> > +there is also a window displaying that buffer.  Otherwise the buffer
> > +point (being used by the program) will get overwritten by the
> > +window point.
> > +
> > +If the value is a list of buffer names (recommended), only those
> > +buffers will have their buffer points restored.  Otherwise, t means
> > +restore all buffers\\=' points, and nil means none.

> If we indeed need such an option, why shouldn't it be Edebug's
> business to automatically keep point in all buffers that are displayed
> in some window?  It doesn't strike me as the best UI to burden the
> user with that task.

It would be intolerable for users.  Say during an edebug session, the
user makes some notes in buffer my-notes.txt.  Execute an instruction,
then go back to my-notes.txt.  Point has been "restored" to before the
new notes.  That would happen in every buffer.

> And like Stefan, I think we still need to understand better what
> exactly happens here and why.  I don't think I understood that from
> your original description.

Sorry, I had some difficulty getting from my original problem to a
reproducible test case.  That's here:

With test-edebug.el being
#########################################################################
(defun test-edebug ()
  (let ((A "*scratch*") (B "emacs.README"))
    (set-buffer A)
    (set-buffer B)
    (goto-char (point-max))
    (insert "(2022-11-01)\n")
    ;; B's buffer-point is at point-max.

    (set-buffer A)
    (set-buffer B)
    ;; B's buffer-point is no longer at point-max.
    (insert "(2022-11-01)a\n")))
#########################################################################
,
(i) Emacs -Q.
(ii) On a single frame, arrange buffers *scratch*, test-edebug.el, and
  some other substantial buffer, that I call emacs.README.
(iii) Put point in emacs.README somewhere other than point-max.
(iv) Instrument test-edebug for edebug with C-u C-M-x.
(v) M-: (test-edebug).
(vi) Step through test-edebug using the space key.
(vii) Note that the second text insertion happens where point was in the
  window, not at point-max.  This is the bug.

My patch from yesterday evening, though not in its final form, appears
to solve the bug, providing the user does C-x X F in emacs.README.

-- 
Alan Mackenzie (Nuremberg, Germany).