undo boundaries

undo boundaries

[Nikolaj Schumacher]

Hello,

according I brief discussion on help-gnu-emacs, this appears to be a bug in
Emacs 22:

(defun test ()
  (interactive)
  (insert "a")
  (undo-boundary)
  (insert "b"))

If I do M-x test, I get:
ab
  ^

Then, if I undo once in Emacs 22, I get:
a
^

What I was expecting (and what happens in Emacs 21):
a
 ^


Also note:

bojohan+news@dd.chalmers.se (Johan Bockgård) wrote:

> The likely suspect is this change:
>
> undo.c
> revision 1.55
> date: 2002-04-04 22:42:56 +0200;  author: monnier;
> (record_point): New fun.
> (record_delete, record_insert): Use it.



regards,
Nikolaj Schumacher

Re: undo boundaries

[martin rudalics]

 > (defun test ()
 >   (interactive)
 >   (insert "a")
 >   (undo-boundary)
 >   (insert "b"))
 >
 > If I do M-x test, I get:
 > ab
 >   ^
 >
 > Then, if I undo once in Emacs 22, I get:
 > a
 > ^
 >
 > What I was expecting (and what happens in Emacs 21):
 > a
 >  ^
 >
 >
 > Also note:
 >
 > bojohan+news@dd.chalmers.se (Johan Bockgård) wrote:
 >
 >
 >>The likely suspect is this change:
 >>
 >>undo.c
 >>revision 1.55
 >>date: 2002-04-04 22:42:56 +0200;  author: monnier;
 >>(record_point): New fun.
 >>(record_delete, record_insert): Use it.

Your example is a bit contrived.  If you do

a M-: (undo-boundary) RET b

subsequent undoing behaves as expected.  Hence, the culprit is probably
`execute-extended-command'.

FWIW, your problem is caused by the (insert "b").  Due to the preceding
(undo-boundary) record_point called by record_insert sets at_boundary to
1, the test last_point_position != pt succeeds, and last_point_position
(the position of `point' before `test' is executed) gets recorded in
undo_list.  However, that position (in your example the position before
the "a") is inherently wrong here since it gets recorded _after_ the
insertion of "a".  Maybe someone wants to debug this to verify my claim.

We could try to convince record_insert and record_delete to set another
variable, say last_point_position_is_valid, to nil, set that variable to
t at the time last_point_position is recorded in the command loop, and
inhibit point recording when last_point_position_is_valid is nil.

BTW, with Emacs 23 you can use

(defun test ()
   (interactive)
   (let ((undo-inhibit-record-point t))
     (insert "a")			
     (undo-boundary)		
     (insert "b")))