bug#71415: 30.0.50; vtable-current-object off by one

[john muhl <jm@pub.pink>]

Adam Porter <adam@alphapapa.net> writes:

> Hi Eli, John,
>
> My first thought i> Hi Eli, John,
>
> On 6/15/24 03:40, Eli Zaretskii wrote:
>>> From: john muhl <jm@pub.pink>
>>> Date: Fri, 07 Jun 2024 09:15:07 -0500
>>>
>>> After updating an object, ‘vtable-current-object’ in the displayer
>>> and formatter functions points to the object after the one that
>>> was updated. Running the following:
>>>
>>>    (with-current-buffer (get-buffer-create "vtable-demo")
>>>      (require 'vtable)
>>>      (erase-buffer)
>>>      (make-vtable
>>>       :columns '("Index" "Number")
>>>       :objects `((1 ,(random)) (2 ,(random)) (3 ,(random)))
>>>       :displayer (lambda (value &rest _)
>>>                    ;; vtable-current-object is off by one; e.g.
>>>                    ;; update the first row and v-c-o will be the second row;
>>>                    ;; update the final row and v-c-o will be nil.
>>>                    (message "%s %s" value (vtable-current-object))
>>>                    (propertize (number-to-string value) 'face 'vtable)))
>>>      (let* ((table (vtable-current-table))
>>>             (first (nth 0 (vtable-objects table)))
>>>             (other (nth 1 (vtable-objects table)))
>>>             (final (nth 2 (vtable-objects table))))
>>>        (setf (nth 1 first) 1)
>>>        (setf (nth 1 other) 2)
>>>        (setf (nth 1 final) 3)
>>>        (vtable-update-object table first)
>>>        (vtable-update-object table other)
>>>        (vtable-update-object table final)))
>>>
>>> I see in the messages buffer:
>>>
>>>    1 (2 2) [2 times]
>>>    2 (3 3) [2 times]
>>>    3 nil [2 times]
>>>
>>> when I expect to see:
>>>
>>>    1 (1 1) [2 times]
>>>    2 (2 2) [2 times]
>>>    3 (3 3) [2 times]
>> Adam, any comments or suggestions?
>
s that this code had some changes recently, and the
> bug report indicates that the version of Emacs used was compiled from
> revision f2309557ff366a81183859bda88b9a0012a5d9d6, but I can't find
> that in emacs.git/master.  So I'd suggest trying to reproduce it with
> a build of Emacs from master after the related changes.
>
> Secondly, it seems likely to be a violation of intent to call
> `vtable-current-object' inside the table's displayer function.  I
> don't think that function is intended to be used within the process of
> updating a vtable.  It's intended to be used from outside vtable code
> to find what object's representation point is on in a vtable.
>
> So if it works correctly in that case but not in this apparently
> contrived one, I doubt that this would indicate a bug.  And in that
> case, I'd suggest that we be told what the user is actually trying to
> accomplish, in case there's a real bug elsewhere that was
> "rabbit-trailed" into a misunderstanding here.  :)
>
> Thanks,
> Adam

Right. It’s my mistake. Sorry for the bother.