bug#61735: 29.0.50; String object in margin not associated correctly with buffer text

[Eli Zaretskii <eliz@gnu.org>]

> From: dalanicolai <dalanicolai@gmail.com>
> Date: Thu, 23 Feb 2023 17:32:50 +0100
> 
> (defun baleen-render (data)
>   (pop-to-buffer (get-buffer-create "*baleen*"))
>   (set-window-margins nil 5)
>   (dolist (page data)
>     (dolist (match (cdr page))
>       (let ((o (make-overlay (point)
>                              (progn (insert match)
>                                     (point)))))
>         (let ((s " "))
>           (put-text-property 0 1
>                              'display `((margin left-margin) ,(format " %d" (car page)))
>                              s)
>           (overlay-put o 'before-string s)))
>       (insert "\n"))))
> 
> (baleen-render '((1 "test1" "test2") (2 "test3")))
> ```
> 
> Here, for every 'match' in a 'page' I am creating a new string 's'
> to which I add the margin display property to 'associate' it
> with some buffer text by using it as the value for its before-string
> property.
> However, although each 's' should get a different display property value
> via (format " %d" (car page)), all margin entries end up showing the
> same value of 2 (while the first two lines should show page number 1).
> 
> To reproduce the error, simply evaluate the code above. Using edebug on
> 'baleen-render' it can be seen that the code seems correct, i.e. (car
> page) correctly returns the correct page number.
> 
> It seems that although I am creating a different string object on each
> iteration, somehow the object put in the marging seems to be always the
> same.

Is it really true that you create a different string object every
time?  Add a copy-sequence call there, like this:

        (let ((s (copy-sequence " ")))

and the code does what you expect.

Stefan, am I missing something here?