bug#38812: 28.0.50; Custom: Problem with reverting some session's customizations

[Eli Zaretskii <eliz@gnu.org>]

> From: Mauro Aranda <maurooaranda@gmail.com>
> Date: Tue, 31 Dec 2019 14:21:09 -0300
> Cc: 38812@debbugs.gnu.org
> 
> > And I have a question: isn't it better not to use setcar in
> > custom-push-theme instead?
> 
> I thought of doing that, and use setf with alist-get to make the change
> instead.  But I think we'll be better off if we avoid sharing the cons
> cell inadvertedly, since that is prone to have bugs like this one.

And I actually think that a problem should be fixed where it is
caused.  There's nothing wrong per se with sharing portions of Lisp
data structures.

> Alternatively, we could create the list in custom-theme-recalc-variable,
> to accomplish the same thing without changing the return value of
> custom-variable-theme-value.  In that case, I think it would be
> convenient to change the doc string of custom-variable-theme-value, to
> say it returns some cdr.
> 
> To me, either the patch I posted (with an additional explanatory
> comment, of course) or the latter option sound better, but I won't argue
> too much if you think otherwise.

My alternative patch is below.  WDYT?

diff --git a/lisp/custom.el b/lisp/custom.el
index 26bdaae2c2..7ed85b22e8 100644
--- a/lisp/custom.el
+++ b/lisp/custom.el
@@ -886,7 +886,10 @@ custom-push-theme
 	(put theme 'theme-settings
 	     (cons (list prop symbol theme value)
 		   (delq res theme-settings)))
-	(setcar (cdr setting) value)))
+        ;; It's tempting to use setcar here, but that could
+        ;; inadvertently modify other properties in SYMBOL's proplist,
+        ;; if those just happen to share elements with the value of PROP.
+        (put symbol prop (cons (list theme value) (delq setting old)))))
      ;; Add a new setting:
      (t
       (when (custom--should-apply-setting theme)