bug#20153: 24.4.91; destructive add-face-text-property and string deep copying

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: ohwoeowho@gmail.com,  20153@debbugs.gnu.org
> Date: Wed, 09 Oct 2019 19:45:54 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> Instead I've now changed add_properties (and add_text_properties_1) to
> >> take a bool parameter to say whether they're allowed to be destructive
> >> or not, and make the add-face-text-property call that with false as the
> >> parameter if the object is a string.  This fixes the test case for me
> >> and should hopefully have no measurable performance impact.
> >
> > Isn't that a backward-incompatible change?
> 
> It is, but the previous behaviour was a bug.  If you have a copy of a
> string and modify the copy, the original string shouldn't change.

AFAIU, the string didn't change, only its plist did.  Right?

Please bring this up on emacs-devel, and if no one objects to the
change, we should at least mention it in NEWS.

Thanks.