bug#39865: 28.0.50; Emacs crash

[Robert Pluim]

>>>>> On Fri, 06 Mar 2020 19:53:32 +0200, Eli Zaretskii <eliz@gnu.org> said:

    >> From: Vinicius José Latorre <viniciusjl@gmail.com>
    >> Date: Fri, 6 Mar 2020 14:20:42 -0300
    >> Cc: 39865@debbugs.gnu.org
    >> 
    >> > OK, then the crash is a known problem: you cannot change the font
    >> > backend once the session started, you can only do that in X resources.
    >> > Previously, XFT was your original backend, but now it's HarfBuzz.
    >> 
    >> I think that emacs should be more robust about this and do not crash for 
any bad value.

    Eli> I agree.  It is not a bad value, we just couldn't yet find a way of
    Eli> switching the font-backend.

Eli, would you accept a patch for emacs-27 that signals a user-error
if trying to set a font-backend thatʼs not supported by the running
emacs? Or that ignores it (with a message)?

Robert