bug#23386: Segfault when messing with font-backend

[Dmitry Antipov <dmantipov@yandex.ru>]

On 07/09/2016 10:11 PM, npostavs@users.sourceforge.net wrote:

> Here is a patch that attempts to fix the issue by resetting the font
> after the backend is changed.  It does let Emacs successfully open the
> frame with the new font (no funny box characters in the modeline), but
> I'm not sure if it's the best way of marking the font object invalid.

IMHO the original trick (request to drop font backend when there is a font
opened by using this backend) is practically meaningless, so why just not
prohibit it explicitly? For example, with:

diff --git a/src/frame.c b/src/frame.c
index 22143ab..d8f89ed 100644
--- a/src/frame.c
+++ b/src/frame.c
@@ -3708,7 +3708,19 @@ x_set_font_backend (struct frame *f, Lisp_Object new_value, Lisp_Object old_valu
      return;

    if (FRAME_FONT (f))
-    free_all_realized_faces (Qnil);
+    {
+      if (!NILP (new_value))
+	{
+	  Lisp_Object backend = FRAME_FONT (f)->props[FONT_TYPE_INDEX];
+
+	  /* Do not release the backend used by F's default font.  */
+	  if (NILP (Fmemq (backend, new_value)))
+	    error ("Font backend '%s' is in use by font '%s'",
+		   SDATA (SYMBOL_NAME (backend)),
+		   SDATA (FRAME_FONT (f)->props[FONT_NAME_INDEX]));
+	}
+      free_all_realized_faces (Qnil);
+    }

    new_value = font_update_drivers (f, NILP (new_value) ? Qt : new_value);
    if (NILP (new_value))

Dmitry