Support font driver supersession

* src/font.c (font_update_drivers): If argument NEW_DRIVERS is t, then don't
use superseded drivers.
(syms_of_font) <Qfont_driver_superseded_by>: New DEFSYM.

* src/ftfont.c (syms_of_ftfont) [HAVE_HARFBUZZ]:
* src/ftcrfont.c (syms_of_ftcrfont) [HAVE_HARFBUZZ]:
* src/w32font.c (syms_of_w32font) [HAVE_HARFBUZZ]:
* src/xftfont.c (syms_of_xftfont) [HAVE_HARFBUZZ]: Make Harfbuzz variants
supersede non-Harfbuzz ones.

* src/w32fns.c (Fx_create_frame, w32_create_tip_frame): Remove font backend
determination code.

6 files changed, 38 insertions, 82 deletions

diff --git a/src/font.c b/src/font.c
index 5705758b99f..964028ccf00 100644
--- a/src/font.c
+++ b/src/font.c
@@ -3518,7 +3518,10 @@ free_font_driver_list (struct frame *f)
 
 /* Make the frame F use font backends listed in NEW_DRIVERS (list of
    symbols, e.g. xft, x).  If NEW_DRIVERS is t, make F use all
-   available font drivers.  If NEW_DRIVERS is nil, finalize all drivers.
+   available font drivers that are not superseded by another driver.
+   (A font driver SYMBOL is superseded by the driver specified by
+   SYMBOL's 'font-driver-superseded-by property if it is a non-nil
+   symbol.)  If NEW_DRIVERS is nil, finalize all drivers.
 
    A caller must free all realized faces if any in advance.  The
    return value is a list of font backends actually made used on
@@ -3527,16 +3530,33 @@ free_font_driver_list (struct frame *f)
 Lisp_Object
 font_update_drivers (struct frame *f, Lisp_Object new_drivers)
 {
-  Lisp_Object active_drivers = Qnil;
+  Lisp_Object active_drivers = Qnil, default_drivers = Qnil;
   struct font_driver_list *list;
 
+  /* Collect all unsuperseded driver symbols into
+     `default_drivers'.  */
+  Lisp_Object all_drivers = Qnil;
+  for (list = f->font_driver_list; list; list = list->next)
+    all_drivers = Fcons (list->driver->type, all_drivers);
+  for (Lisp_Object rest = all_drivers; CONSP (rest); rest = XCDR (rest))
+    {
+      Lisp_Object superseded_by
+        = Fget (XCAR (rest), Qfont_driver_superseded_by);
+
+      if (NILP (superseded_by)
+          || NILP (Fmemq (superseded_by, all_drivers)))
+        default_drivers = Fcons (XCAR (rest), default_drivers);
+    }
+
+  if (EQ (new_drivers, Qt))
+    new_drivers = default_drivers;
+
   /* At first, turn off non-requested drivers, and turn on requested
      drivers.  */
   for (list = f->font_driver_list; list; list = list->next)
     {
       struct font_driver const *driver = list->driver;
-      if ((EQ (new_drivers, Qt) || ! NILP (Fmemq (driver->type, new_drivers)))
-          != list->on)
+      if ((! NILP (Fmemq (driver->type, new_drivers))) != list->on)
         {
           if (list->on)
             {
@@ -3559,8 +3579,7 @@ font_update_drivers (struct frame *f, Lisp_Object new_drivers)
 
   if (NILP (new_drivers))
     return Qnil;
-
-  if (! EQ (new_drivers, Qt))
+  else
     {
       /* Re-order the driver list according to new_drivers.  */
       struct font_driver_list **list_table, **next;
@@ -3599,6 +3618,8 @@ font_update_drivers (struct frame *f, Lisp_Object new_drivers)
             {
               struct font_driver const *driver = list->driver;
               eassert (! list->on);
+              if (NILP (Fmemq (driver->type, default_drivers)))
+                continue;
               if (! driver->start_for_frame
                   || driver->start_for_frame (f) == 0)
                 {
@@ -5359,6 +5380,8 @@ syms_of_font (void)
   DEFSYM (QL2R, "L2R");
   DEFSYM (QR2L, "R2L");
 
+  DEFSYM (Qfont_driver_superseded_by, "font-driver-superseded-by");
+
   scratch_font_spec = Ffont_spec (0, NULL);
   staticpro (&scratch_font_spec);
   scratch_font_prefer = Ffont_spec (0, NULL);
diff --git a/src/ftcrfont.c b/src/ftcrfont.c
index 1c28a7ceb67..a019fe8294a 100644
--- a/src/ftcrfont.c
+++ b/src/ftcrfont.c
@@ -599,6 +599,7 @@ syms_of_ftcrfont (void)
   DEFSYM (Qftcr, "ftcr");
 #ifdef HAVE_HARFBUZZ
   DEFSYM (Qftcrhb, "ftcrhb");
+  Fput (Qftcr, Qfont_driver_superseded_by, Qftcrhb);
 #endif  /* HAVE_HARFBUZZ */
   pdumper_do_now_and_after_load (syms_of_ftcrfont_for_pdumper);
 }
diff --git a/src/ftfont.c b/src/ftfont.c
index efd0fcbd8c0..4380a48d8dc 100644
--- a/src/ftfont.c
+++ b/src/ftfont.c
@@ -2970,6 +2970,7 @@ syms_of_ftfont (void)
   DEFSYM (Qfreetype, "freetype");
 #ifdef HAVE_HARFBUZZ
   DEFSYM (Qfreetypehb, "freetypehb");
+  Fput (Qfreetype, Qfont_driver_superseded_by, Qfreetypehb);
 #endif  /* HAVE_HARFBUZZ */
 
   /* Fontconfig's generic families and their aliases.  */
diff --git a/src/w32fns.c b/src/w32fns.c
index 8ebfc119521..83830c616e5 100644
--- a/src/w32fns.c
+++ b/src/w32fns.c
@@ -5844,46 +5844,10 @@ DEFUN ("x-create-frame", Fx_create_frame, Sx_create_frame,
       specbind (Qx_resource_name, name);
     }
 
-  bool register_uniscribe = uniscribe_available;
 #ifdef HAVE_HARFBUZZ
-  /* Register Uniscribe only if HarfBuzz is not available or if
-     explicitly requested.  If Uniscribe is registered, register
-     HarfBuzz only if explicitly requested.  */
-  bool register_harfbuzz = harfbuzz_available;
-  if (register_harfbuzz)
-    register_uniscribe = false;
-  Lisp_Object dflt_backends
-    = gui_display_get_arg (dpyinfo, parameters, Qfont_backend,
-                           "fontBackend", "FontBackend", RES_TYPE_STRING);
-  if (!EQ (dflt_backends, Qunbound))
-    {
-      bool harfbuzz_requested = false, uniscribe_requested = false;
-      if (CONSP (dflt_backends))
-        {
-          if (!NILP (Fmemq (Quniscribe, dflt_backends)))
-            uniscribe_requested = true;
-          if (!NILP (Fmemq (Qharfbuzz, dflt_backends)))
-            harfbuzz_requested = true;
-        }
-      else if (STRINGP (dflt_backends))
-        {
-          if (strcmp (SSDATA (dflt_backends), "uniscribe") == 0)
-            uniscribe_requested = true;
-          else if (strcmp (SSDATA (dflt_backends), "harfbuzz") == 0)
-            harfbuzz_requested = true;
-        }
-      if (uniscribe_requested)
-        {
-          register_uniscribe = uniscribe_available;
-          if (!harfbuzz_requested)
-            register_harfbuzz = false;
-        }
-    }
-  if (register_harfbuzz)
-    register_font_driver (&harfbuzz_font_driver, f);
+  register_font_driver (&harfbuzz_font_driver, f);
 #endif
-  if (register_uniscribe)
-    register_font_driver (&uniscribe_font_driver, f);
+  register_font_driver (&uniscribe_font_driver, f);
   register_font_driver (&w32font_driver, f);
 
   gui_default_parameter (f, parameters, Qfont_backend, Qnil,
@@ -6935,46 +6899,10 @@ w32_create_tip_frame (struct w32_display_info *dpyinfo, Lisp_Object parms)
       specbind (Qx_resource_name, name);
     }
 
-  bool register_uniscribe = uniscribe_available;
 #ifdef HAVE_HARFBUZZ
-  /* Register Uniscribe only if HarfBuzz is not available or if
-     explicitly requested.  If Uniscribe is registered, register
-     HarfBuzz only if explicitly requested.  */
-  bool register_harfbuzz = harfbuzz_available;
-  if (register_harfbuzz)
-    register_uniscribe = false;
-  Lisp_Object dflt_backends
-    = gui_display_get_arg (dpyinfo, parms, Qfont_backend,
-                           "fontBackend", "FontBackend", RES_TYPE_STRING);
-  if (!EQ (dflt_backends, Qunbound))
-    {
-      bool harfbuzz_requested = false, uniscribe_requested = false;
-      if (CONSP (dflt_backends))
-        {
-          if (!NILP (Fmemq (Quniscribe, dflt_backends)))
-            uniscribe_requested = true;
-          if (!NILP (Fmemq (Qharfbuzz, dflt_backends)))
-            harfbuzz_requested = true;
-        }
-      else if (STRINGP (dflt_backends))
-        {
-          if (strcmp (SSDATA (dflt_backends), "uniscribe") == 0)
-            uniscribe_requested = true;
-          else if (strcmp (SSDATA (dflt_backends), "harfbuzz") == 0)
-            harfbuzz_requested = true;
-        }
-      if (uniscribe_requested)
-        {
-          register_uniscribe = uniscribe_available;
-          if (!harfbuzz_requested)
-            register_harfbuzz = false;
-        }
-    }
-  if (register_harfbuzz)
-    register_font_driver (&harfbuzz_font_driver, f);
+  register_font_driver (&harfbuzz_font_driver, f);
 #endif
-  if (register_uniscribe)
-    register_font_driver (&uniscribe_font_driver, f);
+  register_font_driver (&uniscribe_font_driver, f);
   register_font_driver (&w32font_driver, f);
 
   gui_default_parameter (f, parms, Qfont_backend, Qnil,
diff --git a/src/w32font.c b/src/w32font.c
index 14d49b24d9b..5d54f048fb5 100644
--- a/src/w32font.c
+++ b/src/w32font.c
@@ -2821,6 +2821,8 @@ versions of Windows) characters.  */);
 
   defsubr (&Sx_select_font);
 
+  Fput (Quniscribe, Qfont_driver_superseded_by, Qharfbuzz);
+
   pdumper_do_now_and_after_load (syms_of_w32font_for_pdumper);
 }
 
diff --git a/src/xftfont.c b/src/xftfont.c
index 04cda12fb52..4d2a5f520e0 100644
--- a/src/xftfont.c
+++ b/src/xftfont.c
@@ -679,6 +679,7 @@ syms_of_xftfont (void)
   DEFSYM (Qxft, "xft");
 #ifdef HAVE_HARFBUZZ
   DEFSYM (Qxfthb, "xfthb");
+  Fput (Qxft, Qfont_driver_superseded_by, Qxfthb);
 #endif  /* HAVE_HARFBUZZ */
 
   DEFVAR_BOOL ("xft-font-ascent-descent-override",