Fix redisplay performance problems with some fonts

* src/font.c (font_list_entities): Revert part of the changes
introduced on Apr 2, 2014 to fix bug#17125.  It turns out having
zero_vector in the font-cache is an important indication that
cannot be removed.  (Bug#21028)

1 files changed, 16 insertions, 10 deletions

diff --git a/src/font.c b/src/font.c
index a929509752c..dd6191b2b1f 100644
--- a/src/font.c
+++ b/src/font.c
@@ -2777,21 +2777,27 @@ font_list_entities (struct frame *f, Lisp_Object spec)
           val = XCDR (val);
         else
           {
-            val = driver_list->driver->list (f, scratch_font_spec);
-            if (!NILP (val))
-              {
-                Lisp_Object copy = copy_font_spec (scratch_font_spec);
+            Lisp_Object copy;
 
-                val = Fvconcat (1, &val);
-                ASET (copy, FONT_TYPE_INDEX, driver_list->driver->type);
-                XSETCDR (cache, Fcons (Fcons (copy, val), XCDR (cache)));
-              }
+            val = driver_list->driver->list (f, scratch_font_spec);
+            /* We put zero_vector in the font-cache to indicate that
+               no fonts matching SPEC were found on the system.
+               Failure to have this indication in the font cache can
+               cause severe performance degradation in some rare
+               cases, see bug#21028.  */
+            if (NILP (val))
+              val = zero_vector;
+            else
+              val = Fvconcat (1, &val);
+            copy = copy_font_spec (scratch_font_spec);
+            ASET (copy, FONT_TYPE_INDEX, driver_list->driver->type);
+            XSETCDR (cache, Fcons (Fcons (copy, val), XCDR (cache)));
           }
-        if (VECTORP (val) && ASIZE (val) > 0
+        if (ASIZE (val) > 0
             && (need_filtering
                 || ! NILP (Vface_ignored_fonts)))
           val = font_delete_unmatched (val, need_filtering ? spec : Qnil, size);
-        if (VECTORP (val) && ASIZE (val) > 0)
+        if (ASIZE (val) > 0)
           list = Fcons (val, list);
       }