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);
       }