Stop querying for fonts as soon as a match is found

Scanning through fonts can be very slow, especially with the 'x' font
backend, and the result is almost always not used.  Stop looking for a
font as soon as one is found rather than scanning all the backends.

* src/font.c (font_list_entities): Stop scanning through the font
backends as soon as we find a match unless
'query-all-font-backends is set (Bug#43177).
(syms_of_font): New variable 'query-all-font-backends', default
false.

1 files changed, 14 insertions, 1 deletions

diff --git a/src/font.c b/src/font.c
index 2786a772dc3..779b8520962 100644
--- a/src/font.c
+++ b/src/font.c
@@ -2810,7 +2810,13 @@ font_list_entities (struct frame *f, Lisp_Object spec)
                 || ! NILP (Vface_ignored_fonts)))
           val = font_delete_unmatched (val, need_filtering ? spec : Qnil, size);
         if (ASIZE (val) > 0)
-          list = Fcons (val, list);
+          {
+            list = Fcons (val, list);
+            /* Querying further backends can be very slow, so we only do
+               it if the user has explicitly requested it (Bug#43177).  */
+            if (query_all_font_backends == false)
+              break;
+          }
       }
 
   list = Fnreverse (list);
@@ -5527,6 +5533,13 @@ Non-nil means don't query fontconfig for color fonts, since they often
 cause Xft crashes.  Only has an effect in Xft builds.  */);
   xft_ignore_color_fonts = true;
 
+  DEFVAR_BOOL ("query-all-font-backends", query_all_font_backends,
+               doc: /*
+If non-nil attempt to query all available font backends.
+By default Emacs will stop searching for a matching font at the first
+match.  */);
+  query_all_font_backends = false;
+
 #ifdef HAVE_WINDOW_SYSTEM
 #ifdef HAVE_FREETYPE
   syms_of_ftfont ();