macOS: Fix a character that cannot be rendered in any font causing font fallback for all subsequent characters that cannot be rendered in the main font to fail

Fixes #799

docs/changelog.rst

@@ -6,6 +6,10 @@ Changelog
 0.12.1 [future]
 ------------------------------
 
+- macOS: Fix a character that cannot be rendered in any font causing
+  font fallback for all subsequent characters that cannot be rendered in the
+  main font to fail (:iss:`799`)
+
 - Allow mapping shortcuts using the raw key code from the OS (:iss:`848`)
 
 - Allow mapping of individual keypresses without modifiers as shortcuts

kitty/core_text.m

@@ -171,6 +171,14 @@ find_substitute_face(CFStringRef str, CTFontRef old_font) {
         else start++;
         if (new_font == NULL) { PyErr_SetString(PyExc_ValueError, "Failed to find fallback CTFont"); return NULL; }
         if (new_font == old_font) { CFRelease(new_font); continue; }
+        CFStringRef name = CTFontCopyPostScriptName(new_font);
+        CFComparisonResult cr = CFStringCompare(name, CFSTR("LastResort"), 0);
+        CFRelease(name);
+        if (cr == kCFCompareEqualTo) {
+            CFRelease(new_font);
+            PyErr_SetString(PyExc_ValueError, "Failed to find fallback CTFont other than the LastResort font");
+            return NULL;
+        }
         return new_font;
     }
     PyErr_SetString(PyExc_ValueError, "CoreText returned the same font as a fallback font");

kitty/fonts.c

@@ -1280,6 +1280,7 @@ get_fallback_font(PyObject UNUSED *self, PyObject *args) {
     if (italic) gpu_cell.attrs |= 1 << ITALIC_SHIFT;
     FontGroup *fg = font_groups;
     ssize_t ans = fallback_font(fg, &cpu_cell, &gpu_cell);
+    if (ans == MISSING_FONT) { PyErr_SetString(PyExc_ValueError, "No fallback font found"); return NULL; }
     if (ans < 0) { PyErr_SetString(PyExc_ValueError, "Too many fallback fonts"); return NULL; }
     return fg->fonts[ans].face;
 }

kitty_tests/fonts.py

@@ -2,10 +2,13 @@
 # vim:fileencoding=utf-8
 # License: GPL v3 Copyright: 2017, Kovid Goyal <kovid at kovidgoyal.net>
 
+import sys
+import unittest
+
 from kitty.constants import is_macos
 from kitty.fast_data_types import (
-    DECAWM, sprite_map_set_layout, sprite_map_set_limits, test_render_line,
+    DECAWM, get_fallback_font, sprite_map_set_layout, sprite_map_set_limits,
-    test_sprite_position_for, wcwidth
+    test_render_line, test_sprite_position_for, wcwidth
 )
 from kitty.fonts.box_drawing import box_chars
 from kitty.fonts.render import render_string, setup_for_testing, shape_string
@@ -106,3 +109,16 @@ class Rendering(BaseTest):
         s.draw('\ufe0f')
         self.ae((s.cursor.x, s.cursor.y), (2, 4))
         self.ae(str(s.line(s.cursor.y)), '\u2716\ufe0f')
+
+    @unittest.skipUnless(is_macos, 'Only macOS has a Last Resort font')
+    def test_fallback_font_not_last_resort(self):
+        # Ensure that the LastResort font is not reported as a fallback font on
+        # macOS. See https://github.com/kovidgoyal/kitty/issues/799
+        from io import StringIO
+        orig, buf = sys.stderr, StringIO()
+        sys.stderr = buf
+        try:
+            self.assertRaises(ValueError, get_fallback_font, '\U0010FFFF', False, False)
+        finally:
+            sys.stderr = orig
+        self.assertIn('LastResort', buf.getvalue())