[HarfBuzz] [PATCH] Avoid font fallback with CoreText shaper

[Khaled Hosny]

CoreText does automatic font fallback (AKA "cascading") for  characters
not supported by the requested font, and provides no way to turn it off,
so detect if the returned run uses a font other than the requested one
and fill in the buffer with .notdef glyphs instead of random indices
glyph from a different font.
---
 src/hb-coretext.cc | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/src/hb-coretext.cc b/src/hb-coretext.cc
index ba80136..398b866 100644
--- a/src/hb-coretext.cc
+++ b/src/hb-coretext.cc
@@ -685,6 +685,33 @@ _hb_coretext_shape (hb_shape_plan_t    *shape_plan,
 
 #undef ALLOCATE_ARRAY
 
+    /* CoreText does automatic font fallback (AKA "cascading") for  characters
+     * not supported by the requested font, and provides no way to turn it off,
+     * so we detect if the returned run uses a font other than the requested
+     * one and fill in the buffer with .notdef glyphs instead of random glyph
+     * indices from a different font.
+     */
+    CFDictionaryRef attributes = CTRunGetAttributes(run);
+    if (!CFEqual(CFDictionaryGetValue(attributes, kCTFontAttributeName), font_data->ct_font)) {
+        for (unsigned int j = 0; j < num_glyphs; j++) {
+            CGGlyph notdef = 0;
+            double advance = CTFontGetAdvancesForGlyphs(font_data->ct_font, kCTFontHorizontalOrientation, &notdef, NULL, 1);
+
+            hb_glyph_info_t *info = &buffer->info[buffer->len];
+
+            info->codepoint = notdef;
+            info->cluster = string_indices[0] + j;
+
+            info->mask = advance;
+            info->var1.u32 = 0;
+            info->var2.u32 = 0;
+
+            buffer->len++;
+        }
+        continue;
+    }
+
     double run_width = CTRunGetTypographicBounds (run, range_all, NULL, NULL, NULL);
 
     for (unsigned int j = 0; j < num_glyphs; j++) {
-- 
1.8.4.1