Fix synchronization error when matching font with empty family

We currently match font family names in cross-platform code, before
asking the system to load it. In the case of an empty family name, we
will return the first font in the list which matches the other
conditions.

After that, though, we would pass the empty family name to the
platform font database, and some old platform-specific matching
was invoked. In the case of Windows, we would default to
MS Sans Serif, which is the old Windows 1.0 raster font and which
is probably used by Qt exclusively in the world at this point.

The problem in the end was that the font info would be out of sync.
If the font matched by Qt happened to be smoothly scalable, then
we would treat MS Sans Serif as smoothly scalable, giving us
random glyphs when displaying text in Qt Quick. We would also
overwrite the family name with the one matched by Qt, so even
when MS Sans Serif was used to render the text, we would report
that it was something else.

For empty font families, we therefore pass the family we have
actually matched into the platform database. This will have
the side effect of trusting the font matching in Qt, but only
for the rare case where someone actually explicitly sets the
font family to the empty string.

Note that the random glyph bug that triggered the investigation is not
happening anymore after 1eedfd0e66c7afd682f682a659cda2942485cd08
because we will detect the glyph outlines as unreliable in the
MS Sans Serif font now instead of trusting the smoothly scalable
flag. But there would still be a mismatch between the font used
for rendering and the matched font.

Task-number: QTBUG-63147
Change-Id: I4a09ffddf9f8a0fabe7738e2944b6d874e4728f0
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 9 insertions, 1 deletions

diff --git a/src/gui/text/qfontdatabase.cpp b/src/gui/text/qfontdatabase.cpp
index 288352b370..afbf783c3a 100644
--- a/src/gui/text/qfontdatabase.cpp
+++ b/src/gui/text/qfontdatabase.cpp
@@ -2685,7 +2685,15 @@ QFontEngine *QFontDatabase::findFont(const QFontDef &request, int script)
         index = match(multi ? QChar::Script_Common : script, request, family_name, foundry_name, &desc, blackListed);
     }
     if (index >= 0) {
-        engine = loadEngine(script, request, desc.family, desc.foundry, desc.style, desc.size);
+        QFontDef fontDef = request;
+
+        // Don't pass empty family names to the platform font database, since it will then invoke its own matching
+        // and we will be out of sync with the matched font.
+        if (fontDef.family.isEmpty())
+            fontDef.family = desc.family->name;
+
+        engine = loadEngine(script, fontDef, desc.family, desc.foundry, desc.style, desc.size);
+
         if (engine)
             initFontDef(desc, request, &engine->fontDef, multi);
         else