macOS/iOS: Fix garbled text under some conditions

There seems to be an issue in CoreText which may cause an existing
font descriptor to give unreliable results if it refers to one of
the system theme fonts. Since we do not know all function calls
or events that may trigger this bug, the safe route is to always
create fresh font descriptors when creating fonts for these
descriptors. The impact on performance should be small, as Qt has
its own internal caches.

[ChangeLog][macOS/iOS][Text] Fixed an issue where text using
one of the system theme fonts would under certain circumstances
display random glyphs.

Task-number: QTBUG-63476
Change-Id: I9e9b253018c63976345eec1439a6b78de2cab869
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

2 files changed, 19 insertions, 9 deletions

diff --git a/src/platformsupport/fontdatabases/mac/qcoretextfontdatabase.mm b/src/platformsupport/fontdatabases/mac/qcoretextfontdatabase.mm
index 6347d4d231..237e8a89a5 100644
--- a/src/platformsupport/fontdatabases/mac/qcoretextfontdatabase.mm
+++ b/src/platformsupport/fontdatabases/mac/qcoretextfontdatabase.mm
@@ -416,7 +416,19 @@ extern CGAffineTransform qt_transform_from_fontdef(const QFontDef &fontDef);
 template <>
 QFontEngine *QCoreTextFontDatabaseEngineFactory<QCoreTextFontEngine>::fontEngine(const QFontDef &fontDef, void *usrPtr)
 {
-    CTFontDescriptorRef descriptor = static_cast<CTFontDescriptorRef>(usrPtr);
+    QCFType<CTFontDescriptorRef> descriptor = QCFType<CTFontDescriptorRef>::constructFromGet(
+        static_cast<CTFontDescriptorRef>(usrPtr));
+
+    // CoreText will sometimes invalidate information in font descriptors that refer
+    // to system fonts in certain function calls or application states. While the descriptor
+    // looks the same from the outside, some internal plumbing is different, causing the results
+    // of creating CTFonts from those descriptors unreliable. The work-around for this
+    // is to copy the attributes of those descriptors each time we make a new CTFont
+    // from them instead of referring to the original, as that may trigger the CoreText bug.
+    if (m_systemFontDescriptors.contains(descriptor)) {
+        QCFType<CFDictionaryRef> attributes = CTFontDescriptorCopyAttributes(descriptor);
+        descriptor = CTFontDescriptorCreateWithAttributes(attributes);
+    }
 
     // Since we do not pass in the destination DPI to CoreText when making
     // the font, we need to pass in a point size which is scaled to include
@@ -427,14 +439,10 @@ QFontEngine *QCoreTextFontDatabaseEngineFactory<QCoreTextFontEngine>::fontEngine
     qreal scaledPointSize = fontDef.pixelSize;
 
     CGAffineTransform matrix = qt_transform_from_fontdef(fontDef);
-    CTFontRef font = CTFontCreateWithFontDescriptor(descriptor, scaledPointSize, &matrix);
-    if (font) {
-        QFontEngine *engine = new QCoreTextFontEngine(font, fontDef);
-        CFRelease(font);
-        return engine;
-    }
+    if (QCFType<CTFontRef> font = CTFontCreateWithFontDescriptor(descriptor, scaledPointSize, &matrix))
+        return new QCoreTextFontEngine(font, fontDef);
 
-    return NULL;
+    return nullptr;
 }
 
 #ifndef QT_NO_FREETYPE
diff --git a/src/platformsupport/fontdatabases/mac/qcoretextfontdatabase_p.h b/src/platformsupport/fontdatabases/mac/qcoretextfontdatabase_p.h
index 9612b909f1..e14d1d6e6e 100644
--- a/src/platformsupport/fontdatabases/mac/qcoretextfontdatabase_p.h
+++ b/src/platformsupport/fontdatabases/mac/qcoretextfontdatabase_p.h
@@ -91,12 +91,14 @@ public:
     QFont *themeFont(QPlatformTheme::Font) const;
     const QHash<QPlatformTheme::Font, QFont *> &themeFonts() const;
 
+protected:
+    mutable QSet<CTFontDescriptorRef> m_systemFontDescriptors;
+
 private:
     void populateFromDescriptor(CTFontDescriptorRef font, const QString &familyName = QString());
 
     mutable QString defaultFontName;
 
-    mutable QSet<CTFontDescriptorRef> m_systemFontDescriptors;
     mutable QHash<QPlatformTheme::Font, QFont *> m_themeFonts;
     bool m_hasPopulatedAliases;
 };