macOS: Don't mangle QByteArray settings with @ prefix by decoding as UTF-8

QSettings encodes QVariants as @Type(data) strings. If that data contains
a null-byte, we write the string as UTF-8 encoded CFData. When reading it
back we look for a @ prefix, and then pass it as UTF-8 through stringToVariant.

The problem arises then the user writes raw QByteArrays with a @ prefix.
We can detect this situation by checking the result of stringToVariant,
and if it's just a simple conversion of the string into a QVariant, we
know that stringToVariant hit its fallback path due to not finding any
embedded variants.

If that's the case, we return the raw bytes as a QByteArray.

Change-Id: I4ac5c35d0a6890ebea983b9aca0a3a36b0143de2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 3eac6079d9a02fbedd3740186eaaf21134dc45c2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/io/qsettings_mac.cpp

@@ -247,7 +247,15 @@ static QVariant qtValue(CFPropertyListRef cfvalue)
         }
 
         const QString str = QString::fromUtf8(byteArray.constData(), byteArray.size());
-        return QSettingsPrivate::stringToVariant(str);
+        QVariant variant = QSettingsPrivate::stringToVariant(str);
+        if (variant == QVariant(str)) {
+            // We did not find an encoded variant in the string,
+            // so return the raw byte array instead.
+            byteArray.detach();
+            return byteArray;
+        }
+
+        return variant;
     } else if (typeId == CFDictionaryGetTypeID()) {
         CFDictionaryRef cfdict = static_cast<CFDictionaryRef>(cfvalue);
         CFTypeID arrayTypeId = CFArrayGetTypeID();

tests/auto/corelib/io/qsettings/tst_qsettings.cpp

@@ -709,6 +709,10 @@ void tst_QSettings::embeddedZeroByte_data()
 
     QTest::newRow("@bytearray\\0") << QVariant(bytes);
     QTest::newRow("@string\\0") << QVariant(QString::fromLatin1(bytes.data(), bytes.size()));
+
+    bytes = QByteArray("@\xdd\x7d", 3);
+    QTest::newRow("@-prefixed data") << QVariant(bytes);
+    QTest::newRow("@-prefixed data as string") << QVariant(QString::fromLatin1(bytes.data(), bytes.size()));
 }
 
 void tst_QSettings::embeddedZeroByte()