Qt Labs Settings: warn if failed to initialize internal QSettings instance

First, warn the user that the QSettings instance failed to initialize
if there was an error. Then, if the error was QSettings::AccessError,
print the specific application identifiers that weren't set so they
know what they have to do. For example:

    QML Settings: Failed to initialize QSettings instance. Status code is: 1
    QML Settings: The following application identifiers have not been set: QVector("organizationName", "organizationDomain")

In my case, the QSettings instance wasn't created on
Windows because organizationName wasn't set.

Change-Id: I7970209e09b78f785730422c45978775311b96ac
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

src/imports/settings/qqmlsettings.cpp

@@ -39,10 +39,12 @@
 
 #include "qqmlsettings_p.h"
 #include <qcoreevent.h>
+#include <qcoreapplication.h>
 #include <qloggingcategory.h>
 #include <qsettings.h>
 #include <qpointer.h>
 #include <qjsvalue.h>
+#include <qqmlinfo.h>
 #include <qdebug.h>
 #include <qhash.h>
 
@@ -269,6 +271,26 @@ QSettings *QQmlSettingsPrivate::instance() const
     if (!settings) {
         QQmlSettings *q = const_cast<QQmlSettings*>(q_func());
         settings = new QSettings(q);
+        if (settings->status() != QSettings::NoError) {
+            // TODO: can't print out the enum due to the following error:
+            // error: C2666: 'QQmlInfo::operator <<': 15 overloads have similar conversions
+            qmlWarning(q) << "Failed to initialize QSettings instance. Status code is: " << int(settings->status());
+
+            if (settings->status() == QSettings::AccessError) {
+                QVector<QString> missingIdentifiers;
+                if (QCoreApplication::organizationName().isEmpty())
+                    missingIdentifiers.append(QLatin1String("organizationName"));
+                if (QCoreApplication::organizationDomain().isEmpty())
+                    missingIdentifiers.append(QLatin1String("organizationDomain"));
+                if (QCoreApplication::applicationName().isEmpty())
+                    missingIdentifiers.append(QLatin1String("applicationName"));
+
+                if (!missingIdentifiers.isEmpty())
+                    qmlWarning(q) << "The following application identifiers have not been set: " << missingIdentifiers;
+            }
+            return settings;
+        }
+
         if (!category.isEmpty())
             settings->beginGroup(category);
         if (initialized)

tests/auto/qml/qqmlsettings/tst_qqmlsettings.cpp

@@ -53,6 +53,7 @@ private slots:
     void categories();
     void siblings();
     void initial();
+    void noApplicationIdentifiersSet();
 };
 
 // ### Replace keyValueMap("foo", "bar") with QVariantMap({{"foo", "bar"}})
@@ -147,10 +148,6 @@ void tst_QQmlSettings::initTestCase()
 {
     QQmlDataTest::initTestCase();
 
-    QCoreApplication::setApplicationName("tst_QQmlSettings");
-    QCoreApplication::setOrganizationName("QtProject");
-    QCoreApplication::setOrganizationDomain("qt-project.org");
-
     qmlRegisterType<CppObject>("Qt.test", 1, 0, "CppObject");
 }
 
@@ -158,6 +155,10 @@ void tst_QQmlSettings::init()
 {
     QSettings settings;
     settings.clear();
+
+    QCoreApplication::setApplicationName("tst_QQmlSettings");
+    QCoreApplication::setOrganizationName("QtProject");
+    QCoreApplication::setOrganizationDomain("qt-project.org");
 }
 
 void tst_QQmlSettings::cleanup()
@@ -481,6 +482,31 @@ void tst_QQmlSettings::initial()
     QCOMPARE(settings->property("value").toString(), QStringLiteral("initial"));
 }
 
+void tst_QQmlSettings::noApplicationIdentifiersSet()
+{
+#ifdef Q_OS_MACOS
+    QSKIP("macOS doesn't complain about empty application identifiers");
+#endif
+
+    QCoreApplication::setApplicationName(QString());
+    QCoreApplication::setOrganizationName(QString());
+    QCoreApplication::setOrganizationDomain(QString());
+
+    QTest::ignoreMessage(QtWarningMsg, QRegularExpression(".*QML Settings: Failed to initialize QSettings instance. Status code is: 1"));
+    // Can't set an empty applicationName because QCoreApplication won't allow it, which is why it's not listed here.
+    QTest::ignoreMessage(QtWarningMsg, QRegularExpression(".*QML Settings: The following application identifiers have not been set: QVector\\(\"organizationName\", \"organizationDomain\"\\)"));
+
+    QQmlEngine engine;
+    QQmlComponent component(&engine, testFileUrl("basic.qml"));
+    QScopedPointer<QObject> root(component.create());
+    QVERIFY(root.data());
+    // The value of the QML property will be true because it defaults to it...
+    QVERIFY(root->property("success").toBool());
+    QSettings settings;
+    // ... but the settings' value should be false because it was never loaded.
+    QVERIFY(!settings.value("success").toBool());
+}
+
 QTEST_MAIN(tst_QQmlSettings)
 
 #include "tst_qqmlsettings.moc"