Fix crash when accessing QStyleHints before QGuiApplication is constructed.

Make styleHints a static member variable of QGuiApplicationPrivate and
fix accessor accordingly. Extend tst_QApplication::settableStyleHints()
to run without QApplication instance as well and add a similar test
to  QGuiApplication.

Task-number: QTBUG-44499
Change-Id: I42b92ef38f7dd512d08d70accfa7dd4f09a22f01
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>

3 files changed, 13 insertions, 7 deletions

diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index 61addab9d6..d334bb72fa 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -163,6 +163,7 @@ QWindow *QGuiApplicationPrivate::focus_window = 0;
 
 static QBasicMutex applicationFontMutex;
 QFont *QGuiApplicationPrivate::app_font = 0;
+QStyleHints *QGuiApplicationPrivate::styleHints = Q_NULLPTR;
 bool QGuiApplicationPrivate::obey_desktop_settings = true;
 
 QInputDeviceManager *QGuiApplicationPrivate::m_inputDeviceManager = 0;
@@ -595,7 +596,6 @@ QGuiApplication::~QGuiApplication()
 
 QGuiApplicationPrivate::QGuiApplicationPrivate(int &argc, char **argv, int flags)
     : QCoreApplicationPrivate(argc, argv, flags),
-      styleHints(0),
       inputMethod(0),
       lastTouchType(QEvent::TouchEnd),
       ownGlobalShareContext(false)
@@ -1349,7 +1349,8 @@ QGuiApplicationPrivate::~QGuiApplicationPrivate()
 
     cleanupThreadData();
 
-    delete styleHints;
+    delete QGuiApplicationPrivate::styleHints;
+    QGuiApplicationPrivate::styleHints = Q_NULLPTR;
     delete inputMethod;
 
     qt_cleanupFontDatabase();
@@ -1717,7 +1718,7 @@ void QGuiApplicationPrivate::processMouseEvent(QWindowSystemInterfacePrivate::Mo
         }
         mouse_buttons = buttons = e->buttons;
         if (button & e->buttons) {
-            ulong doubleClickInterval = static_cast<ulong>(qApp->styleHints()->mouseDoubleClickInterval());
+            ulong doubleClickInterval = static_cast<ulong>(QGuiApplication::styleHints()->mouseDoubleClickInterval());
             doubleClick = e->timestamp - mousePressTime < doubleClickInterval && button == mousePressButton;
             type = frameStrut ? QEvent::NonClientAreaMouseButtonPress : QEvent::MouseButtonPress;
             mousePressTime = e->timestamp;
@@ -3294,9 +3295,9 @@ void QGuiApplication::restoreOverrideCursor()
   */
 QStyleHints *QGuiApplication::styleHints()
 {
-    if (!qGuiApp->d_func()->styleHints)
-        qGuiApp->d_func()->styleHints = new QStyleHints();
-    return qGuiApp->d_func()->styleHints;
+    if (!QGuiApplicationPrivate::styleHints)
+        QGuiApplicationPrivate::styleHints = new QStyleHints();
+    return QGuiApplicationPrivate::styleHints;
 }
 
 /*!
diff --git a/src/gui/kernel/qguiapplication_p.h b/src/gui/kernel/qguiapplication_p.h
index ddbc446759..0c00e06499 100644
--- a/src/gui/kernel/qguiapplication_p.h
+++ b/src/gui/kernel/qguiapplication_p.h
@@ -218,7 +218,7 @@ public:
 
     static QFont *app_font;
 
-    QStyleHints *styleHints;
+    static QStyleHints *styleHints;
     static bool obey_desktop_settings;
     QInputMethod *inputMethod;
 
diff --git a/src/gui/kernel/qstylehints.cpp b/src/gui/kernel/qstylehints.cpp
index a0f98383ff..b7af2e759f 100644
--- a/src/gui/kernel/qstylehints.cpp
+++ b/src/gui/kernel/qstylehints.cpp
@@ -35,6 +35,7 @@
 #include <qpa/qplatformintegration.h>
 #include <qpa/qplatformtheme.h>
 #include <private/qguiapplication_p.h>
+#include <qdebug.h>
 
 QT_BEGIN_NAMESPACE
 
@@ -46,6 +47,10 @@ static inline QVariant hint(QPlatformIntegration::StyleHint h)
 static inline QVariant themeableHint(QPlatformTheme::ThemeHint th,
                                      QPlatformIntegration::StyleHint ih)
 {
+    if (!QCoreApplication::instance()) {
+        qWarning() << "Must construct a QGuiApplication before accessing a platform theme hint.";
+        return QVariant();
+    }
     if (const QPlatformTheme *theme = QGuiApplicationPrivate::platformTheme()) {
         const QVariant themeHint = theme->themeHint(th);
         if (themeHint.isValid())