Avoid dangling QGuiApplicationPrivate pointer

The static self pointer of QGuiApplicationPrivate was not reset at
destruction (in constrast to the corresponding
QGuiApplication::self). This could cause crashes when calling Qt API
after QGuiApplication destruction.

Fixing this revealed an issue with QGuiApplication::font(), which
would assert QGuiApplicationPrivate::self. But the QApplication
autotest actually calls this function with no QApplication
instance. That autotest passes only coincidentally, since another
QApplication instance has been created and deleted already, and
the dangling self pointer of that instance was never reset.

To improve the robustness of the api, replace the assert/crash with
just a warning and return an "empty" QFont.

(The assert was added for 5.0 for QTBUG-28306 in order to give a nicer
warning when mixing QWidget and QtCore/GuiApplication. However it
never got that effect in practice, since that issue was fixed at the
same time by another, better patch for the duplicate bug QTBUG-28076).

Fixes: QTBUG-81954
Change-Id: I3fa6cad1625a3e70631b5170d53119d63492b534
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 6 insertions, 1 deletions

diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index 1190bd1936..67b1ed15c5 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -1727,6 +1727,8 @@ QGuiApplicationPrivate::~QGuiApplicationPrivate()
 
     window_list.clear();
     screen_list.clear();
+
+    self = nullptr;
 }
 
 #if 0
@@ -3401,8 +3403,11 @@ void QGuiApplicationPrivate::applyWindowGeometrySpecificationTo(QWindow *window)
 */
 QFont QGuiApplication::font()
 {
-    Q_ASSERT_X(QGuiApplicationPrivate::self, "QGuiApplication::font()", "no QGuiApplication instance");
     const auto locker = qt_scoped_lock(applicationFontMutex);
+    if (!QGuiApplicationPrivate::self && !QGuiApplicationPrivate::app_font) {
+        qWarning("QGuiApplication::font(): no QGuiApplication instance and no application font set.");
+        return QFont();  // in effect: QFont((QFontPrivate*)nullptr), so no recursion
+    }
     initFontUnlocked();
     return *QGuiApplicationPrivate::app_font;
 }