QDialogButtonBox: prevent crashes on deletions in slots

As usual, user code connected to signals emitted from Qt may destroy
an object's internal status while a signal is being emitted.

Guard a bit QDialogButtonBox signal emissions to prevent
crashes in case the button or a dialog get deleted from a slot
connected to clicked(). Also, be sure to emit the corresponding
accepted/rejected/etc. signal.

Change-Id: I7b1888070a8f2f56aa60923a17f90fb5efef145c
Task-number: QTBUG-45835
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/widgets/widgets/qdialogbuttonbox.cpp

@@ -852,9 +852,19 @@ void QDialogButtonBoxPrivate::_q_handleButtonClicked()
 {
     Q_Q(QDialogButtonBox);
     if (QAbstractButton *button = qobject_cast<QAbstractButton *>(q->sender())) {
+        // Can't fetch this *after* emitting clicked, as clicked may destroy the button
+        // or change its role. Now changing the role is not possible yet, but arguably
+        // both clicked and accepted/rejected/etc. should be emitted "atomically"
+        // depending on whatever role the button had at the time of the click.
+        const QDialogButtonBox::ButtonRole buttonRole = q->buttonRole(button);
+        QPointer<QDialogButtonBox> guard(q);
+
         emit q->clicked(button);
 
-        switch (q->buttonRole(button)) {
+        if (!guard)
+            return;
+
+        switch (buttonRole) {
         case QPlatformDialogHelper::AcceptRole:
         case QPlatformDialogHelper::YesRole:
             emit q->accepted();

tests/auto/widgets/widgets/qdialogbuttonbox/tst_qdialogbuttonbox.cpp

@@ -90,6 +90,7 @@ private slots:
 //    void buttons();
 
     void testDelete();
+    void testSignalEmissionAfterDelete_QTBUG_45835();
     void testRemove();
     void testMultipleAdd();
     void testStandardButtonMapping_data();
@@ -111,6 +112,7 @@ private:
 
 tst_QDialogButtonBox::tst_QDialogButtonBox()
 {
+    qRegisterMetaType<QAbstractButton *>();
 }
 
 tst_QDialogButtonBox::~tst_QDialogButtonBox()
@@ -414,6 +416,70 @@ void tst_QDialogButtonBox::testDelete()
     QCOMPARE(buttonBox.buttons().count(), 0);
 }
 
+class ObjectDeleter : public QObject
+{
+    Q_OBJECT
+public slots:
+    void deleteButton(QAbstractButton *button)
+    {
+        delete button;
+    }
+
+    void deleteSender()
+    {
+        delete sender();
+    }
+};
+
+void tst_QDialogButtonBox::testSignalEmissionAfterDelete_QTBUG_45835()
+{
+    {
+        QDialogButtonBox buttonBox;
+        QCOMPARE(buttonBox.buttons().count(), 0);
+
+        QSignalSpy buttonClickedSpy(&buttonBox, &QDialogButtonBox::clicked);
+        QVERIFY(buttonClickedSpy.isValid());
+
+        QSignalSpy buttonBoxAcceptedSpy(&buttonBox, &QDialogButtonBox::accepted);
+        QVERIFY(buttonBoxAcceptedSpy.isValid());
+
+        QPushButton *button = buttonBox.addButton("Test", QDialogButtonBox::AcceptRole);
+        QCOMPARE(buttonBox.buttons().count(), 1);
+
+        ObjectDeleter objectDeleter;
+        connect(&buttonBox, &QDialogButtonBox::clicked, &objectDeleter, &ObjectDeleter::deleteButton);
+
+        button->click();
+
+        QCOMPARE(buttonBox.buttons().count(), 0);
+        QCOMPARE(buttonClickedSpy.count(), 1);
+        QCOMPARE(buttonBoxAcceptedSpy.count(), 1);
+    }
+
+    {
+        QPointer<QDialogButtonBox> buttonBox(new QDialogButtonBox);
+        QCOMPARE(buttonBox->buttons().count(), 0);
+
+        QSignalSpy buttonClickedSpy(buttonBox.data(), &QDialogButtonBox::clicked);
+        QVERIFY(buttonClickedSpy.isValid());
+
+        QSignalSpy buttonBoxAcceptedSpy(buttonBox.data(), &QDialogButtonBox::accepted);
+        QVERIFY(buttonBoxAcceptedSpy.isValid());
+
+        QPushButton *button = buttonBox->addButton("Test", QDialogButtonBox::AcceptRole);
+        QCOMPARE(buttonBox->buttons().count(), 1);
+
+        ObjectDeleter objectDeleter;
+        connect(buttonBox.data(), &QDialogButtonBox::clicked, &objectDeleter, &ObjectDeleter::deleteSender);
+
+        button->click();
+
+        QVERIFY(buttonBox.isNull());
+        QCOMPARE(buttonClickedSpy.count(), 1);
+        QCOMPARE(buttonBoxAcceptedSpy.count(), 0);
+    }
+}
+
 void tst_QDialogButtonBox::testMultipleAdd()
 {
     // Add a button into the thing multiple times.