Fix QML property cache leaks of delegate items

The delegate items are destroyed through an event loop by a call to a
deleteLater(). This, however, doesn't work when the application is
in the process of exiting and the event loop is already closed (i.e.
we're in a stack unwinding part that starts after app.exec())

Combat this situation by setting a parent of the to-be-deleted object
to some QObject that will be destroyed e.g. QCoreApplication::instance()
before the program finishes. As QObjects clean their children on
destruction, this will make sure that we cleanup the previously leaking
thing regardless of the event loop

Added a test to check that delegates are destroyed (as a separate binary
due to differences in main() function)

Fixes: QTBUG-87228
Change-Id: I59066603b77497fe4fd8d051798c3e4b47c119f0
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit 3a5617dc45e281552b9c1f7a04f0561b8fa14d94)

5 files changed, 130 insertions, 1 deletions

diff --git a/src/qmlmodels/qqmldelegatemodel.cpp b/src/qmlmodels/qqmldelegatemodel.cpp
index 725b9e8bc3..12c3d11937 100644
--- a/src/qmlmodels/qqmldelegatemodel.cpp
+++ b/src/qmlmodels/qqmldelegatemodel.cpp
@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2020 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtQml module of the Qt Toolkit.
@@ -2379,6 +2379,15 @@ void QQmlDelegateModelItem::destroyObject()
         data->ownContext = nullptr;
         data->context = nullptr;
     }
+    /* QTBUG-87228: when destroying object at the application exit, the deferred
+     * parent by setting it to QCoreApplication instance if it's nullptr, so
+     * deletion won't work. Not to leak memory, make sure our object has a that
+     * the parent claims the object at the end of the lifetime. When not at the
+     * application exit, normal event loop will handle the deferred deletion
+     * earlier.
+     */
+    if (object->parent() == nullptr)
+        object->setParent(QCoreApplication::instance());
     object->deleteLater();
 
     if (attached) {
diff --git a/tests/auto/quick/qquickview_extra/data/qtbug_87228.qml b/tests/auto/quick/qquickview_extra/data/qtbug_87228.qml
new file mode 100644
index 0000000000..ff10eba23d
--- /dev/null
+++ b/tests/auto/quick/qquickview_extra/data/qtbug_87228.qml
@@ -0,0 +1,30 @@
+import QtQml 2.12
+import QtQml.Models 2.12
+import QtQuick 2.12
+
+Item {
+    height: 480
+    width: 320
+    Rectangle {
+        id: rootRect
+
+        function addItem(desc) {
+            myModel.append({"desc": desc});
+        }
+
+        Rectangle {
+            ListView {
+                objectName: "listView"
+                delegate: Text {
+                    required property string desc
+                    text: desc
+                }
+                model: ListModel { id: myModel }
+            }
+        }
+
+        Component.onCompleted: {
+            addItem("Test creation of a delegate with a property");
+        }
+    }
+}
diff --git a/tests/auto/quick/qquickview_extra/qquickview_extra.pro b/tests/auto/quick/qquickview_extra/qquickview_extra.pro
new file mode 100644
index 0000000000..b40af0ce19
--- /dev/null
+++ b/tests/auto/quick/qquickview_extra/qquickview_extra.pro
@@ -0,0 +1,12 @@
+CONFIG += testcase
+TARGET = tst_qquickview_extra
+macx:CONFIG -= app_bundle
+
+SOURCES += tst_qquickview_extra.cpp
+
+include (../../shared/util.pri)
+include (../shared/util.pri)
+
+TESTDATA = data/*
+
+QT += core-private gui-private qml-private quick-private testlib
diff --git a/tests/auto/quick/qquickview_extra/tst_qquickview_extra.cpp b/tests/auto/quick/qquickview_extra/tst_qquickview_extra.cpp
new file mode 100644
index 0000000000..f697a438bd
--- /dev/null
+++ b/tests/auto/quick/qquickview_extra/tst_qquickview_extra.cpp
@@ -0,0 +1,77 @@
+/****************************************************************************
+**
+** Copyright (C) 2020 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:GPL-EXCEPT$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+#include <qtest.h>
+#include <QtTest/QSignalSpy>
+#include <QtQuick/qquickview.h>
+#include <QtQuick/qquickitem.h>
+#include <QtQml/qqmlengine.h>
+#include "../../shared/util.h"
+#include <QtCore/QDebug>
+#include <QtCore/QTimer>
+
+// Extra app-less tests
+class tst_QQuickViewExtra : public QQmlDataTest
+{
+    Q_OBJECT
+public:
+    tst_QQuickViewExtra();
+
+private slots:
+    void qtbug_87228();
+};
+
+tst_QQuickViewExtra::tst_QQuickViewExtra() { }
+
+void tst_QQuickViewExtra::qtbug_87228()
+{
+    QScopedPointer<QSignalSpy> deletionSpy;
+    {
+        int argc = 0;
+        QGuiApplication app(argc, nullptr);
+        QQuickView view;
+
+        view.setSource(testFileUrl("qtbug_87228.qml"));
+        view.show();
+        QTimer::singleShot(500, &app, QCoreApplication::quit);
+        app.exec();
+
+        QObject *listView = view.findChild<QObject *>("listView");
+        QVERIFY(listView);
+        QQuickItem *contentItem = listView->property("contentItem").value<QQuickItem *>();
+        QVERIFY(contentItem);
+        auto children = contentItem->childItems();
+        QVERIFY(children.size() > 0);
+        // for the sake of this test, any child would be suitable, so pick first
+        deletionSpy.reset(new QSignalSpy(children[0], SIGNAL(destroyed(QObject *))));
+    }
+    QCOMPARE(deletionSpy->count(), 1);
+}
+
+QTEST_APPLESS_MAIN(tst_QQuickViewExtra)
+
+#include "tst_qquickview_extra.moc"
diff --git a/tests/auto/quick/quick.pro b/tests/auto/quick/quick.pro
index 541bfdd527..45bcf8a9ce 100644
--- a/tests/auto/quick/quick.pro
+++ b/tests/auto/quick/quick.pro
@@ -85,6 +85,7 @@ QUICKTESTS += \
     qquicktextinput \
     qquickvisualdatamodel \
     qquickview \
+    qquickview_extra \
     qquickcanvasitem \
     qquickdesignersupport \
     qquickscreen \