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
Pick-to: 6.0 5.15
Change-Id: I59066603b77497fe4fd8d051798c3e4b47c119f0
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

1 files changed, 10 insertions, 1 deletions

diff --git a/src/qmlmodels/qqmldelegatemodel.cpp b/src/qmlmodels/qqmldelegatemodel.cpp
index 1a053e2e31..4f211aea53 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.
@@ -2377,6 +2377,15 @@ 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) {