Fix a crash when node is added and destroyed immediately

When a node is added to the scene it is scheduled for a post-creation
initialization. However, if the node is destroyed before this
post-creation queue has been processed we will crash.

This commit ensures that the node being deleted is removed from the
post-creation queue. Usually the queue will be empty and so this should
not add a performance penalty.

Pick-to: 5.15
Change-Id: Ibe4289e1e54cdb145f8588f15dc4ad894e427582
Reviewed-by: Mike Krus <mike.krus@kdab.com>

2 files changed, 52 insertions, 0 deletions

diff --git a/src/core/nodes/qnode.cpp b/src/core/nodes/qnode.cpp
index 15a2f5978..baf9525cb 100644
--- a/src/core/nodes/qnode.cpp
+++ b/src/core/nodes/qnode.cpp
@@ -134,6 +134,11 @@ void QNodePrivate::notifyDestructionChangesAndRemoveFromScene()
 {
     Q_Q(QNode);
 
+    // Ensure this node is not queued up for post-construction init
+    // to avoid crashing when the event loop spins.
+    if (m_scene && m_scene->postConstructorInit())
+        m_scene->postConstructorInit()->removeNode(q);
+
     // Tell the backend we are about to be destroyed
     if (m_hasBackendNode && m_scene && m_scene->engine())
         QAspectEnginePrivate::get(m_scene->engine())->removeNode(q);
diff --git a/tests/auto/core/qaspectengine/tst_qaspectengine.cpp b/tests/auto/core/qaspectengine/tst_qaspectengine.cpp
index 61e926352..45f91aa44 100644
--- a/tests/auto/core/qaspectengine/tst_qaspectengine.cpp
+++ b/tests/auto/core/qaspectengine/tst_qaspectengine.cpp
@@ -200,6 +200,53 @@ private Q_SLOTS:
         // * destroying the aspect engine
     }
 
+    void shouldNotCrashWhenEntityIsAddedThenImmediatelyDeleted()
+    {
+        // GIVEN
+        // An initialized aspect engine...
+        QAspectEngine engine;
+        // ...and a simple aspect
+        PrintRootAspect *aspect = new PrintRootAspect;
+
+        // WHEN
+        // We register the aspect
+        engine.registerAspect(aspect);
+
+        // THEN
+        const auto registeredAspects = engine.aspects();
+        QCOMPARE(registeredAspects.size(), 1);
+        QCOMPARE(registeredAspects.first(), aspect);
+
+        // WHEN
+        QEntityPtr entity(new QEntity);
+        entity->setObjectName("RootEntity");
+        // we set a scene root entity
+        engine.setRootEntity(entity);
+
+        QEventLoop eventLoop;
+        QTimer::singleShot(1000, &eventLoop, SLOT(quit()));
+        eventLoop.exec();
+
+        // THEN
+        // we don't crash and...
+        const auto rootEntity = engine.rootEntity();
+        QCOMPARE(rootEntity, entity);
+
+        // WHEN
+        // we create a child node and delete within the same spin of
+        // the event loop
+        Qt3DCore::QEntity *childEntity = new Qt3DCore::QEntity(entity.data());
+        delete childEntity;
+        entity = nullptr;
+        QTimer::singleShot(600, &eventLoop, SLOT(quit()));
+
+        // ...and allow events to process...
+        eventLoop.exec();
+
+        // ...and we don't crash when the childEntity is removed from the
+        // post construction init routines
+    }
+
     void shouldNotCrashOnShutdownWhenComponentIsCreatedWithParentBeforeItsEntity()
     {
         // GIVEN