Race Condition or Crash in Qt3D due to Thread Affinity of NodePostConstructorInit::processNodes()

When creating multiple QNodes, Qt3D may invoke NodePostConstructorInit::processNodes() on either the main thread or the rendering thread.

If it's called on the main thread, the rendering thread appears to be blocked for synchronization. This seems to be the correct approach because the method calls QPrivateNode::_q_postConstructorInit(), which uses a QNodeVisitor that operates on QNodes living on the main thread.

The NodePostConstructorInit::addNode(QNode *node) method queues processNodes() to run on the rendering thread. This is because NodePostConstructorInit has rendering thread affinity.

The crash or race condition occurs when both the main thread and the rendering thread are reading from or writing to the QNode tree structure.

To resolve this, we have a few options:

1. Make NodePostConstructorInit have main thread affinity.
2. Remove the queued processNodes(). This seems like a reasonable solution, as processNodes() is called on every frame.
3. Post processNodes() to run on the main thread.

I've attached a backtrace that highlights the issue. While the crash may present with different backtraces, the root cause consistently stems from NodePostConstructorInit::processNodes().