Also, if you don't switch to VR, but just try to quit Slicer, it hangs, and if I force quite, here's where it gets stuck:

Julien -- is there anything wrong with that VR DIsplay node initialization code that I use?

Thread 2835cc DispatchQueue 1
User stack:
9 start + 52 (in Slicer) [0x10001ea68]
9 main + 3044 (in Slicer) [0x100021064]
9 qSlicerAppMainWindow::~qSlicerAppMainWindow() + 433 (in libqSlicerApp.dylib) [0x10004e031]
9 QWidget::~QWidget() + 681 (in QtGui) [0x106b0b319]
9 QObjectPrivate::deleteChildren() + 95 (in QtCore) [0x1077362ef]
9 QWidget::~QWidget() + 681 (in QtGui) [0x106b0ae59]
9 QObjectPrivate::deleteChildren() + 95 (in QtCore) [0x1077362ef]
9 QFrame::~QFrame() + 41 (in QtGui) [0x106e76cb9]
9 QWidget::~QWidget() + 681 (in QtGui) [0x106b0b319]
9 QObjectPrivate::deleteChildren() + 95 (in QtCore) [0x1077362ef]
9 qMRMLThreeDWidget::~qMRMLThreeDWidget() + 75 (in libqMRMLWidgets.dylib) [0x100523acb]
9 QWidget::~QWidget() + 681 (in QtGui) [0x106b0b319]
9 QObjectPrivate::deleteChildren() + 95 (in QtCore) [0x1077362ef]
9 qMRMLThreeDView::~qMRMLThreeDView() + 58 (in libqMRMLWidgets.dylib) [0x10051d8ea]
9 qMRMLThreeDViewPrivate::~qMRMLThreeDViewPrivate() + 48 (in libqMRMLWidgets.dylib) [0x10051d390]
9 vtkMRMLDisplayableManagerGroup::~vtkMRMLDisplayableManagerGroup() + 101 (in libMRMLDisplayableManager.dylib) [0x103132465]
9 vtkMRMLVolumeRenderingDisplayableManager::~vtkMRMLVolumeRenderingDisplayableManager() + 69 (in libvtkSlicerVolumeRenderingModuleMRMLDisplayableManager.dylib) [0x12b7efd25]
5 vtkMRMLVolumeRenderingDisplayableManager::RemoveDisplayNode(vtkMRMLVolumeRenderingDisplayNode) + 53 (in libvtkSlicerVolumeRenderingModuleMRMLDisplayableManager.dylib) [0x12b7ef8e5]
4 std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const, std::allocator<char> const&) + 67 (in libstdc++.6.dylib) [0x7fff8415055f]
3 char std::string::_S_construct<char const>(char const, char const, std::allocator<char> const&, std::forward_iterator_tag) + 69 (in libstdc++.6.dylib) [0x7fff8415017d]
1 std::string::_Rep::_S_create(unsigned long, unsigned long, std::allocator<char> const&) + 131 (in libstdc++.6.dylib) [0x7fff8414e4db]
1 std::string::_Rep::_S_create(unsigned long, unsigned long, std::allocator<char> const&) + 142 (in libstdc++.6.dylib) [0x7fff8414e4e6]
1 std::string::_Rep::_S_create(unsigned long, unsigned long, std::allocator<char> const&) + 121 (in libstdc++.6.dylib) [0x7fff8414e4d1]
1 operator new(unsigned long) + 97 (in libstdc++.6.dylib) [0x7fff84168f05]
1 malloc + 39 (in libSystem.B.dylib) [0x7fff8788dc83]
1 std::string::_Rep::_S_create(unsigned long, unsigned long, std::allocator<char> const&) + 148 (in libstdc++.6.dylib) [0x7fff8414e4ec]
1 strlen + 29 (in libSystem.B.dylib) [0x7fff8788ec0d]
3 vtkMRMLVolumeRenderingDisplayableManager::RemoveDisplayNode(vtkMRMLVolumeRenderingDisplayNode) + 591 (in libvtkSlicerVolumeRenderingModuleMRMLDisplayableManager.dylib) [0x12b7efaff]
2 free + 56 (in libSystem.B.dylib) [0x7fff8789214d]
1 szone_size + 215 (in libSystem.B.dylib) [0x7fff878922fe]
1 szone_size + 388 (in libSystem.B.dylib) [0x7fff878923ab]
1 __spin_lock + 13 (in commpage [libSystem.B.dylib]) [0x7fffffe0026d]
1 vtkMRMLVolumeRenderingDisplayableManager::RemoveDisplayNode(vtkMRMLVolumeRenderingDisplayNode) + 574 (in libvtkSlicerVolumeRenderingModuleMRMLDisplayableManager.dylib) [0x12b7efaee]

Crash is fixed in r21118:
http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&amp;revision=21128

For the hang, the problem comes from your code:
You shouldn't call the last line "slicer.mrmlScene.AddNode(vrNode)".
As it is mentioned in the comment of vtkSlicerVolumeRenderingLogic::CreateVolumeRenderingDisplayNode(), the returned node is already added into the scene. (see
http://slicer.org/doc/html/classvtkSlicerVolumeRenderingLogic.html#a3ee5dc9a38aa9503b9494cc1d0789ec0)
Adding an existing node twice into the scene shouldn't be allowed and it leads to problems such as the hang. I just created an issue to fix the hang in the code, however, I would prefer to do it after 4.2 if it is not urgent (it's only a hang because something illegal happened).
In order to reduce the confusion in the future,

• r21129 warns the developer that the node already exist, however I'm hesitant to add the check "IsNodePresent" in release mode as it would slow down node insertion. The more I think about it the more I think we should add it though.
• do you think the name of the function CreateVolumeRenderingDisplayNode() was misleading ? From the name you wouldn't expect the node from being added into the scene ? Feel free to create a new issue that would require the issue developer to make a pass in the code of Slicer to make sure all the functions that are named "CreateXXX" return a node that is not added into the scene. Note that it would have the drawback that you would need to manually "dereference" it in your python or C++ code (because the returned node has an "artificial" reference count of 1)

Julien, I admit I didn't read the code or doxygen. The same code was there last year, and I don't remember this issue come up.

From my perspective, it is not clear from the "CreateNode" name that the node would be added to the scene. I would call it something like "CreateAndAdd". To me the use of "CreateNode" is ambiguous, since there are other similar calls (e.g., MRMLScene:CreateNodeByClass()) that create but do not add the node to the scene.

Now, back to the issue:

1) if I run the script and then switch to VT, volume rendering visibility is toggled. Is this expected behavior? I would think not, because I set visibility on display node.

2) if I run the script (excluding the AddNode line), switch to VR and quit Slicer, I have the following memory leaks. If I don't switch to VR and exit, no leaks.

Class "vtkCellData" has 10 instances still around.
Class "vtkCollection" has 2 instances still around.
Class "vtkInformationIntegerVectorValue" has 9 instances still around.
Class "vtkInformationVector" has 80 instances still around.
Class "vtkPerspectiveTransform" has 4 instances still around.
Class "vtkMRMLScalarVolumeNode" has 1 instance still around.
Class "vtkImageBimodalAnalysis" has 2 instances still around.
Class "vtkPointData" has 10 instances still around.
Class "vtkObservation" has 50 instances still around.
Class "vtkTrivialProducer" has 2 instances still around.
Class "vtkMatrix4x4" has 56 instances still around.
Class "vtkPassThrough" has 3 instances still around.
Class "vtkImageMapToColors" has 1 instance still around.
Class "vtkSimpleTransform" has 8 instances still around.
Class "vtkColorTransferFunction" has 1 instance still around.
Class "vtkObserverManager" has 19 instances still around.
Class "vtkIntArray" has 3 instances still around.
Class "vtkPoints" has 5 instances still around.
Class "vtkInformation" has 100 instances still around.
Class "vtkInformationStringVectorValue" has 2 instances still around.
Class "vtkLine" has 2 instances still around.
Class "vtkMultiThreader" has 10 instances still around.
Class "vtkInformationIntegerPointerValue" has 5 instances still around.
Class "vtkEventBroker" has 1 instance still around.
Class "vtkPolyData" has 5 instances still around.
Class "vtkLookupTable" has 1 instance still around.
Class "vtkMRMLCPURayCastVolumeRenderingDisplayNode" has 1 instance still around.
Class "vtkMRMLSliceNode" has 6 instances still around.
Class "vtkPiecewiseFunction" has 2 instances still around.
Class "vtkImageThreshold" has 1 instance still around.
Class "vtkMRMLAnnotationTextDisplayNode" has 1 instance still around.
Class "vtkOpenGLCamera" has 2 instances still around.
Class "vtkImageAccumulateDiscrete" has 2 instances still around.
Class "vtkIdList" has 3 instances still around.
Class "vtkDoubleArray" has 3 instances still around.
Class "vtkAlgorithmOutput" has 12 instances still around.
Class "vtkImageCast" has 1 instance still around.
Class "vtkMRMLColorTableNode" has 1 instance still around.
Class "vtkMRMLAnnotationROINode" has 2 instances still around.
Class "vtkInformationIntegerValue" has 119 instances still around.
Class "vtkExtentTranslator" has 1 instance still around.
Class "vtkTagTable" has 5 instances still around.
Class "vtkCellArray" has 3 instances still around.
Class "vtkImageData" has 5 instances still around.
Class "vtkMRMLAnnotationLineDisplayNode" has 1 instance still around.
Class "vtkFloatArray" has 3 instances still around.
Class "vtkMRMLScalarVolumeDisplayNode" has 1 instance still around.
Class "vtkInformationStringValue" has 7 instances still around.
Class "vtkVolumeProperty" has 1 instance still around.
Class "vtkStringArray" has 4 instances still around.
Class "vtkImageMapToWindowLevelColors" has 1 instance still around.
Class "vtkInformationExecutivePortVectorValue" has 11 instances still around.
Class "vtkTransform" has 6 instances still around.
Class "vtkIdTypeArray" has 3 instances still around.
Class "vtkShortArray" has 1 instance still around.
Class "vtkVoxel" has 1 instance still around.
Class "vtkFieldData" has 12 instances still around.
Class "vtkMRMLAnnotationPointDisplayNode" has 1 instance still around.
Class "vtkAssignAttribute" has 3 instances still around.
Class "vtkInformationDoubleVectorValue" has 7 instances still around.
Class "vtkMRMLCameraNode" has 2 instances still around.
Class "vtkMRMLVolumeArchetypeStorageNode" has 1 instance still around.
Class "vtkUnsignedCharArray" has 1 instance still around.
Class "vtkStreamingDemandDrivenPipeline" has 18 instances still around.
Class "vtkMRMLVolumePropertyNode" has 1 instance still around.
Class "vtkBitArray" has 8 instances still around.
Class "vtkImageAppendComponents" has 1 instance still around.
Class "vtkTimerLog" has 1 instance still around.
Class "vtkUnsignedLongArray" has 13 instances still around.
Class "vtkCommand or subclass" has 96 instances still around.
Class "vtkInformationExecutivePortValue" has 14 instances still around.
Class "vtkImageLogic" has 1 instance still around.

0) "CreateXXX": See issue 0002626
1) When you open VR, it creates a new VR display node because the current volume doesn't have a VR display node. And by default, the created VR display node is not visible.
You need to add in your script:
v.AddAndObserveDisplayNodeID(vrNode.GetID())
2) I get memory leaks even if I don't open the VR module.

You are absolutely correct. The irony is, I just fixed this in another place before you updated this, but missed right in front of my face in this test!