Fixing Windows Screenshot Test Crashes: VTK OpenGL Issue

by Alex Johnson 57 views

Introduction

This article addresses a critical issue encountered during screenshot tests on Windows Continuous Integration (CI) environments, specifically related to VTK OpenGL initialization failures. The problem manifests as a segmentation fault (exit code 139) when the 3D viewer attempts to open, primarily due to VTK's inability to initialize OpenGL in a headless Windows environment. This article provides a detailed analysis of the issue, steps to reproduce it, the current and expected behaviors, the context in which the problem arises, failure logs, and a proposed solution. Understanding and resolving this issue is crucial for maintaining the stability and reliability of automated testing pipelines on Windows platforms.

Summary of the Failure

The core issue lies in the incompatibility between the VTK library and headless Windows runners. When screenshot tests are executed on Windows CI, the attempt to open the 3D viewer leads to a crash. This crash is triggered by VTK's failure to initialize OpenGL because headless Windows environments lack the necessary GPU and display adapter support. The result is a segmentation fault, indicated by exit code 139, which halts the testing process. This issue primarily affects automated testing environments where graphical interfaces are not available, highlighting the need for a workaround to ensure tests can run smoothly without requiring a physical display.

Steps to Reproduce the Issue

To reproduce this issue, follow these detailed steps within a Windows CI environment:

  1. Run screenshot tests on Windows CI: Initiate the test suite on a Windows-based Continuous Integration system. This environment typically operates without a graphical interface, simulating a headless setup.
  2. Execute the specific test: Target the mantidimaging/eyes_tests/mi_3d_viewer_test.py::MI3DViewerTest::test_3d_viewer_opens_without_data test. This particular test is designed to open the 3D viewer, triggering VTK's OpenGL initialization.
  3. Observe the crash: The test will crash with a segmentation fault, producing exit code 139. This indicates that the VTK library failed to initialize OpenGL due to the lack of proper graphical support in the headless environment. The crash log will typically include error messages related to OpenGL initialization failures, such as those from vtkWin32OpenGLRenderWindow and vtkOpenGLRenderWindow.

By following these steps, developers and testers can reliably reproduce the issue in a controlled environment, making it easier to verify potential fixes and workarounds. This reproducibility is essential for effective debugging and resolution of the problem.

Current Behavior

Currently, the 3D viewer tests crash consistently on Windows CI environments, presenting a significant obstacle to automated testing workflows. The error sequence observed includes specific messages that pinpoint the root cause of the problem:

  • vtkWin32OpenGLRenderWindow: failed to get wglChoosePixelFormatARB: This error indicates that VTK could not retrieve a suitable pixel format for OpenGL rendering in the Windows environment. The wglChoosePixelFormatARB function is crucial for selecting the appropriate pixel format, and its failure suggests a fundamental issue with OpenGL initialization.
  • vtkWin32OpenGLRenderWindow: failed to get valid pixel format: Following the previous error, this message confirms that VTK was unable to find a pixel format that meets its requirements, further highlighting the initialization failure.
  • vtkOpenGLRenderWindow: GLEW could not be initialized: Missing GL version: This error indicates that the OpenGL Extension Wrangler Library (GLEW), which helps manage OpenGL extensions, could not be initialized. The "Missing GL version" message suggests that the required OpenGL version is not available in the environment.
  • Windows fatal exception: access violation: This is the culminating error, indicating a segmentation fault. The access violation occurs when VTK attempts to initialize an OpenGL rendering context in ccpi.viewer.CILViewerBase.setInteractorStyle(), leading to the termination of the process with exit code 139 (SIGSEGV). This fatal exception underscores the severity of the issue, as it directly results in the test's failure.

These errors collectively demonstrate that the lack of proper OpenGL support in headless Windows environments directly leads to the failure of VTK initialization, causing the 3D viewer tests to crash. Understanding this sequence of errors is critical for developing an effective solution.

Expected Behavior

The expected behavior in this scenario is to bypass the 3D viewer tests on headless Windows CI environments. Since VTK requires actual OpenGL/GPU support, which is unavailable on platforms like GitHub Actions Windows runners, these tests should be skipped rather than executed and subsequently failed. This approach ensures that the test suite can run without crashing, providing reliable results for other tests that do not depend on OpenGL.

To achieve this, a common practice is to use conditional skipping based on the environment. In Python testing frameworks like pytest, this can be implemented using decorators that check the platform or environment variables before running a test. For example, the proposed solution involves adding a @pytest.mark.skipif decorator to the test class:

@pytest.mark.skipif(sys.platform == "win32", reason="VTK requires OpenGL, not available on headless Windows CI")
class MI3DViewerTest(unittest.TestCase):

This decorator tells pytest to skip the MI3DViewerTest class if the platform is Windows (`sys.platform ==