Flaky `test_flight_animation_export_gif`: intermittent bus error in VTK off-screen rendering

[thc1006]

What happens

tests/integration/test_plots.py::test_flight_animation_export_gif, and the other VTK/PyVista off-screen animation tests near it, sometimes crash the whole pytest process:

Fatal Python error: Bus error
  File ".../tests/integration/test_plots.py", line 79 in test_flight_animation_export_gif

It is intermittent. The same commit and the same environment can pass one run and crash the next. When it does crash, the interpreter dies rather than a test failing cleanly, so the coverage upload is skipped and the whole matrix goes red.

Why it looks like a flake rather than a code regression

• It also hit develop's own Tests run on 2026-07-19 (https://github.com/RocketPy-Team/RocketPy/actions/runs/29697106388), at the same file and line.
• On a branch I was working on, a passing run and a crashing run installed the same rendering stack (vtk 9.6.2, pyvista 0.48.4, matplotlib 3.11.1, pillow 12.3.0), so it is not tied to a dependency bump.
• A bus error here is a native crash inside VTK's off-screen OpenGL, which tends to be a fragile spot on headless CI runners rather than something in the Python test logic.

Possible directions

• Retry only the animation tests, for example with pytest-rerunfailures (@pytest.mark.flaky(reruns=2)), so an occasional OpenGL hiccup does not take down the whole matrix.
• Or run the rendering tests in their own step or subprocess, so a crash there does not end the rest of the suite.
• Or, if it can be pinned down, look at the off-screen GL setup on the runners (software rendering / mesa).

I ran into this while working on #1054, which is a seeding change that does not touch plotting. Happy to send a small PR marking the animation tests with a rerun if that is the approach you would prefer.

[Gui-FernandesBR]

@thc1006 which branch and commit were you working at?

[thc1006]

Thanks for picking this up, @Gui-FernandesBR.

I was on my PR #1054, branch enh/reproducible-montecarlo-seeding, at commit da2ba5cf. The useful part is that it is not specific to my branch: the same crash happens on develop. The Tests run on develop d31822d3 failed the same way.

Both crash in the Pytest (macos-latest, 3.14) job, during pytest tests/integration:

Fatal Python error: Bus error
  File ".../tests/integration/test_plots.py", line 79 in test_flight_animation_export_gif
... Bus error: 10   pytest tests/integration --cov=rocketpy --cov-append

• develop d31822d3: run 29697106388, the Pytest (macos-latest, 3.14) job
• my ENH: reproducible Monte Carlo via per-simulation-index seeding #1054 da2ba5cf: run 29728954730, the same job

A few things point to a flake rather than a real failure:

• The other matrix jobs in each run show The operation was canceled, not a test failure. That is fail-fast cancelling the siblings once the macOS job dies (it leaves orphan pytest and Xvfb processes behind), so a run can look like every job failed when only macos-3.14 actually crashed.
• It is intermittent on the same code: an earlier commit of mine, e5ba9404, passed Pytest (macos-latest, 3.14), and da2ba5cf crashed it.
• Same rendering stack in the passing and crashing runs: vtk 9.6.2, pyvista 0.48.4, matplotlib 3.11.1, pillow 12.3.0, so it is not a version bump.
• My change only touches Monte Carlo seeding, nothing in the plotting path.

So it reads as a native crash in the VTK off-screen GIF export on the macOS runner. Happy to send a small PR marking the animation tests with a rerun (e.g. pytest-rerunfailures) if that is the direction you would prefer.