Deadlock in pytest tests

I encountered an issue when building this software with the Nix package manager on NixOS. Our package uses commit 536b6e1414c8a93f0815dbae85d03749daaa5432 because there is no recent release.

nix-build '<nixpkgs>' -A python310Packages.opensfm --check

The build is often stuck at a very specific point:

...
opensfm/test/test_dense.py ..                                            [ 55%]
opensfm/test/test_geo.py .....                                           [ 57%]
opensfm/test/test_geometry.py ....                                       [ 59%]
opensfm/test/test_io.py .........                                        [ 62%]
opensfm/test/test_matching.py ......                                     [ 65%]
opensfm/test/test_multiview.py .........                                 [ 68%]
opensfm/test/test_pairs_selection.py .........                           [ 72%]
opensfm/test/test_reconstruction_alignment.py ........                   [ 75%]
opensfm/test/test_reconstruction_incremental.py Terminated

So i have to kill the build process. I waited over 3.5 hours once!

Maybe the last test creates a deadlock?

When i look at the process with strace, it only does sched_yield calls.

I have seen similar issues with other python packages doing pytest tests and only doing futex calls at some point.

This seem to happen especially when multiple python packages are built at the same time.

Downstream issue: https://github.com/NixOS/nixpkgs/issues/217247
Our package definition: https://github.com/NixOS/nixpkgs/blob/nixos-unstable/pkgs/development/python-modules/opensfm/default.nix

Reproduce the Nix check with nix-build '<nixpkgs>' -A python310Packages.opensfm --check, then focus on opensfm/test/test_reconstruction_incremental.py, where the run stops. Compare behavior when Python packages build concurrently and inspect the process with strace. Done means the pytest suite completes reliably instead of hanging at that test.