test: ignore Python 3.15 fork-with-threads DeprecationWarning in test_multiprocessing#221
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test: ignore Python 3.15 fork-with-threads DeprecationWarning in test_multiprocessing#221d-v-b wants to merge 2 commits into
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…_multiprocessing Python 3.15 makes os.fork() emit a DeprecationWarning when the process has multiple threads (python/cpython#84559). zarr's sync event-loop thread is always running by the time test_multiprocessing forks, and the suite runs with filterwarnings = ["error"], so the fork parametrizations fail on 3.15. Fork-safety despite those threads is exactly what this test pins down (zarr registers an os.register_at_fork handler), so keep the coverage and ignore the warning with a targeted per-param filter. Fixes zarr-developers#4086 Assisted-by: ClaudeCode:claude-fable-5
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Fixes zarr-developers#4086
Problem
Python 3.15 makes
os.fork()emitDeprecationWarning: This process is multi-threaded, use of fork() may lead to deadlocks in the childwhenever the forking process has more than one thread (python/cpython#84559). By the timetest_multiprocessingcallsctx.Pool(), zarr's sync bridge has already started its dedicatedzarr_ioevent-loop thread, so the parent is always multi-threaded — even under plain single-process pytest (the issue attributes the threads to pytest-xdist, but xdist is incidental). Because the suite runs withfilterwarnings = ["error"], the twoforkparametrizations fail on 3.15.Fix
Add a targeted
pytest.mark.filterwarningsignore to theforkparam only, matching the exact 3.15 message. The warning is emitted in the parent duringos.fork(), so a per-test marker catches it. Note the message part is a real regex here: pytest's ini/marker filter parser does not escape it (unlike stdlib-W), so the escaped parens/\d+are correct.Skipping fork on 3.15+ was rejected: zarr deliberately supports fork via its
os.register_at_forkhandler inzarr/core/sync.py, and this test is what pins that behavior. The coverage should keep running for as long as CPython supports fork-with-threads.Verification
Reproduced and verified on CPython 3.15.0b3 (uv-installed, numpy/numcodecs built from source):
['MainThread', 'zarr_io', 'asyncio_0']— zarr's own threads trigger the deprecation, no xdist involved.DeprecationWarningfrom the issue is raised under warnings-as-errors.Poolroundtrip completes on 3.15 and the child processes return correct data — i.e. zarr'sregister_at_forkhandling still works there.pytest tests/test_array.py::test_multiprocessingpasses on macOS (fork params skip; spawn/forkserver pass).🤖 Generated with Claude Code