`threadpool_limits` is not thread-safe under `ThreadPoolExecutor`

Start by reading the threadpool_limits context manager and its tests; the issue does not name their file paths. Reproduce the reported behavior with the supplied ThreadPoolExecutor example and inspect how limits are changed and restored across overlapping contexts. Done means concurrent workers consistently observe the intended limit and the original limits are restored afterward.

When using threadpoolctl.threadpool_limits inside multiple Python threads (e.g. via ThreadPoolExecutor), the thread limit is not reliably enforced.
Despite using with threadpool_limits(limits=2) inside each worker, some threads observe OpenBLAS using the full default thread count.

Reproducible Example

#!/usr/bin/env python3
from threadpoolctl import threadpool_limits, threadpool_info
import torch

from concurrent.futures import ThreadPoolExecutor, as_completed


def worker(i):
    # NOTE: this appears to be process-global, not thread-local
    with threadpool_limits(limits=2):
        a = torch.randn(512, 256, device="cpu")
        b = torch.randn(256, 512, device="cpu")
        _ = a @ b
        info = threadpool_info()
        print(f"[worker {i}] threadpool_info:", info)
        return info


results = []

with ThreadPoolExecutor(max_workers=8) as executor:
    futures = [executor.submit(worker, i) for i in range(8)]
    for fut in as_completed(futures):
        results.append(fut.result())

print("final threadpool_info:")
print(threadpool_info())

Actual Output

[worker 6] threadpool_info: [{'user_api': 'blas', 'internal_api': 'openblas', 'num_threads': 2, 'prefix': 'libscipy_openblas', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/numpy.libs/libscipy_openblas64_-56d6093b.so', 'version': '0.3.29', 'threading_layer': 'pthreads', 'architecture': 'Haswell'}, {'user_api': 'openmp', 'internal_api': 'openmp', 'num_threads': 2, 'prefix': 'libgomp', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/torch/lib/libgomp.so.1', 'version': None}]
[worker 1] threadpool_info: [{'user_api': 'blas', 'internal_api': 'openblas', 'num_threads': 2, 'prefix': 'libscipy_openblas', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/numpy.libs/libscipy_openblas64_-56d6093b.so', 'version': '0.3.29', 'threading_layer': 'pthreads', 'architecture': 'Haswell'}, {'user_api': 'openmp', 'internal_api': 'openmp', 'num_threads': 2, 'prefix': 'libgomp', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/torch/lib/libgomp.so.1', 'version': None}]
[worker 5] threadpool_info: [{'user_api': 'blas', 'internal_api': 'openblas', 'num_threads': 24, 'prefix': 'libscipy_openblas', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/numpy.libs/libscipy_openblas64_-56d6093b.so', 'version': '0.3.29', 'threading_layer': 'pthreads', 'architecture': 'Haswell'}, {'user_api': 'openmp', 'internal_api': 'openmp', 'num_threads': 2, 'prefix': 'libgomp', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/torch/lib/libgomp.so.1', 'version': None}]
[worker 3] threadpool_info: [{'user_api': 'blas', 'internal_api': 'openblas', 'num_threads': 2, 'prefix': 'libscipy_openblas', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/numpy.libs/libscipy_openblas64_-56d6093b.so', 'version': '0.3.29', 'threading_layer': 'pthreads', 'architecture': 'Haswell'}, {'user_api': 'openmp', 'internal_api': 'openmp', 'num_threads': 2, 'prefix': 'libgomp', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/torch/lib/libgomp.so.1', 'version': None}]
[worker 4] threadpool_info: [{'user_api': 'blas', 'internal_api': 'openblas', 'num_threads': 2, 'prefix': 'libscipy_openblas', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/numpy.libs/libscipy_openblas64_-56d6093b.so', 'version': '0.3.29', 'threading_layer': 'pthreads', 'architecture': 'Haswell'}, {'user_api': 'openmp', 'internal_api': 'openmp', 'num_threads': 2, 'prefix': 'libgomp', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/torch/lib/libgomp.so.1', 'version': None}]
[worker 0] threadpool_info: [{'user_api': 'blas', 'internal_api': 'openblas', 'num_threads': 2, 'prefix': 'libscipy_openblas', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/numpy.libs/libscipy_openblas64_-56d6093b.so', 'version': '0.3.29', 'threading_layer': 'pthreads', 'architecture': 'Haswell'}, {'user_api': 'openmp', 'internal_api': 'openmp', 'num_threads': 2, 'prefix': 'libgomp', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/torch/lib/libgomp.so.1', 'version': None}]
[worker 7] threadpool_info: [{'user_api': 'blas', 'internal_api': 'openblas', 'num_threads': 2, 'prefix': 'libscipy_openblas', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/numpy.libs/libscipy_openblas64_-56d6093b.so', 'version': '0.3.29', 'threading_layer': 'pthreads', 'architecture': 'Haswell'}, {'user_api': 'openmp', 'internal_api': 'openmp', 'num_threads': 2, 'prefix': 'libgomp', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/torch/lib/libgomp.so.1', 'version': None}]
[worker 2] threadpool_info: [{'user_api': 'blas', 'internal_api': 'openblas', 'num_threads': 2, 'prefix': 'libscipy_openblas', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/numpy.libs/libscipy_openblas64_-56d6093b.so', 'version': '0.3.29', 'threading_layer': 'pthreads', 'architecture': 'Haswell'}, {'user_api': 'openmp', 'internal_api': 'openmp', 'num_threads': 2, 'prefix': 'libgomp', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/torch/lib/libgomp.so.1', 'version': None}]
final threadpool_info:
[{'user_api': 'blas', 'internal_api': 'openblas', 'num_threads': 2, 'prefix': 'libscipy_openblas', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/numpy.libs/libscipy_openblas64_-56d6093b.so', 'version': '0.3.29', 'threading_layer': 'pthreads', 'architecture': 'Haswell'}, {'user_api': 'openmp', 'internal_api': 'openmp', 'num_threads': 24, 'prefix': 'libgomp', 'filepath': '/root/miniconda3/envs/gp_py311/lib/python3.11/site-packages/torch/lib/libgomp.so.1', 'version': None}]

Some workers correctly observe num_threads = 2, while others observe num_threads = 24, even though all are wrapped in with threadpool_limits(limits=2).

Environment

Distributor ID: Ubuntu
Description: Ubuntu 22.04.5 LTS
Release: 22.04
Codename: jammy

torch: 2.9.0
numpy: 2.2.6
scipy: 1.16.2
threadpoolctl: 3.6.0