os.fork() was called. os.fork() is incompatible with multithreaded code,

[mschwamb]

Showing up in the linux runners as a warning:

/opt/hostedtoolcache/Python/3.11.14/x64/lib/python3.11/multiprocessing/popen_fork.py:66: RuntimeWarning: os.fork() was called. os.fork() is incompatible with multithreaded code, and JAX is multithreaded, so this will likely lead to a deadlock.
self.pid = os.fork()

[mschwamb]

Python's changed how it does multithreading as of v3.13 - this all goes away and it seems jax plays nicely in python 3.14 so going to try and upgrade the CI to 3.13 and 3.14

[matthewholman]

Fixed by #336, which pins the ProcessPoolExecutor workers to the spawn start method (same root cause as #256). No
more os.fork() in the multithreaded/JAX path. Closing.