[itamarst]

The expected joblib usage pattern is that it has a thread-local counter, which gets incremented on ThreadedBackend.configure(), and decremented on ThreadedBackend.finalize(), and if it hits zero then OpenBLASController.clear_controlling_thread() is called.

The result is that so long as you have Parallel() running in a particular thread, typically the main thread, other threads can't set OpenBLAS-pthreads thread pool size.

A separate companion PR will expose an API to allow joblib to override thread limits so they're either 1 or all cores, since we want that too, I think.

Haven't written all the relevant tests since seems worth some design feedback first @ogrisel

TODO

• CI passes
• Tests
• Changelog
• actually write the corresponding joblib logic and make sure it works

[ogrisel]

I like this approach. If we ever have a second case of process wide shared threadpool, we could even introduce a mixin class for those controller methods. Or we could use ducktyping on the controllers from joblib.