> Thanks for testing! It wouldn't entirely surprise me if this turned out to be an Apple bug somewhere.

I'm ran threading_hang.py on Mac OS X 10.10 Yosemite (darwin 14.5) on Python 2.7 compiled in release mode.

After 28 iterations (~4 hours, 9 minutes per iteration), the bug was still not reproduced.

It looks like my OS X version is more recent than yours. You should run your test on various OS X versions to try to identify the bug.

Since it's likely a race condition, I would not be surprised that the bug is very hard to reproduce and may exist on other platforms, but may be less likely on other platforms or even less likely on some computers for various reasons...