When we spawn a task, we inject a magic local variable into the top stack frame's local namespace, as part of our tricky control-C magic:

However, this doesn't work for async def functions defined in C or Cython, because they don't have a Python frame. So we a simple workaround, we detect this case and inject a no-op Python frame where we can insert our magic local (cf #550):

However, the bug came back at some point with newer versions of Cython:

             |   File "/root/.pyenv/versions/3.11.6/lib/python3.11/site-packages/trio/_core/_run.py", line 1599, in spawn_impl
             |     coro.cr_frame.f_locals.setdefault(LOCALS_KEY_KI_PROTECTION_ENABLED, system_task)
             |     ^^^^^^^^^^^^^^^^^^^^^^
             | AttributeError: 'NoneType' object has no attribute 'f_locals'

From the error message, it appears that Cython has added a cr_frame attribute to its fake coroutine objects... just it's None. So it's still not really there, but it's enough to sneak past our hasattr(coro, "cr_frame") test.

I think the fix here is just to replace the if not hasattr(coro, "cr_frame"): with if getattr(coro, "cr_frame", None) is None:. But it would be good to also run a test for this in CI. We do have a test for the interpreter's built-in C coroutines (test_async_function_implemented_in_C) and I originally assumed that would be enough, but obviously it wasn't, so I guess we need to do the legwork to install cython in CI and build a tiny Cython module.

Edit by CoolCat467:
Things to do:

• Fix existing issue (Cython Workaround #2911 is handling)
• Add CI runner for cython so this sort of thing doesn't happen again in the future (Add Cython to CI #2942)