to_thread.run_sync() foils meta-class destructors

[belm0]

Invoking to_thread.run_sync() seems to prevent meta-class destructors of completely unrelated classes from being called on shutdown.

import trio

class deltype(type):
    def __del__(self):
        print(f"{self.__name__} class deleted")

Test = deltype("Test", (object,), {})

async def async_main():
    await trio.to_thread.run_sync(lambda: print('hi'))
    await trio.sleep(0)

trio.run(async_main)

expected output:

hi
Test class deleted

actual output:

hi

if the run_sync() is commented out, the meta-class destructor is called as expected:

Test class deleted

[smurfix]

"git bisect" says the thread cache is the culprit.

1d1f871

[smurfix]

That was reasonably easy ;-)