Possible leak when exception is raised in inner coroutine #3346

Hi, I have been debugging a memory leak in an application that uses Tornado and I was able to narrow down the behavior to what I believe is a representative minimum example, based on an existing unit test.

Edit:

• Reproducing with CPython 3.9.16 on macOS, although the original issue was on Linux with CPython 3.6.8 (I know, super old! but hopefully any workaround would still apply).
• Tested against master as of a6dfd70 but from what I can tell this reproduces very far back to old versions of Tornado (e.g. 4.5+)

Possibly related, from what I can gather (although closed for a long time now): #1872 #2229

I have a couple questions:

1. Is this expected behavior / is there some reason I'm missing for why a reference cycle forms here? I don't have a ton of experience with Tornado directly so maybe this is just a side effect of how the IOLoop is implemented or something, and it's expected that GC is needed to clean up in this scenario.
2. Is there a "correct" workaround that would help free the exception / traceback, etc.? I can del the local variables, but my fear is that in that case the remaining stack frames, tracebacks etc. would stick around, possibly resulting in other leaked variables that aren't deleted (particularly if the exception is bubbled through several coroutines in a more complicated example).

Here is the test I'm reproducing with, adapted from the existing one in gen_test.py:

    @skipNotCPython
    @unittest.skipIf(
        (3,) < sys.version_info < (3, 6), "asyncio.Future has reference cycles"
    )
    def test_coroutine_refcounting(self):
        # On CPython, tasks and their arguments should be released immediately
        # without waiting for garbage collection.
        @gen.coroutine
        def raise_exc():
            yield
            raise ValueError("Some error")

        @gen.coroutine
        def inner():
            class Foo(object):
                pass

            local_var = Foo()
            self.local_ref = weakref.ref(local_var)

            try:
                yield raise_exc()
            except ValueError:
                # del local_var # <- this works to free the variable
                pass

        @gen.coroutine
        def inner2():
            yield inner()

            # Error! There is a still a strong reference to the local variable via
            # the ValueError traceback -> `inner` frame, but why is that?
            self.assertIsNone(self.local_ref())

        self.io_loop.run_sync(inner2, timeout=3)

        self.assertIsNone(self.local_ref())
        self.finished = True

Thanks!