gen: Release frames after caught exceptions - #3694
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When a coroutine catches an exception from a yielded Future, gen.throw() adds the receiving generator frame to the exception traceback. The Future retains that exception, forming a cycle that keeps frame locals alive until cyclic garbage collection runs. Restore the pre-throw traceback after the generator handles the exception, covering both return and yield paths. Add a refcounting regression test. Fixes tornadoweb#3346
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Fixes #3346.
When a
@gen.coroutinecatches an exception from a yielded Future,gen.throw()leaves the receiving generator frame on the exception traceback. The Future retains that exception, closing a cycle through the Runner callback stack and keeping frame locals alive until cyclic GC.Restore the pre-throw traceback once the coroutine handles the exception, whether it returns or yields again. The regression test verifies CPython releases those locals immediately in both paths.
Tests:
gen_testpasses on Python 3.10, 3.13, and 3.14; flake8, black, and mypy pass. The full suite's sole failure (StaticDefaultFilenameRootTest.test_no_open_redirecton macOS) reproduces unchanged on master.