coroutine: catch asyncio.CancelledError alongside Exception so cancellations surface on the returned future#3664
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…lations surface on the returned future The @gen.coroutine wrapper and the Runner both caught only Exception. asyncio.CancelledError inherits from BaseException in Python 3.8- and from Exception in 3.8+, so the wrapper's outer try/except Exception around ctx_run(func, ...) and the Runner's except Exception around future.result() silently dropped the cancellation. The pre-fix behaviour either let the CancelledError leak out (3.8-) or vanished into the result_future without ever surfacing on the consumer's await (3.8+). Catch (Exception, asyncio.CancelledError) explicitly at both call sites so an asyncio cancellation of a yielded future or a sync cancellation in the coroutine body is propagated to the returned future the same way any other exception is. Regression test for tornadoweb#3537.
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Fixes #3537.
The @gen.coroutine wrapper and the Runner both caught only Exception. asyncio.CancelledError inherits from BaseException in Python 3.7- and from Exception in 3.8+, so the wrapper's outer try/except Exception around ctx_run(func, ...) and the Runner's except Exception around future.result() silently dropped the cancellation. The pre-fix behaviour either let the CancelledError leak out (3.7-) or vanished into the result_future without ever surfacing on the consumer's await (3.8+).
Catch (Exception, asyncio.CancelledError) explicitly at both call sites so an asyncio cancellation of a yielded future or a sync cancellation in the coroutine body is propagated to the returned future the same way any other exception is. Three new tests cover the sync-raise, post-yield-raise, and yield-a-cancelled-future shapes.