The current solutions doesn't look correct. It swallows cancelling causing task to hang: https://bugs.python.org/issue42130 .

Proposed test case calls cancel for inner future and set_result for outer task in the same loop step. The old (prior to this patch) behaviour in this case looks correct to me. We have to elaborate on original problem to find the source of actual race.

Therefore I've tried to reproduce the problem with original code sample and published the adapted version of it here: https://github.com/ods/bpo-37658 . Nikita Ilyasov, please correct me if I've misunderstood what you meant. My result is that proposed solution actually doesn't solve the problem, but rather lowers the chance to get it. Namely, I get the traceback quoted in README both with 3.8.5, 3.9.4, and current code in main.