asyncio: initial exception traceback frames are lost for Future.result()

[iaroslavkrasikov]

Since gh-90082 (PR #30274), the C implementation of Future stores it's exception and traceback separately. However, Future.result() clears the traceback after first call, which means the initial traceback frames are lost on subsequent awaits or result() calls. Pure-python Future implementation is unaffected.

async def fail():
    await asyncio.sleep(0.1)
    1 / 0

async def main():
    task = asyncio.create_task(fail())
    await asyncio.sleep(0.2)
    with contextlib.suppress(ZeroDivisionError):
        await task
    await task

_CFuture traceback (current):

Traceback (most recent call last):
  ...
  File "<python-input-0>", line 9, in main
    await task
ZeroDivisionError

_PyFuture traceback (expected):

Traceback (most recent call last):
  ...
  File "<python-input-7>", line 9, in main
    await task
  ...
  File "<python-input-7>", line 3, in fail
    1 / 0
ZeroDivisionError

Linked PRs

• gh-154791: Fix asyncio Future losing traceback on repeated result() call #154798