Weirdness with traceback when resuming coroutines with coro.throw() #93592
Description
Bug report
When a coroutine is resumed using coro.throw(), e.g. by using Task.cancel(), the frame.f_back chain becomes
mysteriously truncated at arbitrary places in the call stack.
Your environment
- CPython versions tested on: 3.10.4
- Operating system and architecture: Win 11
The following example pytest code demonstrates the problem. A recursive stack of coroutines is resumed, either normally (after a sleep) or by sending a CancelledError in. In the latter case, a traceback is generated and it is, in all cases, curiously truncated after just a few steps. Manually walking the stack will result in a f.f_back == None after a few steps.
I employ a few different recursions, including a manual await-like method operating directly on the coroutine protocol. Initially I suspected that coro.throw() was deliberatly engineeded to mess with the f.f_back of the frame chain, but even call stacks using regular coroutine recursion (the await keyword on async functions) appear truncated.
traceback.print_stack() is ultimately doing a f = sys._getframe() and following the f = f.f_back chain. It can just as easily be verified that this chain is broken with a None after a short bit.
import asyncio
import pytest
import types
import traceback
@types.coroutine
def myawait(coro):
try:
val = coro.send(None)
except StopIteration as e:
return e.value
while True:
try:
val = yield val
except BaseException as e:
try:
val = coro.throw(e)
except StopIteration as e:
return e.value
else:
try:
val = coro.send(val)
except StopIteration as e:
return e.value
async def realawait(coro):
return await coro
async def bas(result):
return await bar(result)
async def foo1(result, n=2):
if n:
return await foo1(result, n-1)
return await bas(result)
async def foo2(result, n=2):
if n:
return await foo2(result, n-1)
return await realawait(bar(result))
async def foo3(result, n=2):
if n:
return await foo3(result, n-1)
return await myawait(bar(result))
async def bar(result):
try:
await asyncio.sleep(0.1)
except asyncio.CancelledError:
traceback.print_stack(limit=5)
result.append(False)
result.append(traceback.format_stack())
else:
traceback.print_stack(limit=5)
result.append(True)
result.append(traceback.format_stack())
@pytest.mark.parametrize("func", [foo1, foo2, foo3])
async def test_regular(func):
result = []
t = asyncio.Task(func(result))
await asyncio.sleep(0)
await t
ok, stack = result
assert ok
assert len(stack) > 5
@pytest.mark.xfail()
@pytest.mark.parametrize("func", [foo1, foo2, foo3])
async def test_truncated(func):
result = []
t = asyncio.Task(func(result))
await asyncio.sleep(0)
t.cancel()
await t
ok, stack = result
assert not ok
assert len(stack) > 5