potential pypy mutability bug

[iscai-msft]

Describe the bug

when using pypy and mutating a CIMultiDict, I get a RuntimeError of "Dictionary changed during iteration". This doesn't happen using regular python, and I don't see this error when using pypy and other kinds of dictionaries.

To Reproduce

I've added the following tests to repro with pypy

@pytest.mark.asyncio
async def test_headers_response_items_multidict_compare():
    h = CIMultiDict({'a': '123, 456', 'b': '789'})
    before_mutation_items = h.items()
    h['c'] = '000'
    with pytest.raises(RuntimeError):
        list(before_mutation_items)

def test_headers_response_items_requests_compare():
    from requests.structures import CaseInsensitiveDict
    h = CaseInsensitiveDict({'a': '123, 456', 'b': '789'})
    before_mutation_items = h.items()
    h['c'] = '000'
    list(before_mutation_items)

def test_headers_response_items_python_compare():
    h = {'a': '123, 456', 'b': '789'}
    before_mutation_items = h.items()
    h['c'] = '000'
    list(before_mutation_items)

Expected behavior

My thinking is that if mutating the dictionary doesn't raise for regular python is that it shouldn't raise for pypy. Is this expected behavior?

Logs/tracebacks

self = _ItemsView('a': '123, 456', 'b': '789', 'c': '000')

    def _iter(self):
        for i, k, v in self._impl._items:
            if self._version != self._impl._version:
>               raise RuntimeError("Dictionary changed during iteration")
E               RuntimeError: Dictionary changed during iteration

envs/corepypy3venv/site-packages/multidict/_multidict_py.py:459: RuntimeError

Python Version

$ python --version
Python 3.7.10 (51efa818fd9b24f625078c65e8e2f6a5ac24d572, Apr 08 2021, 17:43:00)
[PyPy 7.3.4 with GCC Apple LLVM 12.0.0 (clang-1200.0.32.29)]

aiohttp Version

$ python -m pip show aiohttp
Version: 3.7.4.post0

multidict Version

$ python -m pip show multidict
Version: 5.1.0

yarl Version

$ python -m pip show yarl
Version: 1.6.3

OS

macOS

Related component

Client

Additional context

No response

Code of Conduct

• I agree to follow the aio-libs Code of Conduct

[Dreamsorcerer]

Looks like we have pypy running on the CI. So, if you could make a PR with the test reproducing the issue, that would be great.

[iscai-msft]

hey apologies @Dreamsorcerer for the delayed response, I made a PR here #627. Still waiting on CI, also not sure if I'm supposed to add a changelog entry for just adding a test. Thanks!

[Dreamsorcerer]

I'm not really familiar with the code, but this appears to be an explicit check:
https://github.com/aio-libs/multidict/blob/master/multidict/_multidict_py.py#L483

I suspect one option would be to reset the version at the beginning of that _iter() method, rather than causing an error any time that is reused. It may also be fine to just remove that check, as I suspect the same error would occur if changing the items while iterating over in the for loop.