return-in-finally in multiprocessing/connection.py

https://github.com/python/cpython/blob/7fd61607cd28ec466717c78adfb1eb5b63add1f0/Lib/multiprocessing/connection.py#L330C1-L340C68

This is clearly a bug, because the except: clause contains a naked raise, i.e., asks for the exception to propagate on. But the return in finally swallows that exception.

                    try:
                              ...
                    except:
                        ov.cancel()
                        raise
                    finally:
                        nread, err = ov.GetOverlappedResult(True)
                        if err == 0:
                            f = io.BytesIO()
                            f.write(ov.getbuffer())
                            return f
                        elif err == _winapi.ERROR_MORE_DATA:
                            return self._get_more_data(ov, maxsize)

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gh-131233: remove return-in-finally in multiprocessing/connection.py #131416

I don't believe this is actually a bug.

The logic as written:

If an exception occurs while waiting for overlapped IO, cancel the overlapped IO request
Regardless of whether an exception occurred while waiting or not, get the overlapped IO result
If the IO operation was successful (in one of two possible ways), return a BytesIO instance containing the received data, even if an exception occurred while waiting for the operation
If the IO operation wasn't successful, re-raise any raised exception

While it's a genuinely cryptic way of writing the code, I'm not seeing an immediately obvious way to rewrite it to eliminate the return-in-finally without duplicating the Win32 overlapped IO result and error code checking logic.

Yes, I see - the return is conditional so it's possible that what you wrote was the intention.

Would this be a way to rewrite it?

                sentinel = object()
                return_value = sentinel
                try:
                    try:
                        if err == _winapi.ERROR_IO_PENDING:
                            waitres = _winapi.WaitForMultipleObjects(
                                [ov.event], False, INFINITE)
                            assert waitres == WAIT_OBJECT_0
                    except:
                        ov.cancel()
                        raise
                    finally:
                        nread, err = ov.GetOverlappedResult(True)
                        if err == 0:
                            f = io.BytesIO()
                            f.write(ov.getbuffer())
                            return_value = f
                        elif err == _winapi.ERROR_MORE_DATA:
                            return_value = self._get_more_data(ov, maxsize)
                except:
                    if return_value == sentinel:
                        raise

                if return_value != sentinel:
                    return return_value

pythongh-131233: remove return-in-finally in multiprocessing/connecti…

Merge branch 'main' into pythongh-131233

gh-131233: remove return-in-finally in multiprocessing/connection.py (#…

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return-in-finally in multiprocessing/connection.py #131233

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ncoghlan commented on Mar 14, 2025

iritkatriel commented on Mar 14, 2025

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