Comment 7 for bug 1702160

These days, there is one other user of exception_cb: DataSourceScaleway.
It uses
            exception_cb=lambda _, exc: exc.code == 404 or (
                isinstance(exc.cause, requests.exceptions.ConnectionError)
            )
which returns True on ConnectionErrors and on 404, causing it NOT to retry on 404 according to the current logi in readurl() which is probably the intended behavior. (A file not found error won't change by retrying ...)

Given the history, my preferred resolution of the issue would be to leave the OpenStack helper alone, reinstate the old logic in url_helper.readurl, update the commnet and revert the logic in DataSourceScaleway.