Can't use XMLRPC errors with multiprocessing and pickle

Can't use XMLRPC errors with multiprocessing and pickle modules. Multiprocessing freezes.

Bug example

from multiprocessing import Pool
from xmlrpc.client import ProtocolError


def raise_error(_):
    raise ProtocolError("1", 2, "3", {})


Pool(1).map(raise_error, (["A", 5]))

Expected output

Traceback (most recent call last):
  File "C:\Program Files\Python310\lib\multiprocessing\pool.py", line 125, in worker
    result = (True, func(*args, **kwds))
  File "C:\Program Files\Python310\lib\multiprocessing\pool.py", line 48, in mapstar
    return list(map(*args))
  File "C:\Users\ppatrin\Projects\test.py", line 6, in raise_error
    raise ProtocolError("1", 2, "3", {})
xmlrpc.client.ProtocolError: <ProtocolError for 1: 2 3>

Actual output

"C:\Program Files\Python310\python.exe" C:/Users/ppatrin/Projects/test.py
Exception in thread Thread-3 (_handle_results):
Traceback (most recent call last):
  File "C:\Program Files\Python310\lib\threading.py", line 1009, in _bootstrap_inner
    self.run()
  File "C:\Program Files\Python310\lib\threading.py", line 946, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Program Files\Python310\lib\multiprocessing\pool.py", line 576, in _handle_results
    task = get()
  File "C:\Program Files\Python310\lib\multiprocessing\connection.py", line 256, in recv
    return _ForkingPickler.loads(buf.getbuffer())
TypeError: ProtocolError.__init__() missing 4 required positional arguments: 'url', 'errcode', 'errmsg', and 'headers'

And multiprocessing processes frozes.

Real problem

import pickle
from xmlrpc.client import ProtocolError

pickle.loads(pickle.dumps(ProtocolError(1, "2", 3, {})))

>>> TypeError: __init__() missing 4 required positional arguments: 'url', 'errcode', 'errmsg', and 'headers'

How to fix

We should fix a super call in exception method initializer.

class ProtocolError(Error):
    """Indicates an HTTP protocol error."""
    def __init__(self, url, errcode, errmsg, headers):
---        Error.__init__(self)
+++        Error.__init__(self, url, errcode, errmsg, headers)
        self.url = url
        self.errcode = errcode
        self.errmsg = errmsg
        self.headers = headers

Your environment

• CPython versions tested on: 3.10.4
• Operating system and architecture: Windows 11 and Ubuntu 20.04.

从 xmlrpc.client 模块中的 ProtocolError 初始化器开始，复现 issue 中展示的 pickle 往返过程。当 ProtocolError 可以成功进行 pickle 和 unpickle，并且 multiprocessing 示例在返回预期错误时不再冻结时，工作就完成了。