opencv-python 时，子进程不能正常启动

Question

一个生产者函数负责从摄像头中读取 frame，一个消费者函数负责使用 opencv 识别里面是否有人。
假如是单进程，可以正常运行，但是假如通过一个 Manager.Queue 通讯，试图使用生产者消费者模式的时候，只要调用 cv2 就报错。

代码如下：

import cv2
import time
import os
import sys
from multiprocessing import Process, Queue, Pool, Manager
from functools import wraps
from collections import defaultdict


basedir = os.path.abspath(os.path.dirname(__file__))
xml = os.path.join(basedir, 'opencv-master/data/haarcascades/haarcascade_frontalface_default.xml')
face_cascade = cv2.CascadeClassifier(xml)
cap = cv2.VideoCapture(0)
start = time.time()
total_fps = 0
manager = Manager()
queue = manager.Queue()


def producer(queue):
    print("producer start")
    fcount = 0
    max_fcount = 20
    sample_rate = 10
    while cap.isOpened() and cv2.waitKey(1) != 27 and fcount < max_fcount:
        ret, frame = cap.read()
        if not ret:
            break
        queue.put(frame)
        cv2.imshow('My Camera',frame)
        fcount += 1
    print("producer stop")
    queue.put('stop')


@timing
def consumer(queue):
    print("consumer start")
    i = -1
    while True:
        info = queue.get()
        i += 1
        print(f'{i}:{type(info)}')
        if isinstance(info, str):
            break
        
        gray = cv2.cvtColor(info, cv2.COLOR_BGR2GRAY)
        faces = face_cascade.detectMultiScale(
            gray,
            scaleFactor=1.15,
            minNeighbors=5,
            minSize=(15, 15),
        )
        
        for x, y, w, h in faces:
            top_left = (x, y)
            bottom = (x + w, y + h)
            cv2.rectangle(info, top_left, bottom, (125, 255, 255), 1)
        global total_fps
        global start
        total_fps += 1
        fps = 'fps:' + str(round(total_fps / (time.time() - start), 0))
        cv2.putText(info, fps, (10, 10), 1, 1, (175, 255, 175), thickness=1)

        cv2.imshow('My Camera',info)

        if not isinstance(faces, tuple):
            # 如果检测到人脸
            print(f'@{time.ctime()}, {len(faces)} detected!')
            file_name = str(int(time.time())) + '.jpg'
            cv2.imwrite(os.path.join(basedir, 'Persons', file_name), info)
        else:
            print('没有检测到人脸')
    print("consumer stop")


if __name__ == '__main__':
    c = Process(target=consumer, args=(queue,))
    # p.daemon = True
    c.daemon = True
    # p.start()
    c.start()
    # p.join()
    producer(queue)
    c.join()

症状就是，假如在constumer 中的下面这行代码之前输入 continue，就各种正常
但是只要用到了cv2，就会报错。
gray = cv2.cvtColor(info, cv2.COLOR_BGR2GRAY)

报错如下图，还看不出什么有效信息：

请问是哪里用错了呢？