当我在Python中使用多个进程时，为什么主进程无法退出？

Question

我希望所有子进程都完成，然后主进程退出，但它无法退出，为什么？

#!/usr/bin/env python
# coding=utf-8

import os

from multiprocessing import Manager
from multiprocessing import Pool


def write_file_name_to_queue(q, src_folder):
    print('Process to write: %s' % os.getpid())
    if not os.path.exists(src_folder):
        print "Please input folder path"
        return

    for (dirpath, dirnames, filelist) in os.walk(src_folder):
        for name in filelist:
            if name[0] == '.':
                continue
            q.put(os.path.join(dirpath, name))


def read_file_name_from_queue(q):
    print('Process to read: %s' % os.getpid())
    while True:
        value = q.get(True)
        print('Get %s from queue.' % value)


if __name__ == "__main__":
    mg = Manager()
    q = mg.Queue()
    p = Pool()

    p.apply_async(func=write_file_name_to_queue, args=(q, "./test/"))

    for i in xrange(8):
        p.apply_async(func=read_file_name_from_queue, args=(q,))

    p.close()
    p.join()

运行它并获得以下结果：

➜  check python check_process.py
Process to write: 3918
Process to read: 3919
Process to read: 3920
Get ./test/a from queue.
Get ./test/b from queue.
Get ./test/c from queue.
Get ./test/e from queue.
Get ./test/f from queue.
Process to read: 3921
Process to read: 3918

这个过程还在等待。