为什么这个 client 端的 socket 收不到 server 发来的信息?

下面是代码, server 端会在 recv 上挂住 这个是为什么?

server:

# -*- coding: UTF-8 -*-

# 导入库
import socket, threading, os, uuid, sys

SIZE = 1024 * 1024 * 5
filename = str(uuid.uuid4()) + '.jpg'

# 检查当前目录下是否有等下要命名的图片,有的话删除之
def checkFile():
    list = os.listdir('.')
    for iterm in list:
        if iterm == filename:
            os.remove(iterm)
            print('remove')
        else:
            pass

# 接受数据线程
def tcplink(sock, addr):
# def tcplink():
    print('Accept new connection from %s:%s...' % addr)
    # sock.send(b'Welcome from server!')
    print('receiving, please wait for a second ...')
    while True:
        try:
            data = sock.recv(SIZE)
            if not data :
                print('reach the end of file')
                break
            elif data == 'begin to send':
                print('create file')
                checkFile()
                with open(filename, 'wb') as f:
                    pass
            else:
                with open(filename, 'ab') as f:
                    f.write(data)
        except:
            print("Unexpected error:", sys.exc_info()[0])
            break
    print('msg send')
    sock.send(b'Welcome from server!')
    sock.close()
    print('receive finished')
    print('Connection from %s:%s closed.' % addr)


# 创建一个 socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# 监听端口（这里的 ip 要在不同的情况下更改）
s.bind(('127.0.0.1', 9999))
# 每次只允许一个客户端接入
s.listen(2)
print('Waiting for connection...')
while True:
    # 建立一个线程用来监听收到的数据
    sock, addr = s.accept()
    t = threading.Thread(target = tcplink, args = (sock, addr))
    # 线程运行
    t.start()

client:

# -*- coding: UTF-8 -*-
import socket

SIZE = 1024 * 1024 * 5

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# 建立连接:
s.connect(('127.0.0.1', 9999))

# 接收欢迎消息:
with open('./image.png', 'rb') as f:
   data = f.read()
   s.sendall(data)

print('sended !')
data = s.recv(SIZE)
# print(data)

s.close()
print('connection closed')