Simple two way socket communication-CodePudding

I want to implement a simple two way communication between server and client using sockets. The Client sends messages and waits for replies. The Server reads messages and replies with appropriate results. I have the below codes, but both of them are hanging. Can you explain what is wrong here?

Server:

import socket
import time

HOST = "127.0.0.1"
PORT = 18735

# create socket and listen
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((HOST, PORT))
s.listen()
print(f"Listening on {HOST}:{PORT}")

# accept connection
conn, addr = s.accept()
print(f"Accepted connection from {HOST}:{PORT}")

cmd = ""
while True:
    data = conn.recv(2)
    time.sleep(1)
    if not data:
        break
    print(data.decode())
    # conn.sendall(b"OK")  This line fixes the problem!

conn.sendall(b"Finished")
s.close()

Client:

import socket

HOST = "127.0.0.1"
PORT = 18735

# create socket and connect
cs = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
cs.connect((HOST, PORT))

# send data
cs.sendall(b"123456")

# wait for a result
data = cs.recv(1024)
print("result: ", data)
cs.close()

EDIT: Now, I can solve the problem by sending the short message from the server. However, I wonder whether this is necessary or not?

CodePudding user response：

The server won't break out of its while loop until the client either disconnects or at least shuts down the write half of its connection (with cs.shutdown(socket.SHUT_WR)).

The client won't disconnect until it receives a message from the server, which the server sends outside of the while loop (i.e. after the client disconnects/shuts down the write half of its connection).

You've created a deadlock.

Fundamentally, the issue is that it doesn't make sense to wait until the client has disconnected before you send a response to that client. Either the client needs to not expect a response back, or the server needs to send it while the client is still connected. In the case of the latter, you probably need to implement one of the following:

The application protocol you're implementing needs a defined message structure so the server knows when the client has finished sending.
The client needs a special sequence it can send to indicate the end of a message.
The client needs to shut down the write half of its connection to indicate to the server that it won't send any more data.