How to establish a 2 way communication using sockets in Python? - python-3.x

Question

So, I have a server script that receives an image from a client script and is supposed to send an acknowledgement saying "OK". But the acknowledgement never goes through.
Server script -
import socket,sys
s = socket.socket()
print("Socket successfully created")
port =80
s.bind(('', port))
print("socket binded to %s" %(port))
s.listen(5)
print("socket is listening")
while True:
c, addr = s.accept()
print('Got connection from', addr)
file_name=s.recv(1024)
file_name=fil_ename.decode("utf-8")
with open(file_name,"wb")as f:
while True:
data=c.recv(1024)
if not data:
break
f.write(data)
c.send(bytes('Thank you ! File received.',"utf-8"))
c.close()
Client script -
import socket
s = socket.socket()
# Define the port on which you want to connect
port = 80
s.connect(('IP address of my server', port))
s.send(bytes("hand.jpeg","utf-8"))
f=open("back.jpeg","rb")
data=f.read(512)
while data:
s.send(data)
data=f.read(512)
f.close()
print(s.recv(10))
The server does not send any acknowledgement and seems to get stuck in the for loop. But if i remove the line c.send(bytes('Thank you ! File received.',"utf-8")) from the server script, the code works well. Also, if I remove the receive part from server side and just send the acknowledgent part , i.e c.send(bytes('Thank you ! File received.',"utf-8")) , the client receives the message. But If a combination of receive(the image file) and the acknowledgement is made in the server side as shown in the code, the server side fails to respond.
Important thing to note is that on KeyBoardInterrupt-ing the above program, it shows that the server side script is hanged/stuck in the data=c.recv(1024) line. But the same problem vanishes if the acknowledgement line is removed.
Note:- The client side script is running on my local machine and the server side script is running on a Google Cloud VM Instance.
Please help.
Thank you.

Answer 1

Hmm... I don't think I completely believe your description of the behavior. But I do know what's wrong. It's entirely reasonable that your server is sitting in the receive loop, because the client hasn't signaled EOF to the connection. Under what circumstances do you believe that this will actually break?
if not data:
break
The answer is that the client needs to either close the socket, or use shutdown(SHUT_WR) to indicate that it will not be sending any more data. So to do what you want, on the client side:
...
f.close()
s.shutdown(socket.SHUT_WR)
...
Now the next time the server calls recv, it will get an empty string returned and the break above will be taken.
This leaves the connection open in one direction but not the other. So the client will not be able to send any more data. However, the server will still be able to send to the client until it closes the socket (or uses shutdown itself).
There is another more subtle problem. You are assuming that your first server-side recv will receive only and exactly the bytes containing your file name. 99.9% of the time that will work. But it's possible that the data from your next client-side send will also be available when the server calls recv for the first time. That will likely give you a bogus file name (though not necessarily an illegal one) and will certainly mean that your file is not transferred faithfully.
You should never assume that the data provided by a single send by one peer will be received by a corresponding single recv on the other side. The data received could be more or less and it's up to the application to frame the data to ensure that it receives exactly the intended amount.

Answer 2

Here
while True:
data=c.recv(1024)
if not data:
break
f.write(data)
it loops back to waiting for a message after it has received one because you don't break the while loop after receiving data. the if not data: doesn't do anything since recv() stops and waits until it gets a message, and thus data is never nothing. You should either break the loop after receiving a message by adding a break after f.write(data), or send the OK in the loop.