Socket python doesn't send data if program is alive

I'm trying to run a client/server script, where the client sends a file to the server and waits for responses until the server sends a stop message. The problem is: once the connection is established the client starts sending data but until I press CTRL-C the server cannot recreate the file. Only after CTRL-C print "file is fully created" and the file becomes visible, instead, before it's seems to be waiting for something. idk where the problem is. Also tried changing condition on send loop using len(), but doesn't work. Anyone know how to fix it ?

client.py :

import socket               # Import socket module
# from threading import Thread

s = socket.socket()         # Create a socket object
HOST = "101.xx.x.xxx"       # public IP address
PORT = 4243                 # Reserve a port for your service.
PDF_PATH = "exam.pdf"
s.connect((HOST, PORT))

def send():
    f = open(PDF_PATH, "rb")
    while data := f.read(4096):
        s.send(data)
    f.close()
    return
    
def receive():
    while 1:
        exercise = s.recv(4096)
        if exercise == "stop!":
            s.close()
            break
        f = open(f"{exercise}.txt", "wb")
        while data := f.read(4096):
            f.write(data)
    return

def main():
    send()
    receive()
    
if __name__ == "__main__":
    main()

server.py :

import socket
from threading import Thread

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
HOST = socket.gethostname()
IP = socket.gethostbyname(HOST)
PORT = 4243
s.bind(('', PORT))
s.listen(5)

def receive_file(conn, i):
    f = open(f"exam.pdf", "wb")
    while received := conn.recv(4096):
        f.write(received)
    print("File is fully copied\n")
    f.close()

def send_result(conn,i):
    while 1:
        nbr = str(input("which exercise? "))
        if nbr == "stop!":
            break
        f = open(f"exercise{nbr}.txt", "rb")
        conn.send(bytes(f"exercise{nbr}.txt", encoding="utf-8"))
        while data := f.read(4096):
            conn.send(data)
        f.close()

def main():
    try:
        while 1:
            i = 0
            conn, addr = s.accept()
            print("Got connection from", addr)
            # c.send(b"Thank you for connecting")
            t = Thread(target=receive_file, args=(conn, i))
            t.start()
            t.join()
            t = Thread(target=send_result, args=(conn, i))
            t.start()
            t.join()
    except KeyboardInterrupt:
        print("interrupting \n")
        conn.close()
        s.close()


if _name_ == '_main_':
    main()