Vegar Østhus Vegar Østhus - 2 months ago 8
Python Question

Thread synchronization in Python

I am currently working on a school project where the assignment, among other things, is to set up a threaded server/client system. Each client in the system is supposed to be assigned its own thread on the server when connecting to it. In addition i would like the server to run other threads, one concerning input from the command line and another concerning broadcasting messages to all clients. However, I can't get this to run as i want to. It seems like the threads are blocking each other. I would like my program to take inputs from the command line, at the "same time" as the server listens to connected clients, and so on.

I am new to python programming and multithreading, and allthough I think my idea is good, I'm not suprised my code doesn't work. Thing is I'm not exactly sure how I'm going to implement the message passing between the different threads. Nor am I sure exactly how to implement the resource lock commands properly. I'm going to post the code for my server file and my client file here, and I hope someone could help me with this. I think this actually should be two relative simple scripts. I have tried to comment on my code as good as possible to some extend.

import select
import socket
import sys
import threading
import client

class Server:

#initializing server socket
def __init__(self, event):
self.host = 'localhost'
self.port = 50000
self.backlog = 5
self.size = 1024
self.server = None
self.server_running = False
self.listen_threads = []
self.local_threads = []
self.clients = []
self.serverSocketLock = None
self.cmdLock = None
#here i have also declared some events for the command line input
#and the receive function respectively, not sure if correct
self.cmd_event = event
self.socket_event = event

def openSocket(self):
#binding server to port
try:
self.server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
self.server.bind((self.host, self.port))
self.server.listen(5)
print "Listening to port " + str(self.port) + "..."
except socket.error, (value,message):
if self.server:
self.server.close()
print "Could not open socket: " + message
sys.exit(1)

def run(self):
self.openSocket()

#making Rlocks for the socket and for the command line input

self.serverSocketLock = threading.RLock()
self.cmdLock = threading.RLock()

#set blocking to non-blocking
self.server.setblocking(0)
#making two threads always running on the server,
#one for the command line input, and one for broadcasting (sending)
cmd_thread = threading.Thread(target=self.server_cmd)
broadcast_thread = threading.Thread(target=self.broadcast,args=[self.clients])
cmd_thread.daemon = True
broadcast_thread.daemon = True
#append the threads to thread list
self.local_threads.append(cmd_thread)
self.local_threads.append(broadcast_thread)

cmd_thread.start()
broadcast_thread.start()


self.server_running = True
while self.server_running:

#connecting to "knocking" clients
try:
c = client.Client(self.server.accept())
self.clients.append(c)
print "Client " + str(c.address) + " connected"

#making a thread for each clientn and appending it to client list
listen_thread = threading.Thread(target=self.listenToClient,args=[c])
self.listen_threads.append(listen_thread)
listen_thread.daemon = True
listen_thread.start()
#setting event "client has connected"
self.socket_event.set()

except socket.error, (value, message):
continue

#close threads

self.server.close()
print "Closing client threads"
for c in self.listen_threads:
c.join()

def listenToClient(self, c):

while self.server_running:

#the idea here is to wait until the thread gets the message "client
#has connected"
self.socket_event.wait()
#then clear the event immidiately...
self.socket_event.clear()
#and aquire the socket resource
self.serverSocketLock.acquire()

#the below is the receive thingy

try:
recvd_data = c.client.recv(self.size)
if recvd_data == "" or recvd_data == "close\n":
print "Client " + str(c.address) + (" disconnected...")
self.socket_event.clear()
self.serverSocketLock.release()
return

print recvd_data

#I put these here to avoid locking the resource if no message
#has been received
self.socket_event.clear()
self.serverSocketLock.release()
except socket.error, (value, message):
continue

def server_cmd(self):

#this is a simple command line utility
while self.server_running:

#got to have a smart way to make this work

self.cmd_event.wait()
self.cmd_event.clear()
self.cmdLock.acquire()


cmd = sys.stdin.readline()
if cmd == "":
continue
if cmd == "close\n":
print "Server shutting down..."
self.server_running = False

self.cmdLock.release()


def broadcast(self, clients):
while self.server_running:

#this function will broadcast a message received from one
#client, to all other clients, but i guess any thread
#aspects applied to the above, will work here also

try:
send_data = sys.stdin.readline()
if send_data == "":
continue
else:
for c in clients:
c.client.send(send_data)
self.serverSocketLock.release()
self.cmdLock.release()
except socket.error, (value, message):
continue

if __name__ == "__main__":
e = threading.Event()
s = Server(e)
s.run()


And then the client file

import select
import socket
import sys
import server
import threading

class Client(threading.Thread):

#initializing client socket

def __init__(self,(client,address)):
threading.Thread.__init__(self)
self.client = client
self.address = address
self.size = 1024
self.client_running = False
self.running_threads = []
self.ClientSocketLock = None

def run(self):

#connect to server
self.client.connect(('localhost',50000))

#making a lock for the socket resource
self.clientSocketLock = threading.Lock()
self.client.setblocking(0)
self.client_running = True

#making two threads, one for receiving messages from server...
listen = threading.Thread(target=self.listenToServer)

#...and one for sending messages to server
speak = threading.Thread(target=self.speakToServer)

#not actually sure wat daemon means
listen.daemon = True
speak.daemon = True

#appending the threads to the thread-list
self.running_threads.append(listen)
self.running_threads.append(speak)
listen.start()
speak.start()

#this while-loop is just for avoiding the script terminating
while self.client_running:
dummy = 1

#closing the threads if the client goes down
print "Client operating on its own"
self.client.close()

#close threads
for t in self.running_threads:
t.join()
return

#defining "listen"-function
def listenToServer(self):
while self.client_running:

#here i acquire the socket to this function, but i realize I also
#should have a message passing wait()-function or something
#somewhere
self.clientSocketLock.acquire()

try:
data_recvd = self.client.recv(self.size)
print data_recvd
except socket.error, (value,message):
continue

#releasing the socket resource
self.clientSocketLock.release()

#defining "speak"-function, doing much the same as for the above function
def speakToServer(self):
while self.client_running:
self.clientSocketLock.acquire()
try:
send_data = sys.stdin.readline()
if send_data == "close\n":
print "Disconnecting..."
self.client_running = False
else:
self.client.send(send_data)
except socket.error, (value,message):
continue

self.clientSocketLock.release()

if __name__ == "__main__":
c = Client((socket.socket(socket.AF_INET, socket.SOCK_STREAM),'localhost'))
c.run()


I realize this is quite a few code lines for you to read through, but as I said, I think the concept and the script in it self should be quite simple to understand. It would be very much appriciated if someone could help me synchronize my threads in a proper way =)

Thanks in advance

---Edit---

OK. So I now have simplified my code to just containing send and receive functions in both the server and the client modules. The clients connecting to the server gets their own threads, and the send and receive functions in both modules operetes in their own separate threads. This works like a charm, with the broadcast function in the server module echoing strings it gets from one client to all clients. So far so good!

The next thing i want my script to do, is taking specific commands, i.e. "close", in the client module to shut down the client, and join all running threads in the thread list. Im using an event flag to notify the listenToServer and the main thread that the speakToServer thread has read the input "close". It seems like the main thread jumps out of its while loop and starts the for loop that is supposed to join the other threads. But here it hangs. It seems like the while loop in the listenToServer thread never stops even though server_running should be set to False when the event flag is set.

I'm posting only the client module here, because I guess an answer to get these two threads to synchronize will relate to synchronizing more threads in both the client and the server module also.

import select
import socket
import sys
import server_bygg0203
import threading
from time import sleep

class Client(threading.Thread):

#initializing client socket

def __init__(self,(client,address)):

threading.Thread.__init__(self)
self.client = client
self.address = address
self.size = 1024
self.client_running = False
self.running_threads = []
self.ClientSocketLock = None
self.disconnected = threading.Event()

def run(self):

#connect to server
self.client.connect(('localhost',50000))

#self.client.setblocking(0)
self.client_running = True

#making two threads, one for receiving messages from server...
listen = threading.Thread(target=self.listenToServer)

#...and one for sending messages to server
speak = threading.Thread(target=self.speakToServer)

#not actually sure what daemon means
listen.daemon = True
speak.daemon = True

#appending the threads to the thread-list
self.running_threads.append((listen,"listen"))
self.running_threads.append((speak, "speak"))
listen.start()
speak.start()

while self.client_running:

#check if event is set, and if it is
#set while statement to false

if self.disconnected.isSet():
self.client_running = False

#closing the threads if the client goes down
print "Client operating on its own"
self.client.shutdown(1)
self.client.close()

#close threads

#the script hangs at the for-loop below, and
#refuses to close the listen-thread (and possibly
#also the speak thread, but it never gets that far)

for t in self.running_threads:
print "Waiting for " + t[1] + " to close..."
t[0].join()
self.disconnected.clear()
return

#defining "speak"-function
def speakToServer(self):

#sends strings to server
while self.client_running:
try:
send_data = sys.stdin.readline()
self.client.send(send_data)

#I want the "close" command
#to set an event flag, which is being read by all other threads,
#and, at the same time set the while statement to false

if send_data == "close\n":
print "Disconnecting..."
self.disconnected.set()
self.client_running = False
except socket.error, (value,message):
continue
return

#defining "listen"-function
def listenToServer(self):

#receives strings from server
while self.client_running:

#check if event is set, and if it is
#set while statement to false

if self.disconnected.isSet():
self.client_running = False
try:
data_recvd = self.client.recv(self.size)
print data_recvd
except socket.error, (value,message):
continue
return

if __name__ == "__main__":
c = Client((socket.socket(socket.AF_INET, socket.SOCK_STREAM),'localhost'))
c.run()


Later on, when I get this server/client system up and running, I will use this system on some elevator models we have here on the lab, with each client receiving floor orders or "up" and "down" calls. The server will be running an distribution algorithm and updating the elevator queues on the clients that are most appropriate for the requested order. I realize it's a long way to go, but I guess one should just take one step at the time =)

Hope someone has the time to look into this. Thanks in advance.

jdi jdi
Answer

The biggest problem I see with this code is that you have far too much going on right away to easily debug your problem. Threading can get extremely complicated because of how non-linear the logic becomes. Especially when you have to worry about synchronizing with locks.

The reason you are seeing clients blocking on each other is because of the way you are using your serverSocketLock in your listenToClient() loop in the server. To be honest this isn't exactly your problem right now with your code, but it became the problem when I started to debug it and turned the sockets into blocking sockets. If you are putting each connection into its own thread and reading from them, then there is no reason to use a global server lock here. They can all read from their own sockets at the same time, which is the purpose of the thread.

Here is my recommendation to you:

  1. Get rid of all the locks and extra threads that you don't need, and start from the beginning
  2. Have the clients connect as you do, and put them in their thread as you do. And simply have them send data every second. Verify that you can get more than one client connecting and sending, and that your server is looping and receiving. Once you have this part working, you can move on to the next part.
  3. Right now you have your sockets set to non-blocking. This is causing them all to spin really fast over their loops when data is not ready. Since you are threading, you should set them to block. Then the reader threads will simply sit and wait for data and respond immediately.

Locks are used when threads will be accessing shared resources. You obviously need to for any time a thread will try and modify a server attribute like a list or a value. But not when they are working on their own private sockets.

The event you are using to trigger your readers doesn't seem necessary here. You have received the client, and you start the thread afterwards. So it is ready to go.

In a nutshell...simplify and test one bit at a time. When its working, add more. There are too many threads and locks right now.

Here is a simplified example of your listenToClient method:

def listenToClient(self, c):
    while self.server_running:
        try:
            recvd_data = c.client.recv(self.size)
            print "received:", c, recvd_data
            if recvd_data == "" or recvd_data == "close\n":
                print "Client " + str(c.address) + (" disconnected...")
                return

            print recvd_data

        except socket.error, (value, message):
            if value == 35:
                continue 
            else:
                print "Error:", value, message  
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