Here is my problem. I have a server on lua which sends data via socket. Data go constantly - it is a stream of exchange transactions. My python script as client should receive data.
def listen():
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
host = socket.gethostname()
sock.connect(("localhost", 1111))
with BytesIO() as response_bytes:
while True:
try:
fragment = sock.recv(8196)
print("Фрагмент: {}".format(fragment))
except Exception:
pass
if __name__ == "__main__":
t2 = threading.Thread(listen())
t2.start()
while True:
print ("test")
Main thread wait sock.recv(8196) line. I want that data from a socket were accepted in parallel, and the main stream continued to work. The current code blocks performance of main until listen is executed. I am not familiar with multiple tasks in Python. What decisions can be?
You have to pass the listen function to threading.Thread(). You're calling the function in the main thread, waiting for it to finish, and then passing its return value (which is just None, since the function never returns anything).
t2 = threading.Thread(target = listen)
I am writing a threaded Python script for the first time and running into some trouble. The general idea is that a Raspberry Pi receives data from a Bluetooth connection, this data is then used to create a thread that calls the start_laps method. Once this thread is started, I need to continue listening for new data to determine if the thread should be killed. However, my code is not continuing execution after the thread is started. What would cause this?
import json
import bluetooth
import threading
import timed_LEDs
import subprocess
import ast
def start_laps(delay, lap_times):
timed_LEDs.start_LEDs(delay, lap_times)
# put pi in discoverable
subprocess.call(['sudo', 'hciconfig', 'hci0', 'piscan'])
server_socket = bluetooth.BluetoothSocket(bluetooth.RFCOMM)
port = 1
server_socket.bind(("", port))
server_socket.listen(1)
client_socket, address = server_socket.accept()
print("Accepted connection from ", address)
threads = []
while True:
print("RECEIVING")
data = client_socket.recv(1024)
data = json.loads(data.decode())
print(data)
if(data["lap_times"]):
print("STARTING THREAD")
t = threading.Thread(target=start_laps(int(data["delay"]), ast.literal_eval(data["lap_times"])))
threads.append(t)
t.start()
elif data == "stop":
print("Stop dat lap")
else:
print(data)
client_socket.close()
You are using the threading module wrong.
This line
threading.Thread(target=start_laps(int(data["delay"]), ast.literal_eval(data["lap_times"])))
executes the function start_laps, which obviously blocks the program. What you want is the following:
threading.Thread(target=start_laps, args=(int(data["delay"]), ast.literal_eval(data["lap_times"])))
This executes the function in the created Thread with the given args
This is my code:
socketcheck.py
import time
import subprocess
subprocess.Popen(["python", "server.py"])
for i in range(10):
time.sleep(2)
print i
def print_from_server(data):
print data
server.py
import socket
from socketcheck import print_from_server
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind(('localhost',3005))
client_connected = 1
while 1:
s.listen(1)
conn, addr = s.accept()
data = conn.recv(1024)
if data:
client_connected = 0
else: break
if client_connected == 0:
print 'data received'
print_from_server(data)
client_connected = 1
conn.sendall(data)
client.py
import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('localhost',3005))
s.sendall('Hello, world')
data = s.recv(1024)
#s.close()
print 'Received', repr(data)
What I am trying to do here is, run socketcheck.py which runs server.py in background and listens for a client connection. So whatever data the client sends, I want to pass it on to socketcheck.py. Is this valid? If so, then how do I achieve it?
Now when I try and run socketcheck.py, the for loop is running indefinitely.
Thanks :)
EDIT:
This initially I tried as a single program, but until the client gets connected, the rest of the program doesn't execute(blocking), with the setblocking(0) the program flow wouldn't stop but when the client connects to server it doesn't print(do anything). The server code looked something like this:
import socket
from socketcheck import print_from_server
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind(('localhost',3005))
s.setblocking(0)
while 1:
try:
s.listen(1)
conn, addr = s.accept()
conn.setblocking(0)
data = conn.recv(1024)
if not data: break
print 'data received'
conn.sendall(data)
except:
print 'non blocking'
print 'the lengthy program continues from here'
The reason why your program crashes your computer is simple:
You have a while loop which calls print_from_server(data), and each time it calls it, a new subprocess gets created via subprocess.Popen(["python", "server.py"]).
The reason for creating a new popen each time is a bit more complicated: You open a new server.py program in socketcheck.py. If this new server.py program calls print_from_server(data), this is the first time print_from_server(data) gets called (for the new server.py program). So the global commands (such as popen) are executed, since they are always executed once.
The number of processes running will explode quickly and you computer crashes.
One additional remark: You cannot print to console with a print command in a subprocess, since there is no console attached to that subprocess, you can only print to file. If you do that, you'll see that this output explodes quickly from all the processes.
Put socketcheck.py and server.py into one program and everything works fine, or explain why you need two programs.
The functionality can be easily achieved with multithreading :)
I'm currently working to add support for gevent-socketio to an existing django project. I'm finding that gevent.monkey.patch_all() call is breaking the cancellation mechanism of a thread which is responsible for receiving data from a socket, we'll call the class SocketReadThread for now.
SocketReadThread is pretty simple, it calls recv() on a blocking socket. When it receives data is processes it and calls recv() again. The thread stops when an exception occurs or when recv() returns 0 bytes as occurs when socket.shutdown(SHUT_RDWR) is called in SocketReadThread.stop_reading()
The problem occurs when the gevent.monkey.patch_all() replaces the default socket implementation. Instead of shutting down nicely I get the following exception:
error: [Errno 9] File descriptor was closed in another greenlet
I'm assuming this is occurring because gevent makes my socket non-blocking in order to work its magic. This means that when I call socket.shutdown(socket.SHUT_RDWR) the greenlet that was doing the work for the monkey patched socket.recv call tried to read from the closed file descriptor.
I coded an example to isolate this issue:
from gevent import monkey
monkey.patch_all()
import socket
import sys
import threading
import time
class SocketReadThread(threading.Thread):
def __init__(self, socket):
super(SocketReadThread, self).__init__()
self._socket = socket
def run(self):
connected = True
while connected:
try:
print "calling socket.recv"
data = self._socket.recv(1024)
if (len(data) < 1):
print "received nothing, assuming socket shutdown"
connected = False
else :
print "Recieved something: {}".format(data)
except socket.timeout as e:
print "Socket timeout: {}".format(e)
connected = false
except :
ex = sys.exc_info()[1]
print "Unexpected exception occurrred: {}".format(str(ex))
raise ex
def stop_reading(self):
self._socket.shutdown(socket.SHUT_RDWR)
self._socket.close()
if __name__ == '__main__':
sock = socket.socket()
sock.connect(('127.0.0.1', 4242))
st = SocketReadThread(sock)
st.start()
time.sleep(3)
st.stop_reading()
st.join()
If you open a terminal an run nc -lp 4242 & (to give this program something to connect to) and then run this program you will see the exception mentioned above. If you remove the call to monkey.patch_all() you will see that it works just fine.
My question is: How can support cancellation of the SocketReadThread in a way that works with or without gevent monkey patching and doesn't require the use of an arbitrary timeout that would make cancellation slow (i.e. calling recv() with a timeout and checking a conditional)?
I found that there were two different workarounds for this. The first was to simply catch and suppress the exception. This appears to work fine since it is common practice for one thread to close a socket in order to cause another thread to exit from a blocking read. I don't know or understand why greenlets would complain about this other than a debugging aid. It is really just an annoyance.
The second option was to use the self-pipe trick (a quick search yields many explanations) as a mechanism to wake up a blocked thread. Essentially we create a second file descriptor (a socket is like a type of file descriptor to the OS) for signaling cancellation. We then use select as our blocking to wait for either incoming data on the socket or a cancellation request to come in on the cancellation file descriptor. See the example code below.
from gevent import monkey
monkey.patch_all()
import os
import select
import socket
import sys
import threading
import time
class SocketReadThread(threading.Thread):
def __init__(self, socket):
super(SocketReadThread, self).__init__()
self._socket = socket
self._socket.setblocking(0)
r, w = os.pipe()
self._cancelpipe_r = os.fdopen(r, 'r')
self._cancelpipe_w = os.fdopen(w, 'w')
def run(self):
connected = True
read_fds = [self._socket, self._cancelpipe_r]
while connected:
print "Calling select"
read_list, write_list, x_list = select.select(read_fds, [], [])
print "Select returned"
if self._cancelpipe_r in read_list :
print "exiting"
self._cleanup()
connected = False
elif self._socket in read_list:
print "calling socket.recv"
data = self._socket.recv(1024)
if (len(data) < 1):
print "received nothing, assuming socket shutdown"
connected = False
self._cleanup()
else :
print "Recieved something: {}".format(data)
def stop_reading(self):
print "writing to pipe"
self._cancelpipe_w.write("\n")
self._cancelpipe_w.flush()
print "joining"
self.join()
print "joined"
def _cleanup(self):
self._cancelpipe_r.close()
self._cancelpipe_w.close()
self._socket.shutdown(socket.SHUT_RDWR)
self._socket.close()
if __name__ == '__main__':
sock = socket.socket()
sock.connect(('127.0.0.1', 4242))
st = SocketReadThread(sock)
st.start()
time.sleep(3)
st.stop_reading()
Again, before running the above program run netcat -lp 4242 & to give it a listening socket to connect to.
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.
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:
Get rid of all the locks and extra threads that you don't need, and start from the beginning
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.
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
Backup your work, then toss it - partially.
You need to implement your program in pieces, and test each piece as you go. First, tackle the input part of your program. Don't worry about how to broadcast the input you received. Instead worry that you are able to successfully and repeatedly receive input over your socket. So far - so good.
Now, I assume you would like to react to this input by broadcasting to the other attached clients. Well too bad, you can't do that yet! Because, I left one minor detail out of the paragraph above. You have to design a PROTOCOL.
What is a protocol? It's a set of rules for communication. How does your server know when the client had finished sending it's data? Is it terminated by some special character? Or perhaps you encode the size of the message to be sent as the first byte or two of the message.
This is turning out to be a lot of work, isn't it? :-)
What's a simple protocol. A line-oriented protocol is simple. Read 1 character at a time until you get to the end of record terminator - '\n'. So, clients would send records like this to your server --
HELO\n
MSG DAVE Where Are Your Kids?\n
So, assuming you have this simple protocol designed, implement it. For now, DON'T WORRY ABOUT THE MULTITHREADING STUFF! Just worry about making it work.
Your current protocol is to read 1024 bytes. Which may not be bad, just make sure you send 1024 byte messages from the client.
Once you have the protocol stuff setup, move on to reacting to the input. But for now you need something that will read input. Once that is done, we can worry about doing something with it.
jdi is right, you have too much program to work with. Pieces are easier to fix.