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How to kill a Server thread in Python

I have an application which runs multiple servers all on their own threads. I want to be able to tell a thread to stop running. To do this though I would need to tell the thread to stop, the thread would then need to tell the server to stop and the server would then close its own socket (which is in a receiving loop, getting data from all the connected clients). How would I do this?
I have tried using passed stop variables, however I think the issue is in the socket needing to be closed. I can't find a way to tell the server to close the socket without sending a direct message to the server telling it to do so, which seems inefficient.
Here is my server code:
import socket
import threading
class Server:
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
connections = []
def __init__(self, port):
self.sock.bind(('0.0.0.0', port))
self.sock.listen(1)
def handler(self, c, a):
while True:
try:
data = c.recv(1024) #loop won't run until recieved dat
except:
c.shutdown(socket.SHUT_RDWR)
c.close()
break
print("Chat: ", str(data, 'utf-8'))
if not data:
c.close()
break
def run(self):
self._stop = False
while not self._stop:
c, a = self.sock.accept() ##c is client and a is address
cThread = threading.Thread(target=self.handler, args=(c,a))
cThread.daemon = True
cThread.start()
self.connections.append(c)
print("Server: ", str(a[0]) + ':' + str(a[1]), "connected")
self.close()
def shutdownServer(self):
self._stop = True
def close(self):
print('Closing server')
if self.sock:
self.sock.close()
self.sock = None
def serverRun(port, stop):
while True:
print("server port: " + str(port))
actual_server = Server(port)
actual_server.run()
if(stop):
print("Stopping server thread")
break
Here is the code which sets up the thread and runs the server:
def main():
stopThreads = False
thread = threading.Thread(target = server.serverRun, args=(1, lambda : stopThreads,))
thread.start()
time.sleep(1)
stopThreads = True
thread.join()
print("server thread killed")
main()
Any help would be appreciated.
Edit: Edited to clarify the problem is less so closing the thread and more so passing a variable to the class being run in the thread, so it can close its socket when the thread is trying to be stopped.

Okay, so I figured out the blocker was the socket.accept() function. So for anyone who may have the same issue with terminating server threads, you can just use a sock.select() before your sock.accept() to check if there are any incoming connections. If you use a sock.select() and add a timeout to it, the whole loop will run after the allotted time it waits for connections, so the thread can be killed if the event has told it to do so and if it hasn't, it will look for connections again.
You can use the thread event function (which stovfl mentioned in comments on the main thread) to tell the thread when to stop.
Here is how I changed my code so it can now self terminate:
def run(self, running):
while running.is_set():
timeout = 2
readable, writable, errored = select.select([self.sock], [], [], timeout)
for s in readable:
if s is self.sock:
client_socket, a = self.sock.accept() ##c is client and a is address
cThread = threading.Thread(target=self.handler, args=(client_socket, a))
cThread.daemon = True
cThread.start()
self.connections.append(client_socket)
print("Server: ", str(a[0]) + ':' + str(a[1]), "connected")
self.close()
def serverRun(running, port):
while running.is_set():
print("server port: " + str(port))
actual_server = Server(port)
actual_server.run(running)
And main was changed to:
def main():
running = threading.Event()
running.set()
thread = threading.Thread(target=server.serverRun, args=(running, 1))
thread.start()
time.sleep(30)
print("Event running.clear()")
running.clear()
print('Wait until Thread is terminating')
thread.join()
print("EXIT __main__")
main()

Shutdown for socketserver based Python 3 server hangs

I am working on a "simple" server using a threaded SocketServer in Python 3.
I am going through a lot of trouble implementing shutdown for this. The code below I found on the internet and shutdown works initially but stops working after sending a few commands from the client via telnet. Some investigation tells me it hangs in threading._shutdown... threading._wait_for_tstate_lock but so far this does not ring a bell.
My research tells me that there are ~42 different solutions, frameworks, etc. on how to do this in different python versions. So far I could not find a working approach for python3. E.g. I love telnetsrv
(https://pypi.python.org/pypi/telnetsrv/0.4) for python 2.7 (it uses greenlets from gevent) but this one does not work for python 3. So if there is a more pythonic, std lib approach or something that works reliably I would love to hear about it!
My bet currently is with socketserver but I could not figure out yet how to deal with the hanging server. I removed all the log statements and most functionality so I can post this minimal server which exposes the issue:
# -*- coding: utf-8 -*-
import socketserver
import threading
SERVER = None
def shutdown_cmd(request):
global SERVER
request.send(bytes('server shutdown requested\n', 'utf-8'))
request.close()
SERVER.shutdown()
print('after shutdown!!')
#SERVER.server_close()
class service(socketserver.BaseRequestHandler):
def handle(self):
while True:
try:
msg = str(self.request.recv(1024).strip(), 'utf-8')
if msg == 'shutdown':
shutdown_cmd(msg, self.request)
else:
self.request.send(bytes("You said '{}'\n".format(msg), "utf-8"))
except Exception as e:
pass
class ThreadedTCPServer(socketserver.ThreadingMixIn, socketserver.TCPServer):
pass
def run():
global SERVER
SERVER = ThreadedTCPServer(('', 1520), service)
server_thread = threading.Thread(target=SERVER.serve_forever)
server_thread.daemon = True
server_thread.start()
input("Press enter to shutdown")
SERVER.shutdown()
if __name__ == '__main__':
run()
It would be great being able to stop the server from the handler, too (see shutdown_cmd)

shutdown() works as expected, the server has stopped accepting new connections, but python still waiting for alive threads to terminate.
By default, socketserver.ThreadingMixIn will create new threads to handle incoming connection and by default, those are non-daemon threads, so python will wait for all alive non-daemon threads to terminate.
Of course, you could make the server spawn daemon threads, then python will not waiting:
The ThreadingMixIn class defines an attribute daemon_threads, which indicates whether or not the server should wait for thread termination. You should set the flag explicitly if you would like threads to behave autonomously; the default is False, meaning that Python will not exit until all threads created by ThreadingMixIn have exited.
class ThreadedTCPServer(socketserver.ThreadingMixIn, socketserver.TCPServer):
daemon_threads = True
But that is not the ideal solution, you should check why threads never terminate, usually, the server should stop processing connection when no new data available or client shutdown connection:
import socketserver
import threading
shutdown_evt = threading.Event()
class service(socketserver.BaseRequestHandler):
def handle(self):
self.request.setblocking(False)
while True:
try:
msg = self.request.recv(1024)
if msg == b'shutdown':
shutdown_evt.set()
break
elif msg:
self.request.send(b'you said: ' + msg)
if shutdown_evt.wait(0.1):
break
except Exception as e:
break
class ThreadedTCPServer(socketserver.ThreadingMixIn, socketserver.TCPServer):
pass
def run():
SERVER = ThreadedTCPServer(('127.0.0.1', 10000), service)
server_thread = threading.Thread(target=SERVER.serve_forever)
server_thread.daemon = True
server_thread.start()
input("Press enter to shutdown")
shutdown_evt.set()
SERVER.shutdown()
if __name__ == '__main__':
run()

I tried two solutions to implement a tcp server which runs on Python 3 on both Linux and Windows (I tried Windows 7):
using socketserver (my question) - shutdown is not working
using asyncio (posted an answer for that) - does not work on Windows
Both solutions have been based upon search results on the web. In the end I had to give up on the idea of finding a proven solution because I could not find one. Consequently I implemented my own solution (based on gevent). I post it here because I hope it will be helpful for others to avoid stuggeling the way I did.
# -*- coding: utf-8 -*-
from gevent.server import StreamServer
from gevent.pool import Pool
class EchoServer(StreamServer):
def __init__(self, listener, handle=None, spawn='default'):
StreamServer.__init__(self, listener, handle=handle, spawn=spawn)
def handle(self, socket, address):
print('New connection from %s:%s' % address[:2])
socket.sendall(b'Welcome to the echo server! Type quit to exit.\r\n')
# using a makefile because we want to use readline()
rfileobj = socket.makefile(mode='rb')
while True:
line = rfileobj.readline()
if not line:
print("client disconnected")
break
if line.strip().lower() == b'quit':
print("client quit")
break
if line.strip().lower() == b'shutdown':
print("client initiated server shutdown")
self.stop()
break
socket.sendall(line)
print("echoed %r" % line.decode().strip())
rfileobj.close()
srv = EchoServer(('', 1520), spawn=Pool(20))
srv.serve_forever()

after more research I found a sample that works using asyncio:
# -*- coding: utf-8 -*-
import asyncio
# after further research I found this relevant europython talk:
# https://www.youtube.com/watch?v=pi49aiLBas8
# * protocols and transport are useful if you do not have tons of socket based code
# * event loop pushes data in
# * transport used to push data back to the client
# found decent sample in book by wrox "professional python"
class ServerProtocol(asyncio.Protocol):
def connection_made(self, transport):
self.transport = transport
self.write('Welcome')
def connection_lost(self, exc):
self.transport = None
def data_received(self, data):
if not data or data == '':
return
message = data.decode('ascii')
command = message.strip().split(' ')[0].lower()
args = message.strip().split(' ')[1:]
#sanity check
if not hasattr(self, 'command_%s' % command):
self.write('Invalid command: %s' % command)
return
# run command
try:
return getattr(self, 'command_%s' % command)(*args)
except Exception as ex:
self.write('Error: %s' % str(ex))
def write(self, msg):
self.transport.write((msg + '\n').encode('ascii', 'ignore'))
def command_shutdown(self):
self.write('Okay. shutting down')
raise KeyboardInterrupt
def command_bye(self):
self.write('bye then!')
self.transport.close()
self.transport = None
if __name__ == '__main__':
loop = asyncio.get_event_loop()
coro = loop.create_server(ServerProtocol, '127.0.0.1', 8023)
asyncio.async(coro)
try:
loop.run_forever()
except KeyboardInterrupt:
pass
I understand that this is the most useful way to do this kind of network programming. If necessary the performance could be improved using the same code with uvloop (https://magic.io/blog/uvloop-blazing-fast-python-networking/).

Another way to shut down the server is by creating a process/thread for the serve_forever call.
After server_forever is started, simply wait for a custom flag to trigger and use server_close on the server, and terminate the process.
streaming_server = StreamingServer(('', 8000), StreamingHandler)
FLAG_KEEP_ALIVE.value = True
process_serve_forever = Process(target=streaming_server.serve_forever)
process_serve_forever.start()
while FLAG_KEEP_ALIVE.value:
pass
streaming_server.server_close()
process_serve_forever.terminate()

Keyboard interrupt, sockets and Threads

I'm working on an interface that communicates using sockets. I need to use Threads in order to receive and send data at the same time.
The below code is a cut down version of the module that continuously sends data.
My issue is that during development, I need to start and stop this frequently, but it seems threads makes it impossible to stop the application like you can a normal script. The process needs to be killed in process manager in order to restart it.
I have a feeling I am misunderstanding something.
I've looked around but I'm seeing loads of inconsistent answers which I've tried to implement to no avail. I've tried handling the KeyboardInterrupt exception and that doesn't work.
Any ideas?
import socket
import sys
import threading
running = True
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server_address = ('localhost', 50000)
print ('connecting to %s port %s' % server_address)
sock.connect(server_address)
class TelemetryPacket:
state = "N/A"
def __init__(self, stringvalue):
self.state = stringvalue
def serialise(self):
return "%s" % (self.state)
def send(running):
try:
while (running):
telemetry = TelemetryPacket(0).serialise()
sock.sendto(telemetry.encode('utf-8'), server_address)
except KeyboardInterrupt:
print ("threads successfully closed")
running = False
def recv(running):
try:
while (running):
data = sock.recv(1)
if (data):
print("data received: %s" % data)
except KeyboardInterrupt:
print ("threads successfully closed")
running = False
a = threading.Thread(target=send, args=(running,))
a.start()
b = threading.Thread(target=recv, args=(running,))
b.start()

You should be capturing your your keyboard interrupt in the main thread and then sending a signal to your child threads to exit and join back to the main thread. You should also use something more thread safe for signaling like the basic threading.Event(), so:
import threading
import time
exit_signal = threading.Event() # your global exit signal
def send():
while not exit_signal.is_set():
pass # whatever you need
def recv():
while not exit_signal.is_set():
pass # whatever you need
# create the threads
a = threading.Thread(target=send)
b = threading.Thread(target=recv)
# start the threads
a.start()
b.start()
# create a main thread loop
try:
while not exit_signal.is_set(): # enable children threads to exit the main thread, too
time.sleep(0.1) # let it breathe a little
except KeyboardInterrupt: # on keyboard interrupt...
exit_signal.set() # send signal to all listening threads
# join back the threads
a.join()
b.join()
# and you're done...

The problem is that on keyboard exception only the main thread is killed.
So you need to notify the threads that the main thread has been killed.
Instead of passing the running as an argument, use it to signal the threads.Here is an example code. Only difference is that function doesn't take running as an argument but it uses it to keep track. So when the main thread dies, we set it to false(at the end of the code).
import socket
import sys
import threading
running = True
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server_address = ('localhost', 50000)
print ('connecting to %s port %s' % server_address)
sock.bind(server_address)
class TelemetryPacket:
state = "N/A"
def __init__(self, stringvalue):
self.state = stringvalue
def serialise(self):
return "%s" % (self.state)
def send():
try:
while (running):
telemetry = TelemetryPacket(0).serialise()
sock.sendto(telemetry.encode('utf-8'), server_address)
except KeyboardInterrupt:
print ("threads successfully closed")
def recv():
try:
while (running):
data = sock.recv(1)
if (data):
print("data received: %s" % data)
except KeyboardInterrupt:
print ("threads successfully closed")
try:
a = threading.Thread(target=send)
a.start()
b = threading.Thread(target=recv)
b.start()
while True:
pass
except KeyboardInterrupt:
print ("threads successfully closed")
running = False # Kill the threads

How to exit a multithreaded program?

I was just messing around with threading in python, wrote this basic IM thingy [code at bottom]
I noticed that when I kill the program with C-c it doesn't exit, it just hangs forever.
I'm just guessing it's waiting for each thread to finish what they are doing, but since it's an endless loop that will never happen.
So I guess I need to kill each thread manually, or end the loop when the killsignal comes in.
How would I do that?
#!/usr/bin/env python
import threading
import socket
class Listen(threading.Thread):
def run(self):
conn = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
conn.bind(('', 2727))
conn.listen(1)
while True:
channel, details = conn.accept()
print str(details)+": "+channel.recv(250)
channel.send("got it")
channel.close()
class Shout(threading.Thread):
def run(self):
while True:
try:
address = raw_input("who u talking to? ")
conn = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
conn.connect((address, 2727))
break
except:
print "can't connect to "+ str(address)
while True:
conn = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
conn.connect((address, 2727))
conn.send(raw_input())
conn.close()
listen = Listen().start()
shout = Shout().start()

I see several causes of the misbehavior in your code.
Ctrl+C causes a "KeyboardInterrupt" exception in the main thread. So you should handle it there.
Your socket is in blocking mode. This causes several socket functions to block the calling thread until the function returns. During this state the thread cannot react to any termination event.
As you already said: your endless loop in the thread's run() function is ... really endless. So the thread execution is never ending (at least not without an unexpected exception). You should use some kind of synchronization object, like an threading.Event object, to be able to tell a thread externally that it should terminate itself.
I would discourage the use of raw_input() out of the main thread. Imagine what happens when you have more than one Shout thread.
Why are you always closing and reconnecting the socket when a message has been transmitted in your Shout class? Network connections should be re-established only in special cases because of the setup costs.
Without a frame protocol for the communication you can never expect to have received all data that was sent by the other host when the recv() function returns.
The start() function of the thread object does not return a value or object. So saving the returned value (=None) doesn't make much sense.
You can never expect the send() function to transmit all passed data. Therefore you must check the result of the function and appropriately handle the situation when not all bytes were really transmitted.
To learn threading there are surely better problems to solve than network communication, since that topic is in itself really complex.
Beside all these things, here is my try for a solution. Still there is much that can be improved. You should consider the answer from Mark Tolonen too, since the SocketServer class is surely provided to ease several things in handling this kind of stuff. But you should keep on studying the basics too.
#!/usr/bin/env python
import threading
import socket
import time
import errno
class StoppableThread(threading.Thread):
def __init__(self):
threading.Thread.__init__(self)
self.stop_event = threading.Event()
def stop(self):
if self.isAlive() == True:
# set event to signal thread to terminate
self.stop_event.set()
# block calling thread until thread really has terminated
self.join()
class Accept(StoppableThread):
def __init__(self, port):
StoppableThread.__init__(self)
self.port = port
self.threads = []
def run(self):
# handle connection acception
conn = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
conn.bind(('', self.port ))
conn.listen(5)
# set socket timeout to ~10ms
conn.settimeout(0.01)
while self.stop_event.is_set() == False:
try:
csock, caddr = conn.accept()
# spawn a new thread to handle the client connection
listen_thread = Listen(csock, caddr)
self.threads.append(listen_thread)
listen_thread.start()
except socket.timeout:
# socket operation timeout
# clear all terminated threads from thread list
for thread in self.threads:
if thread.isAlive() == False:
self.threads.remove(thread)
self.stop_threads()
def stop_threads(self):
# stop all running threads
for listen_thread in self.threads:
if listen_thread.isAlive() == True:
listen_thread.stop()
self.threads = []
class Listen(StoppableThread):
def __init__(self, csock, caddr):
StoppableThread.__init__(self)
self.csock = csock
self.caddr = caddr
self.csock.setblocking(False)
def run(self):
while self.stop_event.is_set() == False:
try:
recv_data = self.csock.recv(250)
if len(recv_data) > 0:
print str(self.caddr)+": " + recv_data
self.csock.send("got it")
else:
# connection was closed by foreign host
self.stop_event.set()
except socket.error as (sock_errno, sock_errstr):
if (sock_errno == errno.EWOULDBLOCK):
# socket would block - sleep sometime
time.sleep(0.1)
else:
# unexpected / unhandled error - terminate thread
self.stop_event.set()
channel.close()
class Shout(StoppableThread):
def __init__(self, sport):
StoppableThread.__init__(self)
self.sport = sport
def run(self):
while self.stop_event.is_set() == False:
try:
address = raw_input("who u talking to? ")
conn = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
conn.connect((address, self.sport))
break
except socket.error:
# handle connection problems
print "can't connect to "+ str(address)
except:
# exit thread in case of an unexpected error
self.stop_event.set()
while self.stop_event.is_set() == False:
try:
# chat loop: send messages to remote host
print "what to send? :",
msg = raw_input()
# beware: send() function may block indefinitly here and it might not send all bytes as expected !!
conn.send(msg)
except:
# exit thread in case of an unexpected error
self.stop_event.set()
# close socket before thread terminates
conn.close()
def main():
do_exit = False
server_port = 2727
# start server socket thread
accept = Accept(server_port)
accept.start()
# start transmitting client socket thread
shout = Shout(server_port)
shout.start()
while do_exit == False:
try:
# sleep some time
time.sleep(0.1)
except KeyboardInterrupt:
# Ctrl+C was hit - exit program
do_exit = True
# stop all running threads
shout.stop()
accept.stop()
# exit main program after all threads were terminated gracefully
if __name__ == "__main__":
main()

Look at the Python library source for SocketServer.py, in particular the implementation of server_forever() to see how a server implements a quit. It uses select() to poll the server socket for new connections and tests a quit flag. Here's a hack on your source to use SocketServer, and I added a quit flag to Shout(). It will run the Shout and Listen threads for 5 seconds and then stop them.
import socket
import SocketServer
import threading
import time
class Handler(SocketServer.StreamRequestHandler):
def handle(self):
print str(self.client_address) + ": " + self.request.recv(250)
self.request.send("got it\n")
class Listen(threading.Thread):
def run(self):
self.server = SocketServer.TCPServer(('',2727),Handler)
self.server.serve_forever()
def stop(self):
self.server.shutdown()
class Shout(threading.Thread):
def __init__(self):
threading.Thread.__init__(self)
self.quit = False
def run(self):
while not self.quit:
conn = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
conn.connect(('localhost', 2727))
conn.send('sending\n')
print conn.recv(100)
conn.close()
def stop(self):
self.quit = True
listen = Listen()
listen.start()
shout = Shout()
shout.start()
time.sleep(5)
shout.stop()
listen.stop()

Can you explain the following signal handling behavior in python?

I have the following program:
import socket
import sys
import threading
import signal
class serve(threading.Thread):
def __init__(self):
super(serve, self).__init__()
self.s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
self.host = ''
self.port = int(sys.argv[1])
def run(self):
self.s.bind((self.host, self.port))
self.s.listen(1)
conn, addr = self.s.accept()
# Call blocks in the following recv
data = conn.recv(1000000)
conn.close()
self.s.close()
def handler(signum, frame):
print "I am the handler: "
signal.signal(signal.SIGHUP, handler)
background = serve()
background.start()
background.join()
There is a client program that connects to this but does not send any data. The problem is when a SIGHUP is sent, and "Interrupted System call" exception is being thrown. Any idea why? It is happening in python 2.6+ and on FreeBSD. I suspect it is related to http://bugs.python.org/issue1975.

If a system call is executing when a signal arrives, the system call is interrupted. I believe this is in part to prevent escalation attacks and in part to keep the process in a consistent state when the signal handler is invoked. It also would allow you to wake up a process that's hung on a system call.
To instead restart system calls after a signal is handled, use signal.siginterrupt after you set a signal handler:
signal.signal(signal.SIGHUP, handler)
signal.siginterrupt(signal.SIGHUP, false)