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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()

How to serve no more that one HTTP request in python?

I have a simple HTTP server setup like this one. It processes a slow 40 second request to open and then close gates (real metallic gates). If second HTTP query is made during execution of the first one, it is placed in queue and then executed after first run. I don't need this behavior, I need to reply with error if gate open/close procedure is in progress now.
How can I do that? There's a parameter 'request_queue_size' - but I'm not sure how to set it.

You need to follow a different strategy designing your server service. You need to keep the state of the door either in memory or in a database. Then, each time you receive a request to do something on the door, you check the current state of the door in your persistence, and then you execute the action if it is possible to do on the current state, otherwise you return an error. Also, don't forget to update the state of the door once an action completes.

'request_queue_size' seems to have no effect.
The solution was to make server multithreaded, and implement locking variable 'busy':
from socketserver import ThreadingMixIn
from http.server import BaseHTTPRequestHandler, HTTPServer
import time
from gpiozero import DigitalOutputDevice
import logging
from time import sleep
logging.basicConfig(format='%(asctime)s %(levelname)s:%(message)s', level=logging.INFO)
hostName = ''
hostPort = 9001
busy = False
class ThreadingServer(ThreadingMixIn, HTTPServer):
pass
class MyServer(BaseHTTPRequestHandler):
def do_GET(self):
global busy
self.send_response(200)
self.send_header("Content-type", "text/html")
self.end_headers()
self.wfile.write(bytes("Hello!<br>", "utf-8"))
if self.path == '/gates':
if not busy:
busy = True
relay = DigitalOutputDevice(17) # Initialize GPIO 17
relay.on()
logging.info('Cycle started')
self.wfile.write(bytes("Cycle started<br>", "utf-8"))
sleep(2)
relay.close()
sleep(20)
relay = DigitalOutputDevice(17)
relay.on()
sleep(2)
relay.close()
logging.info('Cycle finished')
self.wfile.write(bytes("Cycle finished", "utf-8"))
busy = False
else:
# self.wfile.write(bytes("Busy now!<br>", "utf-8"))
self.send_error(503)
myServer = ThreadingServer((hostName, hostPort), MyServer)
print(time.asctime(), "Server Starts - %s:%s" % (hostName, hostPort))
try:
myServer.serve_forever()
except KeyboardInterrupt:
pass
myServer.server_close()
print(time.asctime(), "Server Stops - %s:%s" % (hostName, hostPort))

In general, the idea you're looking for is called request throttling. There are lots of implementations of this kind of thing which shouldn't be hard to dig up out there on the Web: here's one for Flask, my microframework of choice - https://flask-limiter.readthedocs.io/en/stable/
Quick usage example:
#app.route("/open_gate")
#limiter.limit("1 per minute")
def slow():
gate_robot.open_gate()
return

Gevent.monkey.patch_all breaks code that relies on socket.shutdown()

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.

socket.accept() hangs my program

I am trying to create applications that can communicate through sockets in python.I was already able to achieve this in a console program and was trying to do the same in WPF.It is quite a simple program that should act as the server,where a socket is created,bound to a port then listens for connections.The problem i am facing now is that everytime i include the code for the socket to accept a connection,my program hangs. I have tried to look for the solution online but everyone seems to be doing it exactly the same way i am doing it.Has anyone encountered this problem before and if so how did you solve it.
Thanks in advance for your help.
This is the code:
import wpf
import socket
import sys
import clr
import System
HOST=''
PORT = 8888
from System.Windows import Application, Window
s=socket.socket(socket.AF_INET,socket.SOCK_STREAM)
class MyWindow(Window):
def __init__(self):
wpf.LoadComponent(self, 'managementToolSimulator2.xaml')
def Button1_Click(self, sender, e):
pass
def Window_Loaded(self, sender, e):
s.bind((HOST,PORT))
s.listen(5)
self.ListBox1.AddText('Listening...')
while 1:
conn,addr=s.accept()
self.ListBox1.AddText('Got connection')
pass
if __name__ == '__main__':
Application().Run(MyWindow())
If i run the program without the line:
while 1:
conn,addr=s.accept()
self.ListBox1.AddText('Got connection')
it runs fine but if i include the line the program hangs and has to be stopped from the task manager.

Thread off the server.
You must not make blocking calls in GUI event handlers. The GUI thread must handle Windows messages in a timely manner - you cannot just block execution in a handler.

Using Processes as Threads with Networking in Python

Basically, my idea was to write some sort of basic server where I could connect to my computer and then run a command remotely. This didn't seem to be much of a problem; but then I had the bright idea that the next step would logically be to add some sort of threading so I could spawn multiple connections.
I read that, because of the GIL, multiprocessing.Process would be the best to try to do this. I don't completely understand threading and it's hard to find good documentation on it; so I'm kind of just throwing stuff and trying to figure out how it works.
Well, it seems like I might be close to doing this right; but I have a feeling I'm just as likely to be no where near doing this correctly. My program now does allow multiple connections, which it didn't when I first started working with threading; but once a connection is established, and then another is established, the first connection is no longer able to send a command to the server. I would appreciate it if someone could give me any help, or point me in the right direction on what I need to learn and understand.
Here's my code:
class server:
def __init__(self):
self.s = socket.socket()
try:
self.s.bind(("",69696))
self.s.listen(1)
except socket.error,(value,message):
if self.s:
self.s.close()
def connection(self):
while True:
client , address = self.s.accept()
data = client.recv(5)
password = 'hello'
while 1:
if data == password:
subprocess.call('firefox')
client.close()
else:
client.send('wrong password')
data = client.recv(5)
p = Process(target=x.connection())
p.start()
x = server()
if __name__ == '__main':
main()

Well, this answer only applies if you're on a unix or unix-like operating system(windows does not have os.fork() which we use).
One of the most common approaches for doing these things on unix platforms is to fork a new process to handle the client connection while the master process continues to listen for requests.
Below is code for a simple echo server that can handle multiple simultaneous connections. You just need to modify handle_client_connection() to fit your needs
import socket
import os
class ForkingServer:
def serve_forever(self):
self.s = socket.socket()
try:
self.s.bind(("", 9000))
self.s.listen(1)
except socket.error, (value,message):
print "error:", message
if self.s:
self.s.close()
return
while True:
client,address = self.s.accept()
pid = os.fork()
# You should read the documentation for how fork() works if you don't
# know it already
# The short version is that at this point in the code, there are 2 processes
# completely identical to each other which are simulatenously executing
# The only difference is that the parent process gets the pid of the child
# returned from fork() and the child process gets a value of 0 returned
if pid == 0:
# only the newly spawned process will execute this
self.handle_client_connection(client, address)
break
# In the meantime the parent process will continue on to here
# thus it will go back to the beginning of the loop and accept a new connection
def handle_client_connection(self, client,address):
#simple echo server
print "Got a connection from:", address
while True:
data = client.recv(5)
if not data:
# client closed the connection
break
client.send(data)
print "Connection from", address, "closed"
server = ForkingServer()
server.serve_forever()