How can I ensure closing all connection in loadbalancer when something fails or hangs?

Question

I'm trying to write a simple load-balancer. It works ok till one of servers (BalanceServer) doesn't close connection then... Client (ReverseProxy) disconnects but the connection in with BalanceServer stays open. I tried to add callback (#3) to ReverseProxy.connectionLost to close the connection with one of the servers as I do with closing connection when server disconnects (clientLoseConnection), but at that time the ServerWriter is Null and I cannot terminate it at #1 and #2

---

How can I ensure that all connections are closed when one of sides disconnects? I guess that also some kind of timeout here would be nice when both client and one of servers hang, but how can I add it so it works on both connections?

---

from twisted.internet.protocol import Protocol, Factory, ClientCreator
from twisted.internet import reactor, defer
from collections import namedtuple

BalanceServer = namedtuple('BalanceServer', 'host port')

SERVER_LIST = [BalanceServer('127.0.0.1', 8000), BalanceServer('127.0.0.1', 8001)]

def getServer(servers):
    while True:
        for server in servers:
            yield server

# this writes to one of balance servers and responds to client with callback 'clientWrite'
class ServerWriter(Protocol):
    def sendData(self, data):
        self.transport.write(data)

    def dataReceived(self, data):
        self.clientWrite(data)

    def connectionLost( self, reason ):
        self.clientLoseConnection()

# callback for reading data from client to send it to server and get response to client again    
def transferData(serverWriter, clientWrite, clientLoseConnection, data):
    if serverWriter:
        serverWriter.clientWrite = clientWrite
        serverWriter.clientLoseConnection = clientLoseConnection
        serverWriter.sendData(data)

def closeConnection(serverWriter):
    if serverWriter: #1 this is null
        #2 So connection is not closed and hangs there, till BalanceServer close it 
        serverWriter.transport.loseConnection()

# accepts clients
class ReverseProxy(Protocol):
    def connectionMade(self):
        server = self.factory.getServer()
        self.serverWriter = ClientCreator(reactor, ServerWriter)
        self.client = self.serverWriter.connectTCP( server.host, server.port )

    def dataReceived(self, data):
        self.client.addCallback(transferData, self.transport.write, 
                    self.transport.loseConnection, data )

    def connectionLost(self, reason):
        self.client.addCallback(closeConnection) #3 adding close doesn't work


class ReverseProxyFactory(Factory):
    protocol = ReverseProxy
    def __init__(self, serverGenerator):
        self.getServer = serverGenerator

plainFactory = ReverseProxyFactory( getServer(SERVER_LIST).next )
reactor.listenTCP( 7777, plainFactory )
reactor.run()