使用paramiko端口转发演示时如何避免无限循环？

import paramiko
import forward
import MySQLdb
import cfg
import sys

client = paramiko.SSHClient()
client.load_system_host_keys()
client.set_missing_host_key_policy(paramiko.WarningPolicy())

try:
    client.connect(cfg.remhost, cfg.remport, username=cfg.user, password=cfg.password)
except Exception, e:
    print \'*** Failed to connect to %s:%d: %r\' % (cfg.remhost, cfg.remport, e)
    sys.exit(1)


try:
    forward.forward_tunnel(3306, cfg.remhost, 3306, client.get_transport())
except KeyboardInterrupt:
    print \'C-c: Port forwarding stopped.\'
    sys.exit(0)

try:
    db = MySQLdb.connect(\'127.0.0.1\', cfg.dbuser, cfg.dbpass, cfg.dbname)
except Exception, e:
    print \'Failed to connect to database\'
    sys.exit(1)

try:
    cursor = self.db.cursor(MySQLdb.cursors.DictCursor)
    sql = \'SELECT * FROM  \' + cfg.dbtable
    cursor.execute(sql)
    results = cursor.fetchall()
    print str(len(results))
except Exception, e:
    print \'Failed to query database\'
    sys.exit(1)

class ForwardServer (SocketServer.ThreadingTCPServer):
    daemon_threads = True
    allow_reuse_address = True


class Handler (SocketServer.BaseRequestHandler):

    def handle(self):
        try:
            chan = self.ssh_transport.open_channel(\'direct-tcpip\',
                                                   (self.chain_host, self.chain_port),
                                                   self.request.getpeername())
        except Exception, e:
            verbose(\'Incoming request to %s:%d failed: %s\' % (self.chain_host,
                                                              self.chain_port,
                                                              repr(e)))
            return
        if chan is None:
            verbose(\'Incoming request to %s:%d was rejected by the SSH server.\' %
                    (self.chain_host, self.chain_port))
            return

        verbose(\'Connected!  Tunnel open %r -> %r -> %r\' % (self.request.getpeername(),
                                                            chan.getpeername(), (self.chain_host, self.chain_port)))
        while True:
            r, w, x = select.select([self.request, chan], [], [])
            if self.request in r:
                data = self.request.recv(1024)
                if len(data) == 0:
                    break
                chan.send(data)
            if chan in r:
                data = chan.recv(1024)
                if len(data) == 0:
                    break
                self.request.send(data)
        chan.close()
        self.request.close()
        verbose(\'Tunnel closed from %r\' % (self.request.getpeername(),))


def forward_tunnel(local_port, remote_host, remote_port, transport):
    # this is a little convoluted, but lets me configure things for the Handler
    # object.  (SocketServer doesn\'t give Handlers any way to access the outer
    # server normally.)
    class SubHander (Handler):
        chain_host = remote_host
        chain_port = remote_port
        ssh_transport = transport
    ForwardServer((\'\', local_port), SubHander).serve_forever()


def verbose(s):
    if g_verbose:
        print s