How to use the foolscap.tokens.NegotiationError function in foolscap

# parse the decision here, create the connection parameters dict
        ver = int(decision['banana-decision-version'])
        vocab_index_string = decision.get('initial-vocab-table-index')
        if vocab_index_string:
            vocab_index, vocab_hash = vocab_index_string.split()
            vocab_index = int(vocab_index)
        else:
            vocab_index = 0
        check_inrange(self.initialVocabTableRange[0],
                      self.initialVocabTableRange[1],
                      vocab_index, "initial vocab table index")
        our_hash = vocab.hashVocabTable(vocab_index)
        if vocab_index > 0 and our_hash != vocab_hash:
            msg = ("Our hash for vocab-table-index %d (%s) does not match "
                   "your hash (%s)" % (vocab_index, our_hash, vocab_hash))
            raise NegotiationError(msg)

        if self.theirTubRef in self.tub.brokers:
            # we're the slave, so we need to drop our existing connection and
            # use the one picked by the master
            self.log("master told us to use a new connection, "
                     "so we must drop the existing one", level=UNUSUAL)
            err = DeadReferenceError("replaced by a new connection")
            why = Failure(err)
            self.tub.brokers[self.theirTubRef].shutdown(why)

        current_connection = decision.get('current-connection')
        if current_connection:
            tubID = self.theirTubRef.getTubID()
            if tubID != "":
                self.tub.slave_table[tubID] = tuple(current_connection.split())
        else:

the connection parameters we will use ourselves.

            - We are the master, we can't accomodate their request: raise
            NegotiationError

            - We are not the master: DECISION is None
        """

        self.log("evaluateHello(isClient=%s): offer=%s" %
                 (self.isClient, offer))
        if not offer.has_key('banana-negotiation-range'):
            if offer.has_key('banana-negotiation-version'):
                msg = ("Peer is speaking foolscap-0.0.5 or earlier, "
                       "which is not compatible with this version. "
                       "Please upgrade the peer.")
                raise NegotiationError(msg)
            raise NegotiationError("No valid banana-negotiation sequence seen")
        min_s, max_s = offer['banana-negotiation-range'].split()
        theirMinVer = int(min_s)
        theirMaxVer = int(max_s)
        # best_overlap() might raise a NegotiationError
        best = best_overlap(self.minVersion, self.maxVersion,
                            theirMinVer, theirMaxVer,
                            "banana version")

        negfunc = getattr(self, "evaluateNegotiationVersion%d" % best)
        self.decision_version = best
        return negfunc(offer)

- We are the master, we can't accomodate their request: raise
            NegotiationError

            - We are not the master: DECISION is None
        """

        self.log("evaluateHello(isClient=%s): offer=%s" %
                 (self.isClient, offer))
        if not offer.has_key('banana-negotiation-range'):
            if offer.has_key('banana-negotiation-version'):
                msg = ("Peer is speaking foolscap-0.0.5 or earlier, "
                       "which is not compatible with this version. "
                       "Please upgrade the peer.")
                raise NegotiationError(msg)
            raise NegotiationError("No valid banana-negotiation sequence seen")
        min_s, max_s = offer['banana-negotiation-range'].split()
        theirMinVer = int(min_s)
        theirMaxVer = int(max_s)
        # best_overlap() might raise a NegotiationError
        best = best_overlap(self.minVersion, self.maxVersion,
                            theirMinVer, theirMaxVer,
                            "banana version")

        negfunc = getattr(self, "evaluateNegotiationVersion%d" % best)
        self.decision_version = best
        return negfunc(offer)

"""
        # the client sends us a GET message
        lines = header.split("\r\n")
        if not lines[0].startswith("GET "):
            raise BananaError("not right")
        command, url, version = lines[0].split()
        if not url.startswith("/id/"):
            # probably a web browser
            raise BananaError("not right")
        targetTubID = url[4:]

        Message.log(event_type=u"handlePLAINTEXTServer", tub_id=targetTubID)

        if targetTubID == "":
            # they're asking for an old UnauthenticatedTub. Refuse.
            raise NegotiationError("secure Tubs require encryption")
        if isSubstring("Upgrade: TLS/1.0\r\n", header):
            wantEncrypted = True
        else:
            wantEncrypted = False

        Message.log(event_type=u"handlePLAINTEXTServer", want_encrypted=wantEncrypted)

        self._handleTubRequest(header, targetTubID)

raise NegotiationError("secure Tubs require encryption")

        # now that we know which Tub the client wants to connect to, either
        # send a Redirect, or start the ENCRYPTED phase

        tub, redirect = self.listener.lookupTubID(targetTubID)
        if tub:
            self.tub = tub # our tub
            self.options.update(self.tub.options)
            self.brokerClass = self.tub.brokerClass
            self.myTubID = tub.tubID
            self.sendPlaintextServerAndStartENCRYPTED(wantEncrypted)
        elif redirect:
            self.sendRedirect(redirect)
        else:
            raise NegotiationError("unknown TubID %s" % targetTubID)

def check_inrange(my_min, my_max, decision, name):
    if decision < my_min or decision > my_max:
        raise NegotiationError("I can't handle %s %d" % (name, decision))

level=UNUSUAL)
                existing = self.tub.brokers[theirTubRef]
                acceptOffer = self.compareOfferAndExisting(offer, existing, lp)
                if acceptOffer:
                    # drop the old one
                    self.log("accepting new offer, dropping existing connection",
                             parent=lp)
                    err = DeadReferenceError("[%s] replaced by a new connection"
                                             % theirTubRef.getShortTubID())
                    why = Failure(err)
                    existing.shutdown(why)
                else:
                    # reject the new one
                    self.log("rejecting the offer: we already have one",
                             parent=lp)
                    raise NegotiationError("Duplicate connection")

            if theirTubRef:
                # generate a new seqnum, one higher than the last one we've
                # used. Note that UnauthenticatedTubs all share the same
                # index, so we leak certain information about how many
                # connections we've established.
                old_seqnum = self.tub.master_table.get(theirTubRef.getTubID(),
                                                       0)
                new_seqnum = old_seqnum + 1
                new_slave_IR = offer.get('my-incarnation', None)
                self.tub.master_table[theirTubRef.getTubID()] = new_seqnum
                my_IR = self.tub.getIncarnationString()
                decision['current-connection'] = "%s %s" % (my_IR, new_seqnum)
                # these params will be copied into the Broker where we can
                # retrieve them later, when we need to compare it against a new
                # offer.

def _handleTubRequest(self, header, targetTubID):
        """
        Proxy to the destination which is responsible for the target TubID.

        :param bytes header: The first part of the Foolscap negotiation which
            will need to be passed along to the proxy target.

        :param bytes targetTubID: The TubID which was requested.
        """
        try:
            _, (ip, port_number) = self.factory.route_mapping()[targetTubID]
        except KeyError:
            raise NegotiationError("unknown TubID %s" % (targetTubID,))

        if not ip:
            raise NegotiationError("TubID not yet available %s" % (targetTubID,))

        # Now proxy to ip:port_number
        proxy(self, TCP4ClientEndpoint(self.factory.reactor, ip, port_number), header)

def _handleTubRequest(self, header, targetTubID):
        """
        Proxy to the destination which is responsible for the target TubID.

        :param bytes header: The first part of the Foolscap negotiation which
            will need to be passed along to the proxy target.

        :param bytes targetTubID: The TubID which was requested.
        """
        try:
            _, (ip, port_number) = self.factory.route_mapping()[targetTubID]
        except KeyError:
            raise NegotiationError("unknown TubID %s" % (targetTubID,))

        if not ip:
            raise NegotiationError("TubID not yet available %s" % (targetTubID,))

        # Now proxy to ip:port_number
        proxy(self, TCP4ClientEndpoint(self.factory.reactor, ip, port_number), header)