forked-synapse/synapse/http/endpoint.py

# -*- coding: utf-8 -*-
# Copyright 2014-2016 OpenMarket Ltd
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import collections
import logging
import random
import re
import time

from twisted.internet import defer
from twisted.internet.endpoints import HostnameEndpoint, wrapClientTLS
from twisted.internet.error import ConnectError
from twisted.names import client, dns
from twisted.names.error import DNSNameError, DomainError

logger = logging.getLogger(__name__)

SERVER_CACHE = {}

# our record of an individual server which can be tried to reach a destination.
#
# "host" is the hostname acquired from the SRV record. Except when there's
# no SRV record, in which case it is the original hostname.
_Server = collections.namedtuple(
    "_Server", "priority weight host port expires"
)


def parse_server_name(server_name):
    """Split a server name into host/port parts.

    Args:
        server_name (str): server name to parse

    Returns:
        Tuple[str, int|None]: host/port parts.

    Raises:
        ValueError if the server name could not be parsed.
    """
    try:
        if server_name[-1] == ']':
            # ipv6 literal, hopefully
            return server_name, None

        domain_port = server_name.rsplit(":", 1)
        domain = domain_port[0]
        port = int(domain_port[1]) if domain_port[1:] else None
        return domain, port
    except Exception:
        raise ValueError("Invalid server name '%s'" % server_name)


VALID_HOST_REGEX = re.compile(
    "\\A[0-9a-zA-Z.-]+\\Z",
)


def parse_and_validate_server_name(server_name):
    """Split a server name into host/port parts and do some basic validation.

    Args:
        server_name (str): server name to parse

    Returns:
        Tuple[str, int|None]: host/port parts.

    Raises:
        ValueError if the server name could not be parsed.
    """
    host, port = parse_server_name(server_name)

    # these tests don't need to be bulletproof as we'll find out soon enough
    # if somebody is giving us invalid data. What we *do* need is to be sure
    # that nobody is sneaking IP literals in that look like hostnames, etc.

    # look for ipv6 literals
    if host[0] == '[':
        if host[-1] != ']':
            raise ValueError("Mismatched [...] in server name '%s'" % (
                server_name,
            ))
        return host, port

    # otherwise it should only be alphanumerics.
    if not VALID_HOST_REGEX.match(host):
        raise ValueError("Server name '%s' contains invalid characters" % (
            server_name,
        ))

    return host, port


def matrix_federation_endpoint(reactor, destination, tls_client_options_factory=None,
                               timeout=None):
    """Construct an endpoint for the given matrix destination.

    Args:
        reactor: Twisted reactor.
        destination (unicode): The name of the server to connect to.
        tls_client_options_factory
            (synapse.crypto.context_factory.ClientTLSOptionsFactory):
            Factory which generates TLS options for client connections.
        timeout (int): connection timeout in seconds
    """

    domain, port = parse_server_name(destination)

    endpoint_kw_args = {}

    if timeout is not None:
        endpoint_kw_args.update(timeout=timeout)

    if tls_client_options_factory is None:
        transport_endpoint = HostnameEndpoint
        default_port = 8008
    else:
        # the SNI string should be the same as the Host header, minus the port.
        # as per https://github.com/matrix-org/synapse/issues/2525#issuecomment-336896777,
        # the Host header and SNI should therefore be the server_name of the remote
        # server.
        tls_options = tls_client_options_factory.get_options(domain)

        def transport_endpoint(reactor, host, port, timeout):
            return wrapClientTLS(
                tls_options,
                HostnameEndpoint(reactor, host, port, timeout=timeout),
            )
        default_port = 8448

    if port is None:
        return SRVClientEndpoint(
            reactor, "matrix", domain, protocol="tcp",
            default_port=default_port, endpoint=transport_endpoint,
            endpoint_kw_args=endpoint_kw_args
        )
    else:
        return transport_endpoint(
            reactor, domain, port, **endpoint_kw_args
        )


class SRVClientEndpoint(object):
    """An endpoint which looks up SRV records for a service.
    Cycles through the list of servers starting with each call to connect
    picking the next server.
    Implements twisted.internet.interfaces.IStreamClientEndpoint.
    """

    def __init__(self, reactor, service, domain, protocol="tcp",
                 default_port=None, endpoint=HostnameEndpoint,
                 endpoint_kw_args={}):
        self.reactor = reactor
        self.service_name = "_%s._%s.%s" % (service, protocol, domain)

        if default_port is not None:
            self.default_server = _Server(
                host=domain,
                port=default_port,
                priority=0,
                weight=0,
                expires=0,
            )
        else:
            self.default_server = None

        self.endpoint = endpoint
        self.endpoint_kw_args = endpoint_kw_args

        self.servers = None
        self.used_servers = None

    @defer.inlineCallbacks
    def fetch_servers(self):
        self.used_servers = []
        self.servers = yield resolve_service(self.service_name)

    def pick_server(self):
        if not self.servers:
            if self.used_servers:
                self.servers = self.used_servers
                self.used_servers = []
                self.servers.sort()
            elif self.default_server:
                return self.default_server
            else:
                raise ConnectError(
                    "No server available for %s" % self.service_name
                )

        # look for all servers with the same priority
        min_priority = self.servers[0].priority
        weight_indexes = list(
            (index, server.weight + 1)
            for index, server in enumerate(self.servers)
            if server.priority == min_priority
        )

        total_weight = sum(weight for index, weight in weight_indexes)
        target_weight = random.randint(0, total_weight)
        for index, weight in weight_indexes:
            target_weight -= weight
            if target_weight <= 0:
                server = self.servers[index]
                # XXX: this looks totally dubious:
                #
                # (a) we never reuse a server until we have been through
                #     all of the servers at the same priority, so if the
                #     weights are A: 100, B:1, we always do ABABAB instead of
                #     AAAA...AAAB (approximately).
                #
                # (b) After using all the servers at the lowest priority,
                #     we move onto the next priority. We should only use the
                #     second priority if servers at the top priority are
                #     unreachable.
                #
                del self.servers[index]
                self.used_servers.append(server)
                return server

    @defer.inlineCallbacks
    def connect(self, protocolFactory):
        if self.servers is None:
            yield self.fetch_servers()
        server = self.pick_server()
        logger.info("Connecting to %s:%s", server.host, server.port)
        endpoint = self.endpoint(
            self.reactor, server.host, server.port, **self.endpoint_kw_args
        )
        connection = yield endpoint.connect(protocolFactory)
        defer.returnValue(connection)


@defer.inlineCallbacks
def resolve_service(service_name, dns_client=client, cache=SERVER_CACHE, clock=time):
    cache_entry = cache.get(service_name, None)
    if cache_entry:
        if all(s.expires > int(clock.time()) for s in cache_entry):
            servers = list(cache_entry)
            defer.returnValue(servers)

    servers = []

    try:
        try:
            answers, _, _ = yield dns_client.lookupService(service_name)
        except DNSNameError:
            defer.returnValue([])

        if (len(answers) == 1
                and answers[0].type == dns.SRV
                and answers[0].payload
                and answers[0].payload.target == dns.Name(b'.')):
            raise ConnectError("Service %s unavailable" % service_name)

        for answer in answers:
            if answer.type != dns.SRV or not answer.payload:
                continue

            payload = answer.payload

            servers.append(_Server(
                host=str(payload.target),
                port=int(payload.port),
                priority=int(payload.priority),
                weight=int(payload.weight),
                expires=int(clock.time()) + answer.ttl,
            ))

        servers.sort()
        cache[service_name] = list(servers)
    except DomainError as e:
        # We failed to resolve the name (other than a NameError)
        # Try something in the cache, else rereaise
        cache_entry = cache.get(service_name, None)
        if cache_entry:
            logger.warn(
                "Failed to resolve %r, falling back to cache. %r",
                service_name, e
            )
            servers = list(cache_entry)
        else:
            raise e

    defer.returnValue(servers)