matrixfederationclient.py

# -*- coding: utf-8 -*-
# Copyright 2014-2016 OpenMarket Ltd
# Copyright 2018 New Vector 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 cgi
import logging
import random
import sys
import urllib
from io import BytesIO

import attr
import treq
from canonicaljson import encode_canonical_json
from prometheus_client import Counter
from signedjson.sign import sign_json
from zope.interface import implementer

from twisted.internet import defer, protocol
from twisted.internet.error import DNSLookupError
from twisted.internet.interfaces import IReactorPluggableNameResolver, IReactorTime
from twisted.internet.task import _EPSILON, Cooperator
from twisted.web._newclient import ResponseDone
from twisted.web.http_headers import Headers
from twisted.web.iweb import IResponse

import synapse.metrics
import synapse.util.retryutils
from synapse.api.errors import (
    Codes,
    FederationDeniedError,
    HttpResponseException,
    RequestSendFailed,
    SynapseError,
)
from synapse.http import QuieterFileBodyProducer
from synapse.http.client import BlacklistingAgentWrapper, IPBlacklistingResolver
from synapse.http.federation.matrix_federation_agent import MatrixFederationAgent
from synapse.logging.context import make_deferred_yieldable
from synapse.logging.opentracing import (
    inject_active_span_byte_dict,
    set_tag,
    start_active_span,
    tags,
)
from synapse.util import json_decoder
from synapse.util.async_helpers import timeout_deferred
from synapse.util.metrics import Measure

logger = logging.getLogger(__name__)

outgoing_requests_counter = Counter(
    "synapse_http_matrixfederationclient_requests", "", ["method"]
)
incoming_responses_counter = Counter(
    "synapse_http_matrixfederationclient_responses", "", ["method", "code"]
)


MAX_LONG_RETRIES = 10
MAX_SHORT_RETRIES = 3
MAXINT = sys.maxsize


_next_id = 1


@attr.s(slots=True, frozen=True)
class MatrixFederationRequest:
    method = attr.ib()
    """HTTP method
    :type: str
    """

    path = attr.ib()
    """HTTP path
    :type: str
    """

    destination = attr.ib()
    """The remote server to send the HTTP request to.
    :type: str"""

    json = attr.ib(default=None)
    """JSON to send in the body.
    :type: dict|None
    """

    json_callback = attr.ib(default=None)
    """A callback to generate the JSON.
    :type: func|None
    """

    query = attr.ib(default=None)
    """Query arguments.
    :type: dict|None
    """

    txn_id = attr.ib(default=None)
    """Unique ID for this request (for logging)
    :type: str|None
    """

    uri = attr.ib(init=False, type=bytes)
    """The URI of this request
    """

    def __attrs_post_init__(self):
        global _next_id
        txn_id = "%s-O-%s" % (self.method, _next_id)
        _next_id = (_next_id + 1) % (MAXINT - 1)

        object.__setattr__(self, "txn_id", txn_id)

        destination_bytes = self.destination.encode("ascii")
        path_bytes = self.path.encode("ascii")
        if self.query:
            query_bytes = encode_query_args(self.query)
        else:
            query_bytes = b""

        # The object is frozen so we can pre-compute this.
        uri = urllib.parse.urlunparse(
            (b"matrix", destination_bytes, path_bytes, None, query_bytes, b"")
        )
        object.__setattr__(self, "uri", uri)

    def get_json(self):
        if self.json_callback:
            return self.json_callback()
        return self.json


async def _handle_json_response(
    reactor: IReactorTime,
    timeout_sec: float,
    request: MatrixFederationRequest,
    response: IResponse,
    start_ms: int,
):
    """
    Reads the JSON body of a response, with a timeout

    Args:
        reactor: twisted reactor, for the timeout
        timeout_sec: number of seconds to wait for response to complete
        request: the request that triggered the response
        response: response to the request
        start_ms: Timestamp when request was made

    Returns:
        dict: parsed JSON response
    """
    try:
        check_content_type_is_json(response.headers)

        # Use the custom JSON decoder (partially re-implements treq.json_content).
        d = treq.text_content(response, encoding="utf-8")
        d.addCallback(json_decoder.decode)
        d = timeout_deferred(d, timeout=timeout_sec, reactor=reactor)

        body = await make_deferred_yieldable(d)
    except TimeoutError as e:
        logger.warning(
            "{%s} [%s] Timed out reading response - %s %s",
            request.txn_id,
            request.destination,
            request.method,
            request.uri.decode("ascii"),
        )
        raise RequestSendFailed(e, can_retry=True) from e
    except Exception as e:
        logger.warning(
            "{%s} [%s] Error reading response %s %s: %s",
            request.txn_id,
            request.destination,
            request.method,
            request.uri.decode("ascii"),
            e,
        )
        raise

    time_taken_secs = reactor.seconds() - start_ms / 1000

    logger.info(
        "{%s} [%s] Completed request: %d %s in %.2f secs - %s %s",
        request.txn_id,
        request.destination,
        response.code,
        response.phrase.decode("ascii", errors="replace"),
        time_taken_secs,
        request.method,
        request.uri.decode("ascii"),
    )
    return body


class MatrixFederationHttpClient:
    """HTTP client used to talk to other homeservers over the federation
    protocol. Send client certificates and signs requests.

    Attributes:
        agent (twisted.web.client.Agent): The twisted Agent used to send the
            requests.
    """

    def __init__(self, hs, tls_client_options_factory):
        self.hs = hs
        self.signing_key = hs.signing_key
        self.server_name = hs.hostname

        real_reactor = hs.get_reactor()

        # We need to use a DNS resolver which filters out blacklisted IP
        # addresses, to prevent DNS rebinding.
        nameResolver = IPBlacklistingResolver(
            real_reactor, None, hs.config.federation_ip_range_blacklist
        )

        @implementer(IReactorPluggableNameResolver)
        class Reactor:
            def __getattr__(_self, attr):
                if attr == "nameResolver":
                    return nameResolver
                else:
                    return getattr(real_reactor, attr)

        self.reactor = Reactor()

        user_agent = hs.version_string
        if hs.config.user_agent_suffix:
            user_agent = "%s %s" % (user_agent, hs.config.user_agent_suffix)
        user_agent = user_agent.encode("ascii")

        self.agent = MatrixFederationAgent(
            self.reactor, tls_client_options_factory, user_agent
        )

        # Use a BlacklistingAgentWrapper to prevent circumventing the IP
        # blacklist via IP literals in server names
        self.agent = BlacklistingAgentWrapper(
            self.agent,
            self.reactor,
            ip_blacklist=hs.config.federation_ip_range_blacklist,
        )

        self.clock = hs.get_clock()
        self._store = hs.get_datastore()
        self.version_string_bytes = hs.version_string.encode("ascii")
        self.default_timeout = 60

        def schedule(x):
            self.reactor.callLater(_EPSILON, x)

        self._cooperator = Cooperator(scheduler=schedule)

    async def _send_request_with_optional_trailing_slash(
        self, request, try_trailing_slash_on_400=False, **send_request_args
    ):
        """Wrapper for _send_request which can optionally retry the request
        upon receiving a combination of a 400 HTTP response code and a
        'M_UNRECOGNIZED' errcode. This is a workaround for Synapse <= v0.99.3
        due to #3622.

        Args:
            request (MatrixFederationRequest): details of request to be sent
            try_trailing_slash_on_400 (bool): Whether on receiving a 400
                'M_UNRECOGNIZED' from the server to retry the request with a
                trailing slash appended to the request path.
            send_request_args (Dict): A dictionary of arguments to pass to
                `_send_request()`.

        Raises:
            HttpResponseException: If we get an HTTP response code >= 300
                (except 429).

        Returns:
            Dict: Parsed JSON response body.
        """
        try:
            response = await self._send_request(request, **send_request_args)
        except HttpResponseException as e:
            # Received an HTTP error > 300. Check if it meets the requirements
            # to retry with a trailing slash
            if not try_trailing_slash_on_400:
                raise

            if e.code != 400 or e.to_synapse_error().errcode != "M_UNRECOGNIZED":
                raise

            # Retry with a trailing slash if we received a 400 with
            # 'M_UNRECOGNIZED' which some endpoints can return when omitting a
            # trailing slash on Synapse <= v0.99.3.
            logger.info("Retrying request with trailing slash")

            # Request is frozen so we create a new instance
            request = attr.evolve(request, path=request.path + "/")

            response = await self._send_request(request, **send_request_args)

        return response

    async def _send_request(
        self,
        request,
        retry_on_dns_fail=True,
        timeout=None,
        long_retries=False,
        ignore_backoff=False,
        backoff_on_404=False,
    ):
        """
        Sends a request to the given server.

        Args:
            request (MatrixFederationRequest): details of request to be sent

            timeout (int|None): number of milliseconds to wait for the response headers
                (including connecting to the server), *for each attempt*.
                60s by default.

            long_retries (bool): whether to use the long retry algorithm.

                The regular retry algorithm makes 4 attempts, with intervals
                [0.5s, 1s, 2s].

                The long retry algorithm makes 11 attempts, with intervals
                [4s, 16s, 60s, 60s, ...]

                Both algorithms add -20%/+40% jitter to the retry intervals.

                Note that the above intervals are *in addition* to the time spent
                waiting for the request to complete (up to `timeout` ms).

                NB: the long retry algorithm takes over 20 minutes to complete, with
                a default timeout of 60s!

            ignore_backoff (bool): true to ignore the historical backoff data
                and try the request anyway.

            backoff_on_404 (bool): Back off if we get a 404

        Returns:
            twisted.web.client.Response: resolves with the HTTP
            response object on success.

        Raises:
            HttpResponseException: If we get an HTTP response code >= 300
                (except 429).
            NotRetryingDestination: If we are not yet ready to retry this
                server.
            FederationDeniedError: If this destination  is not on our
                federation whitelist
            RequestSendFailed: If there were problems connecting to the
                remote, due to e.g. DNS failures, connection timeouts etc.
        """
        if timeout:
            _sec_timeout = timeout / 1000
        else:
            _sec_timeout = self.default_timeout

        if (
            self.hs.config.federation_domain_whitelist is not None
            and request.destination not in self.hs.config.federation_domain_whitelist
        ):
            raise FederationDeniedError(request.destination)

        limiter = await synapse.util.retryutils.get_retry_limiter(
            request.destination,
            self.clock,
            self._store,
            backoff_on_404=backoff_on_404,
            ignore_backoff=ignore_backoff,
        )

        method_bytes = request.method.encode("ascii")
        destination_bytes = request.destination.encode("ascii")
        path_bytes = request.path.encode("ascii")
        if request.query:
            query_bytes = encode_query_args(request.query)
        else:
            query_bytes = b""

        scope = start_active_span(
            "outgoing-federation-request",
            tags={
                tags.SPAN_KIND: tags.SPAN_KIND_RPC_CLIENT,
                tags.PEER_ADDRESS: request.destination,
                tags.HTTP_METHOD: request.method,
                tags.HTTP_URL: request.path,
            },
            finish_on_close=True,
        )

        # Inject the span into the headers
        headers_dict = {}
        inject_active_span_byte_dict(headers_dict, request.destination)

        headers_dict[b"User-Agent"] = [self.version_string_bytes]

        with limiter, scope:
            # XXX: Would be much nicer to retry only at the transaction-layer
            # (once we have reliable transactions in place)
            if long_retries:
                retries_left = MAX_LONG_RETRIES
            else:
                retries_left = MAX_SHORT_RETRIES

            url_bytes = request.uri
            url_str = url_bytes.decode("ascii")

            url_to_sign_bytes = urllib.parse.urlunparse(
                (b"", b"", path_bytes, None, query_bytes, b"")
            )

            while True:
                try:
                    json = request.get_json()
                    if json:
                        headers_dict[b"Content-Type"] = [b"application/json"]
                        auth_headers = self.build_auth_headers(
                            destination_bytes, method_bytes, url_to_sign_bytes, json
                        )
                        data = encode_canonical_json(json)
                        producer = QuieterFileBodyProducer(
                            BytesIO(data), cooperator=self._cooperator
                        )
                    else:
                        producer = None
                        auth_headers = self.build_auth_headers(
                            destination_bytes, method_bytes, url_to_sign_bytes
                        )

                    headers_dict[b"Authorization"] = auth_headers

                    logger.debug(
                        "{%s} [%s] Sending request: %s %s; timeout %fs",
                        request.txn_id,
                        request.destination,
                        request.method,
                        url_str,
                        _sec_timeout,
                    )

                    outgoing_requests_counter.labels(request.method).inc()

                    try:
                        with Measure(self.clock, "outbound_request"):
                            # we don't want all the fancy cookie and redirect handling
                            # that treq.request gives: just use the raw Agent.
                            request_deferred = self.agent.request(
                                method_bytes,
                                url_bytes,
                                headers=Headers(headers_dict),
                                bodyProducer=producer,
                            )

                            request_deferred = timeout_deferred(
                                request_deferred,
                                timeout=_sec_timeout,
                                reactor=self.reactor,
                            )

                            response = await request_deferred
                    except TimeoutError as e:
                        raise RequestSendFailed(e, can_retry=True) from e
                    except DNSLookupError as e:
                        raise RequestSendFailed(e, can_retry=retry_on_dns_fail) from e
                    except Exception as e:
                        raise RequestSendFailed(e, can_retry=True) from e

                    incoming_responses_counter.labels(
                        request.method, response.code
                    ).inc()

                    set_tag(tags.HTTP_STATUS_CODE, response.code)
                    response_phrase = response.phrase.decode("ascii", errors="replace")

                    if 200 <= response.code < 300:
                        logger.debug(
                            "{%s} [%s] Got response headers: %d %s",
                            request.txn_id,
                            request.destination,
                            response.code,
                            response_phrase,
                        )
                        pass
                    else:
                        logger.info(
                            "{%s} [%s] Got response headers: %d %s",
                            request.txn_id,
                            request.destination,
                            response.code,
                            response_phrase,
                        )
                        # :'(
                        # Update transactions table?
                        d = treq.content(response)
                        d = timeout_deferred(
                            d, timeout=_sec_timeout, reactor=self.reactor
                        )

                        try:
                            body = await make_deferred_yieldable(d)
                        except Exception as e:
                            # Eh, we're already going to raise an exception so lets
                            # ignore if this fails.
                            logger.warning(
                                "{%s} [%s] Failed to get error response: %s %s: %s",
                                request.txn_id,
                                request.destination,
                                request.method,
                                url_str,
                                _flatten_response_never_received(e),
                            )
                            body = None

                        e = HttpResponseException(response.code, response_phrase, body)

                        # Retry if the error is a 429 (Too Many Requests),
                        # otherwise just raise a standard HttpResponseException
                        if response.code == 429:
                            raise RequestSendFailed(e, can_retry=True) from e
                        else:
                            raise e

                    break
                except RequestSendFailed as e:
                    logger.info(
                        "{%s} [%s] Request failed: %s %s: %s",
                        request.txn_id,
                        request.destination,
                        request.method,
                        url_str,
                        _flatten_response_never_received(e.inner_exception),
                    )

                    if not e.can_retry:
                        raise

                    if retries_left and not timeout:
                        if long_retries:
                            delay = 4 ** (MAX_LONG_RETRIES + 1 - retries_left)
                            delay = min(delay, 60)
                            delay *= random.uniform(0.8, 1.4)
                        else:
                            delay = 0.5 * 2 ** (MAX_SHORT_RETRIES - retries_left)
                            delay = min(delay, 2)
                            delay *= random.uniform(0.8, 1.4)

                        logger.debug(
                            "{%s} [%s] Waiting %ss before re-sending...",
                            request.txn_id,
                            request.destination,
                            delay,
                        )

                        await self.clock.sleep(delay)
                        retries_left -= 1
                    else:
                        raise

                except Exception as e:
                    logger.warning(
                        "{%s} [%s] Request failed: %s %s: %s",
                        request.txn_id,
                        request.destination,
                        request.method,
                        url_str,
                        _flatten_response_never_received(e),
                    )
                    raise
        return response

    def build_auth_headers(
        self, destination, method, url_bytes, content=None, destination_is=None
    ):
        """
        Builds the Authorization headers for a federation request
        Args:
            destination (bytes|None): The desination homeserver of the request.
                May be None if the destination is an identity server, in which case
                destination_is must be non-None.
            method (bytes): The HTTP method of the request
            url_bytes (bytes): The URI path of the request
            content (object): The body of the request
            destination_is (bytes): As 'destination', but if the destination is an
                identity server

        Returns:
            list[bytes]: a list of headers to be added as "Authorization:" headers
        """
        request = {
            "method": method.decode("ascii"),
            "uri": url_bytes.decode("ascii"),
            "origin": self.server_name,
        }

        if destination is not None:
            request["destination"] = destination.decode("ascii")

        if destination_is is not None:
            request["destination_is"] = destination_is.decode("ascii")

        if content is not None:
            request["content"] = content

        request = sign_json(request, self.server_name, self.signing_key)

        auth_headers = []

        for key, sig in request["signatures"][self.server_name].items():
            auth_headers.append(
                (
                    'X-Matrix origin=%s,key="%s",sig="%s"'
                    % (self.server_name, key, sig)
                ).encode("ascii")
            )
        return auth_headers

    async def put_json(
        self,
        destination,
        path,
        args={},
        data={},
        json_data_callback=None,
        long_retries=False,
        timeout=None,
        ignore_backoff=False,
        backoff_on_404=False,
        try_trailing_slash_on_400=False,
    ):
        """ Sends the specifed json data using PUT

        Args:
            destination (str): The remote server to send the HTTP request
                to.
            path (str): The HTTP path.
            args (dict): query params
            data (dict): A dict containing the data that will be used as
                the request body. This will be encoded as JSON.
            json_data_callback (callable): A callable returning the dict to
                use as the request body.

            long_retries (bool): whether to use the long retry algorithm. See
                docs on _send_request for details.

            timeout (int|None): number of milliseconds to wait for the response headers
                (including connecting to the server), *for each attempt*.
                self._default_timeout (60s) by default.

            ignore_backoff (bool): true to ignore the historical backoff data
                and try the request anyway.
            backoff_on_404 (bool): True if we should count a 404 response as
                a failure of the server (and should therefore back off future
                requests).
            try_trailing_slash_on_400 (bool): True if on a 400 M_UNRECOGNIZED
                response we should try appending a trailing slash to the end
                of the request. Workaround for #3622 in Synapse <= v0.99.3. This
                will be attempted before backing off if backing off has been
                enabled.

        Returns:
            dict|list: Succeeds when we get a 2xx HTTP response. The
            result will be the decoded JSON body.

        Raises:
            HttpResponseException: If we get an HTTP response code >= 300
                (except 429).
            NotRetryingDestination: If we are not yet ready to retry this
                server.
            FederationDeniedError: If this destination  is not on our
                federation whitelist
            RequestSendFailed: If there were problems connecting to the
                remote, due to e.g. DNS failures, connection timeouts etc.
        """
        request = MatrixFederationRequest(
            method="PUT",
            destination=destination,
            path=path,
            query=args,
            json_callback=json_data_callback,
            json=data,
        )

        start_ms = self.clock.time_msec()

        response = await self._send_request_with_optional_trailing_slash(
            request,
            try_trailing_slash_on_400,
            backoff_on_404=backoff_on_404,
            ignore_backoff=ignore_backoff,
            long_retries=long_retries,
            timeout=timeout,
        )

        body = await _handle_json_response(
            self.reactor, self.default_timeout, request, response, start_ms
        )

        return body

    async def post_json(
        self,
        destination,
        path,
        data={},
        long_retries=False,
        timeout=None,
        ignore_backoff=False,
        args={},
    ):
        """ Sends the specifed json data using POST

        Args:
            destination (str): The remote server to send the HTTP request
                to.

            path (str): The HTTP path.

            data (dict): A dict containing the data that will be used as
                the request body. This will be encoded as JSON.

            long_retries (bool): whether to use the long retry algorithm. See
                docs on _send_request for details.

            timeout (int|None): number of milliseconds to wait for the response headers
                (including connecting to the server), *for each attempt*.
                self._default_timeout (60s) by default.

            ignore_backoff (bool): true to ignore the historical backoff data and
                try the request anyway.

            args (dict): query params
        Returns:
            dict|list: Succeeds when we get a 2xx HTTP response. The
            result will be the decoded JSON body.

        Raises:
            HttpResponseException: If we get an HTTP response code >= 300
                (except 429).
            NotRetryingDestination: If we are not yet ready to retry this
                server.
            FederationDeniedError: If this destination  is not on our
                federation whitelist
            RequestSendFailed: If there were problems connecting to the
                remote, due to e.g. DNS failures, connection timeouts etc.
        """

        request = MatrixFederationRequest(
            method="POST", destination=destination, path=path, query=args, json=data
        )

        start_ms = self.clock.time_msec()

        response = await self._send_request(
            request,
            long_retries=long_retries,
            timeout=timeout,
            ignore_backoff=ignore_backoff,
        )

        if timeout:
            _sec_timeout = timeout / 1000
        else:
            _sec_timeout = self.default_timeout

        body = await _handle_json_response(
            self.reactor, _sec_timeout, request, response, start_ms,
        )
        return body

    async def get_json(
        self,
        destination,
        path,
        args=None,
        retry_on_dns_fail=True,
        timeout=None,
        ignore_backoff=False,
        try_trailing_slash_on_400=False,
    ):
        """ GETs some json from the given host homeserver and path

        Args:
            destination (str): The remote server to send the HTTP request
                to.

            path (str): The HTTP path.

            args (dict|None): A dictionary used to create query strings, defaults to
                None.

            timeout (int|None): number of milliseconds to wait for the response headers
                (including connecting to the server), *for each attempt*.
                self._default_timeout (60s) by default.

            ignore_backoff (bool): true to ignore the historical backoff data
                and try the request anyway.

            try_trailing_slash_on_400 (bool): True if on a 400 M_UNRECOGNIZED
                response we should try appending a trailing slash to the end of
                the request. Workaround for #3622 in Synapse <= v0.99.3.
        Returns:
            dict|list: Succeeds when we get a 2xx HTTP response. The
            result will be the decoded JSON body.

        Raises:
            HttpResponseException: If we get an HTTP response code >= 300
                (except 429).
            NotRetryingDestination: If we are not yet ready to retry this
                server.
            FederationDeniedError: If this destination  is not on our
                federation whitelist
            RequestSendFailed: If there were problems connecting to the
                remote, due to e.g. DNS failures, connection timeouts etc.
        """
        request = MatrixFederationRequest(
            method="GET", destination=destination, path=path, query=args
        )

        start_ms = self.clock.time_msec()

        response = await self._send_request_with_optional_trailing_slash(
            request,
            try_trailing_slash_on_400,
            backoff_on_404=False,
            ignore_backoff=ignore_backoff,
            retry_on_dns_fail=retry_on_dns_fail,
            timeout=timeout,
        )

        body = await _handle_json_response(
            self.reactor, self.default_timeout, request, response, start_ms
        )

        return body

    async def delete_json(
        self,
        destination,
        path,
        long_retries=False,
        timeout=None,
        ignore_backoff=False,
        args={},
    ):
        """Send a DELETE request to the remote expecting some json response

        Args:
            destination (str): The remote server to send the HTTP request
                to.
            path (str): The HTTP path.

            long_retries (bool): whether to use the long retry algorithm. See
                docs on _send_request for details.

            timeout (int|None): number of milliseconds to wait for the response headers
                (including connecting to the server), *for each attempt*.
                self._default_timeout (60s) by default.

            ignore_backoff (bool): true to ignore the historical backoff data and
                try the request anyway.

            args (dict): query params
        Returns:
            dict|list: Succeeds when we get a 2xx HTTP response. The
            result will be the decoded JSON body.

        Raises:
            HttpResponseException: If we get an HTTP response code >= 300
                (except 429).
            NotRetryingDestination: If we are not yet ready to retry this
                server.
            FederationDeniedError: If this destination  is not on our
                federation whitelist
            RequestSendFailed: If there were problems connecting to the
                remote, due to e.g. DNS failures, connection timeouts etc.
        """
        request = MatrixFederationRequest(
            method="DELETE", destination=destination, path=path, query=args
        )

        start_ms = self.clock.time_msec()

        response = await self._send_request(
            request,
            long_retries=long_retries,
            timeout=timeout,
            ignore_backoff=ignore_backoff,
        )

        body = await _handle_json_response(
            self.reactor, self.default_timeout, request, response, start_ms
        )
        return body

    async def get_file(
        self,
        destination,
        path,
        output_stream,
        args={},
        retry_on_dns_fail=True,
        max_size=None,
        ignore_backoff=False,
    ):
        """GETs a file from a given homeserver
        Args:
            destination (str): The remote server to send the HTTP request to.
            path (str): The HTTP path to GET.
            output_stream (file): File to write the response body to.
            args (dict): Optional dictionary used to create the query string.
            ignore_backoff (bool): true to ignore the historical backoff data
                and try the request anyway.

        Returns:
            tuple[int, dict]: Resolves with an (int,dict) tuple of
            the file length and a dict of the response headers.

        Raises:
            HttpResponseException: If we get an HTTP response code >= 300
                (except 429).
            NotRetryingDestination: If we are not yet ready to retry this
                server.
            FederationDeniedError: If this destination  is not on our
                federation whitelist
            RequestSendFailed: If there were problems connecting to the
                remote, due to e.g. DNS failures, connection timeouts etc.
        """
        request = MatrixFederationRequest(
            method="GET", destination=destination, path=path, query=args
        )

        response = await self._send_request(
            request, retry_on_dns_fail=retry_on_dns_fail, ignore_backoff=ignore_backoff
        )

        headers = dict(response.headers.getAllRawHeaders())

        try:
            d = _readBodyToFile(response, output_stream, max_size)
            d.addTimeout(self.default_timeout, self.reactor)
            length = await make_deferred_yieldable(d)
        except Exception as e:
            logger.warning(
                "{%s} [%s] Error reading response: %s",
                request.txn_id,
                request.destination,
                e,
            )
            raise
        logger.info(
            "{%s} [%s] Completed: %d %s [%d bytes] %s %s",
            request.txn_id,
            request.destination,
            response.code,
            response.phrase.decode("ascii", errors="replace"),
            length,
            request.method,
            request.uri.decode("ascii"),
        )
        return (length, headers)


class _ReadBodyToFileProtocol(protocol.Protocol):
    def __init__(self, stream, deferred, max_size):
        self.stream = stream
        self.deferred = deferred
        self.length = 0
        self.max_size = max_size

    def dataReceived(self, data):
        self.stream.write(data)
        self.length += len(data)
        if self.max_size is not None and self.length >= self.max_size:
            self.deferred.errback(
                SynapseError(
                    502,
                    "Requested file is too large > %r bytes" % (self.max_size,),
                    Codes.TOO_LARGE,
                )
            )
            self.deferred = defer.Deferred()
            self.transport.loseConnection()

    def connectionLost(self, reason):
        if reason.check(ResponseDone):
            self.deferred.callback(self.length)
        else:
            self.deferred.errback(reason)


def _readBodyToFile(response, stream, max_size):
    d = defer.Deferred()
    response.deliverBody(_ReadBodyToFileProtocol(stream, d, max_size))
    return d


def _flatten_response_never_received(e):
    if hasattr(e, "reasons"):
        reasons = ", ".join(
            _flatten_response_never_received(f.value) for f in e.reasons
        )

        return "%s:[%s]" % (type(e).__name__, reasons)
    else:
        return repr(e)


def check_content_type_is_json(headers):
    """
    Check that a set of HTTP headers have a Content-Type header, and that it
    is application/json.

    Args:
        headers (twisted.web.http_headers.Headers): headers to check

    Raises:
        RequestSendFailed: if the Content-Type header is missing or isn't JSON

    """
    c_type = headers.getRawHeaders(b"Content-Type")
    if c_type is None:
        raise RequestSendFailed(RuntimeError("No Content-Type header"), can_retry=False)

    c_type = c_type[0].decode("ascii")  # only the first header
    val, options = cgi.parse_header(c_type)
    if val != "application/json":
        raise RequestSendFailed(
            RuntimeError("Content-Type not application/json: was '%s'" % c_type),
            can_retry=False,
        )


def encode_query_args(args):
    if args is None:
        return b""

    encoded_args = {}
    for k, vs in args.items():
        if isinstance(vs, str):
            vs = [vs]
        encoded_args[k] = [v.encode("UTF-8") for v in vs]

    query_bytes = urllib.parse.urlencode(encoded_args, True)

    return query_bytes.encode("utf8")