anonymousland-synapse/synapse/http/matrixfederationclient.py

# Copyright 2014-2021 The Matrix.org Foundation C.I.C.
#
# 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 abc
import cgi
import codecs
import logging
import random
import sys
import typing
import urllib.parse
from io import BytesIO, StringIO
from typing import (
    Callable,
    Dict,
    Generic,
    List,
    Optional,
    Tuple,
    TypeVar,
    Union,
    overload,
)

import attr
import treq
from canonicaljson import encode_canonical_json
from prometheus_client import Counter
from signedjson.sign import sign_json
from typing_extensions import Literal

from twisted.internet import defer
from twisted.internet.error import DNSLookupError
from twisted.internet.interfaces import IReactorTime
from twisted.internet.task import _EPSILON, Cooperator
from twisted.web.client import ResponseFailed
from twisted.web.http_headers import Headers
from twisted.web.iweb import IBodyProducer, 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,
    BodyExceededMaxSize,
    ByteWriteable,
    encode_query_args,
    read_body_with_max_size,
)
from synapse.http.federation.matrix_federation_agent import MatrixFederationAgent
from synapse.logging import opentracing
from synapse.logging.context import make_deferred_yieldable
from synapse.logging.opentracing import set_tag, start_active_span, tags
from synapse.types import JsonDict
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"]
)

# a federation response can be rather large (eg a big state_ids is 50M or so), so we
# need a generous limit here.
MAX_RESPONSE_SIZE = 100 * 1024 * 1024

MAX_LONG_RETRIES = 10
MAX_SHORT_RETRIES = 3
MAXINT = sys.maxsize


_next_id = 1


QueryArgs = Dict[str, Union[str, List[str]]]


T = TypeVar("T")


class ByteParser(ByteWriteable, Generic[T], abc.ABC):
    """A `ByteWriteable` that has an additional `finish` function that returns
    the parsed data.
    """

    CONTENT_TYPE: str = abc.abstractproperty()  # type: ignore
    """The expected content type of the response, e.g. `application/json`. If
    the content type doesn't match we fail the request.
    """

    @abc.abstractmethod
    def finish(self) -> T:
        """Called when response has finished streaming and the parser should
        return the final result (or error).
        """
        pass


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

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

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

    json = attr.ib(default=None, type=Optional[JsonDict])
    """JSON to send in the body.
    """

    json_callback = attr.ib(default=None, type=Optional[Callable[[], JsonDict]])
    """A callback to generate the JSON.
    """

    query = attr.ib(default=None, type=Optional[dict])
    """Query arguments.
    """

    txn_id = attr.ib(default=None, type=Optional[str])
    """Unique ID for this request (for logging)
    """

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

    def __attrs_post_init__(self) -> None:
        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) -> Optional[JsonDict]:
        if self.json_callback:
            return self.json_callback()
        return self.json


class JsonParser(ByteParser[Union[JsonDict, list]]):
    """A parser that buffers the response and tries to parse it as JSON."""

    CONTENT_TYPE = "application/json"

    def __init__(self):
        self._buffer = StringIO()
        self._binary_wrapper = BinaryIOWrapper(self._buffer)

    def write(self, data: bytes) -> int:
        return self._binary_wrapper.write(data)

    def finish(self) -> Union[JsonDict, list]:
        return json_decoder.decode(self._buffer.getvalue())


async def _handle_response(
    reactor: IReactorTime,
    timeout_sec: float,
    request: MatrixFederationRequest,
    response: IResponse,
    start_ms: int,
    parser: ByteParser[T],
    max_response_size: Optional[int] = None,
) -> T:
    """
    Reads the body of a response with a timeout and sends it to a parser

    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
        parser: The parser for the response
        max_response_size: The maximum size to read from the response, if None
            uses the default.

    Returns:
        The parsed response
    """

    if max_response_size is None:
        max_response_size = MAX_RESPONSE_SIZE

    try:
        check_content_type_is(response.headers, parser.CONTENT_TYPE)

        d = read_body_with_max_size(response, parser, max_response_size)
        d = timeout_deferred(d, timeout=timeout_sec, reactor=reactor)

        length = await make_deferred_yieldable(d)

        value = parser.finish()
    except BodyExceededMaxSize as e:
        # The response was too big.
        logger.warning(
            "{%s} [%s] JSON response exceeded max size %i - %s %s",
            request.txn_id,
            request.destination,
            MAX_RESPONSE_SIZE,
            request.method,
            request.uri.decode("ascii"),
        )
        raise RequestSendFailed(e, can_retry=False) from e
    except ValueError as e:
        # The content was invalid.
        logger.warning(
            "{%s} [%s] Failed to parse response - %s %s",
            request.txn_id,
            request.destination,
            request.method,
            request.uri.decode("ascii"),
        )
        raise RequestSendFailed(e, can_retry=False) from e
    except defer.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 ResponseFailed as e:
        logger.warning(
            "{%s} [%s] Failed to read 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, got %d bytes - %s %s",
        request.txn_id,
        request.destination,
        response.code,
        response.phrase.decode("ascii", errors="replace"),
        time_taken_secs,
        length,
        request.method,
        request.uri.decode("ascii"),
    )
    return value


class BinaryIOWrapper:
    """A wrapper for a TextIO which converts from bytes on the fly."""

    def __init__(self, file: typing.TextIO, encoding="utf-8", errors="strict"):
        self.decoder = codecs.getincrementaldecoder(encoding)(errors)
        self.file = file

    def write(self, b: Union[bytes, bytearray]) -> int:
        self.file.write(self.decoder.decode(b))
        return len(b)


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

        self.reactor = hs.get_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")

        federation_agent = MatrixFederationAgent(
            self.reactor,
            tls_client_options_factory,
            user_agent,
            hs.config.federation_ip_range_whitelist,
            hs.config.federation_ip_range_blacklist,
        )

        # Use a BlacklistingAgentWrapper to prevent circumventing the IP
        # blacklist via IP literals in server names
        self.agent = BlacklistingAgentWrapper(
            federation_agent,
            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: MatrixFederationRequest,
        try_trailing_slash_on_400: bool = False,
        **send_request_args,
    ) -> IResponse:
        """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: details of request to be sent
            try_trailing_slash_on_400: 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: A dictionary of arguments to pass to `_send_request()`.

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

        Returns:
            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: MatrixFederationRequest,
        retry_on_dns_fail: bool = True,
        timeout: Optional[int] = None,
        long_retries: bool = False,
        ignore_backoff: bool = False,
        backoff_on_404: bool = False,
    ) -> IResponse:
        """
        Sends a request to the given server.

        Args:
            request: details of request to be sent

            retry_on_dns_fail: true if the request should be retied on DNS failures

            timeout: number of milliseconds to wait for the response headers
                (including connecting to the server), *for each attempt*.
                60s by default.

            long_retries: 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: true to ignore the historical backoff data
                and try the request anyway.

            backoff_on_404: Back off if we get a 404

        Returns:
            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: Dict[bytes, List[bytes]] = {}
        opentracing.inject_header_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: Optional[IBodyProducer] = 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 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

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

                        # Retry if the error is a 5xx or a 429 (Too Many
                        # Requests), otherwise just raise a standard
                        # `HttpResponseException`
                        if 500 <= response.code < 600 or response.code == 429:
                            raise RequestSendFailed(exc, can_retry=True) from exc
                        else:
                            raise exc

                    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: Optional[bytes],
        method: bytes,
        url_bytes: bytes,
        content: Optional[JsonDict] = None,
        destination_is: Optional[bytes] = None,
    ) -> List[bytes]:
        """
        Builds the Authorization headers for a federation request
        Args:
            destination: The destination homeserver of the request.
                May be None if the destination is an identity server, in which case
                destination_is must be non-None.
            method: The HTTP method of the request
            url_bytes: The URI path of the request
            content: The body of the request
            destination_is: As 'destination', but if the destination is an
                identity server

        Returns:
            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

    @overload
    async def put_json(
        self,
        destination: str,
        path: str,
        args: Optional[QueryArgs] = None,
        data: Optional[JsonDict] = None,
        json_data_callback: Optional[Callable[[], JsonDict]] = None,
        long_retries: bool = False,
        timeout: Optional[int] = None,
        ignore_backoff: bool = False,
        backoff_on_404: bool = False,
        try_trailing_slash_on_400: bool = False,
        parser: Literal[None] = None,
        max_response_size: Optional[int] = None,
    ) -> Union[JsonDict, list]:
        ...

    @overload
    async def put_json(
        self,
        destination: str,
        path: str,
        args: Optional[QueryArgs] = None,
        data: Optional[JsonDict] = None,
        json_data_callback: Optional[Callable[[], JsonDict]] = None,
        long_retries: bool = False,
        timeout: Optional[int] = None,
        ignore_backoff: bool = False,
        backoff_on_404: bool = False,
        try_trailing_slash_on_400: bool = False,
        parser: Optional[ByteParser[T]] = None,
        max_response_size: Optional[int] = None,
    ) -> T:
        ...

    async def put_json(
        self,
        destination: str,
        path: str,
        args: Optional[QueryArgs] = None,
        data: Optional[JsonDict] = None,
        json_data_callback: Optional[Callable[[], JsonDict]] = None,
        long_retries: bool = False,
        timeout: Optional[int] = None,
        ignore_backoff: bool = False,
        backoff_on_404: bool = False,
        try_trailing_slash_on_400: bool = False,
        parser: Optional[ByteParser] = None,
        max_response_size: Optional[int] = None,
    ):
        """Sends the specified json data using PUT

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

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

            timeout: number of milliseconds to wait for the response.
                self._default_timeout (60s) by default.

                Note that we may make several attempts to send the request; this
                timeout applies to the time spent waiting for response headers for
                *each* attempt (including connection time) as well as the time spent
                reading the response body after a 200 response.

            ignore_backoff: true to ignore the historical backoff data
                and try the request anyway.
            backoff_on_404: 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: 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.
            parser: The parser to use to decode the response. Defaults to
                parsing as JSON.
            max_response_size: The maximum size to read from the response, if None
                uses the default.

        Returns:
            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,
        )

        if timeout is not None:
            _sec_timeout = timeout / 1000
        else:
            _sec_timeout = self.default_timeout

        if parser is None:
            parser = JsonParser()

        body = await _handle_response(
            self.reactor,
            _sec_timeout,
            request,
            response,
            start_ms,
            parser=parser,
            max_response_size=max_response_size,
        )

        return body

    async def post_json(
        self,
        destination: str,
        path: str,
        data: Optional[JsonDict] = None,
        long_retries: bool = False,
        timeout: Optional[int] = None,
        ignore_backoff: bool = False,
        args: Optional[QueryArgs] = None,
    ) -> Union[JsonDict, list]:
        """Sends the specified json data using POST

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

            path: The HTTP path.

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

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

            timeout: number of milliseconds to wait for the response.
                self._default_timeout (60s) by default.

                Note that we may make several attempts to send the request; this
                timeout applies to the time spent waiting for response headers for
                *each* attempt (including connection time) as well as the time spent
                reading the response body after a 200 response.

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

            args: 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_response(
            self.reactor, _sec_timeout, request, response, start_ms, parser=JsonParser()
        )
        return body

    async def get_json(
        self,
        destination: str,
        path: str,
        args: Optional[QueryArgs] = None,
        retry_on_dns_fail: bool = True,
        timeout: Optional[int] = None,
        ignore_backoff: bool = False,
        try_trailing_slash_on_400: bool = False,
    ) -> Union[JsonDict, list]:
        """GETs some json from the given host homeserver and path

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

            path: The HTTP path.

            args: A dictionary used to create query strings, defaults to
                None.

            timeout: number of milliseconds to wait for the response.
                self._default_timeout (60s) by default.

                Note that we may make several attempts to send the request; this
                timeout applies to the time spent waiting for response headers for
                *each* attempt (including connection time) as well as the time spent
                reading the response body after a 200 response.

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

            try_trailing_slash_on_400: 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:
            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,
        )

        if timeout is not None:
            _sec_timeout = timeout / 1000
        else:
            _sec_timeout = self.default_timeout

        body = await _handle_response(
            self.reactor, _sec_timeout, request, response, start_ms, parser=JsonParser()
        )

        return body

    async def delete_json(
        self,
        destination: str,
        path: str,
        long_retries: bool = False,
        timeout: Optional[int] = None,
        ignore_backoff: bool = False,
        args: Optional[QueryArgs] = None,
    ) -> Union[JsonDict, list]:
        """Send a DELETE request to the remote expecting some json response

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

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

            timeout: number of milliseconds to wait for the response.
                self._default_timeout (60s) by default.

                Note that we may make several attempts to send the request; this
                timeout applies to the time spent waiting for response headers for
                *each* attempt (including connection time) as well as the time spent
                reading the response body after a 200 response.

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

            args: query params
        Returns:
            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,
        )

        if timeout is not None:
            _sec_timeout = timeout / 1000
        else:
            _sec_timeout = self.default_timeout

        body = await _handle_response(
            self.reactor, _sec_timeout, request, response, start_ms, parser=JsonParser()
        )
        return body

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

        Returns:
            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 = read_body_with_max_size(response, output_stream, max_size)
            d.addTimeout(self.default_timeout, self.reactor)
            length = await make_deferred_yieldable(d)
        except BodyExceededMaxSize:
            msg = "Requested file is too large > %r bytes" % (max_size,)
            logger.warning(
                "{%s} [%s] %s",
                request.txn_id,
                request.destination,
                msg,
            )
            raise SynapseError(502, msg, Codes.TOO_LARGE)
        except defer.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 ResponseFailed as e:
            logger.warning(
                "{%s} [%s] Failed to read 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",
                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)


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(headers: Headers, expected_content_type: str) -> None:
    """
    Check that a set of HTTP headers have a Content-Type header, and that it
    is the expected value..

    Args:
        headers: headers to check

    Raises:
        RequestSendFailed: if the Content-Type header is missing or doesn't match

    """
    content_type_headers = headers.getRawHeaders(b"Content-Type")
    if content_type_headers is None:
        raise RequestSendFailed(
            RuntimeError("No Content-Type header received from remote server"),
            can_retry=False,
        )

    c_type = content_type_headers[0].decode("ascii")  # only the first header
    val, options = cgi.parse_header(c_type)
    if val != expected_content_type:
        raise RequestSendFailed(
            RuntimeError(
                f"Remote server sent Content-Type header of '{c_type}', not '{expected_content_type}'",
            ),
            can_retry=False,
        )