# Buggy code
async def send_request(destination) -> Dict[str, Any]:
    content = await self._do_send_join(destination, pdu)

    logger.debug("Got content: %s", content)

    state = [
        event_from_pdu_json(p, room_version, outlier=True)
        for p in content.get("state", [])
    ]

    auth_chain = [
        event_from_pdu_json(p, room_version, outlier=True)
        for p in content.get("auth_chain", [])
    ]

    pdus = {p.event_id: p for p in itertools.chain(state, auth_chain)}

    create_event = None
    for e in state:
        if (e.type, e.state_key) == (EventTypes.Create, ""):
            create_event = e
            break

    if create_event is None:
        # If the state doesn't have a create event then the room is
        # invalid, and it would fail auth checks anyway.
        raise SynapseError(400, "No create event in state")

    # the room version should be sane.
    create_room_version = create_event.content.get(
        "room_version", RoomVersions.V1.identifier
    )
    if create_room_version != room_version.identifier:
        # either the server that fulfilled the make_join, or the server that is
        # handling the send_join, is lying.
        raise InvalidResponseError(
            "Unexpected room version %s in create event" % (create_room_version,)
        )

    valid_pdus = await self._check_sigs_and_hash_and_fetch(
        destination,
        list(pdus.values()),
        outlier=True,
        room_version=room_version.identifier,
    )

    valid_pdus_map = {p.event_id: p for p in valid_pdus}

    # NB: We *need* to copy to ensure that we don't have multiple
    # references being passed on, as that causes... issues.
    signed_state = [
        copy.copy(valid_pdus_map[p.event_id])
        for p in state
        if p.event_id in valid_pdus_map
    ]

    signed_auth = [
        valid_pdus_map[p.event_id] for p in auth_chain if p.event_id in valid_pdus_map
    ]

    # NB: We *need* to copy to ensure that we don't have multiple
    # references being passed on, as that causes... issues.
    for s in signed_state:
        s.internal_metadata = copy.deepcopy(s.internal_metadata)

    # double-check that the same create event has ended up in the auth chain
    auth_chain_create_events = [
        e.event_id
        for e in signed_auth
        if (e.type, e.state_key) == (EventTypes.Create, "")
    ]
    if auth_chain_create_events != [create_event.event_id]:
        raise InvalidResponseError(
            "Unexpected create event(s) in auth chain" % (auth_chain_create_events,)
        )

    return {
        "state": signed_state,
        "auth_chain": signed_auth,
        "origin": destination,
    }


# Traceback (AttributeError)
2020-02-24 14:00:10,088 - synapse.federation.federation_client - 421 - WARNING - POST-1957- Failed to send_join via matrix.org
...
Traceback (most recent call last):
File "/home/synapse/matrixtest/synapse/federation/federation_client.py", line 402, in _try_destination_list
res = await callback(destination)
File "/home/synapse/matrixtest/synapse/federation/federation_client.py", line 589, in send_request
valid_pdus_map = {p.event_id: p for p in valid_pdus}
File "/home/synapse/matrixtest/synapse/federation/federation_client.py", line 589, in <dictcomp>
valid_pdus_map = {p.event_id: p for p in valid_pdus}
AttributeError: 'coroutine' object has no attribute 'event_id'
2020-02-24 14:00:10,154 - synapse.http.server - 81 - INFO - POST-1957- <SynapseRequest at 0x7ff1bf3efd30 method='POST' uri='/_matrix/client/r0/join/%23synapse%3Amatrix.org' clientproto='HTTP/1.1' site=8447> SynapseError: 502 - Failed to send_join via any server

# Fixed code
async def send_request(destination) -> Dict[str, Any]:
    content = await self._do_send_join(destination, pdu)

    logger.debug("Got content: %s", content)

    state = [
        event_from_pdu_json(p, room_version, outlier=True)
        for p in content.get("state", [])
    ]

    auth_chain = [
        event_from_pdu_json(p, room_version, outlier=True)
        for p in content.get("auth_chain", [])
    ]

    pdus = {p.event_id: p for p in itertools.chain(state, auth_chain)}

    create_event = None
    for e in state:
        if (e.type, e.state_key) == (EventTypes.Create, ""):
            create_event = e
            break

    if create_event is None:
        # If the state doesn't have a create event then the room is
        # invalid, and it would fail auth checks anyway.
        raise SynapseError(400, "No create event in state")

    # the room version should be sane.
    create_room_version = create_event.content.get(
        "room_version", RoomVersions.V1.identifier
    )
    if create_room_version != room_version.identifier:
        # either the server that fulfilled the make_join, or the server that is
        # handling the send_join, is lying.
        raise InvalidResponseError(
            "Unexpected room version %s in create event" % (create_room_version,)
        )

    valid_pdus = await self._check_sigs_and_hash_and_fetch(
        destination,
        list(pdus.values()),
        outlier=True,
        room_version=room_version.identifier,
    )

    valid_pdus_map = {p.event_id: p for p in valid_pdus}

    # NB: We *need* to copy to ensure that we don't have multiple
    # references being passed on, as that causes... issues.
    signed_state = [
        copy.copy(valid_pdus_map[p.event_id])
        for p in state
        if p.event_id in valid_pdus_map
    ]

    signed_auth = [
        valid_pdus_map[p.event_id] for p in auth_chain if p.event_id in valid_pdus_map
    ]

    # NB: We *need* to copy to ensure that we don't have multiple
    # references being passed on, as that causes... issues.
    for s in signed_state:
        s.internal_metadata = copy.deepcopy(s.internal_metadata)

    # double-check that the same create event has ended up in the auth chain
    auth_chain_create_events = [
        e.event_id
        for e in signed_auth
        if (e.type, e.state_key) == (EventTypes.Create, "")
    ]
    if auth_chain_create_events != [create_event.event_id]:
        raise InvalidResponseError(
            "Unexpected create event(s) in auth chain: %s" % (auth_chain_create_events,)
        )

    return {
        "state": signed_state,
        "auth_chain": signed_auth,
        "origin": destination,
    }