path: root/synapse/events/utils.py

#
# This file is licensed under the Affero General Public License (AGPL) version 3.
#
# Copyright 2021 The Matrix.org Foundation C.I.C.
# Copyright 2014-2016 OpenMarket Ltd
# Copyright (C) 2023 New Vector, Ltd
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero General Public License as
# published by the Free Software Foundation, either version 3 of the
# License, or (at your option) any later version.
#
# See the GNU Affero General Public License for more details:
# <https://www.gnu.org/licenses/agpl-3.0.html>.
#
# Originally licensed under the Apache License, Version 2.0:
# <http://www.apache.org/licenses/LICENSE-2.0>.
#
# [This file includes modifications made by New Vector Limited]
#
#
import collections.abc
import re
from typing import (
    TYPE_CHECKING,
    Any,
    Awaitable,
    Callable,
    Dict,
    Iterable,
    List,
    Mapping,
    Match,
    MutableMapping,
    Optional,
    Union,
)

import attr
from canonicaljson import encode_canonical_json

from synapse.api.constants import (
    MAX_PDU_SIZE,
    EventContentFields,
    EventTypes,
    RelationTypes,
)
from synapse.api.errors import Codes, SynapseError
from synapse.api.room_versions import RoomVersion
from synapse.types import JsonDict, Requester

from . import EventBase, make_event_from_dict

if TYPE_CHECKING:
    from synapse.handlers.relations import BundledAggregations
    from synapse.server import HomeServer


# Split strings on "." but not "\." (or "\\\.").
SPLIT_FIELD_REGEX = re.compile(r"\\*\.")
# Find escaped characters, e.g. those with a \ in front of them.
ESCAPE_SEQUENCE_PATTERN = re.compile(r"\\(.)")

CANONICALJSON_MAX_INT = (2**53) - 1
CANONICALJSON_MIN_INT = -CANONICALJSON_MAX_INT


# Module API callback that allows adding fields to the unsigned section of
# events that are sent to clients.
ADD_EXTRA_FIELDS_TO_UNSIGNED_CLIENT_EVENT_CALLBACK = Callable[
    [EventBase], Awaitable[JsonDict]
]


def prune_event(event: EventBase) -> EventBase:
    """Returns a pruned version of the given event, which removes all keys we
    don't know about or think could potentially be dodgy.

    This is used when we "redact" an event. We want to remove all fields that
    the user has specified, but we do want to keep necessary information like
    type, state_key etc.
    """
    pruned_event_dict = prune_event_dict(event.room_version, event.get_dict())

    pruned_event = make_event_from_dict(
        pruned_event_dict, event.room_version, event.internal_metadata.get_dict()
    )

    # Copy the bits of `internal_metadata` that aren't returned by `get_dict`
    pruned_event.internal_metadata.stream_ordering = (
        event.internal_metadata.stream_ordering
    )
    pruned_event.internal_metadata.instance_name = event.internal_metadata.instance_name
    pruned_event.internal_metadata.outlier = event.internal_metadata.outlier

    # Mark the event as redacted
    pruned_event.internal_metadata.redacted = True

    return pruned_event


def clone_event(event: EventBase) -> EventBase:
    """Take a copy of the event.

    This is mostly useful because it does a *shallow* copy of the `unsigned` data,
    which means it can then be updated without corrupting the in-memory cache. Note that
    other properties of the event, such as `content`, are *not* (currently) copied here.
    """
    # XXX: We rely on at least one of `event.get_dict()` and `make_event_from_dict()`
    #   making a copy of `unsigned`. Currently, both do, though I don't really know why.
    #   Still, as long as they do, there's not much point doing yet another copy here.
    new_event = make_event_from_dict(
        event.get_dict(), event.room_version, event.internal_metadata.get_dict()
    )

    # Copy the bits of `internal_metadata` that aren't returned by `get_dict`.
    new_event.internal_metadata.stream_ordering = (
        event.internal_metadata.stream_ordering
    )
    new_event.internal_metadata.instance_name = event.internal_metadata.instance_name
    new_event.internal_metadata.outlier = event.internal_metadata.outlier

    return new_event


def prune_event_dict(room_version: RoomVersion, event_dict: JsonDict) -> JsonDict:
    """Redacts the event_dict in the same way as `prune_event`, except it
    operates on dicts rather than event objects

    Returns:
        A copy of the pruned event dict
    """

    allowed_keys = [
        "event_id",
        "sender",
        "room_id",
        "hashes",
        "signatures",
        "content",
        "type",
        "state_key",
        "depth",
        "prev_events",
        "auth_events",
        "origin_server_ts",
    ]

    # Earlier room versions from had additional allowed keys.
    if not room_version.updated_redaction_rules:
        allowed_keys.extend(["prev_state", "membership", "origin"])

    event_type = event_dict["type"]

    new_content = {}

    def add_fields(*fields: str) -> None:
        for field in fields:
            if field in event_dict["content"]:
                new_content[field] = event_dict["content"][field]

    if event_type == EventTypes.Member:
        add_fields("membership")
        if room_version.restricted_join_rule_fix:
            add_fields(EventContentFields.AUTHORISING_USER)
        if room_version.updated_redaction_rules:
            # Preserve the signed field under third_party_invite.
            third_party_invite = event_dict["content"].get("third_party_invite")
            if isinstance(third_party_invite, collections.abc.Mapping):
                new_content["third_party_invite"] = {}
                if "signed" in third_party_invite:
                    new_content["third_party_invite"]["signed"] = third_party_invite[
                        "signed"
                    ]

    elif event_type == EventTypes.Create:
        if room_version.updated_redaction_rules:
            # MSC2176 rules state that create events cannot have their `content` redacted.
            new_content = event_dict["content"]
        elif not room_version.implicit_room_creator:
            # Some room versions give meaning to `creator`
            add_fields("creator")

    elif event_type == EventTypes.JoinRules:
        add_fields("join_rule")
        if room_version.restricted_join_rule:
            add_fields("allow")
    elif event_type == EventTypes.PowerLevels:
        add_fields(
            "users",
            "users_default",
            "events",
            "events_default",
            "state_default",
            "ban",
            "kick",
            "redact",
        )

        if room_version.updated_redaction_rules:
            add_fields("invite")

    elif event_type == EventTypes.Aliases and room_version.special_case_aliases_auth:
        add_fields("aliases")
    elif event_type == EventTypes.RoomHistoryVisibility:
        add_fields("history_visibility")
    elif event_type == EventTypes.Redaction and room_version.updated_redaction_rules:
        add_fields("redacts")

    # Protect the rel_type and event_id fields under the m.relates_to field.
    if room_version.msc3389_relation_redactions:
        relates_to = event_dict["content"].get("m.relates_to")
        if isinstance(relates_to, collections.abc.Mapping):
            new_relates_to = {}
            for field in ("rel_type", "event_id"):
                if field in relates_to:
                    new_relates_to[field] = relates_to[field]
            # Only include a non-empty relates_to field.
            if new_relates_to:
                new_content["m.relates_to"] = new_relates_to

    allowed_fields = {k: v for k, v in event_dict.items() if k in allowed_keys}

    allowed_fields["content"] = new_content

    unsigned: JsonDict = {}
    allowed_fields["unsigned"] = unsigned

    event_unsigned = event_dict.get("unsigned", {})

    if "age_ts" in event_unsigned:
        unsigned["age_ts"] = event_unsigned["age_ts"]
    if "replaces_state" in event_unsigned:
        unsigned["replaces_state"] = event_unsigned["replaces_state"]

    return allowed_fields


def _copy_field(src: JsonDict, dst: JsonDict, field: List[str]) -> None:
    """Copy the field in 'src' to 'dst'.

    For example, if src={"foo":{"bar":5}} and dst={}, and field=["foo","bar"]
    then dst={"foo":{"bar":5}}.

    Args:
        src: The dict to read from.
        dst: The dict to modify.
        field: List of keys to drill down to in 'src'.
    """
    if len(field) == 0:  # this should be impossible
        return
    if len(field) == 1:  # common case e.g. 'origin_server_ts'
        if field[0] in src:
            dst[field[0]] = src[field[0]]
        return

    # Else is a nested field e.g. 'content.body'
    # Pop the last field as that's the key to move across and we need the
    # parent dict in order to access the data. Drill down to the right dict.
    key_to_move = field.pop(-1)
    sub_dict = src
    for sub_field in field:  # e.g. sub_field => "content"
        if sub_field in sub_dict and isinstance(
            sub_dict[sub_field], collections.abc.Mapping
        ):
            sub_dict = sub_dict[sub_field]
        else:
            return

    if key_to_move not in sub_dict:
        return

    # Insert the key into the output dictionary, creating nested objects
    # as required. We couldn't do this any earlier or else we'd need to delete
    # the empty objects if the key didn't exist.
    sub_out_dict = dst
    for sub_field in field:
        sub_out_dict = sub_out_dict.setdefault(sub_field, {})
    sub_out_dict[key_to_move] = sub_dict[key_to_move]


def _escape_slash(m: Match[str]) -> str:
    """
    Replacement function; replace a backslash-backslash or backslash-dot with the
    second character. Leaves any other string alone.
    """
    if m.group(1) in ("\\", "."):
        return m.group(1)
    return m.group(0)


def _split_field(field: str) -> List[str]:
    """
    Splits strings on unescaped dots and removes escaping.

    Args:
        field: A string representing a path to a field.

    Returns:
        A list of nested fields to traverse.
    """

    # Convert the field and remove escaping:
    #
    # 1. "content.body.thing\.with\.dots"
    # 2. ["content", "body", "thing\.with\.dots"]
    # 3. ["content", "body", "thing.with.dots"]

    # Find all dots (and their preceding backslashes). If the dot is unescaped
    # then emit a new field part.
    result = []
    prev_start = 0
    for match in SPLIT_FIELD_REGEX.finditer(field):
        # If the match is an *even* number of characters than the dot was escaped.
        if len(match.group()) % 2 == 0:
            continue

        # Add a new part (up to the dot, exclusive) after escaping.
        result.append(
            ESCAPE_SEQUENCE_PATTERN.sub(
                _escape_slash, field[prev_start : match.end() - 1]
            )
        )
        prev_start = match.end()

    # Add any part of the field after the last unescaped dot. (Note that if the
    # character is a dot this correctly adds a blank string.)
    result.append(re.sub(r"\\(.)", _escape_slash, field[prev_start:]))

    return result


def only_fields(dictionary: JsonDict, fields: List[str]) -> JsonDict:
    """Return a new dict with only the fields in 'dictionary' which are present
    in 'fields'.

    If there are no event fields specified then all fields are included.
    The entries may include '.' characters to indicate sub-fields.
    So ['content.body'] will include the 'body' field of the 'content' object.
    A literal '.' or '\' character in a field name may be escaped using a '\'.

    Args:
        dictionary: The dictionary to read from.
        fields: A list of fields to copy over. Only shallow refs are
        taken.
    Returns:
        A new dictionary with only the given fields. If fields was empty,
        the same dictionary is returned.
    """
    if len(fields) == 0:
        return dictionary

    # for each field, convert it:
    # ["content.body.thing\.with\.dots"] => [["content", "body", "thing\.with\.dots"]]
    split_fields = [_split_field(f) for f in fields]

    output: JsonDict = {}
    for field_array in split_fields:
        _copy_field(dictionary, output, field_array)
    return output


def format_event_raw(d: JsonDict) -> JsonDict:
    return d


def format_event_for_client_v1(d: JsonDict) -> JsonDict:
    d = format_event_for_client_v2(d)

    sender = d.get("sender")
    if sender is not None:
        d["user_id"] = sender

    copy_keys = (
        "age",
        "redacted_because",
        "replaces_state",
        "prev_content",
        "invite_room_state",
        "knock_room_state",
    )
    for key in copy_keys:
        if key in d["unsigned"]:
            d[key] = d["unsigned"][key]

    return d


def format_event_for_client_v2(d: JsonDict) -> JsonDict:
    drop_keys = (
        "auth_events",
        "prev_events",
        "hashes",
        "signatures",
        "depth",
        "origin",
        "prev_state",
    )
    for key in drop_keys:
        d.pop(key, None)
    return d


def format_event_for_client_v2_without_room_id(d: JsonDict) -> JsonDict:
    d = format_event_for_client_v2(d)
    d.pop("room_id", None)
    return d


@attr.s(slots=True, frozen=True, auto_attribs=True)
class SerializeEventConfig:
    as_client_event: bool = True
    # Function to convert from federation format to client format
    event_format: Callable[[JsonDict], JsonDict] = format_event_for_client_v1
    # The entity that requested the event. This is used to determine whether to include
    # the transaction_id in the unsigned section of the event.
    requester: Optional[Requester] = None
    # List of event fields to include. If empty, all fields will be returned.
    only_event_fields: Optional[List[str]] = None
    # Some events can have stripped room state stored in the `unsigned` field.
    # This is required for invite and knock functionality. If this option is
    # False, that state will be removed from the event before it is returned.
    # Otherwise, it will be kept.
    include_stripped_room_state: bool = False


_DEFAULT_SERIALIZE_EVENT_CONFIG = SerializeEventConfig()


def serialize_event(
    e: Union[JsonDict, EventBase],
    time_now_ms: int,
    *,
    config: SerializeEventConfig = _DEFAULT_SERIALIZE_EVENT_CONFIG,
) -> JsonDict:
    """Serialize event for clients

    Args:
        e
        time_now_ms
        config: Event serialization config

    Returns:
        The serialized event dictionary.
    """

    # FIXME(erikj): To handle the case of presence events and the like
    if not isinstance(e, EventBase):
        return e

    time_now_ms = int(time_now_ms)

    # Should this strip out None's?
    d = dict(e.get_dict().items())

    d["event_id"] = e.event_id

    if "age_ts" in d["unsigned"]:
        d["unsigned"]["age"] = time_now_ms - d["unsigned"]["age_ts"]
        del d["unsigned"]["age_ts"]

    if "redacted_because" in e.unsigned:
        d["unsigned"]["redacted_because"] = serialize_event(
            e.unsigned["redacted_because"],
            time_now_ms,
            config=config,
        )

    # If we have a txn_id saved in the internal_metadata, we should include it in the
    # unsigned section of the event if it was sent by the same session as the one
    # requesting the event.
    txn_id: Optional[str] = getattr(e.internal_metadata, "txn_id", None)
    if (
        txn_id is not None
        and config.requester is not None
        and config.requester.user.to_string() == e.sender
    ):
        # Some events do not have the device ID stored in the internal metadata,
        # this includes old events as well as those created by appservice, guests,
        # or with tokens minted with the admin API. For those events, fallback
        # to using the access token instead.
        event_device_id: Optional[str] = getattr(e.internal_metadata, "device_id", None)
        if event_device_id is not None:
            if event_device_id == config.requester.device_id:
                d["unsigned"]["transaction_id"] = txn_id

        else:
            # Fallback behaviour: only include the transaction ID if the event
            # was sent from the same access token.
            #
            # For regular users, the access token ID can be used to determine this.
            # This includes access tokens minted with the admin API.
            #
            # For guests and appservice users, we can't check the access token ID
            # so assume it is the same session.
            event_token_id: Optional[int] = getattr(
                e.internal_metadata, "token_id", None
            )
            if (
                (
                    event_token_id is not None
                    and config.requester.access_token_id is not None
                    and event_token_id == config.requester.access_token_id
                )
                or config.requester.is_guest
                or config.requester.app_service
            ):
                d["unsigned"]["transaction_id"] = txn_id

    # invite_room_state and knock_room_state are a list of stripped room state events
    # that are meant to provide metadata about a room to an invitee/knocker. They are
    # intended to only be included in specific circumstances, such as down sync, and
    # should not be included in any other case.
    if not config.include_stripped_room_state:
        d["unsigned"].pop("invite_room_state", None)
        d["unsigned"].pop("knock_room_state", None)

    if config.as_client_event:
        d = config.event_format(d)

    # If the event is a redaction, the field with the redacted event ID appears
    # in a different location depending on the room version. e.redacts handles
    # fetching from the proper location; copy it to the other location for forwards-
    # and backwards-compatibility with clients.
    if e.type == EventTypes.Redaction and e.redacts is not None:
        if e.room_version.updated_redaction_rules:
            d["redacts"] = e.redacts
        else:
            d["content"] = dict(d["content"])
            d["content"]["redacts"] = e.redacts

    only_event_fields = config.only_event_fields
    if only_event_fields:
        if not isinstance(only_event_fields, list) or not all(
            isinstance(f, str) for f in only_event_fields
        ):
            raise TypeError("only_event_fields must be a list of strings")
        d = only_fields(d, only_event_fields)

    return d


class EventClientSerializer:
    """Serializes events that are to be sent to clients.

    This is used for bundling extra information with any events to be sent to
    clients.
    """

    def __init__(self, hs: "HomeServer") -> None:
        self._store = hs.get_datastores().main
        self._add_extra_fields_to_unsigned_client_event_callbacks: List[
            ADD_EXTRA_FIELDS_TO_UNSIGNED_CLIENT_EVENT_CALLBACK
        ] = []

    async def serialize_event(
        self,
        event: Union[JsonDict, EventBase],
        time_now: int,
        *,
        config: SerializeEventConfig = _DEFAULT_SERIALIZE_EVENT_CONFIG,
        bundle_aggregations: Optional[Dict[str, "BundledAggregations"]] = None,
    ) -> JsonDict:
        """Serializes a single event.

        Args:
            event: The event being serialized.
            time_now: The current time in milliseconds
            config: Event serialization config
            bundle_aggregations: A map from event_id to the aggregations to be bundled
               into the event.

        Returns:
            The serialized event
        """
        # To handle the case of presence events and the like
        if not isinstance(event, EventBase):
            return event

        serialized_event = serialize_event(event, time_now, config=config)

        new_unsigned = {}
        for callback in self._add_extra_fields_to_unsigned_client_event_callbacks:
            u = await callback(event)
            new_unsigned.update(u)

        if new_unsigned:
            # We do the `update` this way round so that modules can't clobber
            # existing fields.
            new_unsigned.update(serialized_event["unsigned"])
            serialized_event["unsigned"] = new_unsigned

        # Check if there are any bundled aggregations to include with the event.
        if bundle_aggregations:
            if event.event_id in bundle_aggregations:
                await self._inject_bundled_aggregations(
                    event,
                    time_now,
                    config,
                    bundle_aggregations,
                    serialized_event,
                )

        return serialized_event

    async def _inject_bundled_aggregations(
        self,
        event: EventBase,
        time_now: int,
        config: SerializeEventConfig,
        bundled_aggregations: Dict[str, "BundledAggregations"],
        serialized_event: JsonDict,
    ) -> None:
        """Potentially injects bundled aggregations into the unsigned portion of the serialized event.

        Args:
            event: The event being serialized.
            time_now: The current time in milliseconds
            config: Event serialization config
            bundled_aggregations: Bundled aggregations to be injected.
                A map from event_id to aggregation data. Must contain at least an
                entry for `event`.

                While serializing the bundled aggregations this map may be searched
                again for additional events in a recursive manner.
            serialized_event: The serialized event which may be modified.
        """

        # We have already checked that aggregations exist for this event.
        event_aggregations = bundled_aggregations[event.event_id]

        # The JSON dictionary to be added under the unsigned property of the event
        # being serialized.
        serialized_aggregations = {}

        if event_aggregations.references:
            serialized_aggregations[RelationTypes.REFERENCE] = (
                event_aggregations.references
            )

        if event_aggregations.replace:
            # Include information about it in the relations dict.
            #
            # Matrix spec v1.5 (https://spec.matrix.org/v1.5/client-server-api/#server-side-aggregation-of-mreplace-relationships)
            # said that we should only include the `event_id`, `origin_server_ts` and
            # `sender` of the edit; however MSC3925 proposes extending it to the whole
            # of the edit, which is what we do here.
            serialized_aggregations[RelationTypes.REPLACE] = await self.serialize_event(
                event_aggregations.replace,
                time_now,
                config=config,
            )

        # Include any threaded replies to this event.
        if event_aggregations.thread:
            thread = event_aggregations.thread

            serialized_latest_event = await self.serialize_event(
                thread.latest_event,
                time_now,
                config=config,
                bundle_aggregations=bundled_aggregations,
            )

            thread_summary = {
                "latest_event": serialized_latest_event,
                "count": thread.count,
                "current_user_participated": thread.current_user_participated,
            }
            serialized_aggregations[RelationTypes.THREAD] = thread_summary

        # Include the bundled aggregations in the event.
        if serialized_aggregations:
            # There is likely already an "unsigned" field, but a filter might
            # have stripped it off (via the event_fields option). The server is
            # allowed to return additional fields, so add it back.
            serialized_event.setdefault("unsigned", {}).setdefault(
                "m.relations", {}
            ).update(serialized_aggregations)

    async def serialize_events(
        self,
        events: Iterable[Union[JsonDict, EventBase]],
        time_now: int,
        *,
        config: SerializeEventConfig = _DEFAULT_SERIALIZE_EVENT_CONFIG,
        bundle_aggregations: Optional[Dict[str, "BundledAggregations"]] = None,
    ) -> List[JsonDict]:
        """Serializes multiple events.

        Args:
            event
            time_now: The current time in milliseconds
            config: Event serialization config
            bundle_aggregations: Whether to include the bundled aggregations for this
                event. Only applies to non-state events. (State events never include
                bundled aggregations.)

        Returns:
            The list of serialized events
        """
        return [
            await self.serialize_event(
                event,
                time_now,
                config=config,
                bundle_aggregations=bundle_aggregations,
            )
            for event in events
        ]

    def register_add_extra_fields_to_unsigned_client_event_callback(
        self, callback: ADD_EXTRA_FIELDS_TO_UNSIGNED_CLIENT_EVENT_CALLBACK
    ) -> None:
        """Register a callback that returns additions to the unsigned section of
        serialized events.
        """
        self._add_extra_fields_to_unsigned_client_event_callbacks.append(callback)


_PowerLevel = Union[str, int]
PowerLevelsContent = Mapping[str, Union[_PowerLevel, Mapping[str, _PowerLevel]]]


def copy_and_fixup_power_levels_contents(
    old_power_levels: PowerLevelsContent,
) -> Dict[str, Union[int, Dict[str, int]]]:
    """Copy the content of a power_levels event, unfreezing immutabledicts along the way.

    We accept as input power level values which are strings, provided they represent an
    integer, e.g. `"`100"` instead of 100. Such strings are converted to integers
    in the returned dictionary (hence "fixup" in the function name).

    Note that future room versions will outlaw such stringy power levels (see
    https://github.com/matrix-org/matrix-spec/issues/853).

    Raises:
        TypeError if the input does not look like a valid power levels event content
    """
    if not isinstance(old_power_levels, collections.abc.Mapping):
        raise TypeError("Not a valid power-levels content: %r" % (old_power_levels,))

    power_levels: Dict[str, Union[int, Dict[str, int]]] = {}

    for k, v in old_power_levels.items():
        if isinstance(v, collections.abc.Mapping):
            h: Dict[str, int] = {}
            power_levels[k] = h
            for k1, v1 in v.items():
                _copy_power_level_value_as_integer(v1, h, k1)

        else:
            _copy_power_level_value_as_integer(v, power_levels, k)

    return power_levels


def _copy_power_level_value_as_integer(
    old_value: object,
    power_levels: MutableMapping[str, Any],
    key: str,
) -> None:
    """Set `power_levels[key]` to the integer represented by `old_value`.

    :raises TypeError: if `old_value` is neither an integer nor a base-10 string
        representation of an integer.
    """
    if type(old_value) is int:  # noqa: E721
        power_levels[key] = old_value
        return

    if isinstance(old_value, str):
        try:
            parsed_value = int(old_value, base=10)
        except ValueError:
            # Fall through to the final TypeError.
            pass
        else:
            power_levels[key] = parsed_value
            return

    raise TypeError(f"Invalid power_levels value for {key}: {old_value}")


def validate_canonicaljson(value: Any) -> None:
    """
    Ensure that the JSON object is valid according to the rules of canonical JSON.

    See the appendix section 3.1: Canonical JSON.

    This rejects JSON that has:
    * An integer outside the range of [-2 ^ 53 + 1, 2 ^ 53 - 1]
    * Floats
    * NaN, Infinity, -Infinity
    """
    if type(value) is int:  # noqa: E721
        if value < CANONICALJSON_MIN_INT or CANONICALJSON_MAX_INT < value:
            raise SynapseError(400, "JSON integer out of range", Codes.BAD_JSON)

    elif isinstance(value, float):
        # Note that Infinity, -Infinity, and NaN are also considered floats.
        raise SynapseError(400, "Bad JSON value: float", Codes.BAD_JSON)

    elif isinstance(value, collections.abc.Mapping):
        for v in value.values():
            validate_canonicaljson(v)

    elif isinstance(value, (list, tuple)):
        for i in value:
            validate_canonicaljson(i)

    elif not isinstance(value, (bool, str)) and value is not None:
        # Other potential JSON values (bool, None, str) are safe.
        raise SynapseError(400, "Unknown JSON value", Codes.BAD_JSON)


def maybe_upsert_event_field(
    event: EventBase, container: JsonDict, key: str, value: object
) -> bool:
    """Upsert an event field, but only if this doesn't make the event too large.

    Returns true iff the upsert took place.
    """
    if key in container:
        old_value: object = container[key]
        container[key] = value
        # NB: here and below, we assume that passing a non-None `time_now` argument to
        # get_pdu_json doesn't increase the size of the encoded result.
        upsert_okay = len(encode_canonical_json(event.get_pdu_json())) <= MAX_PDU_SIZE
        if not upsert_okay:
            container[key] = old_value
    else:
        container[key] = value
        upsert_okay = len(encode_canonical_json(event.get_pdu_json())) <= MAX_PDU_SIZE
        if not upsert_okay:
            del container[key]

    return upsert_okay


def strip_event(event: EventBase) -> JsonDict:
    """
    Used for "stripped state" events which provide a simplified view of the state of a
    room intended to help a potential joiner identify the room (relevant when the user
    is invited or knocked).

    Stripped state events can only have the `sender`, `type`, `state_key` and `content`
    properties present.
    """

    return {
        "type": event.type,
        "state_key": event.state_key,
        "content": event.content,
        "sender": event.sender,
    }