Protect against malformed initial join room state

[kegsay]

It is possible for Synapse to send back malformed sync v2 responses, which look like this:

{
    "join": {
        "!IcnzdHPPHhnQSIYddY:hs1": {
            "account_data": {
                "events": []
            },
            "ephemeral": {
                "events": []
            },
            "state": {
                "events": [
                    {
                        "content": {
                            "join_rule": "invite"
                        },
                        "event_id": "$-s3dp1zqApBugW6UOBWzGRFlRf1uvFKSqSeHVDl7I_w",
                        "origin_server_ts": 1699375767822,
                        "sender": "@user-1:hs1",
                        "state_key": "",
                        "type": "m.room.join_rules",
                        "unsigned": {
                            "age": 977
                        }
                    },
                    {
                        "content": {
                            "ban": 50,
                            "events": {
                                "m.room.avatar": 50,
                                "m.room.canonical_alias": 50,
                                "m.room.encryption": 100,
                                "m.room.history_visibility": 100,
                                "m.room.name": 50,
                                "m.room.power_levels": 100,
                                "m.room.server_acl": 100,
                                "m.room.tombstone": 100
                            },
                            "events_default": 0,
                            "historical": 100,
                            "invite": 0,
                            "kick": 50,
                            "redact": 50,
                            "state_default": 50,
                            "users": {
                                "@user-1:hs1": 100
                            },
                            "users_default": 0
                        },
                        "event_id": "$0rs4J6X4oQdER16GeFd5dVJnzuzZd7Gk3lO5n-ZidVk",
                        "origin_server_ts": 1699375767818,
                        "sender": "@user-1:hs1",
                        "state_key": "",
                        "type": "m.room.power_levels",
                        "unsigned": {
                            "age": 981
                        }
                    },
                    {
                        "content": {
                            "guest_access": "can_join"
                        },
                        "event_id": "$QumnUTkpI1r2HqBaBwVsR1M8RhnlRuyW6wHV5dh4Jek",
                        "origin_server_ts": 1699375767823,
                        "sender": "@user-1:hs1",
                        "state_key": "",
                        "type": "m.room.guest_access",
                        "unsigned": {
                            "age": 976
                        }
                    },
                    {
                        "content": {
                            "history_visibility": "shared"
                        },
                        "event_id": "$jtaiXEi4M_5z_gcPjSqmCTLr346jYtGcLriyPGsMINA",
                        "origin_server_ts": 1699375767822,
                        "sender": "@user-1:hs1",
                        "state_key": "",
                        "type": "m.room.history_visibility",
                        "unsigned": {
                            "age": 977
                        }
                    }
                ]
            },
            "summary": {},
            "timeline": {
                "events": [
                    {
                        "content": {
                            "creator": "@user-1:hs1",
                            "room_version": "10"
                        },
                        "event_id": "$ZVJ-2IpB4_Q_SekOHpfS5TYfXJsO09MOLjn7e9Mb0TU",
                        "origin_server_ts": 1699375767666,
                        "sender": "@user-1:hs1",
                        "state_key": "",
                        "type": "m.room.create",
                        "unsigned": {
                            "age": 1133
                        }
                    },
                    {
                        "content": {
                            "displayname": "user-1",
                            "membership": "join"
                        },
                        "event_id": "$wkt0vwF8jnp5-9PZWlcUJLqC_nrod_njGfd7J92TUB8",
                        "origin_server_ts": 1699375767774,
                        "sender": "@user-1:hs1",
                        "state_key": "@user-1:hs1",
                        "type": "m.room.member",
                        "unsigned": {
                            "age": 1025
                        }
                    },
                    {
                        "content": {
                            "avatar_url": null,
                            "displayname": "user-2",
                            "membership": "join"
                        },
                        "event_id": "$xXr5h3VwkV213gif8JEBzjWHKp6zzCl7H0dRsJKy7rY",
                        "origin_server_ts": 1699375768754,
                        "sender": "@user-2:hs2",
                        "state_key": "@user-2:hs2",
                        "type": "m.room.member",
                        "unsigned": {
                            "age": 8,
                            "prev_content": {
                                "displayname": "user-2",
                                "membership": "invite"
                            },
                            "prev_sender": "@user-1:hs1",
                            "replaces_state": "$k_kVHm4t3-ct_esCvXcuHDoY5PATx9SuppXkqeahU30"
                        }
                    }
                ],
                "limited": true,
                "prev_batch": "s5_3_0_1_1_1_1_2_0_2"
            },
            "unread_notifications": {
                "highlight_count": 0,
                "notification_count": 0
            }
        }
    }
}

This is malformed because the m.room.create event is objectively in the wrong place. It should be in state, not timeline. Unfortunately, this precise response causes a sequence of failures which ends up failing to process this room and the proxy not noticing it.

Currently the proxy will:

• Attempt to parse state.
• The Initialilse function fails to parse state due to lack of a create event. This will cause an internal.DataError to be returned as it is a problem with the data. This means "do not retry this sync request, skip over".
• By returning an error from Initialise, we never look at the timeline: https://github.com/matrix-org/sliding-sync/blob/main/sync2/poller.go#L795
• This means we never see the m.room.create event in the timeline, which could mean we actually process this room correctly.
• Due to the DataError being returned, we now increment since, permanently losing the room.

To fix this, the proxy should:

• Upon getting a DataError from Initialise, look for an m.room.create in the timeline.
• If it exists, move it to state, along with other state events bar the last event.
• Keep the last event in the timeline
• Retry Initialise.
• If that fails, give up. If it succeeds, log loudly that we have caught this.

[kegsay]

xref matrix-org/complement#690