WIP: Ensure create event appears in a valid section of the v2 response

[kegsay]

This fails occasionally locally on postgres/sqlite no worker mode with: (also seen in the wild on matrix.org)

    federation_room_join_test.go:682: m.room.create event is in an invalid position in the response => {
            "device_lists": {
                "changed": [
                    "@user-2:hs2",
                    "@user-1:hs1"
                ]
            },
            "device_one_time_keys_count": {
                "signed_curve25519": 0
            },
            "device_unused_fallback_key_types": [],
            "next_batch": "s5_3_0_1_1_1_1_2_0_2",
            "org.matrix.msc2732.device_unused_fallback_key_types": [],
            "presence": {
                "events": [
                    {
                        "content": {
                            "currently_active": true,
                            "last_active_ago": 217,
                            "presence": "online"
                        },
                        "sender": "@user-1:hs1",
                        "type": "m.presence"
                    }
                ]
            },
            "rooms": {
                "join": {
                    "!BPwVMIJhHZgejuqiXE:hs1": {
                        "account_data": {
                            "events": []
                        },
                        "ephemeral": {
                            "events": []
                        },
                        "state": {
                            "events": [
                                {
                                    "content": {
                                        "guest_access": "can_join"
                                    },
                                    "event_id": "$7korYoqgIvIiZjDx7-Ve4ZzIlZ3v9zX7uDOIqsbH49I",
                                    "origin_server_ts": 1699379145170,
                                    "sender": "@user-1:hs1",
                                    "state_key": "",
                                    "type": "m.room.guest_access",
                                    "unsigned": {
                                        "age": 816
                                    }
                                },
                                {
                                    "content": {
                                        "history_visibility": "shared"
                                    },
                                    "event_id": "$6HQc2YUT8xvuKN4ktFOyfBM1QuXWU-r1hxEU0w5FU4c",
                                    "origin_server_ts": 1699379145169,
                                    "sender": "@user-1:hs1",
                                    "state_key": "",
                                    "type": "m.room.history_visibility",
                                    "unsigned": {
                                        "age": 817
                                    }
                                },
                                {
                                    "content": {
                                        "join_rule": "invite"
                                    },
                                    "event_id": "$2t6w40Wn8xJSgU0BHrbmC7Deq9y0KlKF-hPXHhqsuc4",
                                    "origin_server_ts": 1699379145169,
                                    "sender": "@user-1:hs1",
                                    "state_key": "",
                                    "type": "m.room.join_rules",
                                    "unsigned": {
                                        "age": 817
                                    }
                                },
                                {
                                    "content": {
                                        "ban": 50,
                                        "events": {
                                            "m.room.avatar": 50,
                                            "m.room.canonical_alias": 50,
                                            "m.room.encryption": 100,
                                            "m.room.history_visibility": 100,
                                            "m.room.name": 50,
                                            "m.room.power_levels": 100,
                                            "m.room.server_acl": 100,
                                            "m.room.tombstone": 100
                                        },
                                        "events_default": 0,
                                        "historical": 100,
                                        "invite": 0,
                                        "kick": 50,
                                        "redact": 50,
                                        "state_default": 50,
                                        "users": {
                                            "@user-1:hs1": 100
                                        },
                                        "users_default": 0
                                    },
                                    "event_id": "$LEsPRN6gtF82r_qsR4RoLcuGy9R-4D7dIiqDXN7UgEQ",
                                    "origin_server_ts": 1699379145165,
                                    "sender": "@user-1:hs1",
                                    "state_key": "",
                                    "type": "m.room.power_levels",
                                    "unsigned": {
                                        "age": 821
                                    }
                                }
                            ]
                        },
                        "summary": {},
                        "timeline": {
                            "events": [
                                {
                                    "content": {
                                        "creator": "@user-1:hs1",
                                        "room_version": "10"
                                    },
                                    "event_id": "$OBt3NDgimNbtITAKAOrRYEhSY5uwpykCFNS6tBorVy4",
                                    "origin_server_ts": 1699379145110,
                                    "sender": "@user-1:hs1",
                                    "state_key": "",
                                    "type": "m.room.create",
                                    "unsigned": {
                                        "age": 876
                                    }
                                },
                                {
                                    "content": {
                                        "displayname": "user-1",
                                        "membership": "join"
                                    },
                                    "event_id": "$is1f6hIu79vI5Cs9OEtL6dvJikPu7fTTJXLumNDqCLM",
                                    "origin_server_ts": 1699379145145,
                                    "sender": "@user-1:hs1",
                                    "state_key": "@user-1:hs1",
                                    "type": "m.room.member",
                                    "unsigned": {
                                        "age": 841
                                    }
                                },
                                {
                                    "content": {
                                        "avatar_url": null,
                                        "displayname": "user-2",
                                        "membership": "join"
                                    },
                                    "event_id": "$BOoylaWCMbAw1SYkL2TC0LuSQ7EVkni_1Pe11GBuI5I",
                                    "origin_server_ts": 1699379145954,
                                    "sender": "@user-2:hs2",
                                    "state_key": "@user-2:hs2",
                                    "type": "m.room.member",
                                    "unsigned": {
                                        "age": 2,
                                        "prev_content": {
                                            "displayname": "user-2",
                                            "membership": "invite"
                                        },
                                        "prev_sender": "@user-1:hs1",
                                        "replaces_state": "$o16n7D2VKzt6TVjJtcekseD5wub5Ijdw2J3zpdj4f_4"
                                    }
                                }
                            ],
                            "limited": true,
                            "prev_batch": "s5_3_0_1_1_1_1_2_0_2"
                        },
                        "unread_notifications": {
                            "highlight_count": 0,
                            "notification_count": 0
                        }
                    }
                }
            }
        }

[DMRobertson]

To spell this out, the spec says that state is

Updates to the state, between the time indicated by the since parameter, and the start of the timeline (or all state up to the start of the timeline, if since is not given, or full_state is true).

This means that the state block is a subset of the room state before the first timeline entry.

On m.room.create:

This is the first event in a room and cannot be changed. It acts as the root of all other events.

This means that the state before the create event is empty.

The invariant to enforce is thus

If m.room.create appears in a timeline block, it must be the first entry in the timeline and there must be no state block.

[DMRobertson]

... so I agree with https://github.com/matrix-org/complement/pull/690/files#diff-1d4586d8fb593a2139a64386f33a3ad90c45f93281f98cc34cf0217e38522cb2R667-R674

[clokep]

Clearing review flags since this is a draft? Shout if that was the wrong thing to do and you wanted some sort of initial review.