m.room.create events are not sent in response timelines to clients in an initial sliding sync

[DMRobertson]

I was surprised by this, but it may not be a bug given that clients can explicitly request the m.room.create state.

Integration test to reproduce:

func TestRoomCreationTimelineSeenInSync(t *testing.T) {
	pqString := testutils.PrepareDBConnectionString()
	v2 := runTestV2Server(t)
	defer v2.close()
	v3 := runTestServer(t, v2, pqString)
	defer v3.close()

	v2.addAccount(t, alice, aliceToken)
	timeline := createRoomState(t, alice, time.Now())
	v2.queueResponse(aliceToken, sync2.SyncResponse{
		NextBatch: "dummy",
		Rooms: sync2.SyncRoomsResponse{
			Join: map[string]sync2.SyncV2JoinResponse{
				"!unimportant": {
					Timeline: sync2.TimelineResponse{
						Events:    timeline,
						PrevBatch: "prevBatch",
					},
				},
			}},
	})

	res := v3.mustDoV3Request(t, aliceToken, sync3.Request{
		RoomSubscriptions: map[string]sync3.RoomSubscription{
			"!unimportant": {
				TimelineLimit: 20,
			},
		},
	})
	m.MatchResponse(t, res, m.MatchRoomSubscription("!unimportant", m.MatchRoomTimeline(timeline)))
}

which fails with

  poller_test.go:1014: MatchResponse: MatchRoomSubscription[!unimportant]: timeline length mismatch: got 3 want 4
        {
            "lists": {},
            "rooms": {
                "!unimportant": {
                    "name": "Empty Room",
                    "avatar": null,
                    "timeline": [
                        {
                            "type": "m.room.member",
                            "state_key": "@alice:localhost",
                            "sender": "@alice:localhost",
                            "content": {
                                "membership": "join"
                            },
                            "event_id": "$event_2_TestRoomCreationTimelineSeenInSync",
                            "origin_server_ts": 1697631424107
                        },
                        {
                            "type": "m.room.power_levels",
                            "state_key": "",
                            "sender": "@alice:localhost",
                            "content": {
                                "ban": 50,
                                "invite": 50,
                                "kick": 50,
                                "redact": 50,
                                "users": {
                                    "@alice:localhost": 100
                                },
                                "users_default": 0,
                                "events": null,
                                "events_default": 0,
                                "state_default": 50,
                                "notifications": {
                                    "room": 50
                                }
                            },
                            "event_id": "$event_3_TestRoomCreationTimelineSeenInSync",
                            "origin_server_ts": 1697631424107
                        },
                        {
                            "type": "m.room.join_rules",
                            "state_key": "",
                            "sender": "@alice:localhost",
                            "content": {
                                "join_rule": "public"
                            },
                            "event_id": "$event_4_TestRoomCreationTimelineSeenInSync",
                            "origin_server_ts": 1697631424107
                        }
                    ],
                    "notification_count": 0,
                    "highlight_count": 0,
                    "initial": true,
                    "joined_count": 1,
                    "invited_count": 0,
                    "timestamp": 1697631424107
                }
            },
            "extensions": {},
            "pos": "1"
        }

I think this is because we load timelines here

sliding-sync/sync3/handler/connstate.go

Line 567 in ae9777d

timelines := s.userCache.LazyLoadTimelines(ctx, s.anchorLoadPosition, roomIDs, int(roomSub.TimelineLimit))

where anchorloadPos is 0. That calls

sliding-sync/sync3/caches/user.go

Line 318 in db7e94d

roomIDToLatestEvents, err := c.store.LatestEventsInRooms(c.UserID, roomIDs, loadPos, maxTimelineEvents)

with loadPos 0, maxTimelineEvents 10. Next we call

sliding-sync/state/storage.go

Line 698 in c3dfd66

roomIDToRanges, err := s.visibleEventNIDsBetweenForRooms(userID, roomIDs, 0, to)

with to = 10. The next level is

sliding-sync/state/storage.go

Line 772 in c3dfd66

return s.visibleEventNIDsWithData(joinTimingsAtFromByRoomID, membershipEvents, userID, from, to)

with from = 0, to=10. This ultimately returns a range of (2, 10), because our join event has NID 2. But the create event has NID 1, and so is not loaded from the database.