Changing the required state but not the timeline limit re-sends the timeline

[DMRobertson]

In this situation I'd expect to see a new required_state blob, but only new entries in the timeline.

Test case, from #329:

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

	alice := "alice"
	aliceToken := "alicetoken"
	bob := "bob"
	roomID := "!unimportant"

	v2.addAccount(t, alice, aliceToken)

	t.Log("alice creates a public room.")
	timeline1 := createRoomState(t, alice, time.Now())
	v2.queueResponse(aliceToken, sync2.SyncResponse{
		Rooms: sync2.SyncRoomsResponse{
			Join: map[string]sync2.SyncV2JoinResponse{
				roomID: {
					Timeline: sync2.TimelineResponse{
						Events:    timeline1,
						PrevBatch: "alicePublicPrevBatch1",
					},
				},
			},
		},
		NextBatch: "aliceSync1",
	})

	t.Log("alice sliding syncs.")
	aliceRes := v3.mustDoV3Request(t, aliceToken, sync3.Request{
		RoomSubscriptions: map[string]sync3.RoomSubscription{
			roomID: {TimelineLimit: 20},
		},
	})

	t.Log("She sees herself joined to her room, with an appropriate timeline.")
	// Note: we only expect timeline1[1:] here, excluding the create event. See
	// https://github.com/matrix-org/sliding-sync/issues/343
	m.MatchResponse(t, aliceRes,
		m.LogResponse(t),
		m.MatchRoomSubscription(roomID, m.MatchRoomTimeline(timeline1[1:])),
	)

	t.Logf("Alice's poller gets a gappy sync response for the public room. bob's membership is now join, and alice has sent 10 messages.")
	timeline2 := make([]json.RawMessage, 10)
	for i := range timeline2 {
		timeline2[i] = testutils.NewMessageEvent(t, alice, fmt.Sprintf("hello %d", i))
	}

	newMembership := testutils.NewJoinEvent(t, bob)

	v2.queueResponse(aliceToken, sync2.SyncResponse{
		NextBatch: "alice2",
		Rooms: sync2.SyncRoomsResponse{
			Join: map[string]sync2.SyncV2JoinResponse{
				roomID: {
					State: sync2.EventsResponse{
						Events: []json.RawMessage{newMembership},
					},
					Timeline: sync2.TimelineResponse{
						Events:    timeline2,
						Limited:   true,
						PrevBatch: "alicePublicPrevBatch2",
					},
				},
			},
		},
	})
	v2.waitUntilEmpty(t, aliceToken)

	t.Log("alice syncs, requesting Bob's membership. She sees it, and her new timeline.")
	aliceRes = v3.mustDoV3RequestWithPos(t, aliceToken, aliceRes.Pos, sync3.Request{
		RoomSubscriptions: map[string]sync3.RoomSubscription{
			roomID: {
				TimelineLimit: 20,
				RequiredState: [][2]string{{"m.room.member", bob}},
			},
		},
	})
	m.MatchResponse(t, aliceRes, m.MatchRoomSubscription(roomID,
		m.MatchRoomTimeline(timeline2),
	))
}

Notes on how to fix:

• Move the timeline limit changed method onto room subscription for tidying up.
• Core problem: RoomDelta.Subs is a []string (roomIDs). Would need to make this a struct that describes how the subscription has changed.

Related: if I shrink the timeline limit, I would only expect to see new entries in the timeline. If I grow the timeline limit... not sure, maybe it's fine to resend the timeline in this situation.

[DMRobertson]

Recording for posterity, but we're not going to prioritise fixing this.