Add logic for determining when device lists have changed due to membership changes (#1220)

* Flesh out structure for handling device list updates for room membership changes

* First cut untested algorithm

* Add tests for determining changed/left device lists

* Linting

* Unbreak tests

* Sigh.. linting

internal/consumers.go

@@ -58,11 +58,17 @@ var ErrShutdown = fmt.Errorf("shutdown")
 // Returns nil once all the goroutines are started.
 // Returns an error if it can't start consuming for any of the partitions.
 func (c *ContinualConsumer) Start() error {
+	_, err := c.StartOffsets()
+	return err
+}
+
+// StartOffsets is the same as Start but returns the loaded offsets as well.
+func (c *ContinualConsumer) StartOffsets() ([]sqlutil.PartitionOffset, error) {
 	offsets := map[int32]int64{}
 
 	partitions, err := c.Consumer.Partitions(c.Topic)
 	if err != nil {
-		return err
+		return nil, err
 	}
 	for _, partition := range partitions {
 		// Default all the offsets to the beginning of the stream.
@@ -71,7 +77,7 @@ func (c *ContinualConsumer) Start() error {
 
 	storedOffsets, err := c.PartitionStore.PartitionOffsets(context.TODO(), c.Topic)
 	if err != nil {
-		return err
+		return nil, err
 	}
 	for _, offset := range storedOffsets {
 		// We've already processed events from this partition so advance the offset to where we got to.
@@ -87,7 +93,7 @@ func (c *ContinualConsumer) Start() error {
 			for _, p := range partitionConsumers {
 				p.Close() // nolint: errcheck
 			}
-			return err
+			return nil, err
 		}
 		partitionConsumers = append(partitionConsumers, pc)
 	}
@@ -95,7 +101,7 @@ func (c *ContinualConsumer) Start() error {
 		go c.consumePartition(pc)
 	}
 
-	return nil
+	return storedOffsets, nil
 }
 
 // consumePartition consumes the room events for a single partition of the kafkaesque stream.

syncapi/consumers/keychange.go

@@ -17,13 +17,14 @@ package consumers
 import (
 	"context"
 	"encoding/json"
+	"sync"
 
 	"github.com/Shopify/sarama"
 	currentstateAPI "github.com/matrix-org/dendrite/currentstateserver/api"
 	"github.com/matrix-org/dendrite/internal"
-	"github.com/matrix-org/dendrite/internal/config"
 	"github.com/matrix-org/dendrite/keyserver/api"
 	"github.com/matrix-org/dendrite/syncapi/storage"
+	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrixserverlib"
 	log "github.com/sirupsen/logrus"
 )
@@ -35,19 +36,22 @@ type OutputKeyChangeEventConsumer struct {
 	serverName        gomatrixserverlib.ServerName // our server name
 	currentStateAPI   currentstateAPI.CurrentStateInternalAPI
 	// keyAPI            api.KeyInternalAPI
+	partitionToOffset   map[int32]int64
+	partitionToOffsetMu sync.Mutex
 }
 
 // NewOutputKeyChangeEventConsumer creates a new OutputKeyChangeEventConsumer.
 // Call Start() to begin consuming from the key server.
 func NewOutputKeyChangeEventConsumer(
-	cfg *config.Dendrite,
+	serverName gomatrixserverlib.ServerName,
+	topic string,
 	kafkaConsumer sarama.Consumer,
 	currentStateAPI currentstateAPI.CurrentStateInternalAPI,
 	store storage.Database,
 ) *OutputKeyChangeEventConsumer {
 
 	consumer := internal.ContinualConsumer{
-		Topic:          string(cfg.Kafka.Topics.OutputKeyChangeEvent),
+		Topic:          topic,
 		Consumer:       kafkaConsumer,
 		PartitionStore: store,
 	}
@@ -55,8 +59,10 @@ func NewOutputKeyChangeEventConsumer(
 	s := &OutputKeyChangeEventConsumer{
 		keyChangeConsumer:   &consumer,
 		db:                  store,
-		serverName:        cfg.Matrix.ServerName,
+		serverName:          serverName,
 		currentStateAPI:     currentStateAPI,
+		partitionToOffset:   make(map[int32]int64),
+		partitionToOffsetMu: sync.Mutex{},
 	}
 
 	consumer.ProcessMessage = s.onMessage
@@ -66,10 +72,25 @@ func NewOutputKeyChangeEventConsumer(
 
 // Start consuming from the key server
 func (s *OutputKeyChangeEventConsumer) Start() error {
-	return s.keyChangeConsumer.Start()
+	offsets, err := s.keyChangeConsumer.StartOffsets()
+	s.partitionToOffsetMu.Lock()
+	for _, o := range offsets {
+		s.partitionToOffset[o.Partition] = o.Offset
+	}
+	s.partitionToOffsetMu.Unlock()
+	return err
+}
+
+func (s *OutputKeyChangeEventConsumer) updateOffset(msg *sarama.ConsumerMessage) {
+	s.partitionToOffsetMu.Lock()
+	defer s.partitionToOffsetMu.Unlock()
+	s.partitionToOffset[msg.Partition] = msg.Offset
 }
 
 func (s *OutputKeyChangeEventConsumer) onMessage(msg *sarama.ConsumerMessage) error {
+	defer func() {
+		s.updateOffset(msg)
+	}()
 	var output api.DeviceKeys
 	if err := json.Unmarshal(msg.Value, &output); err != nil {
 		// If the message was invalid, log it and move on to the next message in the stream
@@ -78,18 +99,190 @@ func (s *OutputKeyChangeEventConsumer) onMessage(msg *sarama.ConsumerMessage) er
 	}
 	// work out who we need to notify about the new key
 	var queryRes currentstateAPI.QuerySharedUsersResponse
-	err := s.currentStateAPI.QuerySharedUsers(context.Background(), &currentstateAPI.QuerySharedUsersRequest{}, &queryRes)
+	err := s.currentStateAPI.QuerySharedUsers(context.Background(), &currentstateAPI.QuerySharedUsersRequest{
+		UserID: output.UserID,
+	}, &queryRes)
 	if err != nil {
 		log.WithError(err).Error("syncapi: failed to QuerySharedUsers for key change event from key server")
 		return err
 	}
-	// TODO: notify users by waking up streams
+	// TODO: f.e queryRes.UserIDsToCount : notify users by waking up streams
 	return nil
 }
 
-// Catchup returns a list of user IDs of users who have changed their device keys between the partition|offset given and now.
+// Catchup fills in the given response for the given user ID to bring it up-to-date with device lists. hasNew=true if the response
-// Returns the new offset for this partition.
+// was filled in, else false if there are no new device list changes because there is nothing to catch up on. The response MUST
-func (s *OutputKeyChangeEventConsumer) Catchup(parition int32, offset int64) (userIDs []string, newOffset int, err error) {
+// be already filled in with join/leave information.
-	//return s.keyAPI.QueryKeyChangeCatchup(ctx, partition, offset)
+func (s *OutputKeyChangeEventConsumer) Catchup(
+	ctx context.Context, userID string, res *types.Response, tok types.StreamingToken,
+) (hasNew bool, err error) {
+	// Track users who we didn't track before but now do by virtue of sharing a room with them, or not.
+	newlyJoinedRooms := joinedRooms(res, userID)
+	newlyLeftRooms := leftRooms(res)
+	if len(newlyJoinedRooms) > 0 || len(newlyLeftRooms) > 0 {
+		changed, left, err := s.trackChangedUsers(ctx, userID, newlyJoinedRooms, newlyLeftRooms)
+		if err != nil {
+			return false, err
+		}
+		res.DeviceLists.Changed = changed
+		res.DeviceLists.Left = left
+		hasNew = len(changed) > 0 || len(left) > 0
+	}
+
+	// TODO: now also track users who we already share rooms with but who have updated their devices between the two tokens
 	return
 }
+
+func (s *OutputKeyChangeEventConsumer) OnJoinEvent(ev *gomatrixserverlib.HeaderedEvent) {
+	// work out who we are now sharing rooms with which we previously were not and notify them about the joining
+	// users keys:
+	changed, _, err := s.trackChangedUsers(context.Background(), *ev.StateKey(), []string{ev.RoomID()}, nil)
+	if err != nil {
+		log.WithError(err).Error("OnJoinEvent: failed to work out changed users")
+		return
+	}
+	// TODO: f.e changed, wake up stream
+	for _, userID := range changed {
+		log.Infof("OnJoinEvent:Notify %s that %s should have device lists tracked", userID, *ev.StateKey())
+	}
+}
+
+func (s *OutputKeyChangeEventConsumer) OnLeaveEvent(ev *gomatrixserverlib.HeaderedEvent) {
+	// work out who we are no longer sharing any rooms with and notify them about the leaving user
+	_, left, err := s.trackChangedUsers(context.Background(), *ev.StateKey(), nil, []string{ev.RoomID()})
+	if err != nil {
+		log.WithError(err).Error("OnLeaveEvent: failed to work out left users")
+		return
+	}
+	// TODO: f.e left, wake up stream
+	for _, userID := range left {
+		log.Infof("OnLeaveEvent:Notify %s that %s should no longer track device lists", userID, *ev.StateKey())
+	}
+
+}
+
+// nolint:gocyclo
+func (s *OutputKeyChangeEventConsumer) trackChangedUsers(
+	ctx context.Context, userID string, newlyJoinedRooms, newlyLeftRooms []string,
+) (changed, left []string, err error) {
+	// process leaves first, then joins afterwards so if we join/leave/join/leave we err on the side of including users.
+
+	// Leave algorithm:
+	// - Get set of users and number of times they appear in rooms prior to leave. - QuerySharedUsersRequest with 'IncludeRoomID'.
+	// - Get users in newly left room. - QueryCurrentState
+	// - Loop set of users and decrement by 1 for each user in newly left room.
+	// - If count=0 then they share no more rooms so inform BOTH parties of this via 'left'=[...] in /sync.
+	var queryRes currentstateAPI.QuerySharedUsersResponse
+	err = s.currentStateAPI.QuerySharedUsers(ctx, &currentstateAPI.QuerySharedUsersRequest{
+		UserID:         userID,
+		IncludeRoomIDs: newlyLeftRooms,
+	}, &queryRes)
+	if err != nil {
+		return nil, nil, err
+	}
+	var stateRes currentstateAPI.QueryBulkStateContentResponse
+	err = s.currentStateAPI.QueryBulkStateContent(ctx, &currentstateAPI.QueryBulkStateContentRequest{
+		RoomIDs: newlyLeftRooms,
+		StateTuples: []gomatrixserverlib.StateKeyTuple{
+			{
+				EventType: gomatrixserverlib.MRoomMember,
+				StateKey:  "*",
+			},
+		},
+		AllowWildcards: true,
+	}, &stateRes)
+	if err != nil {
+		return nil, nil, err
+	}
+	for _, state := range stateRes.Rooms {
+		for tuple, membership := range state {
+			if membership != gomatrixserverlib.Join {
+				continue
+			}
+			queryRes.UserIDsToCount[tuple.StateKey]--
+		}
+	}
+	for userID, count := range queryRes.UserIDsToCount {
+		if count <= 0 {
+			left = append(left, userID) // left is returned
+		}
+	}
+
+	// Join algorithm:
+	// - Get the set of all joined users prior to joining room - QuerySharedUsersRequest with 'ExcludeRoomID'.
+	// - Get users in newly joined room - QueryCurrentState
+	// - Loop set of users in newly joined room, do they appear in the set of users prior to joining?
+	// - If yes: then they already shared a room in common, do nothing.
+	// - If no: then they are a brand new user so inform BOTH parties of this via 'changed=[...]'
+	err = s.currentStateAPI.QuerySharedUsers(ctx, &currentstateAPI.QuerySharedUsersRequest{
+		UserID:         userID,
+		ExcludeRoomIDs: newlyJoinedRooms,
+	}, &queryRes)
+	if err != nil {
+		return nil, left, err
+	}
+	err = s.currentStateAPI.QueryBulkStateContent(ctx, &currentstateAPI.QueryBulkStateContentRequest{
+		RoomIDs: newlyJoinedRooms,
+		StateTuples: []gomatrixserverlib.StateKeyTuple{
+			{
+				EventType: gomatrixserverlib.MRoomMember,
+				StateKey:  "*",
+			},
+		},
+		AllowWildcards: true,
+	}, &stateRes)
+	if err != nil {
+		return nil, left, err
+	}
+	for _, state := range stateRes.Rooms {
+		for tuple, membership := range state {
+			if membership != gomatrixserverlib.Join {
+				continue
+			}
+			// new user who we weren't previously sharing rooms with
+			if _, ok := queryRes.UserIDsToCount[tuple.StateKey]; !ok {
+				changed = append(changed, tuple.StateKey) // changed is returned
+			}
+		}
+	}
+	return changed, left, nil
+}
+
+func joinedRooms(res *types.Response, userID string) []string {
+	var roomIDs []string
+	for roomID, join := range res.Rooms.Join {
+		// we would expect to see our join event somewhere if we newly joined the room.
+		// Normal events get put in the join section so it's not enough to know the room ID is present in 'join'.
+		newlyJoined := membershipEventPresent(join.State.Events, userID)
+		if newlyJoined {
+			roomIDs = append(roomIDs, roomID)
+			continue
+		}
+		newlyJoined = membershipEventPresent(join.Timeline.Events, userID)
+		if newlyJoined {
+			roomIDs = append(roomIDs, roomID)
+		}
+	}
+	return roomIDs
+}
+
+func leftRooms(res *types.Response) []string {
+	roomIDs := make([]string, len(res.Rooms.Leave))
+	i := 0
+	for roomID := range res.Rooms.Leave {
+		roomIDs[i] = roomID
+		i++
+	}
+	return roomIDs
+}
+
+func membershipEventPresent(events []gomatrixserverlib.ClientEvent, userID string) bool {
+	for _, ev := range events {
+		// it's enough to know that we have our member event here, don't need to check membership content
+		// as it's implied by being in the respective section of the sync response.
+		if ev.Type == gomatrixserverlib.MRoomMember && ev.StateKey != nil && *ev.StateKey == userID {
+			return true
+		}
+	}
+	return false
+}

syncapi/consumers/keychange_test.go

@@ -0,0 +1,400 @@
+package consumers
+
+import (
+	"context"
+	"reflect"
+	"sort"
+	"testing"
+
+	"github.com/matrix-org/dendrite/currentstateserver/api"
+	"github.com/matrix-org/dendrite/syncapi/types"
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+var (
+	syncingUser = "@alice:localhost"
+)
+
+type mockCurrentStateAPI struct {
+	roomIDToJoinedMembers map[string][]string
+}
+
+func (s *mockCurrentStateAPI) QueryCurrentState(ctx context.Context, req *api.QueryCurrentStateRequest, res *api.QueryCurrentStateResponse) error {
+	return nil
+}
+
+// QueryRoomsForUser retrieves a list of room IDs matching the given query.
+func (s *mockCurrentStateAPI) QueryRoomsForUser(ctx context.Context, req *api.QueryRoomsForUserRequest, res *api.QueryRoomsForUserResponse) error {
+	return nil
+}
+
+// QueryBulkStateContent does a bulk query for state event content in the given rooms.
+func (s *mockCurrentStateAPI) QueryBulkStateContent(ctx context.Context, req *api.QueryBulkStateContentRequest, res *api.QueryBulkStateContentResponse) error {
+	res.Rooms = make(map[string]map[gomatrixserverlib.StateKeyTuple]string)
+	if req.AllowWildcards && len(req.StateTuples) == 1 && req.StateTuples[0].EventType == gomatrixserverlib.MRoomMember && req.StateTuples[0].StateKey == "*" {
+		for _, roomID := range req.RoomIDs {
+			res.Rooms[roomID] = make(map[gomatrixserverlib.StateKeyTuple]string)
+			for _, userID := range s.roomIDToJoinedMembers[roomID] {
+				res.Rooms[roomID][gomatrixserverlib.StateKeyTuple{
+					EventType: gomatrixserverlib.MRoomMember,
+					StateKey:  userID,
+				}] = "join"
+			}
+		}
+	}
+	return nil
+}
+
+// QuerySharedUsers returns a list of users who share at least 1 room in common with the given user.
+func (s *mockCurrentStateAPI) QuerySharedUsers(ctx context.Context, req *api.QuerySharedUsersRequest, res *api.QuerySharedUsersResponse) error {
+	roomsToQuery := req.IncludeRoomIDs
+	for roomID, members := range s.roomIDToJoinedMembers {
+		exclude := false
+		for _, excludeRoomID := range req.ExcludeRoomIDs {
+			if roomID == excludeRoomID {
+				exclude = true
+				break
+			}
+		}
+		if exclude {
+			continue
+		}
+		for _, userID := range members {
+			if userID == req.UserID {
+				roomsToQuery = append(roomsToQuery, roomID)
+				break
+			}
+		}
+	}
+
+	res.UserIDsToCount = make(map[string]int)
+	for _, roomID := range roomsToQuery {
+		for _, userID := range s.roomIDToJoinedMembers[roomID] {
+			res.UserIDsToCount[userID]++
+		}
+	}
+	return nil
+}
+
+type wantCatchup struct {
+	hasNew  bool
+	changed []string
+	left    []string
+}
+
+func assertCatchup(t *testing.T, hasNew bool, syncResponse *types.Response, want wantCatchup) {
+	if hasNew != want.hasNew {
+		t.Errorf("got hasNew=%v want %v", hasNew, want.hasNew)
+	}
+	sort.Strings(syncResponse.DeviceLists.Left)
+	if !reflect.DeepEqual(syncResponse.DeviceLists.Left, want.left) {
+		t.Errorf("device_lists.left got %v want %v", syncResponse.DeviceLists.Left, want.left)
+	}
+	sort.Strings(syncResponse.DeviceLists.Changed)
+	if !reflect.DeepEqual(syncResponse.DeviceLists.Changed, want.changed) {
+		t.Errorf("device_lists.changed got %v want %v", syncResponse.DeviceLists.Changed, want.changed)
+	}
+}
+
+func joinResponseWithRooms(syncResponse *types.Response, userID string, roomIDs []string) *types.Response {
+	for _, roomID := range roomIDs {
+		roomEvents := []gomatrixserverlib.ClientEvent{
+			{
+				Type:     "m.room.member",
+				StateKey: &userID,
+				EventID:  "$something:here",
+				Sender:   userID,
+				RoomID:   roomID,
+				Content:  []byte(`{"membership":"join"}`),
+			},
+		}
+
+		jr := syncResponse.Rooms.Join[roomID]
+		jr.State.Events = roomEvents
+		syncResponse.Rooms.Join[roomID] = jr
+	}
+	return syncResponse
+}
+
+func leaveResponseWithRooms(syncResponse *types.Response, userID string, roomIDs []string) *types.Response {
+	for _, roomID := range roomIDs {
+		roomEvents := []gomatrixserverlib.ClientEvent{
+			{
+				Type:     "m.room.member",
+				StateKey: &userID,
+				EventID:  "$something:here",
+				Sender:   userID,
+				RoomID:   roomID,
+				Content:  []byte(`{"membership":"leave"}`),
+			},
+		}
+
+		lr := syncResponse.Rooms.Leave[roomID]
+		lr.Timeline.Events = roomEvents
+		syncResponse.Rooms.Leave[roomID] = lr
+	}
+	return syncResponse
+}
+
+// tests that joining a room which results in sharing a new user includes that user in `changed`
+func TestKeyChangeCatchupOnJoinShareNewUser(t *testing.T) {
+	newShareUser := "@bill:localhost"
+	newlyJoinedRoom := "!TestKeyChangeCatchupOnJoinShareNewUser:bar"
+	consumer := NewOutputKeyChangeEventConsumer(gomatrixserverlib.ServerName("localhost"), "some_topic", nil, &mockCurrentStateAPI{
+		roomIDToJoinedMembers: map[string][]string{
+			newlyJoinedRoom: {syncingUser, newShareUser},
+			"!another:room": {syncingUser},
+		},
+	}, nil)
+	syncResponse := types.NewResponse()
+	syncResponse = joinResponseWithRooms(syncResponse, syncingUser, []string{newlyJoinedRoom})
+
+	hasNew, err := consumer.Catchup(context.Background(), syncingUser, syncResponse, types.NewStreamToken(0, 0))
+	if err != nil {
+		t.Fatalf("Catchup returned an error: %s", err)
+	}
+	assertCatchup(t, hasNew, syncResponse, wantCatchup{
+		hasNew:  true,
+		changed: []string{newShareUser},
+	})
+}
+
+// tests that leaving a room which results in sharing no rooms with a user includes that user in `left`
+func TestKeyChangeCatchupOnLeaveShareLeftUser(t *testing.T) {
+	removeUser := "@bill:localhost"
+	newlyLeftRoom := "!TestKeyChangeCatchupOnLeaveShareLeftUser:bar"
+	consumer := NewOutputKeyChangeEventConsumer(gomatrixserverlib.ServerName("localhost"), "some_topic", nil, &mockCurrentStateAPI{
+		roomIDToJoinedMembers: map[string][]string{
+			newlyLeftRoom:   {removeUser},
+			"!another:room": {syncingUser},
+		},
+	}, nil)
+	syncResponse := types.NewResponse()
+	syncResponse = leaveResponseWithRooms(syncResponse, syncingUser, []string{newlyLeftRoom})
+
+	hasNew, err := consumer.Catchup(context.Background(), syncingUser, syncResponse, types.NewStreamToken(0, 0))
+	if err != nil {
+		t.Fatalf("Catchup returned an error: %s", err)
+	}
+	assertCatchup(t, hasNew, syncResponse, wantCatchup{
+		hasNew: true,
+		left:   []string{removeUser},
+	})
+}
+
+// tests that joining a room which doesn't result in sharing a new user results in no changes.
+func TestKeyChangeCatchupOnJoinShareNoNewUsers(t *testing.T) {
+	existingUser := "@bob:localhost"
+	newlyJoinedRoom := "!TestKeyChangeCatchupOnJoinShareNoNewUsers:bar"
+	consumer := NewOutputKeyChangeEventConsumer(gomatrixserverlib.ServerName("localhost"), "some_topic", nil, &mockCurrentStateAPI{
+		roomIDToJoinedMembers: map[string][]string{
+			newlyJoinedRoom: {syncingUser, existingUser},
+			"!another:room": {syncingUser, existingUser},
+		},
+	}, nil)
+	syncResponse := types.NewResponse()
+	syncResponse = joinResponseWithRooms(syncResponse, syncingUser, []string{newlyJoinedRoom})
+
+	hasNew, err := consumer.Catchup(context.Background(), syncingUser, syncResponse, types.NewStreamToken(0, 0))
+	if err != nil {
+		t.Fatalf("Catchup returned an error: %s", err)
+	}
+	assertCatchup(t, hasNew, syncResponse, wantCatchup{
+		hasNew: false,
+	})
+}
+
+// tests that leaving a room which doesn't result in sharing no rooms with a user results in no changes.
+func TestKeyChangeCatchupOnLeaveShareNoUsers(t *testing.T) {
+	existingUser := "@bob:localhost"
+	newlyLeftRoom := "!TestKeyChangeCatchupOnLeaveShareNoUsers:bar"
+	consumer := NewOutputKeyChangeEventConsumer(gomatrixserverlib.ServerName("localhost"), "some_topic", nil, &mockCurrentStateAPI{
+		roomIDToJoinedMembers: map[string][]string{
+			newlyLeftRoom:   {existingUser},
+			"!another:room": {syncingUser, existingUser},
+		},
+	}, nil)
+	syncResponse := types.NewResponse()
+	syncResponse = leaveResponseWithRooms(syncResponse, syncingUser, []string{newlyLeftRoom})
+
+	hasNew, err := consumer.Catchup(context.Background(), syncingUser, syncResponse, types.NewStreamToken(0, 0))
+	if err != nil {
+		t.Fatalf("Catchup returned an error: %s", err)
+	}
+	assertCatchup(t, hasNew, syncResponse, wantCatchup{
+		hasNew: false,
+	})
+}
+
+// tests that not joining any rooms (but having messages in the response) do not result in changes.
+func TestKeyChangeCatchupNoNewJoinsButMessages(t *testing.T) {
+	existingUser := "@bob1:localhost"
+	roomID := "!TestKeyChangeCatchupNoNewJoinsButMessages:bar"
+	consumer := NewOutputKeyChangeEventConsumer(gomatrixserverlib.ServerName("localhost"), "some_topic", nil, &mockCurrentStateAPI{
+		roomIDToJoinedMembers: map[string][]string{
+			roomID: {syncingUser, existingUser},
+		},
+	}, nil)
+	syncResponse := types.NewResponse()
+	empty := ""
+	roomStateEvents := []gomatrixserverlib.ClientEvent{
+		{
+			Type:     "m.room.name",
+			StateKey: &empty,
+			EventID:  "$something:here",
+			Sender:   existingUser,
+			RoomID:   roomID,
+			Content:  []byte(`{"name":"The Room Name"}`),
+		},
+	}
+	roomTimelineEvents := []gomatrixserverlib.ClientEvent{
+		{
+			Type:    "m.room.message",
+			EventID: "$something1:here",
+			Sender:  existingUser,
+			RoomID:  roomID,
+			Content: []byte(`{"body":"Message 1"}`),
+		},
+		{
+			Type:    "m.room.message",
+			EventID: "$something2:here",
+			Sender:  syncingUser,
+			RoomID:  roomID,
+			Content: []byte(`{"body":"Message 2"}`),
+		},
+		{
+			Type:    "m.room.message",
+			EventID: "$something3:here",
+			Sender:  existingUser,
+			RoomID:  roomID,
+			Content: []byte(`{"body":"Message 3"}`),
+		},
+	}
+
+	jr := syncResponse.Rooms.Join[roomID]
+	jr.State.Events = roomStateEvents
+	jr.Timeline.Events = roomTimelineEvents
+	syncResponse.Rooms.Join[roomID] = jr
+
+	hasNew, err := consumer.Catchup(context.Background(), syncingUser, syncResponse, types.NewStreamToken(0, 0))
+	if err != nil {
+		t.Fatalf("Catchup returned an error: %s", err)
+	}
+	assertCatchup(t, hasNew, syncResponse, wantCatchup{
+		hasNew: false,
+	})
+}
+
+// tests that joining/leaving multiple rooms can result in both `changed` and `left` and they are not duplicated.
+func TestKeyChangeCatchupChangeAndLeft(t *testing.T) {
+	newShareUser := "@berta:localhost"
+	newShareUser2 := "@bobby:localhost"
+	newlyLeftUser := "@charlie:localhost"
+	newlyLeftUser2 := "@debra:localhost"
+	newlyJoinedRoom := "!join:bar"
+	newlyLeftRoom := "!left:bar"
+	consumer := NewOutputKeyChangeEventConsumer(gomatrixserverlib.ServerName("localhost"), "some_topic", nil, &mockCurrentStateAPI{
+		roomIDToJoinedMembers: map[string][]string{
+			newlyJoinedRoom: {syncingUser, newShareUser, newShareUser2},
+			newlyLeftRoom:   {newlyLeftUser, newlyLeftUser2},
+			"!another:room": {syncingUser},
+		},
+	}, nil)
+	syncResponse := types.NewResponse()
+	syncResponse = joinResponseWithRooms(syncResponse, syncingUser, []string{newlyJoinedRoom})
+	syncResponse = leaveResponseWithRooms(syncResponse, syncingUser, []string{newlyLeftRoom})
+
+	hasNew, err := consumer.Catchup(context.Background(), syncingUser, syncResponse, types.NewStreamToken(0, 0))
+	if err != nil {
+		t.Fatalf("Catchup returned an error: %s", err)
+	}
+	assertCatchup(t, hasNew, syncResponse, wantCatchup{
+		hasNew:  true,
+		changed: []string{newShareUser, newShareUser2},
+		left:    []string{newlyLeftUser, newlyLeftUser2},
+	})
+}
+
+// tests that joining/leaving the SAME room puts users in `left` if the final state is leave.
+// NB: Consider the case:
+// - Alice and Bob are in a room.
+// - Alice goes offline, Charlie joins, sends encrypted messages then leaves the room.
+// - Alice comes back online. Technically nothing has changed in the set of users between those two points in time,
+//   it's still just (Alice,Bob) but then we won't be tracking Charlie -- is this okay though? It's device keys
+//   which are only relevant when actively sending events I think? And if Alice does need the keys she knows
+//   charlie's (user_id, device_id) so can just hit /keys/query - no need to keep updated about it because she
+//   doesn't share any rooms with him.
+// Ergo, we put them in `left` as it is simpler.
+func TestKeyChangeCatchupChangeAndLeftSameRoom(t *testing.T) {
+	newShareUser := "@berta:localhost"
+	newShareUser2 := "@bobby:localhost"
+	roomID := "!join:bar"
+	consumer := NewOutputKeyChangeEventConsumer(gomatrixserverlib.ServerName("localhost"), "some_topic", nil, &mockCurrentStateAPI{
+		roomIDToJoinedMembers: map[string][]string{
+			roomID:          {newShareUser, newShareUser2},
+			"!another:room": {syncingUser},
+		},
+	}, nil)
+	syncResponse := types.NewResponse()
+	roomEvents := []gomatrixserverlib.ClientEvent{
+		{
+			Type:     "m.room.member",
+			StateKey: &syncingUser,
+			EventID:  "$something:here",
+			Sender:   syncingUser,
+			RoomID:   roomID,
+			Content:  []byte(`{"membership":"join"}`),
+		},
+		{
+			Type:    "m.room.message",
+			EventID: "$something2:here",
+			Sender:  syncingUser,
+			RoomID:  roomID,
+			Content: []byte(`{"body":"now I leave you"}`),
+		},
+		{
+			Type:     "m.room.member",
+			StateKey: &syncingUser,
+			EventID:  "$something3:here",
+			Sender:   syncingUser,
+			RoomID:   roomID,
+			Content:  []byte(`{"membership":"leave"}`),
+		},
+		{
+			Type:     "m.room.member",
+			StateKey: &syncingUser,
+			EventID:  "$something4:here",
+			Sender:   syncingUser,
+			RoomID:   roomID,
+			Content:  []byte(`{"membership":"join"}`),
+		},
+		{
+			Type:    "m.room.message",
+			EventID: "$something5:here",
+			Sender:  syncingUser,
+			RoomID:  roomID,
+			Content: []byte(`{"body":"now I am back, and I leave you for good"}`),
+		},
+		{
+			Type:     "m.room.member",
+			StateKey: &syncingUser,
+			EventID:  "$something6:here",
+			Sender:   syncingUser,
+			RoomID:   roomID,
+			Content:  []byte(`{"membership":"leave"}`),
+		},
+	}
+
+	lr := syncResponse.Rooms.Leave[roomID]
+	lr.Timeline.Events = roomEvents
+	syncResponse.Rooms.Leave[roomID] = lr
+
+	hasNew, err := consumer.Catchup(context.Background(), syncingUser, syncResponse, types.NewStreamToken(0, 0))
+	if err != nil {
+		t.Fatalf("Catchup returned an error: %s", err)
+	}
+	assertCatchup(t, hasNew, syncResponse, wantCatchup{
+		hasNew: true,
+		left:   []string{newShareUser, newShareUser2},
+	})
+}

syncapi/types/types.go

@@ -302,6 +302,10 @@ type Response struct {
 	ToDevice struct {
 		Events []gomatrixserverlib.SendToDeviceEvent `json:"events"`
 	} `json:"to_device"`
+	DeviceLists struct {
+		Changed []string `json:"changed,omitempty"`
+		Left    []string `json:"left,omitempty"`
+	} `json:"device_lists,omitempty"`
 }
 
 // NewResponse creates an empty response with initialised maps.