diff --git a/syncapi/storage/syncserver.go b/syncapi/storage/syncserver.go
index b4d7ccbd..20fa8a4e 100644
--- a/syncapi/storage/syncserver.go
+++ b/syncapi/storage/syncserver.go
@@ -35,6 +35,12 @@ import (
 	"github.com/matrix-org/gomatrixserverlib"
 )
 
+const (
+	membershipJoin  = "join"
+	membershipLeave = "leave"
+	membershipBan   = "ban"
+)
+
 type stateDelta struct {
 	roomID      string
 	stateEvents []gomatrixserverlib.Event
@@ -248,14 +254,12 @@ func (d *SyncServerDatasource) addPDUDeltaToResponse(
 	// joined rooms, but also which rooms have membership transitions for this user between the 2 PDU stream positions.
 	// This works out what the 'state' key should be for each room as well as which membership block
 	// to put the room into.
-	deltas, err := d.getStateDeltas(ctx, &device, txn, fromPos, toPos, device.UserID)
+	deltas, joinedRoomIDs, err := d.getStateDeltas(ctx, &device, txn, fromPos, toPos, device.UserID)
 	if err != nil {
 		return nil, err
 	}
 
-	joinedRoomIDs := make([]string, 0, len(deltas))
 	for _, delta := range deltas {
-		joinedRoomIDs = append(joinedRoomIDs, delta.roomID)
 		err = d.addRoomDeltaToResponse(ctx, &device, txn, fromPos, toPos, delta, numRecentEventsPerRoom, res)
 		if err != nil {
 			return nil, err
@@ -344,7 +348,7 @@ func (d *SyncServerDatasource) IncrementalSync(
 		)
 	} else {
 		joinedRoomIDs, err = d.roomstate.selectRoomIDsWithMembership(
-			ctx, nil, device.UserID, "join",
+			ctx, nil, device.UserID, membershipJoin,
 		)
 	}
 	if err != nil {
@@ -393,7 +397,7 @@ func (d *SyncServerDatasource) getResponseWithPDUsForCompleteSync(
 	res = types.NewResponse(toPos)
 
 	// Extract room state and recent events for all rooms the user is joined to.
-	joinedRoomIDs, err = d.roomstate.selectRoomIDsWithMembership(ctx, txn, userID, "join")
+	joinedRoomIDs, err = d.roomstate.selectRoomIDsWithMembership(ctx, txn, userID, membershipJoin)
 	if err != nil {
 		return
 	}
@@ -571,7 +575,7 @@ func (d *SyncServerDatasource) addRoomDeltaToResponse(
 	res *types.Response,
 ) error {
 	endPos := toPos
-	if delta.membershipPos > 0 && delta.membership == "leave" {
+	if delta.membershipPos > 0 && delta.membership == membershipLeave {
 		// make sure we don't leak recent events after the leave event.
 		// TODO: History visibility makes this somewhat complex to handle correctly. For example:
 		// TODO: This doesn't work for join -> leave in a single /sync request (see events prior to join).
@@ -595,7 +599,7 @@ func (d *SyncServerDatasource) addRoomDeltaToResponse(
 	}
 
 	switch delta.membership {
-	case "join":
+	case membershipJoin:
 		jr := types.NewJoinResponse()
 		if prevPDUPos := recentStreamEvents[0].streamPosition - 1; prevPDUPos > 0 {
 			// Use the short form of batch token for prev_batch
@@ -608,9 +612,9 @@ func (d *SyncServerDatasource) addRoomDeltaToResponse(
 		jr.Timeline.Limited = false // TODO: if len(events) >= numRecents + 1 and then set limited:true
 		jr.State.Events = gomatrixserverlib.ToClientEvents(delta.stateEvents, gomatrixserverlib.FormatSync)
 		res.Rooms.Join[delta.roomID] = *jr
-	case "leave":
+	case membershipLeave:
 		fallthrough // transitions to leave are the same as ban
-	case "ban":
+	case membershipBan:
 		// TODO: recentEvents may contain events that this user is not allowed to see because they are
 		//       no longer in the room.
 		lr := types.NewLeaveResponse()
@@ -716,10 +720,14 @@ func (d *SyncServerDatasource) fetchMissingStateEvents(
 	return events, nil
 }
 
+// getStateDeltas returns the state deltas between fromPos and toPos,
+// exclusive of oldPos, inclusive of newPos, for the rooms in which
+// the user has new membership events.
+// A list of joined room IDs is also returned in case the caller needs it.
 func (d *SyncServerDatasource) getStateDeltas(
 	ctx context.Context, device *authtypes.Device, txn *sql.Tx,
 	fromPos, toPos int64, userID string,
-) ([]stateDelta, error) {
+) ([]stateDelta, []string, error) {
 	// Implement membership change algorithm: https://github.com/matrix-org/synapse/blob/v0.19.3/synapse/handlers/sync.py#L821
 	// - Get membership list changes for this user in this sync response
 	// - For each room which has membership list changes:
@@ -733,11 +741,11 @@ func (d *SyncServerDatasource) getStateDeltas(
 	// get all the state events ever between these two positions
 	stateNeeded, eventMap, err := d.events.selectStateInRange(ctx, txn, fromPos, toPos)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 	state, err := d.fetchStateEvents(ctx, txn, stateNeeded, eventMap)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	for roomID, stateStreamEvents := range state {
@@ -748,12 +756,12 @@ func (d *SyncServerDatasource) getStateDeltas(
 			//       the 'state' part of the response though, so is transparent modulo bandwidth concerns as it is not added to
 			//       the timeline.
 			if membership := getMembershipFromEvent(&ev.Event, userID); membership != "" {
-				if membership == "join" {
+				if membership == membershipJoin {
 					// send full room state down instead of a delta
 					var allState []gomatrixserverlib.Event
 					allState, err = d.roomstate.selectCurrentState(ctx, txn, roomID)
 					if err != nil {
-						return nil, err
+						return nil, nil, err
 					}
 					s := make([]streamEvent, len(allState))
 					for i := 0; i < len(s); i++ {
@@ -775,19 +783,19 @@ func (d *SyncServerDatasource) getStateDeltas(
 	}
 
 	// Add in currently joined rooms
-	joinedRoomIDs, err := d.roomstate.selectRoomIDsWithMembership(ctx, txn, userID, "join")
+	joinedRoomIDs, err := d.roomstate.selectRoomIDsWithMembership(ctx, txn, userID, membershipJoin)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 	for _, joinedRoomID := range joinedRoomIDs {
 		deltas = append(deltas, stateDelta{
-			membership:  "join",
+			membership:  membershipJoin,
 			stateEvents: streamEventsToEvents(device, state[joinedRoomID]),
 			roomID:      joinedRoomID,
 		})
 	}
 
-	return deltas, nil
+	return deltas, joinedRoomIDs, nil
 }
 
 // streamEventsToEvents converts streamEvent to Event. If device is non-nil and