Update sync position

syncapi/storage/shared/syncserver.go

@@ -1070,18 +1070,20 @@ func (d *Database) SendToDeviceUpdatesForSync(
 		return nil, fmt.Errorf("d.SendToDevice.SelectSendToDeviceMessages: %w", err)
 	}
 
-	// Start by cleaning up any send-to-device messages that have older sent-by-tokens.
+	// Work out whether we need to update any of the database entries.
-	// This means that they were sent in a previous /sync and the client has happily
-	// progressed onto newer sync tokens.
 	toUpdate := []types.SendToDeviceNID{}
 	toDelete := []types.SendToDeviceNID{}
 	for pos, event := range events {
 		if event.SentByToken == nil {
-			// Mark the event for update and keep it in our list of return events.
+			// If the event has no sent-by token yet then we haven't attempted to send
+			// it. Record the current requested sync token in the database.
 			toUpdate = append(toUpdate, event.ID)
 			event.SentByToken = &token
 		} else if token.IsAfter(*event.SentByToken) {
-			// Mark the event for deletion and remove it from our list of return events.
+			// The event had a sync token, therefore we've sent it before. The current
+			// sync token is now after the stored one so we can assume that the client
+			// successfully completed the previous sync (it would re-request it otherwise)
+			// so we can remove the entry from the database.
 			toDelete = append(toDelete, event.ID)
 			events = append(events[:pos], events[pos+1:]...)
 		}

syncapi/sync/requestpool.go

@@ -245,6 +245,11 @@ func (rp *RequestPool) appendAccountData(
 func (rp *RequestPool) appendSendToDeviceMessages(
 	data *types.Response, userID string, req syncRequest, currentPos types.StreamingToken,
 ) (*types.Response, error) {
+	nextPos, err := types.NewStreamTokenFromString(data.NextBatch)
+	if err != nil {
+		return nil, err
+	}
+
 	events, err := rp.db.SendToDeviceUpdatesForSync(
 		context.TODO(),
 		userID,
@@ -257,7 +262,9 @@ func (rp *RequestPool) appendSendToDeviceMessages(
 
 	for _, event := range events {
 		data.ToDevice.Events = append(data.ToDevice.Events, event.SendToDeviceEvent)
+		nextPos.Positions[1]++
 	}
+	data.NextBatch = nextPos.String()
 	return data, nil
 }