Fix /sync when we have no events (#341)

* Fix /sync when we have no events

We used a since token of 0 to mean that no token was given. However, naffka
streams start at 0. This causes clients to get stuck spinning forever until an
event is sent.

This changes it so that we pass around pointers instead, with nil meaning a
since token wasn't given.

* Comment

* Fix unit tests

* Comments

* Fix typo

src/github.com/matrix-org/dendrite/syncapi/sync/notifier_test.go

@@ -264,7 +264,7 @@ func waitForEvents(n *Notifier, req syncRequest) (types.StreamPosition, error) {
 		return types.StreamPosition(0), fmt.Errorf(
 			"waitForEvents timed out waiting for %s (pos=%d)", req.userID, req.since,
 		)
-	case <-listener.GetNotifyChannel(req.since):
+	case <-listener.GetNotifyChannel(*req.since):
 		p := listener.GetStreamPosition()
 		return p, nil
 	}
@@ -282,7 +282,7 @@ func newTestSyncRequest(userID string, since types.StreamPosition) syncRequest {
 	return syncRequest{
 		userID:        userID,
 		timeout:       1 * time.Minute,
-		since:         since,
+		since:         &since,
 		wantFullState: false,
 		limit:         defaultTimelineLimit,
 		log:           util.GetLogger(context.TODO()),

src/github.com/matrix-org/dendrite/syncapi/sync/request.go

@@ -34,7 +34,7 @@ type syncRequest struct {
 	userID        string
 	limit         int
 	timeout       time.Duration
-	since         types.StreamPosition
+	since         *types.StreamPosition // nil means that no since token was supplied
 	wantFullState bool
 	log           *log.Entry
 }
@@ -70,13 +70,16 @@ func getTimeout(timeoutMS string) time.Duration {
 	return time.Duration(i) * time.Millisecond
 }
 
-func getSyncStreamPosition(since string) (types.StreamPosition, error) {
+// getSyncStreamPosition tries to parse a 'since' token taken from the API to a
+// stream position. If the string is empty then (nil, nil) is returned.
+func getSyncStreamPosition(since string) (*types.StreamPosition, error) {
 	if since == "" {
-		return types.StreamPosition(0), nil
+		return nil, nil
 	}
 	i, err := strconv.Atoi(since)
 	if err != nil {
-		return types.StreamPosition(0), err
+		return nil, err
 	}
-	return types.StreamPosition(i), nil
+	token := types.StreamPosition(i)
+	return &token, nil
 }

src/github.com/matrix-org/dendrite/syncapi/sync/requestpool.go

@@ -64,7 +64,7 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 	currPos := rp.notifier.CurrentPosition()
 
 	// If this is an initial sync or timeout=0 we return immediately
-	if syncReq.since == types.StreamPosition(0) || syncReq.timeout == 0 {
+	if syncReq.since == nil || syncReq.timeout == 0 {
 		syncData, err := rp.currentSyncForUser(*syncReq, currPos)
 		if err != nil {
 			return httputil.LogThenError(req, err)
@@ -93,7 +93,7 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 		case <-timer.C:
 			return util.JSONResponse{
 				Code: 200,
-				JSON: types.NewResponse(syncReq.since),
+				JSON: types.NewResponse(currPos),
 			}
 		// Or for the request to be cancelled
 		case <-req.Context().Done():
@@ -121,10 +121,10 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 
 func (rp *RequestPool) currentSyncForUser(req syncRequest, currentPos types.StreamPosition) (res *types.Response, err error) {
 	// TODO: handle ignored users
-	if req.since == types.StreamPosition(0) {
+	if req.since == nil {
 		res, err = rp.db.CompleteSync(req.ctx, req.userID, req.limit)
 	} else {
-		res, err = rp.db.IncrementalSync(req.ctx, req.userID, req.since, currentPos, req.limit)
+		res, err = rp.db.IncrementalSync(req.ctx, req.userID, *req.since, currentPos, req.limit)
 	}
 
 	if err != nil {
@@ -148,7 +148,7 @@ func (rp *RequestPool) appendAccountData(
 		return nil, err
 	}
 
-	if req.since == types.StreamPosition(0) {
+	if req.since == nil {
 		// If this is the initial sync, we don't need to check if a data has
 		// already been sent. Instead, we send the whole batch.
 		var global []gomatrixserverlib.ClientEvent
@@ -170,7 +170,7 @@ func (rp *RequestPool) appendAccountData(
 	}
 
 	// Sync is not initial, get all account data since the latest sync
-	dataTypes, err := rp.db.GetAccountDataInRange(req.ctx, userID, req.since, currentPos)
+	dataTypes, err := rp.db.GetAccountDataInRange(req.ctx, userID, *req.since, currentPos)
 	if err != nil {
 		return nil, err
 	}