Fix sync position with partial info used as complete one in syncserver and notifier

Signed-off-by: Alex Chen <minecnly@gmail.com>

syncapi/storage/syncserver.go

@@ -331,7 +331,8 @@ func (d *SyncServerDatasource) IncrementalSync(
 	fromPos, toPos types.SyncPosition,
 	numRecentEventsPerRoom int,
 ) (*types.Response, error) {
-	res := types.NewResponse(toPos)
+	nextBatchPos := fromPos.WithUpdates(toPos)
+	res := types.NewResponse(nextBatchPos)
 
 	var joinedRoomIDs []string
 	var err error

syncapi/sync/notifier.go

@@ -66,7 +66,7 @@ func (n *Notifier) OnNewEvent(ev *gomatrixserverlib.Event, roomID string, userID
 	// This needs to be done PRIOR to waking up users as they will read this value.
 	n.streamLock.Lock()
 	defer n.streamLock.Unlock()
-	n.currPos = pos
+	n.currPos = n.currPos.WithUpdates(pos)
 
 	n.removeEmptyUserStreams()
 

syncapi/types/types.go

@@ -41,6 +41,19 @@ func (sp SyncPosition) IsAfter(other SyncPosition) bool {
 		sp.TypingPosition > other.TypingPosition
 }
 
+// WithUpdates returns a copy of sp with updates represented by other applied.
+// If a fieldn is not 0 in other, it is considered an update.
+func (sp SyncPosition) WithUpdates(other SyncPosition) SyncPosition {
+	ret := sp
+	if other.PDUPosition != 0 {
+		ret.PDUPosition = other.PDUPosition
+	}
+	if other.TypingPosition != 0 {
+		ret.TypingPosition = other.TypingPosition
+	}
+	return ret
+}
+
 // PrevEventRef represents a reference to a previous event in a state event upgrade
 type PrevEventRef struct {
 	PrevContent   json.RawMessage `json:"prev_content"`