fix : add synchronized on the returned list when traversing it Spliterator (ShiftLeftSecurity#425)

Co-authored-by: jubatusd <[email protected]>

Author: jubatusd

core/src/main/java/overflowdb/ReferenceManager.java

@@ -152,8 +152,10 @@ private void safelyClearReferences(final List<NodeRef> refsToClear) {
    * useful when saving the graph
    */
   public void clearAllReferences() {
-    nodesWriter.writeAndClearBatched(clearableRefs.spliterator(), clearableRefs.size());
-    logger.debug("cleared all clearable references");
+    synchronized (clearableRefs){
+      nodesWriter.writeAndClearBatched(clearableRefs.spliterator(), clearableRefs.size());
+      logger.debug("cleared all clearable references");
+    }
   }
 
   @Override