no-jira: hard defense against infinite loop in parent line

Defense against infinite loop in calculation of the parent line in the
flexi tree table, prevent OOM if the tree is not consistent (element
which is a parent of itself)

src/main/java/org/olat/core/gui/components/form/flexible/impl/elements/table/FlexiTreeNodeComparator.java

@@ -24,6 +24,8 @@ import java.util.Collections;
 import java.util.Comparator;
 import java.util.List;
 
+import org.olat.core.logging.AssertException;
+
 /**
  * 
  * Initial date: 15 nov. 2017<br>
@@ -102,7 +104,8 @@ public class FlexiTreeNodeComparator implements Comparator<FlexiTreeTableNode> {
 	
 	private FlexiTreeTableNode root(FlexiTreeTableNode node) {
 		FlexiTreeTableNode root = node;
-		for(FlexiTreeTableNode parent=node.getParent(); parent != null; parent = parent.getParent()) {
+		int i = 0;
+		for(FlexiTreeTableNode parent=node.getParent(); parent != null && i < 100; parent = parent.getParent(), i++) {
 			root = parent;
 		}
 		return root;
@@ -112,6 +115,9 @@ public class FlexiTreeNodeComparator implements Comparator<FlexiTreeTableNode> {
 		List<FlexiTreeTableNode> nodes = new ArrayList<>();
 		for(FlexiTreeTableNode parent=node; parent != null; parent = parent.getParent()) {
 			nodes.add(parent);
+			if(nodes.size() > 255) {
+				throw new AssertException("Flexi tree parent line in an infinite loop");
+			}
 		}
 		Collections.reverse(nodes);
 		return nodes;