Fixed synchronization in computing children

Author: jisedlac

visualvm/heapviewer/src/org/graalvm/visualvm/heapviewer/model/HeapViewerNode.java

@@ -189,15 +189,17 @@ protected HeapViewerNode[] doInBackground() throws Exception {
                 return ret;
             }
             protected void done() {
-                if (children != null) try {
-                    HeapViewerNode[] newChildren = get();
-                    // newChildren may be null, for example if the worker thread has been interrupted
-                    if (newChildren != null) {
-                        setChildren(newChildren);
-                        root.updateChildren(HeapViewerNode.this);
+                synchronized (HeapViewerNode.this) {
+                    if (currentWorker == null) try {
+                        HeapViewerNode[] newChildren = get();
+                        // newChildren may be null, for example if the worker thread has been interrupted
+                        if (newChildren != null) {
+                            setChildren(newChildren);
+                            root.updateChildren(HeapViewerNode.this);
+                        }
+                    } catch (Exception ex) {
+                        Exceptions.printStackTrace(ex);
                     }
-                } catch (Exception ex) {
-                    Exceptions.printStackTrace(ex);
                 }
             }
         };