Index: lucene/contrib/facet/src/test/org/apache/lucene/facet/taxonomy/TestTaxonomyCombined.java
===================================================================
--- lucene/contrib/facet/src/test/org/apache/lucene/facet/taxonomy/TestTaxonomyCombined.java	(revision 1178702)
+++ lucene/contrib/facet/src/test/org/apache/lucene/facet/taxonomy/TestTaxonomyCombined.java	(working copy)
@@ -491,6 +491,43 @@
     indexDir.close();
   }
   
+  @Test
+  public void testWriterParentThreadSafety() throws Exception { //nocommit 
+    Directory indexDir = newDirectory();
+    TaxonomyWriter tw1 = new LuceneTaxonomyWriter(indexDir);
+    fillTaxonomy(tw1);
+    tw1.commit();
+    tw1.close();
+    
+    final TaxonomyWriter tw = new LuceneTaxonomyWriter(indexDir); // reopen tw
+    Thread[] t = new Thread[2];
+    final Exception[] error = new Exception[] {null}; 
+    for (int i = 0; i < t.length; i++) {
+      t[i] = new Thread("taxo-writer-"+i) {
+        @Override
+        public void run() {
+          try {
+            tw.getParent(0);
+          } catch (IOException e) {
+            error[0] = e;
+          }
+        }
+      };
+    }
+    for (int i = 0; i < t.length; i++) {
+      t[i].start();
+    }
+    for (int i = 0; i < t.length; i++) {
+      t[i].join();
+    }
+    TaxonomyReader tr = new LuceneTaxonomyReader(indexDir);
+    checkWriterParent(tr, tw);
+    
+    tw.close();
+    tr.close();
+    indexDir.close();
+  }
+  
   private void checkWriterParent(TaxonomyReader tr, TaxonomyWriter tw) throws Exception {
     // check that the parent of the root ordinal is the invalid ordinal:
     assertEquals(TaxonomyReader.INVALID_ORDINAL, tw.getParent(0));
Index: lucene/contrib/facet/src/java/org/apache/lucene/facet/taxonomy/lucene/LuceneTaxonomyWriter.java
===================================================================
--- lucene/contrib/facet/src/java/org/apache/lucene/facet/taxonomy/lucene/LuceneTaxonomyWriter.java	(revision 1178702)
+++ lucene/contrib/facet/src/java/org/apache/lucene/facet/taxonomy/lucene/LuceneTaxonomyWriter.java	(working copy)
@@ -705,11 +705,13 @@
   // TODO (Facet): synchronization of some sort?
   private ParentArray parentArray;
   private ParentArray getParentArray() throws IOException {
+    if ("taxo-writer-0".equals(Thread.currentThread().getName())) try { Thread.sleep(100);  } catch (InterruptedException e) { /* nocommit */ }
     if (parentArray==null) {
       if (reader == null) {
         reader = openReader();
       }
       parentArray = new ParentArray();
+      if ("taxo-writer-1".equals(Thread.currentThread().getName())) try { Thread.sleep(100);  } catch (InterruptedException e) { /* nocommit */ }
       parentArray.refresh(reader);
     }
     return parentArray;