Index: src/main/java/org/apache/jackrabbit/oak/plugins/index/lucene/directory/CopyOnReadDirectory.java
===================================================================
--- src/main/java/org/apache/jackrabbit/oak/plugins/index/lucene/directory/CopyOnReadDirectory.java	(revision 1868860)
+++ src/main/java/org/apache/jackrabbit/oak/plugins/index/lucene/directory/CopyOnReadDirectory.java	(working copy)
@@ -263,6 +263,8 @@
     }
 
     /**
+     * Close the files _after_ the method returns (asynchronously).
+     * 
      * On close file which are not present in remote are removed from local.
      * CopyOnReadDir is opened at different revisions of the index state
      *
@@ -274,8 +276,6 @@
      * be ensured that any currently opened IndexSearcher does not get affected.
      * The way IndexSearchers get created in IndexTracker it ensures that new searcher
      * pinned to newer revision gets opened first and then existing ones are closed.
-     *
-     *
      */
     @Override
     public void close() throws IOException {
Index: src/main/java/org/apache/jackrabbit/oak/plugins/index/lucene/directory/DefaultDirectoryFactory.java
===================================================================
--- src/main/java/org/apache/jackrabbit/oak/plugins/index/lucene/directory/DefaultDirectoryFactory.java	(revision 1868860)
+++ src/main/java/org/apache/jackrabbit/oak/plugins/index/lucene/directory/DefaultDirectoryFactory.java	(working copy)
@@ -66,7 +66,19 @@
                 // (copy from the remote directory to the local directory)
                 // to avoid having to stream it when merging
                 String indexPath = definition.getIndexPath();
-                Directory d = indexCopier.wrapForRead(indexPath, definition, directory, dirName);
+
+                // Here we create a new index directory, because
+                // re-using the existing directory (opened above) would
+                // mean that the directory returned by the method
+                // shares the same OakDirectory instance.
+                // That in turn could result in concurrently changing
+                // the NodeBuilder on close, as d.close() closes the directory
+                // _asynchronously_ (after the method returned).
+                // With newIndexDirectory, the internal OakDirectory is not shared
+                Directory readerDir = newIndexDirectory(definition, builder, dirName);
+                Directory d = indexCopier.wrapForRead(indexPath, definition, readerDir, dirName);
+                
+                // closing is done asynchronously
                 d.close();
             }
             directory = indexCopier.wrapForWrite(definition, directory, reindex, dirName, cowDirectoryTracker);
Index: src/main/java/org/apache/jackrabbit/oak/plugins/index/lucene/directory/DirectoryFactory.java
===================================================================
--- src/main/java/org/apache/jackrabbit/oak/plugins/index/lucene/directory/DirectoryFactory.java	(revision 1868860)
+++ src/main/java/org/apache/jackrabbit/oak/plugins/index/lucene/directory/DirectoryFactory.java	(working copy)
@@ -25,8 +25,24 @@
 import org.apache.jackrabbit.oak.spi.state.NodeBuilder;
 import org.apache.lucene.store.Directory;
 
+/**
+ * A builder for Lucene directories.
+ */
 public interface DirectoryFactory {
 
+    /**
+     * Open a new directory.
+     * 
+     * Internally, it read the data from the index definition. It writes to the
+     * builder, for example when closing the directory.
+     * 
+     * @param definition the index definition
+     * @param builder the builder pointing to the index definition (see above
+     *            for usage)
+     * @param dirName the name of the directory (in the file system)
+     * @param reindex whether reindex is needed
+     * @return the Lucene directory
+     */
     Directory newInstance(LuceneIndexDefinition definition, NodeBuilder builder, String dirName,
                           boolean reindex) throws IOException;