Index: lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
===================================================================
--- lucene/core/src/java/org/apache/lucene/index/IndexWriter.java	(revision 1415054)
+++ lucene/core/src/java/org/apache/lucene/index/IndexWriter.java	(working copy)
@@ -2638,6 +2638,10 @@
             synchronized(this) {
               maybeApplyDeletes(true);
 
+              if (commitUserData != null) {
+                setCommitData(commitUserData);
+              }
+
               readerPool.commit(segmentInfos);
 
               // Must clone the segmentInfos while we still
@@ -2688,10 +2692,23 @@
         }
       }
 
-      startCommit(toCommit, commitUserData);
+      startCommit(toCommit);
     }
   }
 
+  /**
+   * Sets the commit user data map. That method is considered a transaction by
+   * {@link IndexWriter} and will be {@link #commit() committed} even if no other
+   * changes were made to the writer instance.
+   * <p>
+   * <b>NOTE:</b> the map is cloned internally, therefore altering the map's
+   * contents after calling this method has no effect.
+   */
+  public final void setCommitData(Map<String,String> commitUserData) {
+    segmentInfos.setUserData(new HashMap<String,String>(commitUserData));
+    ++changeCount;
+  }
+  
   // Used only by commit and prepareCommit, below; lock
   // order is commitLock -> IW
   private final Object commitLock = new Object();
@@ -3906,7 +3923,7 @@
    *  if it wasn't already.  If that succeeds, then we
    *  prepare a new segments_N file but do not fully commit
    *  it. */
-  private void startCommit(final SegmentInfos toSync, final Map<String,String> commitUserData) throws IOException {
+  private void startCommit(final SegmentInfos toSync) throws IOException {
 
     assert testPoint("startStartCommit");
     assert pendingCommit == null;
@@ -3939,10 +3956,6 @@
         }
 
         assert filesExist(toSync);
-
-        if (commitUserData != null) {
-          toSync.setUserData(commitUserData);
-        }
       }
 
       assert testPoint("midStartCommit");
Index: lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java
===================================================================
--- lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java	(revision 1415054)
+++ lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java	(working copy)
@@ -23,8 +23,10 @@
 import java.io.StringReader;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Map;
 
 import org.apache.lucene.analysis.*;
 import org.apache.lucene.analysis.tokenattributes.CharTermAttribute;
@@ -1932,4 +1934,25 @@
     w.close();
     dir.close();
   }
+  
+  // LUCENE-4575
+  public void testCommitWithUserDataOnly() throws Exception {
+    Directory dir = newDirectory();
+    IndexWriter writer = new IndexWriter(dir, newIndexWriterConfig(TEST_VERSION_CURRENT, null));
+    writer.commit(); // first commit to complete IW create transaction.
+    
+    // this should store the commit data, even though no other changes were made
+    writer.setCommitData(new HashMap<String,String>() {{
+      put("key", "value");
+    }});
+    writer.commit();
+    
+    DirectoryReader r = DirectoryReader.open(dir);
+    assertEquals("value", r.getIndexCommit().getUserData().get("key"));
+
+    writer.close();
+    r.close();
+    dir.close();
+  }
+  
 }
Index: lucene/facet/src/java/org/apache/lucene/facet/taxonomy/directory/DirectoryTaxonomyWriter.java
===================================================================
--- lucene/facet/src/java/org/apache/lucene/facet/taxonomy/directory/DirectoryTaxonomyWriter.java	(revision 1415054)
+++ lucene/facet/src/java/org/apache/lucene/facet/taxonomy/directory/DirectoryTaxonomyWriter.java	(working copy)
@@ -129,6 +129,8 @@
   private volatile ParentArray parentArray;
   private volatile int nextID;
 
+  private Map<String,String> commitData;
+  
   /** Reads the commit data from a Directory. */
   private static Map<String, String> readCommitData(Directory dir) throws IOException {
     SegmentInfos infos = new SegmentInfos();
@@ -353,7 +355,7 @@
   @Override
   public synchronized void close() throws IOException {
     if (!isClosed) {
-      indexWriter.commit(combinedCommitData(null));
+      indexWriter.commit(combinedCommitData());
       doClose();
     }
   }
@@ -668,17 +670,16 @@
    */ 
   @Override
   public synchronized void commit() throws IOException {
-    ensureOpen();
-    indexWriter.commit(combinedCommitData(null));
+    commit(null);
   }
 
   /**
    * Combine original user data with that of the taxonomy creation time
    */
-  private Map<String,String> combinedCommitData(Map<String,String> userData) {
+  private Map<String,String> combinedCommitData() {
     Map<String,String> m = new HashMap<String, String>();
-    if (userData != null) {
-      m.putAll(userData);
+    if (commitData != null) {
+      m.putAll(commitData);
     }
     m.put(INDEX_EPOCH, Long.toString(indexEpoch));
     return m;
@@ -692,7 +693,8 @@
   @Override
   public synchronized void commit(Map<String,String> commitUserData) throws IOException {
     ensureOpen();
-    indexWriter.commit(combinedCommitData(commitUserData));
+    this.commitData = commitUserData;
+    indexWriter.commit(combinedCommitData());
   }
   
   /**
@@ -701,8 +703,7 @@
    */
   @Override
   public synchronized void prepareCommit() throws IOException {
-    ensureOpen();
-    indexWriter.prepareCommit(combinedCommitData(null));
+    prepareCommit(null);
   }
 
   /**
@@ -712,7 +713,8 @@
   @Override
   public synchronized void prepareCommit(Map<String,String> commitUserData) throws IOException {
     ensureOpen();
-    indexWriter.prepareCommit(combinedCommitData(commitUserData));
+    this.commitData = commitUserData;
+    indexWriter.prepareCommit(combinedCommitData());
   }
   
   @Override