Index: CHANGES.txt
===================================================================
--- CHANGES.txt	(revision 833886)
+++ CHANGES.txt	(working copy)
@@ -188,6 +188,10 @@
    infoStream on IndexWriter and then add an empty document and commit
    (Shai Erera via Mike McCandless)
 
+ * LUCENE-2046: IndexReader should not see the index as changed, after
+   IndexWriter.prepareCommit has been called but before
+   IndexWriter.commit is called. (Peter Keegan via Mike McCandless)
+
 API Changes
 
  * Un-deprecate search(Weight weight, Filter filter, int n) from
Index: src/test/org/apache/lucene/index/TestIndexReader.java
===================================================================
--- src/test/org/apache/lucene/index/TestIndexReader.java	(revision 833884)
+++ src/test/org/apache/lucene/index/TestIndexReader.java	(working copy)
@@ -1778,4 +1778,24 @@
     r2.close();
     dir.close();
   }
+
+  // LUCENE-2046
+  public void testPrepareCommitIsCurrent() throws Throwable {
+    Directory dir = new MockRAMDirectory();
+    IndexWriter writer = new IndexWriter(dir, new WhitespaceAnalyzer(), IndexWriter.MaxFieldLength.UNLIMITED);
+    Document doc = new Document();
+    writer.addDocument(doc);
+    IndexReader r = IndexReader.open(dir, true);
+    assertTrue(r.isCurrent());
+    writer.addDocument(doc);
+    writer.prepareCommit();
+    assertTrue(r.isCurrent());
+    IndexReader r2 = r.reopen();
+    assertTrue(r == r2);
+    writer.commit();
+    assertFalse(r.isCurrent());
+    writer.close();
+    r.close();
+    dir.close();
+  }
 }
Index: src/java/org/apache/lucene/index/SegmentInfos.java
===================================================================
--- src/java/org/apache/lucene/index/SegmentInfos.java	(revision 833884)
+++ src/java/org/apache/lucene/index/SegmentInfos.java	(working copy)
@@ -402,36 +402,14 @@
   public static long readCurrentVersion(Directory directory)
     throws CorruptIndexException, IOException {
 
-    return ((Long) new FindSegmentsFile(directory) {
-        @Override
-        protected Object doBody(String segmentFileName) throws CorruptIndexException, IOException {
-
-          IndexInput input = directory.openInput(segmentFileName);
-
-          int format = 0;
-          long version = 0;
-          try {
-            format = input.readInt();
-            if(format < 0){
-              if (format < CURRENT_FORMAT)
-                throw new CorruptIndexException("Unknown format version: " + format);
-              version = input.readLong(); // read version
-            }
-          }
-          finally {
-            input.close();
-          }
-     
-          if(format < 0)
-            return Long.valueOf(version);
-
-          // We cannot be sure about the format of the file.
-          // Therefore we have to read the whole file and cannot simply seek to the version entry.
-          SegmentInfos sis = new SegmentInfos();
-          sis.read(directory, segmentFileName);
-          return Long.valueOf(sis.getVersion());
-        }
-      }.run()).longValue();
+    // Fully read the segments file: this ensures that it's
+    // completely written so that if
+    // IndexWriter.prepareCommit has been called (but not
+    // yet commit), then the reader will still see itself as
+    // current:
+    SegmentInfos sis = new SegmentInfos();
+    sis.read(directory);
+    return sis.version;
   }
 
   /**