Index: lucene/CHANGES.txt
===================================================================
--- lucene/CHANGES.txt	(revision 1140705)
+++ lucene/CHANGES.txt	(working copy)
@@ -10,6 +10,10 @@
 * LUCENE-3251: Directory#copy failed to close target output if opening the
   source stream failed. (Simon Willnauer)
 
+* LUCENE-3255: If segments_N file is all zeros (due to file
+  corruption), don't read that to mean the index is empty.  (Gregory
+  Tarr, Mark Harwood, Simon Willnauer, Mike McCandless)
+
 Optimizations
   
 * LUCENE-3201, LUCENE-3218: CompoundFileSystem code has been consolidated 
Index: lucene/src/test/org/apache/lucene/index/TestAllZerosSegmentsFile.java
===================================================================
--- lucene/src/test/org/apache/lucene/index/TestAllZerosSegmentsFile.java	(revision 0)
+++ lucene/src/test/org/apache/lucene/index/TestAllZerosSegmentsFile.java	(revision 0)
@@ -0,0 +1,47 @@
+package org.apache.lucene.index;
+
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import org.apache.lucene.document.Document;
+import org.apache.lucene.store.Directory;
+import org.apache.lucene.store.IndexOutput;
+import org.apache.lucene.util.LuceneTestCase;
+
+public class TestAllZerosSegmentsFile extends LuceneTestCase {
+
+  public void test() throws Exception {
+    Directory dir = newDirectory();
+    RandomIndexWriter w = new RandomIndexWriter(random, dir);
+    w.addDocument(new Document());
+    w.close();
+
+    String nextSegmentsFile = IndexFileNames.fileNameFromGeneration(IndexFileNames.SEGMENTS,
+                                                                    "",
+                                                                    SegmentInfos.getCurrentSegmentGeneration(dir)+1);
+    IndexOutput out = dir.createOutput(nextSegmentsFile);
+    for(int idx=0;idx<8;idx++) {
+      out.writeByte((byte) 0);
+    }
+    out.close();
+
+    IndexReader r= IndexReader.open(dir,true);
+    assertEquals(r.numDocs(), 1);
+    r.close();
+    dir.close();
+  }
+}

Property changes on: lucene/src/test/org/apache/lucene/index/TestAllZerosSegmentsFile.java
___________________________________________________________________
Added: svn:eol-style
   + native

Index: lucene/src/java/org/apache/lucene/index/SegmentInfos.java
===================================================================
--- lucene/src/java/org/apache/lucene/index/SegmentInfos.java	(revision 1140705)
+++ lucene/src/java/org/apache/lucene/index/SegmentInfos.java	(working copy)
@@ -101,6 +101,9 @@
   
   /* This must always point to the most recent file format. */
   public static final int CURRENT_FORMAT = FORMAT_3_1;
+
+  public static final int FORMAT_MINIMUM = FORMAT;
+  public static final int FORMAT_MAXIMUM = CURRENT_FORMAT;
   
   public int counter = 0;    // used to name new segments
   /**
@@ -267,16 +270,17 @@
 
     try {
       int format = input.readInt();
-      if(format < 0){     // file contains explicit format info
-        // check that it is a format we can understand
-        if (format < CURRENT_FORMAT)
-          throw new CorruptIndexException("Unknown format version: " + format);
-        version = input.readLong(); // read version
-        counter = input.readInt(); // read counter
+      // check that it is a format we can understand
+      if (format > FORMAT_MINIMUM) {
+        throw new IndexFormatTooOldException(segmentFileName, format,
+          FORMAT_MINIMUM, FORMAT_MAXIMUM);
       }
-      else{     // file is in old format without explicit format info
-        counter = format;
+      if (format < FORMAT_MAXIMUM) {
+        throw new IndexFormatTooNewException(segmentFileName, format,
+          FORMAT_MINIMUM, FORMAT_MAXIMUM);
       }
+      version = input.readLong(); // read version
+      counter = input.readInt(); // read counter
       
       for (int i = input.readInt(); i > 0; i--) { // read segmentInfos
         SegmentInfo si = new SegmentInfo(directory, format, input);