Index: lucene/CHANGES.txt
===================================================================
--- lucene/CHANGES.txt	(revision 1061498)
+++ lucene/CHANGES.txt	(working copy)
@@ -464,6 +464,9 @@
 * LUCENE-2864: Add getMaxTermFrequency (maximum within-document TF) to 
   FieldInvertState so that it can be used in Similarity.computeNorm.
   (Robert Muir)
+
+* LUCENE-2720: Segments now record the code version which wrote them. 
+  (Shai Erera, Mike McCandless, Uwe Schindler)
   
 Optimizations
 
Index: lucene/src/java/org/apache/lucene/index/FieldsReader.java
===================================================================
--- lucene/src/java/org/apache/lucene/index/FieldsReader.java	(revision 1061498)
+++ lucene/src/java/org/apache/lucene/index/FieldsReader.java	(working copy)
@@ -75,6 +75,26 @@
     ensureOpen();
     return new FieldsReader(fieldInfos, numTotalDocs, size, format, formatSize, docStoreOffset, cloneableFieldsStream, cloneableIndexStream);
   }
+
+  /**
+   * Detects the code version this segment was written with. Returns either
+   * "2.x" for all pre-3.0 segments, or "3.0" for 3.0 segments. This method
+   * should not be called for 3.1+ segments since they already record their code
+   * version.
+   */
+  static String detectCodeVersion(Directory dir, String segment) throws IOException {
+    IndexInput idxStream = dir.openInput(IndexFileNames.segmentFileName(segment, IndexFileNames.FIELDS_INDEX_EXTENSION), 1024);
+    try {
+      int format = idxStream.readInt();
+      if (format < FieldsWriter.FORMAT_LUCENE_3_0_NO_COMPRESSED_FIELDS) {
+        return "2.x";
+      } else {
+        return "3.0";
+      }
+    } finally {
+      idxStream.close();
+    }
+  }
   
   // Used only by clone
   private FieldsReader(FieldInfos fieldInfos, int numTotalDocs, int size, int format, int formatSize,
Index: lucene/src/java/org/apache/lucene/index/SegmentInfo.java
===================================================================
--- lucene/src/java/org/apache/lucene/index/SegmentInfo.java	(revision 1061498)
+++ lucene/src/java/org/apache/lucene/index/SegmentInfo.java	(working copy)
@@ -21,6 +21,8 @@
 import org.apache.lucene.store.IndexOutput;
 import org.apache.lucene.store.IndexInput;
 import org.apache.lucene.util.BitVector;
+import org.apache.lucene.util.Constants;
+
 import java.io.IOException;
 import java.util.HashSet;
 import java.util.List;
@@ -95,6 +97,9 @@
 
   private Map<String,String> diagnostics;
 
+  // Tracks the Lucene version this segment was created with. Since 3.1
+  private String version;
+  
   public SegmentInfo(String name, int docCount, Directory dir, boolean isCompoundFile, boolean hasSingleNormFile,
                      boolean hasProx, boolean hasVectors) { 
     this.name = name;
@@ -108,6 +113,7 @@
     delCount = 0;
     this.hasProx = hasProx;
     this.hasVectors = hasVectors;
+    this.version = Constants.LUCENE_MAIN_VERSION;
   }
 
   /**
@@ -115,6 +121,7 @@
    */
   void reset(SegmentInfo src) {
     clearFiles();
+    version = src.version;
     name = src.name;
     docCount = src.docCount;
     dir = src.dir;
@@ -153,6 +160,9 @@
    */
   SegmentInfo(Directory dir, int format, IndexInput input) throws IOException {
     this.dir = dir;
+    if (format <= SegmentInfos.FORMAT_3_1) {
+      version = input.readString();
+    }
     name = input.readString();
     docCount = input.readInt();
     if (format <= SegmentInfos.FORMAT_LOCKLESS) {
@@ -361,6 +371,7 @@
     if (normGen != null) {
       si.normGen = normGen.clone();
     }
+    si.version = version;
     return si;
   }
 
@@ -572,6 +583,8 @@
   void write(IndexOutput output)
     throws IOException {
     assert delCount <= docCount: "delCount=" + delCount + " docCount=" + docCount + " segment=" + name;
+    // Write the Lucene version that created this segment, since 3.1
+    output.writeString(version); 
     output.writeString(name);
     output.writeInt(docCount);
     output.writeLong(delGen);
@@ -743,7 +756,7 @@
   public String toString(Directory dir, int pendingDelCount) {
 
     StringBuilder s = new StringBuilder();
-    s.append(name).append(':');
+    s.append(name).append('(').append(version == null ? "?" : version).append(')').append(':');
 
     char cfs;
     try {
@@ -813,4 +826,24 @@
   public int hashCode() {
     return dir.hashCode() + name.hashCode();
   }
+
+  /**
+   * Used by SegmentInfos to upgrade segments that do not record their code
+   * version (either "2.x" or "3.0").
+   * <p>
+   * <b>NOTE:</b> this method is used for internal purposes only - you should
+   * not modify the version of a SegmentInfo, or it may result in unexpected
+   * exceptions thrown when you attempt to open the index.
+   * 
+   * @lucene.internal
+   */
+  void setVersion(String version) {
+    this.version = version;
+  }
+  
+  /** Returns the version of the code which wrote the segment. */
+  public String getVersion() {
+    return version;
+  }
+  
 }
Index: lucene/src/java/org/apache/lucene/index/SegmentInfos.java
===================================================================
--- lucene/src/java/org/apache/lucene/index/SegmentInfos.java	(revision 1061498)
+++ lucene/src/java/org/apache/lucene/index/SegmentInfos.java	(working copy)
@@ -92,8 +92,11 @@
   /** Each segment records whether it has term vectors */
   public static final int FORMAT_HAS_VECTORS = -10;
 
+  /** Each segment records the Lucene version that created it. */
+  public static final int FORMAT_3_1 = -11;
+  
   /* This must always point to the most recent file format. */
-  public static final int CURRENT_FORMAT = FORMAT_HAS_VECTORS;
+  public static final int CURRENT_FORMAT = FORMAT_3_1;
   
   public int counter = 0;    // used to name new segments
   /**
@@ -267,7 +270,30 @@
       }
       
       for (int i = input.readInt(); i > 0; i--) { // read segmentInfos
-        add(new SegmentInfo(directory, format, input));
+        SegmentInfo si = new SegmentInfo(directory, format, input);
+        if (si.getVersion() == null) {
+          // It's a pre-3.1 segment, upgrade its version to either 3.0 or 2.x
+          Directory dir = directory;
+          if (si.getDocStoreOffset() != -1) {
+            if (si.getDocStoreIsCompoundFile()) {
+              dir = new CompoundFileReader(dir, IndexFileNames.segmentFileName(
+                  si.getDocStoreSegment(),
+                  IndexFileNames.COMPOUND_FILE_STORE_EXTENSION), 1024);
+            }
+          } else if (si.getUseCompoundFile()) {
+            dir = new CompoundFileReader(dir, IndexFileNames.segmentFileName(
+                si.name, IndexFileNames.COMPOUND_FILE_EXTENSION), 1024);
+          }
+
+          try {
+            String store = si.getDocStoreOffset() != -1 ? si.getDocStoreSegment() : si.name;
+            si.setVersion(FieldsReader.detectCodeVersion(dir, store));
+          } finally {
+            // If we opened the directory, close it
+            if (dir != directory) dir.close();
+          }
+        }
+        add(si);
       }
       
       if(format >= 0){    // in old format the version number may be at the end of the file
@@ -351,8 +377,8 @@
       segnOutput.writeLong(version); 
       segnOutput.writeInt(counter); // write counter
       segnOutput.writeInt(size()); // write infos
-      for (int i = 0; i < size(); i++) {
-        info(i).write(segnOutput);
+      for (SegmentInfo si : this) {
+        si.write(segnOutput);
       }
       segnOutput.writeStringStringMap(userData);
       segnOutput.prepareCommit();
Index: lucene/src/java/org/apache/lucene/util/Constants.java
===================================================================
--- lucene/src/java/org/apache/lucene/util/Constants.java	(revision 1061498)
+++ lucene/src/java/org/apache/lucene/util/Constants.java	(working copy)
@@ -70,6 +70,9 @@
     return s.toString();
   }
   
+  // NOTE: we track per-segment version as a String with the "X.Y" format, e.g.
+  // "4.0", "3.1", "3.0". Therefore when we change this constant, we should keep
+  // the format.
   public static final String LUCENE_MAIN_VERSION = ident("3.1");
 
   public static final String LUCENE_VERSION;