docs/attachments/LUCENE-8508/LUCENE-8508.patch

diff --git a/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java b/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
index 5affd85..93672f4 100644
--- a/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
+++ b/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
@@ -781,7 +781,7 @@ public class IndexWriter implements Closeable, TwoPhaseCommit, Accountable,
         // against an index that's currently open for
         // searching.  In this case we write the next
         // segments_N file with no segments:
-        final SegmentInfos sis = new SegmentInfos(Version.LATEST.major);
+        final SegmentInfos sis = new SegmentInfos(config.getIndexCreatedVersionMajor());
         if (indexExists) {
           final SegmentInfos previous = SegmentInfos.readLatestCommit(directory);
           sis.updateGenerationVersionAndCounter(previous);
diff --git a/lucene/core/src/java/org/apache/lucene/index/IndexWriterConfig.java b/lucene/core/src/java/org/apache/lucene/index/IndexWriterConfig.java
index d657d52..a72aeab 100644
--- a/lucene/core/src/java/org/apache/lucene/index/IndexWriterConfig.java
+++ b/lucene/core/src/java/org/apache/lucene/index/IndexWriterConfig.java
@@ -33,6 +33,7 @@ import org.apache.lucene.search.similarities.Similarity;
 import org.apache.lucene.util.InfoStream;
 import org.apache.lucene.util.PrintStreamInfoStream;
 import org.apache.lucene.util.SetOnce.AlreadySetException;
+import org.apache.lucene.util.Version;
 import org.apache.lucene.util.SetOnce;
 
 /**
@@ -173,6 +174,35 @@ public final class IndexWriterConfig extends LiveIndexWriterConfig {
   }
 
   /**
+   * Expert: set the compatibility version to use for this index. In case the
+   * index is created, it will use the given major version for compatibility.
+   * It is sometimes useful to set the previous major version for compatibility
+   * due to the fact that {@link IndexWriter#addIndexes} only accepts indices
+   * that have been written with the same major version as the current index.
+   * If the index already exists, then this value is ignored.
+   * Default value is the {@link Version#major major} of the
+   * {@link Version#LATEST latest version}.
+   * <p><b>NOTE</b>: Changing the creation version reduces backward
+   * compatibility guarantees. For instance an index created with Lucene 8 with
+   * a compatibility version of 7 can't be read with Lucene 9 due to the fact
+   * that Lucene only supports reading indices created with the current or
+   * previous major release.
+   * @param indexCreatedVersionMajor the major version to use for compatibility
+   */
+  public IndexWriterConfig setIndexCreatedVersionMajor(int indexCreatedVersionMajor) {
+    if (indexCreatedVersionMajor > Version.LATEST.major) {
+      throw new IllegalArgumentException("indexCreatedVersionMajor may not be in the future: current major version is " +
+          Version.LATEST.major + ", but got: " + indexCreatedVersionMajor);
+    }
+    if (indexCreatedVersionMajor < Version.LATEST.major - 1) {
+      throw new IllegalArgumentException("indexCreatedVersionMajor may not be less than the minimum supported version: " +
+          (Version.LATEST.major-1) + ", but got: " + indexCreatedVersionMajor);
+    }
+    this.createdVersionMajor = indexCreatedVersionMajor;
+    return this;
+  }
+
+  /**
    * Expert: allows an optional {@link IndexDeletionPolicy} implementation to be
    * specified. You can use this to control when prior commits are deleted from
    * the index. The default policy is {@link KeepOnlyLastCommitDeletionPolicy}
diff --git a/lucene/core/src/java/org/apache/lucene/index/LiveIndexWriterConfig.java b/lucene/core/src/java/org/apache/lucene/index/LiveIndexWriterConfig.java
index 016e880..259a020 100644
--- a/lucene/core/src/java/org/apache/lucene/index/LiveIndexWriterConfig.java
+++ b/lucene/core/src/java/org/apache/lucene/index/LiveIndexWriterConfig.java
@@ -29,6 +29,7 @@ import org.apache.lucene.search.IndexSearcher;
 import org.apache.lucene.search.Sort;
 import org.apache.lucene.search.similarities.Similarity;
 import org.apache.lucene.util.InfoStream;
+import org.apache.lucene.util.Version;
 
 /**
  * Holds all the configuration used by {@link IndexWriter} with few setters for
@@ -57,6 +58,9 @@ public class LiveIndexWriterConfig {
    *  with. */
   protected volatile OpenMode openMode;
 
+  /** Compatibility version to use for this index. */
+  protected int createdVersionMajor = Version.LATEST.major;
+
   /** {@link Similarity} to use when encoding norms. */
   protected volatile Similarity similarity;
 
@@ -285,7 +289,15 @@ public class LiveIndexWriterConfig {
   public OpenMode getOpenMode() {
     return openMode;
   }
-  
+
+  /**
+   * Return the compatibility version to use for this index.
+   * @see IndexWriterConfig#setIndexCreatedVersionMajor
+   */
+  public int getIndexCreatedVersionMajor() {
+    return createdVersionMajor;
+  }
+
   /**
    * Returns the {@link IndexDeletionPolicy} specified in
    * {@link IndexWriterConfig#setIndexDeletionPolicy(IndexDeletionPolicy)} or
diff --git a/lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java b/lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java
index 08b3dc1..20d3532 100644
--- a/lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java
+++ b/lucene/core/src/test/org/apache/lucene/index/TestIndexWriter.java
@@ -3601,4 +3601,35 @@ public class TestIndexWriter extends LuceneTestCase {
       }
     }
   }
+
+  public void testSetIndexCreatedVersion() throws IOException {
+    IllegalArgumentException e = expectThrows(IllegalArgumentException.class,
+        () -> new IndexWriterConfig().setIndexCreatedVersionMajor(Version.LATEST.major+1));
+    assertEquals("indexCreatedVersionMajor may not be in the future: current major version is " +
+        Version.LATEST.major + ", but got: " + (Version.LATEST.major+1), e.getMessage());
+    e = expectThrows(IllegalArgumentException.class,
+        () -> new IndexWriterConfig().setIndexCreatedVersionMajor(Version.LATEST.major-2));
+    assertEquals("indexCreatedVersionMajor may not be less than the minimum supported version: " +
+        (Version.LATEST.major-1) + ", but got: " + (Version.LATEST.major-2), e.getMessage());
+
+    for (int previousMajor = Version.LATEST.major - 1; previousMajor <= Version.LATEST.major; previousMajor++) {
+      for (int newMajor = Version.LATEST.major - 1; newMajor <= Version.LATEST.major; newMajor++) {
+        for (OpenMode openMode : OpenMode.values()) {
+          try (Directory dir = newDirectory()) {
+            try (IndexWriter w = new IndexWriter(dir, newIndexWriterConfig().setIndexCreatedVersionMajor(previousMajor))) {}
+            SegmentInfos infos = SegmentInfos.readLatestCommit(dir);
+            assertEquals(previousMajor, infos.getIndexCreatedVersionMajor());
+            try (IndexWriter w = new IndexWriter(dir, newIndexWriterConfig().setOpenMode(openMode).setIndexCreatedVersionMajor(newMajor))) {}
+            infos = SegmentInfos.readLatestCommit(dir);
+            if (openMode == OpenMode.CREATE) {
+              assertEquals(newMajor, infos.getIndexCreatedVersionMajor());
+            } else {
+              assertEquals(previousMajor, infos.getIndexCreatedVersionMajor());
+            }
+          }
+        }
+      }
+    }
+  }
+
 }