Use the smallest version rather than the default version

src/main/java/org/elasticsearch/index/shard/IndexShard.java

@@ -756,13 +756,13 @@ public org.apache.lucene.util.Version upgrade(UpgradeRequest upgrade) {
     }
 
     public org.apache.lucene.util.Version minimumCompatibleVersion() {
-        org.apache.lucene.util.Version luceneVersion = org.apache.lucene.util.Version.LUCENE_3_6;
+        org.apache.lucene.util.Version luceneVersion = null;
         for(Segment segment : engine().segments()) {
-            if (luceneVersion.onOrAfter(segment.getVersion())) {
+            if (luceneVersion == null || luceneVersion.onOrAfter(segment.getVersion())) {
                 luceneVersion = segment.getVersion();
             }
         }
-        return luceneVersion;
+        return luceneVersion == null ?  Version.indexCreated(indexSettings).luceneVersion : luceneVersion;
     }
 
     public SnapshotIndexCommit snapshotIndex() throws EngineException {

src/test/java/org/elasticsearch/index/shard/IndexShardTests.java

@@ -37,6 +37,7 @@
 
 import static org.elasticsearch.cluster.metadata.IndexMetaData.SETTING_NUMBER_OF_REPLICAS;
 import static org.elasticsearch.cluster.metadata.IndexMetaData.SETTING_NUMBER_OF_SHARDS;
+import static org.elasticsearch.cluster.metadata.IndexMetaData.SETTING_VERSION_CREATED;
 import static org.elasticsearch.common.settings.ImmutableSettings.settingsBuilder;
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAcked;
 import static org.hamcrest.Matchers.equalTo;
@@ -148,5 +149,19 @@ public void testDeleteByQueryBWC() {
             assertEquals(numDocs, searcher.reader().numDocs());
         }
     }
-
+
+    public void testMinimumCompatVersion() {
+        Version versionCreated = randomVersion();
+        assertAcked(client().admin().indices().prepareCreate("test")
+                .setSettings(SETTING_NUMBER_OF_SHARDS, 1, SETTING_NUMBER_OF_REPLICAS, 0, SETTING_VERSION_CREATED, versionCreated.id));
+        client().prepareIndex("test", "test").setSource("{}").get();
+        ensureGreen("test");
+        IndicesService indicesService = getInstanceFromNode(IndicesService.class);
+        IndexShard test = indicesService.indexService("test").shard(0);
+        assertEquals(versionCreated.luceneVersion, test.minimumCompatibleVersion());
+        client().prepareIndex("test", "test").setSource("{}").get();
+        assertEquals(versionCreated.luceneVersion, test.minimumCompatibleVersion());
+        test.engine().flush();
+        assertEquals(Version.CURRENT.luceneVersion, test.minimumCompatibleVersion());
+    }
 }

src/test/java/org/elasticsearch/rest/action/admin/indices/upgrade/UpgradeReallyOldIndexTest.java

@@ -19,7 +19,11 @@
 
 package org.elasticsearch.rest.action.admin.indices.upgrade;
 
+import org.elasticsearch.Version;
 import org.elasticsearch.bwcompat.StaticIndexBackwardCompatibilityTest;
+import org.elasticsearch.cluster.metadata.IndexMetaData;
+import org.elasticsearch.index.IndexService;
+import org.elasticsearch.indices.IndicesService;
 
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertNoFailures;
 
@@ -28,20 +32,23 @@ public class UpgradeReallyOldIndexTest extends StaticIndexBackwardCompatibilityT
     public void testUpgrade_0_20() throws Exception {
         String indexName = "test";
         loadIndex("index-0.20.zip", indexName);
-
+        assertMinVersion(indexName, org.apache.lucene.util.Version.parse("3.6.2"));
         assertTrue(UpgradeTest.hasAncientSegments(client(), indexName));
         UpgradeTest.assertNotUpgraded(client(), indexName);
         assertNoFailures(client().admin().indices().prepareUpgrade(indexName).setUpgradeOnlyAncientSegments(true).get());
         assertFalse(UpgradeTest.hasAncientSegments(client(), indexName));
 
         // This index has entirely ancient segments so the whole index should now be upgraded:
         UpgradeTest.assertUpgraded(client(), indexName);
+        assertEquals(Version.CURRENT.luceneVersion.toString(), client().admin().indices().prepareGetSettings(indexName).get().getSetting(indexName, IndexMetaData.SETTING_VERSION_MINIMUM_COMPATIBLE));
+        assertMinVersion(indexName, Version.CURRENT.luceneVersion);
+
     }
 
     public void testUpgradeMixed_0_20_6_and_0_90_6() throws Exception {
         String indexName = "index-0.20.6-and-0.90.6";
         loadIndex(indexName + ".zip", indexName);
-
+        assertMinVersion(indexName, org.apache.lucene.util.Version.parse("3.6.2"));
         // Has ancient segments?:
         assertTrue(UpgradeTest.hasAncientSegments(client(), indexName));
 
@@ -59,5 +66,18 @@ public void testUpgradeMixed_0_20_6_and_0_90_6() throws Exception {
 
         // We succeeded in upgrading only the ancient segments but leaving the "merely old" ones untouched:
         assertTrue(UpgradeTest.hasOldButNotAncientSegments(client(), indexName));
+        assertEquals(org.apache.lucene.util.Version.LUCENE_4_5_1.toString(), client().admin().indices().prepareGetSettings(indexName).get().getSetting(indexName, IndexMetaData.SETTING_VERSION_MINIMUM_COMPATIBLE));
+        assertMinVersion(indexName, org.apache.lucene.util.Version.LUCENE_4_5_1);
+
+    }
+
+    private void assertMinVersion(String index, org.apache.lucene.util.Version version) {
+        for (IndicesService services : internalCluster().getInstances(IndicesService.class)) {
+            IndexService indexService = services.indexService(index);
+            if (indexService != null) {
+                assertEquals(version, indexService.shard(0).minimumCompatibleVersion());
+            }
+        }
+
     }
 }