Only flush for checkindex if we have uncommitted changes

Today we force a flush before check index to ensure we have an index
to check on. Yet if the index is large and the FS is slow this can have
significant impact on the index deletion performance. This commit introduces
a check if there are any uncommitted changes in order to skip the additional commit.

Closes #10505

src/main/java/org/elasticsearch/index/engine/Engine.java

@@ -1061,4 +1061,10 @@ public abstract class Engine implements Closeable {
             }
         }
     }
+
+    /**
+     * Returns <code>true</code> the internal writer has any uncommitted changes. Otherwise <code>false</code>
+     * @return
+     */
+    public abstract boolean hasUncommittedChanges();
 }

src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -940,6 +940,11 @@ public class InternalEngine extends Engine {
         }
     }
 
+    @Override
+    public boolean hasUncommittedChanges() {
+        return indexWriter.hasUncommittedChanges();
+    }
+
     @Override
     protected SearcherManager getSearcherManager() {
         return searcherManager;

src/main/java/org/elasticsearch/index/engine/ShadowEngine.java

@@ -216,4 +216,9 @@ public class ShadowEngine extends Engine {
             }
         }
     }
+
+    @Override
+    public boolean hasUncommittedChanges() {
+        return false;
+    }
 }

src/test/java/org/elasticsearch/bwcompat/OldIndexBackwardsCompatibilityTests.java

@@ -158,7 +158,6 @@ public class OldIndexBackwardsCompatibilityTests extends ElasticsearchIntegratio
     }
 
     void unloadIndex(String indexName) throws Exception {
-        client().admin().indices().prepareFlush(indexName).setWaitIfOngoing(true).setForce(true).get(); // temporary for debugging
         ElasticsearchAssertions.assertAcked(client().admin().indices().prepareDelete(indexName).get());
         ElasticsearchAssertions.assertAllFilesClosed();
     }
@@ -201,20 +200,6 @@ public class OldIndexBackwardsCompatibilityTests extends ElasticsearchIntegratio
 
         Collections.shuffle(indexes, getRandom());
         for (String index : indexes) {
-            if (index.equals("index-0.90.13.zip") == false) {
-                long startTime = System.currentTimeMillis();
-                logger.info("--> Testing old index " + index);
-                assertOldIndexWorks(index);
-                logger.info("--> Done testing " + index + ", took " + ((System.currentTimeMillis() - startTime) / 1000.0) + " seconds");
-            }
-        }
-    }
-
-    @TestLogging("test.engine:TRACE,index.engine:TRACE,test.engine.lucene:TRACE,index.engine.lucene:TRACE")
-    public void testShitSlowIndex() throws Exception {
-        setupCluster();
-        for (int i = 0; i < 5; i++) {
-            String index = "index-0.90.13.zip";
             long startTime = System.currentTimeMillis();
             logger.info("--> Testing old index " + index);
             assertOldIndexWorks(index);

src/test/java/org/elasticsearch/test/store/MockFSDirectoryService.java

@@ -26,17 +26,13 @@ import org.apache.lucene.store.Directory;
 import org.apache.lucene.store.LockFactory;
 import org.apache.lucene.store.StoreRateLimiting;
 import org.apache.lucene.util.AbstractRandomizedTest;
-import org.apache.lucene.util.IOUtils;
 import org.elasticsearch.cluster.metadata.IndexMetaData;
 import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.inject.Inject;
 import org.elasticsearch.common.io.stream.BytesStreamOutput;
-import org.elasticsearch.common.lease.Releasable;
 import org.elasticsearch.common.lease.Releasables;
 import org.elasticsearch.common.lucene.Lucene;
 import org.elasticsearch.common.settings.Settings;
-import org.elasticsearch.index.engine.Engine;
-import org.elasticsearch.index.engine.InternalEngine;
 import org.elasticsearch.index.settings.IndexSettings;
 import org.elasticsearch.index.shard.IndexShardException;
 import org.elasticsearch.index.shard.IndexShardState;
@@ -91,8 +87,10 @@ public class MockFSDirectoryService extends FsDirectoryService {
                             // When the the internal engine closes we do a rollback, which removes uncommitted segments
                             // By doing a commit flush we perform a Lucene commit, but don't clear the translog,
                             // so that even in tests where don't flush we can check the integrity of the Lucene index
-                            logger.info("{} flushing in order to run checkindex", indexShard.shardId());
-                            Releasables.close(indexShard.engine().snapshotIndex()); // Keep translog for tests that rely on replaying it
+                            if (indexShard.engine().hasUncommittedChanges()) { // only if we have any changes
+                                logger.info("{} flushing in order to run checkindex", indexShard.shardId());
+                                Releasables.close(indexShard.engine().snapshotIndex()); // Keep translog for tests that rely on replaying it
+                            }
                             logger.info("{} flush finished in beforeIndexShardClosed", indexShard.shardId());
                             canRun = true;
                         }