[ENGINE] Wait until engine is started up when acquireing searcher

Today we have a small window where a searcher can be acquired but the
engine is in the state of starting up. This causes a NPE triggering a
shard failure if we are fast enough. This commit fixes this situation
gracefully.

Closes #7455

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

@@ -689,12 +689,23 @@ public class InternalEngine extends AbstractIndexShardComponent implements Engin
     @Override
     public final Searcher acquireSearcher(String source) throws EngineException {
         boolean success = false;
+         /* Acquire order here is store -> manager since we need
+          * to make sure that the store is not closed before
+          * the searcher is acquired. */
+        store.incRef();
         try {
-            /* Acquire order here is store -> manager since we need
-            * to make sure that the store is not closed before
-            * the searcher is acquired. */
-            store.incRef();
-            final SearcherManager manager = this.searcherManager;
+            SearcherManager manager = this.searcherManager;
+            if (manager == null) {
+                ensureOpen();
+                try (InternalLock _ = this.readLock.acquire()) {
+                    // we might start up right now and the searcherManager is not initialized
+                    // we take the read lock and retry again since write lock is taken
+                    // while start() is called and otherwise the ensureOpen() call will
+                    // barf.
+                    manager = this.searcherManager;
+                    assert manager != null : "SearcherManager is null but shouldn't";
+                }
+            }
             /* This might throw NPE but that's fine we will run ensureOpen()
             *  in the catch block and throw the right exception */
             final IndexSearcher searcher = manager.acquire();
@@ -707,6 +718,8 @@ public class InternalEngine extends AbstractIndexShardComponent implements Engin
                     manager.release(searcher);
                 }
             }
+        } catch (EngineClosedException ex) {
+            throw ex;
         } catch (Throwable ex) {
             ensureOpen(); // throw EngineCloseException here if we are already closed
             logger.error("failed to acquire searcher, source {}", ex, source);

src/test/java/org/elasticsearch/index/engine/internal/InternalEngineTests.java

@@ -31,6 +31,7 @@ import org.apache.lucene.index.CorruptIndexException;
 import org.apache.lucene.index.IndexDeletionPolicy;
 import org.apache.lucene.index.Term;
 import org.apache.lucene.search.TermQuery;
+import org.apache.lucene.store.AlreadyClosedException;
 import org.apache.lucene.util.LuceneTestCase.Slow;
 import org.elasticsearch.ExceptionsHelper;
 import org.elasticsearch.common.bytes.BytesArray;
@@ -80,6 +81,7 @@ import java.io.IOException;
 import java.util.Arrays;
 import java.util.List;
 import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 
 import static org.elasticsearch.common.settings.ImmutableSettings.Builder.EMPTY_SETTINGS;
@@ -321,6 +323,33 @@ public class InternalEngineTests extends ElasticsearchTestCase {
         assertThat(segments.get(2).isCompound(), equalTo(true));
     }
 
+    public void testStartAndAcquireConcurrently() {
+        ConcurrentMergeSchedulerProvider mergeSchedulerProvider = new ConcurrentMergeSchedulerProvider(shardId, EMPTY_SETTINGS, threadPool, new IndexSettingsService(shardId.index(), EMPTY_SETTINGS));
+        final Engine engine = createEngine(engineSettingsService, store, createTranslog(), mergeSchedulerProvider);
+        final AtomicBoolean startPending = new AtomicBoolean(true);
+        Thread thread = new Thread() {
+            public void run() {
+                try {
+                    Thread.yield();
+                    engine.start();
+                } finally {
+                    startPending.set(false);
+                }
+
+            }
+        };
+        thread.start();
+        while(startPending.get()) {
+            try {
+                engine.acquireSearcher("foobar").close();
+                break;
+            } catch (EngineClosedException ex) {
+                // all good
+            }
+        }
+        engine.close();
+    }
+
 
     @Test
     public void testSegmentsWithMergeFlag() throws Exception {