Add ssh command to restart online indexer

In case the online indexer fails, this command allows admin to restart
it without having to restart Gerrit.

Change-Id: I73435de62c357537d173c5eafd6ad60d3b49fc73

gerrit-lucene/src/main/java/com/google/gerrit/lucene/LuceneVersionManager.java

@@ -43,7 +43,7 @@ import java.util.List;
 import java.util.TreeMap;
 
 @Singleton
-class LuceneVersionManager implements LifecycleListener {
+public class LuceneVersionManager implements LifecycleListener {
   private static final Logger log = LoggerFactory
       .getLogger(LuceneVersionManager.class);
 
@@ -90,6 +90,7 @@ class LuceneVersionManager implements LifecycleListener {
   private final LuceneChangeIndex.Factory indexFactory;
   private final IndexCollection indexes;
   private final OnlineReindexer.Factory reindexerFactory;
+  private OnlineReindexer reindexer;
 
   @Inject
   LuceneVersionManager(
@@ -160,7 +161,37 @@ class LuceneVersionManager implements LifecycleListener {
 
     int latest = write.get(0).version;
     if (latest != search.version) {
-      reindexerFactory.create(latest).start();
+      reindexer = reindexerFactory.create(latest);
+      reindexer.start();
+    }
+  }
+
+  /**
+   * Start the online reindexer if the current index is not already the latest.
+   *
+   * @return true if started, otherwise false.
+   * @throws ReindexerAlreadyRunningException
+   */
+  public synchronized boolean startReindexer()
+      throws ReindexerAlreadyRunningException {
+    validateReindexerNotRunning();
+    if (!isCurrentIndexVersionLatest()) {
+      reindexer.start();
+      return true;
+    }
+    return false;
+  }
+
+  private boolean isCurrentIndexVersionLatest() {
+    return reindexer == null
+        || reindexer.getVersion() == indexes.getSearchIndex().getSchema()
+            .getVersion();
+  }
+
+  private void validateReindexerNotRunning()
+      throws ReindexerAlreadyRunningException {
+    if (reindexer != null && reindexer.isRunning()) {
+      throw new ReindexerAlreadyRunningException();
     }
   }
 

gerrit-lucene/src/main/java/com/google/gerrit/lucene/OnlineReindexer.java

@@ -29,6 +29,7 @@ import org.slf4j.LoggerFactory;
 
 import java.io.IOException;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicBoolean;
 
 public class OnlineReindexer {
   private static final Logger log = LoggerFactory
@@ -42,6 +43,7 @@ public class OnlineReindexer {
   private final SiteIndexer batchIndexer;
   private final ProjectCache projectCache;
   private final int version;
+  private final AtomicBoolean running = new AtomicBoolean();
 
   @Inject
   OnlineReindexer(
@@ -56,15 +58,29 @@ public class OnlineReindexer {
   }
 
   public void start() {
-    Thread t = new Thread() {
-      @Override
-      public void run() {
-        reindex();
-      }
-    };
-    t.setName(String.format("Reindex v%d-v%d",
-        version(indexes.getSearchIndex()), version));
-    t.start();
+    if (running.compareAndSet(false, true)) {
+      Thread t = new Thread() {
+        @Override
+        public void run() {
+          try {
+            reindex();
+          } finally {
+            running.set(false);
+          }
+        }
+      };
+      t.setName(String.format("Reindex v%d-v%d",
+          version(indexes.getSearchIndex()), version));
+      t.start();
+    }
+  }
+
+  public boolean isRunning() {
+    return running.get();
+  }
+
+  public int getVersion() {
+    return version;
   }
 
   private static int version(ChangeIndex i) {

gerrit-lucene/src/main/java/com/google/gerrit/lucene/ReindexerAlreadyRunningException.java

@@ -0,0 +1,24 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.lucene;
+
+public class ReindexerAlreadyRunningException extends Exception {
+
+  private static final long serialVersionUID = 1L;
+
+  public ReindexerAlreadyRunningException() {
+    super("Reindexer is already running.");
+  }
+}