Add config option to disable online reindexing

Change-Id: I18c48008778df86355a0bd57977e13e080f5fc90

Documentation/config-gerrit.txt

@@ -2176,6 +2176,18 @@ If not set or set to a negative value, defaults to using the same
 thread pool as interactive operations (unless
 link:#changeMerge.threadPoolSize[changeMerge.threadPoolSize] is set).
 
+[[index.onlineUpgrade]]index.onlineUpgrade::
++
+Whether to upgrade to new index schema versions while the server is
+running. This is recommended as it prevents additional downtime during
+Gerrit version upgrades (avoiding the need for an offline reindex step
+using Reindex), but can add additional server load during the upgrade.
++
+If set to false, there is no way to upgrade the index schema to take
+advantage of new search features without restarting the server.
++
+Defaults to true.
+
 ==== Lucene configuration
 
 Open and closed changes are indexed in separate indexes named

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

@@ -20,6 +20,7 @@ import com.google.common.collect.Lists;
 import com.google.common.collect.Maps;
 import com.google.common.primitives.Ints;
 import com.google.gerrit.extensions.events.LifecycleListener;
+import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.config.SitePaths;
 import com.google.gerrit.server.index.ChangeSchemas;
 import com.google.gerrit.server.index.IndexCollection;
@@ -93,9 +94,11 @@ class LuceneVersionManager implements LifecycleListener {
   private final LuceneChangeIndex.Factory indexFactory;
   private final IndexCollection indexes;
   private final OnlineReindexer.Factory reindexerFactory;
+  private final boolean onlineUpgrade;
 
   @Inject
   LuceneVersionManager(
+      @GerritServerConfig Config cfg,
       SitePaths sitePaths,
       LuceneChangeIndex.Factory indexFactory,
       IndexCollection indexes,
@@ -104,6 +107,7 @@ class LuceneVersionManager implements LifecycleListener {
     this.indexFactory = indexFactory;
     this.indexes = indexes;
     this.reindexerFactory = reindexerFactory;
+    this.onlineUpgrade = cfg.getBoolean("index", null, "onlineUpgrade", true);
   }
 
   @Override
@@ -133,7 +137,7 @@ class LuceneVersionManager implements LifecycleListener {
       if (v.schema == null) {
         continue;
       }
-      if (write.isEmpty()) {
+      if (write.isEmpty() && onlineUpgrade) {
         write.add(v);
       }
       if (v.ready) {
@@ -162,7 +166,7 @@ class LuceneVersionManager implements LifecycleListener {
     }
 
     int latest = write.get(0).version;
-    if (latest != search.version) {
+    if (onlineUpgrade && latest != search.version) {
       reindexerFactory.create(latest).start();
     }
   }