Merge branch 'stable-2.16' into stable-3.0

* origin/stable-2.16:
  Add missing "--migrate-draft-to" flag on init doc
  Fix buggy Index-Interactive Executor
  reindex: Use thread count specified on command line
  Disable auto flushing during offline Lucene indexing
  Avoid creating loose objects in Schema 146
  Avoid creating loose objects in schema 167
  Fix schemas to use the appropriate ObjectInserter
  Update schemas 115,119 to create a desired initial commit
  Create initial commit in schema 146 only when necessary
  Avoid creating loose objects in schemas 115 and 119
  Parallelize inserts into accountPatchReviewDb in schema 127
  Avoid listing repositories multiple times during schema migrations
  Parallelize Schema 108
  Avoid creating loose objects in Schema 154
  Parallelize Schema 130 and 131
  Avoid creating loose objects in Schema 144
  Avoid creating loose objects in Schema 139
  Avoid creating loose objects in Schema 124
  Update schemas 115,139,144 to ignore entries not in the 'accounts' table
  Introduce flag '--migrate-draft-to'
  OutgoingEmail: Handle null accountId in getUserNameEmailFor(Account.Id)
  NoteDbMigrator: Make shuffling project slices optional
  NoteDbMigrator: Make GC on repositories optional
  OutgoingEmail: Handle null accountId in getNameEmailFor(Account.Id) method
  OnlineNoteDbMigration: allow per-project migration
  Update jgit to 5.1.16.202106041830-r
  NoteDbMigrator: Warm PostgreSQL DB before migration

Change-Id: I9fac6c0eaf690a6c25af04c5bf89249cc35a9ca5

19 files changed, 212 insertions, 49 deletions

diff --git a/Documentation/config-gerrit.txt b/Documentation/config-gerrit.txt
index 3594626906..6d98877225 100644
--- a/Documentation/config-gerrit.txt
+++ b/Documentation/config-gerrit.txt
@@ -2748,7 +2748,7 @@ by the JVM. If set to a negative value, defaults to a direct executor.
 [[index.batchThreads]]index.batchThreads::
 +
 Number of threads to use for indexing in background operations, such as
-online schema upgrades, and also for offline reindexing.
+online schema upgrades, and also the default for offline reindexing.
 +
 If not set or set to a zero, defaults to the number of logical CPUs as returned
 by the JVM. If set to a negative value, defaults to a direct executor.
@@ -2964,6 +2964,11 @@ Lucene documentation] for further details.
 +
 Defaults to true (throttling enabled).
 
+During offline reindexing, setting ramBufferSize greater than the size
+of index (size of specific index folder under <site_dir>/index) and
+maxBufferedDocs as -1 avoids unnecessary flushes and triggers only a
+single flush at the end of the process.
+
 Sample Lucene index configuration:
 ----
 [index]
diff --git a/Documentation/pgm-reindex.txt b/Documentation/pgm-reindex.txt
index 5167277b8e..5392564df3 100644
--- a/Documentation/pgm-reindex.txt
+++ b/Documentation/pgm-reindex.txt
@@ -20,7 +20,8 @@ Rebuilds the secondary index.
 
 == OPTIONS
 --threads::
-	Number of threads to use for indexing.
+	Number of threads to use for indexing. Default is
+	link:config-gerrit.html#index.batchThreads[index.batchThreads]
 
 --changes-schema-version::
 	Schema version to reindex; default is most recent version.
diff --git a/java/com/google/gerrit/acceptance/GerritServer.java b/java/com/google/gerrit/acceptance/GerritServer.java
index 5e8698f15a..34f514c590 100644
--- a/java/com/google/gerrit/acceptance/GerritServer.java
+++ b/java/com/google/gerrit/acceptance/GerritServer.java
@@ -41,6 +41,7 @@ import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.config.SitePath;
 import com.google.gerrit.server.git.receive.AsyncReceiveCommits;
 import com.google.gerrit.server.schema.JdbcAccountPatchReviewStore;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.ssh.NoSshModule;
 import com.google.gerrit.server.util.SocketUtil;
 import com.google.gerrit.server.util.SystemLog;
@@ -374,7 +375,8 @@ public class GerritServer implements AutoCloseable {
     cfg.setString(
         "accountPatchReviewDb", null, "url", JdbcAccountPatchReviewStore.TEST_IN_MEMORY_URL);
     daemon.setEnableHttpd(desc.httpd());
-    daemon.setLuceneModule(LuceneIndexModule.singleVersionAllLatest(0, isSlave(baseConfig)));
+    daemon.setLuceneModule(
+        LuceneIndexModule.singleVersionAllLatest(0, isSlave(baseConfig), AutoFlush.ENABLED));
     daemon.setDatabaseForTesting(
         ImmutableList.of(
             new InMemoryTestingDatabaseModule(cfg, site, inMemoryRepoManager),
diff --git a/java/com/google/gerrit/httpd/init/WebAppInitializer.java b/java/com/google/gerrit/httpd/init/WebAppInitializer.java
index bf5cd2a9b8..c9dfc5817a 100644
--- a/java/com/google/gerrit/httpd/init/WebAppInitializer.java
+++ b/java/com/google/gerrit/httpd/init/WebAppInitializer.java
@@ -73,6 +73,7 @@ import com.google.gerrit.server.git.GarbageCollectionModule;
 import com.google.gerrit.server.git.GitRepositoryManagerModule;
 import com.google.gerrit.server.git.SearchingChangeCacheImpl;
 import com.google.gerrit.server.git.WorkQueue;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexModule;
 import com.google.gerrit.server.index.IndexModule.IndexType;
 import com.google.gerrit.server.index.OnlineUpgrader;
@@ -344,7 +345,7 @@ public class WebAppInitializer extends GuiceServletContextListener implements Fi
   private Module createIndexModule() {
     switch (indexType) {
       case LUCENE:
-        return LuceneIndexModule.latestVersion(false);
+        return LuceneIndexModule.latestVersion(false, AutoFlush.ENABLED);
       case ELASTICSEARCH:
         return ElasticIndexModule.latestVersion(false);
       default:
diff --git a/java/com/google/gerrit/lucene/AbstractLuceneIndex.java b/java/com/google/gerrit/lucene/AbstractLuceneIndex.java
index 7a0430c701..93c0fada6d 100644
--- a/java/com/google/gerrit/lucene/AbstractLuceneIndex.java
+++ b/java/com/google/gerrit/lucene/AbstractLuceneIndex.java
@@ -43,6 +43,7 @@ import com.google.gerrit.index.query.FieldBundle;
 import com.google.gerrit.index.query.ListResultSet;
 import com.google.gerrit.index.query.ResultSet;
 import com.google.gerrit.server.config.SitePaths;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexUtils;
 import com.google.gerrit.server.logging.LoggingContextAwareExecutorService;
 import com.google.gerrit.server.logging.LoggingContextAwareScheduledExecutorService;
@@ -101,6 +102,7 @@ public abstract class AbstractLuceneIndex<K, V> implements Index<K, V> {
   private final ReferenceManager<IndexSearcher> searcherManager;
   private final ControlledRealTimeReopenThread<IndexSearcher> reopenThread;
   private final Set<NrtFuture> notDoneNrtFutures;
+  private final AutoFlush autoFlush;
   private ScheduledExecutorService autoCommitExecutor;
 
   AbstractLuceneIndex(
@@ -110,12 +112,14 @@ public abstract class AbstractLuceneIndex<K, V> implements Index<K, V> {
       String name,
       String subIndex,
       GerritIndexWriterConfig writerConfig,
-      SearcherFactory searcherFactory)
+      SearcherFactory searcherFactory,
+      AutoFlush autoFlush)
       throws IOException {
     this.schema = schema;
     this.sitePaths = sitePaths;
     this.dir = dir;
     this.name = name;
+    this.autoFlush = autoFlush;
     String index = Joiner.on('_').skipNulls().join(name, subIndex);
     long commitPeriod = writerConfig.getCommitWithinMs();
 
@@ -209,7 +213,9 @@ public abstract class AbstractLuceneIndex<K, V> implements Index<K, V> {
           }
         });
 
-    reopenThread.start();
+    if (autoFlush.equals(AutoFlush.ENABLED)) {
+      reopenThread.start();
+    }
   }
 
   @Override
@@ -459,6 +465,9 @@ public abstract class AbstractLuceneIndex<K, V> implements Index<K, V> {
     }
 
     private boolean isGenAvailableNowForCurrentSearcher() {
+      if (autoFlush.equals(AutoFlush.DISABLED)) {
+        return true;
+      }
       try {
         return reopenThread.waitForGeneration(gen, 0);
       } catch (InterruptedException e) {
diff --git a/java/com/google/gerrit/lucene/ChangeSubIndex.java b/java/com/google/gerrit/lucene/ChangeSubIndex.java
index 98424b5e1c..8fcd0b1d04 100644
--- a/java/com/google/gerrit/lucene/ChangeSubIndex.java
+++ b/java/com/google/gerrit/lucene/ChangeSubIndex.java
@@ -29,6 +29,7 @@ import com.google.gerrit.index.query.Predicate;
 import com.google.gerrit.index.query.QueryParseException;
 import com.google.gerrit.reviewdb.client.Change;
 import com.google.gerrit.server.config.SitePaths;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.change.ChangeField;
 import com.google.gerrit.server.index.change.ChangeIndex;
 import com.google.gerrit.server.query.change.ChangeData;
@@ -48,7 +49,8 @@ public class ChangeSubIndex extends AbstractLuceneIndex<Change.Id, ChangeData>
       SitePaths sitePaths,
       Path path,
       GerritIndexWriterConfig writerConfig,
-      SearcherFactory searcherFactory)
+      SearcherFactory searcherFactory,
+      AutoFlush autoFlush)
       throws IOException {
     this(
         schema,
@@ -56,7 +58,8 @@ public class ChangeSubIndex extends AbstractLuceneIndex<Change.Id, ChangeData>
         FSDirectory.open(path),
         path.getFileName().toString(),
         writerConfig,
-        searcherFactory);
+        searcherFactory,
+        autoFlush);
   }
 
   ChangeSubIndex(
@@ -65,9 +68,10 @@ public class ChangeSubIndex extends AbstractLuceneIndex<Change.Id, ChangeData>
       Directory dir,
       String subIndex,
       GerritIndexWriterConfig writerConfig,
-      SearcherFactory searcherFactory)
+      SearcherFactory searcherFactory,
+      AutoFlush autoFlush)
       throws IOException {
-    super(schema, sitePaths, dir, NAME, subIndex, writerConfig, searcherFactory);
+    super(schema, sitePaths, dir, NAME, subIndex, writerConfig, searcherFactory, autoFlush);
   }
 
   @Override
diff --git a/java/com/google/gerrit/lucene/LuceneAccountIndex.java b/java/com/google/gerrit/lucene/LuceneAccountIndex.java
index 0b787b6e18..272c774754 100644
--- a/java/com/google/gerrit/lucene/LuceneAccountIndex.java
+++ b/java/com/google/gerrit/lucene/LuceneAccountIndex.java
@@ -32,6 +32,7 @@ import com.google.gerrit.server.account.AccountCache;
 import com.google.gerrit.server.account.AccountState;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.config.SitePaths;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexUtils;
 import com.google.gerrit.server.index.account.AccountIndex;
 import com.google.inject.Inject;
@@ -87,7 +88,8 @@ public class LuceneAccountIndex extends AbstractLuceneIndex<Account.Id, AccountS
       @GerritServerConfig Config cfg,
       SitePaths sitePaths,
       Provider<AccountCache> accountCache,
-      @Assisted Schema<AccountState> schema)
+      @Assisted Schema<AccountState> schema,
+      AutoFlush autoFlush)
       throws IOException {
     super(
         schema,
@@ -96,7 +98,8 @@ public class LuceneAccountIndex extends AbstractLuceneIndex<Account.Id, AccountS
         ACCOUNTS,
         null,
         new GerritIndexWriterConfig(cfg, ACCOUNTS),
-        new SearcherFactory());
+        new SearcherFactory(),
+        autoFlush);
     this.accountCache = accountCache;
 
     indexWriterConfig = new GerritIndexWriterConfig(cfg, ACCOUNTS);
diff --git a/java/com/google/gerrit/lucene/LuceneChangeIndex.java b/java/com/google/gerrit/lucene/LuceneChangeIndex.java
index 87f9396a90..a9e431ef12 100644
--- a/java/com/google/gerrit/lucene/LuceneChangeIndex.java
+++ b/java/com/google/gerrit/lucene/LuceneChangeIndex.java
@@ -55,6 +55,7 @@ import com.google.gerrit.reviewdb.converter.ProtoConverter;
 import com.google.gerrit.server.StarredChangesUtil;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.config.SitePaths;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexExecutor;
 import com.google.gerrit.server.index.IndexUtils;
 import com.google.gerrit.server.index.change.ChangeField;
@@ -155,7 +156,8 @@ public class LuceneChangeIndex implements ChangeIndex {
       SitePaths sitePaths,
       @IndexExecutor(INTERACTIVE) ListeningExecutorService executor,
       ChangeData.Factory changeDataFactory,
-      @Assisted Schema<ChangeData> schema)
+      @Assisted Schema<ChangeData> schema,
+      AutoFlush autoFlush)
       throws IOException {
     this.executor = executor;
     this.changeDataFactory = changeDataFactory;
@@ -170,18 +172,35 @@ public class LuceneChangeIndex implements ChangeIndex {
     if (LuceneIndexModule.isInMemoryTest(cfg)) {
       openIndex =
           new ChangeSubIndex(
-              schema, sitePaths, new RAMDirectory(), "ramOpen", openConfig, searcherFactory);
+              schema,
+              sitePaths,
+              new RAMDirectory(),
+              "ramOpen",
+              openConfig,
+              searcherFactory,
+              autoFlush);
       closedIndex =
           new ChangeSubIndex(
-              schema, sitePaths, new RAMDirectory(), "ramClosed", closedConfig, searcherFactory);
+              schema,
+              sitePaths,
+              new RAMDirectory(),
+              "ramClosed",
+              closedConfig,
+              searcherFactory,
+              autoFlush);
     } else {
       Path dir = LuceneVersionManager.getDir(sitePaths, CHANGES, schema);
       openIndex =
           new ChangeSubIndex(
-              schema, sitePaths, dir.resolve(CHANGES_OPEN), openConfig, searcherFactory);
+              schema, sitePaths, dir.resolve(CHANGES_OPEN), openConfig, searcherFactory, autoFlush);
       closedIndex =
           new ChangeSubIndex(
-              schema, sitePaths, dir.resolve(CHANGES_CLOSED), closedConfig, searcherFactory);
+              schema,
+              sitePaths,
+              dir.resolve(CHANGES_CLOSED),
+              closedConfig,
+              searcherFactory,
+              autoFlush);
     }
   }
 
diff --git a/java/com/google/gerrit/lucene/LuceneGroupIndex.java b/java/com/google/gerrit/lucene/LuceneGroupIndex.java
index 0fdef77b96..47a5038812 100644
--- a/java/com/google/gerrit/lucene/LuceneGroupIndex.java
+++ b/java/com/google/gerrit/lucene/LuceneGroupIndex.java
@@ -30,6 +30,7 @@ import com.google.gerrit.server.account.GroupCache;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.config.SitePaths;
 import com.google.gerrit.server.group.InternalGroup;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexUtils;
 import com.google.gerrit.server.index.group.GroupIndex;
 import com.google.inject.Inject;
@@ -83,7 +84,8 @@ public class LuceneGroupIndex extends AbstractLuceneIndex<AccountGroup.UUID, Int
       @GerritServerConfig Config cfg,
       SitePaths sitePaths,
       Provider<GroupCache> groupCache,
-      @Assisted Schema<InternalGroup> schema)
+      @Assisted Schema<InternalGroup> schema,
+      AutoFlush autoFlush)
       throws IOException {
     super(
         schema,
@@ -92,7 +94,8 @@ public class LuceneGroupIndex extends AbstractLuceneIndex<AccountGroup.UUID, Int
         GROUPS,
         null,
         new GerritIndexWriterConfig(cfg, GROUPS),
-        new SearcherFactory());
+        new SearcherFactory(),
+        autoFlush);
     this.groupCache = groupCache;
 
     indexWriterConfig = new GerritIndexWriterConfig(cfg, GROUPS);
diff --git a/java/com/google/gerrit/lucene/LuceneIndexModule.java b/java/com/google/gerrit/lucene/LuceneIndexModule.java
index 302a2da851..10b58c0894 100644
--- a/java/com/google/gerrit/lucene/LuceneIndexModule.java
+++ b/java/com/google/gerrit/lucene/LuceneIndexModule.java
@@ -19,6 +19,7 @@ import com.google.gerrit.index.IndexConfig;
 import com.google.gerrit.index.project.ProjectIndex;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.index.AbstractIndexModule;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.VersionManager;
 import com.google.gerrit.server.index.account.AccountIndex;
 import com.google.gerrit.server.index.change.ChangeIndex;
@@ -28,25 +29,36 @@ import org.apache.lucene.search.BooleanQuery;
 import org.eclipse.jgit.lib.Config;
 
 public class LuceneIndexModule extends AbstractIndexModule {
-  public static LuceneIndexModule singleVersionAllLatest(int threads, boolean slave) {
-    return new LuceneIndexModule(ImmutableMap.of(), threads, slave);
+  private final AutoFlush autoFlush;
+
+  public static LuceneIndexModule singleVersionAllLatest(
+      int threads, boolean slave, AutoFlush autoFlush) {
+    return new LuceneIndexModule(ImmutableMap.of(), threads, slave, autoFlush);
   }
 
   public static LuceneIndexModule singleVersionWithExplicitVersions(
-      Map<String, Integer> versions, int threads, boolean slave) {
-    return new LuceneIndexModule(versions, threads, slave);
+      Map<String, Integer> versions, int threads, boolean slave, AutoFlush autoFlush) {
+    return new LuceneIndexModule(versions, threads, slave, autoFlush);
   }
 
-  public static LuceneIndexModule latestVersion(boolean slave) {
-    return new LuceneIndexModule(null, 0, slave);
+  public static LuceneIndexModule latestVersion(boolean slave, AutoFlush autoFlush) {
+    return new LuceneIndexModule(null, 0, slave, autoFlush);
   }
 
   static boolean isInMemoryTest(Config cfg) {
     return cfg.getBoolean("index", "lucene", "testInmemory", false);
   }
 
-  private LuceneIndexModule(Map<String, Integer> singleVersions, int threads, boolean slave) {
+  private LuceneIndexModule(
+      Map<String, Integer> singleVersions, int threads, boolean slave, AutoFlush autoFlush) {
     super(singleVersions, threads, slave);
+    this.autoFlush = autoFlush;
+  }
+
+  @Override
+  protected void configure() {
+    super.configure();
+    bind(AutoFlush.class).toInstance(autoFlush);
   }
 
   @Override
diff --git a/java/com/google/gerrit/lucene/LuceneProjectIndex.java b/java/com/google/gerrit/lucene/LuceneProjectIndex.java
index 950e2060ca..601ed5fb3f 100644
--- a/java/com/google/gerrit/lucene/LuceneProjectIndex.java
+++ b/java/com/google/gerrit/lucene/LuceneProjectIndex.java
@@ -30,6 +30,7 @@ import com.google.gerrit.index.query.QueryParseException;
 import com.google.gerrit.reviewdb.client.Project;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.config.SitePaths;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexUtils;
 import com.google.gerrit.server.project.ProjectCache;
 import com.google.gerrit.server.project.ProjectState;
@@ -83,7 +84,8 @@ public class LuceneProjectIndex extends AbstractLuceneIndex<Project.NameKey, Pro
       @GerritServerConfig Config cfg,
       SitePaths sitePaths,
       Provider<ProjectCache> projectCache,
-      @Assisted Schema<ProjectData> schema)
+      @Assisted Schema<ProjectData> schema,
+      AutoFlush autoFlush)
       throws IOException {
     super(
         schema,
@@ -92,7 +94,8 @@ public class LuceneProjectIndex extends AbstractLuceneIndex<Project.NameKey, Pro
         PROJECTS,
         null,
         new GerritIndexWriterConfig(cfg, PROJECTS),
-        new SearcherFactory());
+        new SearcherFactory(),
+        autoFlush);
     this.projectCache = projectCache;
 
     indexWriterConfig = new GerritIndexWriterConfig(cfg, PROJECTS);
diff --git a/java/com/google/gerrit/pgm/Daemon.java b/java/com/google/gerrit/pgm/Daemon.java
index 8887385bf6..6b4a065f3f 100644
--- a/java/com/google/gerrit/pgm/Daemon.java
+++ b/java/com/google/gerrit/pgm/Daemon.java
@@ -82,6 +82,7 @@ import com.google.gerrit.server.git.GarbageCollectionModule;
 import com.google.gerrit.server.git.SearchingChangeCacheImpl;
 import com.google.gerrit.server.git.WorkQueue;
 import com.google.gerrit.server.group.PeriodicGroupIndexer;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexModule;
 import com.google.gerrit.server.index.IndexModule.IndexType;
 import com.google.gerrit.server.index.OnlineUpgrader;
@@ -498,7 +499,7 @@ public class Daemon extends SiteProgram {
     }
     switch (indexType) {
       case LUCENE:
-        return LuceneIndexModule.latestVersion(slave);
+        return LuceneIndexModule.latestVersion(slave, AutoFlush.ENABLED);
       case ELASTICSEARCH:
         return ElasticIndexModule.latestVersion(slave);
       default:
diff --git a/java/com/google/gerrit/pgm/Reindex.java b/java/com/google/gerrit/pgm/Reindex.java
index 6fd542406c..544c61d47d 100644
--- a/java/com/google/gerrit/pgm/Reindex.java
+++ b/java/com/google/gerrit/pgm/Reindex.java
@@ -30,6 +30,7 @@ import com.google.gerrit.pgm.util.BatchProgramModule;
 import com.google.gerrit.pgm.util.SiteProgram;
 import com.google.gerrit.server.change.ChangeResource;
 import com.google.gerrit.server.config.GerritServerConfig;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexModule;
 import com.google.gerrit.server.index.IndexModule.IndexType;
 import com.google.gerrit.server.index.change.ChangeSchemaDefinitions;
@@ -51,8 +52,10 @@ import org.eclipse.jgit.util.io.NullOutputStream;
 import org.kohsuke.args4j.Option;
 
 public class Reindex extends SiteProgram {
-  @Option(name = "--threads", usage = "Number of threads to use for indexing")
-  private int threads = Runtime.getRuntime().availableProcessors();
+  @Option(
+      name = "--threads",
+      usage = "Number of threads to use for indexing. Default is index.batchThreads from config.")
+  private int threads = 0;
 
   @Option(
       name = "--changes-schema-version",
@@ -149,7 +152,9 @@ public class Reindex extends SiteProgram {
     Module indexModule;
     switch (IndexModule.getIndexType(dbInjector)) {
       case LUCENE:
-        indexModule = LuceneIndexModule.singleVersionWithExplicitVersions(versions, threads, slave);
+        indexModule =
+            LuceneIndexModule.singleVersionWithExplicitVersions(
+                versions, threads, slave, AutoFlush.DISABLED);
         break;
       case ELASTICSEARCH:
         indexModule =
diff --git a/java/com/google/gerrit/pgm/init/index/lucene/LuceneIndexModuleOnInit.java b/java/com/google/gerrit/pgm/init/index/lucene/LuceneIndexModuleOnInit.java
index 12a44dc535..54618a19e5 100644
--- a/java/com/google/gerrit/pgm/init/index/lucene/LuceneIndexModuleOnInit.java
+++ b/java/com/google/gerrit/pgm/init/index/lucene/LuceneIndexModuleOnInit.java
@@ -17,6 +17,7 @@ package com.google.gerrit.pgm.init.index.lucene;
 import com.google.gerrit.lucene.LuceneAccountIndex;
 import com.google.gerrit.lucene.LuceneGroupIndex;
 import com.google.gerrit.pgm.init.index.IndexModuleOnInit;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.account.AccountIndex;
 import com.google.gerrit.server.index.group.GroupIndex;
 import com.google.inject.AbstractModule;
@@ -36,5 +37,7 @@ public class LuceneIndexModuleOnInit extends AbstractModule {
             .build(GroupIndex.Factory.class));
 
     install(new IndexModuleOnInit());
+
+    bind(AutoFlush.class).toInstance(AutoFlush.DISABLED);
   }
 }
diff --git a/java/com/google/gerrit/server/git/meta/VersionedMetaData.java b/java/com/google/gerrit/server/git/meta/VersionedMetaData.java
index 86942998a6..cce81b0036 100644
--- a/java/com/google/gerrit/server/git/meta/VersionedMetaData.java
+++ b/java/com/google/gerrit/server/git/meta/VersionedMetaData.java
@@ -213,6 +213,27 @@ public abstract class VersionedMetaData {
   }
 
   /**
+   * Update this metadata branch, recording a new commit on its reference. This method mutates its
+   * receiver.
+   *
+   * @param update helper information to define the update that will occur.
+   * @param objInserter Shared object inserter.
+   * @param objReader Shared object reader.
+   * @param revWalk Shared rev walk.
+   * @return the commit that was created
+   * @throws IOException if there is a storage problem and the update cannot be executed as
+   *     requested or if it failed because of a concurrent update to the same reference
+   */
+  public RevCommit commit(
+      MetaDataUpdate update, ObjectInserter objInserter, ObjectReader objReader, RevWalk revWalk)
+      throws IOException {
+    try (BatchMetaDataUpdate batch = openUpdate(update, objInserter, objReader, revWalk)) {
+      batch.write(update.getCommitBuilder());
+      return batch.commit();
+    }
+  }
+
+  /**
    * Creates a new commit and a new ref based on this commit. This method mutates its receiver.
    *
    * @param update helper information to define the update that will occur.
@@ -259,11 +280,39 @@ public abstract class VersionedMetaData {
    * @throws IOException if the update failed.
    */
   public BatchMetaDataUpdate openUpdate(MetaDataUpdate update) throws IOException {
+    return openUpdate(update, null, null, null);
+  }
+
+  /**
+   * Open a batch of updates to the same metadata ref.
+   *
+   * <p>This allows making multiple commits to a single metadata ref, at the end of which is a
+   * single ref update. For batching together updates to multiple refs (each consisting of one or
+   * more commits against their respective refs), create the {@link MetaDataUpdate} with a {@link
+   * BatchRefUpdate}.
+   *
+   * <p>A ref update produced by this {@link BatchMetaDataUpdate} is only committed if there is no
+   * associated {@link BatchRefUpdate}. As a result, the configured ref updated event is not fired
+   * if there is an associated batch.
+   *
+   * <p>If object inserter, reader and revwalk are provided, then the updates are not flushed,
+   * allowing callers the flexibility to flush only once after several updates.
+   *
+   * @param update helper info about the update.
+   * @param objInserter Shared object inserter.
+   * @param objReader Shared object reader.
+   * @param revWalk Shared rev walk.
+   * @throws IOException if the update failed.
+   */
+  public BatchMetaDataUpdate openUpdate(
+      MetaDataUpdate update, ObjectInserter objInserter, ObjectReader objReader, RevWalk revWalk)
+      throws IOException {
     final Repository db = update.getRepository();
 
-    inserter = db.newObjectInserter();
-    reader = inserter.newReader();
-    final RevWalk rw = new RevWalk(reader);
+    inserter = objInserter == null ? db.newObjectInserter() : objInserter;
+    reader = objReader == null ? inserter.newReader() : objReader;
+    final RevWalk rw = revWalk == null ? new RevWalk(reader) : revWalk;
+
     final RevTree tree = revision != null ? rw.parseTree(revision) : null;
     newTree = readTree(tree);
     return new BatchMetaDataUpdate() {
@@ -376,7 +425,7 @@ public abstract class VersionedMetaData {
         newTree = null;
 
         rw.close();
-        if (inserter != null) {
+        if (objInserter == null && inserter != null) {
           inserter.close();
           inserter = null;
         }
@@ -392,7 +441,9 @@ public abstract class VersionedMetaData {
         BatchRefUpdate bru = update.getBatch();
         if (bru != null) {
           bru.addCommand(new ReceiveCommand(oldId.toObjectId(), newId.toObjectId(), refName));
-          inserter.flush();
+          if (objInserter == null) {
+            inserter.flush();
+          }
           revision = rw.parseCommit(newId);
           return revision;
         }
diff --git a/java/com/google/gerrit/server/index/AutoFlush.java b/java/com/google/gerrit/server/index/AutoFlush.java
new file mode 100644
index 0000000000..3d131105dd
--- /dev/null
+++ b/java/com/google/gerrit/server/index/AutoFlush.java
@@ -0,0 +1,20 @@
+// Copyright (C) 2021 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.server.index;
+
+public enum AutoFlush {
+  ENABLED,
+  DISABLED
+}
diff --git a/java/com/google/gerrit/server/index/IndexModule.java b/java/com/google/gerrit/server/index/IndexModule.java
index e7b892d611..612c637dfa 100644
--- a/java/com/google/gerrit/server/index/IndexModule.java
+++ b/java/com/google/gerrit/server/index/IndexModule.java
@@ -224,13 +224,14 @@ public class IndexModule extends LifecycleModule {
       return interactiveExecutor;
     }
     int threads = this.threads;
-    if (threads < 0) {
-      return MoreExecutors.newDirectExecutorService();
-    } else if (threads == 0) {
+    if (threads == 0) {
       threads =
           config.getInt(
               "index", null, "threads", Runtime.getRuntime().availableProcessors() / 2 + 1);
     }
+    if (threads < 0) {
+      return MoreExecutors.newDirectExecutorService();
+    }
     return MoreExecutors.listeningDecorator(
         workQueue.createQueue(threads, "Index-Interactive", true));
   }
@@ -243,11 +244,13 @@ public class IndexModule extends LifecycleModule {
     if (batchExecutor != null) {
       return batchExecutor;
     }
-    int threads = config.getInt("index", null, "batchThreads", 0);
+    int threads = this.threads;
+    if (threads == 0) {
+      threads =
+          config.getInt("index", null, "batchThreads", Runtime.getRuntime().availableProcessors());
+    }
     if (threads < 0) {
       return MoreExecutors.newDirectExecutorService();
-    } else if (threads == 0) {
-      threads = Runtime.getRuntime().availableProcessors();
     }
     return MoreExecutors.listeningDecorator(workQueue.createQueue(threads, "Index-Batch", true));
   }
diff --git a/java/com/google/gerrit/server/mail/send/OutgoingEmail.java b/java/com/google/gerrit/server/mail/send/OutgoingEmail.java
index 3bb710bb02..97035f23b4 100644
--- a/java/com/google/gerrit/server/mail/send/OutgoingEmail.java
+++ b/java/com/google/gerrit/server/mail/send/OutgoingEmail.java
@@ -21,6 +21,7 @@ import static java.util.Objects.requireNonNull;
 import com.google.common.collect.Sets;
 import com.google.common.flogger.FluentLogger;
 import com.google.gerrit.exceptions.EmailException;
+import com.google.gerrit.common.Nullable;
 import com.google.gerrit.extensions.api.changes.RecipientType;
 import com.google.gerrit.extensions.client.GeneralPreferencesInfo;
 import com.google.gerrit.extensions.client.GeneralPreferencesInfo.EmailFormat;
@@ -333,7 +334,7 @@ public abstract class OutgoingEmail {
   }
 
   /** Lookup a human readable name for an account, usually the "full name". */
-  protected String getNameFor(Account.Id accountId) {
+  protected String getNameFor(@Nullable Account.Id accountId) {
     if (accountId == null) {
       return args.gerritPersonIdent.getName();
     }
@@ -359,7 +360,14 @@ public abstract class OutgoingEmail {
    * @param accountId user to fetch.
    * @return name/email of account, or Anonymous Coward if unset.
    */
-  protected String getNameEmailFor(Account.Id accountId) {
+  protected String getNameEmailFor(@Nullable Account.Id accountId) {
+    if (accountId == null) {
+      return args.gerritPersonIdent.getName()
+          + " <"
+          + args.gerritPersonIdent.getEmailAddress()
+          + ">";
+    }
+
     Optional<Account> account = args.accountCache.get(accountId).map(AccountState::getAccount);
     if (account.isPresent()) {
       String name = account.get().getFullName();
@@ -380,9 +388,13 @@ public abstract class OutgoingEmail {
    * username. If no username is set, this function returns null.
    *
    * @param accountId user to fetch.
-   * @return name/email of account, username, or null if unset.
+   * @return name/email of account, username, or null if unset or the accountId is null.
    */
-  protected String getUserNameEmailFor(Account.Id accountId) {
+  protected String getUserNameEmailFor(@Nullable Account.Id accountId) {
+    if (accountId == null) {
+      return null;
+    }
+
     Optional<AccountState> accountState = args.accountCache.get(accountId);
     if (!accountState.isPresent()) {
       return null;
diff --git a/java/com/google/gerrit/testing/InMemoryModule.java b/java/com/google/gerrit/testing/InMemoryModule.java
index 83b9e2b6e0..9d07780af4 100644
--- a/java/com/google/gerrit/testing/InMemoryModule.java
+++ b/java/com/google/gerrit/testing/InMemoryModule.java
@@ -61,6 +61,7 @@ import com.google.gerrit.server.git.GitRepositoryManager;
 import com.google.gerrit.server.git.PerThreadRequestScope;
 import com.google.gerrit.server.git.SearchingChangeCacheImpl;
 import com.google.gerrit.server.git.WorkQueue;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexModule.IndexType;
 import com.google.gerrit.server.index.account.AccountSchemaDefinitions;
 import com.google.gerrit.server.index.account.AllAccountsIndexer;
@@ -304,8 +305,13 @@ public class InMemoryModule extends FactoryModule {
       boolean slave = cfg.getBoolean("container", "slave", false);
       Class<?> clazz = Class.forName(moduleClassName);
       Method m =
-          clazz.getMethod("singleVersionWithExplicitVersions", Map.class, int.class, boolean.class);
-      return (Module) m.invoke(null, getSingleSchemaVersions(), 0, slave);
+          clazz.getMethod(
+              "singleVersionWithExplicitVersions",
+              Map.class,
+              int.class,
+              boolean.class,
+              AutoFlush.class);
+      return (Module) m.invoke(null, getSingleSchemaVersions(), 0, slave, AutoFlush.ENABLED);
     } catch (ClassNotFoundException
         | SecurityException
         | NoSuchMethodException