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diff --git a/java/com/google/gerrit/server/index/group/StalenessChecker.java b/java/com/google/gerrit/server/index/group/StalenessChecker.java
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+// Copyright (C) 2017 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.group;
+
+import com.google.common.collect.ImmutableSet;
+import com.google.gerrit.index.IndexConfig;
+import com.google.gerrit.index.query.FieldBundle;
+import com.google.gerrit.reviewdb.client.AccountGroup;
+import com.google.gerrit.reviewdb.client.RefNames;
+import com.google.gerrit.server.config.AllUsersName;
+import com.google.gerrit.server.git.GitRepositoryManager;
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+import java.io.IOException;
+import java.util.Optional;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.Repository;
+
+/**
+ * Checks if documents in the group index are stale.
+ *
+ * <p>An index document is considered stale if the stored SHA1 differs from the HEAD SHA1 of the
+ * groups branch.
+ *
+ * <p>Note: This only applies to NoteDb.
+ */
+@Singleton
+public class StalenessChecker {
+  public static final ImmutableSet<String> FIELDS =
+      ImmutableSet.of(GroupField.UUID.getName(), GroupField.REF_STATE.getName());
+
+  private final GroupIndexCollection indexes;
+  private final GitRepositoryManager repoManager;
+  private final IndexConfig indexConfig;
+  private final AllUsersName allUsers;
+
+  @Inject
+  StalenessChecker(
+      GroupIndexCollection indexes,
+      GitRepositoryManager repoManager,
+      IndexConfig indexConfig,
+      AllUsersName allUsers) {
+    this.indexes = indexes;
+    this.repoManager = repoManager;
+    this.indexConfig = indexConfig;
+    this.allUsers = allUsers;
+  }
+
+  public boolean isStale(AccountGroup.UUID uuid) throws IOException {
+    GroupIndex i = indexes.getSearchIndex();
+    if (i == null) {
+      return false; // No index; caller couldn't do anything if it is stale.
+    }
+
+    Optional<FieldBundle> result =
+        i.getRaw(uuid, IndexedGroupQuery.createOptions(indexConfig, 0, 1, FIELDS));
+    if (!result.isPresent()) {
+      // The document is missing in the index.
+      try (Repository repo = repoManager.openRepository(allUsers)) {
+        Ref ref = repo.exactRef(RefNames.refsGroups(uuid));
+
+        // Stale if the group actually exists.
+        return ref != null;
+      }
+    }
+
+    try (Repository repo = repoManager.openRepository(allUsers)) {
+      Ref ref = repo.exactRef(RefNames.refsGroups(uuid));
+      ObjectId head = ref == null ? ObjectId.zeroId() : ref.getObjectId();
+      return !head.equals(ObjectId.fromString(result.get().getValue(GroupField.REF_STATE), 0));
+    }
+  }
+}