java/com/google/gerrit/server/group/db/GroupNameNotes.java - gerrit - Git at Google

 // 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.group.db;

 import static com.google.common.collect.ImmutableBiMap.toImmutableBiMap;
 import static java.nio.charset.StandardCharsets.UTF_8;
 import static java.util.Objects.requireNonNull;
 import static org.eclipse.jgit.lib.Constants.OBJ_BLOB;

 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Strings;
 import com.google.common.collect.HashMultiset;
 import com.google.common.collect.ImmutableBiMap;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Multiset;
 import com.google.common.flogger.FluentLogger;
 import com.google.common.hash.Hashing;
 import com.google.gerrit.common.Nullable;
 import com.google.gerrit.common.data.GroupReference;
 import com.google.gerrit.entities.AccountGroup;
 import com.google.gerrit.entities.Project;
 import com.google.gerrit.entities.RefNames;
 import com.google.gerrit.exceptions.DuplicateKeyException;
 import com.google.gerrit.git.ObjectIds;
 import com.google.gerrit.server.git.meta.VersionedMetaData;
 import java.io.IOException;
 import java.util.Collection;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
 import org.eclipse.jgit.errors.ConfigInvalidException;
 import org.eclipse.jgit.lib.BatchRefUpdate;
 import org.eclipse.jgit.lib.CommitBuilder;
 import org.eclipse.jgit.lib.Config;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectInserter;
 import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.PersonIdent;
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.notes.Note;
 import org.eclipse.jgit.notes.NoteMap;
 import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.revwalk.RevWalk;
 import org.eclipse.jgit.transport.ReceiveCommand;

 /**
  * An enforcer of unique names for groups in NoteDb.
  *
  * <p>The way groups are stored in NoteDb (see {@link GroupConfig}) doesn't enforce unique names,
  * even though groups in Gerrit must not have duplicate names. The storage format doesn't allow to
  * quickly look up whether a name has already been used either. That's why we additionally keep a
  * map of name/UUID pairs and manage it with this class.
  *
  * <p>To claim the name for a new group, create an instance of {@code GroupNameNotes} via {@link
  * #forNewGroup(Project.NameKey, Repository, AccountGroup.UUID, AccountGroup.NameKey)} and call
  * {@link #commit(com.google.gerrit.server.git.meta.MetaDataUpdate) commit(MetaDataUpdate)} on it.
  * For renaming, call {@link #forRename(Project.NameKey, Repository, AccountGroup.UUID,
  * AccountGroup.NameKey, AccountGroup.NameKey)} and also commit the returned {@code GroupNameNotes}.
  * Both times, the creation of the {@code GroupNameNotes} will fail if the (new) name is already
  * used. Committing the {@code GroupNameNotes} is necessary to make the adjustments for real.
  *
  * <p>The map has an additional benefit: We can quickly iterate over all group name/UUID pairs
  * without having to load all groups completely (which is costly).
  *
  * <p><em>Internal details</em>
  *
  * <p>The map of names is represented by Git {@link Note notes}. They are stored on the branch
  * {@link RefNames#REFS_GROUPNAMES}. Each commit on the branch reflects one moment in time of the
  * complete map.
  *
  * <p>As key for the notes, we use the SHA-1 of the name. As data, they contain a text version of a
  * JGit {@link Config} file. That config file has two entries:
  *
  * <ul>
  *   <li>the name of the group (as clear text)
  *   <li>the UUID of the group which currently has this name
  * </ul>
  */
 public class GroupNameNotes extends VersionedMetaData {
   private static final FluentLogger logger = FluentLogger.forEnclosingClass();

   private static final String SECTION_NAME = "group";
   private static final String UUID_PARAM = "uuid";
   private static final String NAME_PARAM = "name";

   @VisibleForTesting
   static final String UNIQUE_REF_ERROR = "GroupReference collection must contain unique references";

   /**
    * Creates an instance of {@code GroupNameNotes} for use when renaming a group.
    *
    * <p><strong>Note: </strong>The returned instance of {@code GroupNameNotes} has to be committed
    * via {@link #commit(com.google.gerrit.server.git.meta.MetaDataUpdate) commit(MetaDataUpdate)} in
    * order to claim the new name and free up the old one.
    *
    * @param projectName the name of the project which holds the commits of the notes
    * @param repository the repository which holds the commits of the notes
    * @param groupUuid the UUID of the group which is renamed
    * @param oldName the current name of the group
    * @param newName the new name of the group
    * @return an instance of {@code GroupNameNotes} configured for a specific renaming of a group
    * @throws IOException if the repository can't be accessed for some reason
    * @throws ConfigInvalidException if the note for the specified group doesn't exist or is in an
    *     invalid state
    * @throws DuplicateKeyException if a group with the new name already exists
    */
   public static GroupNameNotes forRename(
       Project.NameKey projectName,
       Repository repository,
       AccountGroup.UUID groupUuid,
       AccountGroup.NameKey oldName,
       AccountGroup.NameKey newName)
       throws IOException, ConfigInvalidException, DuplicateKeyException {
     requireNonNull(oldName);
     requireNonNull(newName);

     GroupNameNotes groupNameNotes = new GroupNameNotes(groupUuid, oldName, newName);
     groupNameNotes.load(projectName, repository);
     groupNameNotes.ensureNewNameIsNotUsed();
     return groupNameNotes;
   }

   /**
    * Creates an instance of {@code GroupNameNotes} for use when creating a new group.
    *
    * <p><strong>Note: </strong>The returned instance of {@code GroupNameNotes} has to be committed
    * via {@link #commit(com.google.gerrit.server.git.meta.MetaDataUpdate) commit(MetaDataUpdate)} in
    * order to claim the new name.
    *
    * @param projectName the name of the project which holds the commits of the notes
    * @param repository the repository which holds the commits of the notes
    * @param groupUuid the UUID of the new group
    * @param groupName the name of the new group
    * @return an instance of {@code GroupNameNotes} configured for a specific group creation
    * @throws IOException if the repository can't be accessed for some reason
    * @throws ConfigInvalidException in no case so far
    * @throws DuplicateKeyException if a group with the new name already exists
    */
   public static GroupNameNotes forNewGroup(
       Project.NameKey projectName,
       Repository repository,
       AccountGroup.UUID groupUuid,
       AccountGroup.NameKey groupName)
       throws IOException, ConfigInvalidException, DuplicateKeyException {
     requireNonNull(groupName);

     GroupNameNotes groupNameNotes = new GroupNameNotes(groupUuid, null, groupName);
     groupNameNotes.load(projectName, repository);
     groupNameNotes.ensureNewNameIsNotUsed();
     return groupNameNotes;
   }

   /**
    * Loads the {@code GroupReference} (name/UUID pair) for the group with the specified name.
    *
    * @param repository the repository which holds the commits of the notes
    * @param groupName the name of the group
    * @return the corresponding {@code GroupReference} if a group/note with the given name exists
    * @throws IOException if the repository can't be accessed for some reason
    * @throws ConfigInvalidException if the note for the specified group is in an invalid state
    */
   public static Optional<GroupReference> loadGroup(
       Repository repository, AccountGroup.NameKey groupName)
       throws IOException, ConfigInvalidException {
     Ref ref = repository.exactRef(RefNames.REFS_GROUPNAMES);
     if (ref == null) {
       return Optional.empty();
     }

     try (RevWalk revWalk = new RevWalk(repository);
         ObjectReader reader = revWalk.getObjectReader()) {
       RevCommit notesCommit = revWalk.parseCommit(ref.getObjectId());
       NoteMap noteMap = NoteMap.read(reader, notesCommit);
       ObjectId noteDataBlobId = noteMap.get(getNoteKey(groupName));
       if (noteDataBlobId == null) {
         return Optional.empty();
       }
       return Optional.of(getGroupReference(reader, noteDataBlobId));
     }
   }

   /**
    * Loads the {@code GroupReference}s (name/UUID pairs) for all groups.
    *
    * <p>Even though group UUIDs should be unique, this class doesn't enforce it. For this reason,
    * it's technically possible that two of the {@code GroupReference}s have a duplicate UUID but a
    * different name. In practice, this shouldn't occur unless we introduce a bug in the future.
    *
    * @param repository the repository which holds the commits of the notes
    * @return the {@code GroupReference}s of all existing groups/notes
    * @throws IOException if the repository can't be accessed for some reason
    * @throws ConfigInvalidException if one of the notes is in an invalid state
    */
   public static ImmutableList<GroupReference> loadAllGroups(Repository repository)
       throws IOException, ConfigInvalidException {
     Ref ref = repository.exactRef(RefNames.REFS_GROUPNAMES);
     if (ref == null) {
       return ImmutableList.of();
     }
     try (RevWalk revWalk = new RevWalk(repository);
         ObjectReader reader = revWalk.getObjectReader()) {
       RevCommit notesCommit = revWalk.parseCommit(ref.getObjectId());
       NoteMap noteMap = NoteMap.read(reader, notesCommit);

       Multiset<GroupReference> groupReferences = HashMultiset.create();
       for (Note note : noteMap) {
         GroupReference groupReference = getGroupReference(reader, note.getData());
         int numOfOccurrences = groupReferences.add(groupReference, 1);
         if (numOfOccurrences > 1) {
           GroupsNoteDbConsistencyChecker.logConsistencyProblemAsWarning(
               "The UUID of group %s (%s) is duplicate in group name notes",
               groupReference.getName(), groupReference.getUUID());
         }
       }

       return ImmutableList.copyOf(groupReferences);
     }
   }

   /**
    * Replaces the map of name/UUID pairs with a new version which matches exactly the passed {@code
    * GroupReference}s.
    *
    * <p>All old entries are discarded and replaced by the new ones.
    *
    * <p>This operation also works if the previous map has invalid entries or can't be read anymore.
    *
    * <p><strong>Note: </strong>This method doesn't flush the {@code ObjectInserter}. It doesn't
    * execute the {@code BatchRefUpdate} either.
    *
    * @param repository the repository which holds the commits of the notes
    * @param inserter an {@code ObjectInserter} for that repository
    * @param bru a {@code BatchRefUpdate} to which this method adds commands
    * @param groupReferences all {@code GroupReference}s (name/UUID pairs) which should be contained
    *     in the map of name/UUID pairs
    * @param ident the {@code PersonIdent} which is used as author and committer for commits
    * @throws IOException if the repository can't be accessed for some reason
    */
   public static void updateAllGroups(
       Repository repository,
       ObjectInserter inserter,
       BatchRefUpdate bru,
       Collection<GroupReference> groupReferences,
       PersonIdent ident)
       throws IOException {
     // Not strictly necessary for iteration; throws IAE if it encounters duplicates, which is nice.
     ImmutableBiMap<AccountGroup.UUID, String> biMap = toBiMap(groupReferences);

     try (ObjectReader reader = inserter.newReader();
         RevWalk rw = new RevWalk(reader)) {
       // Always start from an empty map, discarding old notes.
       NoteMap noteMap = NoteMap.newEmptyMap();
       Ref ref = repository.exactRef(RefNames.REFS_GROUPNAMES);
       RevCommit oldCommit = ref != null ? rw.parseCommit(ref.getObjectId()) : null;

       for (Map.Entry<AccountGroup.UUID, String> e : biMap.entrySet()) {
         AccountGroup.NameKey nameKey = AccountGroup.nameKey(e.getValue());
         ObjectId noteKey = getNoteKey(nameKey);
         noteMap.set(noteKey, getAsNoteData(e.getKey(), nameKey), inserter);
       }

       ObjectId newTreeId = noteMap.writeTree(inserter);
       if (oldCommit != null && newTreeId.equals(oldCommit.getTree())) {
         return;
       }
       CommitBuilder cb = new CommitBuilder();
       if (oldCommit != null) {
         cb.addParentId(oldCommit);
       }
       cb.setTreeId(newTreeId);
       cb.setAuthor(ident);
       cb.setCommitter(ident);
       int n = groupReferences.size();
       cb.setMessage("Store " + n + " group name" + (n != 1 ? "s" : ""));
       ObjectId newId = inserter.insert(cb).copy();

       ObjectId oldId = ObjectIds.copyOrZero(oldCommit);
       bru.addCommand(new ReceiveCommand(oldId, newId, RefNames.REFS_GROUPNAMES));
     }
   }

   // Returns UUID <=> Name bimap.
   private static ImmutableBiMap<AccountGroup.UUID, String> toBiMap(
       Collection<GroupReference> groupReferences) {
     try {
       return groupReferences.stream()
           .collect(toImmutableBiMap(GroupReference::getUUID, GroupReference::getName));
     } catch (IllegalArgumentException e) {
       throw new IllegalArgumentException(UNIQUE_REF_ERROR, e);
     }
   }

   private final AccountGroup.UUID groupUuid;
   private Optional<AccountGroup.NameKey> oldGroupName;
   private Optional<AccountGroup.NameKey> newGroupName;

   private boolean nameConflicting;

   private GroupNameNotes(
       AccountGroup.UUID groupUuid,
       @Nullable AccountGroup.NameKey oldGroupName,
       @Nullable AccountGroup.NameKey newGroupName) {
     this.groupUuid = requireNonNull(groupUuid);

     if (Objects.equals(oldGroupName, newGroupName)) {
       this.oldGroupName = Optional.empty();
       this.newGroupName = Optional.empty();
     } else {
       this.oldGroupName = Optional.ofNullable(oldGroupName);
       this.newGroupName = Optional.ofNullable(newGroupName);
     }
   }

   @Override
   protected String getRefName() {
     return RefNames.REFS_GROUPNAMES;
   }

   @Override
   protected void onLoad() throws IOException, ConfigInvalidException {
     nameConflicting = false;

     logger.atFine().log("Reading group notes");

     if (revision != null) {
       NoteMap noteMap = NoteMap.read(reader, revision);
       if (newGroupName.isPresent()) {
         ObjectId newNameId = getNoteKey(newGroupName.get());
         nameConflicting = noteMap.contains(newNameId);
       }
       ensureOldNameIsPresent(noteMap);
     }
   }

   private void ensureOldNameIsPresent(NoteMap noteMap) throws IOException, ConfigInvalidException {
     if (oldGroupName.isPresent()) {
       AccountGroup.NameKey oldName = oldGroupName.get();
       ObjectId noteKey = getNoteKey(oldName);
       ObjectId noteDataBlobId = noteMap.get(noteKey);
       if (noteDataBlobId == null) {
         throw new ConfigInvalidException(
             String.format("Group name '%s' doesn't exist in the list of all names", oldName));
       }
       GroupReference group = getGroupReference(reader, noteDataBlobId);
       AccountGroup.UUID foundUuid = group.getUUID();
       if (!Objects.equals(groupUuid, foundUuid)) {
         throw new ConfigInvalidException(
             String.format(
                 "Name '%s' points to UUID '%s' and not to '%s'", oldName, foundUuid, groupUuid));
       }
     }
   }

   private void ensureNewNameIsNotUsed() throws DuplicateKeyException {
     if (newGroupName.isPresent() && nameConflicting) {
       throw new DuplicateKeyException(
           String.format("Name '%s' is already used", newGroupName.get().get()));
     }
   }

   @Override
   protected boolean onSave(CommitBuilder commit) throws IOException, ConfigInvalidException {
     if (!oldGroupName.isPresent() && !newGroupName.isPresent()) {
       return false;
     }

     logger.atFine().log("Updating group notes");

     NoteMap noteMap = revision == null ? NoteMap.newEmptyMap() : NoteMap.read(reader, revision);
     if (oldGroupName.isPresent()) {
       removeNote(noteMap, oldGroupName.get(), inserter);
     }

     if (newGroupName.isPresent()) {
       addNote(noteMap, newGroupName.get(), groupUuid, inserter);
     }

     commit.setTreeId(noteMap.writeTree(inserter));
     commit.setMessage(getCommitMessage());

     oldGroupName = Optional.empty();
     newGroupName = Optional.empty();

     return true;
   }

   private static void removeNote(
       NoteMap noteMap, AccountGroup.NameKey groupName, ObjectInserter inserter) throws IOException {
     ObjectId noteKey = getNoteKey(groupName);
     noteMap.set(noteKey, null, inserter);
   }

   private static void addNote(
       NoteMap noteMap,
       AccountGroup.NameKey groupName,
       AccountGroup.UUID groupUuid,
       ObjectInserter inserter)
       throws IOException {
     ObjectId noteKey = getNoteKey(groupName);
     noteMap.set(noteKey, getAsNoteData(groupUuid, groupName), inserter);
   }

   // Use the same approach as ExternalId.Key.sha1().
   @SuppressWarnings("deprecation")
   @VisibleForTesting
   public static ObjectId getNoteKey(AccountGroup.NameKey groupName) {
     return ObjectId.fromRaw(Hashing.sha1().hashString(groupName.get(), UTF_8).asBytes());
   }

   private static String getAsNoteData(AccountGroup.UUID uuid, AccountGroup.NameKey groupName) {
     Config config = new Config();
     config.setString(SECTION_NAME, null, UUID_PARAM, uuid.get());
     config.setString(SECTION_NAME, null, NAME_PARAM, groupName.get());
     return config.toText();
   }

   private static GroupReference getGroupReference(ObjectReader reader, ObjectId noteDataBlobId)
       throws IOException, ConfigInvalidException {
     byte[] noteData = reader.open(noteDataBlobId, OBJ_BLOB).getCachedBytes();
     return getFromNoteData(noteData);
   }

   static GroupReference getFromNoteData(byte[] noteData) throws ConfigInvalidException {
     Config config = new Config();
     config.fromText(new String(noteData, UTF_8));

     String uuid = config.getString(SECTION_NAME, null, UUID_PARAM);
     String name = Strings.nullToEmpty(config.getString(SECTION_NAME, null, NAME_PARAM));
     if (uuid == null) {
       throw new ConfigInvalidException(String.format("UUID for group '%s' must be defined", name));
     }

     return new GroupReference(AccountGroup.uuid(uuid), name);
   }

   private String getCommitMessage() {
     if (oldGroupName.isPresent() && newGroupName.isPresent()) {
       return String.format(
           "Rename group from '%s' to '%s'", oldGroupName.get(), newGroupName.get());
     }
     if (newGroupName.isPresent()) {
       return String.format("Create group '%s'", newGroupName.get());
     }
     if (oldGroupName.isPresent()) {
       return String.format("Delete group '%s'", oldGroupName.get());
     }
     return "No-op";
   }
 }
	// 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.group.db;

	import static com.google.common.collect.ImmutableBiMap.toImmutableBiMap;
	import static java.nio.charset.StandardCharsets.UTF_8;
	import static java.util.Objects.requireNonNull;
	import static org.eclipse.jgit.lib.Constants.OBJ_BLOB;

	import com.google.common.annotations.VisibleForTesting;
	import com.google.common.base.Strings;
	import com.google.common.collect.HashMultiset;
	import com.google.common.collect.ImmutableBiMap;
	import com.google.common.collect.ImmutableList;
	import com.google.common.collect.Multiset;
	import com.google.common.flogger.FluentLogger;
	import com.google.common.hash.Hashing;
	import com.google.gerrit.common.Nullable;
	import com.google.gerrit.common.data.GroupReference;
	import com.google.gerrit.entities.AccountGroup;
	import com.google.gerrit.entities.Project;
	import com.google.gerrit.entities.RefNames;
	import com.google.gerrit.exceptions.DuplicateKeyException;
	import com.google.gerrit.git.ObjectIds;
	import com.google.gerrit.server.git.meta.VersionedMetaData;
	import java.io.IOException;
	import java.util.Collection;
	import java.util.Map;
	import java.util.Objects;
	import java.util.Optional;
	import org.eclipse.jgit.errors.ConfigInvalidException;
	import org.eclipse.jgit.lib.BatchRefUpdate;
	import org.eclipse.jgit.lib.CommitBuilder;
	import org.eclipse.jgit.lib.Config;
	import org.eclipse.jgit.lib.ObjectId;
	import org.eclipse.jgit.lib.ObjectInserter;
	import org.eclipse.jgit.lib.ObjectReader;
	import org.eclipse.jgit.lib.PersonIdent;
	import org.eclipse.jgit.lib.Ref;
	import org.eclipse.jgit.lib.Repository;
	import org.eclipse.jgit.notes.Note;
	import org.eclipse.jgit.notes.NoteMap;
	import org.eclipse.jgit.revwalk.RevCommit;
	import org.eclipse.jgit.revwalk.RevWalk;
	import org.eclipse.jgit.transport.ReceiveCommand;

	/**
	* An enforcer of unique names for groups in NoteDb.
	*
	* <p>The way groups are stored in NoteDb (see {@link GroupConfig}) doesn't enforce unique names,
	* even though groups in Gerrit must not have duplicate names. The storage format doesn't allow to
	* quickly look up whether a name has already been used either. That's why we additionally keep a
	* map of name/UUID pairs and manage it with this class.
	*
	* <p>To claim the name for a new group, create an instance of {@code GroupNameNotes} via {@link
	* #forNewGroup(Project.NameKey, Repository, AccountGroup.UUID, AccountGroup.NameKey)} and call
	* {@link #commit(com.google.gerrit.server.git.meta.MetaDataUpdate) commit(MetaDataUpdate)} on it.
	* For renaming, call {@link #forRename(Project.NameKey, Repository, AccountGroup.UUID,
	* AccountGroup.NameKey, AccountGroup.NameKey)} and also commit the returned {@code GroupNameNotes}.
	* Both times, the creation of the {@code GroupNameNotes} will fail if the (new) name is already
	* used. Committing the {@code GroupNameNotes} is necessary to make the adjustments for real.
	*
	* <p>The map has an additional benefit: We can quickly iterate over all group name/UUID pairs
	* without having to load all groups completely (which is costly).
	*
	* <p><em>Internal details</em>
	*
	* <p>The map of names is represented by Git {@link Note notes}. They are stored on the branch
	* {@link RefNames#REFS_GROUPNAMES}. Each commit on the branch reflects one moment in time of the
	* complete map.
	*
	* <p>As key for the notes, we use the SHA-1 of the name. As data, they contain a text version of a
	* JGit {@link Config} file. That config file has two entries:
	*
	* <ul>
	* <li>the name of the group (as clear text)
	* <li>the UUID of the group which currently has this name
	* </ul>
	*/
	public class GroupNameNotes extends VersionedMetaData {
	private static final FluentLogger logger = FluentLogger.forEnclosingClass();

	private static final String SECTION_NAME = "group";
	private static final String UUID_PARAM = "uuid";
	private static final String NAME_PARAM = "name";

	@VisibleForTesting
	static final String UNIQUE_REF_ERROR = "GroupReference collection must contain unique references";

	/**
	* Creates an instance of {@code GroupNameNotes} for use when renaming a group.
	*
	* <p><strong>Note: </strong>The returned instance of {@code GroupNameNotes} has to be committed
	* via {@link #commit(com.google.gerrit.server.git.meta.MetaDataUpdate) commit(MetaDataUpdate)} in
	* order to claim the new name and free up the old one.
	*
	* @param projectName the name of the project which holds the commits of the notes
	* @param repository the repository which holds the commits of the notes
	* @param groupUuid the UUID of the group which is renamed
	* @param oldName the current name of the group
	* @param newName the new name of the group
	* @return an instance of {@code GroupNameNotes} configured for a specific renaming of a group
	* @throws IOException if the repository can't be accessed for some reason
	* @throws ConfigInvalidException if the note for the specified group doesn't exist or is in an
	* invalid state
	* @throws DuplicateKeyException if a group with the new name already exists
	*/
	public static GroupNameNotes forRename(
	Project.NameKey projectName,
	Repository repository,
	AccountGroup.UUID groupUuid,
	AccountGroup.NameKey oldName,
	AccountGroup.NameKey newName)
	throws IOException, ConfigInvalidException, DuplicateKeyException {
	requireNonNull(oldName);
	requireNonNull(newName);

	GroupNameNotes groupNameNotes = new GroupNameNotes(groupUuid, oldName, newName);
	groupNameNotes.load(projectName, repository);
	groupNameNotes.ensureNewNameIsNotUsed();
	return groupNameNotes;
	}

	/**
	* Creates an instance of {@code GroupNameNotes} for use when creating a new group.
	*
	* <p><strong>Note: </strong>The returned instance of {@code GroupNameNotes} has to be committed
	* via {@link #commit(com.google.gerrit.server.git.meta.MetaDataUpdate) commit(MetaDataUpdate)} in
	* order to claim the new name.
	*
	* @param projectName the name of the project which holds the commits of the notes
	* @param repository the repository which holds the commits of the notes
	* @param groupUuid the UUID of the new group
	* @param groupName the name of the new group
	* @return an instance of {@code GroupNameNotes} configured for a specific group creation
	* @throws IOException if the repository can't be accessed for some reason
	* @throws ConfigInvalidException in no case so far
	* @throws DuplicateKeyException if a group with the new name already exists
	*/
	public static GroupNameNotes forNewGroup(
	Project.NameKey projectName,
	Repository repository,
	AccountGroup.UUID groupUuid,
	AccountGroup.NameKey groupName)
	throws IOException, ConfigInvalidException, DuplicateKeyException {
	requireNonNull(groupName);

	GroupNameNotes groupNameNotes = new GroupNameNotes(groupUuid, null, groupName);
	groupNameNotes.load(projectName, repository);
	groupNameNotes.ensureNewNameIsNotUsed();
	return groupNameNotes;
	}

	/**
	* Loads the {@code GroupReference} (name/UUID pair) for the group with the specified name.
	*
	* @param repository the repository which holds the commits of the notes
	* @param groupName the name of the group
	* @return the corresponding {@code GroupReference} if a group/note with the given name exists
	* @throws IOException if the repository can't be accessed for some reason
	* @throws ConfigInvalidException if the note for the specified group is in an invalid state
	*/
	public static Optional<GroupReference> loadGroup(
	Repository repository, AccountGroup.NameKey groupName)
	throws IOException, ConfigInvalidException {
	Ref ref = repository.exactRef(RefNames.REFS_GROUPNAMES);
	if (ref == null) {
	return Optional.empty();
	}

	try (RevWalk revWalk = new RevWalk(repository);
	ObjectReader reader = revWalk.getObjectReader()) {
	RevCommit notesCommit = revWalk.parseCommit(ref.getObjectId());
	NoteMap noteMap = NoteMap.read(reader, notesCommit);
	ObjectId noteDataBlobId = noteMap.get(getNoteKey(groupName));
	if (noteDataBlobId == null) {
	return Optional.empty();
	}
	return Optional.of(getGroupReference(reader, noteDataBlobId));
	}
	}

	/**
	* Loads the {@code GroupReference}s (name/UUID pairs) for all groups.
	*
	* <p>Even though group UUIDs should be unique, this class doesn't enforce it. For this reason,
	* it's technically possible that two of the {@code GroupReference}s have a duplicate UUID but a
	* different name. In practice, this shouldn't occur unless we introduce a bug in the future.
	*
	* @param repository the repository which holds the commits of the notes
	* @return the {@code GroupReference}s of all existing groups/notes
	* @throws IOException if the repository can't be accessed for some reason
	* @throws ConfigInvalidException if one of the notes is in an invalid state
	*/
	public static ImmutableList<GroupReference> loadAllGroups(Repository repository)
	throws IOException, ConfigInvalidException {
	Ref ref = repository.exactRef(RefNames.REFS_GROUPNAMES);
	if (ref == null) {
	return ImmutableList.of();
	}
	try (RevWalk revWalk = new RevWalk(repository);
	ObjectReader reader = revWalk.getObjectReader()) {
	RevCommit notesCommit = revWalk.parseCommit(ref.getObjectId());
	NoteMap noteMap = NoteMap.read(reader, notesCommit);

	Multiset<GroupReference> groupReferences = HashMultiset.create();
	for (Note note : noteMap) {
	GroupReference groupReference = getGroupReference(reader, note.getData());
	int numOfOccurrences = groupReferences.add(groupReference, 1);
	if (numOfOccurrences > 1) {
	GroupsNoteDbConsistencyChecker.logConsistencyProblemAsWarning(
	"The UUID of group %s (%s) is duplicate in group name notes",
	groupReference.getName(), groupReference.getUUID());
	}
	}

	return ImmutableList.copyOf(groupReferences);
	}
	}

	/**
	* Replaces the map of name/UUID pairs with a new version which matches exactly the passed {@code
	* GroupReference}s.
	*
	* <p>All old entries are discarded and replaced by the new ones.
	*
	* <p>This operation also works if the previous map has invalid entries or can't be read anymore.
	*
	* <p><strong>Note: </strong>This method doesn't flush the {@code ObjectInserter}. It doesn't
	* execute the {@code BatchRefUpdate} either.
	*
	* @param repository the repository which holds the commits of the notes
	* @param inserter an {@code ObjectInserter} for that repository
	* @param bru a {@code BatchRefUpdate} to which this method adds commands
	* @param groupReferences all {@code GroupReference}s (name/UUID pairs) which should be contained
	* in the map of name/UUID pairs
	* @param ident the {@code PersonIdent} which is used as author and committer for commits
	* @throws IOException if the repository can't be accessed for some reason
	*/
	public static void updateAllGroups(
	Repository repository,
	ObjectInserter inserter,
	BatchRefUpdate bru,
	Collection<GroupReference> groupReferences,
	PersonIdent ident)
	throws IOException {
	// Not strictly necessary for iteration; throws IAE if it encounters duplicates, which is nice.
	ImmutableBiMap<AccountGroup.UUID, String> biMap = toBiMap(groupReferences);

	try (ObjectReader reader = inserter.newReader();
	RevWalk rw = new RevWalk(reader)) {
	// Always start from an empty map, discarding old notes.
	NoteMap noteMap = NoteMap.newEmptyMap();
	Ref ref = repository.exactRef(RefNames.REFS_GROUPNAMES);
	RevCommit oldCommit = ref != null ? rw.parseCommit(ref.getObjectId()) : null;

	for (Map.Entry<AccountGroup.UUID, String> e : biMap.entrySet()) {
	AccountGroup.NameKey nameKey = AccountGroup.nameKey(e.getValue());
	ObjectId noteKey = getNoteKey(nameKey);
	noteMap.set(noteKey, getAsNoteData(e.getKey(), nameKey), inserter);
	}

	ObjectId newTreeId = noteMap.writeTree(inserter);
	if (oldCommit != null && newTreeId.equals(oldCommit.getTree())) {
	return;
	}
	CommitBuilder cb = new CommitBuilder();
	if (oldCommit != null) {
	cb.addParentId(oldCommit);
	}
	cb.setTreeId(newTreeId);
	cb.setAuthor(ident);
	cb.setCommitter(ident);
	int n = groupReferences.size();
	cb.setMessage("Store " + n + " group name" + (n != 1 ? "s" : ""));
	ObjectId newId = inserter.insert(cb).copy();

	ObjectId oldId = ObjectIds.copyOrZero(oldCommit);
	bru.addCommand(new ReceiveCommand(oldId, newId, RefNames.REFS_GROUPNAMES));
	}
	}

	// Returns UUID <=> Name bimap.
	private static ImmutableBiMap<AccountGroup.UUID, String> toBiMap(
	Collection<GroupReference> groupReferences) {
	try {
	return groupReferences.stream()
	.collect(toImmutableBiMap(GroupReference::getUUID, GroupReference::getName));
	} catch (IllegalArgumentException e) {
	throw new IllegalArgumentException(UNIQUE_REF_ERROR, e);
	}
	}

	private final AccountGroup.UUID groupUuid;
	private Optional<AccountGroup.NameKey> oldGroupName;
	private Optional<AccountGroup.NameKey> newGroupName;

	private boolean nameConflicting;

	private GroupNameNotes(
	AccountGroup.UUID groupUuid,
	@Nullable AccountGroup.NameKey oldGroupName,
	@Nullable AccountGroup.NameKey newGroupName) {
	this.groupUuid = requireNonNull(groupUuid);

	if (Objects.equals(oldGroupName, newGroupName)) {
	this.oldGroupName = Optional.empty();
	this.newGroupName = Optional.empty();
	} else {
	this.oldGroupName = Optional.ofNullable(oldGroupName);
	this.newGroupName = Optional.ofNullable(newGroupName);
	}
	}

	@Override
	protected String getRefName() {
	return RefNames.REFS_GROUPNAMES;
	}

	@Override
	protected void onLoad() throws IOException, ConfigInvalidException {
	nameConflicting = false;

	logger.atFine().log("Reading group notes");

	if (revision != null) {
	NoteMap noteMap = NoteMap.read(reader, revision);
	if (newGroupName.isPresent()) {
	ObjectId newNameId = getNoteKey(newGroupName.get());
	nameConflicting = noteMap.contains(newNameId);
	}
	ensureOldNameIsPresent(noteMap);
	}
	}

	private void ensureOldNameIsPresent(NoteMap noteMap) throws IOException, ConfigInvalidException {
	if (oldGroupName.isPresent()) {
	AccountGroup.NameKey oldName = oldGroupName.get();
	ObjectId noteKey = getNoteKey(oldName);
	ObjectId noteDataBlobId = noteMap.get(noteKey);
	if (noteDataBlobId == null) {
	throw new ConfigInvalidException(
	String.format("Group name '%s' doesn't exist in the list of all names", oldName));
	}
	GroupReference group = getGroupReference(reader, noteDataBlobId);
	AccountGroup.UUID foundUuid = group.getUUID();
	if (!Objects.equals(groupUuid, foundUuid)) {
	throw new ConfigInvalidException(
	String.format(
	"Name '%s' points to UUID '%s' and not to '%s'", oldName, foundUuid, groupUuid));
	}
	}
	}

	private void ensureNewNameIsNotUsed() throws DuplicateKeyException {
	if (newGroupName.isPresent() && nameConflicting) {
	throw new DuplicateKeyException(
	String.format("Name '%s' is already used", newGroupName.get().get()));
	}
	}

	@Override
	protected boolean onSave(CommitBuilder commit) throws IOException, ConfigInvalidException {
	if (!oldGroupName.isPresent() && !newGroupName.isPresent()) {
	return false;
	}

	logger.atFine().log("Updating group notes");

	NoteMap noteMap = revision == null ? NoteMap.newEmptyMap() : NoteMap.read(reader, revision);
	if (oldGroupName.isPresent()) {
	removeNote(noteMap, oldGroupName.get(), inserter);
	}

	if (newGroupName.isPresent()) {
	addNote(noteMap, newGroupName.get(), groupUuid, inserter);
	}

	commit.setTreeId(noteMap.writeTree(inserter));
	commit.setMessage(getCommitMessage());

	oldGroupName = Optional.empty();
	newGroupName = Optional.empty();

	return true;
	}

	private static void removeNote(
	NoteMap noteMap, AccountGroup.NameKey groupName, ObjectInserter inserter) throws IOException {
	ObjectId noteKey = getNoteKey(groupName);
	noteMap.set(noteKey, null, inserter);
	}

	private static void addNote(
	NoteMap noteMap,
	AccountGroup.NameKey groupName,
	AccountGroup.UUID groupUuid,
	ObjectInserter inserter)
	throws IOException {
	ObjectId noteKey = getNoteKey(groupName);
	noteMap.set(noteKey, getAsNoteData(groupUuid, groupName), inserter);
	}

	// Use the same approach as ExternalId.Key.sha1().
	@SuppressWarnings("deprecation")
	@VisibleForTesting
	public static ObjectId getNoteKey(AccountGroup.NameKey groupName) {
	return ObjectId.fromRaw(Hashing.sha1().hashString(groupName.get(), UTF_8).asBytes());
	}

	private static String getAsNoteData(AccountGroup.UUID uuid, AccountGroup.NameKey groupName) {
	Config config = new Config();
	config.setString(SECTION_NAME, null, UUID_PARAM, uuid.get());
	config.setString(SECTION_NAME, null, NAME_PARAM, groupName.get());
	return config.toText();
	}

	private static GroupReference getGroupReference(ObjectReader reader, ObjectId noteDataBlobId)
	throws IOException, ConfigInvalidException {
	byte[] noteData = reader.open(noteDataBlobId, OBJ_BLOB).getCachedBytes();
	return getFromNoteData(noteData);
	}

	static GroupReference getFromNoteData(byte[] noteData) throws ConfigInvalidException {
	Config config = new Config();
	config.fromText(new String(noteData, UTF_8));

	String uuid = config.getString(SECTION_NAME, null, UUID_PARAM);
	String name = Strings.nullToEmpty(config.getString(SECTION_NAME, null, NAME_PARAM));
	if (uuid == null) {
	throw new ConfigInvalidException(String.format("UUID for group '%s' must be defined", name));
	}

	return new GroupReference(AccountGroup.uuid(uuid), name);
	}

	private String getCommitMessage() {
	if (oldGroupName.isPresent() && newGroupName.isPresent()) {
	return String.format(
	"Rename group from '%s' to '%s'", oldGroupName.get(), newGroupName.get());
	}
	if (newGroupName.isPresent()) {
	return String.format("Create group '%s'", newGroupName.get());
	}
	if (oldGroupName.isPresent()) {
	return String.format("Delete group '%s'", oldGroupName.get());
	}
	return "No-op";
	}
	}