// Copyright (C) 2008 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.git.receive;

import static com.google.common.base.MoreObjects.firstNonNull;
import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkState;
import static com.google.common.collect.ImmutableSet.toImmutableSet;
import static com.google.common.flogger.LazyArgs.lazy;
import static com.google.gerrit.common.FooterConstants.CHANGE_ID;
import static com.google.gerrit.reviewdb.client.RefNames.REFS_CHANGES;
import static com.google.gerrit.reviewdb.client.RefNames.isConfigRef;
import static com.google.gerrit.server.change.HashtagsUtil.cleanupHashtag;
import static com.google.gerrit.server.git.MultiProgressMonitor.UNKNOWN;
import static com.google.gerrit.server.git.receive.ReceiveConstants.COMMAND_REJECTION_MESSAGE_FOOTER;
import static com.google.gerrit.server.git.receive.ReceiveConstants.ONLY_CHANGE_OWNER_OR_PROJECT_OWNER_CAN_MODIFY_WIP;
import static com.google.gerrit.server.git.receive.ReceiveConstants.PUSH_OPTION_SKIP_VALIDATION;
import static com.google.gerrit.server.git.receive.ReceiveConstants.SAME_CHANGE_ID_IN_MULTIPLE_CHANGES;
import static com.google.gerrit.server.git.validators.CommitValidators.NEW_PATCHSET_PATTERN;
import static com.google.gerrit.server.mail.MailUtil.getRecipientsFromFooters;
import static java.nio.charset.StandardCharsets.UTF_8;
import static java.util.Objects.requireNonNull;
import static java.util.stream.Collectors.joining;
import static java.util.stream.Collectors.toList;
import static org.eclipse.jgit.lib.Constants.R_HEADS;
import static org.eclipse.jgit.transport.ReceiveCommand.Result.NOT_ATTEMPTED;
import static org.eclipse.jgit.transport.ReceiveCommand.Result.OK;
import static org.eclipse.jgit.transport.ReceiveCommand.Result.REJECTED_MISSING_OBJECT;
import static org.eclipse.jgit.transport.ReceiveCommand.Result.REJECTED_OTHER_REASON;

import com.google.common.base.Function;
import com.google.common.base.Joiner;
import com.google.common.base.Splitter;
import com.google.common.base.Strings;
import com.google.common.base.Throwables;
import com.google.common.collect.BiMap;
import com.google.common.collect.HashBiMap;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.ImmutableSetMultimap;
import com.google.common.collect.Iterables;
import com.google.common.collect.LinkedListMultimap;
import com.google.common.collect.ListMultimap;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.common.collect.MultimapBuilder;
import com.google.common.collect.Sets;
import com.google.common.collect.SortedSetMultimap;
import com.google.common.collect.Streams;
import com.google.common.flogger.FluentLogger;
import com.google.gerrit.common.FooterConstants;
import com.google.gerrit.common.Nullable;
import com.google.gerrit.common.data.LabelType;
import com.google.gerrit.common.data.LabelTypes;
import com.google.gerrit.exceptions.StorageException;
import com.google.gerrit.extensions.api.changes.HashtagsInput;
import com.google.gerrit.extensions.api.changes.NotifyHandling;
import com.google.gerrit.extensions.api.changes.RecipientType;
import com.google.gerrit.extensions.api.changes.SubmitInput;
import com.google.gerrit.extensions.api.projects.ProjectConfigEntryType;
import com.google.gerrit.extensions.client.GeneralPreferencesInfo;
import com.google.gerrit.extensions.registration.DynamicItem;
import com.google.gerrit.extensions.registration.DynamicMap;
import com.google.gerrit.extensions.registration.Extension;
import com.google.gerrit.extensions.restapi.AuthException;
import com.google.gerrit.extensions.restapi.BadRequestException;
import com.google.gerrit.extensions.restapi.ResourceConflictException;
import com.google.gerrit.extensions.restapi.RestApiException;
import com.google.gerrit.extensions.restapi.UnprocessableEntityException;
import com.google.gerrit.reviewdb.client.Account;
import com.google.gerrit.reviewdb.client.BooleanProjectConfig;
import com.google.gerrit.reviewdb.client.Branch;
import com.google.gerrit.reviewdb.client.Change;
import com.google.gerrit.reviewdb.client.PatchSet;
import com.google.gerrit.reviewdb.client.PatchSetInfo;
import com.google.gerrit.reviewdb.client.Project;
import com.google.gerrit.reviewdb.client.RefNames;
import com.google.gerrit.reviewdb.client.RevId;
import com.google.gerrit.server.ApprovalsUtil;
import com.google.gerrit.server.ChangeUtil;
import com.google.gerrit.server.CreateGroupPermissionSyncer;
import com.google.gerrit.server.IdentifiedUser;
import com.google.gerrit.server.PatchSetUtil;
import com.google.gerrit.server.account.AccountResolver;
import com.google.gerrit.server.change.ChangeInserter;
import com.google.gerrit.server.change.NotifyResolver;
import com.google.gerrit.server.change.SetHashtagsOp;
import com.google.gerrit.server.change.SetPrivateOp;
import com.google.gerrit.server.config.AllProjectsName;
import com.google.gerrit.server.config.GerritServerConfig;
import com.google.gerrit.server.config.PluginConfig;
import com.google.gerrit.server.config.ProjectConfigEntry;
import com.google.gerrit.server.edit.ChangeEdit;
import com.google.gerrit.server.edit.ChangeEditUtil;
import com.google.gerrit.server.git.BanCommit;
import com.google.gerrit.server.git.ChangeReportFormatter;
import com.google.gerrit.server.git.GroupCollector;
import com.google.gerrit.server.git.MergedByPushOp;
import com.google.gerrit.server.git.MultiProgressMonitor;
import com.google.gerrit.server.git.MultiProgressMonitor.Task;
import com.google.gerrit.server.git.ReceivePackInitializer;
import com.google.gerrit.server.git.TagCache;
import com.google.gerrit.server.git.ValidationError;
import com.google.gerrit.server.git.validators.CommitValidationMessage;
import com.google.gerrit.server.git.validators.RefOperationValidationException;
import com.google.gerrit.server.git.validators.RefOperationValidators;
import com.google.gerrit.server.git.validators.ValidationMessage;
import com.google.gerrit.server.index.change.ChangeIndexer;
import com.google.gerrit.server.logging.RequestId;
import com.google.gerrit.server.logging.TraceContext;
import com.google.gerrit.server.mail.MailUtil.MailRecipients;
import com.google.gerrit.server.notedb.ChangeNotes;
import com.google.gerrit.server.notedb.Sequences;
import com.google.gerrit.server.patch.PatchSetInfoFactory;
import com.google.gerrit.server.permissions.ChangePermission;
import com.google.gerrit.server.permissions.GlobalPermission;
import com.google.gerrit.server.permissions.PermissionBackend;
import com.google.gerrit.server.permissions.PermissionBackendException;
import com.google.gerrit.server.permissions.PermissionDeniedException;
import com.google.gerrit.server.permissions.ProjectPermission;
import com.google.gerrit.server.permissions.RefPermission;
import com.google.gerrit.server.plugincontext.PluginSetContext;
import com.google.gerrit.server.project.CreateRefControl;
import com.google.gerrit.server.project.NoSuchChangeException;
import com.google.gerrit.server.project.NoSuchProjectException;
import com.google.gerrit.server.project.ProjectCache;
import com.google.gerrit.server.project.ProjectConfig;
import com.google.gerrit.server.project.ProjectState;
import com.google.gerrit.server.query.change.ChangeData;
import com.google.gerrit.server.query.change.InternalChangeQuery;
import com.google.gerrit.server.submit.MergeOp;
import com.google.gerrit.server.submit.MergeOpRepoManager;
import com.google.gerrit.server.submit.SubmoduleException;
import com.google.gerrit.server.submit.SubmoduleOp;
import com.google.gerrit.server.update.BatchUpdate;
import com.google.gerrit.server.update.BatchUpdateOp;
import com.google.gerrit.server.update.ChangeContext;
import com.google.gerrit.server.update.Context;
import com.google.gerrit.server.update.RepoContext;
import com.google.gerrit.server.update.RepoOnlyOp;
import com.google.gerrit.server.update.RetryHelper;
import com.google.gerrit.server.update.RetryHelper.Action;
import com.google.gerrit.server.update.RetryHelper.ActionType;
import com.google.gerrit.server.update.UpdateException;
import com.google.gerrit.server.util.LabelVote;
import com.google.gerrit.server.util.MagicBranch;
import com.google.gerrit.server.util.RequestScopePropagator;
import com.google.gerrit.server.util.time.TimeUtil;
import com.google.gerrit.util.cli.CmdLineParser;
import com.google.inject.Inject;
import com.google.inject.Provider;
import com.google.inject.assistedinject.Assisted;
import com.google.inject.util.Providers;
import java.io.IOException;
import java.io.StringWriter;
import java.io.UnsupportedEncodingException;
import java.net.URLDecoder;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.Optional;
import java.util.Set;
import java.util.TreeSet;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.regex.Matcher;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import org.eclipse.jgit.errors.ConfigInvalidException;
import org.eclipse.jgit.errors.IncorrectObjectTypeException;
import org.eclipse.jgit.errors.MissingObjectException;
import org.eclipse.jgit.lib.Config;
import org.eclipse.jgit.lib.Constants;
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.NoteMap;
import org.eclipse.jgit.revwalk.FooterLine;
import org.eclipse.jgit.revwalk.RevCommit;
import org.eclipse.jgit.revwalk.RevObject;
import org.eclipse.jgit.revwalk.RevSort;
import org.eclipse.jgit.revwalk.RevWalk;
import org.eclipse.jgit.revwalk.filter.RevFilter;
import org.eclipse.jgit.transport.ReceiveCommand;
import org.eclipse.jgit.transport.ReceiveCommand.Result;
import org.eclipse.jgit.transport.ReceivePack;
import org.kohsuke.args4j.CmdLineException;
import org.kohsuke.args4j.Option;

/**
 * Receives change upload using the Git receive-pack protocol.
 *
 * <p>Conceptually, most use of Gerrit is a push of some commits to refs/for/BRANCH. However, the
 * receive-pack protocol that this is based on allows multiple ref updates to be processed at once.
 * So we have to be prepared to also handle normal pushes (refs/heads/BRANCH), and legacy pushes
 * (refs/changes/CHANGE). It is hard to split this class up further, because normal pushes can also
 * result in updates to reviews, through the autoclose mechanism.
 */
class ReceiveCommits {
  private static final FluentLogger logger = FluentLogger.forEnclosingClass();

  private static final String CODE_REVIEW_ERROR =
      "You need 'Push' rights to upload code review requests.\n"
          + "Verify that you are pushing to the right branch.";
  private static final String CANNOT_DELETE_CHANGES = "Cannot delete from '" + REFS_CHANGES + "'";
  private static final String CANNOT_DELETE_CONFIG =
      "Cannot delete project configuration from '" + RefNames.REFS_CONFIG + "'";
  private static final String INTERNAL_SERVER_ERROR = "internal server error";

  interface Factory {
    ReceiveCommits create(
        ProjectState projectState,
        IdentifiedUser user,
        ReceivePack receivePack,
        AllRefsWatcher allRefsWatcher,
        MessageSender messageSender,
        ResultChangeIds resultChangeIds);
  }

  private class ReceivePackMessageSender implements MessageSender {
    @Override
    public void sendMessage(String what) {
      receivePack.sendMessage(what);
    }

    @Override
    public void sendError(String what) {
      receivePack.sendError(what);
    }

    @Override
    public void sendBytes(byte[] what) {
      sendBytes(what, 0, what.length);
    }

    @Override
    public void sendBytes(byte[] what, int off, int len) {
      try {
        receivePack.getMessageOutputStream().write(what, off, len);
      } catch (IOException e) {
        // Ignore write failures (matching JGit behavior).
      }
    }

    @Override
    public void flush() {
      try {
        receivePack.getMessageOutputStream().flush();
      } catch (IOException e) {
        // Ignore write failures (matching JGit behavior).
      }
    }
  }

  private static final Function<Exception, RestApiException> INSERT_EXCEPTION =
      input -> {
        if (input instanceof RestApiException) {
          return (RestApiException) input;
        } else if ((input instanceof ExecutionException)
            && (input.getCause() instanceof RestApiException)) {
          return (RestApiException) input.getCause();
        }
        return new RestApiException("Error inserting change/patchset", input);
      };

  // ReceiveCommits has a lot of fields, sorry. Here and in the constructor they are split up
  // somewhat, and kept sorted lexicographically within sections, except where later assignments
  // depend on previous ones.

  // Injected fields.
  private final AccountResolver accountResolver;
  private final AllProjectsName allProjectsName;
  private final BatchUpdate.Factory batchUpdateFactory;
  private final ChangeEditUtil editUtil;
  private final ChangeIndexer indexer;
  private final ChangeInserter.Factory changeInserterFactory;
  private final ChangeNotes.Factory notesFactory;
  private final ChangeReportFormatter changeFormatter;
  private final CmdLineParser.Factory optionParserFactory;
  private final BranchCommitValidator.Factory commitValidatorFactory;
  private final CreateGroupPermissionSyncer createGroupPermissionSyncer;
  private final CreateRefControl createRefControl;
  private final DynamicMap<ProjectConfigEntry> pluginConfigEntries;
  private final PluginSetContext<ReceivePackInitializer> initializers;
  private final MergedByPushOp.Factory mergedByPushOpFactory;
  private final PatchSetInfoFactory patchSetInfoFactory;
  private final PatchSetUtil psUtil;
  private final PermissionBackend permissionBackend;
  private final ProjectCache projectCache;
  private final Provider<InternalChangeQuery> queryProvider;
  private final Provider<MergeOp> mergeOpProvider;
  private final Provider<MergeOpRepoManager> ormProvider;
  private final ReceiveConfig receiveConfig;
  private final RefOperationValidators.Factory refValidatorsFactory;
  private final ReplaceOp.Factory replaceOpFactory;
  private final RetryHelper retryHelper;
  private final RequestScopePropagator requestScopePropagator;
  private final Sequences seq;
  private final SetHashtagsOp.Factory hashtagsFactory;
  private final SubmoduleOp.Factory subOpFactory;
  private final TagCache tagCache;
  private final ProjectConfig.Factory projectConfigFactory;
  private final SetPrivateOp.Factory setPrivateOpFactory;

  // Assisted injected fields.
  private final AllRefsWatcher allRefsWatcher;
  private final ProjectState projectState;
  private final IdentifiedUser user;
  private final ReceivePack receivePack;

  // Immutable fields derived from constructor arguments.
  private final boolean allowProjectOwnersToChangeParent;
  private final boolean allowPushToRefsChanges;
  private final LabelTypes labelTypes;
  private final NoteMap rejectCommits;
  private final PermissionBackend.ForProject permissions;
  private final Project project;
  private final Repository repo;

  // Collections populated during processing.
  private final List<UpdateGroupsRequest> updateGroups;
  private final List<ValidationMessage> messages;
  /** Multimap of error text to refnames that produced that error. */
  private final ListMultimap<String, String> errors;

  private final ListMultimap<String, String> pushOptions;
  private final Map<Change.Id, ReplaceRequest> replaceByChange;

  // Collections lazily populated during processing.
  private ListMultimap<Change.Id, Ref> refsByChange;
  private ListMultimap<ObjectId, Ref> refsById;

  // Other settings populated during processing.
  private MagicBranchInput magicBranch;
  private boolean newChangeForAllNotInTarget;
  private boolean setChangeAsPrivate;
  private Optional<NoteDbPushOption> noteDbPushOption;
  private Optional<String> tracePushOption;

  private MessageSender messageSender;
  private ResultChangeIds resultChangeIds;

  @Inject
  ReceiveCommits(
      AccountResolver accountResolver,
      AllProjectsName allProjectsName,
      BatchUpdate.Factory batchUpdateFactory,
      ProjectConfig.Factory projectConfigFactory,
      @GerritServerConfig Config cfg,
      ChangeEditUtil editUtil,
      ChangeIndexer indexer,
      ChangeInserter.Factory changeInserterFactory,
      ChangeNotes.Factory notesFactory,
      DynamicItem<ChangeReportFormatter> changeFormatterProvider,
      CmdLineParser.Factory optionParserFactory,
      BranchCommitValidator.Factory commitValidatorFactory,
      CreateGroupPermissionSyncer createGroupPermissionSyncer,
      CreateRefControl createRefControl,
      DynamicMap<ProjectConfigEntry> pluginConfigEntries,
      PluginSetContext<ReceivePackInitializer> initializers,
      MergedByPushOp.Factory mergedByPushOpFactory,
      PatchSetInfoFactory patchSetInfoFactory,
      PatchSetUtil psUtil,
      PermissionBackend permissionBackend,
      ProjectCache projectCache,
      Provider<InternalChangeQuery> queryProvider,
      Provider<MergeOp> mergeOpProvider,
      Provider<MergeOpRepoManager> ormProvider,
      ReceiveConfig receiveConfig,
      RefOperationValidators.Factory refValidatorsFactory,
      ReplaceOp.Factory replaceOpFactory,
      RetryHelper retryHelper,
      RequestScopePropagator requestScopePropagator,
      Sequences seq,
      SetHashtagsOp.Factory hashtagsFactory,
      SubmoduleOp.Factory subOpFactory,
      TagCache tagCache,
      SetPrivateOp.Factory setPrivateOpFactory,
      @Assisted ProjectState projectState,
      @Assisted IdentifiedUser user,
      @Assisted ReceivePack rp,
      @Assisted AllRefsWatcher allRefsWatcher,
      @Nullable @Assisted MessageSender messageSender,
      @Assisted ResultChangeIds resultChangeIds)
      throws IOException {
    // Injected fields.
    this.accountResolver = accountResolver;
    this.allProjectsName = allProjectsName;
    this.batchUpdateFactory = batchUpdateFactory;
    this.changeFormatter = changeFormatterProvider.get();
    this.changeInserterFactory = changeInserterFactory;
    this.commitValidatorFactory = commitValidatorFactory;
    this.createRefControl = createRefControl;
    this.createGroupPermissionSyncer = createGroupPermissionSyncer;
    this.editUtil = editUtil;
    this.hashtagsFactory = hashtagsFactory;
    this.indexer = indexer;
    this.initializers = initializers;
    this.mergeOpProvider = mergeOpProvider;
    this.mergedByPushOpFactory = mergedByPushOpFactory;
    this.notesFactory = notesFactory;
    this.optionParserFactory = optionParserFactory;
    this.ormProvider = ormProvider;
    this.patchSetInfoFactory = patchSetInfoFactory;
    this.permissionBackend = permissionBackend;
    this.pluginConfigEntries = pluginConfigEntries;
    this.projectCache = projectCache;
    this.psUtil = psUtil;
    this.queryProvider = queryProvider;
    this.receiveConfig = receiveConfig;
    this.refValidatorsFactory = refValidatorsFactory;
    this.replaceOpFactory = replaceOpFactory;
    this.retryHelper = retryHelper;
    this.requestScopePropagator = requestScopePropagator;
    this.seq = seq;
    this.subOpFactory = subOpFactory;
    this.tagCache = tagCache;
    this.projectConfigFactory = projectConfigFactory;
    this.setPrivateOpFactory = setPrivateOpFactory;

    // Assisted injected fields.
    this.allRefsWatcher = allRefsWatcher;
    this.projectState = projectState;
    this.user = user;
    this.receivePack = rp;

    // Immutable fields derived from constructor arguments.
    allowPushToRefsChanges = cfg.getBoolean("receive", "allowPushToRefsChanges", false);
    repo = rp.getRepository();
    project = projectState.getProject();
    labelTypes = projectState.getLabelTypes();
    permissions = permissionBackend.user(user).project(project.getNameKey());
    rejectCommits = BanCommit.loadRejectCommitsMap(rp.getRepository(), rp.getRevWalk());

    // Collections populated during processing.
    errors = MultimapBuilder.linkedHashKeys().arrayListValues().build();
    messages = new ArrayList<>();
    pushOptions = LinkedListMultimap.create();
    replaceByChange = new LinkedHashMap<>();
    updateGroups = new ArrayList<>();

    this.allowProjectOwnersToChangeParent =
        cfg.getBoolean("receive", "allowProjectOwnersToChangeParent", false);

    // Other settings populated during processing.
    newChangeForAllNotInTarget =
        projectState.is(BooleanProjectConfig.CREATE_NEW_CHANGE_FOR_ALL_NOT_IN_TARGET);

    // Handles for outputting back over the wire to the end user.
    this.messageSender = messageSender != null ? messageSender : new ReceivePackMessageSender();
    this.resultChangeIds = resultChangeIds;
  }

  void init() {
    initializers.runEach(i -> i.init(projectState.getNameKey(), receivePack));
  }

  MessageSender getMessageSender() {
    return messageSender;
  }

  Project getProject() {
    return project;
  }

  private void addMessage(String message, ValidationMessage.Type type) {
    messages.add(new CommitValidationMessage(message, type));
  }

  private void addMessage(String message) {
    messages.add(new CommitValidationMessage(message, ValidationMessage.Type.OTHER));
  }

  private void addError(String error) {
    addMessage(error, ValidationMessage.Type.ERROR);
  }

  void sendMessages() {
    for (ValidationMessage m : messages) {
      String msg = m.getType().getPrefix() + m.getMessage();

      // Avoid calling sendError which will add its own error: prefix.
      messageSender.sendMessage(msg);
    }
  }

  void processCommands(Collection<ReceiveCommand> commands, MultiProgressMonitor progress) {
    Task commandProgress = progress.beginSubTask("refs", UNKNOWN);
    commands = commands.stream().map(c -> wrapReceiveCommand(c, commandProgress)).collect(toList());
    processCommandsUnsafe(commands, progress);
    rejectRemaining(commands, INTERNAL_SERVER_ERROR);

    // This sends error messages before the 'done' string of the progress monitor is sent.
    // Currently, the test framework relies on this ordering to understand if pushes completed
    // successfully.
    sendErrorMessages();

    commandProgress.end();
    progress.end();
  }

  // Process as many commands as possible, but may leave some commands in state NOT_ATTEMPTED.
  private void processCommandsUnsafe(
      Collection<ReceiveCommand> commands, MultiProgressMonitor progress) {
    parsePushOptions();
    try (TraceContext traceContext =
        TraceContext.newTrace(
            tracePushOption.isPresent(),
            tracePushOption.orElse(null),
            (tagName, traceId) -> addMessage(tagName + ": " + traceId))) {
      traceContext.addTag(RequestId.Type.RECEIVE_ID, new RequestId(project.getNameKey().get()));

      logger.atFinest().log("Calling user: %s", user.getLoggableName());

      // Log the push options here, rather than in parsePushOptions(), so that they are included
      // into the trace if tracing is enabled.
      logger.atFine().log("push options: %s", receivePack.getPushOptions());

      if (!projectState.getProject().getState().permitsWrite()) {
        for (ReceiveCommand cmd : commands) {
          reject(cmd, "prohibited by Gerrit: project state does not permit write");
        }
        return;
      }

      logger.atFine().log("Parsing %d commands", commands.size());

      List<ReceiveCommand> magicCommands = new ArrayList<>();
      List<ReceiveCommand> directPatchSetPushCommands = new ArrayList<>();
      List<ReceiveCommand> regularCommands = new ArrayList<>();

      for (ReceiveCommand cmd : commands) {
        if (MagicBranch.isMagicBranch(cmd.getRefName())) {
          magicCommands.add(cmd);
        } else if (isDirectChangesPush(cmd.getRefName())) {
          directPatchSetPushCommands.add(cmd);
        } else {
          regularCommands.add(cmd);
        }
      }

      int commandTypes =
          (magicCommands.isEmpty() ? 0 : 1)
              + (directPatchSetPushCommands.isEmpty() ? 0 : 1)
              + (regularCommands.isEmpty() ? 0 : 1);

      if (commandTypes > 1) {
        rejectRemaining(commands, "cannot combine normal pushes and magic pushes");
        return;
      }

      try {
        if (!regularCommands.isEmpty()) {
          handleRegularCommands(regularCommands, progress);
          return;
        }

        for (ReceiveCommand cmd : directPatchSetPushCommands) {
          parseDirectChangesPush(cmd);
        }

        boolean first = true;
        for (ReceiveCommand cmd : magicCommands) {
          if (first) {
            parseMagicBranch(cmd);
            first = false;
          } else {
            reject(cmd, "duplicate request");
          }
        }
      } catch (PermissionBackendException | NoSuchProjectException | IOException err) {
        logger.atSevere().withCause(err).log("Failed to process refs in %s", project.getName());
        return;
      }

      Task newProgress = progress.beginSubTask("new", UNKNOWN);
      Task replaceProgress = progress.beginSubTask("updated", UNKNOWN);

      List<CreateRequest> newChanges = Collections.emptyList();
      if (magicBranch != null && magicBranch.cmd.getResult() == NOT_ATTEMPTED) {
        newChanges = selectNewAndReplacedChangesFromMagicBranch(newProgress);
      }

      // Commit validation has already happened, so any changes without Change-Id are for the
      // deprecated feature.
      warnAboutMissingChangeId(newChanges);
      preparePatchSetsForReplace(newChanges);
      insertChangesAndPatchSets(newChanges, replaceProgress);
      newProgress.end();
      replaceProgress.end();
      queueSuccessMessages(newChanges);

      logger.atFine().log(
          "Command results: %s",
          lazy(() -> commands.stream().map(ReceiveCommits::commandToString).collect(joining(","))));
    }
  }

  private void sendErrorMessages() {
    if (!errors.isEmpty()) {
      logger.atFine().log("Handling error conditions: %s", errors.keySet());
      for (String error : errors.keySet()) {
        receivePack.sendMessage("error: " + buildError(error, errors.get(error)));
      }
      receivePack.sendMessage(String.format("User: %s", user.getLoggableName()));
      receivePack.sendMessage(COMMAND_REJECTION_MESSAGE_FOOTER);
    }
  }

  private void handleRegularCommands(List<ReceiveCommand> cmds, MultiProgressMonitor progress)
      throws PermissionBackendException, IOException, NoSuchProjectException {
    resultChangeIds.setMagicPush(false);
    for (ReceiveCommand cmd : cmds) {
      parseRegularCommand(cmd);
    }

    try (BatchUpdate bu =
            batchUpdateFactory.create(
                project.getNameKey(), user.materializedCopy(), TimeUtil.nowTs());
        ObjectInserter ins = repo.newObjectInserter();
        ObjectReader reader = ins.newReader();
        RevWalk rw = new RevWalk(reader)) {
      bu.setRepository(repo, rw, ins);
      bu.setRefLogMessage("push");

      int added = 0;
      for (ReceiveCommand cmd : cmds) {
        if (cmd.getResult() == NOT_ATTEMPTED) {
          bu.addRepoOnlyOp(new UpdateOneRefOp(cmd));
          added++;
        }
      }
      logger.atFine().log("Added %d additional ref updates", added);
      bu.execute();
    } catch (UpdateException | RestApiException e) {
      rejectRemaining(cmds, INTERNAL_SERVER_ERROR);
      logger.atSevere().withCause(e).log("update failed:");
    }

    Set<Branch.NameKey> branches = new HashSet<>();
    for (ReceiveCommand c : cmds) {
      // Most post-update steps should happen in UpdateOneRefOp#postUpdate. The only steps that
      // should happen in this loop are things that can't happen within one BatchUpdate because
      // they involve kicking off an additional BatchUpdate.
      if (c.getResult() != OK) {
        continue;
      }
      if (isHead(c) || isConfig(c)) {
        switch (c.getType()) {
          case CREATE:
          case UPDATE:
          case UPDATE_NONFASTFORWARD:
            Task closeProgress = progress.beginSubTask("closed", UNKNOWN);
            autoCloseChanges(c, closeProgress);
            closeProgress.end();
            branches.add(new Branch.NameKey(project.getNameKey(), c.getRefName()));
            break;

          case DELETE:
            break;
        }
      }
    }

    // Update superproject gitlinks if required.
    if (!branches.isEmpty()) {
      try (MergeOpRepoManager orm = ormProvider.get()) {
        orm.setContext(TimeUtil.nowTs(), user, NotifyResolver.Result.none());
        SubmoduleOp op = subOpFactory.create(branches, orm);
        op.updateSuperProjects();
      } catch (SubmoduleException e) {
        logger.atSevere().withCause(e).log("Can't update the superprojects");
      }
    }
  }

  /** Appends messages for successful change creation/updates. */
  private void queueSuccessMessages(List<CreateRequest> newChanges) {
    // adjacency list for commit => parent
    Map<String, String> adjList = new HashMap<>();
    for (CreateRequest cr : newChanges) {
      String parent = cr.commit.getParentCount() == 0 ? null : cr.commit.getParent(0).name();
      adjList.put(cr.commit.name(), parent);
    }
    for (ReplaceRequest rr : replaceByChange.values()) {
      String parent = null;
      if (rr.revCommit != null) {
        parent = rr.revCommit.getParentCount() == 0 ? null : rr.revCommit.getParent(0).name();
      }
      adjList.put(rr.newCommitId.name(), parent);
    }

    // get commits that are not parents
    Set<String> leafs = new TreeSet<>(adjList.keySet());
    leafs.removeAll(adjList.values());
    // go backwards from the last commit to its parent(s)
    Set<String> ordered = new LinkedHashSet<>();
    for (String leaf : leafs) {
      if (ordered.contains(leaf)) {
        continue;
      }
      while (leaf != null) {
        if (!ordered.contains(leaf)) {
          ordered.add(leaf);
        }
        leaf = adjList.get(leaf);
      }
    }
    // reverse the order to start with earliest commit
    List<String> orderedCommits = new ArrayList<>(ordered);
    Collections.reverse(orderedCommits);

    Map<String, CreateRequest> created =
        newChanges.stream()
            .filter(r -> r.change != null)
            .collect(Collectors.toMap(r -> r.commit.name(), r -> r));
    Map<String, ReplaceRequest> updated =
        replaceByChange.values().stream()
            .filter(r -> r.inputCommand.getResult() == OK)
            .collect(Collectors.toMap(r -> r.newCommitId.name(), r -> r));

    if (created.isEmpty() && updated.isEmpty()) {
      return;
    }

    addMessage("");
    addMessage("SUCCESS");
    addMessage("");

    boolean edit = false;
    Boolean isPrivate = null;
    Boolean wip = null;
    if (!updated.isEmpty()) {
      edit = magicBranch != null && (magicBranch.edit || magicBranch.draft);
      if (magicBranch != null) {
        if (magicBranch.isPrivate) {
          isPrivate = true;
        } else if (magicBranch.removePrivate) {
          isPrivate = false;
        }
        if (magicBranch.workInProgress) {
          wip = true;
        } else if (magicBranch.ready) {
          wip = false;
        }
      }
    }

    for (String commit : orderedCommits) {
      if (created.get(commit) != null) {
        addCreatedMessage(created.get(commit));
      } else if (updated.get(commit) != null) {
        addReplacedMessage(updated.get(commit), edit, isPrivate, wip);
      }
    }
    addMessage("");
  }

  private void addCreatedMessage(CreateRequest c) {
    addMessage(
        changeFormatter.newChange(
            ChangeReportFormatter.Input.builder().setChange(c.change).build()));
  }

  private void addReplacedMessage(ReplaceRequest u, boolean edit, Boolean isPrivate, Boolean wip) {
    String subject;
    if (edit) {
      subject =
          u.revCommit == null ? u.notes.getChange().getSubject() : u.revCommit.getShortMessage();
    } else {
      subject = u.info.getSubject();
    }

    if (isPrivate == null) {
      isPrivate = u.notes.getChange().isPrivate();
    }
    if (wip == null) {
      wip = u.notes.getChange().isWorkInProgress();
    }

    ChangeReportFormatter.Input input =
        ChangeReportFormatter.Input.builder()
            .setChange(u.notes.getChange())
            .setSubject(subject)
            .setIsEdit(edit)
            .setIsPrivate(isPrivate)
            .setIsWorkInProgress(wip)
            .build();
    addMessage(changeFormatter.changeUpdated(input));
  }

  private void insertChangesAndPatchSets(List<CreateRequest> newChanges, Task replaceProgress) {
    ReceiveCommand magicBranchCmd = magicBranch != null ? magicBranch.cmd : null;
    if (magicBranchCmd != null && magicBranchCmd.getResult() != NOT_ATTEMPTED) {
      logger.atWarning().log(
          "Skipping change updates on %s because ref update failed: %s %s",
          project.getName(),
          magicBranchCmd.getResult(),
          Strings.nullToEmpty(magicBranchCmd.getMessage()));
      return;
    }

    try (BatchUpdate bu =
            batchUpdateFactory.create(
                project.getNameKey(), user.materializedCopy(), TimeUtil.nowTs());
        ObjectInserter ins = repo.newObjectInserter();
        ObjectReader reader = ins.newReader();
        RevWalk rw = new RevWalk(reader)) {
      bu.setRepository(repo, rw, ins);
      bu.setRefLogMessage("push");
      if (magicBranch != null) {
        bu.setNotify(magicBranch.getNotifyForNewChange());
      }

      logger.atFine().log("Adding %d replace requests", newChanges.size());
      for (ReplaceRequest replace : replaceByChange.values()) {
        if (magicBranch != null) {
          bu.setNotifyHandling(replace.ontoChange, magicBranch.getNotifyHandling(replace.notes));
        }
        replace.addOps(bu, replaceProgress);
      }

      logger.atFine().log("Adding %d create requests", newChanges.size());
      for (CreateRequest create : newChanges) {
        create.addOps(bu);
      }

      logger.atFine().log("Adding %d group update requests", newChanges.size());
      updateGroups.forEach(r -> r.addOps(bu));

      logger.atFine().log("Executing batch");
      try {
        bu.execute();
      } catch (UpdateException e) {
        throw INSERT_EXCEPTION.apply(e);
      }

      replaceByChange.values().stream()
          .forEach(req -> resultChangeIds.add(ResultChangeIds.Key.REPLACED, req.ontoChange));
      newChanges.stream()
          .forEach(req -> resultChangeIds.add(ResultChangeIds.Key.CREATED, req.changeId));

      if (magicBranchCmd != null) {
        magicBranchCmd.setResult(OK);
      }
      for (ReplaceRequest replace : replaceByChange.values()) {
        String rejectMessage = replace.getRejectMessage();
        if (rejectMessage == null) {
          if (replace.inputCommand.getResult() == NOT_ATTEMPTED) {
            // Not necessarily the magic branch, so need to set OK on the original value.
            replace.inputCommand.setResult(OK);
          }
        } else {
          logger.atFine().log("Rejecting due to message from ReplaceOp");
          reject(replace.inputCommand, rejectMessage);
        }
      }

    } catch (ResourceConflictException e) {
      addError(e.getMessage());
      reject(magicBranchCmd, "conflict");
    } catch (BadRequestException | UnprocessableEntityException | AuthException e) {
      logger.atFine().withCause(e).log("Rejecting due to client error");
      reject(magicBranchCmd, e.getMessage());
    } catch (RestApiException | IOException e) {
      logger.atSevere().withCause(e).log("Can't insert change/patch set for %s", project.getName());
      reject(magicBranchCmd, String.format("%s: %s", INTERNAL_SERVER_ERROR, e.getMessage()));
    }

    if (magicBranch != null && magicBranch.submit) {
      try {
        submit(newChanges, replaceByChange.values());
      } catch (ResourceConflictException e) {
        addError(e.getMessage());
        reject(magicBranchCmd, "conflict");
      } catch (RestApiException
          | StorageException
          | UpdateException
          | IOException
          | ConfigInvalidException
          | PermissionBackendException e) {
        logger.atSevere().withCause(e).log("Error submitting changes to %s", project.getName());
        reject(magicBranchCmd, "error during submit");
      }
    }
  }

  private String buildError(String error, List<String> branches) {
    StringBuilder sb = new StringBuilder();
    if (branches.size() == 1) {
      sb.append("branch ").append(branches.get(0)).append(":\n");
      sb.append(error);
      return sb.toString();
    }
    sb.append("branches ").append(Joiner.on(", ").join(branches));
    return sb.append(":\n").append(error).toString();
  }

  /** Parses push options specified as "git push -o OPTION" */
  private void parsePushOptions() {
    List<String> optionList = receivePack.getPushOptions();
    if (optionList != null) {
      for (String option : optionList) {
        int e = option.indexOf('=');
        if (e > 0) {
          pushOptions.put(option.substring(0, e), option.substring(e + 1));
        } else {
          pushOptions.put(option, "");
        }
      }
    }

    List<String> noteDbValues = pushOptions.get("notedb");
    if (!noteDbValues.isEmpty()) {
      // These semantics for duplicates/errors are somewhat arbitrary and may not match e.g. the
      // CmdLineParser behavior used by MagicBranchInput.
      String value = noteDbValues.get(noteDbValues.size() - 1);
      noteDbPushOption = NoteDbPushOption.parse(value);
      if (!noteDbPushOption.isPresent()) {
        addError("Invalid value in -o " + NoteDbPushOption.OPTION_NAME + "=" + value);
      }
    } else {
      noteDbPushOption = Optional.of(NoteDbPushOption.DISALLOW);
    }

    List<String> traceValues = pushOptions.get("trace");
    if (!traceValues.isEmpty()) {
      String value = traceValues.get(traceValues.size() - 1);
      tracePushOption = Optional.of(value);
    } else {
      tracePushOption = Optional.empty();
    }
  }

  private static boolean isDirectChangesPush(String refname) {
    Matcher m = NEW_PATCHSET_PATTERN.matcher(refname);
    return m.matches();
  }

  private void parseDirectChangesPush(ReceiveCommand cmd) {
    Matcher m = NEW_PATCHSET_PATTERN.matcher(cmd.getRefName());
    checkArgument(m.matches());

    if (allowPushToRefsChanges) {
      // The referenced change must exist and must still be open.
      Change.Id changeId = Change.Id.parse(m.group(1));
      parseReplaceCommand(cmd, changeId);
      messages.add(new ValidationMessage("warning: pushes to refs/changes are deprecated", false));
    } else {
      reject(cmd, "upload to refs/changes not allowed");
    }
  }

  // Wrap ReceiveCommand so the progress counter works automatically.
  private ReceiveCommand wrapReceiveCommand(ReceiveCommand cmd, Task progress) {
    String refname = cmd.getRefName();

    if (projectState.isAllUsers() && RefNames.REFS_USERS_SELF.equals(cmd.getRefName())) {
      refname = RefNames.refsUsers(user.getAccountId());
      logger.atFine().log("Swapping out command for %s to %s", RefNames.REFS_USERS_SELF, refname);
    }

    // We must also update the original, because callers may inspect it afterwards to decide if
    // the command went through or not.
    return new ReceiveCommand(cmd.getOldId(), cmd.getNewId(), refname, cmd.getType()) {
      @Override
      public void setResult(Result s, String m) {
        if (getResult() == NOT_ATTEMPTED) { // Only report the progress update once.
          progress.update(1);
        }
        // Counter intuitively, we don't check that results == NOT_ATTEMPTED here.
        // This is so submit-on-push can still reject the update if the change is created
        // successfully
        // (status OK) but the submit failed (merge failed: REJECTED_OTHER_REASON).
        super.setResult(s, m);
        cmd.setResult(s, m);
      }
    };
  }

  /*
   * Interpret a normal push.
   */
  private void parseRegularCommand(ReceiveCommand cmd)
      throws PermissionBackendException, NoSuchProjectException, IOException {
    if (cmd.getResult() != NOT_ATTEMPTED) {
      // Already rejected by the core receive process.
      logger.atFine().log("Already processed by core: %s %s", cmd.getResult(), cmd);
      return;
    }

    if (!Repository.isValidRefName(cmd.getRefName()) || cmd.getRefName().contains("//")) {
      reject(cmd, "not valid ref");
      return;
    }
    if (RefNames.isNoteDbMetaRef(cmd.getRefName())) {
      // Reject pushes to NoteDb refs without a special option and permission. Note that this
      // prohibition doesn't depend on NoteDb being enabled in any way, since all sites will
      // migrate to NoteDb eventually, and we don't want garbage data waiting there when the
      // migration finishes.
      logger.atFine().log(
          "%s NoteDb ref %s with %s=%s",
          cmd.getType(), cmd.getRefName(), NoteDbPushOption.OPTION_NAME, noteDbPushOption);
      if (!Optional.of(NoteDbPushOption.ALLOW).equals(noteDbPushOption)) {
        // Only reject this command, not the whole push. This supports the use case of "git clone
        // --mirror" followed by "git push --mirror", when the user doesn't really intend to clone
        // or mirror the NoteDb data; there is no single refspec that describes all refs *except*
        // NoteDb refs.
        reject(
            cmd,
            "NoteDb update requires -o "
                + NoteDbPushOption.OPTION_NAME
                + "="
                + NoteDbPushOption.ALLOW.value());
        return;
      }
      try {
        permissionBackend.user(user).check(GlobalPermission.ACCESS_DATABASE);
      } catch (AuthException e) {
        reject(cmd, "NoteDb update requires access database permission");
        return;
      }
    }

    switch (cmd.getType()) {
      case CREATE:
        parseCreate(cmd);
        break;

      case UPDATE:
        parseUpdate(cmd);
        break;

      case DELETE:
        parseDelete(cmd);
        break;

      case UPDATE_NONFASTFORWARD:
        parseRewind(cmd);
        break;

      default:
        reject(cmd, "prohibited by Gerrit: unknown command type " + cmd.getType());
        return;
    }

    if (cmd.getResult() != NOT_ATTEMPTED) {
      return;
    }

    if (isConfig(cmd)) {
      validateConfigPush(cmd);
    }
  }

  /** Validates a push to refs/meta/config, and reject the command if it fails. */
  private void validateConfigPush(ReceiveCommand cmd) throws PermissionBackendException {
    logger.atFine().log("Processing %s command", cmd.getRefName());
    try {
      permissions.check(ProjectPermission.WRITE_CONFIG);
    } catch (AuthException e) {
      reject(
          cmd,
          String.format(
              "must be either project owner or have %s permission",
              ProjectPermission.WRITE_CONFIG.describeForException()));
      return;
    }

    switch (cmd.getType()) {
      case CREATE:
      case UPDATE:
      case UPDATE_NONFASTFORWARD:
        try {
          ProjectConfig cfg = projectConfigFactory.create(project.getNameKey());
          cfg.load(project.getNameKey(), receivePack.getRevWalk(), cmd.getNewId());
          if (!cfg.getValidationErrors().isEmpty()) {
            addError("Invalid project configuration:");
            for (ValidationError err : cfg.getValidationErrors()) {
              addError("  " + err.getMessage());
            }
            reject(cmd, "invalid project configuration");
            logger.atSevere().log(
                "User %s tried to push invalid project configuration %s for %s",
                user.getLoggableName(), cmd.getNewId().name(), project.getName());
            return;
          }
          Project.NameKey newParent = cfg.getProject().getParent(allProjectsName);
          Project.NameKey oldParent = project.getParent(allProjectsName);
          if (oldParent == null) {
            // update of the 'All-Projects' project
            if (newParent != null) {
              reject(cmd, "invalid project configuration: root project cannot have parent");
              return;
            }
          } else {
            if (!oldParent.equals(newParent)) {
              if (allowProjectOwnersToChangeParent) {
                try {
                  permissionBackend
                      .user(user)
                      .project(project.getNameKey())
                      .check(ProjectPermission.WRITE_CONFIG);
                } catch (AuthException e) {
                  reject(cmd, "invalid project configuration: only project owners can set parent");
                  return;
                }
              } else {
                try {
                  permissionBackend.user(user).check(GlobalPermission.ADMINISTRATE_SERVER);
                } catch (AuthException e) {
                  reject(cmd, "invalid project configuration: only Gerrit admin can set parent");
                  return;
                }
              }
            }

            if (projectCache.get(newParent) == null) {
              reject(cmd, "invalid project configuration: parent does not exist");
              return;
            }
          }
          validatePluginConfig(cmd, cfg);
        } catch (Exception e) {
          reject(cmd, "invalid project configuration");
          logger.atSevere().withCause(e).log(
              "User %s tried to push invalid project configuration %s for %s",
              user.getLoggableName(), cmd.getNewId().name(), project.getName());
          return;
        }
        break;

      case DELETE:
        break;

      default:
        reject(
            cmd,
            "prohibited by Gerrit: don't know how to handle config update of type "
                + cmd.getType());
    }
  }

  /**
   * validates a push to refs/meta/config for plugin configuration, and rejects the push if it
   * fails.
   */
  private void validatePluginConfig(ReceiveCommand cmd, ProjectConfig cfg) {
    for (Extension<ProjectConfigEntry> e : pluginConfigEntries) {
      PluginConfig pluginCfg = cfg.getPluginConfig(e.getPluginName());
      ProjectConfigEntry configEntry = e.getProvider().get();
      String value = pluginCfg.getString(e.getExportName());
      String oldValue =
          projectState.getConfig().getPluginConfig(e.getPluginName()).getString(e.getExportName());
      if (configEntry.getType() == ProjectConfigEntryType.ARRAY) {
        oldValue =
            Arrays.stream(
                    projectState
                        .getConfig()
                        .getPluginConfig(e.getPluginName())
                        .getStringList(e.getExportName()))
                .collect(joining("\n"));
      }

      if ((value == null ? oldValue != null : !value.equals(oldValue))
          && !configEntry.isEditable(projectState)) {
        reject(
            cmd,
            String.format(
                "invalid project configuration: Not allowed to set parameter"
                    + " '%s' of plugin '%s' on project '%s'.",
                e.getExportName(), e.getPluginName(), project.getName()));
        continue;
      }

      if (ProjectConfigEntryType.LIST.equals(configEntry.getType())
          && value != null
          && !configEntry.getPermittedValues().contains(value)) {
        reject(
            cmd,
            String.format(
                "invalid project configuration: The value '%s' is "
                    + "not permitted for parameter '%s' of plugin '%s'.",
                value, e.getExportName(), e.getPluginName()));
      }
    }
  }

  private void parseCreate(ReceiveCommand cmd)
      throws PermissionBackendException, NoSuchProjectException, IOException {
    RevObject obj;
    try {
      obj = receivePack.getRevWalk().parseAny(cmd.getNewId());
    } catch (IOException err) {
      logger.atSevere().withCause(err).log(
          "Invalid object %s for %s creation", cmd.getNewId().name(), cmd.getRefName());
      reject(cmd, "invalid object");
      return;
    }
    logger.atFine().log("Creating %s", cmd);

    if (isHead(cmd) && !isCommit(cmd)) {
      return;
    }

    Branch.NameKey branch = new Branch.NameKey(project.getName(), cmd.getRefName());
    try {
      // Must pass explicit user instead of injecting a provider into CreateRefControl, since
      // Provider<CurrentUser> within ReceiveCommits will always return anonymous.
      createRefControl.checkCreateRef(Providers.of(user), receivePack.getRepository(), branch, obj);
    } catch (AuthException denied) {
      rejectProhibited(cmd, denied);
      return;
    } catch (ResourceConflictException denied) {
      reject(cmd, "prohibited by Gerrit: " + denied.getMessage());
      return;
    }

    if (validRefOperation(cmd)) {
      validateRegularPushCommits(new Branch.NameKey(project.getNameKey(), cmd.getRefName()), cmd);
    }
  }

  private void parseUpdate(ReceiveCommand cmd) throws PermissionBackendException {
    logger.atFine().log("Updating %s", cmd);
    Optional<AuthException> err = checkRefPermission(cmd, RefPermission.UPDATE);
    if (!err.isPresent()) {
      if (isHead(cmd) && !isCommit(cmd)) {
        reject(cmd, "head must point to commit");
        return;
      }
      if (validRefOperation(cmd)) {
        validateRegularPushCommits(new Branch.NameKey(project.getNameKey(), cmd.getRefName()), cmd);
      }
    } else {
      rejectProhibited(cmd, err.get());
    }
  }

  private boolean isCommit(ReceiveCommand cmd) {
    RevObject obj;
    try {
      obj = receivePack.getRevWalk().parseAny(cmd.getNewId());
    } catch (IOException err) {
      logger.atSevere().withCause(err).log(
          "Invalid object %s for %s", cmd.getNewId().name(), cmd.getRefName());
      reject(cmd, "invalid object");
      return false;
    }

    if (obj instanceof RevCommit) {
      return true;
    }
    reject(cmd, "not a commit");
    return false;
  }

  private void parseDelete(ReceiveCommand cmd) throws PermissionBackendException {
    logger.atFine().log("Deleting %s", cmd);
    if (cmd.getRefName().startsWith(REFS_CHANGES)) {
      errors.put(CANNOT_DELETE_CHANGES, cmd.getRefName());
      reject(cmd, "cannot delete changes");
    } else if (isConfigRef(cmd.getRefName())) {
      errors.put(CANNOT_DELETE_CONFIG, cmd.getRefName());
      reject(cmd, "cannot delete project configuration");
    }

    Optional<AuthException> err = checkRefPermission(cmd, RefPermission.DELETE);
    if (!err.isPresent()) {
      validRefOperation(cmd);

    } else {
      rejectProhibited(cmd, err.get());
    }
  }

  private void parseRewind(ReceiveCommand cmd) throws PermissionBackendException {
    try {
      receivePack.getRevWalk().parseCommit(cmd.getNewId());
    } catch (IOException err) {
      logger.atSevere().withCause(err).log(
          "Invalid object %s for %s forced update", cmd.getNewId().name(), cmd.getRefName());
      reject(cmd, "invalid object");
      return;
    }
    logger.atFine().log("Rewinding %s", cmd);

    if (!validRefOperation(cmd)) {
      return;
    }
    validateRegularPushCommits(new Branch.NameKey(project.getNameKey(), cmd.getRefName()), cmd);
    if (cmd.getResult() != NOT_ATTEMPTED) {
      return;
    }

    checkRefPermission(cmd, RefPermission.FORCE_UPDATE)
        .ifPresent((AuthException err) -> rejectProhibited(cmd, err));
  }

  private Optional<AuthException> checkRefPermission(ReceiveCommand cmd, RefPermission perm)
      throws PermissionBackendException {
    return checkRefPermission(permissions.ref(cmd.getRefName()), perm);
  }

  private Optional<AuthException> checkRefPermission(
      PermissionBackend.ForRef forRef, RefPermission perm) throws PermissionBackendException {
    try {
      forRef.check(perm);
      return Optional.empty();
    } catch (AuthException e) {
      return Optional.of(e);
    }
  }

  private void rejectProhibited(ReceiveCommand cmd, AuthException err) {
    err.getAdvice().ifPresent(a -> errors.put(a, cmd.getRefName()));
    reject(cmd, prohibited(err, cmd.getRefName()));
  }

  private static String prohibited(AuthException e, String alreadyDisplayedResource) {
    String msg = e.getMessage();
    if (e instanceof PermissionDeniedException) {
      PermissionDeniedException pde = (PermissionDeniedException) e;
      if (pde.getResource().isPresent()
          && pde.getResource().get().equals(alreadyDisplayedResource)) {
        // Avoid repeating resource name if exactly the given name was already displayed by the
        // generic git push machinery.
        msg = PermissionDeniedException.MESSAGE_PREFIX + pde.describePermission();
      }
    }
    return "prohibited by Gerrit: " + msg;
  }

  static class MagicBranchInput {
    private static final Splitter COMMAS = Splitter.on(',').omitEmptyStrings();

    boolean deprecatedTopicSeen;
    final ReceiveCommand cmd;
    final LabelTypes labelTypes;
    private final boolean defaultPublishComments;
    Branch.NameKey dest;
    PermissionBackend.ForRef perm;
    Set<String> reviewer = Sets.newLinkedHashSet();
    Set<String> cc = Sets.newLinkedHashSet();
    Map<String, Short> labels = new HashMap<>();
    String message;
    List<RevCommit> baseCommit;
    CmdLineParser cmdLineParser;
    Set<String> hashtags = new HashSet<>();

    @Option(name = "--trace", metaVar = "NAME", usage = "enable tracing")
    String trace;

    @Option(name = "--base", metaVar = "BASE", usage = "merge base of changes")
    List<ObjectId> base;

    @Option(name = "--topic", metaVar = "NAME", usage = "attach topic to changes")
    String topic;

    @Option(
        name = "--draft",
        usage =
            "Will be removed. Before that, this option will be mapped to '--private'"
                + "for new changes and '--edit' for existing changes")
    boolean draft;

    @Option(name = "--private", usage = "mark new/updated change as private")
    boolean isPrivate;

    @Option(name = "--remove-private", usage = "remove privacy flag from updated change")
    boolean removePrivate;

    @Option(
        name = "--wip",
        aliases = {"-work-in-progress"},
        usage = "mark change as work in progress")
    boolean workInProgress;

    @Option(name = "--ready", usage = "mark change as ready")
    boolean ready;

    @Option(
        name = "--edit",
        aliases = {"-e"},
        usage = "upload as change edit")
    boolean edit;

    @Option(name = "--submit", usage = "immediately submit the change")
    boolean submit;

    @Option(name = "--merged", usage = "create single change for a merged commit")
    boolean merged;

    @Option(name = "--publish-comments", usage = "publish all draft comments on updated changes")
    private boolean publishComments;

    @Option(
        name = "--no-publish-comments",
        aliases = {"--np"},
        usage = "do not publish draft comments")
    private boolean noPublishComments;

    @Option(
        name = "--notify",
        usage =
            "Notify handling that defines to whom email notifications "
                + "should be sent. Allowed values are NONE, OWNER, "
                + "OWNER_REVIEWERS, ALL. If not set, the default is ALL.")
    private NotifyHandling notifyHandling;

    @Option(
        name = "--notify-to",
        metaVar = "USER",
        usage = "user that should be notified one time by email")
    List<Account.Id> notifyTo = new ArrayList<>();

    @Option(
        name = "--notify-cc",
        metaVar = "USER",
        usage = "user that should be CC'd one time by email")
    List<Account.Id> notifyCc = new ArrayList<>();

    @Option(
        name = "--notify-bcc",
        metaVar = "USER",
        usage = "user that should be BCC'd one time by email")
    List<Account.Id> notifyBcc = new ArrayList<>();

    @Option(
        name = "--reviewer",
        aliases = {"-r"},
        metaVar = "REVIEWER",
        usage = "add reviewer to changes")
    void reviewer(String str) {
      reviewer.add(str);
    }

    @Option(name = "--cc", metaVar = "CC", usage = "add CC to changes")
    void cc(String str) {
      cc.add(str);
    }

    @Option(
        name = "--label",
        aliases = {"-l"},
        metaVar = "LABEL+VALUE",
        usage = "label(s) to assign (defaults to +1 if no value provided)")
    void addLabel(String token) throws CmdLineException {
      LabelVote v = LabelVote.parse(token);
      try {
        LabelType.checkName(v.label());
        ApprovalsUtil.checkLabel(labelTypes, v.label(), v.value());
      } catch (BadRequestException e) {
        throw cmdLineParser.reject(e.getMessage());
      }
      labels.put(v.label(), v.value());
    }

    @Option(
        name = "--message",
        aliases = {"-m"},
        metaVar = "MESSAGE",
        usage = "Comment message to apply to the review")
    void addMessage(String token) {
      // Many characters have special meaning in the context of a git ref.
      //
      // Clients can use underscores to represent spaces.
      message = token.replace("_", " ");
      try {
        // Other characters can be represented using percent-encoding.
        message = URLDecoder.decode(message, UTF_8.name());
      } catch (IllegalArgumentException e) {
        // Ignore decoding errors; leave message as percent-encoded.
      } catch (UnsupportedEncodingException e) {
        // This shouldn't happen; surely URLDecoder recognizes UTF-8.
        throw new IllegalStateException(e);
      }
    }

    @Option(
        name = "--hashtag",
        aliases = {"-t"},
        metaVar = "HASHTAG",
        usage = "add hashtag to changes")
    void addHashtag(String token) {
      String hashtag = cleanupHashtag(token);
      if (!hashtag.isEmpty()) {
        hashtags.add(hashtag);
      }
      // TODO(dpursehouse): validate hashtags
    }

    MagicBranchInput(IdentifiedUser user, ReceiveCommand cmd, LabelTypes labelTypes) {
      this.deprecatedTopicSeen = false;
      this.cmd = cmd;
      this.draft = cmd.getRefName().startsWith(MagicBranch.NEW_DRAFT_CHANGE);
      this.labelTypes = labelTypes;
      GeneralPreferencesInfo prefs = user.state().getGeneralPreferences();
      this.defaultPublishComments =
          prefs != null
              ? firstNonNull(user.state().getGeneralPreferences().publishCommentsOnPush, false)
              : false;
    }

    /**
     * Get reviewer strings from magic branch options, combined with additional recipients computed
     * from some other place.
     *
     * <p>The set of reviewers on a change includes strings passed explicitly via options as well as
     * account IDs computed from the commit message itself.
     *
     * @param additionalRecipients recipients parsed from the commit.
     * @return set of reviewer strings to pass to {@code ReviewerAdder}.
     */
    ImmutableSet<String> getCombinedReviewers(MailRecipients additionalRecipients) {
      return getCombinedReviewers(reviewer, additionalRecipients.getReviewers());
    }

    /**
     * Get CC strings from magic branch options, combined with additional recipients computed from
     * some other place.
     *
     * <p>The set of CCs on a change includes strings passed explicitly via options as well as
     * account IDs computed from the commit message itself.
     *
     * @param additionalRecipients recipients parsed from the commit.
     * @return set of CC strings to pass to {@code ReviewerAdder}.
     */
    ImmutableSet<String> getCombinedCcs(MailRecipients additionalRecipients) {
      return getCombinedReviewers(cc, additionalRecipients.getCcOnly());
    }

    private static ImmutableSet<String> getCombinedReviewers(
        Set<String> strings, Set<Account.Id> ids) {
      return Streams.concat(strings.stream(), ids.stream().map(Account.Id::toString))
          .collect(toImmutableSet());
    }

    boolean shouldPublishComments() {
      if (publishComments) {
        return true;
      } else if (noPublishComments) {
        return false;
      }
      return defaultPublishComments;
    }

    /**
     * returns the destination ref of the magic branch, and populates options in the cmdLineParser.
     */
    String parse(Repository repo, Set<String> refs, ListMultimap<String, String> pushOptions)
        throws CmdLineException {
      String ref = RefNames.fullName(MagicBranch.getDestBranchName(cmd.getRefName()));

      ListMultimap<String, String> options = LinkedListMultimap.create(pushOptions);

      // Process and lop off the "%OPTION" suffix.
      int optionStart = ref.indexOf('%');
      if (0 < optionStart) {
        for (String s : COMMAS.split(ref.substring(optionStart + 1))) {
          int e = s.indexOf('=');
          if (0 < e) {
            options.put(s.substring(0, e), s.substring(e + 1));
          } else {
            options.put(s, "");
          }
        }
        ref = ref.substring(0, optionStart);
      }

      if (!options.isEmpty()) {
        cmdLineParser.parseOptionMap(options);
      }

      // We accept refs/for/BRANCHNAME/TOPIC. Since we don't know
      // for sure where the branch ends and the topic starts, look
      // backward for a split that works. This behavior is deprecated.
      String head = readHEAD(repo);
      int split = ref.length();
      for (; ; ) {
        String name = ref.substring(0, split);
        if (refs.contains(name) || name.equals(head)) {
          break;
        }

        split = name.lastIndexOf('/', split - 1);
        if (split <= Constants.R_REFS.length()) {
          return ref;
        }
      }
      if (split < ref.length()) {
        topic = Strings.emptyToNull(ref.substring(split + 1));
        deprecatedTopicSeen = true;
      }
      return ref.substring(0, split);
    }

    NotifyResolver.Result getNotifyForNewChange() {
      return NotifyResolver.Result.create(
          firstNonNull(notifyHandling, workInProgress ? NotifyHandling.OWNER : NotifyHandling.ALL),
          ImmutableSetMultimap.<RecipientType, Account.Id>builder()
              .putAll(RecipientType.TO, notifyTo)
              .putAll(RecipientType.CC, notifyCc)
              .putAll(RecipientType.BCC, notifyBcc)
              .build());
    }

    NotifyHandling getNotifyHandling(ChangeNotes notes) {
      requireNonNull(notes);
      if (notifyHandling != null) {
        return notifyHandling;
      }
      if (workInProgress || (!ready && notes.getChange().isWorkInProgress())) {
        return NotifyHandling.OWNER;
      }
      return NotifyHandling.ALL;
    }
  }

  /**
   * Parse the magic branch data (refs/for/BRANCH/OPTIONALTOPIC%OPTIONS) into the magicBranch
   * member.
   *
   * <p>Assumes we are handling a magic branch here.
   */
  private void parseMagicBranch(ReceiveCommand cmd) throws PermissionBackendException {
    logger.atFine().log("Found magic branch %s", cmd.getRefName());
    MagicBranchInput magicBranch = new MagicBranchInput(user, cmd, labelTypes);

    String ref;
    magicBranch.cmdLineParser = optionParserFactory.create(magicBranch);

    try {
      ref = magicBranch.parse(repo, receivePack.getAdvertisedRefs().keySet(), pushOptions);
    } catch (CmdLineException e) {
      if (!magicBranch.cmdLineParser.wasHelpRequestedByOption()) {
        logger.atFine().log("Invalid branch syntax");
        reject(cmd, e.getMessage());
        return;
      }
      ref = null; // never happens
    }

    if (magicBranch.topic != null && magicBranch.topic.length() > ChangeUtil.TOPIC_MAX_LENGTH) {
      reject(
          cmd, String.format("topic length exceeds the limit (%d)", ChangeUtil.TOPIC_MAX_LENGTH));
    }

    if (magicBranch.cmdLineParser.wasHelpRequestedByOption()) {
      StringWriter w = new StringWriter();
      w.write("\nHelp for refs/for/branch:\n\n");
      magicBranch.cmdLineParser.printUsage(w, null);
      addMessage(w.toString());
      reject(cmd, "see help");
      return;
    }
    if (projectState.isAllUsers() && RefNames.REFS_USERS_SELF.equals(ref)) {
      logger.atFine().log("Handling %s", RefNames.REFS_USERS_SELF);
      ref = RefNames.refsUsers(user.getAccountId());
    }
    // Pushing changes for review usually requires that the target branch exists, but there is an
    // exception for the branch to which HEAD points to and for refs/meta/config. Pushing for
    // review to these branches is allowed even if the branch does not exist yet. This allows to
    // push initial code for review to an empty repository and to review an initial project
    // configuration.
    if (!receivePack.getAdvertisedRefs().containsKey(ref)
        && !ref.equals(readHEAD(repo))
        && !ref.equals(RefNames.REFS_CONFIG)) {
      logger.atFine().log("Ref %s not found", ref);
      if (ref.startsWith(Constants.R_HEADS)) {
        String n = ref.substring(Constants.R_HEADS.length());
        reject(cmd, "branch " + n + " not found");
      } else {
        reject(cmd, ref + " not found");
      }
      return;
    }

    magicBranch.dest = new Branch.NameKey(project.getNameKey(), ref);
    magicBranch.perm = permissions.ref(ref);

    Optional<AuthException> err = checkRefPermission(magicBranch.perm, RefPermission.CREATE_CHANGE);
    if (err.isPresent()) {
      rejectProhibited(cmd, err.get());
      return;
    }

    // TODO(davido): Remove legacy support for drafts magic branch option
    // after repo-tool supports private and work-in-progress changes.
    if (magicBranch.draft && !receiveConfig.allowDrafts) {
      errors.put(CODE_REVIEW_ERROR, ref);
      reject(cmd, "draft workflow is disabled");
      return;
    }

    if (magicBranch.isPrivate && magicBranch.removePrivate) {
      reject(cmd, "the options 'private' and 'remove-private' are mutually exclusive");
      return;
    }

    boolean privateByDefault =
        projectCache.get(project.getNameKey()).is(BooleanProjectConfig.PRIVATE_BY_DEFAULT);
    setChangeAsPrivate =
        magicBranch.draft
            || magicBranch.isPrivate
            || (privateByDefault && !magicBranch.removePrivate);

    if (receiveConfig.disablePrivateChanges && setChangeAsPrivate) {
      reject(cmd, "private changes are disabled");
      return;
    }

    if (magicBranch.workInProgress && magicBranch.ready) {
      reject(cmd, "the options 'wip' and 'ready' are mutually exclusive");
      return;
    }
    if (magicBranch.publishComments && magicBranch.noPublishComments) {
      reject(
          cmd, "the options 'publish-comments' and 'no-publish-comments' are mutually exclusive");
      return;
    }

    if (magicBranch.submit) {
      err = checkRefPermission(magicBranch.perm, RefPermission.UPDATE_BY_SUBMIT);
      if (err.isPresent()) {
        rejectProhibited(cmd, err.get());
        return;
      }
    }

    RevWalk walk = receivePack.getRevWalk();
    RevCommit tip;
    try {
      tip = walk.parseCommit(magicBranch.cmd.getNewId());
      logger.atFine().log("Tip of push: %s", tip.name());
    } catch (IOException ex) {
      magicBranch.cmd.setResult(REJECTED_MISSING_OBJECT);
      logger.atSevere().withCause(ex).log("Invalid pack upload; one or more objects weren't sent");
      return;
    }

    String destBranch = magicBranch.dest.get();
    try {
      if (magicBranch.merged) {
        if (magicBranch.base != null) {
          reject(cmd, "cannot use merged with base");
          return;
        }
        RevCommit branchTip = readBranchTip(magicBranch.dest);
        if (branchTip == null) {
          reject(cmd, magicBranch.dest.get() + " not found");
          return;
        }
        if (!walk.isMergedInto(tip, branchTip)) {
          reject(cmd, "not merged into branch");
          return;
        }
      }

      // If tip is a merge commit, or the root commit or
      // if %base or %merged was specified, ignore newChangeForAllNotInTarget.
      if (tip.getParentCount() > 1
          || magicBranch.base != null
          || magicBranch.merged
          || tip.getParentCount() == 0) {
        logger.atFine().log("Forcing newChangeForAllNotInTarget = false");
        newChangeForAllNotInTarget = false;
      }

      if (magicBranch.base != null) {
        logger.atFine().log("Handling %%base: %s", magicBranch.base);
        magicBranch.baseCommit = Lists.newArrayListWithCapacity(magicBranch.base.size());
        for (ObjectId id : magicBranch.base) {
          try {
            magicBranch.baseCommit.add(walk.parseCommit(id));
          } catch (IncorrectObjectTypeException notCommit) {
            reject(cmd, "base must be a commit");
            return;
          } catch (MissingObjectException e) {
            reject(cmd, "base not found");
            return;
          } catch (IOException e) {
            logger.atWarning().withCause(e).log(
                "Project %s cannot read %s", project.getName(), id.name());
            reject(cmd, INTERNAL_SERVER_ERROR);
            return;
          }
        }
      } else if (newChangeForAllNotInTarget) {
        RevCommit branchTip = readBranchTip(magicBranch.dest);
        if (branchTip != null) {
          magicBranch.baseCommit = Collections.singletonList(branchTip);
          logger.atFine().log("Set baseCommit = %s", magicBranch.baseCommit.get(0).name());
        } else {
          // The target branch does not exist. Usually pushing changes for review requires that the
          // target branch exists, but there is an exception for the branch to which HEAD points to
          // and for refs/meta/config. Pushing for review to these branches is allowed even if the
          // branch does not exist yet. This allows to push initial code for review to an empty
          // repository and to review an initial project configuration.
          if (!ref.equals(readHEAD(repo)) && !ref.equals(RefNames.REFS_CONFIG)) {
            reject(cmd, magicBranch.dest.get() + " not found");
            return;
          }
        }
      }
    } catch (IOException ex) {
      logger.atWarning().withCause(ex).log(
          "Error walking to %s in project %s", destBranch, project.getName());
      reject(cmd, INTERNAL_SERVER_ERROR);
      return;
    }

    if (magicBranch.deprecatedTopicSeen) {
      messages.add(
          new ValidationMessage(
              "WARNING: deprecated topic syntax. Use -o topic=TOPIC instead", false));
      logger.atInfo().log("deprecated topic push seen for project %s", project.getName());
    }

    if (validateConnected(magicBranch.cmd, magicBranch.dest, tip)) {
      this.magicBranch = magicBranch;
      this.resultChangeIds.setMagicPush(true);
    }
  }

  // Validate that the new commits are connected with the target
  // branch.  If they aren't, we want to abort. We do this check by
  // looking to see if we can compute a merge base between the new
  // commits and the target branch head.
  private boolean validateConnected(ReceiveCommand cmd, Branch.NameKey dest, RevCommit tip) {
    RevWalk walk = receivePack.getRevWalk();
    try {
      Ref targetRef = receivePack.getAdvertisedRefs().get(dest.get());
      if (targetRef == null || targetRef.getObjectId() == null) {
        // The destination branch does not yet exist. Assume the
        // history being sent for review will start it and thus
        // is "connected" to the branch.
        logger.atFine().log("Branch is unborn");

        // This is not an error condition.
        return true;
      }

      RevCommit h = walk.parseCommit(targetRef.getObjectId());
      logger.atFine().log("Current branch tip: %s", h.name());
      RevFilter oldRevFilter = walk.getRevFilter();
      try {
        walk.reset();
        walk.setRevFilter(RevFilter.MERGE_BASE);
        walk.markStart(tip);
        walk.markStart(h);
        if (walk.next() == null) {
          reject(cmd, "no common ancestry");
          return false;
        }
      } finally {
        walk.reset();
        walk.setRevFilter(oldRevFilter);
      }
    } catch (IOException e) {
      cmd.setResult(REJECTED_MISSING_OBJECT);
      logger.atSevere().withCause(e).log("Invalid pack upload; one or more objects weren't sent");
      return false;
    }
    return true;
  }

  private static String readHEAD(Repository repo) {
    try {
      String head = repo.getFullBranch();
      logger.atFine().log("HEAD = %s", head);
      return head;
    } catch (IOException e) {
      logger.atSevere().withCause(e).log("Cannot read HEAD symref");
      return null;
    }
  }

  private RevCommit readBranchTip(Branch.NameKey branch) throws IOException {
    Ref r = allRefs().get(branch.get());
    if (r == null) {
      return null;
    }
    return receivePack.getRevWalk().parseCommit(r.getObjectId());
  }

  // Handle an upload to refs/changes/XX/CHANGED-NUMBER.
  private void parseReplaceCommand(ReceiveCommand cmd, Change.Id changeId) {
    logger.atFine().log("Parsing replace command");
    if (cmd.getType() != ReceiveCommand.Type.CREATE) {
      reject(cmd, "invalid usage");
      return;
    }

    RevCommit newCommit;
    try {
      newCommit = receivePack.getRevWalk().parseCommit(cmd.getNewId());
      logger.atFine().log("Replacing with %s", newCommit);
    } catch (IOException e) {
      logger.atSevere().withCause(e).log("Cannot parse %s as commit", cmd.getNewId().name());
      reject(cmd, "invalid commit");
      return;
    }

    Change changeEnt;
    try {
      changeEnt = notesFactory.createChecked(project.getNameKey(), changeId).getChange();
    } catch (NoSuchChangeException e) {
      logger.atSevere().withCause(e).log("Change not found %s", changeId);
      reject(cmd, "change " + changeId + " not found");
      return;
    } catch (StorageException e) {
      logger.atSevere().withCause(e).log("Cannot lookup existing change %s", changeId);
      reject(cmd, "database error");
      return;
    }
    if (!project.getNameKey().equals(changeEnt.getProject())) {
      reject(cmd, "change " + changeId + " does not belong to project " + project.getName());
      return;
    }

    BranchCommitValidator validator =
        commitValidatorFactory.create(projectState, changeEnt.getDest(), user);
    try {
      if (validator.validCommit(
          receivePack.getRevWalk().getObjectReader(),
          cmd,
          newCommit,
          false,
          messages,
          rejectCommits,
          changeEnt)) {
        logger.atFine().log("Replacing change %s", changeEnt.getId());
        requestReplace(cmd, true, changeEnt, newCommit);
      }
    } catch (IOException e) {
      reject(cmd, "I/O exception validating commit");
    }
  }

  /**
   * Add an update for an existing change. Returns true if it succeeded; rejects the command if it
   * failed.
   */
  private boolean requestReplace(
      ReceiveCommand cmd, boolean checkMergedInto, Change change, RevCommit newCommit) {
    if (change.isClosed()) {
      reject(
          cmd,
          changeFormatter.changeClosed(
              ChangeReportFormatter.Input.builder().setChange(change).build()));
      return false;
    }

    ReplaceRequest req = new ReplaceRequest(change.getId(), newCommit, cmd, checkMergedInto);
    if (replaceByChange.containsKey(req.ontoChange)) {
      reject(cmd, "duplicate request");
      return false;
    }
    replaceByChange.put(req.ontoChange, req);
    return true;
  }

  private void warnAboutMissingChangeId(List<CreateRequest> newChanges) {
    for (CreateRequest create : newChanges) {
      try {
        receivePack.getRevWalk().parseBody(create.commit);
      } catch (IOException e) {
        continue;
      }
      List<String> idList = create.commit.getFooterLines(FooterConstants.CHANGE_ID);

      if (idList.isEmpty()) {
        messages.add(
            new ValidationMessage("warning: pushing without Change-Id is deprecated", false));
        break;
      }
    }
  }

  private List<CreateRequest> selectNewAndReplacedChangesFromMagicBranch(Task newProgress) {
    logger.atFine().log("Finding new and replaced changes");
    List<CreateRequest> newChanges = new ArrayList<>();

    ListMultimap<ObjectId, Ref> existing = changeRefsById();
    GroupCollector groupCollector =
        GroupCollector.create(changeRefsById(), psUtil, notesFactory, project.getNameKey());

    BranchCommitValidator validator =
        commitValidatorFactory.create(projectState, magicBranch.dest, user);

    try {
      RevCommit start = setUpWalkForSelectingChanges();
      if (start == null) {
        return Collections.emptyList();
      }

      LinkedHashMap<RevCommit, ChangeLookup> pending = new LinkedHashMap<>();
      Set<Change.Key> newChangeIds = new HashSet<>();
      int maxBatchChanges = receiveConfig.getEffectiveMaxBatchChangesLimit(user);
      int total = 0;
      int alreadyTracked = 0;
      boolean rejectImplicitMerges =
          start.getParentCount() == 1
              && projectCache
                  .get(project.getNameKey())
                  .is(BooleanProjectConfig.REJECT_IMPLICIT_MERGES)
              // Don't worry about implicit merges when creating changes for
              // already-merged commits; they're already in history, so it's too
              // late.
              && !magicBranch.merged;
      Set<RevCommit> mergedParents;
      if (rejectImplicitMerges) {
        mergedParents = new HashSet<>();
      } else {
        mergedParents = null;
      }

      for (; ; ) {
        RevCommit c = receivePack.getRevWalk().next();
        if (c == null) {
          break;
        }
        total++;
        receivePack.getRevWalk().parseBody(c);
        String name = c.name();
        groupCollector.visit(c);
        Collection<Ref> existingRefs = existing.get(c);

        if (rejectImplicitMerges) {
          Collections.addAll(mergedParents, c.getParents());
          mergedParents.remove(c);
        }

        boolean commitAlreadyTracked = !existingRefs.isEmpty();
        if (commitAlreadyTracked) {
          alreadyTracked++;
          // Corner cases where an existing commit might need a new group:
          // A) Existing commit has a null group; wasn't assigned during schema
          //    upgrade, or schema upgrade is performed on a running server.
          // B) Let A<-B<-C, then:
          //      1. Push A to refs/heads/master
          //      2. Push B to refs/for/master
          //      3. Force push A~ to refs/heads/master
          //      4. Push C to refs/for/master.
          //      B will be in existing so we aren't replacing the patch set. It
          //      used to have its own group, but now needs to to be changed to
          //      A's group.
          // C) Commit is a PatchSet of a pre-existing change uploaded with a
          //    different target branch.
          for (Ref ref : existingRefs) {
            updateGroups.add(new UpdateGroupsRequest(ref, c));
          }
          if (!(newChangeForAllNotInTarget || magicBranch.base != null)) {
            continue;
          }
        }

        List<String> idList = c.getFooterLines(CHANGE_ID);
        if (!idList.isEmpty()) {
          pending.put(
              c, lookupByChangeKey(c, new Change.Key(idList.get(idList.size() - 1).trim())));
        } else {
          pending.put(c, lookupByCommit(c));
        }

        int n = pending.size() + newChanges.size();
        if (maxBatchChanges != 0 && n > maxBatchChanges) {
          logger.atFine().log("%d changes exceeds limit of %d", n, maxBatchChanges);
          reject(
              magicBranch.cmd,
              "the number of pushed changes in a batch exceeds the max limit " + maxBatchChanges);
          return Collections.emptyList();
        }

        if (commitAlreadyTracked) {
          boolean changeExistsOnDestBranch = false;
          for (ChangeData cd : pending.get(c).destChanges) {
            if (cd.change().getDest().equals(magicBranch.dest)) {
              changeExistsOnDestBranch = true;
              break;
            }
          }
          if (changeExistsOnDestBranch) {
            continue;
          }

          logger.atFine().log("Creating new change for %s even though it is already tracked", name);
        }

        if (!validator.validCommit(
            receivePack.getRevWalk().getObjectReader(),
            magicBranch.cmd,
            c,
            magicBranch.merged,
            messages,
            rejectCommits,
            null)) {
          // Not a change the user can propose? Abort as early as possible.
          logger.atFine().log("Aborting early due to invalid commit");
          return Collections.emptyList();
        }

        // Don't allow merges to be uploaded in commit chain via all-not-in-target
        if (newChangeForAllNotInTarget && c.getParentCount() > 1) {
          reject(
              magicBranch.cmd,
              "Pushing merges in commit chains with 'all not in target' is not allowed,\n"
                  + "to override please set the base manually");
          logger.atFine().log("Rejecting merge commit %s with newChangeForAllNotInTarget", name);
          // TODO(dborowitz): Should we early return here?
        }

        if (idList.isEmpty()) {
          newChanges.add(new CreateRequest(c, magicBranch.dest.get(), newProgress));
          continue;
        }
      }
      logger.atFine().log(
          "Finished initial RevWalk with %d commits total: %d already"
              + " tracked, %d new changes with no Change-Id, and %d deferred"
              + " lookups",
          total, alreadyTracked, newChanges.size(), pending.size());

      if (rejectImplicitMerges) {
        rejectImplicitMerges(mergedParents);
      }

      for (Iterator<ChangeLookup> itr = pending.values().iterator(); itr.hasNext(); ) {
        ChangeLookup p = itr.next();
        if (p.changeKey == null) {
          continue;
        }

        if (newChangeIds.contains(p.changeKey)) {
          logger.atFine().log("Multiple commits with Change-Id %s", p.changeKey);
          reject(magicBranch.cmd, SAME_CHANGE_ID_IN_MULTIPLE_CHANGES);
          return Collections.emptyList();
        }

        List<ChangeData> changes = p.destChanges;
        if (changes.size() > 1) {
          logger.atFine().log(
              "Multiple changes in branch %s with Change-Id %s: %s",
              magicBranch.dest,
              p.changeKey,
              changes.stream().map(cd -> cd.getId().toString()).collect(joining()));
          // WTF, multiple changes in this branch have the same key?
          // Since the commit is new, the user should recreate it with
          // a different Change-Id. In practice, we should never see
          // this error message as Change-Id should be unique per branch.
          //
          reject(magicBranch.cmd, p.changeKey.get() + " has duplicates");
          return Collections.emptyList();
        }

        if (changes.size() == 1) {
          // Schedule as a replacement to this one matching change.
          //

          RevId currentPs = changes.get(0).currentPatchSet().getRevision();
          // If Commit is already current PatchSet of target Change.
          if (p.commit.name().equals(currentPs.get())) {
            if (pending.size() == 1) {
              // There are no commits left to check, all commits in pending were already
              // current PatchSet of the corresponding target changes.
              reject(magicBranch.cmd, "commit(s) already exists (as current patchset)");
            } else {
              // Commit is already current PatchSet.
              // Remove from pending and try next commit.
              itr.remove();
              continue;
            }
          }
          if (requestReplace(magicBranch.cmd, false, changes.get(0).change(), p.commit)) {
            continue;
          }
          return Collections.emptyList();
        }

        if (changes.isEmpty()) {
          if (!isValidChangeId(p.changeKey.get())) {
            reject(magicBranch.cmd, "invalid Change-Id");
            return Collections.emptyList();
          }

          // In case the change look up from the index failed,
          // double check against the existing refs
          if (foundInExistingRef(existing.get(p.commit))) {
            if (pending.size() == 1) {
              reject(magicBranch.cmd, "commit(s) already exists (as current patchset)");
              return Collections.emptyList();
            }
            itr.remove();
            continue;
          }
          newChangeIds.add(p.changeKey);
        }
        newChanges.add(new CreateRequest(p.commit, magicBranch.dest.get(), newProgress));
      }
      logger.atFine().log(
          "Finished deferred lookups with %d updates and %d new changes",
          replaceByChange.size(), newChanges.size());
    } catch (IOException e) {
      // Should never happen, the core receive process would have
      // identified the missing object earlier before we got control.
      //
      magicBranch.cmd.setResult(REJECTED_MISSING_OBJECT);
      logger.atSevere().withCause(e).log("Invalid pack upload; one or more objects weren't sent");
      return Collections.emptyList();
    } catch (StorageException e) {
      logger.atSevere().withCause(e).log("Cannot query database to locate prior changes");
      reject(magicBranch.cmd, "database error");
      return Collections.emptyList();
    }

    if (newChanges.isEmpty() && replaceByChange.isEmpty()) {
      reject(magicBranch.cmd, "no new changes");
      return Collections.emptyList();
    }
    if (!newChanges.isEmpty() && magicBranch.edit) {
      reject(magicBranch.cmd, "edit is not supported for new changes");
      return newChanges;
    }

    try {
      SortedSetMultimap<ObjectId, String> groups = groupCollector.getGroups();
      List<Integer> newIds = seq.nextChangeIds(newChanges.size());
      for (int i = 0; i < newChanges.size(); i++) {
        CreateRequest create = newChanges.get(i);
        create.setChangeId(newIds.get(i));
        create.groups = ImmutableList.copyOf(groups.get(create.commit));
      }
      for (ReplaceRequest replace : replaceByChange.values()) {
        replace.groups = ImmutableList.copyOf(groups.get(replace.newCommitId));
      }
      for (UpdateGroupsRequest update : updateGroups) {
        update.groups = ImmutableList.copyOf((groups.get(update.commit)));
      }
      logger.atFine().log("Finished updating groups from GroupCollector");
    } catch (StorageException e) {
      logger.atSevere().withCause(e).log("Error collecting groups for changes");
      reject(magicBranch.cmd, INTERNAL_SERVER_ERROR);
    }
    return newChanges;
  }

  private boolean foundInExistingRef(Collection<Ref> existingRefs) {
    for (Ref ref : existingRefs) {
      ChangeNotes notes =
          notesFactory.create(project.getNameKey(), Change.Id.fromRef(ref.getName()));
      Change change = notes.getChange();
      if (change.getDest().equals(magicBranch.dest)) {
        logger.atFine().log("Found change %s from existing refs.", change.getKey());
        // Reindex the change asynchronously, ignoring errors.
        @SuppressWarnings("unused")
        Future<?> possiblyIgnoredError = indexer.indexAsync(project.getNameKey(), change.getId());
        return true;
      }
    }
    return false;
  }

  private RevCommit setUpWalkForSelectingChanges() throws IOException {
    RevWalk rw = receivePack.getRevWalk();
    RevCommit start = rw.parseCommit(magicBranch.cmd.getNewId());

    rw.reset();
    rw.sort(RevSort.TOPO);
    rw.sort(RevSort.REVERSE, true);
    receivePack.getRevWalk().markStart(start);
    if (magicBranch.baseCommit != null) {
      markExplicitBasesUninteresting();
    } else if (magicBranch.merged) {
      logger.atFine().log("Marking parents of merged commit %s uninteresting", start.name());
      for (RevCommit c : start.getParents()) {
        rw.markUninteresting(c);
      }
    } else {
      markHeadsAsUninteresting(rw, magicBranch.dest != null ? magicBranch.dest.get() : null);
    }
    return start;
  }

  private void markExplicitBasesUninteresting() throws IOException {
    logger.atFine().log("Marking %d base commits uninteresting", magicBranch.baseCommit.size());
    for (RevCommit c : magicBranch.baseCommit) {
      receivePack.getRevWalk().markUninteresting(c);
    }
    Ref targetRef = allRefs().get(magicBranch.dest.get());
    if (targetRef != null) {
      logger.atFine().log(
          "Marking target ref %s (%s) uninteresting",
          magicBranch.dest.get(), targetRef.getObjectId().name());
      receivePack
          .getRevWalk()
          .markUninteresting(receivePack.getRevWalk().parseCommit(targetRef.getObjectId()));
    }
  }

  private void rejectImplicitMerges(Set<RevCommit> mergedParents) throws IOException {
    if (!mergedParents.isEmpty()) {
      Ref targetRef = allRefs().get(magicBranch.dest.get());
      if (targetRef != null) {
        RevWalk rw = receivePack.getRevWalk();
        RevCommit tip = rw.parseCommit(targetRef.getObjectId());
        boolean containsImplicitMerges = true;
        for (RevCommit p : mergedParents) {
          containsImplicitMerges &= !rw.isMergedInto(p, tip);
        }

        if (containsImplicitMerges) {
          rw.reset();
          for (RevCommit p : mergedParents) {
            rw.markStart(p);
          }
          rw.markUninteresting(tip);
          RevCommit c;
          while ((c = rw.next()) != null) {
            rw.parseBody(c);
            messages.add(
                new CommitValidationMessage(
                    "Implicit Merge of " + c.abbreviate(7).name() + " " + c.getShortMessage(),
                    ValidationMessage.Type.ERROR));
          }
          reject(magicBranch.cmd, "implicit merges detected");
        }
      }
    }
  }

  // Mark all branch tips as uninteresting in the given revwalk,
  // so we get only the new commits when walking rw.
  private void markHeadsAsUninteresting(RevWalk rw, @Nullable String forRef) {
    int i = 0;
    for (Ref ref : allRefs().values()) {
      if ((ref.getName().startsWith(R_HEADS) || ref.getName().equals(forRef))
          && ref.getObjectId() != null) {
        try {
          rw.markUninteresting(rw.parseCommit(ref.getObjectId()));
          i++;
        } catch (IOException e) {
          logger.atWarning().withCause(e).log(
              "Invalid ref %s in %s", ref.getName(), project.getName());
        }
      }
    }
    logger.atFine().log("Marked %d heads as uninteresting", i);
  }

  private static boolean isValidChangeId(String idStr) {
    return idStr.matches("^I[0-9a-fA-F]{40}$") && !idStr.matches("^I00*$");
  }

  private static class ChangeLookup {
    final RevCommit commit;

    @Nullable final Change.Key changeKey;
    final List<ChangeData> destChanges;

    ChangeLookup(RevCommit c, @Nullable Change.Key key, final List<ChangeData> destChanges) {
      this.commit = c;
      this.changeKey = key;
      this.destChanges = destChanges;
    }
  }

  private ChangeLookup lookupByChangeKey(RevCommit c, Change.Key key) {
    return new ChangeLookup(c, key, queryProvider.get().byBranchKey(magicBranch.dest, key));
  }

  private ChangeLookup lookupByCommit(RevCommit c) {
    return new ChangeLookup(
        c, null, queryProvider.get().byBranchCommit(magicBranch.dest, c.getName()));
  }

  /** Represents a commit for which a Change should be created. */
  private class CreateRequest {
    final RevCommit commit;
    final Task progress;
    final String refName;

    Change.Id changeId;
    ReceiveCommand cmd;
    ChangeInserter ins;
    List<String> groups = ImmutableList.of();

    Change change;

    CreateRequest(RevCommit commit, String refName, Task progress) {
      this.commit = commit;
      this.refName = refName;
      this.progress = progress;
    }

    private void setChangeId(int id) {
      possiblyOverrideWorkInProgress();

      changeId = new Change.Id(id);
      ins =
          changeInserterFactory
              .create(changeId, commit, refName)
              .setTopic(magicBranch.topic)
              .setPrivate(setChangeAsPrivate)
              .setWorkInProgress(magicBranch.workInProgress)
              // Changes already validated in validateNewCommits.
              .setValidate(false);

      if (magicBranch.merged) {
        ins.setStatus(Change.Status.MERGED);
      }
      cmd = new ReceiveCommand(ObjectId.zeroId(), commit, ins.getPatchSetId().toRefName());
      if (receivePack.getPushCertificate() != null) {
        ins.setPushCertificate(receivePack.getPushCertificate().toTextWithSignature());
      }
    }

    private void possiblyOverrideWorkInProgress() {
      // When wip or ready explicitly provided, leave it as is.
      if (magicBranch.workInProgress || magicBranch.ready) {
        return;
      }
      magicBranch.workInProgress =
          projectState.is(BooleanProjectConfig.WORK_IN_PROGRESS_BY_DEFAULT)
              || firstNonNull(user.state().getGeneralPreferences().workInProgressByDefault, false);
    }

    private void addOps(BatchUpdate bu) throws RestApiException {
      checkState(changeId != null, "must call setChangeId before addOps");
      try {
        RevWalk rw = receivePack.getRevWalk();
        rw.parseBody(commit);
        final PatchSet.Id psId = ins.setGroups(groups).getPatchSetId();
        Account.Id me = user.getAccountId();
        List<FooterLine> footerLines = commit.getFooterLines();
        requireNonNull(magicBranch);

        // TODO(dborowitz): Support reviewers by email from footers? Maybe not: kernel developers
        // with AOSP accounts already complain about these notifications, and that would make it
        // worse. Might be better to get rid of the feature entirely:
        // https://groups.google.com/d/topic/repo-discuss/tIFxY7L4DXk/discussion
        MailRecipients fromFooters = getRecipientsFromFooters(accountResolver, footerLines);
        fromFooters.remove(me);

        Map<String, Short> approvals = magicBranch.labels;
        StringBuilder msg =
            new StringBuilder(
                ApprovalsUtil.renderMessageWithApprovals(
                    psId.get(), approvals, Collections.emptyMap()));
        msg.append('.');
        if (!Strings.isNullOrEmpty(magicBranch.message)) {
          msg.append("\n").append(magicBranch.message);
        }

        bu.setNotify(magicBranch.getNotifyForNewChange());
        bu.insertChange(
            ins.setReviewersAndCcsAsStrings(
                    magicBranch.getCombinedReviewers(fromFooters),
                    magicBranch.getCombinedCcs(fromFooters))
                .setApprovals(approvals)
                .setMessage(msg.toString())
                .setRequestScopePropagator(requestScopePropagator)
                .setSendMail(true)
                .setPatchSetDescription(magicBranch.message));
        if (!magicBranch.hashtags.isEmpty()) {
          // Any change owner is allowed to add hashtags when creating a change.
          bu.addOp(
              changeId,
              hashtagsFactory.create(new HashtagsInput(magicBranch.hashtags)).setFireEvent(false));
        }
        if (!Strings.isNullOrEmpty(magicBranch.topic)) {
          bu.addOp(
              changeId,
              new BatchUpdateOp() {
                @Override
                public boolean updateChange(ChangeContext ctx) {
                  ctx.getUpdate(psId).setTopic(magicBranch.topic);
                  return true;
                }
              });
        }
        bu.addOp(
            changeId,
            new BatchUpdateOp() {
              @Override
              public boolean updateChange(ChangeContext ctx) {
                CreateRequest.this.change = ctx.getChange();
                return false;
              }
            });
        bu.addOp(changeId, new ChangeProgressOp(progress));
      } catch (Exception e) {
        throw INSERT_EXCEPTION.apply(e);
      }
    }
  }

  private void submit(Collection<CreateRequest> create, Collection<ReplaceRequest> replace)
      throws RestApiException, UpdateException, IOException, ConfigInvalidException,
          PermissionBackendException {
    Map<ObjectId, Change> bySha = Maps.newHashMapWithExpectedSize(create.size() + replace.size());
    for (CreateRequest r : create) {
      requireNonNull(
          r.change,
          () -> String.format("cannot submit new change %s; op may not have run", r.changeId));
      bySha.put(r.commit, r.change);
    }
    for (ReplaceRequest r : replace) {
      bySha.put(r.newCommitId, r.notes.getChange());
    }
    Change tipChange = bySha.get(magicBranch.cmd.getNewId());
    requireNonNull(
        tipChange,
        () ->
            String.format(
                "tip of push does not correspond to a change; found these changes: %s", bySha));
    logger.atFine().log(
        "Processing submit with tip change %s (%s)", tipChange.getId(), magicBranch.cmd.getNewId());
    try (MergeOp op = mergeOpProvider.get()) {
      op.merge(tipChange, user, false, new SubmitInput(), false);
    }
  }

  private void preparePatchSetsForReplace(List<CreateRequest> newChanges) {
    try {
      readChangesForReplace();
      for (ReplaceRequest req : replaceByChange.values()) {
        if (req.inputCommand.getResult() == NOT_ATTEMPTED) {
          req.validateNewPatchSet();
        }
      }
    } catch (StorageException err) {
      logger.atSevere().withCause(err).log(
          "Cannot read database before replacement for project %s", project.getName());
      rejectRemainingRequests(replaceByChange.values(), INTERNAL_SERVER_ERROR);
    } catch (IOException | PermissionBackendException err) {
      logger.atSevere().withCause(err).log(
          "Cannot read repository before replacement for project %s", project.getName());
      rejectRemainingRequests(replaceByChange.values(), INTERNAL_SERVER_ERROR);
    }
    logger.atFine().log("Read %d changes to replace", replaceByChange.size());

    if (magicBranch != null && magicBranch.cmd.getResult() != NOT_ATTEMPTED) {
      // Cancel creations tied to refs/for/ or refs/drafts/ command.
      for (ReplaceRequest req : replaceByChange.values()) {
        if (req.inputCommand == magicBranch.cmd && req.cmd != null) {
          req.cmd.setResult(Result.REJECTED_OTHER_REASON, "aborted");
        }
      }
      for (CreateRequest req : newChanges) {
        req.cmd.setResult(Result.REJECTED_OTHER_REASON, "aborted");
      }
    }
  }

  private void readChangesForReplace() {
    Collection<ChangeNotes> allNotes =
        notesFactory.create(
            replaceByChange.values().stream().map(r -> r.ontoChange).collect(toList()));
    for (ChangeNotes notes : allNotes) {
      replaceByChange.get(notes.getChangeId()).notes = notes;
    }
  }

  /** Represents a commit that should be stored in a new patchset of an existing change. */
  private class ReplaceRequest {
    final Change.Id ontoChange;
    final ObjectId newCommitId;
    final ReceiveCommand inputCommand;
    final boolean checkMergedInto;
    RevCommit revCommit;
    ChangeNotes notes;
    BiMap<RevCommit, PatchSet.Id> revisions;
    PatchSet.Id psId;
    ReceiveCommand prev;
    ReceiveCommand cmd;
    PatchSetInfo info;
    PatchSet.Id priorPatchSet;
    List<String> groups = ImmutableList.of();
    ReplaceOp replaceOp;

    ReplaceRequest(
        Change.Id toChange, RevCommit newCommit, ReceiveCommand cmd, boolean checkMergedInto) {
      this.ontoChange = toChange;
      this.newCommitId = newCommit.copy();
      this.inputCommand = requireNonNull(cmd);
      this.checkMergedInto = checkMergedInto;

      try {
        revCommit = receivePack.getRevWalk().parseCommit(newCommitId);
      } catch (IOException e) {
        revCommit = null;
      }
      revisions = HashBiMap.create();
      for (Ref ref : refs(toChange)) {
        try {
          revisions.forcePut(
              receivePack.getRevWalk().parseCommit(ref.getObjectId()),
              PatchSet.Id.fromRef(ref.getName()));
        } catch (IOException err) {
          logger.atWarning().withCause(err).log(
              "Project %s contains invalid change ref %s", project.getName(), ref.getName());
        }
      }
    }

    /**
     * Validate the new patch set commit for this change.
     *
     * <p><strong>Side effects:</strong>
     *
     * <ul>
     *   <li>May add error or warning messages to the progress monitor
     *   <li>Will reject {@code cmd} prior to returning false
     *   <li>May reset {@code receivePack.getRevWalk()}; do not call in the middle of a walk.
     * </ul>
     *
     * @return whether the new commit is valid
     * @throws IOException
     * @throws PermissionBackendException
     */
    boolean validateNewPatchSet() throws IOException, PermissionBackendException {
      if (!validateNewPatchSetNoteDb()) {
        return false;
      }
      sameTreeWarning();

      if (magicBranch != null) {
        validateMagicBranchWipStatusChange();
        if (inputCommand.getResult() != NOT_ATTEMPTED) {
          return false;
        }

        if (magicBranch.edit || magicBranch.draft) {
          return newEdit();
        }
      }

      newPatchSet();
      return true;
    }

    boolean validateNewPatchSetForAutoClose() throws IOException, PermissionBackendException {
      if (!validateNewPatchSetNoteDb()) {
        return false;
      }

      newPatchSet();
      return true;
    }

    /** Validates the new PS against permissions and notedb status. */
    private boolean validateNewPatchSetNoteDb() throws IOException, PermissionBackendException {
      if (notes == null) {
        reject(inputCommand, "change " + ontoChange + " not found");
        return false;
      }

      Change change = notes.getChange();
      priorPatchSet = change.currentPatchSetId();
      if (!revisions.containsValue(priorPatchSet)) {
        reject(inputCommand, "change " + ontoChange + " missing revisions");
        return false;
      }

      RevCommit newCommit = receivePack.getRevWalk().parseCommit(newCommitId);

      // Not allowed to create a new patch set if the current patch set is locked.
      if (psUtil.isPatchSetLocked(notes)) {
        reject(inputCommand, "cannot add patch set to " + ontoChange + ".");
        return false;
      }

      try {
        permissions.change(notes).check(ChangePermission.ADD_PATCH_SET);
      } catch (AuthException no) {
        reject(inputCommand, "cannot add patch set to " + ontoChange + ".");
        return false;
      }

      if (change.isClosed()) {
        reject(inputCommand, "change " + ontoChange + " closed");
        return false;
      } else if (revisions.containsKey(newCommit)) {
        reject(inputCommand, "commit already exists (in the change)");
        return false;
      }

      for (Ref r : receivePack.getRepository().getRefDatabase().getRefsByPrefix("refs/changes")) {
        if (r.getObjectId().equals(newCommit)) {
          reject(inputCommand, "commit already exists (in the project)");
          return false;
        }
      }

      for (RevCommit prior : revisions.keySet()) {
        // Don't allow a change to directly depend upon itself. This is a
        // very common error due to users making a new commit rather than
        // amending when trying to address review comments.
        if (receivePack.getRevWalk().isMergedInto(prior, newCommit)) {
          reject(inputCommand, SAME_CHANGE_ID_IN_MULTIPLE_CHANGES);
          return false;
        }
      }

      return true;
    }

    /** Validates whether the WIP change is allowed. Rejects inputCommand if not. */
    private void validateMagicBranchWipStatusChange() throws PermissionBackendException {
      Change change = notes.getChange();
      if ((magicBranch.workInProgress || magicBranch.ready)
          && magicBranch.workInProgress != change.isWorkInProgress()
          && !user.getAccountId().equals(change.getOwner())) {
        boolean hasWriteConfigPermission = false;
        try {
          permissions.check(ProjectPermission.WRITE_CONFIG);
          hasWriteConfigPermission = true;
        } catch (AuthException e) {
          // Do nothing.
        }

        if (!hasWriteConfigPermission) {
          try {
            permissionBackend.user(user).check(GlobalPermission.ADMINISTRATE_SERVER);
          } catch (AuthException e1) {
            reject(inputCommand, ONLY_CHANGE_OWNER_OR_PROJECT_OWNER_CAN_MODIFY_WIP);
          }
        }
      }
    }

    /** prints a warning if the new PS has the same tree as the previous commit. */
    private void sameTreeWarning() throws IOException {
      RevWalk rw = receivePack.getRevWalk();
      RevCommit newCommit = rw.parseCommit(newCommitId);
      RevCommit priorCommit = revisions.inverse().get(priorPatchSet);

      if (newCommit.getTree().equals(priorCommit.getTree())) {
        rw.parseBody(newCommit);
        rw.parseBody(priorCommit);
        boolean messageEq =
            Objects.equals(newCommit.getFullMessage(), priorCommit.getFullMessage());
        boolean parentsEq = parentsEqual(newCommit, priorCommit);
        boolean authorEq = authorEqual(newCommit, priorCommit);
        ObjectReader reader = receivePack.getRevWalk().getObjectReader();

        if (messageEq && parentsEq && authorEq) {
          addMessage(
              String.format(
                  "warning: no changes between prior commit %s and new commit %s",
                  reader.abbreviate(priorCommit).name(), reader.abbreviate(newCommit).name()));
        } else {
          StringBuilder msg = new StringBuilder();
          msg.append("warning: ").append(reader.abbreviate(newCommit).name());
          msg.append(":");
          msg.append(" no files changed");
          if (!authorEq) {
            msg.append(", author changed");
          }
          if (!messageEq) {
            msg.append(", message updated");
          }
          if (!parentsEq) {
            msg.append(", was rebased");
          }
          addMessage(msg.toString());
        }
      }
    }

    /**
     * Sets cmd and prev to the ReceiveCommands for change edits. Returns false if there was a
     * failure.
     */
    private boolean newEdit() {
      psId = notes.getChange().currentPatchSetId();
      Optional<ChangeEdit> edit;

      try {
        edit = editUtil.byChange(notes, user);
      } catch (AuthException | IOException e) {
        logger.atSevere().withCause(e).log("Cannot retrieve edit");
        return false;
      }

      if (edit.isPresent()) {
        if (edit.get().getBasePatchSet().getId().equals(psId)) {
          // replace edit
          cmd =
              new ReceiveCommand(edit.get().getEditCommit(), newCommitId, edit.get().getRefName());
        } else {
          // delete old edit ref on rebase
          prev =
              new ReceiveCommand(
                  edit.get().getEditCommit(), ObjectId.zeroId(), edit.get().getRefName());
          createEditCommand();
        }
      } else {
        createEditCommand();
      }

      return true;
    }

    /** Creates a ReceiveCommand for a new edit. */
    private void createEditCommand() {
      cmd =
          new ReceiveCommand(
              ObjectId.zeroId(),
              newCommitId,
              RefNames.refsEdit(user.getAccountId(), notes.getChangeId(), psId));
    }

    /** Updates 'this' to add a new patchset. */
    private void newPatchSet() throws IOException {
      RevCommit newCommit = receivePack.getRevWalk().parseCommit(newCommitId);
      psId =
          ChangeUtil.nextPatchSetIdFromAllRefsMap(allRefs(), notes.getChange().currentPatchSetId());
      info = patchSetInfoFactory.get(receivePack.getRevWalk(), newCommit, psId);
      cmd = new ReceiveCommand(ObjectId.zeroId(), newCommitId, psId.toRefName());
    }

    void addOps(BatchUpdate bu, @Nullable Task progress) throws IOException {
      if (magicBranch != null && (magicBranch.edit || magicBranch.draft)) {
        bu.addOp(notes.getChangeId(), new ReindexOnlyOp());
        if (prev != null) {
          bu.addRepoOnlyOp(new UpdateOneRefOp(prev));
        }
        bu.addRepoOnlyOp(new UpdateOneRefOp(cmd));
        return;
      }
      RevWalk rw = receivePack.getRevWalk();
      // TODO(dborowitz): Move to ReplaceOp#updateRepo.
      RevCommit newCommit = rw.parseCommit(newCommitId);
      rw.parseBody(newCommit);

      RevCommit priorCommit = revisions.inverse().get(priorPatchSet);
      replaceOp =
          replaceOpFactory
              .create(
                  projectState,
                  notes.getChange().getDest(),
                  checkMergedInto,
                  checkMergedInto ? inputCommand.getNewId().name() : null,
                  priorPatchSet,
                  priorCommit,
                  psId,
                  newCommit,
                  info,
                  groups,
                  magicBranch,
                  receivePack.getPushCertificate())
              .setRequestScopePropagator(requestScopePropagator);
      bu.addOp(notes.getChangeId(), replaceOp);
      if (progress != null) {
        bu.addOp(notes.getChangeId(), new ChangeProgressOp(progress));
      }
    }

    String getRejectMessage() {
      return replaceOp != null ? replaceOp.getRejectMessage() : null;
    }
  }

  private class UpdateGroupsRequest {
    final PatchSet.Id psId;
    final RevCommit commit;
    List<String> groups = ImmutableList.of();

    UpdateGroupsRequest(Ref ref, RevCommit commit) {
      this.psId = requireNonNull(PatchSet.Id.fromRef(ref.getName()));
      this.commit = commit;
    }

    private void addOps(BatchUpdate bu) {
      bu.addOp(
          psId.getParentKey(),
          new BatchUpdateOp() {
            @Override
            public boolean updateChange(ChangeContext ctx) {
              PatchSet ps = psUtil.get(ctx.getNotes(), psId);
              List<String> oldGroups = ps.getGroups();
              if (oldGroups == null) {
                if (groups == null) {
                  return false;
                }
              } else if (sameGroups(oldGroups, groups)) {
                return false;
              }
              psUtil.setGroups(ctx.getUpdate(psId), ps, groups);
              return true;
            }
          });
    }

    private boolean sameGroups(List<String> a, List<String> b) {
      return Sets.newHashSet(a).equals(Sets.newHashSet(b));
    }
  }

  private class UpdateOneRefOp implements RepoOnlyOp {
    final ReceiveCommand cmd;

    private UpdateOneRefOp(ReceiveCommand cmd) {
      this.cmd = requireNonNull(cmd);
    }

    @Override
    public void updateRepo(RepoContext ctx) throws IOException {
      ctx.addRefUpdate(cmd);
    }

    @Override
    public void postUpdate(Context ctx) {
      String refName = cmd.getRefName();
      if (cmd.getType() == ReceiveCommand.Type.UPDATE) { // aka fast-forward
        logger.atFine().log("Updating tag cache on fast-forward of %s", cmd.getRefName());
        tagCache.updateFastForward(project.getNameKey(), refName, cmd.getOldId(), cmd.getNewId());
      }
      if (isConfig(cmd)) {
        logger.atFine().log("Reloading project in cache");
        projectCache.evict(project);
        ProjectState ps = projectCache.get(project.getNameKey());
        try {
          logger.atFine().log("Updating project description");
          repo.setGitwebDescription(ps.getProject().getDescription());
        } catch (IOException e) {
          logger.atWarning().withCause(e).log("cannot update description of %s", project.getName());
        }
        if (allProjectsName.equals(project.getNameKey())) {
          try {
            createGroupPermissionSyncer.syncIfNeeded();
          } catch (IOException | ConfigInvalidException e) {
            logger.atSevere().withCause(e).log("Can't sync create group permissions");
          }
        }
      }
    }
  }

  private static class ReindexOnlyOp implements BatchUpdateOp {
    @Override
    public boolean updateChange(ChangeContext ctx) {
      // Trigger reindexing even though change isn't actually updated.
      return true;
    }
  }

  private List<Ref> refs(Change.Id changeId) {
    return refsByChange().get(changeId);
  }

  private void initChangeRefMaps() {
    if (refsByChange == null) {
      int estRefsPerChange = 4;
      refsById = MultimapBuilder.hashKeys().arrayListValues().build();
      refsByChange =
          MultimapBuilder.hashKeys(allRefs().size() / estRefsPerChange)
              .arrayListValues(estRefsPerChange)
              .build();
      for (Ref ref : allRefs().values()) {
        ObjectId obj = ref.getObjectId();
        if (obj != null) {
          PatchSet.Id psId = PatchSet.Id.fromRef(ref.getName());
          if (psId != null) {
            refsById.put(obj, ref);
            refsByChange.put(psId.getParentKey(), ref);
          }
        }
      }
    }
  }

  private ListMultimap<Change.Id, Ref> refsByChange() {
    initChangeRefMaps();
    return refsByChange;
  }

  private ListMultimap<ObjectId, Ref> changeRefsById() {
    initChangeRefMaps();
    return refsById;
  }

  private static boolean parentsEqual(RevCommit a, RevCommit b) {
    if (a.getParentCount() != b.getParentCount()) {
      return false;
    }
    for (int i = 0; i < a.getParentCount(); i++) {
      if (!a.getParent(i).equals(b.getParent(i))) {
        return false;
      }
    }
    return true;
  }

  private static boolean authorEqual(RevCommit a, RevCommit b) {
    PersonIdent aAuthor = a.getAuthorIdent();
    PersonIdent bAuthor = b.getAuthorIdent();

    if (aAuthor == null && bAuthor == null) {
      return true;
    } else if (aAuthor == null || bAuthor == null) {
      return false;
    }

    return Objects.equals(aAuthor.getName(), bAuthor.getName())
        && Objects.equals(aAuthor.getEmailAddress(), bAuthor.getEmailAddress());
  }

  // Run RefValidators on the command. If any validator fails, the command status is set to
  // REJECTED, and the return value is 'false'
  private boolean validRefOperation(ReceiveCommand cmd) {
    RefOperationValidators refValidators = refValidatorsFactory.create(getProject(), user, cmd);

    try {
      messages.addAll(refValidators.validateForRefOperation());
    } catch (RefOperationValidationException e) {
      messages.addAll(Lists.newArrayList(e.getMessages()));
      reject(cmd, e.getMessage());
      return false;
    }

    return true;
  }

  /**
   * Validates the commits that a regular push brings in.
   *
   * <p>On validation failure, the command is rejected.
   */
  private void validateRegularPushCommits(Branch.NameKey branch, ReceiveCommand cmd)
      throws PermissionBackendException {
    boolean skipValidation =
        !RefNames.REFS_CONFIG.equals(cmd.getRefName())
            && !(MagicBranch.isMagicBranch(cmd.getRefName())
                || NEW_PATCHSET_PATTERN.matcher(cmd.getRefName()).matches())
            && pushOptions.containsKey(PUSH_OPTION_SKIP_VALIDATION);
    if (skipValidation) {
      if (projectState.is(BooleanProjectConfig.USE_SIGNED_OFF_BY)) {
        reject(cmd, "requireSignedOffBy prevents option " + PUSH_OPTION_SKIP_VALIDATION);
        return;
      }

      Optional<AuthException> err =
          checkRefPermission(permissions.ref(branch.get()), RefPermission.SKIP_VALIDATION);
      if (err.isPresent()) {
        rejectProhibited(cmd, err.get());
        return;
      }
      if (!Iterables.isEmpty(rejectCommits)) {
        reject(cmd, "reject-commits prevents " + PUSH_OPTION_SKIP_VALIDATION);
      }
    }

    BranchCommitValidator validator = commitValidatorFactory.create(projectState, branch, user);
    RevWalk walk = receivePack.getRevWalk();
    walk.reset();
    walk.sort(RevSort.NONE);
    try {
      RevObject parsedObject = walk.parseAny(cmd.getNewId());
      if (!(parsedObject instanceof RevCommit)) {
        return;
      }
      ListMultimap<ObjectId, Ref> existing = changeRefsById();
      walk.markStart((RevCommit) parsedObject);
      markHeadsAsUninteresting(walk, cmd.getRefName());
      int limit = receiveConfig.maxBatchCommits;
      int n = 0;
      for (RevCommit c; (c = walk.next()) != null; ) {
        // Even if skipValidation is set, we still get here when at least one plugin
        // commit validator requires to validate all commits. In this case, however,
        // we don't need to check the commit limit.
        if (++n > limit && !skipValidation) {
          logger.atFine().log("Number of new commits exceeds limit of %d", limit);
          reject(
              cmd,
              String.format(
                  "more than %d commits, and %s not set", limit, PUSH_OPTION_SKIP_VALIDATION));
          return;
        }
        if (existing.keySet().contains(c)) {
          continue;
        }

        if (!validator.validCommit(
            walk.getObjectReader(), cmd, c, false, messages, rejectCommits, null, skipValidation)) {
          break;
        }
      }
      logger.atFine().log("Validated %d new commits", n);
    } catch (IOException err) {
      cmd.setResult(REJECTED_MISSING_OBJECT);
      logger.atSevere().withCause(err).log("Invalid pack upload; one or more objects weren't sent");
    }
  }

  private void autoCloseChanges(ReceiveCommand cmd, Task progress) {
    logger.atFine().log("Starting auto-closing of changes");
    String refName = cmd.getRefName();
    Set<Change.Id> ids = new HashSet<>();

    // TODO(dborowitz): Combine this BatchUpdate with the main one in
    // handleRegularCommands
    try {
      retryHelper.execute(
          updateFactory -> {
            try (BatchUpdate bu =
                    updateFactory.create(projectState.getNameKey(), user, TimeUtil.nowTs());
                ObjectInserter ins = repo.newObjectInserter();
                ObjectReader reader = ins.newReader();
                RevWalk rw = new RevWalk(reader)) {
              bu.setRepository(repo, rw, ins);
              // TODO(dborowitz): Teach BatchUpdate to ignore missing changes.

              RevCommit newTip = rw.parseCommit(cmd.getNewId());
              Branch.NameKey branch = new Branch.NameKey(project.getNameKey(), refName);

              rw.reset();
              rw.sort(RevSort.REVERSE);
              rw.markStart(newTip);
              if (!ObjectId.zeroId().equals(cmd.getOldId())) {
                rw.markUninteresting(rw.parseCommit(cmd.getOldId()));
              }

              ListMultimap<ObjectId, Ref> byCommit = changeRefsById();
              Map<Change.Key, ChangeNotes> byKey = null;
              List<ReplaceRequest> replaceAndClose = new ArrayList<>();

              int existingPatchSets = 0;
              int newPatchSets = 0;
              COMMIT:
              for (RevCommit c; (c = rw.next()) != null; ) {
                rw.parseBody(c);

                for (Ref ref : byCommit.get(c.copy())) {
                  PatchSet.Id psId = PatchSet.Id.fromRef(ref.getName());
                  Optional<ChangeNotes> notes = getChangeNotes(psId.getParentKey());
                  if (notes.isPresent() && notes.get().getChange().getDest().equals(branch)) {
                    existingPatchSets++;
                    bu.addOp(notes.get().getChangeId(), setPrivateOpFactory.create(false, null));
                    bu.addOp(
                        psId.getParentKey(),
                        mergedByPushOpFactory.create(
                            requestScopePropagator, psId, refName, newTip.getId().getName()));
                    continue COMMIT;
                  }
                }

                for (String changeId : c.getFooterLines(CHANGE_ID)) {
                  if (byKey == null) {
                    byKey = executeIndexQuery(() -> openChangesByKeyByBranch(branch));
                  }

                  ChangeNotes onto = byKey.get(new Change.Key(changeId.trim()));
                  if (onto != null) {
                    newPatchSets++;
                    // Hold onto this until we're done with the walk, as the call to
                    // req.validate below calls isMergedInto which resets the walk.
                    ReplaceRequest req = new ReplaceRequest(onto.getChangeId(), c, cmd, false);
                    req.notes = onto;
                    replaceAndClose.add(req);
                    continue COMMIT;
                  }
                }
              }

              for (ReplaceRequest req : replaceAndClose) {
                Change.Id id = req.notes.getChangeId();
                if (!req.validateNewPatchSetForAutoClose()) {
                  logger.atFine().log("Not closing %s because validation failed", id);
                  continue;
                }
                req.addOps(bu, null);
                bu.addOp(id, setPrivateOpFactory.create(false, null));
                bu.addOp(
                    id,
                    mergedByPushOpFactory
                        .create(requestScopePropagator, req.psId, refName, newTip.getId().getName())
                        .setPatchSetProvider(req.replaceOp::getPatchSet));
                bu.addOp(id, new ChangeProgressOp(progress));
                ids.add(id);
              }

              logger.atFine().log(
                  "Auto-closing %s changes with existing patch sets and %s with new patch sets",
                  existingPatchSets, newPatchSets);
              bu.execute();
            } catch (IOException | StorageException | PermissionBackendException e) {
              logger.atSevere().withCause(e).log("Failed to auto-close changes");
              return null;
            }

            // If we are here, we didn't throw UpdateException. Record the result.
            // The ordering is indeterminate due to the HashSet; unfortunately, Change.Id doesn't
            // fit into TreeSet.
            ids.stream().forEach(id -> resultChangeIds.add(ResultChangeIds.Key.AUTOCLOSED, id));

            return null;
          },
          // Use a multiple of the default timeout to account for inner retries that may otherwise
          // eat up the whole timeout so that no time is left to retry this outer action.
          RetryHelper.options()
              .timeout(retryHelper.getDefaultTimeout(ActionType.CHANGE_UPDATE).multipliedBy(5))
              .build());
    } catch (RestApiException e) {
      logger.atSevere().withCause(e).log("Can't insert patchset");
    } catch (UpdateException e) {
      logger.atSevere().withCause(e).log("Failed to auto-close changes");
    }
  }

  private Optional<ChangeNotes> getChangeNotes(Change.Id changeId) {
    try {
      return Optional.of(notesFactory.createChecked(project.getNameKey(), changeId));
    } catch (NoSuchChangeException e) {
      return Optional.empty();
    }
  }

  private <T> T executeIndexQuery(Action<T> action) {
    try {
      return retryHelper.execute(
          ActionType.INDEX_QUERY, action, StorageException.class::isInstance);
    } catch (Exception e) {
      Throwables.throwIfUnchecked(e);
      throw new StorageException(e);
    }
  }

  private Map<Change.Key, ChangeNotes> openChangesByKeyByBranch(Branch.NameKey branch) {
    Map<Change.Key, ChangeNotes> r = new HashMap<>();
    for (ChangeData cd : queryProvider.get().byBranchOpen(branch)) {
      try {
        r.put(cd.change().getKey(), cd.notes());
      } catch (NoSuchChangeException e) {
        // Ignore deleted change
      }
    }
    return r;
  }

  // allRefsWatcher hooks into the protocol negotation to get a list of all known refs.
  // This is used as a cache of ref -> sha1 values, and to build an inverse index
  // of (change => list of refs) and a (SHA1 => refs).
  private Map<String, Ref> allRefs() {
    return allRefsWatcher.getAllRefs();
  }

  private static void reject(ReceiveCommand cmd, String why) {
    cmd.setResult(REJECTED_OTHER_REASON, why);
  }

  private static void rejectRemaining(Collection<ReceiveCommand> commands, String why) {
    rejectRemaining(commands.stream(), why);
  }

  private static void rejectRemaining(Stream<ReceiveCommand> commands, String why) {
    commands.filter(cmd -> cmd.getResult() == NOT_ATTEMPTED).forEach(cmd -> reject(cmd, why));
  }

  private static void rejectRemainingRequests(Collection<ReplaceRequest> requests, String why) {
    rejectRemaining(requests.stream().map(req -> req.cmd), why);
  }

  private static boolean isHead(ReceiveCommand cmd) {
    return cmd.getRefName().startsWith(Constants.R_HEADS);
  }

  private static boolean isConfig(ReceiveCommand cmd) {
    return cmd.getRefName().equals(RefNames.REFS_CONFIG);
  }

  private static String commandToString(ReceiveCommand cmd) {
    StringBuilder b = new StringBuilder();
    b.append(cmd);
    b.append("  (").append(cmd.getResult());
    if (cmd.getMessage() != null) {
      b.append(": ").append(cmd.getMessage());
    }
    b.append(")\n");
    return b.toString();
  }
}