// Copyright (C) 2019 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.restapi.change;

import static com.google.common.base.MoreObjects.firstNonNull;
import static com.google.gerrit.extensions.conditions.BooleanCondition.and;
import static com.google.gerrit.server.permissions.RefPermission.CREATE_CHANGE;
import static com.google.gerrit.server.project.ProjectCache.illegalState;
import static java.util.Objects.requireNonNull;

import com.google.common.base.Strings;
import com.google.common.collect.ArrayListMultimap;
import com.google.common.collect.Iterables;
import com.google.common.collect.Multimap;
import com.google.common.flogger.FluentLogger;
import com.google.gerrit.entities.BranchNameKey;
import com.google.gerrit.entities.Change;
import com.google.gerrit.entities.ChangeMessage;
import com.google.gerrit.entities.PatchSet;
import com.google.gerrit.entities.Project;
import com.google.gerrit.entities.RefNames;
import com.google.gerrit.exceptions.StorageException;
import com.google.gerrit.extensions.api.changes.CherryPickInput;
import com.google.gerrit.extensions.api.changes.NotifyHandling;
import com.google.gerrit.extensions.api.changes.RevertInput;
import com.google.gerrit.extensions.common.ChangeInfo;
import com.google.gerrit.extensions.common.RevertSubmissionInfo;
import com.google.gerrit.extensions.restapi.AuthException;
import com.google.gerrit.extensions.restapi.ResourceConflictException;
import com.google.gerrit.extensions.restapi.Response;
import com.google.gerrit.extensions.restapi.RestApiException;
import com.google.gerrit.extensions.restapi.RestModifyView;
import com.google.gerrit.extensions.webui.UiAction;
import com.google.gerrit.server.ChangeMessagesUtil;
import com.google.gerrit.server.ChangeUtil;
import com.google.gerrit.server.CurrentUser;
import com.google.gerrit.server.PatchSetUtil;
import com.google.gerrit.server.change.ChangeJson;
import com.google.gerrit.server.change.ChangeMessages;
import com.google.gerrit.server.change.ChangeResource;
import com.google.gerrit.server.change.NotifyResolver;
import com.google.gerrit.server.change.RevisionResource;
import com.google.gerrit.server.change.WalkSorter;
import com.google.gerrit.server.change.WalkSorter.PatchSetData;
import com.google.gerrit.server.extensions.events.ChangeReverted;
import com.google.gerrit.server.git.CommitUtil;
import com.google.gerrit.server.git.GitRepositoryManager;
import com.google.gerrit.server.mail.send.RevertedSender;
import com.google.gerrit.server.notedb.ChangeNotes;
import com.google.gerrit.server.notedb.Sequences;
import com.google.gerrit.server.permissions.ChangePermission;
import com.google.gerrit.server.permissions.PermissionBackend;
import com.google.gerrit.server.permissions.PermissionBackendException;
import com.google.gerrit.server.project.ContributorAgreementsChecker;
import com.google.gerrit.server.project.NoSuchProjectException;
import com.google.gerrit.server.project.ProjectCache;
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.restapi.change.CherryPickChange.Result;
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.UpdateException;
import com.google.gerrit.server.util.CommitMessageUtil;
import com.google.gerrit.server.util.time.TimeUtil;
import com.google.inject.Inject;
import com.google.inject.Provider;
import java.io.IOException;
import java.sql.Timestamp;
import java.text.MessageFormat;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.Set;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.util.stream.Collectors;
import org.apache.commons.lang.RandomStringUtils;
import org.eclipse.jgit.errors.ConfigInvalidException;
import org.eclipse.jgit.lib.ObjectId;
import org.eclipse.jgit.lib.ObjectInserter;
import org.eclipse.jgit.lib.ObjectReader;
import org.eclipse.jgit.lib.Repository;
import org.eclipse.jgit.revwalk.RevCommit;
import org.eclipse.jgit.revwalk.RevWalk;

public class RevertSubmission
    implements RestModifyView<ChangeResource, RevertInput>, UiAction<ChangeResource> {
  private static final FluentLogger logger = FluentLogger.forEnclosingClass();

  private final Provider<InternalChangeQuery> queryProvider;
  private final Provider<CurrentUser> user;
  private final PermissionBackend permissionBackend;
  private final ProjectCache projectCache;
  private final PatchSetUtil psUtil;
  private final ContributorAgreementsChecker contributorAgreements;
  private final CherryPickChange cherryPickChange;
  private final ChangeJson.Factory json;
  private final GitRepositoryManager repoManager;
  private final WalkSorter sorter;
  private final ChangeMessagesUtil cmUtil;
  private final CommitUtil commitUtil;
  private final ChangeNotes.Factory changeNotesFactory;
  private final ChangeReverted changeReverted;
  private final RevertedSender.Factory revertedSenderFactory;
  private final Sequences seq;
  private final NotifyResolver notifyResolver;
  private final BatchUpdate.Factory updateFactory;
  private final ChangeResource.Factory changeResourceFactory;
  private final GetRelated getRelated;

  private CherryPickInput cherryPickInput;
  private List<ChangeInfo> results;
  private static final Pattern patternRevertSubject = Pattern.compile("Revert \"(.+)\"");
  private static final Pattern patternRevertSubjectWithNum =
      Pattern.compile("Revert\\^(\\d+) \"(.+)\"");

  @Inject
  RevertSubmission(
      Provider<InternalChangeQuery> queryProvider,
      Provider<CurrentUser> user,
      PermissionBackend permissionBackend,
      ProjectCache projectCache,
      PatchSetUtil psUtil,
      ContributorAgreementsChecker contributorAgreements,
      CherryPickChange cherryPickChange,
      ChangeJson.Factory json,
      GitRepositoryManager repoManager,
      WalkSorter sorter,
      ChangeMessagesUtil cmUtil,
      CommitUtil commitUtil,
      ChangeNotes.Factory changeNotesFactory,
      ChangeReverted changeReverted,
      RevertedSender.Factory revertedSenderFactory,
      Sequences seq,
      NotifyResolver notifyResolver,
      BatchUpdate.Factory updateFactory,
      ChangeResource.Factory changeResourceFactory,
      GetRelated getRelated) {
    this.queryProvider = queryProvider;
    this.user = user;
    this.permissionBackend = permissionBackend;
    this.projectCache = projectCache;
    this.psUtil = psUtil;
    this.contributorAgreements = contributorAgreements;
    this.cherryPickChange = cherryPickChange;
    this.json = json;
    this.repoManager = repoManager;
    this.sorter = sorter;
    this.cmUtil = cmUtil;
    this.commitUtil = commitUtil;
    this.changeNotesFactory = changeNotesFactory;
    this.changeReverted = changeReverted;
    this.revertedSenderFactory = revertedSenderFactory;
    this.seq = seq;
    this.notifyResolver = notifyResolver;
    this.updateFactory = updateFactory;
    this.changeResourceFactory = changeResourceFactory;
    this.getRelated = getRelated;
    results = new ArrayList<>();
    cherryPickInput = null;
  }

  @Override
  public Response<RevertSubmissionInfo> apply(ChangeResource changeResource, RevertInput input)
      throws RestApiException, IOException, UpdateException, PermissionBackendException,
          NoSuchProjectException, ConfigInvalidException, StorageException {

    if (!changeResource.getChange().isMerged()) {
      throw new ResourceConflictException(
          String.format("change is %s.", ChangeUtil.status(changeResource.getChange())));
    }

    String submissionId = changeResource.getChange().getSubmissionId();
    if (submissionId == null) {
      throw new ResourceConflictException(
          "This change is merged but doesn't have a submission id,"
              + " meaning it was not submitted through Gerrit.");
    }
    List<ChangeData> changeDatas = queryProvider.get().bySubmissionId(submissionId);

    checkPermissionsForAllChanges(changeResource, changeDatas);
    input.topic = createTopic(input.topic, submissionId);
    return Response.ok(revertSubmission(changeDatas, input));
  }

  private String createTopic(String topic, String submissionId) {
    if (topic != null) {
      topic = Strings.emptyToNull(topic.trim());
    }
    if (topic == null) {
      return String.format(
          "revert-%s-%s", submissionId, RandomStringUtils.randomAlphabetic(10).toUpperCase());
    }
    return topic;
  }

  private void checkPermissionsForAllChanges(
      ChangeResource changeResource, List<ChangeData> changeDatas)
      throws IOException, AuthException, PermissionBackendException, ResourceConflictException {
    for (ChangeData changeData : changeDatas) {
      Change change = changeData.change();

      // Might do the permission tests multiple times, but these are necessary to ensure that the
      // user has permissions to revert all changes. If they lack any permission, no revert will be
      // done.

      contributorAgreements.check(change.getProject(), changeResource.getUser());
      permissionBackend.currentUser().ref(change.getDest()).check(CREATE_CHANGE);
      permissionBackend.currentUser().change(changeData).check(ChangePermission.READ);
      projectCache
          .get(change.getProject())
          .orElseThrow(illegalState(change.getProject()))
          .checkStatePermitsWrite();

      requireNonNull(
          psUtil.get(changeData.notes(), change.currentPatchSetId()),
          String.format(
              "current patch set %s of change %s not found",
              change.currentPatchSetId(), change.currentPatchSetId()));
    }
  }

  private RevertSubmissionInfo revertSubmission(
      List<ChangeData> changeData, RevertInput revertInput)
      throws RestApiException, IOException, UpdateException, ConfigInvalidException,
          StorageException, PermissionBackendException {

    Multimap<BranchNameKey, ChangeData> changesPerProjectAndBranch = ArrayListMultimap.create();
    changeData.stream().forEach(c -> changesPerProjectAndBranch.put(c.change().getDest(), c));
    cherryPickInput = createCherryPickInput(revertInput);
    Timestamp timestamp = TimeUtil.nowTs();

    for (BranchNameKey projectAndBranch : changesPerProjectAndBranch.keySet()) {
      cherryPickInput.base = null;
      Project.NameKey project = projectAndBranch.project();
      cherryPickInput.destination = projectAndBranch.branch();
      Collection<ChangeData> changesInProjectAndBranch =
          changesPerProjectAndBranch.get(projectAndBranch);

      // Sort the changes topologically.
      Iterator<PatchSetData> sortedChangesInProjectAndBranch =
          sorter.sort(changesInProjectAndBranch).iterator();

      Set<ObjectId> commitIdsInProjectAndBranch =
          changesInProjectAndBranch.stream()
              .map(c -> c.currentPatchSet().commitId())
              .collect(Collectors.toSet());

      revertAllChangesInProjectAndBranch(
          revertInput,
          project,
          sortedChangesInProjectAndBranch,
          commitIdsInProjectAndBranch,
          timestamp);
    }
    results.sort(Comparator.comparing(c -> c.revertOf));
    RevertSubmissionInfo revertSubmissionInfo = new RevertSubmissionInfo();
    revertSubmissionInfo.revertChanges = results;
    return revertSubmissionInfo;
  }

  private void revertAllChangesInProjectAndBranch(
      RevertInput revertInput,
      Project.NameKey project,
      Iterator<PatchSetData> sortedChangesInProjectAndBranch,
      Set<ObjectId> commitIdsInProjectAndBranch,
      Timestamp timestamp)
      throws IOException, RestApiException, UpdateException, ConfigInvalidException,
          PermissionBackendException {

    String groupName = null;
    String initialMessage = revertInput.message;
    while (sortedChangesInProjectAndBranch.hasNext()) {
      ChangeNotes changeNotes = sortedChangesInProjectAndBranch.next().data().notes();
      if (cherryPickInput.base == null) {
        // If no base was provided, the first change will be used to find a common base.
        cherryPickInput.base = getBase(changeNotes, commitIdsInProjectAndBranch).name();
      }

      revertInput.message = getMessage(initialMessage, changeNotes);
      if (cherryPickInput.base.equals(changeNotes.getCurrentPatchSet().commitId().getName())) {
        // This is the code in case this is the first revert of this project + branch, and the
        // revert would be on top of the change being reverted.
        craeteNormalRevert(revertInput, changeNotes, timestamp);
        groupName = cherryPickInput.base;
      } else {
        // This is the code in case this is the second revert (or more) of this project + branch.
        if (groupName == null) {
          groupName = cherryPickInput.base;
        }
        createCherryPickedRevert(revertInput, project, groupName, changeNotes, timestamp);
      }
    }
  }

  private void createCherryPickedRevert(
      RevertInput revertInput,
      Project.NameKey project,
      String groupName,
      ChangeNotes changeNotes,
      Timestamp timestamp)
      throws IOException, ConfigInvalidException, UpdateException, RestApiException {
    ObjectId revCommitId =
        commitUtil.createRevertCommit(revertInput.message, changeNotes, user.get(), timestamp);
    // TODO (paiking): As a future change, the revert should just be done directly on the
    // target rather than just creating a commit and then cherry-picking it.
    cherryPickInput.message = revertInput.message;
    ObjectId generatedChangeId = CommitMessageUtil.generateChangeId();
    Change.Id cherryPickRevertChangeId = Change.id(seq.nextChangeId());
    try (BatchUpdate bu = updateFactory.create(project, user.get(), TimeUtil.nowTs())) {
      bu.setNotify(
          notifyResolver.resolve(
              firstNonNull(cherryPickInput.notify, NotifyHandling.ALL),
              cherryPickInput.notifyDetails));
      bu.addOp(
          changeNotes.getChange().getId(),
          new CreateCherryPickOp(
              revCommitId, generatedChangeId, cherryPickRevertChangeId, groupName, timestamp));
      bu.addOp(changeNotes.getChange().getId(), new PostRevertedMessageOp(generatedChangeId));
      bu.addOp(
          cherryPickRevertChangeId,
          new NotifyOp(changeNotes.getChange(), cherryPickRevertChangeId));

      bu.execute();
    }
  }

  private void craeteNormalRevert(
      RevertInput revertInput, ChangeNotes changeNotes, Timestamp timestamp)
      throws IOException, RestApiException, UpdateException, ConfigInvalidException {

    Change.Id revertId =
        commitUtil.createRevertChange(changeNotes, user.get(), revertInput, timestamp);
    results.add(json.noOptions().format(changeNotes.getProjectName(), revertId));
    cherryPickInput.base =
        changeNotesFactory.createChecked(revertId).getCurrentPatchSet().commitId().getName();
  }

  private CherryPickInput createCherryPickInput(RevertInput revertInput) {
    cherryPickInput = new CherryPickInput();
    // To create a revert change, we create a revert commit that is then cherry-picked. The revert
    // change is created for the cherry-picked commit. Notifications are sent only for this change,
    // but not for the intermediately created revert commit.
    cherryPickInput.notify = revertInput.notify;
    cherryPickInput.notifyDetails = revertInput.notifyDetails;
    cherryPickInput.parent = 1;
    cherryPickInput.keepReviewers = true;
    cherryPickInput.topic = revertInput.topic;
    cherryPickInput.allowEmpty = true;
    return cherryPickInput;
  }

  private String getMessage(String initialMessage, ChangeNotes changeNotes) {
    String subject = changeNotes.getChange().getSubject();
    if (subject.length() > 60) {
      subject = subject.substring(0, 56) + "...";
    }
    if (initialMessage == null) {
      initialMessage =
          MessageFormat.format(
              ChangeMessages.get().revertSubmissionDefaultMessage,
              changeNotes.getCurrentPatchSet().commitId().name());
    }

    // For performance purposes: Almost all cases will end here.
    if (!subject.startsWith("Revert")) {
      return MessageFormat.format(
          ChangeMessages.get().revertSubmissionUserMessage, subject, initialMessage);
    }

    Matcher matcher = patternRevertSubjectWithNum.matcher(subject);

    if (matcher.matches()) {
      return MessageFormat.format(
          ChangeMessages.get().revertSubmissionOfRevertSubmissionUserMessage,
          Integer.valueOf(matcher.group(1)) + 1,
          matcher.group(2),
          changeNotes.getCurrentPatchSet().commitId().name());
    }

    matcher = patternRevertSubject.matcher(subject);
    if (matcher.matches()) {
      return MessageFormat.format(
          ChangeMessages.get().revertSubmissionOfRevertSubmissionUserMessage,
          2,
          matcher.group(1),
          changeNotes.getCurrentPatchSet().commitId().name());
    }

    return MessageFormat.format(
        ChangeMessages.get().revertSubmissionUserMessage, subject, initialMessage);
  }

  /**
   * This function finds the base that the first revert in a project + branch should be based on.
   *
   * <p>If there is only one change, we will base the revert on that change. If all changes are
   * related, we will base on the first commit of this submission in the topological order.
   *
   * <p>If none of those special cases applies, the only case left is the case where we have at
   * least 2 independent changes in the same project + branch (and possibly other dependent
   * changes). In this case, it searches using BFS for the first commit that is either: 1. Has 2 or
   * more parents, and has as parents at least one commit that is part of the submission. 2. A
   * commit that is part of the submission. If neither of those are true, it just continues the
   * search by going to the parents.
   *
   * <p>If 1 is true, it means that this merge commit was created when this submission was
   * submitted. It also means that this merge commit is a descendant of all of the changes in this
   * submission and project + branch. Therefore, we return this merge commit.
   *
   * <p>If 2 is true, it will return the commit that WalkSorter has decided that it should be the
   * first commit reverted (e.g changeNotes, which is also the commit that is the first in the
   * topological sorting).
   *
   * <p>It doesn't run through the entire graph since it will stop once it finds at least one commit
   * that is part of the submission.
   *
   * @param changeNotes changeNotes for the change that is found by WalkSorter to be the first one
   *     that should be reverted, the first in the topological sorting.
   * @param commitIds The commitIds of this project and branch.
   * @return the base of the first revert.
   */
  private ObjectId getBase(ChangeNotes changeNotes, Set<ObjectId> commitIds)
      throws StorageException, IOException, PermissionBackendException {
    // If there is only one change in that project and branch, just base the revert on that one
    // change.
    if (commitIds.size() == 1) {
      return Iterables.getOnlyElement(commitIds);
    }
    // If all changes are related, just return the first commit of this submission in the
    // topological sorting.
    if (getRelated.getRelated(getRevisionResource(changeNotes)).stream()
        .map(changes -> ObjectId.fromString(changes.commit.commit))
        .collect(Collectors.toSet())
        .containsAll(commitIds)) {
      return changeNotes.getCurrentPatchSet().commitId();
    }
    // There are independent changes in this submission and repository + branch.
    try (Repository git = repoManager.openRepository(changeNotes.getProjectName());
        ObjectInserter oi = git.newObjectInserter();
        ObjectReader reader = oi.newReader();
        RevWalk revWalk = new RevWalk(reader)) {

      ObjectId startCommit =
          git.getRefDatabase().findRef(changeNotes.getChange().getDest().branch()).getObjectId();
      revWalk.markStart(revWalk.parseCommit(startCommit));
      markChangesParentsUninteresting(commitIds, revWalk);
      Iterator<RevCommit> revWalkIterator = revWalk.iterator();
      while (revWalkIterator.hasNext()) {
        RevCommit revCommit = revWalkIterator.next();
        if (commitIds.contains(revCommit.getId())) {
          return changeNotes.getCurrentPatchSet().commitId();
        }
        if (Arrays.stream(revCommit.getParents())
            .anyMatch(parent -> commitIds.contains(parent.getId()))) {
          // Found a merge commit that at least one parent is in this submission. we should only
          // reach here if both conditions apply:
          // 1. There is more than one change in that project + branch in this submission.
          // 2. Not all changes in that project + branch are related in this submission.
          // Therefore, there are at least 2 unrelated changes in this project + branch that got
          // submitted together,
          // and since we found a merge commit with one of those as parents, this merge commit is
          // the first common descendant of all those changes.
          return revCommit.getId();
        }
      }
      // This should never happen since it can only happen if we go through the entire repository
      // without finding a single commit that matches any commit from the submission.
      throw new StorageException(
          String.format(
              "Couldn't find change %s in the repository %s",
              changeNotes.getChangeId(), changeNotes.getProjectName().get()));
    }
  }

  private RevisionResource getRevisionResource(ChangeNotes changeNotes) {
    return new RevisionResource(
        changeResourceFactory.create(changeNotes, user.get()), psUtil.current(changeNotes));
  }

  // The parents are not interesting since there is no reason to base the reverts on any of the
  // parents or their ancestors.
  private void markChangesParentsUninteresting(Set<ObjectId> commitIds, RevWalk revWalk)
      throws IOException {
    for (ObjectId id : commitIds) {
      RevCommit revCommit = revWalk.parseCommit(id);
      for (int i = 0; i < revCommit.getParentCount(); i++) {
        revWalk.markUninteresting(revCommit.getParent(i));
      }
    }
  }

  @Override
  public Description getDescription(ChangeResource rsrc) {
    Change change = rsrc.getChange();
    boolean projectStatePermitsWrite = false;
    try {
      projectStatePermitsWrite =
          projectCache.get(rsrc.getProject()).map(ProjectState::statePermitsWrite).orElse(false);
    } catch (StorageException e) {
      logger.atSevere().withCause(e).log(
          "Failed to check if project state permits write: %s", rsrc.getProject());
    }
    return new UiAction.Description()
        .setLabel("Revert submission")
        .setTitle(
            "Revert this change and all changes that have been submitted together with this change")
        .setVisible(
            and(
                change.isMerged()
                    && change.getSubmissionId() != null
                    && isChangePartOfSubmission(change.getSubmissionId())
                    && projectStatePermitsWrite,
                permissionBackend
                    .user(rsrc.getUser())
                    .ref(change.getDest())
                    .testCond(CREATE_CHANGE)));
  }

  /**
   * @param submissionId the submission id of the change.
   * @return True if the submission has more than one change, false otherwise.
   */
  private Boolean isChangePartOfSubmission(String submissionId) {
    return (queryProvider.get().setLimit(2).bySubmissionId(submissionId).size() > 1);
  }

  private class CreateCherryPickOp implements BatchUpdateOp {
    private final ObjectId revCommitId;
    private final ObjectId computedChangeId;
    private final Change.Id cherryPickRevertChangeId;
    private final String groupName;
    private final Timestamp timestamp;

    CreateCherryPickOp(
        ObjectId revCommitId,
        ObjectId computedChangeId,
        Change.Id cherryPickRevertChangeId,
        String groupName,
        Timestamp timestamp) {
      this.revCommitId = revCommitId;
      this.computedChangeId = computedChangeId;
      this.cherryPickRevertChangeId = cherryPickRevertChangeId;
      this.groupName = groupName;
      this.timestamp = timestamp;
    }

    @Override
    public boolean updateChange(ChangeContext ctx) throws Exception {
      Change change = ctx.getChange();
      Result cherryPickResult =
          cherryPickChange.cherryPick(
              change,
              change.getProject(),
              revCommitId,
              cherryPickInput,
              BranchNameKey.create(
                  change.getProject(), RefNames.fullName(cherryPickInput.destination)),
              timestamp,
              change.getId(),
              computedChangeId,
              cherryPickRevertChangeId,
              groupName);
      // save the commit as base for next cherryPick of that branch
      cherryPickInput.base =
          changeNotesFactory
              .createChecked(cherryPickResult.changeId())
              .getCurrentPatchSet()
              .commitId()
              .getName();
      results.add(json.noOptions().format(change.getProject(), cherryPickResult.changeId()));
      return true;
    }
  }

  private class NotifyOp implements BatchUpdateOp {
    private final Change change;
    private final Change.Id revertChangeId;

    NotifyOp(Change change, Change.Id revertChangeId) {
      this.change = change;
      this.revertChangeId = revertChangeId;
    }

    @Override
    public void postUpdate(Context ctx) throws Exception {
      changeReverted.fire(
          change, changeNotesFactory.createChecked(revertChangeId).getChange(), ctx.getWhen());
      try {
        RevertedSender cm = revertedSenderFactory.create(ctx.getProject(), change.getId());
        cm.setFrom(ctx.getAccountId());
        cm.setNotify(ctx.getNotify(change.getId()));
        cm.send();
      } catch (Exception err) {
        logger.atSevere().withCause(err).log(
            "Cannot send email for revert change %s", change.getId());
      }
    }
  }

  /**
   * create a message that describes the revert if the cherry-pick is successful, and point the
   * revert of the change towards the cherry-pick. The cherry-pick is the updated change that acts
   * as "revert-of" the original change.
   */
  private class PostRevertedMessageOp implements BatchUpdateOp {
    private final ObjectId computedChangeId;

    PostRevertedMessageOp(ObjectId computedChangeId) {
      this.computedChangeId = computedChangeId;
    }

    @Override
    public boolean updateChange(ChangeContext ctx) throws Exception {
      Change change = ctx.getChange();
      PatchSet.Id patchSetId = change.currentPatchSetId();
      ChangeMessage changeMessage =
          ChangeMessagesUtil.newMessage(
              ctx,
              "Created a revert of this change as I" + computedChangeId.getName(),
              ChangeMessagesUtil.TAG_REVERT);
      cmUtil.addChangeMessage(ctx.getUpdate(patchSetId), changeMessage);
      return true;
    }
  }
}