Merge "Support to delete a comment with 'NoteDbRewriter' in BatchUpdate"

Documentation/rest-api-changes.txt

@@ -4425,6 +4425,62 @@ describes the published comment.
   }
 ----
 
+[[delete-comment]]
+=== Delete Comment
+--
+'DELETE /changes/link:#change-id[\{change-id\}]/revisions/link:#revision-id[\{revision-id\}]/comments/link:#comment-id[\{comment-id\}]' +
+'POST /changes/link:#change-id[\{change-id\}]/revisions/link:#revision-id[\{revision-id\}]/comments/link:#comment-id[\{comment-id\}]/delete'
+--
+
+Deletes a published comment of a revision. Instead of deleting the
+whole comment, this endpoint just replaces the comment's message
+with a new message, which contains the name of the user who deletes
+the comment and the reason why it's deleted. The reason can be
+provided in the request body as a
+link:#delete-comment-input[DeleteCommentInput] entity.
+
+Note that only users with the
+link:access-control.html#capability_administrateServer[Administrate Server]
+global capability are permitted to delete a comment.
+
+Please note that some proxies prohibit request bodies for DELETE
+requests. In this case, if you want to specify options, use a
+POST request:
+
+.Request
+----
+  POST /changes/myProject~master~I8473b95934b5732ac55d26311a706c9c2bde9940/revisions/674ac754f91e64a0efb8087e59a176484bd534d1/comments/TvcXrmjM/delete HTTP/1.0
+  Content-Type: application/json; charset=UTF-8
+
+  {
+    "reason": "contains confidential information"
+  }
+----
+
+As response a link:#comment-info[CommentInfo] entity is returned that
+describes the updated comment.
+
+.Response
+----
+  HTTP/1.1 200 OK
+  Content-Disposition: attachment
+  Content-Type: application/json; charset=UTF-8
+
+  )]}'
+  {
+    "id": "TvcXrmjM",
+    "path": "gerrit-server/src/main/java/com/google/gerrit/server/project/RefControl.java",
+    "line": 23,
+    "message": "Comment removed by: Administrator; Reason: contains confidential information",
+    "updated": "2013-02-26 15:40:43.986000000",
+    "author": {
+      "_account_id": 1000096,
+      "name": "John Doe",
+      "email": "john.doe@example.com"
+    }
+  }
+----
+
 [[list-robot-comments]]
 === List Robot Comments
 --
@@ -5770,6 +5826,20 @@ Links to the commit in external sites as a list of
 link:#web-link-info[WebLinkInfo] entities.
 |===========================
 
+[[delete-comment-input]]
+=== DeleteCommentInput
+The `DeleteCommentInput` entity contains the option for deleting a comment.
+
+[options="header",cols="1,^1,5"]
+|=============================
+|Field Name               ||Description
+|`reason`                 |optional|
+The reason why the comment should be deleted. +
+If set, the comment's message will be replaced with
+"Comment removed by: `name`; Reason: `reason`",
+or just "Comment removed by: `name`." if not set.
+|=============================
+
 [[delete-reviewer-input]]
 === DeleteReviewerInput
 The `DeleteReviewerInput` entity contains options for the deletion of a

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/server/change/CommentsIT.java

@@ -20,12 +20,14 @@ import static com.google.gerrit.acceptance.PushOneCommit.SUBJECT;
 
 import com.google.common.base.Function;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.Iterables;
 import com.google.common.collect.Lists;
 import com.google.gerrit.acceptance.AbstractDaemonTest;
 import com.google.gerrit.acceptance.AcceptanceTestRequestScope;
 import com.google.gerrit.acceptance.NoHttpd;
 import com.google.gerrit.acceptance.PushOneCommit;
+import com.google.gerrit.extensions.api.changes.DeleteCommentInput;
 import com.google.gerrit.extensions.api.changes.DraftInput;
 import com.google.gerrit.extensions.api.changes.ReviewInput;
 import com.google.gerrit.extensions.api.changes.ReviewInput.CommentInput;
@@ -34,27 +36,41 @@ import com.google.gerrit.extensions.client.Comment;
 import com.google.gerrit.extensions.client.Side;
 import com.google.gerrit.extensions.common.ChangeInfo;
 import com.google.gerrit.extensions.common.CommentInfo;
+import com.google.gerrit.extensions.restapi.AuthException;
 import com.google.gerrit.extensions.restapi.BadRequestException;
 import com.google.gerrit.extensions.restapi.IdString;
 import com.google.gerrit.extensions.restapi.ResourceNotFoundException;
 import com.google.gerrit.extensions.restapi.TopLevelResource;
+import com.google.gerrit.reviewdb.client.Change;
 import com.google.gerrit.reviewdb.client.Patch;
+import com.google.gerrit.reviewdb.client.RefNames;
 import com.google.gerrit.server.change.ChangeResource;
 import com.google.gerrit.server.change.ChangesCollection;
 import com.google.gerrit.server.change.PostReview;
 import com.google.gerrit.server.change.RevisionResource;
+import com.google.gerrit.server.notedb.ChangeNoteUtil;
+import com.google.gerrit.server.notedb.DeleteCommentRewriter;
 import com.google.gerrit.testutil.FakeEmailSender;
 import com.google.gerrit.testutil.FakeEmailSender.Message;
 import com.google.inject.Inject;
 import com.google.inject.Provider;
+import java.io.IOException;
 import java.sql.Timestamp;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.function.Supplier;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
+import org.eclipse.jgit.errors.ConfigInvalidException;
+import org.eclipse.jgit.lib.ObjectReader;
+import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.notes.NoteMap;
+import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.revwalk.RevWalk;
 import org.junit.Before;
 import org.junit.Test;
 
@@ -67,6 +83,8 @@ public class CommentsIT extends AbstractDaemonTest {
 
   @Inject private FakeEmailSender email;
 
+  @Inject private ChangeNoteUtil noteUtil;
+
   private final Integer[] lines = {0, 1};
 
   @Before
@@ -739,6 +757,168 @@ public class CommentsIT extends AbstractDaemonTest {
     }
   }
 
+  @Test
+  public void deleteCommentCannotBeAppliedByUser() throws Exception {
+    PushOneCommit.Result result = createChange();
+    CommentInput targetComment = addComment(result.getChangeId(), "My password: abc123");
+
+    Map<String, List<CommentInfo>> commentsMap =
+        getPublishedComments(result.getChangeId(), result.getCommit().name());
+
+    assertThat(commentsMap.size()).isEqualTo(1);
+    assertThat(commentsMap.get(FILE_NAME)).hasSize(1);
+
+    String uuid = commentsMap.get(targetComment.path).get(0).id;
+    DeleteCommentInput input = new DeleteCommentInput("contains confidential information");
+
+    setApiUser(user);
+    exception.expect(AuthException.class);
+    gApi.changes().id(result.getChangeId()).current().comment(uuid).delete(input);
+  }
+
+  @Test
+  public void deleteCommentByRewritingCommitHistory() throws Exception {
+    // Create change (the 1st commit on the change's meta branch).
+    PushOneCommit.Result result = createChange();
+    String changeId = result.getChangeId();
+    Change.Id id = result.getChange().getId();
+
+    // Add two comments in patch set 1 (the 2nd commit on the change's meta branch).
+    ReviewInput reviewInput = new ReviewInput();
+    CommentInput comment1 = newComment(FILE_NAME, Side.REVISION, 0, "My password: abc123", false);
+    CommentInput comment2 = newComment(FILE_NAME, Side.REVISION, 1, "nit: long line", false);
+    reviewInput.comments = ImmutableMap.of(FILE_NAME, Lists.newArrayList(comment1, comment2));
+    reviewInput.label("Code-Review", 1);
+    gApi.changes().id(changeId).current().review(reviewInput);
+
+    // Create patch set 2 (the 3rd commit on the change's meta branch).
+    amendChange(changeId);
+
+    // Add 'comment3' in patch set 2 (the 4th commit on the change's meta branch).
+    CommentInput comment3 = addComment(changeId, "typo");
+
+    Map<String, List<CommentInfo>> commentsMap = getPublishedComments(changeId);
+    assertThat(commentsMap).hasSize(1);
+    assertThat(commentsMap.get(FILE_NAME)).hasSize(3);
+    Optional<CommentInfo> targetCommentInfo =
+        commentsMap
+            .get(FILE_NAME)
+            .stream()
+            .filter(c -> c.message.equals("My password: abc123"))
+            .findFirst();
+    assertThat(targetCommentInfo.isPresent()).isTrue();
+
+    List<RevCommit> commitsBeforeDelete = new ArrayList<>();
+    if (notesMigration.commitChangeWrites()) {
+      commitsBeforeDelete = getCommits(id);
+    }
+
+    String uuid = targetCommentInfo.get().id;
+    // Get the target comment.
+    CommentInfo oldComment =
+        gApi.changes().id(changeId).revision(result.getCommit().getName()).comment(uuid).get();
+
+    // Delete the target comment.
+    DeleteCommentInput input = new DeleteCommentInput("contains confidential information");
+    setApiUser(admin);
+    CommentInfo updatedComment =
+        gApi.changes()
+            .id(changeId)
+            .revision(result.getCommit().getName())
+            .comment(uuid)
+            .delete(input);
+
+    String expectedMsg =
+        String.format("Comment removed by: %s; Reason: %s", admin.fullName, input.reason);
+
+    assertThat(updatedComment.message).isEqualTo(expectedMsg);
+    updatedComment.message = oldComment.message;
+    assertThat(updatedComment).isEqualTo(oldComment);
+
+    // Check the comment's message has been replaced in NoteDb.
+    if (notesMigration.commitChangeWrites()) {
+      assertMetaBranchCommitsAfterRewriting(commitsBeforeDelete, id, uuid, expectedMsg);
+    }
+
+    // Make sure that comments can still be added correctly.
+    CommentInput comment4 = addComment(changeId, "too much space");
+    commentsMap = getPublishedComments(changeId);
+
+    assertThat(commentsMap).hasSize(1);
+    List<CommentInput> comments =
+        Lists.transform(commentsMap.get(FILE_NAME), infoToInput(FILE_NAME));
+
+    // Change comment1's message to the expected message.
+    comment1.message = expectedMsg;
+    assertThat(comments).containsExactly(comment1, comment2, comment3, comment4);
+  }
+
+  private CommentInput addComment(String changeId, String message) throws Exception {
+    ReviewInput input = new ReviewInput();
+    CommentInput comment = newComment(FILE_NAME, Side.REVISION, 0, message, false);
+    input.comments = ImmutableMap.of(comment.path, Lists.newArrayList(comment));
+    gApi.changes().id(changeId).current().review(input);
+    return comment;
+  }
+
+  private List<RevCommit> getCommits(Change.Id changeId)
+      throws IOException, ConfigInvalidException {
+    try (Repository repo = repoManager.openRepository(project);
+        RevWalk revWalk = new RevWalk(repo)) {
+      Ref metaRef = repo.exactRef(RefNames.changeMetaRef(changeId));
+      revWalk.markStart(revWalk.parseCommit(metaRef.getObjectId()));
+      return Lists.newArrayList(revWalk);
+    }
+  }
+
+  /**
+   * All the commits, which contain the target comment before, should still contain the comment with
+   * the updated message. All the other metas of the commits should be exactly the same.
+   */
+  private void assertMetaBranchCommitsAfterRewriting(
+      List<RevCommit> beforeDelete,
+      Change.Id changeId,
+      String targetCommentUuid,
+      String expectedMessage)
+      throws Exception {
+    List<RevCommit> afterDelete = getCommits(changeId);
+    assertThat(afterDelete).hasSize(beforeDelete.size());
+
+    try (Repository repo = repoManager.openRepository(project);
+        ObjectReader reader = repo.newObjectReader()) {
+      for (int i = 0; i < beforeDelete.size(); i++) {
+        RevCommit commitBefore = beforeDelete.get(i);
+        RevCommit commitAfter = afterDelete.get(i);
+
+        Map<String, com.google.gerrit.reviewdb.client.Comment> commentMapBefore =
+            DeleteCommentRewriter.getPublishedComments(
+                noteUtil, changeId, reader, NoteMap.read(reader, commitBefore));
+        Map<String, com.google.gerrit.reviewdb.client.Comment> commentMapAfter =
+            DeleteCommentRewriter.getPublishedComments(
+                noteUtil, changeId, reader, NoteMap.read(reader, commitAfter));
+
+        if (commentMapBefore.containsKey(targetCommentUuid)) {
+          assertThat(commentMapAfter).containsKey(targetCommentUuid);
+          com.google.gerrit.reviewdb.client.Comment comment =
+              commentMapAfter.get(targetCommentUuid);
+          assertThat(comment.message).isEqualTo(expectedMessage);
+          comment.message = commentMapBefore.get(targetCommentUuid).message;
+          commentMapAfter.put(targetCommentUuid, comment);
+          assertThat(commentMapAfter).isEqualTo(commentMapBefore);
+        } else {
+          assertThat(commentMapAfter).doesNotContainKey(targetCommentUuid);
+        }
+
+        // Other metas should be exactly the same.
+        assertThat(commitAfter.getFullMessage()).isEqualTo(commitBefore.getFullMessage());
+        assertThat(commitAfter.getCommitterIdent()).isEqualTo(commitBefore.getCommitterIdent());
+        assertThat(commitAfter.getAuthorIdent()).isEqualTo(commitBefore.getAuthorIdent());
+        assertThat(commitAfter.getEncoding()).isEqualTo(commitBefore.getEncoding());
+        assertThat(commitAfter.getEncodingName()).isEqualTo(commitBefore.getEncodingName());
+      }
+    }
+  }
+
   private static String extractComments(String msg) {
     // Extract lines between start "....." and end "-- ".
     Pattern p = Pattern.compile(".*[.]{5}\n+(.*)\\n+-- \n.*", Pattern.DOTALL);
@@ -803,6 +983,10 @@ public class CommentsIT extends AbstractDaemonTest {
     return gApi.changes().id(changeId).revision(revId).drafts();
   }
 
+  private Map<String, List<CommentInfo>> getPublishedComments(String changeId) throws Exception {
+    return gApi.changes().id(changeId).comments();
+  }
+
   private CommentInfo getDraftComment(String changeId, String revId, String uuid) throws Exception {
     return gApi.changes().id(changeId).revision(revId).draft(uuid).get();
   }

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/CommentApi.java

@@ -21,6 +21,17 @@ import com.google.gerrit.extensions.restapi.RestApiException;
 public interface CommentApi {
   CommentInfo get() throws RestApiException;
 
+  /**
+   * Deletes a published comment of a revision. For NoteDb, it deletes the comment by rewriting the
+   * commit history.
+   *
+   * <p>Note instead of deleting the whole comment, this endpoint just replaces the comment's
+   * message.
+   *
+   * @return the comment with its message updated.
+   */
+  CommentInfo delete(DeleteCommentInput input) throws RestApiException;
+
   /**
    * A default implementation which allows source compatibility when adding new methods to the
    * interface.
@@ -30,5 +41,10 @@ public interface CommentApi {
     public CommentInfo get() {
       throw new NotImplementedException();
     }
+
+    @Override
+    public CommentInfo delete(DeleteCommentInput input) {
+      throw new NotImplementedException();
+    }
   }
 }

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/DeleteCommentInput.java

@@ -0,0 +1,30 @@
+// Copyright (C) 2017 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.extensions.api.changes;
+
+import com.google.common.base.Strings;
+import com.google.gerrit.extensions.restapi.DefaultInput;
+
+public class DeleteCommentInput {
+  @DefaultInput public String reason;
+
+  public DeleteCommentInput() {
+    reason = "";
+  }
+
+  public DeleteCommentInput(String reason) {
+    this.reason = Strings.nullToEmpty(reason);
+  }
+}

gerrit-server/src/main/java/com/google/gerrit/server/CommentsUtil.java

@@ -16,6 +16,7 @@ package com.google.gerrit.server;
 
 import static com.google.common.base.MoreObjects.firstNonNull;
 import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.gerrit.reviewdb.client.PatchLineComment.Status.PUBLISHED;
 import static java.util.stream.Collectors.toList;
 import static java.util.stream.Collectors.toSet;
 
@@ -44,6 +45,7 @@ import com.google.gerrit.server.config.GerritServerId;
 import com.google.gerrit.server.git.GitRepositoryManager;
 import com.google.gerrit.server.notedb.ChangeNotes;
 import com.google.gerrit.server.notedb.ChangeUpdate;
+import com.google.gerrit.server.notedb.NoteDbChangeState.PrimaryStorage;
 import com.google.gerrit.server.notedb.NotesMigration;
 import com.google.gerrit.server.patch.PatchListCache;
 import com.google.gerrit.server.patch.PatchListNotAvailableException;
@@ -223,8 +225,7 @@ public class CommentsUtil {
 
   public List<Comment> publishedByChange(ReviewDb db, ChangeNotes notes) throws OrmException {
     if (!migration.readChanges()) {
-      return sort(
+      return sort(byCommentStatus(db.patchComments().byChange(notes.getChangeId()), PUBLISHED));
-          byCommentStatus(db.patchComments().byChange(notes.getChangeId()), Status.PUBLISHED));
     }
 
     notes.load();
@@ -403,6 +404,25 @@ public class CommentsUtil {
         .delete(toPatchLineComments(update.getId(), PatchLineComment.Status.DRAFT, comments));
   }
 
+  public void deleteCommentByRewritingHistory(
+      ReviewDb db, ChangeUpdate update, Comment.Key commentKey, PatchSet.Id psId, String newMessage)
+      throws OrmException {
+    if (PrimaryStorage.of(update.getChange()).equals(PrimaryStorage.REVIEW_DB)) {
+      PatchLineComment.Key key =
+          new PatchLineComment.Key(new Patch.Key(psId, commentKey.filename), commentKey.uuid);
+      PatchLineComment patchLineComment = db.patchComments().get(key);
+
+      if (!patchLineComment.getStatus().equals(PUBLISHED)) {
+        throw new OrmException(String.format("comment %s is not published", key));
+      }
+
+      patchLineComment.setMessage(newMessage);
+      db.patchComments().upsert(Collections.singleton(patchLineComment));
+    }
+
+    update.deleteCommentByRewritingHistory(commentKey.uuid, newMessage);
+  }
+
   public void deleteAllDraftsFromAllUsers(Change.Id changeId) throws IOException {
     try (Repository repo = repoManager.openRepository(allUsers);
         RevWalk rw = new RevWalk(repo)) {
@@ -533,7 +553,7 @@ public class CommentsUtil {
       ctx.getUser().updateRealAccountId(d::setRealAuthor);
       setCommentRevId(d, patchListCache, notes.getChange(), ps);
     }
-    putComments(ctx.getDb(), ctx.getUpdate(psId), PatchLineComment.Status.PUBLISHED, drafts);
+    putComments(ctx.getDb(), ctx.getUpdate(psId), PUBLISHED, drafts);
   }
 
   private static PatchSet.Id psId(ChangeNotes notes, Comment c) {

gerrit-server/src/main/java/com/google/gerrit/server/api/changes/CommentApiImpl.java

@@ -17,9 +17,11 @@ package com.google.gerrit.server.api.changes;
 import static com.google.gerrit.server.api.ApiUtil.asRestApiException;
 
 import com.google.gerrit.extensions.api.changes.CommentApi;
+import com.google.gerrit.extensions.api.changes.DeleteCommentInput;
 import com.google.gerrit.extensions.common.CommentInfo;
 import com.google.gerrit.extensions.restapi.RestApiException;
 import com.google.gerrit.server.change.CommentResource;
+import com.google.gerrit.server.change.DeleteComment;
 import com.google.gerrit.server.change.GetComment;
 import com.google.inject.Inject;
 import com.google.inject.assistedinject.Assisted;
@@ -30,11 +32,14 @@ class CommentApiImpl implements CommentApi {
   }
 
   private final GetComment getComment;
+  private final DeleteComment deleteComment;
   private final CommentResource comment;
 
   @Inject
-  CommentApiImpl(GetComment getComment, @Assisted CommentResource comment) {
+  CommentApiImpl(
+      GetComment getComment, DeleteComment deleteComment, @Assisted CommentResource comment) {
     this.getComment = getComment;
+    this.deleteComment = deleteComment;
     this.comment = comment;
   }
 
@@ -46,4 +51,13 @@ class CommentApiImpl implements CommentApi {
       throw asRestApiException("Cannot retrieve comment", e);
     }
   }
+
+  @Override
+  public CommentInfo delete(DeleteCommentInput input) throws RestApiException {
+    try {
+      return deleteComment.apply(comment, input);
+    } catch (Exception e) {
+      throw asRestApiException("Cannot delete comment", e);
+    }
+  }
 }

gerrit-server/src/main/java/com/google/gerrit/server/api/changes/DraftApiImpl.java

@@ -16,9 +16,11 @@ package com.google.gerrit.server.api.changes;
 
 import static com.google.gerrit.server.api.ApiUtil.asRestApiException;
 
+import com.google.gerrit.extensions.api.changes.DeleteCommentInput;
 import com.google.gerrit.extensions.api.changes.DraftApi;
 import com.google.gerrit.extensions.api.changes.DraftInput;
 import com.google.gerrit.extensions.common.CommentInfo;
+import com.google.gerrit.extensions.restapi.NotImplementedException;
 import com.google.gerrit.extensions.restapi.RestApiException;
 import com.google.gerrit.server.change.DeleteDraftComment;
 import com.google.gerrit.server.change.DraftCommentResource;
@@ -75,4 +77,9 @@ class DraftApiImpl implements DraftApi {
       throw asRestApiException("Cannot delete draft", e);
     }
   }
+
+  @Override
+  public CommentInfo delete(DeleteCommentInput input) {
+    throw new NotImplementedException();
+  }
 }

gerrit-server/src/main/java/com/google/gerrit/server/change/CommentResource.java

@@ -48,4 +48,8 @@ public class CommentResource implements RestResource {
   Account.Id getAuthorId() {
     return comment.author.getId();
   }
+
+  RevisionResource getRevisionResource() {
+    return rev;
+  }
 }

gerrit-server/src/main/java/com/google/gerrit/server/change/DeleteComment.java

@@ -0,0 +1,145 @@
+// Copyright (C) 2017 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.change;
+
+import com.google.common.base.Strings;
+import com.google.gerrit.common.TimeUtil;
+import com.google.gerrit.extensions.api.changes.DeleteCommentInput;
+import com.google.gerrit.extensions.common.CommentInfo;
+import com.google.gerrit.extensions.restapi.ResourceConflictException;
+import com.google.gerrit.extensions.restapi.ResourceNotFoundException;
+import com.google.gerrit.extensions.restapi.RestApiException;
+import com.google.gerrit.extensions.restapi.RestModifyView;
+import com.google.gerrit.reviewdb.client.Comment;
+import com.google.gerrit.reviewdb.client.PatchSet;
+import com.google.gerrit.reviewdb.server.ReviewDb;
+import com.google.gerrit.server.CommentsUtil;
+import com.google.gerrit.server.CurrentUser;
+import com.google.gerrit.server.PatchSetUtil;
+import com.google.gerrit.server.notedb.ChangeNotes;
+import com.google.gerrit.server.patch.PatchListCache;
+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.update.BatchUpdate;
+import com.google.gerrit.server.update.BatchUpdateOp;
+import com.google.gerrit.server.update.ChangeContext;
+import com.google.gerrit.server.update.UpdateException;
+import com.google.gwtorm.server.OrmException;
+import com.google.inject.Inject;
+import com.google.inject.Provider;
+import com.google.inject.Singleton;
+import java.io.IOException;
+import java.util.List;
+import java.util.Optional;
+import org.eclipse.jgit.errors.ConfigInvalidException;
+
+@Singleton
+public class DeleteComment implements RestModifyView<CommentResource, DeleteCommentInput> {
+
+  private final Provider<CurrentUser> userProvider;
+  private final Provider<ReviewDb> dbProvider;
+  private final PermissionBackend permissionBackend;
+  private final BatchUpdate.Factory batchUpdateFactory;
+  private final CommentsUtil commentsUtil;
+  private final PatchSetUtil psUtil;
+  private final BatchUpdate.Factory updateFactory;
+  private final PatchListCache patchListCache;
+  private final Provider<CommentJson> commentJson;
+  private final ChangeNotes.Factory notesFactory;
+
+  @Inject
+  public DeleteComment(
+      Provider<CurrentUser> userProvider,
+      Provider<ReviewDb> dbProvider,
+      PermissionBackend permissionBackend,
+      BatchUpdate.Factory batchUpdateFactory,
+      CommentsUtil commentsUtil,
+      PatchSetUtil psUtil,
+      BatchUpdate.Factory updateFactory,
+      PatchListCache patchListCache,
+      Provider<CommentJson> commentJson,
+      ChangeNotes.Factory notesFactory) {
+    this.userProvider = userProvider;
+    this.dbProvider = dbProvider;
+    this.permissionBackend = permissionBackend;
+    this.batchUpdateFactory = batchUpdateFactory;
+    this.commentsUtil = commentsUtil;
+    this.psUtil = psUtil;
+    this.updateFactory = updateFactory;
+    this.patchListCache = patchListCache;
+    this.commentJson = commentJson;
+    this.notesFactory = notesFactory;
+  }
+
+  @Override
+  public CommentInfo apply(CommentResource rsrc, DeleteCommentInput input)
+      throws RestApiException, IOException, ConfigInvalidException, OrmException,
+          PermissionBackendException, UpdateException {
+    CurrentUser user = userProvider.get();
+    permissionBackend.user(user).check(GlobalPermission.ADMINISTRATE_SERVER);
+
+    String newMessage = getCommentNewMessage(user.asIdentifiedUser().getName(), input.reason);
+    DeleteCommentOp deleteCommentOp = new DeleteCommentOp(rsrc, newMessage);
+    try (BatchUpdate batchUpdate =
+        batchUpdateFactory.create(
+            dbProvider.get(), rsrc.getRevisionResource().getProject(), user, TimeUtil.nowTs())) {
+      batchUpdate.addOp(rsrc.getRevisionResource().getChange().getId(), deleteCommentOp).execute();
+    }
+
+    ChangeNotes updatedNotes =
+        notesFactory.createChecked(rsrc.getRevisionResource().getChange().getId());
+    List<Comment> changeComments = commentsUtil.publishedByChange(dbProvider.get(), updatedNotes);
+    Optional<Comment> updatedComment =
+        changeComments.stream().filter(c -> c.key.equals(rsrc.getComment().key)).findFirst();
+    if (!updatedComment.isPresent()) {
+      // This should not happen as this endpoint should not remove the whole comment.
+      throw new ResourceNotFoundException("comment not found: " + rsrc.getComment().key);
+    }
+
+    return commentJson.get().newCommentFormatter().format(updatedComment.get());
+  }
+
+  private static String getCommentNewMessage(String name, String reason) {
+    StringBuilder stringBuilder = new StringBuilder("Comment removed by: ").append(name);
+    if (!Strings.isNullOrEmpty(reason)) {
+      stringBuilder.append("; Reason: ").append(reason);
+    }
+    return stringBuilder.toString();
+  }
+
+  private class DeleteCommentOp implements BatchUpdateOp {
+    private final CommentResource rsrc;
+    private final String newMessage;
+
+    DeleteCommentOp(CommentResource rsrc, String newMessage) {
+      this.rsrc = rsrc;
+      this.newMessage = newMessage;
+    }
+
+    @Override
+    public boolean updateChange(ChangeContext ctx)
+        throws ResourceConflictException, OrmException, ResourceNotFoundException {
+      PatchSet.Id psId = ctx.getChange().currentPatchSetId();
+      commentsUtil.deleteCommentByRewritingHistory(
+          ctx.getDb(),
+          ctx.getUpdate(psId),
+          rsrc.getComment().key,
+          rsrc.getPatchSet().getId(),
+          newMessage);
+      return true;
+    }
+  }
+}

gerrit-server/src/main/java/com/google/gerrit/server/change/Module.java

@@ -138,6 +138,8 @@ public class Module extends RestApiModule {
 
     child(REVISION_KIND, "comments").to(Comments.class);
     get(COMMENT_KIND).to(GetComment.class);
+    delete(COMMENT_KIND).to(DeleteComment.class);
+    post(COMMENT_KIND, "delete").to(DeleteComment.class);
 
     child(REVISION_KIND, "robotcomments").to(RobotComments.class);
     get(ROBOT_COMMENT_KIND).to(GetRobotComment.class);

gerrit-server/src/main/java/com/google/gerrit/server/notedb/ChangeUpdate.java

@@ -124,9 +124,10 @@ public class ChangeUpdate extends AbstractChangeUpdate {
   }
 
   private final AccountCache accountCache;
+  private final NoteDbUpdateManager.Factory updateManagerFactory;
   private final ChangeDraftUpdate.Factory draftUpdateFactory;
   private final RobotCommentUpdate.Factory robotCommentUpdateFactory;
-  private final NoteDbUpdateManager.Factory updateManagerFactory;
+  private final DeleteCommentRewriter.Factory deleteCommentRewriterFactory;
 
   private final Table<String, Account.Id, Optional<Short>> approvals;
   private final Map<Account.Id, ReviewerStateInternal> reviewers = new LinkedHashMap<>();
@@ -158,6 +159,7 @@ public class ChangeUpdate extends AbstractChangeUpdate {
 
   private ChangeDraftUpdate draftUpdate;
   private RobotCommentUpdate robotCommentUpdate;
+  private DeleteCommentRewriter deleteCommentRewriter;
 
   @AssistedInject
   private ChangeUpdate(
@@ -169,6 +171,7 @@ public class ChangeUpdate extends AbstractChangeUpdate {
       NoteDbUpdateManager.Factory updateManagerFactory,
       ChangeDraftUpdate.Factory draftUpdateFactory,
       RobotCommentUpdate.Factory robotCommentUpdateFactory,
+      DeleteCommentRewriter.Factory deleteCommentRewriterFactory,
       ProjectCache projectCache,
       @Assisted ChangeControl ctl,
       ChangeNoteUtil noteUtil) {
@@ -181,6 +184,7 @@ public class ChangeUpdate extends AbstractChangeUpdate {
         updateManagerFactory,
         draftUpdateFactory,
         robotCommentUpdateFactory,
+        deleteCommentRewriterFactory,
         projectCache,
         ctl,
         serverIdent.getWhen(),
@@ -197,6 +201,7 @@ public class ChangeUpdate extends AbstractChangeUpdate {
       NoteDbUpdateManager.Factory updateManagerFactory,
       ChangeDraftUpdate.Factory draftUpdateFactory,
       RobotCommentUpdate.Factory robotCommentUpdateFactory,
+      DeleteCommentRewriter.Factory deleteCommentRewriterFactory,
       ProjectCache projectCache,
       @Assisted ChangeControl ctl,
       @Assisted Date when,
@@ -210,6 +215,7 @@ public class ChangeUpdate extends AbstractChangeUpdate {
         updateManagerFactory,
         draftUpdateFactory,
         robotCommentUpdateFactory,
+        deleteCommentRewriterFactory,
         ctl,
         when,
         projectCache.get(getProjectName(ctl)).getLabelTypes().nameComparator(),
@@ -235,15 +241,17 @@ public class ChangeUpdate extends AbstractChangeUpdate {
       NoteDbUpdateManager.Factory updateManagerFactory,
       ChangeDraftUpdate.Factory draftUpdateFactory,
       RobotCommentUpdate.Factory robotCommentUpdateFactory,
+      DeleteCommentRewriter.Factory deleteCommentRewriterFactory,
       @Assisted ChangeControl ctl,
       @Assisted Date when,
       @Assisted Comparator<String> labelNameComparator,
       ChangeNoteUtil noteUtil) {
     super(cfg, migration, ctl, serverIdent, anonymousCowardName, noteUtil, when);
     this.accountCache = accountCache;
+    this.updateManagerFactory = updateManagerFactory;
     this.draftUpdateFactory = draftUpdateFactory;
     this.robotCommentUpdateFactory = robotCommentUpdateFactory;
-    this.updateManagerFactory = updateManagerFactory;
+    this.deleteCommentRewriterFactory = deleteCommentRewriterFactory;
     this.approvals = approvals(labelNameComparator);
   }
 
@@ -257,6 +265,7 @@ public class ChangeUpdate extends AbstractChangeUpdate {
       NoteDbUpdateManager.Factory updateManagerFactory,
       ChangeDraftUpdate.Factory draftUpdateFactory,
       RobotCommentUpdate.Factory robotCommentUpdateFactory,
+      DeleteCommentRewriter.Factory deleteCommentRewriterFactory,
       ChangeNoteUtil noteUtil,
       @Assisted Change change,
       @Assisted("effective") @Nullable Account.Id accountId,
@@ -280,6 +289,7 @@ public class ChangeUpdate extends AbstractChangeUpdate {
     this.draftUpdateFactory = draftUpdateFactory;
     this.robotCommentUpdateFactory = robotCommentUpdateFactory;
     this.updateManagerFactory = updateManagerFactory;
+    this.deleteCommentRewriterFactory = deleteCommentRewriterFactory;
     this.approvals = approvals(labelNameComparator);
   }
 
@@ -394,6 +404,11 @@ public class ChangeUpdate extends AbstractChangeUpdate {
     createDraftUpdateIfNull().deleteComment(c);
   }
 
+  public void deleteCommentByRewritingHistory(String uuid, String newMessage) {
+    deleteCommentRewriter =
+        deleteCommentRewriterFactory.create(getChange().getId(), uuid, newMessage);
+  }
+
   @VisibleForTesting
   ChangeDraftUpdate createDraftUpdateIfNull() {
     if (draftUpdate == null) {
@@ -596,6 +611,8 @@ public class ChangeUpdate extends AbstractChangeUpdate {
   @Override
   protected CommitBuilder applyImpl(RevWalk rw, ObjectInserter ins, ObjectId curr)
       throws OrmException, IOException {
+    checkState(deleteCommentRewriter == null, "cannot update and rewrite ref in one BatchUpdate");
+
     CommitBuilder cb = new CommitBuilder();
 
     int ps = psId != null ? psId.get() : getChange().currentPatchSetId().get();
@@ -798,6 +815,10 @@ public class ChangeUpdate extends AbstractChangeUpdate {
     return robotCommentUpdate;
   }
 
+  public DeleteCommentRewriter getDeleteCommentRewriter() {
+    return deleteCommentRewriter;
+  }
+
   public void setAllowWriteToNewRef(boolean allow) {
     isAllowWriteToNewtRef = allow;
   }

gerrit-server/src/main/java/com/google/gerrit/server/notedb/DeleteCommentRewriter.java

@@ -0,0 +1,246 @@
+// Copyright (C) 2017 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.notedb;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.gerrit.reviewdb.client.PatchLineComment.Status.PUBLISHED;
+import static java.util.stream.Collectors.toList;
+import static org.eclipse.jgit.lib.Constants.OBJ_BLOB;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.gerrit.reviewdb.client.Change;
+import com.google.gerrit.reviewdb.client.Comment;
+import com.google.gerrit.reviewdb.client.RefNames;
+import com.google.gerrit.reviewdb.client.RevId;
+import com.google.gwtorm.server.OrmException;
+import com.google.inject.Inject;
+import com.google.inject.assistedinject.Assisted;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Collectors;
+import org.eclipse.jgit.errors.ConfigInvalidException;
+import org.eclipse.jgit.lib.CommitBuilder;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectInserter;
+import org.eclipse.jgit.lib.ObjectReader;
+import org.eclipse.jgit.notes.NoteMap;
+import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.revwalk.RevSort;
+import org.eclipse.jgit.revwalk.RevWalk;
+
+/**
+ * Deletes a published comment from NoteDb by rewriting the commit history. Instead of deleting the
+ * whole comment, it just replaces the comment's message with a new message.
+ */
+public class DeleteCommentRewriter implements NoteDbRewriter {
+
+  public interface Factory {
+    /**
+     * Creates a DeleteCommentRewriter instance.
+     *
+     * @param id the id of the change which contains the target comment.
+     * @param uuid the uuid of the target comment.
+     * @param newMessage the message used to replace the old message of the target comment.
+     * @return
+     */
+    DeleteCommentRewriter create(
+        Change.Id id, @Assisted("uuid") String uuid, @Assisted("newMessage") String newMessage);
+  }
+
+  private final ChangeNoteUtil noteUtil;
+  private final Change.Id changeId;
+  private final String uuid;
+  private final String newMessage;
+
+  @Inject
+  DeleteCommentRewriter(
+      ChangeNoteUtil noteUtil,
+      @Assisted Change.Id changeId,
+      @Assisted("uuid") String uuid,
+      @Assisted("newMessage") String newMessage) {
+    this.noteUtil = noteUtil;
+    this.changeId = changeId;
+    this.uuid = uuid;
+    this.newMessage = newMessage;
+  }
+
+  @Override
+  public String getRefName() {
+    return RefNames.changeMetaRef(changeId);
+  }
+
+  @Override
+  public ObjectId rewriteCommitHistory(RevWalk revWalk, ObjectInserter inserter, ObjectId currTip)
+      throws IOException, ConfigInvalidException, OrmException {
+    checkArgument(!currTip.equals(ObjectId.zeroId()));
+
+    // Walk from the first commit of the branch.
+    revWalk.reset();
+    revWalk.markStart(revWalk.parseCommit(currTip));
+    revWalk.sort(RevSort.REVERSE);
+
+    ObjectReader reader = revWalk.getObjectReader();
+    ObjectId newTip = revWalk.next(); // The first commit will not be rewritten.
+    NoteMap newTipNoteMap = NoteMap.read(reader, revWalk.parseCommit(newTip));
+    Map<String, Comment> parentComments =
+        getPublishedComments(noteUtil, changeId, reader, newTipNoteMap);
+
+    boolean rewrite = false;
+    RevCommit originalCommit;
+    while ((originalCommit = revWalk.next()) != null) {
+      NoteMap noteMap = NoteMap.read(reader, originalCommit);
+      Map<String, Comment> currComments = getPublishedComments(noteUtil, changeId, reader, noteMap);
+
+      if (!rewrite && currComments.containsKey(uuid)) {
+        rewrite = true;
+      }
+
+      if (!rewrite) {
+        parentComments = currComments;
+        newTip = originalCommit;
+        continue;
+      }
+
+      List<Comment> putInComments = getPutInComments(parentComments, currComments);
+      List<Comment> deletedComments = getDeletedComments(parentComments, currComments);
+      newTip =
+          rewriteCommit(
+              originalCommit,
+              newTipNoteMap,
+              newTip,
+              inserter,
+              reader,
+              putInComments,
+              deletedComments);
+      newTipNoteMap = NoteMap.read(reader, revWalk.parseCommit(newTip));
+      parentComments = currComments;
+    }
+
+    return newTip;
+  }
+
+  /**
+   * Gets all the comments which are presented at a commit. Note they include the comments put in by
+   * the previous commits.
+   */
+  @VisibleForTesting
+  public static Map<String, Comment> getPublishedComments(
+      ChangeNoteUtil noteUtil, Change.Id changeId, ObjectReader reader, NoteMap noteMap)
+      throws IOException, ConfigInvalidException {
+    return RevisionNoteMap.parse(noteUtil, changeId, reader, noteMap, PUBLISHED)
+        .revisionNotes
+        .values()
+        .stream()
+        .flatMap(n -> n.getComments().stream())
+        .collect(Collectors.toMap(c -> c.key.uuid, c -> c));
+  }
+
+  /**
+   * Gets the comments put in by the current commit. The message of the target comment will be
+   * replaced by the new message.
+   *
+   * @param parMap the comment map of the parent commit.
+   * @param curMap the comment map of the current commit.
+   * @return The comments put in by the current commit.
+   */
+  private List<Comment> getPutInComments(Map<String, Comment> parMap, Map<String, Comment> curMap) {
+    List<Comment> comments = new ArrayList<>();
+    for (String key : curMap.keySet()) {
+      if (!parMap.containsKey(key)) {
+        Comment comment = curMap.get(key);
+        if (key.equals(uuid)) {
+          comment.message = newMessage;
+        }
+        comments.add(comment);
+      }
+    }
+    return comments;
+  }
+
+  /**
+   * Gets the comments deleted by the current commit.
+   *
+   * @param parMap the comment map of the parent commit.
+   * @param curMap the comment map of the current commit.
+   * @return The comments deleted by the current commit.
+   */
+  private List<Comment> getDeletedComments(
+      Map<String, Comment> parMap, Map<String, Comment> curMap) {
+    return parMap
+        .entrySet()
+        .stream()
+        .filter(c -> !curMap.containsKey(c.getKey()))
+        .map(c -> c.getValue())
+        .collect(toList());
+  }
+
+  /**
+   * Rewrites one commit.
+   *
+   * @param originalCommit the original commit to be rewritten.
+   * @param parentNoteMap the {@code NoteMap} of the new commit's parent.
+   * @param parentId the {@code ObjectId} of the new commit's parent.
+   * @param inserter the {@code ObjectInserter} for the rewrite process.
+   * @param reader the {@code ObjectReader} for the rewrite process.
+   * @param putInComments the comments put in by this commit.
+   * @param deletedComments the comments deleted by this commit.
+   * @return the {@code objectId} of the new commit.
+   * @throws IOException
+   * @throws ConfigInvalidException
+   */
+  private ObjectId rewriteCommit(
+      RevCommit originalCommit,
+      NoteMap parentNoteMap,
+      ObjectId parentId,
+      ObjectInserter inserter,
+      ObjectReader reader,
+      List<Comment> putInComments,
+      List<Comment> deletedComments)
+      throws IOException, ConfigInvalidException {
+    RevisionNoteMap<ChangeRevisionNote> revNotesMap =
+        RevisionNoteMap.parse(noteUtil, changeId, reader, parentNoteMap, PUBLISHED);
+    RevisionNoteBuilder.Cache cache = new RevisionNoteBuilder.Cache(revNotesMap);
+
+    for (Comment c : putInComments) {
+      cache.get(new RevId(c.revId)).putComment(c);
+    }
+
+    for (Comment c : deletedComments) {
+      cache.get(new RevId(c.revId)).deleteComment(c.key);
+    }
+
+    Map<RevId, RevisionNoteBuilder> builders = cache.getBuilders();
+    for (Map.Entry<RevId, RevisionNoteBuilder> entry : builders.entrySet()) {
+      ObjectId objectId = ObjectId.fromString(entry.getKey().get());
+      byte[] data = entry.getValue().build(noteUtil, noteUtil.getWriteJson());
+      if (data.length == 0) {
+        revNotesMap.noteMap.remove(objectId);
+      }
+      revNotesMap.noteMap.set(objectId, inserter.insert(OBJ_BLOB, data));
+    }
+
+    CommitBuilder cb = new CommitBuilder();
+    cb.setParentId(parentId);
+    cb.setTreeId(revNotesMap.noteMap.writeTree(inserter));
+    cb.setMessage(originalCommit.getFullMessage());
+    cb.setCommitter(originalCommit.getCommitterIdent());
+    cb.setAuthor(originalCommit.getAuthorIdent());
+    cb.setEncoding(originalCommit.getEncoding());
+
+    return inserter.insert(cb);
+  }
+}

gerrit-server/src/main/java/com/google/gerrit/server/notedb/NoteDbModule.java

@@ -49,6 +49,7 @@ public class NoteDbModule extends FactoryModule {
   public void configure() {
     factory(ChangeUpdate.Factory.class);
     factory(ChangeDraftUpdate.Factory.class);
+    factory(DeleteCommentRewriter.Factory.class);
     factory(DraftCommentNotes.Factory.class);
     factory(RobotCommentUpdate.Factory.class);
     factory(RobotCommentNotes.Factory.class);

gerrit-server/src/main/java/com/google/gerrit/server/notedb/NoteDbRewriter.java

@@ -0,0 +1,39 @@
+// Copyright (C) 2017 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.notedb;
+
+import com.google.gwtorm.server.OrmException;
+import java.io.IOException;
+import org.eclipse.jgit.errors.ConfigInvalidException;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectInserter;
+import org.eclipse.jgit.revwalk.RevWalk;
+
+public interface NoteDbRewriter {
+
+  /** Gets the name of the target ref which will be rewritten. */
+  String getRefName();
+
+  /**
+   * Rewrites the commit history.
+   *
+   * @param revWalk a {@code RevWalk} instance.
+   * @param inserter a {@code ObjectInserter} instance.
+   * @param currTip the {@code ObjectId} of the ref's tip commit.
+   * @return the {@code ObjectId} of the ref's new tip commit.
+   */
+  ObjectId rewriteCommitHistory(RevWalk revWalk, ObjectInserter inserter, ObjectId currTip)
+      throws IOException, ConfigInvalidException, OrmException;
+}

gerrit-server/src/main/java/com/google/gerrit/server/notedb/NoteDbUpdateManager.java

@@ -54,6 +54,7 @@ import java.util.HashSet;
 import java.util.Map;
 import java.util.Optional;
 import java.util.Set;
+import org.eclipse.jgit.errors.ConfigInvalidException;
 import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.lib.BatchRefUpdate;
 import org.eclipse.jgit.lib.NullProgressMonitor;
@@ -205,6 +206,7 @@ public class NoteDbUpdateManager implements AutoCloseable {
   private final ListMultimap<String, ChangeUpdate> changeUpdates;
   private final ListMultimap<String, ChangeDraftUpdate> draftUpdates;
   private final ListMultimap<String, RobotCommentUpdate> robotCommentUpdates;
+  private final ListMultimap<String, NoteDbRewriter> rewriters;
   private final Set<Change.Id> toDelete;
 
   private OpenRepo changeRepo;
@@ -232,6 +234,7 @@ public class NoteDbUpdateManager implements AutoCloseable {
     changeUpdates = MultimapBuilder.hashKeys().arrayListValues().build();
     draftUpdates = MultimapBuilder.hashKeys().arrayListValues().build();
     robotCommentUpdates = MultimapBuilder.hashKeys().arrayListValues().build();
+    rewriters = MultimapBuilder.hashKeys().arrayListValues().build();
     toDelete = new HashSet<>();
   }
 
@@ -344,6 +347,7 @@ public class NoteDbUpdateManager implements AutoCloseable {
     return changeUpdates.isEmpty()
         && draftUpdates.isEmpty()
         && robotCommentUpdates.isEmpty()
+        && rewriters.isEmpty()
         && toDelete.isEmpty()
         && !hasCommands(changeRepo)
         && !hasCommands(allUsersRepo);
@@ -377,6 +381,10 @@ public class NoteDbUpdateManager implements AutoCloseable {
     if (rcu != null) {
       robotCommentUpdates.put(rcu.getRefName(), rcu);
     }
+    DeleteCommentRewriter deleteCommentRewriter = update.getDeleteCommentRewriter();
+    if (deleteCommentRewriter != null) {
+      rewriters.put(deleteCommentRewriter.getRefName(), deleteCommentRewriter);
+    }
   }
 
   public void add(ChangeDraftUpdate draftUpdate) {
@@ -603,6 +611,21 @@ public class NoteDbUpdateManager implements AutoCloseable {
     if (!robotCommentUpdates.isEmpty()) {
       addUpdates(robotCommentUpdates, changeRepo);
     }
+    if (!rewriters.isEmpty()) {
+      Optional<String> conflictKey =
+          rewriters
+              .keySet()
+              .stream()
+              .filter(k -> (draftUpdates.containsKey(k) || robotCommentUpdates.containsKey(k)))
+              .findAny();
+      if (conflictKey.isPresent()) {
+        throw new IllegalArgumentException(
+            String.format(
+                "cannot update and rewrite ref %s in one BatchUpdate", conflictKey.get()));
+      }
+      addRewrites(rewriters, changeRepo);
+    }
+
     for (Change.Id id : toDelete) {
       doDelete(id);
     }
@@ -723,6 +746,35 @@ public class NoteDbUpdateManager implements AutoCloseable {
     }
   }
 
+  private static void addRewrites(ListMultimap<String, NoteDbRewriter> rewriters, OpenRepo openRepo)
+      throws OrmException, IOException {
+    for (Map.Entry<String, Collection<NoteDbRewriter>> entry : rewriters.asMap().entrySet()) {
+      String refName = entry.getKey();
+      ObjectId oldTip = openRepo.cmds.get(refName).orElse(ObjectId.zeroId());
+
+      if (oldTip.equals(ObjectId.zeroId())) {
+        throw new OrmException(String.format("Ref %s is empty", refName));
+      }
+
+      ObjectId currTip = oldTip;
+      try {
+        for (NoteDbRewriter noteDbRewriter : entry.getValue()) {
+          ObjectId nextTip =
+              noteDbRewriter.rewriteCommitHistory(openRepo.rw, openRepo.tempIns, currTip);
+          if (nextTip != null) {
+            currTip = nextTip;
+          }
+        }
+      } catch (ConfigInvalidException e) {
+        throw new OrmException("Cannot rewrite commit history", e);
+      }
+
+      if (!oldTip.equals(currTip)) {
+        openRepo.cmds.add(new ReceiveCommand(oldTip, currTip, refName));
+      }
+    }
+  }
+
   private static <U extends AbstractChangeUpdate> boolean allowWrite(
       Collection<U> updates, ObjectId old) {
     if (!old.equals(ObjectId.zeroId())) {