Reduce space needed for merge request diff commits

The table merge_request_diff_commits stores the names and Emails of
commit authors and committers. This data is stored as-is. This means
that if I push 10 commits, GitLab stores my name and Email address 20
times: 10 times for the author details, and 10 times for the committer
details.

This commit adds a set of migrations and code changes to resolve this
problem. Instead of storing names and Emails for every occurrence, we'll
store a unique set of names and Emails in a separate table. The table
merge_request_context_commits in turn has two new columns:
commit_author_id and committer_id.

When creating rows for merge_request_context_commits, we take the author
and committer details and try to find an existing row in the newly
added table. If no row exists, one is created.

The resulting setup is such that given a name and Email pair of (X, Y),
we only store a single occurrence of that pair. This reduces the amount
of space necessary to store all this information. Based on our findings
in https://gitlab.com/gitlab-org/gitlab/-/issues/331823, we estimate
that over 95% of the data currently stored is duplicate data. For
GitLab.com, we estimate this will translate to roughly 500 GB of data we
no longer need to store.

The migration process for GitLab.com will likely take around two weeks,
based on our estimates discussed in
https://gitlab.com/gitlab-org/gitlab/-/issues/331823#note_595865070. The
exact time may differ based on how lucky (or not) we get, and the exact
number of rows that have to be migrated.

See the following issues for more information:

- https://gitlab.com/gitlab-org/gitlab/-/issues/331523#note_583654940
- https://gitlab.com/gitlab-org/gitlab/-/issues/331823

Changelog: performance

app/models/merge_request.rb

@@ -360,7 +360,7 @@ class MergeRequest < ApplicationRecord
   scope :preload_approved_by_users, -> { preload(:approved_by_users) }
   scope :preload_metrics, -> (relation) { preload(metrics: relation) }
   scope :preload_project_and_latest_diff, -> { preload(:source_project, :latest_merge_request_diff) }
-  scope :preload_latest_diff_commit, -> { preload(latest_merge_request_diff: :merge_request_diff_commits) }
+  scope :preload_latest_diff_commit, -> { preload(latest_merge_request_diff: { merge_request_diff_commits: [:commit_author, :committer] }) }
   scope :preload_milestoneish_associations, -> { preload_routables.preload(:assignees, :labels) }
 
   scope :with_web_entity_associations, -> { preload(:author, target_project: [:project_feature, group: [:route, :parent], namespace: :route]) }

app/models/merge_request/diff_commit_user.rb

+# frozen_string_literal: true
+
+class MergeRequest::DiffCommitUser < ApplicationRecord
+  validates :name, length: { maximum: 512 }
+  validates :email, length: { maximum: 512 }
+  validates :name, presence: true, unless: :email
+  validates :email, presence: true, unless: :name
+
+  # Prepares a value to be inserted into a column in the table
+  # `merge_request_diff_commit_users`. Values in this table are limited to
+  # 512 characters.
+  #
+  # We treat empty strings as NULL values, as there's no point in (for
+  # example) storing a row where both the name and Email are an empty
+  # string. In addition, if we treated them differently we could end up with
+  # two rows: one where field X is NULL, and one where field X is an empty
+  # string. This is redundant, so we avoid storing such data.
+  def self.prepare(value)
+    value.present? ? value[0..511] : nil
+  end
+
+  # Creates a new row, or returns an existing one if a row already exists.
+  def self.find_or_create(name, email)
+    find_or_create_by!(name: name, email: email)
+  rescue ActiveRecord::RecordNotUnique
+    retry
+  end
+
+  # Finds many (name, email) pairs in bulk.
+  def self.bulk_find(pairs)
+    queries = {}
+    rows = []
+
+    pairs.each do |(name, email)|
+      queries[[name, email]] = where(name: name, email: email).to_sql
+    end
+
+    # We may end up having to query many users. To ensure we don't hit any
+    # query size limits, we get a fixed number of users at a time.
+    queries.values.each_slice(1_000).map do |slice|
+      rows.concat(from("(#{slice.join("\nUNION ALL\n")}) #{table_name}").to_a)
+    end
+
+    rows
+  end
+
+  # Finds or creates rows for the given pairs of names and Emails.
+  #
+  # The `names_and_emails` argument must be an Array/Set of tuples like so:
+  #
+  #     [
+  #       [name, email],
+  #       [name, email],
+  #       ...
+  #     ]
+  #
+  # This method expects that the names and Emails have already been trimmed to
+  # at most 512 characters.
+  #
+  # The return value is a Hash that maps these tuples to instances of this
+  # model.
+  def self.bulk_find_or_create(pairs)
+    mapping = {}
+    create = []
+
+    # Over time, fewer new rows need to be created. We take advantage of that
+    # here by first finding all rows that already exist, using a limited number
+    # of queries (in most cases only one query will be needed).
+    bulk_find(pairs).each do |row|
+      mapping[[row.name, row.email]] = row
+    end
+
+    pairs.each do |(name, email)|
+      create << { name: name, email: email } unless mapping[[name, email]]
+    end
+
+    return mapping if create.empty?
+
+    # Sometimes we may need to insert new users into the table. We do this in
+    # bulk, so we only need one INSERT for all missing users.
+    insert_all(create, returning: %w[id name email]).each do |row|
+      mapping[[row['name'], row['email']]] =
+        new(id: row['id'], name: row['name'], email: row['email'])
+    end
+
+    # It's possible for (name, email) pairs to be inserted concurrently,
+    # resulting in the above insert not returning anything. Here we get any
+    # remaining users that were created concurrently.
+    bulk_find(pairs.reject { |pair| mapping.key?(pair) }).each do |row|
+      mapping[[row.name, row.email]] = row
+    end
+
+    mapping
+  end
+end

app/models/merge_request_diff.rb

@@ -701,7 +701,7 @@ class MergeRequestDiff < ApplicationRecord
   end
 
   def load_commits(limit: nil)
-    commits = merge_request_diff_commits.limit(limit)
+    commits = merge_request_diff_commits.with_users.limit(limit)
       .map { |commit| Commit.from_hash(commit.to_hash, project) }
 
     CommitCollection

app/models/merge_request_diff_commit.rb

@@ -9,21 +9,51 @@ class MergeRequestDiffCommit < ApplicationRecord
 
   belongs_to :merge_request_diff
 
+  # This relation is called `commit_author` and not `author`, as the project
+  # import/export logic treats relations named `author` as instances of the
+  # `User` class.
+  #
+  # NOTE: these columns are _not_ indexed, nor do they use foreign keys.
+  #
+  # This is deliberate, as creating these indexes on GitLab.com takes a _very_
+  # long time. In addition, there's no real need for them either based on how
+  # this data is used.
+  #
+  # For more information, refer to the following:
+  #
+  # - https://gitlab.com/gitlab-com/gl-infra/production/-/issues/5038#note_614592881
+  # - https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63669
+  belongs_to :commit_author, class_name: 'MergeRequest::DiffCommitUser'
+  belongs_to :committer, class_name: 'MergeRequest::DiffCommitUser'
+
   sha_attribute :sha
   alias_attribute :id, :sha
 
   serialize :trailers, Serializers::Json # rubocop:disable Cop/ActiveRecordSerialize
   validates :trailers, json_schema: { filename: 'git_trailers' }
 
+  scope :with_users, -> { preload(:commit_author, :committer) }
+
+  # A list of keys of which their values need to be trimmed before they can be
+  # inserted into the merge_request_diff_commit_users table.
+  TRIM_USER_KEYS =
+    %i[author_name author_email committer_name committer_email].freeze
+
   # Deprecated; use `bulk_insert!` from `BulkInsertSafe` mixin instead.
   # cf. https://gitlab.com/gitlab-org/gitlab/issues/207989 for progress
   def self.create_bulk(merge_request_diff_id, commits)
-    rows = commits.map.with_index do |commit, index|
-      # See #parent_ids.
-      commit_hash = commit.to_hash.except(:parent_ids)
+    commit_hashes, user_tuples = prepare_commits_for_bulk_insert(commits)
+    users = MergeRequest::DiffCommitUser.bulk_find_or_create(user_tuples)
+
+    rows = commit_hashes.map.with_index do |commit_hash, index|
       sha = commit_hash.delete(:id)
+      author = users[[commit_hash[:author_name], commit_hash[:author_email]]]
+      committer =
+        users[[commit_hash[:committer_name], commit_hash[:committer_email]]]
 
       commit_hash.merge(
+        commit_author_id: author&.id,
+        committer_id: committer&.id,
         merge_request_diff_id: merge_request_diff_id,
         relative_order: index,
         sha: Gitlab::Database::ShaAttribute.serialize(sha), # rubocop:disable Cop/ActiveRecordSerialize
@@ -36,6 +66,24 @@ class MergeRequestDiffCommit < ApplicationRecord
     Gitlab::Database.bulk_insert(self.table_name, rows) # rubocop:disable Gitlab/BulkInsert
   end
 
+  def self.prepare_commits_for_bulk_insert(commits)
+    user_tuples = Set.new
+    hashes = commits.map do |commit|
+      hash = commit.to_hash.except(:parent_ids)
+
+      TRIM_USER_KEYS.each do |key|
+        hash[key] = MergeRequest::DiffCommitUser.prepare(hash[key])
+      end
+
+      user_tuples << [hash[:author_name], hash[:author_email]]
+      user_tuples << [hash[:committer_name], hash[:committer_email]]
+
+      hash
+    end
+
+    [hashes, user_tuples]
+  end
+
   def self.oldest_merge_request_id_per_commit(project_id, shas)
     # This method is defined here and not on MergeRequest, otherwise the SHA
     # values used in the WHERE below won't be encoded correctly.
@@ -54,4 +102,20 @@ class MergeRequestDiffCommit < ApplicationRecord
       )
       .group(:sha)
   end
+
+  def author_name
+    commit_author_id ? commit_author.name : super
+  end
+
+  def author_email
+    commit_author_id ? commit_author.email : super
+  end
+
+  def committer_name
+    committer_id ? committer.name : super
+  end
+
+  def committer_email
+    committer_id ? committer.email : super
+  end
 end

db/migrate/20210602155056_add_merge_request_diff_commit_users.rb

+# frozen_string_literal: true
+
+class AddMergeRequestDiffCommitUsers < ActiveRecord::Migration[6.1]
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  def up
+    create_table_with_constraints :merge_request_diff_commit_users, id: :bigint do |t|
+      t.text :name
+      t.text :email
+
+      t.text_limit :name, 512
+      t.text_limit :email, 512
+
+      t.index [:name, :email], unique: true
+    end
+
+    # Names or Emails can be optional, so in some cases one of these may be
+    # null. But if both are NULL/empty, no row should exist in this table.
+    add_check_constraint(
+      :merge_request_diff_commit_users,
+      "(COALESCE(name, '') != '') OR (COALESCE(email, '') != '')",
+      :merge_request_diff_commit_users_name_or_email_existence
+    )
+  end
+
+  def down
+    drop_table :merge_request_diff_commit_users
+  end
+end

db/migrate/20210602155110_add_merge_request_diff_commit_user_columns.rb

+# frozen_string_literal: true
+
+class AddMergeRequestDiffCommitUserColumns < ActiveRecord::Migration[6.1]
+  include Gitlab::Database::MigrationHelpers
+
+  def up
+    # NOTE: these columns are _not_ indexed, nor do they use foreign keys.
+    #
+    # This is deliberate, as creating these indexes on GitLab.com takes a _very_
+    # long time. In addition, there's no real need for them either based on how
+    # this data is used.
+    #
+    # For more information, refer to the following:
+    #
+    # - https://gitlab.com/gitlab-com/gl-infra/production/-/issues/5038#note_614592881
+    # - https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63669
+    add_column(:merge_request_diff_commits, :commit_author_id, :bigint)
+    add_column(:merge_request_diff_commits, :committer_id, :bigint)
+  end
+
+  def down
+    remove_column(:merge_request_diff_commits, :commit_author_id)
+    remove_column(:merge_request_diff_commits, :committer_id)
+  end
+end

db/post_migrate/20210604133651_schedule_merge_request_diff_users_background_migration.rb

+# frozen_string_literal: true
+
+class ScheduleMergeRequestDiffUsersBackgroundMigration < ActiveRecord::Migration[6.1]
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  # The number of rows to process in a single migration job.
+  #
+  # The minimum interval for background migrations is two minutes. On staging we
+  # observed we can process roughly 20 000 rows in a minute. Based on the total
+  # number of rows on staging, this translates to a total processing time of
+  # roughly 14 days.
+  #
+  # By using a batch size of 40 000, we maintain a rate of roughly 20 000 rows
+  # per minute, hopefully keeping the total migration time under two weeks;
+  # instead of four weeks.
+  BATCH_SIZE = 40_000
+
+  MIGRATION_NAME = 'MigrateMergeRequestDiffCommitUsers'
+
+  class MergeRequestDiff < ActiveRecord::Base
+    self.table_name = 'merge_request_diffs'
+  end
+
+  def up
+    start = MergeRequestDiff.minimum(:id).to_i
+    max = MergeRequestDiff.maximum(:id).to_i
+    delay = BackgroundMigrationWorker.minimum_interval
+
+    # The table merge_request_diff_commits contains _a lot_ of rows (roughly 400
+    # 000 000 on staging). Iterating a table that large to determine job ranges
+    # would take a while.
+    #
+    # To avoid that overhead, we simply schedule fixed ranges according to the
+    # minimum and maximum IDs. The background migration in turn only processes
+    # rows that actually exist.
+    while start < max
+      stop = start + BATCH_SIZE
+
+      migrate_in(delay, MIGRATION_NAME, [start, stop])
+
+      Gitlab::Database::BackgroundMigrationJob
+        .create!(class_name: MIGRATION_NAME, arguments: [start, stop])
+
+      delay += BackgroundMigrationWorker.minimum_interval
+      start += BATCH_SIZE
+    end
+  end
+
+  def down
+    # no-op
+  end
+end

db/schema_migrations/20210602155056

+42b3090efee66f5a7a5c06d8768d1417892c5d6745f60163a09f58e6e3722761
\ No newline at end of file

db/schema_migrations/20210602155110

+aa04d433e400ed3ec11e5d40ada72f122b1d8b7a82f8803d9206da5c94ec5ef9
\ No newline at end of file

db/schema_migrations/20210604133651

+0c01bb41113c468a602649b591e1fd2959a6e3190c835ef2e27351cf69f50fd5
\ No newline at end of file

db/structure.sql

@@ -14760,6 +14760,24 @@ CREATE SEQUENCE merge_request_context_commits_id_seq
 
 ALTER SEQUENCE merge_request_context_commits_id_seq OWNED BY merge_request_context_commits.id;
 
+CREATE TABLE merge_request_diff_commit_users (
+    id bigint NOT NULL,
+    name text,
+    email text,
+    CONSTRAINT check_147358fc42 CHECK ((char_length(name) <= 512)),
+    CONSTRAINT check_f5fa206cf7 CHECK ((char_length(email) <= 512)),
+    CONSTRAINT merge_request_diff_commit_users_name_or_email_existence CHECK (((COALESCE(name, ''::text) <> ''::text) OR (COALESCE(email, ''::text) <> ''::text)))
+);
+
+CREATE SEQUENCE merge_request_diff_commit_users_id_seq
+    START WITH 1
+    INCREMENT BY 1
+    NO MINVALUE
+    NO MAXVALUE
+    CACHE 1;
+
+ALTER SEQUENCE merge_request_diff_commit_users_id_seq OWNED BY merge_request_diff_commit_users.id;
+
 CREATE TABLE merge_request_diff_commits (
     authored_date timestamp without time zone,
     committed_date timestamp without time zone,
@@ -14771,7 +14789,9 @@ CREATE TABLE merge_request_diff_commits (
     committer_name text,
     committer_email text,
     message text,
-    trailers jsonb DEFAULT '{}'::jsonb NOT NULL
+    trailers jsonb DEFAULT '{}'::jsonb NOT NULL,
+    commit_author_id bigint,
+    committer_id bigint
 );
 
 CREATE TABLE merge_request_diff_details (
@@ -20160,6 +20180,8 @@ ALTER TABLE ONLY merge_request_cleanup_schedules ALTER COLUMN merge_request_id S
 
 ALTER TABLE ONLY merge_request_context_commits ALTER COLUMN id SET DEFAULT nextval('merge_request_context_commits_id_seq'::regclass);
 
+ALTER TABLE ONLY merge_request_diff_commit_users ALTER COLUMN id SET DEFAULT nextval('merge_request_diff_commit_users_id_seq'::regclass);
+
 ALTER TABLE ONLY merge_request_diff_details ALTER COLUMN merge_request_diff_id SET DEFAULT nextval('merge_request_diff_details_merge_request_diff_id_seq'::regclass);
 
 ALTER TABLE ONLY merge_request_diffs ALTER COLUMN id SET DEFAULT nextval('merge_request_diffs_id_seq'::regclass);
@@ -21600,6 +21622,9 @@ ALTER TABLE ONLY merge_request_context_commit_diff_files
 ALTER TABLE ONLY merge_request_context_commits
     ADD CONSTRAINT merge_request_context_commits_pkey PRIMARY KEY (id);
 
+ALTER TABLE ONLY merge_request_diff_commit_users
+    ADD CONSTRAINT merge_request_diff_commit_users_pkey PRIMARY KEY (id);
+
 ALTER TABLE ONLY merge_request_diff_commits
     ADD CONSTRAINT merge_request_diff_commits_pkey PRIMARY KEY (merge_request_diff_id, relative_order);
 
@@ -23903,6 +23928,8 @@ CREATE INDEX index_merge_request_blocks_on_blocked_merge_request_id ON merge_req
 
 CREATE UNIQUE INDEX index_merge_request_cleanup_schedules_on_merge_request_id ON merge_request_cleanup_schedules USING btree (merge_request_id);
 
+CREATE UNIQUE INDEX index_merge_request_diff_commit_users_on_name_and_email ON merge_request_diff_commit_users USING btree (name, email);
+
 CREATE INDEX index_merge_request_diff_commits_on_sha ON merge_request_diff_commits USING btree (sha);
 
 CREATE INDEX index_merge_request_diff_details_failed_verification ON merge_request_diff_details USING btree (verification_retry_at NULLS FIRST) WHERE (verification_state = 3);

lib/gitlab/background_migration/migrate_merge_request_diff_commit_users.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    # Migrates author and committer names and emails from
+    # merge_request_diff_commits to two columns that point to
+    # merge_request_diff_commit_users.
+    #
+    # rubocop: disable Metrics/ClassLength
+    class MigrateMergeRequestDiffCommitUsers
+      # The number of user rows in merge_request_diff_commit_users to get in a
+      # single query.
+      USER_ROWS_PER_QUERY = 1_000
+
+      # The number of rows in merge_request_diff_commits to get in a single
+      # query.
+      COMMIT_ROWS_PER_QUERY = 10_000
+
+      # The number of rows in merge_request_diff_commits to update in a single
+      # query.
+      #
+      # Tests in staging revealed that increasing the number of updates per
+      # query translates to a longer total runtime for a migration. For example,
+      # given the same range of rows to migrate, 1000 updates per query required
+      # a total of roughly 15 seconds. On the other hand, 5000 updates per query
+      # required a total of roughly 25 seconds. For this reason, we use a value
+      # of 1000 rows per update.
+      UPDATES_PER_QUERY = 1_000
+
+      # rubocop: disable Style/Documentation
+      class MergeRequestDiffCommit < ActiveRecord::Base
+        include FromUnion
+        extend ::SuppressCompositePrimaryKeyWarning
+
+        self.table_name = 'merge_request_diff_commits'
+
+        # Yields each row to migrate in the given range.
+        #
+        # This method uses keyset pagination to ensure we don't retrieve
+        # potentially tens of thousands (or even hundreds of thousands) of rows
+        # in a single query. Such queries could time out, or increase the amount
+        # of memory needed to process the data.
+        #
+        # We can't use `EachBatch` and similar approaches, as
+        # merge_request_diff_commits doesn't have a single monotonically
+        # increasing primary key.
+        def self.each_row_to_migrate(start_id, stop_id, &block)
+          order = Pagination::Keyset::Order.build(
+            %w[merge_request_diff_id relative_order].map do |col|
+              Pagination::Keyset::ColumnOrderDefinition.new(
+                attribute_name: col,
+                order_expression: self.arel_table[col.to_sym].asc,
+                nullable: :not_nullable,
+                distinct: false
+              )
+            end
+          )
+
+          scope = MergeRequestDiffCommit
+            .where(merge_request_diff_id: start_id...stop_id)
+            .order(order)
+
+          Pagination::Keyset::Iterator
+            .new(scope: scope, use_union_optimization: true)
+            .each_batch(of: COMMIT_ROWS_PER_QUERY) { |rows| rows.each(&block) }
+        end
+      end
+      # rubocop: enable Style/Documentation
+
+      # rubocop: disable Style/Documentation
+      class MergeRequestDiffCommitUser < ActiveRecord::Base
+        self.table_name = 'merge_request_diff_commit_users'
+
+        def self.union(queries)
+          from("(#{queries.join("\nUNION ALL\n")}) #{table_name}")
+        end
+      end
+      # rubocop: enable Style/Documentation
+
+      def perform(start_id, stop_id)
+        # This Hash maps user names + emails to their corresponding rows in
+        # merge_request_diff_commit_users.
+        user_mapping = {}
+
+        user_details, diff_rows_to_update = get_data_to_update(start_id, stop_id)
+
+        get_user_rows_in_batches(user_details, user_mapping)
+        create_missing_users(user_details, user_mapping)
+        update_commit_rows(diff_rows_to_update, user_mapping)
+
+        Database::BackgroundMigrationJob.mark_all_as_succeeded(
+          'MigrateMergeRequestDiffCommitUsers',
+          [start_id, stop_id]
+        )
+      end
+
+      # Returns the data we'll use to determine what merge_request_diff_commits
+      # rows to update, and what data to use for populating their
+      # commit_author_id and committer_id columns.
+      def get_data_to_update(start_id, stop_id)
+        # This Set is used to retrieve users that already exist in
+        # merge_request_diff_commit_users.
+        users = Set.new
+
+        # This Hash maps the primary key of every row in
+        # merge_request_diff_commits to the (trimmed) author and committer
+        # details to use for updating the row.
+        to_update = {}
+
+        MergeRequestDiffCommit.each_row_to_migrate(start_id, stop_id) do |row|
+          author = [prepare(row.author_name), prepare(row.author_email)]
+          committer = [prepare(row.committer_name), prepare(row.committer_email)]
+
+          to_update[[row.merge_request_diff_id, row.relative_order]] =
+            [author, committer]
+
+          users << author if author[0] || author[1]
+          users << committer if committer[0] || committer[1]
+        end
+
+        [users, to_update]
+      end
+
+      # Gets any existing rows in merge_request_diff_commit_users in batches.
+      #
+      # This method may end up having to retrieve lots of rows. To reduce the
+      # overhead, we batch queries into a UNION query. We limit the number of
+      # queries per UNION so we don't end up sending a single query containing
+      # too many SELECT statements.
+      def get_user_rows_in_batches(users, user_mapping)
+        users.each_slice(USER_ROWS_PER_QUERY) do |pairs|
+          queries = pairs.map do |(name, email)|
+            MergeRequestDiffCommitUser.where(name: name, email: email).to_sql
+          end
+
+          MergeRequestDiffCommitUser.union(queries).each do |row|
+            user_mapping[[row.name.to_s, row.email.to_s]] = row
+          end
+        end
+      end
+
+      # Creates any users for which no row exists in
+      # merge_request_diff_commit_users.
+      #
+      # Not all users queried may exist yet, so we need to create any missing
+      # ones; making sure we handle concurrent creations of the same user
+      def create_missing_users(users, mapping)
+        create = []
+
+        users.each do |(name, email)|
+          create << { name: name, email: email } unless mapping[[name, email]]
+        end
+
+        return if create.empty?
+
+        MergeRequestDiffCommitUser
+          .insert_all(create, returning: %w[id name email])
+          .each do |row|
+            mapping[[row['name'], row['email']]] = MergeRequestDiffCommitUser
+              .new(id: row['id'], name: row['name'], email: row['email'])
+          end
+
+        # It's possible for (name, email) pairs to be inserted concurrently,
+        # resulting in the above insert not returning anything. Here we get any
+        # remaining users that were created concurrently.
+        get_user_rows_in_batches(
+          users.reject { |pair| mapping.key?(pair) },
+          mapping
+        )
+      end
+
+      # Updates rows in merge_request_diff_commits with their new
+      # commit_author_id and committer_id values.
+      def update_commit_rows(to_update, user_mapping)
+        MergeRequestDiffCommitUser.transaction do
+          to_update.each_slice(UPDATES_PER_QUERY) do |slice|
+            updates = {}
+
+            slice.each do |(diff_id, order), (author, committer)|
+              author_id = user_mapping[author]&.id
+              committer_id = user_mapping[committer]&.id
+
+              updates[[diff_id, order]] = [author_id, committer_id]
+            end
+
+            bulk_update_commit_rows(updates)
+          end
+        end
+      end
+
+      # Bulk updates rows in the merge_request_diff_commits table with their new
+      # author and/or committer ID values.
+      #
+      # Updates are batched together to reduce the overhead of having to produce
+      # a single UPDATE for every row, as we may end up having to update
+      # thousands of rows at once.
+      #
+      # The query produced by this method is along the lines of the following:
+      #
+      #     UPDATE merge_request_diff_commits
+      #     SET commit_author_id =
+      #       CASE
+      #       WHEN (merge_request_diff_id, relative_order) = (x, y) THEN X
+      #       WHEN ...
+      #       END,
+      #     committer_id =
+      #       CASE
+      #       WHEN (merge_request_diff_id, relative_order) = (x, y) THEN Y
+      #       WHEN ...
+      #       END
+      #     WHERE (merge_request_diff_id, relative_order) IN ( (x, y), ... )
+      #
+      # The `mapping` argument is a Hash in the following format:
+      #
+      #     { [merge_request_diff_id, relative_order] => [author_id, committer_id] }
+      #
+      # rubocop: disable Metrics/AbcSize
+      def bulk_update_commit_rows(mapping)
+        author_case = Arel::Nodes::Case.new
+        committer_case = Arel::Nodes::Case.new
+        primary_values = []
+
+        mapping.each do |diff_id_and_order, (author_id, committer_id)|
+          primary_value = Arel::Nodes::Grouping.new(diff_id_and_order)
+
+          primary_values << primary_value
+
+          if author_id
+            author_case.when(primary_key.eq(primary_value)).then(author_id)
+          end
+
+          if committer_id
+            committer_case.when(primary_key.eq(primary_value)).then(committer_id)
+          end
+        end
+
+        if author_case.conditions.empty? && committer_case.conditions.empty?
+          return
+        end
+
+        fields = []
+
+        # Statements such as `SET x = CASE END` are not valid SQL statements, so
+        # we omit setting an ID field if there are no values to populate it
+        # with.
+        if author_case.conditions.any?
+          fields << [arel_table[:commit_author_id], author_case]
+        end
+
+        if committer_case.conditions.any?
+          fields << [arel_table[:committer_id], committer_case]
+        end
+
+        query = Arel::UpdateManager.new
+          .table(arel_table)
+          .where(primary_key.in(primary_values))
+          .set(fields)
+          .to_sql
+
+        MergeRequestDiffCommit.connection.execute(query)
+      end
+      # rubocop: enable Metrics/AbcSize
+
+      def primary_key
+        Arel::Nodes::Grouping.new(
+          [arel_table[:merge_request_diff_id], arel_table[:relative_order]]
+        )
+      end
+
+      def arel_table
+        MergeRequestDiffCommit.arel_table
+      end
+
+      # Prepares a value to be inserted into a column in the table
+      # `merge_request_diff_commit_users`. Values in this table are limited to
+      # 512 characters.
+      #
+      # We treat empty strings as NULL values, as there's no point in (for
+      # example) storing a row where both the name and Email are an empty
+      # string. In addition, if we treated them differently we could end up with
+      # two rows: one where field X is NULL, and one where field X is an empty
+      # string. This is redundant, so we avoid storing such data.
+      def prepare(value)
+        value.present? ? value[0..511] : nil
+      end
+    end
+    # rubocop: enable Metrics/ClassLength
+  end
+end

lib/gitlab/import_export/project/import_export.yml

@@ -61,7 +61,9 @@ tree:
           - :push_event_payload
         - :suggestions
       - merge_request_diff:
-        - :merge_request_diff_commits
+        - merge_request_diff_commits:
+          - :commit_author
+          - :committer
         - :merge_request_diff_files
       - events:
         - :push_event_payload
@@ -201,6 +203,10 @@ excluded_attributes:
     - :verification_failure
   merge_request_diff_commits:
     - :merge_request_diff_id
+    - :commit_author_id
+    - :committer_id
+  merge_request_diff_commit_user:
+    - :id
   merge_request_diff_detail:
     - :merge_request_diff_id
     - :verification_retry_at

lib/gitlab/import_export/project/object_builder.rb

@@ -28,6 +28,7 @@ module Gitlab
 
         def find
           return if epic? && group.nil?
+          return find_diff_commit_user if diff_commit_user?
 
           super
         end
@@ -81,6 +82,13 @@ module Gitlab
           end
         end
 
+        def find_diff_commit_user
+          find_with_cache do
+            MergeRequest::DiffCommitUser
+              .find_or_create(@attributes['name'], @attributes['email'])
+          end
+        end
+
         def label?
           klass == Label
         end
@@ -101,6 +109,10 @@ module Gitlab
           klass == DesignManagement::Design
         end
 
+        def diff_commit_user?
+          klass == MergeRequest::DiffCommitUser
+        end
+
         # If an existing group milestone used the IID
         # claim the IID back and set the group milestone to use one available
         # This is necessary to fix situations like the following:

lib/gitlab/import_export/project/relation_factory.rb

@@ -31,7 +31,9 @@ module Gitlab
                       ci_cd_settings: 'ProjectCiCdSetting',
                       error_tracking_setting: 'ErrorTracking::ProjectErrorTrackingSetting',
                       links: 'Releases::Link',
-                      metrics_setting: 'ProjectMetricsSetting' }.freeze
+                      metrics_setting: 'ProjectMetricsSetting',
+                      commit_author: 'MergeRequest::DiffCommitUser',
+                      committer: 'MergeRequest::DiffCommitUser' }.freeze
 
         BUILD_MODELS = %i[Ci::Build commit_status].freeze
 
@@ -56,6 +58,7 @@ module Gitlab
           external_pull_request
           external_pull_requests
           DesignManagement::Design
+          MergeRequest::DiffCommitUser
         ].freeze
 
         def create

spec/db/schema_spec.rb

@@ -56,6 +56,7 @@ RSpec.describe 'Database schema' do
     ldap_group_links: %w[group_id],
     members: %w[source_id created_by_id],
     merge_requests: %w[last_edited_by_id state_id],
+    merge_request_diff_commits: %w[commit_author_id committer_id],
     namespaces: %w[owner_id parent_id],
     notes: %w[author_id commit_id noteable_id updated_by_id resolved_by_id confirmed_by_id discussion_id],
     notification_settings: %w[source_id],

spec/factories/merge_request_diff_commit_users.rb

+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :merge_request_diff_commit_user, class: 'MergeRequest::DiffCommitUser' do
+    name { generate(:name) }
+    email { generate(:email) }
+  end
+end

spec/fixtures/lib/gitlab/import_export/complex/project.json

@@ -2799,7 +2799,15 @@
             "author_email": "dmitriy.zaporozhets@gmail.com",
             "committed_date": "2014-08-06T08:35:52.000+02:00",
             "committer_name": "Dmitriy Zaporozhets",
-            "committer_email": "dmitriy.zaporozhets@gmail.com"
+            "committer_email": "dmitriy.zaporozhets@gmail.com",
+            "commit_author": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
+            "committer": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 27,
@@ -2811,7 +2819,15 @@
             "author_email": "dmitriy.zaporozhets@gmail.com",
             "committed_date": "2014-02-27T10:01:38.000+01:00",
             "committer_name": "Dmitriy Zaporozhets",
-            "committer_email": "dmitriy.zaporozhets@gmail.com"
+            "committer_email": "dmitriy.zaporozhets@gmail.com",
+            "commit_author": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
+            "committer": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 27,
@@ -2823,7 +2839,15 @@
             "author_email": "dmitriy.zaporozhets@gmail.com",
             "committed_date": "2014-02-27T09:57:31.000+01:00",
             "committer_name": "Dmitriy Zaporozhets",
-            "committer_email": "dmitriy.zaporozhets@gmail.com"
+            "committer_email": "dmitriy.zaporozhets@gmail.com",
+            "commit_author": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
+            "committer": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 27,
@@ -2835,7 +2859,15 @@
             "author_email": "dmitriy.zaporozhets@gmail.com",
             "committed_date": "2014-02-27T09:54:21.000+01:00",
             "committer_name": "Dmitriy Zaporozhets",
-            "committer_email": "dmitriy.zaporozhets@gmail.com"
+            "committer_email": "dmitriy.zaporozhets@gmail.com",
+            "commit_author": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
+            "committer": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 27,
@@ -2847,7 +2879,15 @@
             "author_email": "dmitriy.zaporozhets@gmail.com",
             "committed_date": "2014-02-27T09:49:50.000+01:00",
             "committer_name": "Dmitriy Zaporozhets",
-            "committer_email": "dmitriy.zaporozhets@gmail.com"
+            "committer_email": "dmitriy.zaporozhets@gmail.com",
+            "commit_author": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
+            "committer": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 27,
@@ -2859,7 +2899,15 @@
             "author_email": "dmitriy.zaporozhets@gmail.com",
             "committed_date": "2014-02-27T09:48:32.000+01:00",
             "committer_name": "Dmitriy Zaporozhets",
-            "committer_email": "dmitriy.zaporozhets@gmail.com"
+            "committer_email": "dmitriy.zaporozhets@gmail.com",
+            "commit_author": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
+            "committer": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            }
           }
         ],
         "merge_request_diff_files": [
@@ -3247,7 +3295,15 @@
             "author_email": "dmitriy.zaporozhets@gmail.com",
             "committed_date": "2014-02-27T09:26:01.000+01:00",
             "committer_name": "Dmitriy Zaporozhets",
-            "committer_email": "dmitriy.zaporozhets@gmail.com"
+            "committer_email": "dmitriy.zaporozhets@gmail.com",
+            "commit_author": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
+            "committer": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            }
           }
         ],
         "merge_request_diff_files": [
@@ -3510,7 +3566,15 @@
             "author_email": "james@jameslopez.es",
             "committed_date": "2016-01-19T13:22:56.000+01:00",
             "committer_name": "James Lopez",
-            "committer_email": "james@jameslopez.es"
+            "committer_email": "james@jameslopez.es",
+            "commit_author": {
+              "name": "James Lopez",
+              "email": "james@jameslopez.es"
+            },
+            "committer": {
+              "name": "James Lopez",
+              "email": "james@jameslopez.es"
+            }
           }
         ],
         "merge_request_diff_files": [
@@ -3773,7 +3837,15 @@
             "author_email": "james@jameslopez.es",
             "committed_date": "2016-01-19T14:14:43.000+01:00",
             "committer_name": "James Lopez",
-            "committer_email": "james@jameslopez.es"
+            "committer_email": "james@jameslopez.es",
+            "commit_author": {
+              "name": "James Lopez",
+              "email": "james@jameslopez.es"
+            },
+            "committer": {
+              "name": "James Lopez",
+              "email": "james@jameslopez.es"
+            }
           },
           {
             "merge_request_diff_id": 14,
@@ -3785,7 +3857,15 @@
             "author_email": "marin@gitlab.com",
             "committed_date": "2015-12-07T12:52:12.000+01:00",
             "committer_name": "Marin Jankovski",
-            "committer_email": "marin@gitlab.com"
+            "committer_email": "marin@gitlab.com",
+            "commit_author": {
+              "name": "Marin Jankovski",
+              "email": "marin@gitlab.com"
+            },
+            "committer": {
+              "name": "Marin Jankovski",
+              "email": "marin@gitlab.com"
+            }
           },
           {
             "merge_request_diff_id": 14,
@@ -3797,7 +3877,15 @@
             "author_email": "maxlazio@gmail.com",
             "committed_date": "2015-12-07T11:54:28.000+01:00",
             "committer_name": "Marin Jankovski",
-            "committer_email": "maxlazio@gmail.com"
+            "committer_email": "maxlazio@gmail.com",
+            "commit_author": {
+              "name": "Marin Jankovski",
+              "email": "maxlazio@gmail.com"
+            },
+            "committer": {
+              "name": "Marin Jankovski",
+              "email": "maxlazio@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 14,
@@ -3809,7 +3897,15 @@
             "author_email": "stanhu@gmail.com",
             "committed_date": "2015-11-13T16:27:12.000+01:00",
             "committer_name": "Stan Hu",
-            "committer_email": "stanhu@gmail.com"
+            "committer_email": "stanhu@gmail.com",
+            "commit_author": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            },
+            "committer": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 14,
@@ -3821,7 +3917,15 @@
             "author_email": "stanhu@gmail.com",
             "committed_date": "2015-11-13T08:50:17.000+01:00",
             "committer_name": "Stan Hu",
-            "committer_email": "stanhu@gmail.com"
+            "committer_email": "stanhu@gmail.com",
+            "commit_author": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            },
+            "committer": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 14,
@@ -3833,7 +3937,15 @@
             "author_email": "stanhu@gmail.com",
             "committed_date": "2015-11-13T08:39:43.000+01:00",
             "committer_name": "Stan Hu",
-            "committer_email": "stanhu@gmail.com"
+            "committer_email": "stanhu@gmail.com",
+            "commit_author": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            },
+            "committer": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 14,
@@ -3845,7 +3957,15 @@
             "author_email": "stanhu@gmail.com",
             "committed_date": "2015-11-13T07:21:40.000+01:00",
             "committer_name": "Stan Hu",
-            "committer_email": "stanhu@gmail.com"
+            "committer_email": "stanhu@gmail.com",
+            "commit_author": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            },
+            "committer": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 14,
@@ -3857,7 +3977,15 @@
             "author_email": "minsik.yoon@samsung.com",
             "committed_date": "2015-11-13T06:01:27.000+01:00",
             "committer_name": "윤민식",
-            "committer_email": "minsik.yoon@samsung.com"
+            "committer_email": "minsik.yoon@samsung.com",
+            "commit_author": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            },
+            "committer": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            }
           },
           {
             "merge_request_diff_id": 14,
@@ -3869,7 +3997,15 @@
             "author_email": "minsik.yoon@samsung.com",
             "committed_date": "2015-11-13T06:00:16.000+01:00",
             "committer_name": "윤민식",
-            "committer_email": "minsik.yoon@samsung.com"
+            "committer_email": "minsik.yoon@samsung.com",
+            "commit_author": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            },
+            "committer": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            }
           },
           {
             "merge_request_diff_id": 14,
@@ -3881,7 +4017,15 @@
             "author_email": "stanhu@gmail.com",
             "committed_date": "2015-11-13T05:23:14.000+01:00",
             "committer_name": "Stan Hu",
-            "committer_email": "stanhu@gmail.com"
+            "committer_email": "stanhu@gmail.com",
+            "commit_author": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            },
+            "committer": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 14,
@@ -3893,7 +4037,15 @@
             "author_email": "minsik.yoon@samsung.com",
             "committed_date": "2015-11-13T05:08:45.000+01:00",
             "committer_name": "윤민식",
-            "committer_email": "minsik.yoon@samsung.com"
+            "committer_email": "minsik.yoon@samsung.com",
+            "commit_author": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            },
+            "committer": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            }
           },
           {
             "merge_request_diff_id": 14,
@@ -3905,7 +4057,15 @@
             "author_email": "minsik.yoon@samsung.com",
             "committed_date": "2015-11-13T05:08:04.000+01:00",
             "committer_name": "윤민식",
-            "committer_email": "minsik.yoon@samsung.com"
+            "committer_email": "minsik.yoon@samsung.com",
+            "commit_author": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            },
+            "committer": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            }
           },
           {
             "merge_request_diff_id": 14,
@@ -3917,7 +4077,15 @@
             "author_email": "stanhu@packetzoom.com",
             "committed_date": "2015-08-25T17:53:12.000+02:00",
             "committer_name": "Stan Hu",
-            "committer_email": "stanhu@packetzoom.com"
+            "committer_email": "stanhu@packetzoom.com",
+            "commit_author": {
+              "name": "Stan Hu",
+              "email": "stanhu@packetzoom.com"
+            },
+            "committer": {
+              "name": "Stan Hu",
+              "email": "stanhu@packetzoom.com"
+            }
           },
           {
             "merge_request_diff_id": 14,
@@ -3929,7 +4097,15 @@
             "author_email": "sytse@gitlab.com",
             "committed_date": "2015-01-10T22:23:29.000+01:00",
             "committer_name": "Sytse Sijbrandij",
-            "committer_email": "sytse@gitlab.com"
+            "committer_email": "sytse@gitlab.com",
+            "commit_author": {
+              "name": "Sytse Sijbrandij",
+              "email": "sytse@gitlab.com"
+            },
+            "committer": {
+              "name": "Sytse Sijbrandij",
+              "email": "sytse@gitlab.com"
+            }
           },
           {
             "merge_request_diff_id": 14,
@@ -3941,7 +4117,15 @@
             "author_email": "marmis85@gmail.com",
             "committed_date": "2015-01-10T21:28:18.000+01:00",
             "committer_name": "marmis85",
-            "committer_email": "marmis85@gmail.com"
+            "committer_email": "marmis85@gmail.com",
+            "commit_author": {
+              "name": "marmis85",
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@@ -3953,7 +4137,15 @@
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-            "committer_email": "dmitriy.zaporozhets@gmail.com"
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+              "email": "dmitriy.zaporozhets@gmail.com"
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+              "email": "dmitriy.zaporozhets@gmail.com"
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@@ -3965,7 +4157,15 @@
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-            "committer_email": "dmitriy.zaporozhets@gmail.com"
+            "committer_email": "dmitriy.zaporozhets@gmail.com",
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+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
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+              "email": "dmitriy.zaporozhets@gmail.com"
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@@ -3977,7 +4177,15 @@
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-            "committer_email": "dmitriy.zaporozhets@gmail.com"
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+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
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+              "email": "dmitriy.zaporozhets@gmail.com"
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@@ -3989,7 +4197,15 @@
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-            "committer_email": "dmitriy.zaporozhets@gmail.com"
+            "committer_email": "dmitriy.zaporozhets@gmail.com",
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+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
+            "committer": {
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+              "email": "dmitriy.zaporozhets@gmail.com"
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@@ -4001,7 +4217,15 @@
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-            "committer_email": "dmitriy.zaporozhets@gmail.com"
+            "committer_email": "dmitriy.zaporozhets@gmail.com",
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+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
+            "committer": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
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+            "committer_email": "james@jameslopez.es",
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+              "email": "james@jameslopez.es"
+            },
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+              "email": "james@jameslopez.es"
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@@ -4470,7 +4702,15 @@
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-            "committer_email": "marin@gitlab.com"
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+              "email": "marin@gitlab.com"
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@@ -4482,7 +4722,15 @@
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+            "committer_email": "maxlazio@gmail.com",
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+              "email": "maxlazio@gmail.com"
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+              "email": "maxlazio@gmail.com"
+            }
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@@ -4494,7 +4742,15 @@
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+            "committer_email": "stanhu@gmail.com",
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+              "email": "stanhu@gmail.com"
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@@ -4506,7 +4762,15 @@
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+              "email": "stanhu@gmail.com"
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@@ -4518,7 +4782,15 @@
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@@ -4542,7 +4822,15 @@
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@@ -4554,7 +4842,15 @@
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-            "committer_email": "minsik.yoon@samsung.com"
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@@ -4566,7 +4862,15 @@
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@@ -4578,7 +4882,15 @@
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+              "email": "minsik.yoon@samsung.com"
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+            "commit_author": {
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+              "email": "stanhu@packetzoom.com"
+            },
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@@ -4614,7 +4942,15 @@
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+              "email": "sytse@gitlab.com"
+            },
+            "committer": {
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+              "email": "sytse@gitlab.com"
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@@ -4967,7 +5311,15 @@
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+            "committer_email": "james@jameslopez.es",
+            "commit_author": {
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+              "email": "james@jameslopez.es"
+            },
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@@ -4979,7 +5331,15 @@
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+              "email": "marin@gitlab.com"
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+            "committer_email": "maxlazio@gmail.com",
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+              "email": "maxlazio@gmail.com"
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@@ -5003,7 +5371,15 @@
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+            "committer_email": "stanhu@gmail.com",
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+              "email": "stanhu@gmail.com"
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+              "email": "stanhu@gmail.com"
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@@ -5027,7 +5411,15 @@
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@@ -5051,7 +5451,15 @@
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@@ -5675,7 +6123,15 @@
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+              "email": "james@jameslopez.es"
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             "committed_date": "2015-12-07T12:52:12.000+01:00",
             "committer_name": "Marin Jankovski",
-            "committer_email": "marin@gitlab.com"
+            "committer_email": "marin@gitlab.com",
+            "commit_author": {
+              "name": "Marin Jankovski",
+              "email": "marin@gitlab.com"
+            },
+            "committer": {
+              "name": "Marin Jankovski",
+              "email": "marin@gitlab.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5699,7 +6163,15 @@
             "author_email": "maxlazio@gmail.com",
             "committed_date": "2015-12-07T11:54:28.000+01:00",
             "committer_name": "Marin Jankovski",
-            "committer_email": "maxlazio@gmail.com"
+            "committer_email": "maxlazio@gmail.com",
+            "commit_author": {
+              "name": "Marin Jankovski",
+              "email": "maxlazio@gmail.com"
+            },
+            "committer": {
+              "name": "Marin Jankovski",
+              "email": "maxlazio@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5711,7 +6183,15 @@
             "author_email": "stanhu@gmail.com",
             "committed_date": "2015-11-13T16:27:12.000+01:00",
             "committer_name": "Stan Hu",
-            "committer_email": "stanhu@gmail.com"
+            "committer_email": "stanhu@gmail.com",
+            "commit_author": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            },
+            "committer": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5723,7 +6203,15 @@
             "author_email": "stanhu@gmail.com",
             "committed_date": "2015-11-13T08:50:17.000+01:00",
             "committer_name": "Stan Hu",
-            "committer_email": "stanhu@gmail.com"
+            "committer_email": "stanhu@gmail.com",
+            "commit_author": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            },
+            "committer": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5735,7 +6223,15 @@
             "author_email": "stanhu@gmail.com",
             "committed_date": "2015-11-13T08:39:43.000+01:00",
             "committer_name": "Stan Hu",
-            "committer_email": "stanhu@gmail.com"
+            "committer_email": "stanhu@gmail.com",
+            "commit_author": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            },
+            "committer": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5747,7 +6243,15 @@
             "author_email": "stanhu@gmail.com",
             "committed_date": "2015-11-13T07:21:40.000+01:00",
             "committer_name": "Stan Hu",
-            "committer_email": "stanhu@gmail.com"
+            "committer_email": "stanhu@gmail.com",
+            "commit_author": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            },
+            "committer": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5759,7 +6263,15 @@
             "author_email": "minsik.yoon@samsung.com",
             "committed_date": "2015-11-13T06:01:27.000+01:00",
             "committer_name": "윤민식",
-            "committer_email": "minsik.yoon@samsung.com"
+            "committer_email": "minsik.yoon@samsung.com",
+            "commit_author": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            },
+            "committer": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5771,7 +6283,15 @@
             "author_email": "minsik.yoon@samsung.com",
             "committed_date": "2015-11-13T06:00:16.000+01:00",
             "committer_name": "윤민식",
-            "committer_email": "minsik.yoon@samsung.com"
+            "committer_email": "minsik.yoon@samsung.com",
+            "commit_author": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            },
+            "committer": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5783,7 +6303,15 @@
             "author_email": "stanhu@gmail.com",
             "committed_date": "2015-11-13T05:23:14.000+01:00",
             "committer_name": "Stan Hu",
-            "committer_email": "stanhu@gmail.com"
+            "committer_email": "stanhu@gmail.com",
+            "commit_author": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            },
+            "committer": {
+              "name": "Stan Hu",
+              "email": "stanhu@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5795,7 +6323,15 @@
             "author_email": "minsik.yoon@samsung.com",
             "committed_date": "2015-11-13T05:08:45.000+01:00",
             "committer_name": "윤민식",
-            "committer_email": "minsik.yoon@samsung.com"
+            "committer_email": "minsik.yoon@samsung.com",
+            "commit_author": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            },
+            "committer": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5807,7 +6343,15 @@
             "author_email": "minsik.yoon@samsung.com",
             "committed_date": "2015-11-13T05:08:04.000+01:00",
             "committer_name": "윤민식",
-            "committer_email": "minsik.yoon@samsung.com"
+            "committer_email": "minsik.yoon@samsung.com",
+            "commit_author": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            },
+            "committer": {
+              "name": "윤민식",
+              "email": "minsik.yoon@samsung.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5819,7 +6363,15 @@
             "author_email": "stanhu@packetzoom.com",
             "committed_date": "2015-08-25T17:53:12.000+02:00",
             "committer_name": "Stan Hu",
-            "committer_email": "stanhu@packetzoom.com"
+            "committer_email": "stanhu@packetzoom.com",
+            "commit_author": {
+              "name": "Stan Hu",
+              "email": "stanhu@packetzoom.com"
+            },
+            "committer": {
+              "name": "Stan Hu",
+              "email": "stanhu@packetzoom.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5831,7 +6383,15 @@
             "author_email": "sytse@gitlab.com",
             "committed_date": "2015-01-10T22:23:29.000+01:00",
             "committer_name": "Sytse Sijbrandij",
-            "committer_email": "sytse@gitlab.com"
+            "committer_email": "sytse@gitlab.com",
+            "commit_author": {
+              "name": "Sytse Sijbrandij",
+              "email": "sytse@gitlab.com"
+            },
+            "committer": {
+              "name": "Sytse Sijbrandij",
+              "email": "sytse@gitlab.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5843,7 +6403,15 @@
             "author_email": "marmis85@gmail.com",
             "committed_date": "2015-01-10T21:28:18.000+01:00",
             "committer_name": "marmis85",
-            "committer_email": "marmis85@gmail.com"
+            "committer_email": "marmis85@gmail.com",
+            "commit_author": {
+              "name": "marmis85",
+              "email": "marmis85@gmail.com"
+            },
+            "committer": {
+              "name": "marmis85",
+              "email": "marmis85@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5855,7 +6423,15 @@
             "author_email": "dmitriy.zaporozhets@gmail.com",
             "committed_date": "2014-02-27T10:01:38.000+01:00",
             "committer_name": "Dmitriy Zaporozhets",
-            "committer_email": "dmitriy.zaporozhets@gmail.com"
+            "committer_email": "dmitriy.zaporozhets@gmail.com",
+            "commit_author": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
+            "committer": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5867,7 +6443,15 @@
             "author_email": "dmitriy.zaporozhets@gmail.com",
             "committed_date": "2014-02-27T09:57:31.000+01:00",
             "committer_name": "Dmitriy Zaporozhets",
-            "committer_email": "dmitriy.zaporozhets@gmail.com"
+            "committer_email": "dmitriy.zaporozhets@gmail.com",
+            "commit_author": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
+            "committer": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5879,7 +6463,15 @@
             "author_email": "dmitriy.zaporozhets@gmail.com",
             "committed_date": "2014-02-27T09:54:21.000+01:00",
             "committer_name": "Dmitriy Zaporozhets",
-            "committer_email": "dmitriy.zaporozhets@gmail.com"
+            "committer_email": "dmitriy.zaporozhets@gmail.com",
+            "commit_author": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
+            "committer": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5891,7 +6483,15 @@
             "author_email": "dmitriy.zaporozhets@gmail.com",
             "committed_date": "2014-02-27T09:49:50.000+01:00",
             "committer_name": "Dmitriy Zaporozhets",
-            "committer_email": "dmitriy.zaporozhets@gmail.com"
+            "committer_email": "dmitriy.zaporozhets@gmail.com",
+            "commit_author": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
+            "committer": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            }
           },
           {
             "merge_request_diff_id": 10,
@@ -5903,7 +6503,15 @@
             "author_email": "dmitriy.zaporozhets@gmail.com",
             "committed_date": "2014-02-27T09:48:32.000+01:00",
             "committer_name": "Dmitriy Zaporozhets",
-            "committer_email": "dmitriy.zaporozhets@gmail.com"
+            "committer_email": "dmitriy.zaporozhets@gmail.com",
+            "commit_author": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            },
+            "committer": {
+              "name": "Dmitriy Zaporozhets",
+              "email": "dmitriy.zaporozhets@gmail.com"
+            }
           }
         ],
         "merge_request_diff_files": [
@@ -6348,7 +6956,15 @@
             "author_email": "james@jameslopez.es",
             "committed_date": "2016-01-19T15:44:02.000+01:00",
             "committer_name": "James Lopez",
-            "committer_email": "james@jameslopez.es"
+            "committer_email": "james@jameslopez.es",
+            "commit_author": {
+              "name": "James Lopez",
+              "email": "james@jameslopez.es"
+            },
+            "committer": {
+              "name": "James Lopez",
+              "email": "james@jameslopez.es"
+            }
           }
         ],
         "merge_request_diff_files": [

spec/lib/gitlab/background_migration/migrate_merge_request_diff_commit_users_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::BackgroundMigration::MigrateMergeRequestDiffCommitUsers do
+  let(:namespaces) { table(:namespaces) }
+  let(:projects) { table(:projects) }
+  let(:users) { table(:users) }
+  let(:merge_requests) { table(:merge_requests) }
+  let(:diffs) { table(:merge_request_diffs) }
+  let(:commits) do
+    table(:merge_request_diff_commits).tap do |t|
+      t.extend(SuppressCompositePrimaryKeyWarning)
+    end
+  end
+
+  let(:commit_users) { described_class::MergeRequestDiffCommitUser }
+
+  let(:namespace) { namespaces.create!(name: 'foo', path: 'foo') }
+  let(:project) { projects.create!(namespace_id: namespace.id) }
+  let(:merge_request) do
+    merge_requests.create!(
+      source_branch: 'x',
+      target_branch: 'master',
+      target_project_id: project.id
+    )
+  end
+
+  let(:diff) { diffs.create!(merge_request_id: merge_request.id) }
+  let(:migration) { described_class.new }
+
+  describe 'MergeRequestDiffCommit' do
+    describe '.each_row_to_migrate' do
+      it 'yields the rows to migrate for a given range' do
+        commit1 = commits.create!(
+          merge_request_diff_id: diff.id,
+          relative_order: 0,
+          sha: Gitlab::Database::ShaAttribute.serialize('123abc'),
+          author_name: 'bob',
+          author_email: 'bob@example.com',
+          committer_name: 'bob',
+          committer_email: 'bob@example.com'
+        )
+
+        commit2 = commits.create!(
+          merge_request_diff_id: diff.id,
+          relative_order: 1,
+          sha: Gitlab::Database::ShaAttribute.serialize('123abc'),
+          author_name: 'Alice',
+          author_email: 'alice@example.com',
+          committer_name: 'Alice',
+          committer_email: 'alice@example.com'
+        )
+
+        # We stub this constant to make sure we run at least two pagination
+        # queries for getting the data. This way we can test if the pagination
+        # is actually working properly.
+        stub_const(
+          'Gitlab::BackgroundMigration::MigrateMergeRequestDiffCommitUsers::COMMIT_ROWS_PER_QUERY',
+          1
+        )
+
+        rows = []
+
+        described_class::MergeRequestDiffCommit.each_row_to_migrate(diff.id, diff.id + 1) do |row|
+          rows << row
+        end
+
+        expect(rows.length).to eq(2)
+
+        expect(rows[0].author_name).to eq(commit1.author_name)
+        expect(rows[1].author_name).to eq(commit2.author_name)
+      end
+    end
+  end
+
+  describe 'MergeRequestDiffCommitUser' do
+    describe '.union' do
+      it 'produces a union of the given queries' do
+        alice = commit_users.create!(name: 'Alice', email: 'alice@example.com')
+        bob = commit_users.create!(name: 'Bob', email: 'bob@example.com')
+        users = commit_users.union([
+          commit_users.where(name: 'Alice').to_sql,
+          commit_users.where(name: 'Bob').to_sql
+        ])
+
+        expect(users).to include(alice)
+        expect(users).to include(bob)
+      end
+    end
+  end
+
+  describe '#perform' do
+    it 'migrates the data in the range' do
+      commits.create!(
+        merge_request_diff_id: diff.id,
+        relative_order: 0,
+        sha: Gitlab::Database::ShaAttribute.serialize('123abc'),
+        author_name: 'bob',
+        author_email: 'bob@example.com',
+        committer_name: 'bob',
+        committer_email: 'bob@example.com'
+      )
+
+      migration.perform(diff.id, diff.id + 1)
+
+      bob = commit_users.find_by(name: 'bob')
+      commit = commits.first
+
+      expect(commit.commit_author_id).to eq(bob.id)
+      expect(commit.committer_id).to eq(bob.id)
+    end
+
+    it 'treats empty names and Emails the same as NULL values' do
+      commits.create!(
+        merge_request_diff_id: diff.id,
+        relative_order: 0,
+        sha: Gitlab::Database::ShaAttribute.serialize('123abc'),
+        author_name: 'bob',
+        author_email: 'bob@example.com',
+        committer_name: '',
+        committer_email: ''
+      )
+
+      migration.perform(diff.id, diff.id + 1)
+
+      bob = commit_users.find_by(name: 'bob')
+      commit = commits.first
+
+      expect(commit.commit_author_id).to eq(bob.id)
+      expect(commit.committer_id).to be_nil
+    end
+
+    it 'does not update rows without a committer and author' do
+      commits.create!(
+        merge_request_diff_id: diff.id,
+        relative_order: 0,
+        sha: Gitlab::Database::ShaAttribute.serialize('123abc')
+      )
+
+      migration.perform(diff.id, diff.id + 1)
+
+      commit = commits.first
+
+      expect(commit_users.count).to eq(0)
+      expect(commit.commit_author_id).to be_nil
+      expect(commit.committer_id).to be_nil
+    end
+
+    it 'marks the background job as done' do
+      Gitlab::Database::BackgroundMigrationJob.create!(
+        class_name: 'MigrateMergeRequestDiffCommitUsers',
+        arguments: [diff.id, diff.id + 1]
+      )
+
+      migration.perform(diff.id, diff.id + 1)
+
+      job = Gitlab::Database::BackgroundMigrationJob.first
+
+      expect(job.status).to eq('succeeded')
+    end
+  end
+
+  describe '#get_data_to_update' do
+    it 'returns the users and commit rows to update' do
+      commits.create!(
+        merge_request_diff_id: diff.id,
+        relative_order: 0,
+        sha: Gitlab::Database::ShaAttribute.serialize('123abc'),
+        author_name: 'bob' + ('a' * 510),
+        author_email: 'bob@example.com',
+        committer_name: 'bob' + ('a' * 510),
+        committer_email: 'bob@example.com'
+      )
+
+      commits.create!(
+        merge_request_diff_id: diff.id,
+        relative_order: 1,
+        sha: Gitlab::Database::ShaAttribute.serialize('456abc'),
+        author_name: 'alice',
+        author_email: 'alice@example.com',
+        committer_name: 'alice',
+        committer_email: 'alice@example.com'
+      )
+
+      users, to_update = migration.get_data_to_update(diff.id, diff.id + 1)
+
+      bob_name = 'bob' + ('a' * 509)
+
+      expect(users).to include(%w[alice alice@example.com])
+      expect(users).to include([bob_name, 'bob@example.com'])
+
+      expect(to_update[[diff.id, 0]])
+        .to eq([[bob_name, 'bob@example.com'], [bob_name, 'bob@example.com']])
+
+      expect(to_update[[diff.id, 1]])
+        .to eq([%w[alice alice@example.com], %w[alice alice@example.com]])
+    end
+
+    it 'does not include a user if both the name and Email are missing' do
+      commits.create!(
+        merge_request_diff_id: diff.id,
+        relative_order: 0,
+        sha: Gitlab::Database::ShaAttribute.serialize('123abc'),
+        author_name: nil,
+        author_email: nil,
+        committer_name: 'bob',
+        committer_email: 'bob@example.com'
+      )
+
+      users, _ = migration.get_data_to_update(diff.id, diff.id + 1)
+
+      expect(users).to eq([%w[bob bob@example.com]].to_set)
+    end
+  end
+
+  describe '#get_user_rows_in_batches' do
+    it 'retrieves all existing users' do
+      alice = commit_users.create!(name: 'alice', email: 'alice@example.com')
+      bob = commit_users.create!(name: 'bob', email: 'bob@example.com')
+
+      users = [[alice.name, alice.email], [bob.name, bob.email]]
+      mapping = {}
+
+      migration.get_user_rows_in_batches(users, mapping)
+
+      expect(mapping[%w[alice alice@example.com]]).to eq(alice)
+      expect(mapping[%w[bob bob@example.com]]).to eq(bob)
+    end
+  end
+
+  describe '#create_missing_users' do
+    it 'creates merge request diff commit users that are missing' do
+      alice = commit_users.create!(name: 'alice', email: 'alice@example.com')
+      users = [%w[alice alice@example.com], %w[bob bob@example.com]]
+      mapping = { %w[alice alice@example.com] => alice }
+
+      migration.create_missing_users(users, mapping)
+
+      expect(mapping[%w[alice alice@example.com]]).to eq(alice)
+      expect(mapping[%w[bob bob@example.com]].name).to eq('bob')
+      expect(mapping[%w[bob bob@example.com]].email).to eq('bob@example.com')
+    end
+  end
+
+  describe '#update_commit_rows' do
+    it 'updates the merge request diff commit rows' do
+      to_update = { [42, 0] => [%w[alice alice@example.com], []] }
+      user_mapping = { %w[alice alice@example.com] => double(:user, id: 1) }
+
+      expect(migration)
+        .to receive(:bulk_update_commit_rows)
+        .with({ [42, 0] => [1, nil] })
+
+      migration.update_commit_rows(to_update, user_mapping)
+    end
+  end
+
+  describe '#bulk_update_commit_rows' do
+    context 'when there are no authors and committers' do
+      it 'does not update any rows' do
+        migration.bulk_update_commit_rows({ [1, 0] => [] })
+
+        expect(described_class::MergeRequestDiffCommit.connection)
+          .not_to receive(:execute)
+      end
+    end
+
+    context 'when there are only authors' do
+      it 'only updates the author IDs' do
+        author = commit_users.create!(name: 'Alice', email: 'alice@example.com')
+        commit = commits.create!(
+          merge_request_diff_id: diff.id,
+          relative_order: 0,
+          sha: Gitlab::Database::ShaAttribute.serialize('123abc')
+        )
+
+        mapping = {
+          [commit.merge_request_diff_id, commit.relative_order] =>
+            [author.id, nil]
+        }
+
+        migration.bulk_update_commit_rows(mapping)
+
+        commit = commits.first
+
+        expect(commit.commit_author_id).to eq(author.id)
+        expect(commit.committer_id).to be_nil
+      end
+    end
+
+    context 'when there are only committers' do
+      it 'only updates the committer IDs' do
+        committer =
+          commit_users.create!(name: 'Alice', email: 'alice@example.com')
+
+        commit = commits.create!(
+          merge_request_diff_id: diff.id,
+          relative_order: 0,
+          sha: Gitlab::Database::ShaAttribute.serialize('123abc')
+        )
+
+        mapping = {
+          [commit.merge_request_diff_id, commit.relative_order] =>
+            [nil, committer.id]
+        }
+
+        migration.bulk_update_commit_rows(mapping)
+
+        commit = commits.first
+
+        expect(commit.committer_id).to eq(committer.id)
+        expect(commit.commit_author_id).to be_nil
+      end
+    end
+
+    context 'when there are both authors and committers' do
+      it 'updates both the author and committer IDs' do
+        author = commit_users.create!(name: 'Bob', email: 'bob@example.com')
+        committer =
+          commit_users.create!(name: 'Alice', email: 'alice@example.com')
+
+        commit = commits.create!(
+          merge_request_diff_id: diff.id,
+          relative_order: 0,
+          sha: Gitlab::Database::ShaAttribute.serialize('123abc')
+        )
+
+        mapping = {
+          [commit.merge_request_diff_id, commit.relative_order] =>
+            [author.id, committer.id]
+        }
+
+        migration.bulk_update_commit_rows(mapping)
+
+        commit = commits.first
+
+        expect(commit.commit_author_id).to eq(author.id)
+        expect(commit.committer_id).to eq(committer.id)
+      end
+    end
+
+    context 'when there are multiple commit rows to update' do
+      it 'updates all the rows' do
+        author = commit_users.create!(name: 'Bob', email: 'bob@example.com')
+        committer =
+          commit_users.create!(name: 'Alice', email: 'alice@example.com')
+
+        commit1 = commits.create!(
+          merge_request_diff_id: diff.id,
+          relative_order: 0,
+          sha: Gitlab::Database::ShaAttribute.serialize('123abc')
+        )
+
+        commit2 = commits.create!(
+          merge_request_diff_id: diff.id,
+          relative_order: 1,
+          sha: Gitlab::Database::ShaAttribute.serialize('456abc')
+        )
+
+        mapping = {
+          [commit1.merge_request_diff_id, commit1.relative_order] =>
+            [author.id, committer.id],
+
+          [commit2.merge_request_diff_id, commit2.relative_order] =>
+            [author.id, nil]
+        }
+
+        migration.bulk_update_commit_rows(mapping)
+
+        commit1 = commits.find_by(relative_order: 0)
+        commit2 = commits.find_by(relative_order: 1)
+
+        expect(commit1.commit_author_id).to eq(author.id)
+        expect(commit1.committer_id).to eq(committer.id)
+
+        expect(commit2.commit_author_id).to eq(author.id)
+        expect(commit2.committer_id).to be_nil
+      end
+    end
+  end
+
+  describe '#primary_key' do
+    it 'returns the primary key for the commits table' do
+      key = migration.primary_key
+
+      expect(key.to_sql).to eq('("merge_request_diff_commits"."merge_request_diff_id", "merge_request_diff_commits"."relative_order")')
+    end
+  end
+
+  describe '#prepare' do
+    it 'trims a value to at most 512 characters' do
+      expect(migration.prepare('€' * 1_000)).to eq('€' * 512)
+    end
+
+    it 'returns nil if the value is an empty string' do
+      expect(migration.prepare('')).to be_nil
+    end
+  end
+end

spec/lib/gitlab/import_export/all_models.yml

@@ -198,6 +198,8 @@ merge_request_diff:
 - merge_request_diff_files
 merge_request_diff_commits:
 - merge_request_diff
+- commit_author
+- committer
 merge_request_diff_detail:
 - merge_request_diff
 merge_request_diff_files:

spec/lib/gitlab/import_export/import_test_coverage_spec.rb

@@ -109,14 +109,14 @@ RSpec.describe 'Test coverage of the Project Import' do
 
   def failure_message(not_tested_relations)
     <<~MSG
-      These relations seem to be added recenty and
+      These relations seem to be added recently and
       they expected to be covered in our Import specs: #{not_tested_relations}.
 
       To do that, expand one of the files listed in `project_json_fixtures`
       (or expand the list if you consider adding a new fixture file).
 
       After that, add a new spec into
-      `spec/lib/gitlab/import_export/project_tree_restorer_spec.rb`
+      `spec/lib/gitlab/import_export/project/tree_restorer_spec.rb`
       to check that the relation is being imported correctly.
 
       In case the spec breaks the master or there is a sense of urgency,

spec/lib/gitlab/import_export/project/object_builder_spec.rb

@@ -150,4 +150,30 @@ RSpec.describe Gitlab::ImportExport::Project::ObjectBuilder do
       expect(merge_request.persisted?).to be true
     end
   end
+
+  context 'merge request diff commit users' do
+    it 'finds the existing user' do
+      user = MergeRequest::DiffCommitUser
+        .find_or_create('Alice', 'alice@example.com')
+
+      found = described_class.build(
+        MergeRequest::DiffCommitUser,
+        'name' => 'Alice',
+        'email' => 'alice@example.com'
+      )
+
+      expect(found).to eq(user)
+    end
+
+    it 'creates a new user' do
+      found = described_class.build(
+        MergeRequest::DiffCommitUser,
+        'name' => 'Alice',
+        'email' => 'alice@example.com'
+      )
+
+      expect(found.name).to eq('Alice')
+      expect(found.email).to eq('alice@example.com')
+    end
+  end
 end

spec/lib/gitlab/import_export/project/tree_restorer_spec.rb

@@ -224,6 +224,27 @@ RSpec.describe Gitlab::ImportExport::Project::TreeRestorer do
           expect(MergeRequestDiffCommit.count).to eq(77)
         end
 
+        it 'assigns committer and author details to all diff commits' do
+          MergeRequestDiffCommit.all.each do |commit|
+            expect(commit.commit_author_id).not_to be_nil
+            expect(commit.committer_id).not_to be_nil
+          end
+        end
+
+        it 'assigns the correct commit users to different diff commits' do
+          commit1 = MergeRequestDiffCommit
+            .find_by(sha: '0b4bc9a49b562e85de7cc9e834518ea6828729b9')
+
+          commit2 = MergeRequestDiffCommit
+            .find_by(sha: 'a4e5dfebf42e34596526acb8611bc7ed80e4eb3f')
+
+          expect(commit1.commit_author.name).to eq('Dmitriy Zaporozhets')
+          expect(commit1.commit_author.email).to eq('dmitriy.zaporozhets@gmail.com')
+
+          expect(commit2.commit_author.name).to eq('James Lopez')
+          expect(commit2.commit_author.email).to eq('james@jameslopez.es')
+        end
+
         it 'has the correct data for merge request latest_merge_request_diff' do
           MergeRequest.find_each do |merge_request|
             expect(merge_request.latest_merge_request_diff_id).to eq(merge_request.merge_request_diffs.maximum(:id))

spec/lib/gitlab/import_export/safe_model_attributes.yml

@@ -235,6 +235,10 @@ MergeRequestDiffCommit:
 - committer_email
 - message
 - trailers
+MergeRequest::DiffCommitUser:
+- id
+- name
+- email
 MergeRequestDiffFile:
 - merge_request_diff_id
 - relative_order

spec/migrations/20210604133651_schedule_merge_request_diff_users_background_migration_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require_migration! 'schedule_merge_request_diff_users_background_migration'
+
+RSpec.describe ScheduleMergeRequestDiffUsersBackgroundMigration, :migration do
+  let(:migration) { described_class.new }
+
+  describe '#up' do
+    before do
+      allow(described_class::MergeRequestDiff)
+        .to receive(:minimum)
+        .with(:id)
+        .and_return(42)
+
+      allow(described_class::MergeRequestDiff)
+        .to receive(:maximum)
+        .with(:id)
+        .and_return(85_123)
+    end
+
+    it 'schedules the migrations in batches' do
+      expect(migration)
+        .to receive(:migrate_in)
+        .ordered
+        .with(2.minutes.to_i, described_class::MIGRATION_NAME, [42, 40_042])
+
+      expect(migration)
+        .to receive(:migrate_in)
+        .ordered
+        .with(4.minutes.to_i, described_class::MIGRATION_NAME, [40_042, 80_042])
+
+      expect(migration)
+        .to receive(:migrate_in)
+        .ordered
+        .with(6.minutes.to_i, described_class::MIGRATION_NAME, [80_042, 120_042])
+
+      migration.up
+    end
+
+    it 'creates rows to track the background migration jobs' do
+      expect(Gitlab::Database::BackgroundMigrationJob)
+        .to receive(:create!)
+        .ordered
+        .with(class_name: described_class::MIGRATION_NAME, arguments: [42, 40_042])
+
+      expect(Gitlab::Database::BackgroundMigrationJob)
+        .to receive(:create!)
+        .ordered
+        .with(class_name: described_class::MIGRATION_NAME, arguments: [40_042, 80_042])
+
+      expect(Gitlab::Database::BackgroundMigrationJob)
+        .to receive(:create!)
+        .ordered
+        .with(class_name: described_class::MIGRATION_NAME, arguments: [80_042, 120_042])
+
+      migration.up
+    end
+  end
+end

spec/models/merge_request/diff_commit_user_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe MergeRequest::DiffCommitUser do
+  describe 'validations' do
+    it 'requires that names are less than 512 characters long' do
+      expect(described_class.new(name: 'a' * 1000)).not_to be_valid
+    end
+
+    it 'requires that Emails are less than 512 characters long' do
+      expect(described_class.new(email: 'a' * 1000)).not_to be_valid
+    end
+
+    it 'requires either a name or Email' do
+      expect(described_class.new).not_to be_valid
+    end
+
+    it 'allows setting of just a name' do
+      expect(described_class.new(name: 'Alice')).to be_valid
+    end
+
+    it 'allows setting of just an Email' do
+      expect(described_class.new(email: 'alice@example.com')).to be_valid
+    end
+
+    it 'allows setting of both a name and Email' do
+      expect(described_class.new(name: 'Alice', email: 'alice@example.com'))
+        .to be_valid
+    end
+  end
+
+  describe '.prepare' do
+    it 'trims a value to at most 512 characters' do
+      expect(described_class.prepare('€' * 1_000)).to eq('€' * 512)
+    end
+
+    it 'returns nil if the value is an empty string' do
+      expect(described_class.prepare('')).to be_nil
+    end
+  end
+
+  describe '.find_or_create' do
+    it 'creates a new row if none exist' do
+      alice = described_class.find_or_create('Alice', 'alice@example.com')
+
+      expect(alice.name).to eq('Alice')
+      expect(alice.email).to eq('alice@example.com')
+    end
+
+    it 'returns an existing row if one exists' do
+      user1 = create(:merge_request_diff_commit_user)
+      user2 = described_class.find_or_create(user1.name, user1.email)
+
+      expect(user1).to eq(user2)
+    end
+
+    it 'handles concurrent inserts' do
+      user = create(:merge_request_diff_commit_user)
+
+      expect(described_class)
+        .to receive(:find_or_create_by!)
+        .ordered
+        .with(name: user.name, email: user.email)
+        .and_raise(ActiveRecord::RecordNotUnique)
+
+      expect(described_class)
+        .to receive(:find_or_create_by!)
+        .ordered
+        .with(name: user.name, email: user.email)
+        .and_return(user)
+
+      expect(described_class.find_or_create(user.name, user.email)).to eq(user)
+    end
+  end
+
+  describe '.bulk_find_or_create' do
+    it 'bulk creates missing rows and reuses existing rows' do
+      bob = create(
+        :merge_request_diff_commit_user,
+        name: 'Bob',
+        email: 'bob@example.com'
+      )
+
+      users = described_class.bulk_find_or_create(
+        [%w[Alice alice@example.com], %w[Bob bob@example.com]]
+      )
+      alice = described_class.find_by(name: 'Alice')
+
+      expect(users[%w[Alice alice@example.com]]).to eq(alice)
+      expect(users[%w[Bob bob@example.com]]).to eq(bob)
+    end
+
+    it 'does not insert any data when all users exist' do
+      bob = create(
+        :merge_request_diff_commit_user,
+        name: 'Bob',
+        email: 'bob@example.com'
+      )
+
+      users = described_class.bulk_find_or_create([%w[Bob bob@example.com]])
+
+      expect(described_class).not_to receive(:insert_all)
+      expect(users[%w[Bob bob@example.com]]).to eq(bob)
+    end
+
+    it 'handles concurrently inserted rows' do
+      bob = create(
+        :merge_request_diff_commit_user,
+        name: 'Bob',
+        email: 'bob@example.com'
+      )
+
+      input = [%w[Bob bob@example.com]]
+
+      expect(described_class)
+        .to receive(:bulk_find)
+        .twice
+        .with(input)
+        .and_return([], [bob])
+
+      users = described_class.bulk_find_or_create(input)
+
+      expect(users[%w[Bob bob@example.com]]).to eq(bob)
+    end
+  end
+end

spec/models/merge_request_diff_commit_spec.rb

@@ -16,6 +16,11 @@ RSpec.describe MergeRequestDiffCommit do
     let(:invalid_items_for_bulk_insertion) { [] } # class does not have any validations defined
   end
 
+  describe 'associations' do
+    it { is_expected.to belong_to(:commit_author) }
+    it { is_expected.to belong_to(:committer) }
+  end
+
   describe '#to_hash' do
     subject { merge_request.commits.first }
 
@@ -46,6 +51,8 @@ RSpec.describe MergeRequestDiffCommit do
           "committed_date": "2014-02-27T10:01:38.000+01:00".to_time,
           "committer_name": "Dmitriy Zaporozhets",
           "committer_email": "dmitriy.zaporozhets@gmail.com",
+          "commit_author_id": an_instance_of(Integer),
+          "committer_id": an_instance_of(Integer),
           "merge_request_diff_id": merge_request_diff_id,
           "relative_order": 0,
           "sha": Gitlab::Database::ShaAttribute.serialize("5937ac0a7beb003549fc5fd26fc247adbce4a52e"),
@@ -59,6 +66,8 @@ RSpec.describe MergeRequestDiffCommit do
           "committed_date": "2014-02-27T09:57:31.000+01:00".to_time,
           "committer_name": "Dmitriy Zaporozhets",
           "committer_email": "dmitriy.zaporozhets@gmail.com",
+          "commit_author_id": an_instance_of(Integer),
+          "committer_id": an_instance_of(Integer),
           "merge_request_diff_id": merge_request_diff_id,
           "relative_order": 1,
           "sha": Gitlab::Database::ShaAttribute.serialize("570e7b2abdd848b95f2f578043fc23bd6f6fd24d"),
@@ -76,6 +85,21 @@ RSpec.describe MergeRequestDiffCommit do
       subject
     end
 
+    it 'creates diff commit users' do
+      diff = create(:merge_request_diff, merge_request: merge_request)
+
+      described_class.create_bulk(diff.id, [commits.first])
+
+      commit_row = MergeRequestDiffCommit
+        .find_by(merge_request_diff_id: diff.id, relative_order: 0)
+
+      commit_user_row =
+        MergeRequest::DiffCommitUser.find_by(name: 'Dmitriy Zaporozhets')
+
+      expect(commit_row.commit_author).to eq(commit_user_row)
+      expect(commit_row.committer).to eq(commit_user_row)
+    end
+
     context 'with dates larger than the DB limit' do
       let(:commits) do
         # This commit's date is "Sun Aug 17 07:12:55 292278994 +0000"
@@ -92,6 +116,8 @@ RSpec.describe MergeRequestDiffCommit do
           "committed_date": timestamp,
           "committer_name": "Alejandro Rodríguez",
           "committer_email": "alejorro70@gmail.com",
+          "commit_author_id": an_instance_of(Integer),
+          "committer_id": an_instance_of(Integer),
           "merge_request_diff_id": merge_request_diff_id,
           "relative_order": 0,
           "sha": Gitlab::Database::ShaAttribute.serialize("ba3343bc4fa403a8dfbfcab7fc1a8c29ee34bd69"),
@@ -107,4 +133,28 @@ RSpec.describe MergeRequestDiffCommit do
       end
     end
   end
+
+  describe '.prepare_commits_for_bulk_insert' do
+    it 'returns the commit hashes and unique user tuples' do
+      commit = double(:commit, to_hash: {
+        parent_ids: %w[foo bar],
+        author_name: 'a' * 1000,
+        author_email: 'a' * 1000,
+        committer_name: 'Alice',
+        committer_email: 'alice@example.com'
+      })
+
+      hashes, tuples = described_class.prepare_commits_for_bulk_insert([commit])
+
+      expect(hashes).to eq([{
+        author_name: 'a' * 512,
+        author_email: 'a' * 512,
+        committer_name: 'Alice',
+        committer_email: 'alice@example.com'
+      }])
+
+      expect(tuples)
+        .to include(['a' * 512, 'a' * 512], %w[Alice alice@example.com])
+    end
+  end
 end