Add cleanup migration for MR's mutliple assignees

The migration steals the remaining background jobs
of populating MRs with assignees, executes them
synchronously and then makes sure that all the
assignees are migrated

changelogs/unreleased/id-clean-up-mr-assignees.yml

+---
+title: Add cleanup migration for MR's multiple assignees
+merge_request: 30261
+author:
+type: changed

db/post_migrate/20190702173936_populate_remaining_merge_request_assignees.rb

+# frozen_string_literal: true
+
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class PopulateRemainingMergeRequestAssignees < ActiveRecord::Migration[5.1]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  BATCH_SIZE = 10_000
+  MIGRATION = 'PopulateMergeRequestAssigneesTable'
+
+  disable_ddl_transaction!
+
+  def up
+    Gitlab::BackgroundMigration.steal(MIGRATION)
+
+    Gitlab::BackgroundMigration::PopulateMergeRequestAssigneesTable.new.perform_all_sync(batch_size: BATCH_SIZE)
+  end
+end

lib/gitlab/background_migration/populate_merge_request_assignees_table.rb

@@ -18,6 +18,14 @@ module Gitlab
         execute("INSERT INTO merge_request_assignees (merge_request_id, user_id) #{select_sql}")
       end
 
+      def perform_all_sync(batch_size:)
+        MergeRequest.each_batch(of: batch_size) do |batch|
+          range = batch.pluck('MIN(id)', 'MAX(id)').first
+
+          perform(*range)
+        end
+      end
+
       private
 
       def merge_request_assignees_not_exists_clause

spec/lib/gitlab/background_migration/populate_merge_request_assignees_table_spec.rb

@@ -27,14 +27,19 @@ describe Gitlab::BackgroundMigration::PopulateMergeRequestAssigneesTable, :migra
     merge_requests.create(params)
   end
 
-  describe '#perform' do
-    it 'creates merge_request_assignees rows according to merge_requests' do
+  before do
     create_merge_request(2, assignee_id: user.id)
     create_merge_request(3, assignee_id: user_2.id)
     create_merge_request(4, assignee_id: user_3.id)
+
+    # Test filtering MRs without assignees
+    create_merge_request(5, assignee_id: nil)
     # Test filtering already migrated row
     merge_request_assignees.create!(merge_request_id: 2, user_id: user_3.id)
+  end
 
+  describe '#perform' do
+    it 'creates merge_request_assignees rows according to merge_requests' do
       subject.perform(1, 4)
 
       rows = merge_request_assignees.order(:id).map { |row| row.attributes.slice('merge_request_id', 'user_id') }
@@ -53,4 +58,13 @@ describe Gitlab::BackgroundMigration::PopulateMergeRequestAssigneesTable, :migra
       end
     end
   end
+
+  describe '#perform_all_sync' do
+    it 'executes peform for all merge requests in batches' do
+      expect(subject).to receive(:perform).with(2, 4).ordered
+      expect(subject).to receive(:perform).with(5, 5).ordered
+
+      subject.perform_all_sync(batch_size: 3)
+    end
+  end
 end