Process all projects synchronously

This simplifies things.
And amount of projects are quite small.

ee/db/post_migrate/20181204040404_migrate_project_approvers.rb

@@ -6,24 +6,25 @@ class MigrateProjectApprovers < ActiveRecord::Migration[5.0]
   DOWNTIME = false
   BATCH_SIZE = 1000
 
-  class Project < ActiveRecord::Base
+  class MergeRequest < ActiveRecord::Base
     include ::EachBatch
-    self.table_name = 'projects'
+    self.table_name = 'merge_requests'
   end
 
-  def up
-    jobs = []
-    Project.each_batch(of: BATCH_SIZE) do |scope|
-      project_ids = scope.pluck(:id)
+  class Approver < ActiveRecord::Base
+    self.table_name = 'approvers'
+  end
 
-      if jobs.length >= BACKGROUND_MIGRATION_JOB_BUFFER_SIZE
-        BackgroundMigrationWorker.bulk_perform_async(jobs)
-        jobs.clear
+  class ApproverGroup < ActiveRecord::Base
+    self.table_name = 'approver_groups'
   end
 
-      jobs << ['MigrateApproverToApprovalRulesInBatch', ['Project', project_ids]]
+  def up
+    get_project_ids.each do |project_id|
+      Gitlab::BackgroundMigration::MigrateApproverToApprovalRules.new.perform('Project', project_id)
     end
-    BackgroundMigrationWorker.bulk_perform_async(jobs)
+
+    bulk_queue_background_migration_jobs_by_range(MergeRequest, 'MigrateApproverToApprovalRulesInBatch')
 
     check_time = Gitlab::BackgroundMigration::MigrateApproverToApprovalRulesCheckProgress::RESCHEDULE_DELAY
     BackgroundMigrationWorker.bulk_perform_in(check_time, [['MigrateApproverToApprovalRulesCheckProgress']])
@@ -31,4 +32,13 @@ class MigrateProjectApprovers < ActiveRecord::Migration[5.0]
 
   def down
   end
+
+  private
+
+  def get_project_ids
+    project_ids = Approver.where('target_type = ?', 'Project').pluck(:target_id)
+    project_ids += ApproverGroup.where('target_type = ?', 'Project').pluck(:target_id)
+    project_ids.uniq!
+    project_ids
+  end
 end

ee/lib/gitlab/background_migration/migrate_approver_to_approval_rules_in_batch.rb

@@ -5,25 +5,13 @@ module Gitlab
     class MigrateApproverToApprovalRulesInBatch
       class MergeRequest < ActiveRecord::Base
         self.table_name = 'merge_requests'
-        include ::EachBatch
       end
 
-      def perform(target_type, target_ids)
-        target_ids.each do |target_id|
-          MigrateApproverToApprovalRules.new.perform(target_type, target_id)
+      def perform(start_id, end_id)
+        merge_request_ids = MergeRequest.where('id >= ? AND id <= ?', start_id, end_id).pluck(:id)
+        merge_request_ids.each do |merge_request_id|
+          MigrateApproverToApprovalRules.new.perform('MergeRequest', merge_request_id)
         end
-
-        schedule_to_migrate_merge_requests(target_ids) if target_type == 'Project'
-      end
-
-      private
-
-      def schedule_to_migrate_merge_requests(project_ids)
-        jobs = []
-        MergeRequest.where(target_project_id: project_ids).each_batch do |scope, _|
-          jobs << [self.class.name, ['MergeRequest', scope.pluck(:id)]]
-        end
-        BackgroundMigrationWorker.bulk_perform_async(jobs) unless jobs.empty?
       end
     end
   end

ee/spec/lib/gitlab/background_migration/migrate_approver_to_approval_rules_in_batch_spec.rb

@@ -5,37 +5,16 @@ require 'spec_helper'
 describe Gitlab::BackgroundMigration::MigrateApproverToApprovalRulesInBatch do
   context 'when there is no more MigrateApproverToApprovalRules jobs' do
     let(:job) { double(:job) }
+    let(:project) { create(:project) }
 
     it 'migrates individual target' do
       allow(Gitlab::BackgroundMigration::MigrateApproverToApprovalRules).to receive(:new).and_return(job)
 
-      expect(job).to receive(:perform).exactly(3).times
-
-      described_class.new.perform('Foo', [1, 2, 3])
-    end
-
-    context 'when targets are projects' do
-      let(:projects) { create_list(:project, 3) }
-
-      context 'when projects contain merge requests' do
-        it 'schedules migrations for merge requests' do
-          merge_requests = projects.flat_map do |project|
-            create(:merge_request, source_project: project, target_project: project)
-          end
-
-          expect(BackgroundMigrationWorker).to receive(:bulk_perform_async).with([[described_class.name, ["MergeRequest", merge_requests.map(&:id)]]])
+      merge_requests = create_list(:merge_request, 3)
 
-          described_class.new.perform('Project', projects.map(&:id))
-        end
-      end
-
-      context 'when merge request do not exist' do
-        it 'does nothing' do
-          expect(BackgroundMigrationWorker).not_to receive(:bulk_perform_async)
+      expect(job).to receive(:perform).exactly(3).times
 
-          described_class.new.perform('Project', projects.map(&:id))
-        end
-      end
+      described_class.new.perform(merge_requests.first.id, merge_requests.last.id)
     end
   end
 end