Convert migration to populate latest merge request ID into a background migration

This is to smear updates over a few hours to avoid causing excessive
replication lag as seen in https://gitlab.com/gitlab-com/infrastructure/issues/3235.

1 files changed, 64 insertions, 0 deletions

diff --git a/spec/migrations/schedule_merge_request_latest_merge_request_diff_id_migrations_spec.rb b/spec/migrations/schedule_merge_request_latest_merge_request_diff_id_migrations_spec.rb
new file mode 100644
index 00000000000..158d0bc02ed
--- /dev/null
+++ b/spec/migrations/schedule_merge_request_latest_merge_request_diff_id_migrations_spec.rb
@@ -0,0 +1,64 @@
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20171026082505_schedule_merge_request_latest_merge_request_diff_id_migrations')
+
+describe ScheduleMergeRequestLatestMergeRequestDiffIdMigrations, :migration, :sidekiq do
+  let(:projects_table) { table(:projects) }
+  let(:merge_requests_table) { table(:merge_requests) }
+  let(:merge_request_diffs_table) { table(:merge_request_diffs) }
+
+  let(:project) { projects_table.create!(name: 'gitlab', path: 'gitlab-org/gitlab-ce') }
+
+  let!(:merge_request_1) { create_mr!('mr_1', diffs: 1) }
+  let!(:merge_request_2) { create_mr!('mr_2', diffs: 2) }
+  let!(:merge_request_migrated) { create_mr!('merge_request_migrated', diffs: 3) }
+  let!(:merge_request_4) { create_mr!('mr_4', diffs: 3) }
+
+  def create_mr!(name, diffs: 0)
+    merge_request =
+      merge_requests_table.create!(target_project_id: project.id,
+                                   target_branch: 'master',
+                                   source_project_id: project.id,
+                                   source_branch: name,
+                                   title: name)
+
+    diffs.times do
+      merge_request_diffs_table.create!(merge_request_id: merge_request.id)
+    end
+
+    merge_request
+  end
+
+  def diffs_for(merge_request)
+    merge_request_diffs_table.where(merge_request_id: merge_request.id)
+  end
+
+  before do
+    stub_const("#{described_class.name}::BATCH_SIZE", 1)
+
+    diff_id = diffs_for(merge_request_migrated).minimum(:id)
+    merge_request_migrated.update!(latest_merge_request_diff_id: diff_id)
+  end
+
+  it 'correctly schedules background migrations' do
+    Sidekiq::Testing.fake! do
+      Timecop.freeze do
+        migrate!
+
+        expect(described_class::MIGRATION).to be_scheduled_migration(5.minutes, merge_request_1.id, merge_request_1.id)
+        expect(described_class::MIGRATION).to be_scheduled_migration(10.minutes, merge_request_2.id, merge_request_2.id)
+        expect(described_class::MIGRATION).to be_scheduled_migration(15.minutes, merge_request_4.id, merge_request_4.id)
+        expect(BackgroundMigrationWorker.jobs.size).to eq 3
+      end
+    end
+  end
+
+  it 'schedules background migrations' do
+    Sidekiq::Testing.inline! do
+      expect(merge_requests_table.where(latest_merge_request_diff_id: nil).count).to eq 3
+
+      migrate!
+
+      expect(merge_requests_table.where(latest_merge_request_diff_id: nil).count).to eq 0
+    end
+  end
+end