Revert "Temporarily remove MR diffs removal migration"

This reverts commit 15ec6a13.

db/post_migrate/20180619121030_enqueue_delete_diff_files_workers.rb

+class EnqueueDeleteDiffFilesWorkers < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  class MergeRequestDiff < ActiveRecord::Base
+    self.table_name = 'merge_request_diffs'
+
+    belongs_to :merge_request
+
+    include EachBatch
+  end
+
+  DOWNTIME = false
+  BATCH_SIZE = 1000
+  MIGRATION = 'DeleteDiffFiles'
+  DELAY_INTERVAL = 8.minutes
+  TMP_INDEX = 'tmp_partial_diff_id_with_files_index'.freeze
+
+  disable_ddl_transaction!
+
+  def up
+    # We add temporary index, to make iteration over batches more performant.
+    # Conditional here is to avoid the need of doing that in a separate
+    # migration file to make this operation idempotent.
+    #
+    unless index_exists_by_name?(:merge_request_diffs, TMP_INDEX)
+      add_concurrent_index(:merge_request_diffs, :id, where: "(state NOT IN ('without_files', 'empty'))", name: TMP_INDEX)
+    end
+
+
+    diffs_with_files = MergeRequestDiff.where.not(state: ['without_files', 'empty'])
+
+    # explain (analyze, buffers) example for the iteration:
+    #
+    # Index Only Scan using tmp_index_20013 on merge_request_diffs  (cost=0.43..1630.19 rows=60567 width=4) (actual time=0.047..9.572 rows=56976 loops=1)
+    #   Index Cond: ((id >= 764586) AND (id < 835298))
+    #   Heap Fetches: 8
+    #   Buffers: shared hit=18188
+    # Planning time: 0.752 ms
+    # Execution time: 12.430 ms
+    #
+    diffs_with_files.each_batch(of: BATCH_SIZE) do |relation, outer_index|
+      ids = relation.pluck(:id)
+
+      ids.each_with_index do |diff_id, inner_index|
+        # This will give some space between batches of workers.
+        interval = DELAY_INTERVAL * outer_index + inner_index.minutes
+
+        # A single `merge_request_diff` can be associated with way too many
+        # `merge_request_diff_files`. It's better to avoid batching these and
+        # schedule one at a time.
+        #
+        # Considering roughly 6M jobs, this should take ~30 days to process all
+        # of them.
+        #
+        BackgroundMigrationWorker.perform_in(interval, MIGRATION, [diff_id])
+      end
+    end
+
+    # We remove temporary index, because it is not required during standard
+    # operations and runtime.
+    #
+    remove_concurrent_index_by_name(:merge_request_diffs, TMP_INDEX)
+  end
+
+  def down
+    if index_exists_by_name?(:merge_request_diffs, TMP_INDEX)
+      remove_concurrent_index_by_name(:merge_request_diffs, TMP_INDEX)
+    end
+  end
+end

spec/lib/gitlab/background_migration/delete_diff_files_spec.rb

 require 'spec_helper'
 
-describe Gitlab::BackgroundMigration::DeleteDiffFiles, :migration, schema: 20180626125654 do
+describe Gitlab::BackgroundMigration::DeleteDiffFiles, :migration, schema: 20180619121030 do
   describe '#perform' do
     context 'when diff files can be deleted' do
       let(:merge_request) { create(:merge_request, :merged) }

spec/migrations/enqueue_delete_diff_files_workers_spec.rb

+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20180619121030_enqueue_delete_diff_files_workers.rb')
+
+describe EnqueueDeleteDiffFilesWorkers, :migration, :sidekiq do
+  let(:merge_request_diffs) { table(:merge_request_diffs) }
+  let(:merge_requests) { table(:merge_requests) }
+  let(:namespaces) { table(:namespaces) }
+  let(:projects) { table(:projects) }
+
+  before do
+    stub_const("#{described_class.name}::BATCH_SIZE", 2)
+
+    namespaces.create!(id: 1, name: 'gitlab', path: 'gitlab')
+    projects.create!(id: 1, namespace_id: 1, name: 'gitlab', path: 'gitlab')
+
+    merge_requests.create!(id: 1, target_project_id: 1, source_project_id: 1, target_branch: 'feature', source_branch: 'master', state: 'merged')
+
+    merge_request_diffs.create!(id: 1, merge_request_id: 1, state: 'collected')
+    merge_request_diffs.create!(id: 2, merge_request_id: 1, state: 'without_files')
+    merge_request_diffs.create!(id: 3, merge_request_id: 1, state: 'collected')
+    merge_request_diffs.create!(id: 4, merge_request_id: 1, state: 'collected')
+    merge_request_diffs.create!(id: 5, merge_request_id: 1, state: 'empty')
+    merge_request_diffs.create!(id: 6, merge_request_id: 1, state: 'collected')
+
+    merge_requests.update(1, latest_merge_request_diff_id: 6)
+  end
+
+  it 'correctly schedules diff file deletion workers' do
+    Sidekiq::Testing.fake! do
+      Timecop.freeze do
+        migrate!
+
+        # 1st batch
+        expect(described_class::MIGRATION).to be_scheduled_delayed_migration(8.minutes, 1)
+        expect(described_class::MIGRATION).to be_scheduled_delayed_migration(9.minutes, 3)
+        # 2nd batch
+        expect(described_class::MIGRATION).to be_scheduled_delayed_migration(16.minutes, 4)
+        expect(described_class::MIGRATION).to be_scheduled_delayed_migration(17.minutes, 6)
+        expect(BackgroundMigrationWorker.jobs.size).to eq(4)
+      end
+    end
+  end
+
+  it 'migrates the data' do
+    expect { migrate! }.to change { merge_request_diffs.where(state: 'without_files').count }
+      .from(1).to(4)
+  end
+end