Change discussion_ids on promoted epics notes

Notes on epics promoted from an issue used to get same discussion_id
as the notes from the issue the epic was promoted from, which would
cause problems when trying to reply to the epic discussion.

db/post_migrate/20190715193142_migrate_discussion_id_on_promoted_epics.rb

@@ -0,0 +1,62 @@
+# 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 MigrateDiscussionIdOnPromotedEpics < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  # We have ~5000 unique discussion_ids -> this migration will take about 102 minutes
+  # (5000/100 * 2 minutes + 2 minutes initial delay) on gitlab.com.
+  DOWNTIME = false
+  BATCH_SIZE = 100
+  DELAY_INTERVAL = 2.minutes
+  MIGRATION = 'FixPromotedEpicsDiscussionIds'
+
+  disable_ddl_transaction!
+
+  class SystemNoteMetadata < ActiveRecord::Base
+    self.table_name = 'system_note_metadata'
+    self.inheritance_column = :_type_disabled
+  end
+
+  class Note < ActiveRecord::Base
+    include EachBatch
+
+    has_one :system_note_metadata, class_name: 'MigrateDiscussionIdOnPromotedEpics::SystemNoteMetadata'
+
+    self.table_name = 'notes'
+    self.inheritance_column = :_type_disabled
+
+    def self.fetch_discussion_ids_query
+      promoted_epics_query = Note
+        .joins(:system_note_metadata)
+        .where(system: true)
+        .where(noteable_type: 'Epic')
+        .where(system_note_metadata: { action: 'moved' })
+        .select("DISTINCT noteable_id")
+
+      Note.where(noteable_type: 'Epic')
+        .where(noteable_id: promoted_epics_query)
+        .distinct.pluck(:discussion_id)
+    end
+  end
+
+  def up
+    add_concurrent_index(:system_note_metadata, :note_id, where: "action='moved'", name: 'temp_index_system_note_metadata_on_moved_note_id')
+    add_concurrent_index(:notes, [:id, :noteable_id], where: "noteable_type='Epic' AND system", name: 'temp_index_notes_on_id_and_noteable_id' )
+
+    all_discussion_ids = Note.fetch_discussion_ids_query
+    all_discussion_ids.in_groups_of(BATCH_SIZE, false).each_with_index do |ids, index|
+      delay = DELAY_INTERVAL * (index + 1)
+      BackgroundMigrationWorker.perform_in(delay, MIGRATION, [ids])
+    end
+
+    remove_concurrent_index(:system_note_metadata, :note_id, where: "action='moved'", name: 'temp_index_system_note_metadata_on_moved_note_id')
+    remove_concurrent_index(:notes, [:id, :noteable_id], where: "noteable_type='Epic' AND system", name: 'temp_index_notes_on_id_and_noteable_id')
+  end
+
+  def down
+    # no-op
+  end
+end

lib/gitlab/background_migration/fix_promoted_epics_discussion_ids.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    # This migration updates discussion ids for epics that were promoted from issue so that the discussion id on epics
+    # is different from discussion id on issue, which was causing problems when replying to epic discussions as it would
+    # identify the discussion as related to an issue and complaint about missing project_id
+    class FixPromotedEpicsDiscussionIds
+      # notes model to iterate through the notes to be updated
+      class Note < ActiveRecord::Base
+        self.table_name = 'notes'
+        self.inheritance_column = :_type_disabled
+      end
+
+      def perform(discussion_ids)
+        Note.where(noteable_type: 'Epic')
+          .where(discussion_id: discussion_ids)
+          .update_all("discussion_id=MD5(discussion_id)||substring(discussion_id from 1 for 8)")
+      end
+    end
+  end
+end

spec/lib/gitlab/background_migration/fix_promoted_epics_discussion_ids_spec.rb

@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::BackgroundMigration::FixPromotedEpicsDiscussionIds, :migration, schema: 20190715193142 do
+  let(:namespaces) { table(:namespaces) }
+  let(:users) { table(:users) }
+  let(:epics) { table(:epics) }
+  let(:notes) { table(:notes) }
+
+  let(:user) { users.create!(email: 'test@example.com', projects_limit: 100, username: 'test') }
+  let(:namespace) { namespaces.create!(name: 'gitlab', path: 'gitlab-org') }
+  let(:epic1) { epics.create!(id: 1, author_id: user.id, iid: 1, group_id: namespace.id, title: 'Epic with discussion', title_html: 'Epic with discussion') }
+
+  def create_note(discussion_id)
+    notes.create!(note: 'note comment',
+                  noteable_id: epic1.id,
+                  noteable_type: 'Epic',
+                  discussion_id: discussion_id)
+  end
+
+  def expect_valid_discussion_id(id)
+    expect(id).to match(/\A\h{40}\z/)
+  end
+
+  describe '#perform with batch of discussion ids' do
+    it 'updates discussion ids' do
+      note1 = create_note('00000000')
+      note2 = create_note('00000000')
+      note3 = create_note('10000000')
+
+      subject.perform(%w(00000000 10000000))
+
+      expect_valid_discussion_id(note1.reload.discussion_id)
+      expect_valid_discussion_id(note2.reload.discussion_id)
+      expect_valid_discussion_id(note3.reload.discussion_id)
+      expect(note1.discussion_id).to eq(note2.discussion_id)
+      expect(note1.discussion_id).not_to eq(note3.discussion_id)
+    end
+
+    it 'skips notes with discussion id not in range' do
+      note4 = create_note('20000000')
+
+      subject.perform(%w(00000000 10000000))
+
+      expect(note4.reload.discussion_id).to eq('20000000')
+    end
+  end
+end

spec/migrations/migrate_discussion_id_on_promoted_epics_spec.rb

@@ -0,0 +1,81 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20190715193142_migrate_discussion_id_on_promoted_epics.rb')
+
+describe MigrateDiscussionIdOnPromotedEpics, :migration, :sidekiq do
+  let(:migration_class) { described_class::MIGRATION }
+  let(:migration_name)  { migration_class.to_s.demodulize }
+
+  let(:namespaces) { table(:namespaces) }
+  let(:projects) { table(:projects) }
+  let(:users) { table(:users) }
+  let(:issues) { table(:issues) }
+  let(:epics) { table(:epics) }
+  let(:notes) { table(:notes) }
+  let(:system_note_metadata) { table(:system_note_metadata) }
+
+  let(:user) { users.create!(email: 'test@example.com', projects_limit: 100, username: 'test') }
+  let(:namespace) { namespaces.create!(name: 'gitlab', path: 'gitlab-org') }
+
+  def create_promotion_note(model, id)
+    note = create_note(model, id, { system: true,
+                                    note: 'promoted from issue XXX' })
+    system_note_metadata.create!(note_id: note.id, action: 'moved')
+  end
+
+  def create_epic
+    epics.create!(author_id: user.id, iid: 1,
+                  group_id: namespace.id,
+                  title: 'Epic with discussion',
+                  title_html: 'Epic with discussion')
+  end
+
+  def create_note(model, id, extra_params = {})
+    params = {
+      note: 'note',
+      noteable_id: model.id,
+      noteable_type: model.class.name,
+      discussion_id: id
+    }.merge(extra_params)
+
+    notes.create!(params)
+  end
+
+  context 'with promoted epic' do
+    let(:epic1) { create_epic }
+    let!(:note1) { create_promotion_note(epic1, 'id1') }
+
+    it 'correctly schedules background migrations in batches' do
+      create_note(epic1, 'id2')
+      create_note(epic1, 'id3')
+
+      stub_const("#{described_class.name}::BATCH_SIZE", 2)
+
+      Sidekiq::Testing.fake! do
+        Timecop.freeze do
+          migrate!
+
+          expect(migration_name).to be_scheduled_delayed_migration(2.minutes, %w(id1 id2))
+          expect(migration_name).to be_scheduled_delayed_migration(4.minutes, %w(id3))
+          expect(BackgroundMigrationWorker.jobs.size).to eq(2)
+        end
+      end
+    end
+
+    it 'schedules only promoted epics' do
+      issue = issues.create!(description: 'first', state: 'opened')
+      create_promotion_note(issue, 'id2')
+      create_note(create_epic, 'id3')
+
+      Sidekiq::Testing.fake! do
+        Timecop.freeze do
+          migrate!
+
+          expect(migration_name).to be_scheduled_delayed_migration(2.minutes, %w(id1))
+          expect(BackgroundMigrationWorker.jobs.size).to eq(1)
+        end
+      end
+    end
+  end
+end