Ensure milestone titles are never empty

If a milestone title is empty we end up with exceptions when trying to
load legacy migrations. Let's avoid them and also migrate any existing
ones.

app/models/milestone.rb

@@ -60,6 +60,7 @@ class Milestone < ApplicationRecord
 
   validates :group, presence: true, unless: :project
   validates :project, presence: true, unless: :group
+  validates :title, presence: true
 
   validate :uniqueness_of_title, if: :title_changed?
   validate :milestone_type_check

changelogs/unreleased/12769-milestone-blank-name-causes-issues.yml

+---
+title: Ensure milestone titles are never empty
+merge_request: 19985
+author:
+type: fixed

db/migrate/20191112232338_ensure_no_empty_milestone_titles.rb

+# frozen_string_literal: true
+
+class EnsureNoEmptyMilestoneTitles < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    loop do
+      rows_updated = exec_update <<~SQL
+        UPDATE milestones SET title = '%BLANK' WHERE id IN (SELECT id FROM milestones WHERE title = '' LIMIT 500)
+      SQL
+      break if rows_updated < 500
+    end
+  end
+
+  def down; end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_11_12_221821) do
+ActiveRecord::Schema.define(version: 2019_11_12_232338) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"

spec/models/milestone_spec.rb

@@ -55,6 +55,17 @@ describe Milestone do
       end
     end
 
+    describe 'title' do
+      it { is_expected.to validate_presence_of(:title) }
+
+      it 'is invalid if title would be empty after sanitation' do
+        milestone = build(:milestone, project: project, title: '<img src=x onerror=prompt(1)>')
+
+        expect(milestone).not_to be_valid
+        expect(milestone.errors[:title]).to include("can't be blank")
+      end
+    end
+
     describe 'milestone_releases' do
       let(:milestone) { build(:milestone, project: project) }