ensure default sections are unique

prevent race conditions possibly creating multiple
default course sections

closes #CORE-2357

Change-Id: I4c3a6fe6a561afd7d67dbdc8da26dfa9b214af61
Reviewed-on: https://gerrit.instructure.com/178286
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
Tested-by: Jenkins

app/models/course.rb

@@ -2083,8 +2083,16 @@ class Course < ActiveRecord::Base
       section.default_section = true
       section.course = self
       section.root_account_id = self.root_account_id
-      Shackles.activate(:master) do
-        section.save unless new_record?
+      unless new_record?
+        Shackles.activate(:master) do
+          CourseSection.unique_constraint_retry do |retry_count|
+            if retry_count > 0
+              section = course_sections.active.where(default_section: true).first
+            else
+              section.save
+            end
+          end
+        end
       end
     end
     section

db/migrate/20190117151238_add_default_sections_unique_index.rb

@@ -0,0 +1,36 @@
+#
+# Copyright (C) 2019 - present Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Affero General Public License as published by the Free
+# Software Foundation, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+class AddDefaultSectionsUniqueIndex < ActiveRecord::Migration[5.1]
+  disable_ddl_transaction!
+  tag :postdeploy
+
+  def up
+    course_ids_to_fix = CourseSection.active.group(:course_id).where(:default_section => true).
+      having("COUNT(*) > 1").pluck(:course_id)
+    course_ids_to_fix.each do |course_id|
+      CourseSection.where(:course_id => course_id, :default_section => true).
+        order(:id).offset(1).update_all(:default_section => false)
+    end
+    add_index :course_sections, :course_id, :unique => true, :where => "default_section = 't' AND workflow_state <> 'deleted'",
+      :name => "index_course_sections_unique_default_section"
+  end
+
+  def down
+    remove_index :course_sections, :name => "index_course_sections_unique_default_section"
+  end
+end