Add instance column to services if missing

This is a corrective migration to keep the instance column and its
index. Upgrade from 12.7 to 12.9 removes the instance column as it was
first added in the normal migration and then removed in the post
migration.

12.8 removed the instance column in a post deployment migration
   https://gitlab.com/gitlab-org/gitlab/-/merge_requests/24885
12.9 added the instance column in a normal migration
   https://gitlab.com/gitlab-org/gitlab/-/merge_requests/25714

db/post_migrate/20200511162057_add_missing_instance_to_servicess.rb

+# frozen_string_literal: true
+
+class AddMissingInstanceToServicess < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  # This is a corrective migration to keep the instance column.
+  # Upgrade from 12.7 to 12.9 removes the instance column as it was first added
+  # in the normal migration and then removed in the post migration.
+  #
+  # 12.8 removed the instance column in a post deployment migration https://gitlab.com/gitlab-org/gitlab/-/merge_requests/24885
+  # 12.9 added the instance column in a normal migration https://gitlab.com/gitlab-org/gitlab/-/merge_requests/25714
+  #
+  # rubocop:disable Migration/AddColumnWithDefault
+  # rubocop:disable Migration/UpdateLargeTable
+  def up
+    unless column_exists?(:services, :instance)
+      add_column_with_default(:services, :instance, :boolean, default: false)
+    end
+  end
+  # rubocop:enable Migration/AddColumnWithDefault
+  # rubocop:enable Migration/UpdateLargeTable
+
+  def down
+    # Does not apply
+  end
+end

db/post_migrate/20200511162115_add_missing_index_to_service_unique_instance_per_type.rb

+# frozen_string_literal: true
+
+class AddMissingIndexToServiceUniqueInstancePerType < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  # This is a corrective migration to keep the index on instance column.
+  # Upgrade from 12.7 to 12.9 removes the instance column as it was first added
+  # in the normal migration and then removed in the post migration.
+  #
+  # 12.8 removed the instance column in a post deployment migration https://gitlab.com/gitlab-org/gitlab/-/merge_requests/24885
+  # 12.9 added the instance column in a normal migration https://gitlab.com/gitlab-org/gitlab/-/merge_requests/25714
+  def up
+    unless index_exists_by_name?(:services, 'index_services_on_type_and_instance')
+      add_concurrent_index(:services, [:type, :instance], unique: true, where: 'instance IS TRUE')
+    end
+  end
+
+  def down
+    # Does not apply
+  end
+end

db/structure.sql

@@ -13768,6 +13768,8 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200508091106
 20200511092714
 20200511145545
+20200511162057
+20200511162115
 20200512085150
 \.