Schedule PopulateUntrackedUploads if needed

To finish migrating untracked files to uploads for installations that were affected by https://gitlab.com/gitlab-org/gitlab-ce/issues/42881

Or just to delete the temp table if it is empty and left behind.

db/migrate/20180208183958_schedule_populate_untracked_uploads_if_needed.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class SchedulePopulateUntrackedUploadsIfNeeded < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  FOLLOW_UP_MIGRATION = 'PopulateUntrackedUploads'.freeze
+
+  class UntrackedFile < ActiveRecord::Base
+    include EachBatch
+
+    self.table_name = 'untracked_files_for_uploads'
+  end
+
+  def up
+    if table_exists?(:untracked_files_for_uploads)
+      process_or_remove_table
+    end
+  end
+
+  def down
+    # nothing
+  end
+
+  private
+
+  def process_or_remove_table
+    if UntrackedFile.all.empty?
+      drop_temp_table
+    else
+      schedule_populate_untracked_uploads_jobs
+    end
+  end
+
+  def drop_temp_table
+    drop_table(:untracked_files_for_uploads, if_exists: true)
+  end
+
+  def schedule_populate_untracked_uploads_jobs
+    say "Scheduling #{FOLLOW_UP_MIGRATION} background migration jobs since there are rows in untracked_files_for_uploads."
+
+    bulk_queue_background_migration_jobs_by_range(
+      UntrackedFile, FOLLOW_UP_MIGRATION)
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180206200543) do
+ActiveRecord::Schema.define(version: 20180208183958) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"