Add a health check for Geo database

app/views/admin/health_check/show.html.haml

@@ -34,6 +34,9 @@
         %code= health_check_url(token: current_application_settings.health_check_access_token, checks: :database)
       %li
         %code= health_check_url(token: current_application_settings.health_check_access_token, checks: :migrations)
+      - if Gitlab::Geo.secondary?
+        %li
+          %code= health_check_url(token: current_application_settings.health_check_access_token, checks: :geo)
 
   %hr
   .panel.panel-default

config/initializers/health_check.rb

 HealthCheck.setup do |config|
   config.standard_checks = %w(database migrations cache)
   config.full_checks = %w(database migrations cache)
+
+  config.add_custom_check('geo') do
+    Gitlab::Geo::HealthCheck.perform_checks
+  end
 end

lib/gitlab/geo/health_check.rb

+module Gitlab
+  module Geo
+    class HealthCheck
+      def self.perform_checks
+        return '' unless Gitlab::Geo.secondary?
+
+        database_version  = self.get_database_version
+        migration_version = self.get_migration_version
+
+        if database_version.to_i != migration_version.to_i
+          "Current database version (#{database_version}) does not match latest migration (#{migration_version})."
+        else
+          ''
+        end
+      rescue => e
+        e.message
+      end
+
+      def self.db_migrate_path
+        # Lazy initialisation so Rails.root will be defined
+        @@db_migrate_path ||= File.join(Rails.root, 'db_geo', 'migrate')
+      end
+
+      def self.get_database_version
+        if defined?(ActiveRecord)
+          connection = ::Geo::BaseRegistry.connection
+          schema_migrations_table_name = ActiveRecord::Base.schema_migrations_table_name
+
+          if connection.table_exists?(schema_migrations_table_name)
+            connection.execute("SELECT MAX(version) AS version FROM #{schema_migrations_table_name}")
+                      .first
+                      .fetch('version')
+          else
+            0
+          end
+        end
+      end
+
+      def self.get_migration_version
+        latest_migration = nil
+
+        Dir[File.join(self.db_migrate_path, "[0-9]*_*.rb")].each do |f|
+          l = f.scan(/0*([0-9]+)_[_.a-zA-Z0-9]*.rb/).first.first rescue -1
+          latest_migration = l if !latest_migration || l.to_i > latest_migration.to_i
+        end
+
+        latest_migration
+      end
+    end
+  end
+end