rails_asset_id

# File actionpack/lib/action_view/helpers/asset_tag_helper.rb, line 787
        def rails_asset_id(source)
          if asset_id = ENV["RAILS_ASSET_ID"]
            asset_id
          else
            if @@cache_asset_timestamps && (asset_id = @@asset_timestamps_cache[source])
              asset_id
            else
              path = File.join(config.assets_dir, source)
              asset_id = File.exist?(path) ? File.mtime(path).to_i.to_s : ''

              if @@cache_asset_timestamps
                @@asset_timestamps_cache_guard.synchronize do
                  @@asset_timestamps_cache[source] = asset_id
                end
              end

              asset_id
            end
          end
        end