autoinstrument_source (NewRelic::Rack::BrowserMonitoring)

# File lib/new_relic/rack/browser_monitoring.rb, line 78
    def autoinstrument_source(response, headers, js_to_inject)
      source = gather_source(response)
      close_old_response(response)
      return nil unless source

      # Only scan the first 50k (roughly) then give up.
      beginning_of_source = source[0..SCAN_LIMIT]

      if body_start = find_body_start(beginning_of_source)
        meta_tag_positions = [
          find_x_ua_compatible_position(beginning_of_source),
          find_charset_position(beginning_of_source)
        ].compact

        if !meta_tag_positions.empty?
          insertion_index = meta_tag_positions.max
        else
          insertion_index = find_end_of_head_open(beginning_of_source) || body_start
        end

        if insertion_index
          source = source[0...insertion_index] <<
            js_to_inject <<
            source[insertion_index..-1]
        else
          NewRelic::Agent.logger.debug "Skipping RUM instrumentation. Could not properly determine location to inject script."
        end
      else
        msg = "Skipping RUM instrumentation. Unable to find <body> tag in first #{SCAN_LIMIT} bytes of document."
        NewRelic::Agent.logger.log_once(:warn, :rum_insertion_failure, msg)
        NewRelic::Agent.logger.debug(msg)
      end

      source
    rescue => e
      NewRelic::Agent.logger.debug "Skipping RUM instrumentation on exception.", e
      nil
    end