trying to make a threadsafe capybara poltergeist searcher

searchers.rb

require 'capybara'
require 'capybara/dsl'
require 'capybara/poltergeist'

Capybara.configure do |config|
  config.run_server     = false
  config.default_driver = :poltergeist
end

module Searchers

  class CaptchaError < StandardError; end

  class Search
    include Capybara::DSL
    DEFAULTS={lim: 100, delay: 3}

    class << self
      def href href = nil
        @href = href if href
        @href
      end
    end

    attr_reader :base_href, :current_session

    def initialize href=self.class.href
      @base_href = URI(href)
      # Capybara requires all absolute URLs to start with http.
      unless @base_href.scheme =~ /^http/
        raise ArgumentError, "base_href must be of http(s) scheme"
      end
      # Overridden, to make sure we have one session per actor.
      @current_session = Capybara::Session.new(:poltergeist)
      visit base_href.path || '/'
    end

    def visit url
      base_href.path = URI(url).path
      super(base_href.to_s)
    end

    def captcha?
      page.has_field? "captcha"
    end

    def setup query
      fill_in "q", :with => query
      click_on "Search"
    end

    def select_results; end

    def extract_url result
      result[:href]
    end

    def next_page; click_on 'Next'; end

    def search query, opts={}
      setup query
      opts=DEFAULTS.merge opts
      urls=[]
      url_count=0
      loop do
        begin
          raise CaptchaError if captcha?
          results=select_results
          break if results.empty?
          results.each {|e|
            url_count+=1
            if block_given?
              yield extract_url( e )
            else
              urls << extract_url( e )
            end
          }
          break if url_count >= opts[:lim]
          sleep( rand(opts[:delay]*10.0)/10 )
          next_page
        rescue Capybara::Poltergeist::JavascriptError
          #ignore
        rescue
         warn "#{self}:#{__method__}: #{$!}"
         break
        end
      end
      return urls unless block_given?
    end
  end

  class Google < Search
    href 'https://www.google.com'

    def select_results
      all "h3.r a"
    end

    def extract_url result
      result[:href][%r{/url\?q=(.*?)&sa},1]
    end

  end

  class Bing < Search

    href 'http://www.bing.com'

    def select_results
      all 'h3 a'
    end

  end

  class DuckDuckGo < Search
    href 'http://www.duckduckgo.com/html'

    def select_results
      all 'div.results_links div.links_deep a'
    end
  end

end