# File lib/rack/lock.rb, line 12
    def call(env)
      old, env[FLAG] = env[FLAG], false
      @mutex.lock
      response = @app.call(env)
      response[2] = BodyProxy.new(response[2]) { @mutex.unlock }
      response
    rescue Exception
      @mutex.unlock
      raise
    ensure
      env[FLAG] = old
    end