```ruby
module Guard
  class Webpack < Plugin
    class Runner

      attr_accessor :options, :thread

      def initialize(options)
        @options = options
      end

      def restart; stop; start; end

      def start
        unless_running do
          @thread = Thread.new { run_webpack }
          ::Guard::UI.info "Webpack watcher started."
        end
      end

      def stop
        if_running do
          @thread.kill
          ::Guard::UI.info "Webpack watcher stopped."
        end
      end

      private

      def local_webpack
        bin = File.join(Dir.pwd,'node_modules/webpack/bin/webpack.js')
        File.exists?(bin) && bin
      end

      def global_webpack
        system("command -v webpack >/dev/null") && 'webpack'
      end

      def no_webpack
        # TODO: This is a bad error.
        raise 'Webpack binary not found'
      end

      def webpack_bin
        local_webpack || global_webpack || no_webpack
      end

      def option_flags
        output = ""
        output += " --colors"   if @options[:colors]
        output += " --progress" if @options[:progress]
        output
      end

      def run_webpack
        begin
          system("#{webpack_bin} --watch #{option_flags}")
        rescue
          # TODO: Be more discerning.
          ::Guard::UI.error "Webpack unable to start (are you sure it's installed?)"
        end
      end

      def unless_running
        yield if !@thread || !@thread.alive?
      end

      def if_running
        yield if @thread && @thread.alive?
      end

    end
  end
end
```

Use fork-exec, to ensure webpack process is killed on thread stop.
user
```ruby
module Guard
  class Webpack < Plugin
    class Runner

      attr_accessor :options, :thread

      def initialize(options)
        @options = options
      end

      def restart; stop; start; end

      def start
        unless_running do
          @thread = Thread.new { run_webpack }
          ::Guard::UI.info "Webpack watcher started."
        end
      end

      def stop
        if_running do
          @thread.kill
          ::Guard::UI.info "Webpack watcher stopped."
        end
      end

      private

      def local_webpack
        bin = File.join(Dir.pwd,'node_modules/webpack/bin/webpack.js')
        File.exists?(bin) && bin
      end

      def global_webpack
        system("command -v webpack >/dev/null") && 'webpack'
      end

      def no_webpack
        # TODO: This is a bad error.
        raise 'Webpack binary not found'
      end

      def webpack_bin
        local_webpack || global_webpack || no_webpack
      end

      def option_flags
        output = ""
        output += " --colors"   if @options[:colors]
        output += " --progress" if @options[:progress]
        output
      end

      def run_webpack
        begin
          pid = fork{ exec("#{webpack_bin} --watch #{option_flags}") }
          Process.wait(pid)
        rescue
          # TODO: Be more discerning.
          ::Guard::UI.error "Webpack unable to start (are you sure it's installed?)"
        ensure
          Process.kill('TERM',pid)
        end
      end

      def unless_running
        yield if !@thread || !@thread.alive?
      end

      def if_running
        yield if @thread && @thread.alive?
      end

    end
  end
end
```
assistant