A buggy Ruby Guard script for running reloadable zero-downtime Crystal Kemal apps

Guardfile

# tested on guard-2.14.0, ruby 2.3.1, Debian 8
# bugs: sometimes rb-inotify crashes on exiting guard CLI
module ::Guard
  class CrystalKemal < Plugin
    def initialize(options = {})
      @path = options.fetch(:path, '.')
      @file = options.fetch(:file, 'app.cr')
      super
      @app_path = File.join(@path, @file)
      @app_name = File.basename(@file, '.*')
    end

    def start
      if build
        launch
      else
        UI.error "build of #{File.basename @new_binary} failed"
      end
    end
   
    def stop
      kill
    end

    def reload
      if build
        kill
        launch
      else
        UI.error "build of #{File.basename @new_binary} failed, leaving #{File.basename @current_binary} alive"
      end
    end

    def run_all
      true
    end

    def run_on_change(paths)
      reload
    end

    private 

    def built?
      @new_binary && File.file?(@new_binary)
    end

    def build
      build_time = Time.now.strftime('%Y%m%d-%H%M%S%L')
      @new_binary = File.expand_path "#{@app_name}.#{build_time}", @path
      build_command = "crystal build '#{@app_path}' -o '#{@new_binary}'"
      UI.info "Building #{File.basename @new_binary}, this may take some time..."
      system build_command
    end

    def launch
      @current_pid = Process.spawn @new_binary
      @current_binary = @new_binary
    end

    def kill
      Process.kill("INT", @current_pid) if @current_pid
      File.unlink(@current_binary) if @current_binary
    end
  end
end

# Watches your main kemal app
guard "CrystalKemal" do
  watch("src/hello.cr")
#  watch(%r{src/.*\.slang})
end

# Watches your sass files
guard "sass", input: "src/assets/styles", output: "public/stylesheets", style: :compressed

# Watches your es6 files
watch(/src\/assets\/scripts\/\.es6$/) { `ruby es2js.rb` }