NoMethodError: undefined method `each' for "[omitted]/app/stylesheets":String

[dnesting]

We are getting this error very rarely:

NoMethodError: undefined method `each' for "[omitted]/app/stylesheets":String
…uby/2.2.0/gems/sass-3.4.16/lib/sass/plugin/compiler.rb: 251:in `file_list'
…uby/2.2.0/gems/sass-3.4.16/lib/sass/plugin/compiler.rb: 205:in `update_stylesheets'
…/bundle/ruby/2.2.0/gems/sass-3.4.16/lib/sass/plugin.rb:  82:in `update_stylesheets'
…/bundle/ruby/2.2.0/gems/sass-3.4.16/lib/sass/plugin.rb:  54:in `check_for_updates'
…le/ruby/2.2.0/gems/sass-3.4.16/lib/sass/plugin/rack.rb:  51:in `call'
…ack-protection-1.5.3/lib/rack/protection/xss_header.rb:  18:in `call'
…rack-protection-1.5.3/lib/rack/protection/json_csrf.rb:  18:in `call'
…gems/rack-protection-1.5.3/lib/rack/protection/base.rb:  49:in `call'
…gems/rack-protection-1.5.3/lib/rack/protection/base.rb:  49:in `call'
…-protection-1.5.3/lib/rack/protection/frame_options.rb:  31:in `call'
…dor/bundle/ruby/2.2.0/gems/rack-1.5.5/lib/rack/head.rb:  11:in `call'
…/ruby/2.2.0/gems/rack-1.5.5/lib/rack/methodoverride.rb:  21:in `call'
…ndle/ruby/2.2.0/gems/sinatra-1.4.6/lib/sinatra/base.rb:2021:in `call'
…ndle/ruby/2.2.0/gems/sinatra-1.4.6/lib/sinatra/base.rb:1486:in `block in call'
…ndle/ruby/2.2.0/gems/sinatra-1.4.6/lib/sinatra/base.rb:1795:in `synchronize'
…ndle/ruby/2.2.0/gems/sinatra-1.4.6/lib/sinatra/base.rb:1486:in `call'
….0/gems/padrino-core-0.12.5/lib/padrino-core/router.rb:  84:in `block in call'
….0/gems/padrino-core-0.12.5/lib/padrino-core/router.rb:  75:in `each'
….0/gems/padrino-core-0.12.5/lib/padrino-core/router.rb:  75:in `call'
…/ruby/2.2.0/gems/puma-2.13.4/lib/puma/configuration.rb:  78:in `call'
…/bundle/ruby/2.2.0/gems/puma-2.13.4/lib/puma/server.rb: 541:in `handle_request'
…/bundle/ruby/2.2.0/gems/puma-2.13.4/lib/puma/server.rb: 388:in `process_client'
…/bundle/ruby/2.2.0/gems/puma-2.13.4/lib/puma/server.rb: 270:in `block in run'
…le/ruby/2.2.0/gems/puma-2.13.4/lib/puma/thread_pool.rb: 106:in `call'
…le/ruby/2.2.0/gems/puma-2.13.4/lib/puma/thread_pool.rb: 106:in `block in spawn_thread'

It's possible the problem is a race condition when executing in a multi-threaded environment. In lib/sass/plugin/configuration.rb:

      def template_location_array
        old_template_location = options[:template_location]
        normalize_template_location!
        options[:template_location]
      ensure
        options[:template_location] = old_template_location
      end

Between execution of normalize_template_location! and the return of options[:template_location], another thread could have executed the same function and executed the 'ensure' block, restoring this value to its single-string form before this method returns.

If you're going to restore its value, consider refactoring normalize_template_location! into a function that simply returns an array, and use that here instead.

[dnesting]

Another way this bug presents:

NoMethodError: undefined method `to_a' for "/home/vcap/app/app/stylesheets":String

….2.0/gems/sass-3.4.16/lib/sass/plugin/configuration.rb: 113:in `normalize_template_location!'
….2.0/gems/sass-3.4.16/lib/sass/plugin/configuration.rb:  92:in `template_location_array'
/home/vcap/app/vendor/bundle/ruby/2.2.0/gems/sass-3.4.16/lib/sass/plugin/compiler.rb: 251:in `file_list'
...

This shouldn't be possible unless options[:template_location] is getting modified concurrently.

[dnesting]

Confirmed this was the problem with this test:

  def test_template_location_array_with_threads
    set_plugin_opts :template_location => template_loc
    threads = []
    1000.times do
      threads << Thread.new do
        1000.times do
          assert_equal [[template_loc, tempfile_loc]], Sass::Plugin.template_location_array
        end
      end
    end
    threads.each { |t| t.join }
  end

This fairly reliably fails with the existing code, and passes consistently with PR #1837.

[chriseppstein]

Patch looks good. we'll get it into the next release.