nginx/passenger/rails needs extra path to make some gems work

2013-05-28

missing runtime paths can break some gems in production environments

Having tested markdown code block highlighting, I was surprised to find Pygments not being able to highlight anything when run via nginx/passenger in FreeBSD production. Blam -- sever error:

Processing by ArticlesController#show as HTML
  Parameters: {"id"=>"9"}
rendering ruby block
  Rendered articles/_full_body.html.haml (113.2ms)
  Rendered articles/show.html.haml within layouts/application (114.4ms)
Completed 500 Internal Server Error in 121ms

ActionView::Template::Error (Failed to get header.):
    7:   = article.summary
    8: - if article.body?
    9:   .article
    10:     =raw article.render_body( internal_images_by_name )
    11: - else
    12:   %p no article body??
    13: %p
  lib/markdown_utils.rb:11:in `block in block_code'
  lib/markdown_utils.rb:10:in `block_code'
  app/models/article.rb:110:in `render'
  app/models/article.rb:110:in `render_body'
  app/views/articles/_full_body.html.haml:10:in `_app_views_articles__full_body_html_haml__4458801067574565172_17264035260'
  app/views/articles/show.html.haml:3:in `_app_views_articles_show_html_haml__3162500010226637938_17263456920'
  app/controllers/articles_controller.rb:73:in `show'

where the line in question looks like this:

 class HTMLwithPygments < Redcarpet::Render::HTML

    def block_code(code, language)
      sha = Digest::SHA1.hexdigest(code)
      Rails.cache.fetch ["code", language, sha].join('-') do
        Pygments.highlight(code, lexer: language)      # line 10 - exception here
      end
    end

  end

More interesting: it works fine from rails/console, even in production:

% rails c
Connecting to database specified by database.yml
Loading production environment (Rails 3.2.13)
irb(main):001:0> Pygments.highlight("puts 'hi'", :lexer => :ruby)
=> "<div class=\"highlight\"><pre><span class=\"nb\">puts</span> <span class=\"s1\">&#39;hi&#39;</span>\n</pre></div>"
irb(main):002:0>

Some tedious debugging explains why Pygments.rb cannot popen4 the Python interpreter under Nginx: Pyhton lives in /usr/local/bin which is not on the path for the Rails process created by Nginx/Passenger. Many ways to fix that. In my case, I added something to the Rails initialization to make sure /usr/local/bin is always on the path for the Rails process.

Now is works like a charm - you're looking at the results in the above code blocks for this tech note.

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