fingerprint.rb

# config/initializers/fingerprint.rb # XXX Breaks Rails asset bundling
#
# Mike Bailey <mike@bailey.net.au>
#
# Updates Rails Helpers to use fingerprint in path instead of timestamp in query string.
#
# Add the following to your Apache config to remove the fingerprints:
#
#   # Remove fingerprints
#   RewriteEngine On
#   RewriteRule (.+?)-fp-\w+\.(css|js|gif|png|jpg)$ $1\.$2 [L]
# 
# This is my attempt to write a super simple Fingerprinter for Rails assets to improve
# speed up my site by improving caching performance. Warning! I've not fully tested this. 
# Read the code and decided for yourself whether it suits your purposes.
#
# Eliot Skyes brought the issue to my attention with his plugin http://bit.ly/cdkIXg
# I had a bit of trouble understanding how the plugin worked so decided to try writing
# my own version. 
#
# Tim Lucas provided some great insights into the issues and solutions http://bit.ly/d9s4Lh
# 
# Although this is piggy backing off the asset timestamps cache I'd like to implement a file 
# based Fingerprint cache so that checksums can optionally be kept with the source.

require 'digest/md5'

module ActionView
  module Helpers
    module AssetTagHelper

      # Use the RAILS_ASSET_ID environment variable or the source's
      # modification time as its cache-busting asset id.
      def rails_asset_id(source)
        if asset_id = ENV["RAILS_ASSET_ID"]
          asset_id
        else
          if @@cache_asset_timestamps && (asset_id = @@asset_timestamps_cache[source])
            asset_id
          else
            path = File.join(ActionView::Helpers::AssetTagHelper::ASSETS_DIR, source)

            # Replace with fingerprint
            # asset_id = File.exist?(path) ? File.mtime(path).to_i.to_s : ''
            asset_id = fingerprint(path)

            if @@cache_asset_timestamps
              @@asset_timestamps_cache_guard.synchronize do
                @@asset_timestamps_cache[source] = asset_id
              end
            end

            asset_id
          end
        end
      end

      def fingerprint(source)
        '-fp-' + Digest::MD5.hexdigest(File.open(source).read)[0,7]
      end

      def rewrite_asset_path(source, path=nil)
        source.gsub(/(\..[^\/]+?)$/, rails_asset_id(source) + '\1')
      end

    end
  end
end