Rake task to hook into assets:precompile in a rails app and remove any BOMs that are inserted by sprockets/sass in CSS files (and their gzipped siblings)

bom-removal.rake

namespace :assets do
  task :remove_bom => :environment do
    require 'sprockets/utils/gzip'

    # object that has enough of the Sprockets::Asset API to be
    # used in the Sprockets::Utils::Gzip class without us needing
    # to use the real class which needs more data
    class DeBOMedCSSAsset < Struct.new(:file_name)
      def content_type
        'text/css'
      end

      def charset
        source.encoding.to_s
      end

      def source
        @source ||= File.read(file_name)
      end
    end

    if Dir.exists?(Rails.application.assets_manifest.directory)
      Dir.chdir(Rails.application.assets_manifest.directory) do
        debomed = []
        # find all css files and remove BOM from any that have it
        Dir[File.join('**', '*.css')].each do |css_file|
          # use binread / binwrite to preserve any encoding
          file_contents = File.binread(css_file)
          if file_contents.bytes[0..2] == [0xEF, 0xBB, 0xBF]
            Rails.logger.info "Removing BOM from #{css_file}"
            File.binwrite(css_file, file_contents[3..-1])
            debomed << css_file
          end
        end
        debomed.each do |debomed|
          # re-write the gziped version using sprockets own gzipper
          if File.exists?("#{debomed}.gz")
            Rails.logger.info "Removing BOM from #{debomed}.gz"
            # TODO: this might be a private API we're using so there's danger
            # this'll go away
            Sprockets::Utils::Gzip.new(DeBOMedCSSAsset.new(debomed)).compress(debomed)
          end
        end
      end
    end
  end
end

Rake::Task['assets:precompile'].enhance do
  Rake::Task['assets:remove_bom'].invoke
end