static_assets_middleware.rb

module Heroku
class StaticAssetsMiddleware
	def initialize(app)
		@app = app
	end

	def call(env)
		# call returns an array containing [response code, header, Rack::Response]
		reply = @app.call(env)

		reply = cache_static_asset(reply)

		reply
	end

	def cache_static_asset(reply)
		return reply unless can_cache?(reply)

		status, headers, response = reply

		# static files are cacheable for 12hrs
		headers['Cache-Control'] = 'public, max-age=43200'

		build_new_reply(status, headers, response)
	end

	def can_cache?(reply)
		response = reply[2]
		status = reply[0]
		response.kind_of?(Rack::File) and status.to_i == 200
	end

	def build_new_reply(status, headers, response)
		headers.delete('Etag') if headers.has_key?('Etag')
		[ status, headers, response ]
	end
end
end