add md5 etags for json

lib/index.js

@@ -7,6 +7,7 @@ var UError = require('./error').UserError;
 var basic_auth = require('./middleware').basic_auth;
 var validate_name = require('./middleware').validate_name;
 var media = require('./middleware').media;
+var etagify = require('./middleware').etagify;
 var expect_json = require('./middleware').expect_json;
 
 module.exports = function(config_hash) {
@@ -28,6 +29,7 @@ module.exports = function(config_hash) {
 
 	var app = express();
 	app.use(express.logger());
+	app.use(etagify);
 	app.use(basic_auth(function(user, pass) {
 		return config.authenticate(user, pass);
    }));

lib/middleware.js

@@ -1,3 +1,4 @@
+var crypto = require('crypto');
 var utils = require('./utils');
 var UError = require('./error').UserError;
 
@@ -77,3 +78,30 @@ module.exports.basic_auth = function basic_auth(callback) {
 	}
 };
 
+// express doesn't do etags with requests <= 1024b
+// we use md5 here, it works well on 1k+ bytes, but sucks with fewer data
+// could improve performance using crc32 after benchmarks
+function md5sum(data) {
+	return crypto.createHash('md5').update(data).digest('hex');
+}
+
+// using it for json only right now
+module.exports.etagify = function(req, res, next) {
+	var _send = res.send;
+	res.send = function(body) {
+		if (typeof(body) === 'string' || typeof(body) === 'object') {
+			res.header('Content-type', 'application/json');
+
+			if (typeof(body) === 'object') {
+				body = JSON.stringify(body, undefined, '\t');
+			}
+			
+			res.header('ETag', '"' + md5sum(body) + '"');
+		} else {
+			// send(null), send(204), etc.
+		}
+		_send.call(res, body);
+	};
+	next();
+}
+