fix: non-overwriting touch semantic support. See issue #27

Readme.md

@@ -50,6 +50,7 @@ http.createServer(app).listen(9341, function() {
 - `ttl` An optional parameter used for setting the default TTL
 - `secret` An optional secret can be used to encrypt/decrypt session contents.
 - `algorithm` An optional algorithm parameter may be used, but must be valid based on returned `crypto.getCiphers()`. The current default is `aes-256-ctr` and was chosen based on the following [information](http://www.daemonology.net/blog/2009-06-11-cryptographic-right-answers.html)
+- `serverSupportsTouch` Set true if your memcached server version is at least 1.4.14 to enable the more efficient touch protocol.
 - ...     Rest of given option will be passed directly to the node-memcached constructor.
 
 For details see [node-memcached](http://github.com/3rd-Eden/node-memcached).

lib/connect-memcached.js

@@ -31,6 +31,7 @@ module.exports = function(session) {
         options = options || {};
         Store.call(this, options);
 
+        this.serverSupportsTouch = options.serverSupportsTouch;
         this.prefix = options.prefix || '';
         this.ttl = options.ttl;
         if (!options.client) {
@@ -107,8 +108,7 @@ module.exports = function(session) {
         sid = this.getKey(sid);
 
         try {
-            var maxAge = sess.cookie.maxAge;
-            var ttl = this.ttl || ('number' == typeof maxAge ? maxAge / 1000 | 0 : oneDay);
+            var ttl = computeTtl.call(this, sess);
             var sess = JSON.stringify((this.secret) ?
                                 encryptData.call(this, JSON.stringify(sess),
                                                  this.secret, this.algorithm) :
@@ -161,10 +161,36 @@ module.exports = function(session) {
 	 * @api public
  	 */
 
-	MemcachedStore.prototype.touch = function (sid, sess, fn) {
-		this.set(sid, sess, fn);
-	}
+    MemcachedStore.prototype.touch = function (sid, sess, fn) {
+        sid = this.getKey(sid);
 
+        try {
+            var ttl = computeTtl.call(this, sess);
+            var self = this;
+
+            // memcached.touch support introduced in 1.4.8 and buggy until 1.4.14
+            // when touch not supported implement it using get+set as modelled in MemoryStore
+            // Memcached.version is not robust so we use options flag to disable work-around
+
+            if (this.serverSupportsTouch) {
+                this.client.touch(sid, ttl, ensureCallback(fn));
+            } else {
+                self.client.get(sid, function(err, data) {
+                    if (err) {
+                        return fn && fn(err);
+                    }
+                    self.client.set(sid, data, ttl, ensureCallback(fn));
+                });
+            }
+        } catch (err) {
+            fn && fn(err);
+        }
+    }
+
+    function computeTtl(sess) {
+        var maxAge = sess.cookie.maxAge;
+        return this.ttl || ('number' == typeof maxAge ? maxAge / 1000 | 0 : oneDay);
+    }
 
     function encryptData(plaintext){
         var pt = encrypt.call(this, this.secret, plaintext, this.algo)