fix(session.js): Refactor session creation code to use promises in or…

…der to fix a possible race condition while inserting a session to the database.

lib/session.js

@@ -3,7 +3,8 @@ var Store = require('./db').Store
 , Cookies = require('cookies')
 , EventEmitter = require('events').EventEmitter
 , crypto = require('crypto')
-, debug = require('debug')('session');
+, debug = require('debug')('session')
+, Q = require("q");
 
 /*!
 * A simple index for storing sesssions in memory.
@@ -67,27 +68,38 @@ SessionStore.prototype.createSession = function(sid, fn) {
     sid = undefined;
   }
   if(sid) {
-    this.find({id: sid}, function(err, s) {
-      if(err) return fn(err);
-      if(!s) {
-        store.insert({id: sid}, function(err, s) {
-          if(err) console.error(err);
+    Q.ninvoke(this, 'find', { id: sid })
+    .then(function (session) {
+      var defer = Q.defer();
+      if (!session) {
+        store.insert({ id: sid }, function (err, s) {
+          if (err) return defer.reject(err);
+          defer.resolve(s);
         });
+      } else {
+        defer.resolve(session);
       }
+      return defer.promise;
+    })
+    .then(function(s) {
       var sess = sessionIndex[sid] || new Session(s, store, socketIndex, store.sockets);
       sessionIndex[sid] = sess;
       // index sessions by user
-      if(s && s.uid) {
+      if (s && s.uid) {
         userSessionIndex[s.uid] = sess;
       }
-      fn(err, sess);
-    });
+      fn(null, sess);
+    })
+    .catch(function (err) {
+      fn(err); // couldn't get a session
+    })
+    .done();
   } else {
     sid = this.createUniqueIdentifier();
     var sess = sessionIndex[sid] = new Session({id: sid}, this, socketIndex, store.sockets);
     this.insert({ id: sid }, function (err, s) {
-      fn(null, sess);        
-      if(err) console.error(err);
+      if (err) return fn(err); // this shouldn't happen
+      fn(err, sess);
     });
   }
 };