Prevent registration race condition

lib/database.js

@@ -738,6 +738,7 @@ Database.prototype.isUsernameTaken = function (name, callback) {
     });
 };
 
+var regInProgress = {};
 Database.prototype.registerUser = function (name, pw, callback) {
     var self = this;
     if(typeof callback !== "function")
@@ -748,37 +749,50 @@ Database.prototype.registerUser = function (name, pw, callback) {
         return;
     }
 
+    if (regInProgress[name]) {
+        callback("Registration is already in progress", null);
+        return;
+    }
+
+    regInProgress[name] = true;
+
     var postRegister = function (err, res) {
         if(err) {
+            delete regInProgress[name];
             callback(err, null);
             return;
         }
 
         self.createLoginSession(name, function (err, hash) {
             if(err) {
+                delete regInProgress[name];
                 // Don't confuse people into thinking the registration
                 // failed when it was the session that failed
                 callback(null, "");
                 return;
             }
 
+            delete regInProgress[name];
             callback(null, hash);
         });
     };
 
     self.isUsernameTaken(name, function (err, taken) {
         if(err) {
+            delete regInProgress[name];
             callback(err, null);
             return;
         }
 
         if(taken) {
+            delete regInProgress[name];
             callback("Username already taken", null);
             return;
         }
 
         bcrypt.hash(pw, 10, function (err, hash) {
             if(err) {
+                delete regInProgress[name];
                 callback(err, null);
                 return;
             }