add deferred request to models or collections for race condutions

backbone.js

@@ -215,6 +215,12 @@
     // began.
     _pending: null,
 
+    // Deferred request is a request that is waiting for
+    // response of currently running model request. 
+    // Initial value is false which means 
+    // there is not an currently running request.
+    _deferredRequest: false,
+
     // The default name for the JSON `id` attribute is `"id"`. MongoDB and
     // CouchDB users may want to set this to `"_id"`.
     idAttribute: 'id',
@@ -571,6 +577,12 @@
     // This should be overridden in most cases.
     model: Model,
 
+    // Deferred request is a request that is waiting for
+    // response of currently running collection request. 
+    // Initial value is false which means 
+    // there is not an currently running request.
+    _deferredRequest: false,
+
     // Initialize is an empty function by default. Override it with your own
     // initialization logic.
     initialize: function(){},
@@ -1387,6 +1399,33 @@
       params.processData = false;
     }
 
+    var success = options.success;
+    options.success = function(){
+      var dr = model._deferredRequest;
+      // running request responsed, 
+      // there is no need to defer another request now
+      model._deferredRequest = false;
+
+      // if there is a request that deferred until response for this model
+      if (!_.isEmpty(dr)) {
+        model.sync(dr[0], dr[1], dr[2]);
+      }
+      if (success) success.apply(null, arguments);
+    };
+
+    // if there is a running request
+    if (_.isArray(model._deferredRequest)) {
+      // defer this request until response of running request.
+      // if there is another deferred request,
+      //override previous deferred request
+      model._deferredRequest = [method, model, options];
+      // FIXME: return sth look like xhr
+      return true;
+    }
+
+    // last requests until response will defer 
+    // but now model._deferredRequest is empty array
+    model._deferredRequest = [];
     // Make the request, allowing the user to override any Ajax options.
     return Backbone.ajax(_.extend(params, options));
   };
@@ -1400,6 +1439,7 @@
   Backbone.wrapError = function(onError, originalModel, options) {
     return function(model, resp) {
       resp = model === originalModel ? resp : model;
+      resp._deferredRequest = false;
       if (onError) {
         onError(originalModel, resp, options);
       } else {

test/sync.js

@@ -20,6 +20,7 @@ $(document).ready(function() {
       library = new Library();
       Backbone.ajax = function(obj) {
         lastRequest = obj;
+        obj.success(obj.data);
       };
       library.create(attrs, {wait: false});
     },
@@ -142,6 +143,28 @@ $(document).ready(function() {
     equal(lastRequest.url, '/one/two');
   });
 
+  test("sync: Defer requests until response", 5, function() {
+    Backbone.ajax = function(obj){
+      lastRequest = obj;
+      // not calling obj.success causes running first request forever 
+    };
+    var model = new Backbone.Model({id:'1'});
+    model.url = '/test';
+    // first long running request that is not deferred
+    model.save({deferred:false});
+    // first deferred request
+    model.save({deferred:true, deferralId:1});
+    strictEqual(model._deferredRequest[1].get('deferred'), true);
+    equal(model._deferredRequest[1].get('deferralId'), 1);
+    // second request that override first deferred request
+    model.save({deferred:true, deferralId:2});
+    equal(model._deferredRequest[1].get('deferralId'), 2);
+    // last deferred request that override previous deferred requests
+    model.destroy();
+    equal(model._deferredRequest[0], 'delete');
+    equal(model._deferredRequest[1].get('deferralId'), 2);
+  });
+
   test("#1052 - `options` is optional.", 0, function() {
     var model = new Backbone.Model();
     model.url = '/test';