Vote buttons should be disabled while in use

Clicking quickly on up/down votes in forums generates a race condition
where the votes can continue going up or down (and I can get e.g. large
negative numbers). This is client-side; the server-side is not affected.

This is caused by safeAjax calling code (including updateWithUndo) not
handling the possible undefined return value. This is returned in the case
where the element that triggers the event is already disabled.

Corrected typo in DiscussionThreadView cleanup: @responseRequest -> @responsesRequest

Resolves TNL-1061

AUTHORS

@@ -195,3 +195,4 @@ Dino Cikatić <dcikatic@edx.org>
 Davorin Šego <dsego@edx.org>
 Marko Jevtić <mjevtic@edx.org>
 Ahsan Ulhaq <ahsan@edx.org>
+Mat Moore <mat@mooresoftware.co.uk>

common/static/coffee/spec/discussion/utils_spec.coffee

@@ -25,3 +25,24 @@ describe 'DiscussionUtil', ->
       # if the ajax call ends in failure, the model state should be reverted
       deferred.reject()
       expect(model.attributes).toEqual({hello: false, number: 42})
+
+    it "rolls back the changes if the associated element is disabled", ->
+      spyOn(DiscussionUtil, "safeAjax").andCallThrough()
+
+      model = new Backbone.Model({hello: false, number: 42})
+      updates = {hello: "world"}
+
+      # This is the element that is disabled/enabled while the ajax request is
+      # in progress
+      $elem = jasmine.createSpyObj('$elem', ['attr'])
+      $elem.attr.andReturn(true)
+
+      res = DiscussionUtil.updateWithUndo(model, updates, {foo: "bar", $elem:$elem}, "error message")
+
+      expect($elem.attr).toHaveBeenCalledWith("disabled")
+      expect(DiscussionUtil.safeAjax).toHaveBeenCalled()
+      expect(model.attributes).toEqual({hello: false, number: 42})
+
+      failed = false
+      res.fail(() => failed = true)
+      expect(failed).toBe(true);

common/static/coffee/src/discussion/utils.coffee

@@ -134,8 +134,12 @@ class @DiscussionUtil
 
   @safeAjax: (params) ->
     $elem = params.$elem
+
     if $elem and $elem.attr("disabled")
-      return
+      deferred = $.Deferred()
+      deferred.reject()
+      return deferred.promise()
+
     params["url"] = URI(params["url"]).addSearch ajax: 1
     params["beforeSend"] = ->
       if $elem

common/static/coffee/src/discussion/views/discussion_thread_view.coffee

@@ -127,7 +127,7 @@ if Backbone?
         $loading: elem
         takeFocus: true
         complete: =>
-          @responseRequest = null
+          @responsesRequest = null
         success: (data, textStatus, xhr) =>
           Content.loadContentInfos(data['annotated_content_info'])
           if @isQuestion()