set position to default value if POST request don't have position argument

TNL-922

common/lib/xmodule/xmodule/seq_module.py

@@ -88,7 +88,13 @@ class SequenceModule(SequenceFields, XModule):
     def handle_ajax(self, dispatch, data):  # TODO: bounds checking
         ''' get = request.POST instance '''
         if dispatch == 'goto_position':
-            self.position = int(data['position'])
+            # set position to default value if either 'position' argument not
+            # found in request or it is a non-positive integer
+            position = data.get('position', u'1')
+            if position.isdigit() and int(position) > 0:
+                self.position = int(position)
+            else:
+                self.position = 1
             return json.dumps({'success': True})
         raise NotFoundError('Unexpected dispatch type')
 

lms/djangoapps/courseware/tests/test_module_render.py

+# -*- coding: utf-8 -*-
 """
 Test for lms courseware app, module render unit
 """
@@ -177,6 +178,44 @@ class ModuleRenderTestCase(ModuleStoreTestCase, LoginEnrollmentTestCase):
         self.assertEquals(403, response.status_code)
         self.assertEquals('Unauthenticated', response.content)
 
+    def test_missing_position_handler(self):
+        """
+        Test that sending POST request without or invalid position argument don't raise server error
+        """
+        self.client.login(username=self.mock_user.username, password="test")
+        dispatch_url = reverse(
+            'xblock_handler',
+            args=[
+                self.course_key.to_deprecated_string(),
+                quote_slashes(self.course_key.make_usage_key('videosequence', 'Toy_Videos').to_deprecated_string()),
+                'xmodule_handler',
+                'goto_position'
+            ]
+        )
+        response = self.client.post(dispatch_url)
+        self.assertEqual(200, response.status_code)
+        self.assertEqual(json.loads(response.content), {'success': True})
+
+        response = self.client.post(dispatch_url, {'position': ''})
+        self.assertEqual(200, response.status_code)
+        self.assertEqual(json.loads(response.content), {'success': True})
+
+        response = self.client.post(dispatch_url, {'position': '-1'})
+        self.assertEqual(200, response.status_code)
+        self.assertEqual(json.loads(response.content), {'success': True})
+
+        response = self.client.post(dispatch_url, {'position': "string"})
+        self.assertEqual(200, response.status_code)
+        self.assertEqual(json.loads(response.content), {'success': True})
+
+        response = self.client.post(dispatch_url, {'position': u"Φυσικά"})
+        self.assertEqual(200, response.status_code)
+        self.assertEqual(json.loads(response.content), {'success': True})
+
+        response = self.client.post(dispatch_url, {'position': None})
+        self.assertEqual(200, response.status_code)
+        self.assertEqual(json.loads(response.content), {'success': True})
+
     @ddt.data('pure', 'vertical')
     @XBlock.register_temp_plugin(PureXBlock, identifier='pure')
     def test_rebinding_same_user(self, block_type):