Validating team size on join, server-side

By doing this, we can prevent the bug where multiple users can join
a team simutaneously and push its enrollment over the defined
maximum value. Tests have also been added to confirm this behavior.

TNL-3061

lms/djangoapps/teams/tests/test_views.py

@@ -143,7 +143,8 @@ class TeamAPITestCase(APITestCase, SharedModuleStoreTestCase):
                     'name': 'Public Profiles',
                     'description': 'Description for topic 6.'
                 },
-            ]
+            ],
+            'max_team_size': 1
         }
         cls.test_course_2 = CourseFactory.create(
             org='MIT',
@@ -185,6 +186,13 @@ class TeamAPITestCase(APITestCase, SharedModuleStoreTestCase):
         profile.year_of_birth = 1970
         profile.save()
 
+        # This student is enrolled in the other course, but not yet a member of a team. This is to allow
+        # course_2 to use a max_team_size of 1 without breaking other tests on course_1
+        self.create_and_enroll_student(
+            courses=[self.test_course_2],
+            username='student_enrolled_other_course_not_on_team'
+        )
+
         # 'solar team' is intentionally lower case to test case insensitivity in name ordering
         self.test_team_1 = CourseTeamFactory.create(
             name=u'sólar team',
@@ -956,6 +964,14 @@ class TestCreateMembershipAPI(TeamAPITestCase):
         )
         self.assertIn('not enrolled', json.loads(response.content)['developer_message'])
 
+    def test_over_max_team_size_in_course_2(self):
+        response = self.post_create_membership(
+            400,
+            self.build_membership_data('student_enrolled_other_course_not_on_team', self.test_team_5),
+            user='student_enrolled_other_course_not_on_team'
+        )
+        self.assertIn('full', json.loads(response.content)['developer_message'])
+
 
 @ddt.ddt
 class TestDetailMembershipAPI(TeamAPITestCase):

lms/djangoapps/teams/views.py

@@ -914,6 +914,13 @@ class MembershipListView(ExpandableFieldViewMixin, GenericAPIView):
         except User.DoesNotExist:
             return Response(status=status.HTTP_404_NOT_FOUND)
 
+        course_module = modulestore().get_course(team.course_id)
+        if course_module.teams_max_size is not None and team.users.count() >= course_module.teams_max_size:
+            return Response(
+                build_api_error(ugettext_noop("This team is already full.")),
+                status=status.HTTP_400_BAD_REQUEST
+            )
+
         try:
             membership = team.add_user(user)
         except AlreadyOnTeamInCourse: