Resolve "subscription ended on submit"

Closes #139 (closed)

core/views/subscription.py

@@ -10,6 +10,8 @@ from core.models import TutorSubmissionAssignment
 from core.permissions import IsReviewer, IsTutorOrReviewer
 from core.serializers import AssignmentDetailSerializer, AssignmentSerializer
 
+from multiprocessing import Lock
+
 log = logging.getLogger(__name__)
 
 
@@ -123,11 +125,13 @@ class AssignmentApiViewSet(
 
     @permission_classes((IsTutorOrReviewer,))
     def create(self, request, *args, **kwargs):
-        context = self.get_serializer_context()
-        serializer = AssignmentDetailSerializer(data=request.data,
-                                                context=context)
-        serializer.is_valid(raise_exception=True)
-        return self._fetch_assignment(serializer)
+        with Lock():
+            context = self.get_serializer_context()
+            serializer = AssignmentDetailSerializer(data=request.data,
+                                                    context=context)
+            serializer.is_valid(raise_exception=True)
+            assignment = self._fetch_assignment(serializer)
+        return assignment
 
     @permission_classes((IsTutorOrReviewer,))
     def retrieve(self, request, *args, **kwargs):