Make settings.BACKGROUND_EXPORT_TASK_CALLBACK capable of aborting bac…

…kground export

docs/installation.rst

@@ -206,7 +206,9 @@ These configuration you can use with django-is-core in your django settings file
 
 .. attribute:: IS_CORE_BACKGROUND_EXPORT_TASK_CALLBACK
 
-  The path to the function which will be called before export task execution.
+  The path to the function which will be called before export task execution. The function
+  is expected to return None on success. Any value other than None causes export to abort
+  with export_failure signal being sent.
 
     # Django settings
     IS_CORE_BACKGROUND_EXPORT_TASK_CALLBACK = 'your.file.perform_action_before_export_task'

is_core/contrib/background_export/signals.py

@@ -1,5 +1,4 @@
 import django.dispatch
 
-
 export_success = django.dispatch.Signal(providing_args=['exported_file'])
-export_failure = django.dispatch.Signal(providing_args=['exception'])
+export_failure = django.dispatch.Signal(providing_args=['exception', 'error'])

is_core/contrib/background_export/tasks.py

@@ -69,7 +69,7 @@ def on_task_success(self, task_id, args, kwargs, retval):
 
     def on_failure(self, exc, task_id, args, kwargs, einfo):
         super().on_failure(exc, task_id, args, kwargs, einfo)
-        export_failure.send(sender=self.__class__, exception=exc)
+        export_failure.send(sender=self.__class__, exception=exc, error=None)
 
 
 @shared_task(base=BackgroundSerializationTask,
@@ -101,12 +101,15 @@ def background_serialization(self, exported_file_pk, rest_context, language, req
 
         # Perform user-defined verification before saving exported file into the database.
         if settings.BACKGROUND_EXPORT_TASK_CALLBACK:
-            import_string(settings.BACKGROUND_EXPORT_TASK_CALLBACK)(
+            error = import_string(settings.BACKGROUND_EXPORT_TASK_CALLBACK)(
                 request=request,
                 queryset=queryset,
                 filename=filename,
                 exported_file=exported_file,
             )
+            if error is not None:
+                export_failure.send(sender=self.__class__, exception=None, error=error)
+                return
 
         exported_file.file.save(filename, ContentFile(''))