Fix #658 and #974 - Rollback database on_return of task

mediagoblin/config_spec.ini

@@ -113,9 +113,7 @@ user_privilege_scheme = string(default="uploader,commenter,reporter")
 
 # Frequency garbage collection will run (setting to 0 or false to disable)
 # Setting units are minutes.
-## NOTE: This is temporarily disabled, but we want to set it back:
-## garbage_collection = integer(default=60)
-garbage_collection = integer(default=0)
+garbage_collection = integer(default=60)
 
 [jinja2]
 # Jinja2 supports more directives than the minimum required by mediagoblin. 

mediagoblin/processing/task.py

@@ -155,5 +155,19 @@ class ProcessMedia(celery.Task):
 
         entry = mgg.database.MediaEntry.query.filter_by(id=entry_id).first()
         json_processing_callback(entry)
+        mgg.database.reset_after_request()
+
+    def after_return(self, *args, **kwargs):
+        """
+        This is called after the task has returned, we should clean up.
+
+        We need to rollback the database to prevent ProgrammingError exceptions
+        from being raised.
+        """
+        # In eager mode we get DetachedInstanceError, we do rollback on_failure
+        # to deal with that case though when in eager mode.
+        if not celery.app.default_app.conf['CELERY_ALWAYS_EAGER']:
+            mgg.database.reset_after_request()
+
 
 tasks.register(ProcessMedia)