Create atomic_update db utility function

In some cases (notably the mark_entry_failed function) it
is useful to have atomic update functionality on the db. On
mongo this requires special syntax.

So created an atomic_update function for mongo and started
to use it in mark_entry_failed.

mediagoblin/db/mongo/util.py

@@ -290,3 +290,14 @@ class MigrationManager(object):
             self.set_current_migration(migration_number)
             if post_callback:
                 post_callback(migration_number, migration_func)
+
+
+##########################
+# Random utility functions
+##########################
+
+
+def atomic_update(table, query_dict, update_values):
+    table.collection.update(
+        query_dict,
+        {"$set": update_values})

mediagoblin/db/util.py

@@ -22,4 +22,5 @@ except ImportError:
 if use_sql:
     from mediagoblin.db.sql.fake import ObjectId, InvalidId, DESCENDING
 else:
-    from mediagoblin.db.mongo.util import ObjectId, InvalidId, DESCENDING
+    from mediagoblin.db.mongo.util import \
+        ObjectId, InvalidId, DESCENDING, atomic_update