rollback the session after UNIQUE constraint failed exception

Here is the error message:
InvalidRequestError: This Session's transaction has been rolled
back due to a previous exception during flush. To begin a new
transaction with this Session, first issue Session.rollback().
Original exception was: (IntegrityError) UNIQUE constraint failed:
version.id u'INSERT INTO version (id) VALUES (?)' (1,)

Signed-off-by: Harrison Feng <harrison_feng@163.com>

Author: harrisonfeng

docker_registry/lib/index/db.py

@@ -113,7 +113,10 @@ def _generate_index(self, session):
         session.add(Version(id=self.version))
         for repository in self._walk_storage(store=store):
             session.add(Repository(**repository))
-        session.commit()
+        try:
+            session.commit()
+        except:
+            session.rollback()
 
     @retry
     def _handle_repository_created(