Currently, the isolator::recover include two parameters:
1. The list of ContainerState, which are the checkpointed conttainers.
2. The hashset of orphans, which are returned from the launcher::recover.
However, to support nested containers in Mesos Pod, this interface is not sufficient. Because unknown nested containers may exist under either the top level alive container or orphan container. We have to include a full list of unknown containers which includes containers from all hierarchy.
We could have added a 3rd parameter to the isolator::recover interface, to guarantee the backward compatibility. However, considering the potential interface changes in the future work and the old orphan hashset should be deprecated, it is the right time to introduce a new protobuf message `ContainerRecoverInfo` for isolator::recover(), which wraps all information for isolators to recover containers.