This was caused by changes in
The problem is that in that change, in the RocksDBKeyedBackend, RegisteredStateMetaInfoBase s were no longer created eagerly for all restored state, but instead only lazily created when the state was accessed again by the user. This causes non-accessed restored state to have empty meta info, and throws NPE when trying to take a snapshot of them.
The rationale behind
FLINK-10679 was that, since RegisteredStateMetaInfoBase holds already serializer instances for state access, creating them eagerly at restore time with restored serializer snapshots did not make sense (because at that point-in-time, we do not have the new serializers yet for state access; the snapshot is only capable of creating the previous state serializer).
I propose the following:
Instead of having final TypeSerializer instances in subclasses of RegisteredStateMetaInfoBase, they should have a StateSerializerProvider.
The StateSerializerProvider would have the following methods:
A StateSerializerProvider can be created either from:
1) A restored serializer snapshot when restoring the state.
2) A fresh, new state's serializer, when registering the state for the first time.
For 1), state that has not been accessed yet after the restore will return the same serializer (i.e. the previous serializer) for both getPreviousSerializer and getCurrentSerializer.
Once a restored state is re-accessed, then updateCurrentSerializer(TypeSerializer<T> newSerializer) should be used to update what serializer the provider returns in getCurrentSerializer.
We could also make use of this new abstraction to move away some of the new serializer's compatibility checks from the state backend to StateSerializerProvider#updateCurrentSerializer.
For tests, apparently we're lacking test coverage for restored state that has not been accessed and being snapshotted again. This should be included as part of the fix.