[resharding] Include state_changes in ApplySplitStateResult for split_state

[shreyan-gupta]

For more context please view #9418

During resharding we need to split the trie/state changes across the child shards. This is mainly handled by the tries.apply_state_changes_to_split_states function which takes the set of trie state updates and distributes it as trie updates across the child shards as per account id.

During pre-processing runtime.apply_update_to_split_states function calls tries.apply_state_changes_to_split_states and saves the state changes as ApplySplitStateResult which would later be applied to the child shards during postprocessing after catchup.

Note that the WrappedTrieChanges (for each child shard) also contains the field called state_changes which wasn't being populated. This would eventually be used in the process_split_state function where we apply and commit the changes to the child shards to DB. We would use the state changes to update flat storage.

[wacban]

Thanks for the nice description! Makes sense.

I couldn't find the reference to state_changes in the process_split_state, can you point me to where is it? Or is that only going to be added later when you also ApplySplitStateResult to the flat storage?

[shreyan-gupta]

@wacban, right, so, this task is to populate state_changes in ApplySplitStateResult and the actual use of the state_changes is a part of this PR #9421

Specifically, we use it here as result.trie_changes.state_changes()

                    if let Some(manager) = self.runtime_adapter.get_flat_storage_manager() {
                        let store_update = manager.save_flat_state_changes(
                            *block_hash,
                            *prev_hash,
                            block.header().height(),
                            result.shard_uid,
                            result.trie_changes.state_changes(),
                        )?;
                        manager.update_flat_storage_for_shard(*shard_uid, block)?;
                        self.chain_store_update.merge(store_update);
                    }