How to validate data from new return data value feature stored in block header

Question

How would one go about validating the data from the block header of a return data value action? It seems like a cool feature, but you'd need a full history node to validate wouldn't you?

https://github.com/EOSIO/eos/releases/tag/v2.1.0

Answer 1

In block header, you can find action_mroot. action_mroot is the root hash of merkle tree of action_receipts. action_receipt contains act_digest, and act_digest is constructed by hashing action itself and action_return_value.

You can validate action_return_value by comparing the hash value (of action and its return value) and act_digest in action receipt.

// eos/libraries/chain/apply_context.cpp

act_digest = generate_action_digest(
  [this](const char* data, uint32_t datalen) {
    return trx_context.hash_with_checktime<digest_type>(data, datalen);
  },
  *act,
  action_return_value // here
);