[ICS02] Design flaw of verify_upgrade_client function of ClientState trait

[riversyang]

Problem Statement

While implementing ICS12 (client for NEAR protocol) based on ibc-rs, I found that the verify_upgrade_client function of ClientState trait will NOT work when the client needs to use a proof verification algorithm other than Merkle proof.

ibc-rs/crates/ibc/src/core/ics02_client/client_state.rs

Lines 124 to 131 in 1c5b50b

	fn verify_upgrade_client(
	&self,
	upgraded_client_state: Any,
	upgraded_consensus_state: Any,
	proof_upgrade_client: MerkleProof,
	proof_upgrade_consensus_state: MerkleProof,
	root: &CommitmentRoot,
	) -> Result<(), ClientError>;

As in NEAR protocol, the state proof is MPT proof rather than Merkle proof, we can not construct a Merkle proof for this function. But we can implement verify_membership and verify_non_membership functions for NEAR client like I did here, because they are using generic data type for proofs.

Solution Proposal

To resolve this issue, the proposed solution is to change the data types of proof_upgrade_client and proof_upgrade_consensus_state to CommitmentProofBytes, similar to the design of verify_membership and verify_non_membership. This modification will allow the implementation of clients to choose their own verification algorithms.

[Farhad-Shabani]

We will handle it through #748
Thanks a lot for pointing this out 🙏