Split CheckHeaderAndUpdateState into 4 functions

[colin-axner]

Summary

The CheckHeaderAndUpdateState is currently overloaded in functionality. It can be easy for light-client developers to unknowingly allow bugs. We should split the functionality up such that light client developers are explicitly told what they need to account for

Problem Definition

CheckHeaderAndUpdateState requires the light client to:

• verify the header
• account for duplicate headers
• automatically detect misbehaviour
• return updated client and consensus states

Detecting misbehaviour and accounting for duplicate headers are easy to overlook, but are vital to the security of a light client implementation.

## Proposal

Split CheckHeaderAndUpdateState into:
- VerifyHeader
- CheckHeaderForMisbehaviour
- UpdateStateFromHeader

Updated proposal

ref

Split CheckHeaderAndUpdateState into 4 functions:

• VerifyHeader
• CheckForMisbehaviour
• UpdateStateOnMisbehaviour
• UpdateState

VerifyHeader: checks header signatures, ensuring everything is constructed correctly and is valid. Should be similar to 07-tendermint checkValidity. Should only return an error

CheckForMisbehaviour: checks to see if a header is evidence of misbehaviour. In 07-tendermint, it'd be these checks. Should return a boolean (foundMisbehaviour)

UpdateStateOnMisbehaviour: freeze the client and updates its state accordingly

UpdateState: Perform a regular update (may do a no-op for duplicate updates)

Splitting PR Recommendation

PR for each light client for each function

Make a PRs for each light client

• solomachine Add 668 functions to solo machine #878
• tendermint Add 668 functions to 07-tendermint #879
• localhost Add new functions from issue 668 to localhost #877

Replace CheckHeaderAndUpdateState with new functions

Add the 4 new functions to the ClientState interface, remove CheckHeaderAndUpdateState and update 02-client code to use the new functions.

Code:

    if err := clientState.VerifyHeader(header); err != nil {
        return err
    }
    
    foundMisbehaviour := clientState.CheckForMisbehaviour(header)
    if foundMisbehaviour {
        clientState.UpdateStateOnMisbehaviour(header)
        // emit misbehaviour event
        return 
    }
    
    clientState.UpdateState(header) // expects no-op on duplicate header
    // emit update event
    return
}

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By splitting the PRs up this way, reviewers don't need to switch context on light client funcitonality or light client vs 02-client.

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged/assigned