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Electra Light Client -- Fork Logic

Introduction
Helper functions
- normalize_merkle_branch
Upgrading light client data
Upgrading the store

Introduction

This document describes how to upgrade existing light client objects based on the Deneb specification to Electra. This is necessary when processing pre-Electra data with a post-Electra LightClientStore. Note that the data being exchanged over the network protocols uses the original format.

Helper functions

`normalize_merkle_branch`

def normalize_merkle_branch(branch: Sequence[Bytes32],
                            gindex: GeneralizedIndex) -> Sequence[Bytes32]:
    depth = floorlog2(gindex)
    num_extra = depth - len(branch)
    return [Bytes32()] * num_extra + [*branch]

Upgrading light client data

A Electra LightClientStore can still process earlier light client data. In order to do so, that pre-Electra data needs to be locally upgraded to Electra before processing.

def upgrade_lc_header_to_electra(pre: deneb.LightClientHeader) -> LightClientHeader:
    return LightClientHeader(
        beacon=pre.beacon,
        execution=pre.execution,
        execution_branch=pre.execution_branch,
    )

def upgrade_lc_bootstrap_to_electra(pre: deneb.LightClientBootstrap) -> LightClientBootstrap:
    return LightClientBootstrap(
        header=upgrade_lc_header_to_electra(pre.header),
        current_sync_committee=pre.current_sync_committee,
        current_sync_committee_branch=normalize_merkle_branch(
            pre.current_sync_committee_branch, CURRENT_SYNC_COMMITTEE_GINDEX_ELECTRA),
    )

def upgrade_lc_update_to_electra(pre: deneb.LightClientUpdate) -> LightClientUpdate:
    return LightClientUpdate(
        attested_header=upgrade_lc_header_to_electra(pre.attested_header),
        next_sync_committee=pre.next_sync_committee,
        next_sync_committee_branch=normalize_merkle_branch(
            pre.next_sync_committee_branch, NEXT_SYNC_COMMITTEE_GINDEX_ELECTRA),
        finalized_header=upgrade_lc_header_to_electra(pre.finalized_header),
        finality_branch=normalize_merkle_branch(
            pre.finality_branch, FINALIZED_ROOT_GINDEX_ELECTRA),
        sync_aggregate=pre.sync_aggregate,
        signature_slot=pre.signature_slot,
    )

def upgrade_lc_finality_update_to_electra(pre: deneb.LightClientFinalityUpdate) -> LightClientFinalityUpdate:
    return LightClientFinalityUpdate(
        attested_header=upgrade_lc_header_to_electra(pre.attested_header),
        finalized_header=upgrade_lc_header_to_electra(pre.finalized_header),
        finality_branch=normalize_merkle_branch(
            pre.finality_branch, FINALIZED_ROOT_GINDEX_ELECTRA),
        sync_aggregate=pre.sync_aggregate,
        signature_slot=pre.signature_slot,
    )

def upgrade_lc_optimistic_update_to_electra(pre: deneb.LightClientOptimisticUpdate) -> LightClientOptimisticUpdate:
    return LightClientOptimisticUpdate(
        attested_header=upgrade_lc_header_to_electra(pre.attested_header),
        sync_aggregate=pre.sync_aggregate,
        signature_slot=pre.signature_slot,
    )

Upgrading the store

Existing LightClientStore objects based on Deneb MUST be upgraded to Electra before Electra based light client data can be processed. The LightClientStore upgrade MAY be performed before ELECTRA_FORK_EPOCH.

def upgrade_lc_store_to_electra(pre: deneb.LightClientStore) -> LightClientStore:
    if pre.best_valid_update is None:
        best_valid_update = None
    else:
        best_valid_update = upgrade_lc_update_to_electra(pre.best_valid_update)
    return LightClientStore(
        finalized_header=upgrade_lc_header_to_electra(pre.finalized_header),
        current_sync_committee=pre.current_sync_committee,
        next_sync_committee=pre.next_sync_committee,
        best_valid_update=best_valid_update,
        optimistic_header=upgrade_lc_header_to_electra(pre.optimistic_header),
        previous_max_active_participants=pre.previous_max_active_participants,
        current_max_active_participants=pre.current_max_active_participants,
    )

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