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eth2.0-specs/specs/deneb/light-client/fork.md
Etan Kissling 2035a9fcad
Update light client specifications for Electra 
Electra introduces two changes that affect light client data handling:

1. The `ExecutionPayloadHeader` is extended with new fields.
   This is handled similarly as before with the Deneb fork.

2. The `BeaconState` generalized indices change due to lack of EIP-6493.
   This is handled by making the generalized index be fork dependent via
   a helper function that computes it dynamically. Furthermore, the case
   where pre-Electra light client data is consumed by an Electra based
   `LightClientStore` requires normalizing the shorter proof of the
   pre-Electra data to fit into the Electra data structure by prepending
   a zero hash.
2024-06-21 11:33:52 +02:00

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Deneb Light Client -- Fork Logic

Table of contents

Introduction

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

Upgrading light client data

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

def upgrade_lc_header_to_deneb(pre: capella.LightClientHeader) -> LightClientHeader:
    return LightClientHeader(
        beacon=pre.beacon,
        execution=ExecutionPayloadHeader(
            parent_hash=pre.execution.parent_hash,
            fee_recipient=pre.execution.fee_recipient,
            state_root=pre.execution.state_root,
            receipts_root=pre.execution.receipts_root,
            logs_bloom=pre.execution.logs_bloom,
            prev_randao=pre.execution.prev_randao,
            block_number=pre.execution.block_number,
            gas_limit=pre.execution.gas_limit,
            gas_used=pre.execution.gas_used,
            timestamp=pre.execution.timestamp,
            extra_data=pre.execution.extra_data,
            base_fee_per_gas=pre.execution.base_fee_per_gas,
            block_hash=pre.execution.block_hash,
            transactions_root=pre.execution.transactions_root,
            withdrawals_root=pre.execution.withdrawals_root,
            blob_gas_used=uint64(0),  # [New in Deneb:EIP4844]
            excess_blob_gas=uint64(0),  # [New in Deneb:EIP4844]
        ),
        execution_branch=pre.execution_branch,
    )

def upgrade_lc_bootstrap_to_deneb(pre: capella.LightClientBootstrap) -> LightClientBootstrap:
    return LightClientBootstrap(
        header=upgrade_lc_header_to_deneb(pre.header),
        current_sync_committee=pre.current_sync_committee,
        current_sync_committee_branch=pre.current_sync_committee_branch,
    )

def upgrade_lc_update_to_deneb(pre: capella.LightClientUpdate) -> LightClientUpdate:
    return LightClientUpdate(
        attested_header=upgrade_lc_header_to_deneb(pre.attested_header),
        next_sync_committee=pre.next_sync_committee,
        next_sync_committee_branch=pre.next_sync_committee_branch,
        finalized_header=upgrade_lc_header_to_deneb(pre.finalized_header),
        finality_branch=pre.finality_branch,
        sync_aggregate=pre.sync_aggregate,
        signature_slot=pre.signature_slot,
    )

def upgrade_lc_finality_update_to_deneb(pre: capella.LightClientFinalityUpdate) -> LightClientFinalityUpdate:
    return LightClientFinalityUpdate(
        attested_header=upgrade_lc_header_to_deneb(pre.attested_header),
        finalized_header=upgrade_lc_header_to_deneb(pre.finalized_header),
        finality_branch=pre.finality_branch,
        sync_aggregate=pre.sync_aggregate,
        signature_slot=pre.signature_slot,
    )

def upgrade_lc_optimistic_update_to_deneb(pre: capella.LightClientOptimisticUpdate) -> LightClientOptimisticUpdate:
    return LightClientOptimisticUpdate(
        attested_header=upgrade_lc_header_to_deneb(pre.attested_header),
        sync_aggregate=pre.sync_aggregate,
        signature_slot=pre.signature_slot,
    )

Upgrading the store

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

def upgrade_lc_store_to_deneb(pre: capella.LightClientStore) -> LightClientStore:
    if pre.best_valid_update is None:
        best_valid_update = None
    else:
        best_valid_update = upgrade_lc_update_to_deneb(pre.best_valid_update)
    return LightClientStore(
        finalized_header=upgrade_lc_header_to_deneb(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_deneb(pre.optimistic_header),
        previous_max_active_participants=pre.previous_max_active_participants,
        current_max_active_participants=pre.current_max_active_participants,
    )