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Ethereum 2.0 Light Client Support -- From Phase 0 to Light Client Patch

Notice: This document is a work-in-progress for researchers and implementers.

Introduction
Configuration
Fork to Light-client patch
- Fork trigger
- Upgrading the state

Introduction

This document describes the process of moving from Phase 0 to Phase 1 of Ethereum 2.0.

Configuration

Warning: this configuration is not definitive.

Name	Value
`LIGHTCLIENT_PATCH_FORK_VERSION`	`Version('0x01000000')`
`LIGHTCLIENT_PATCH_FORK_SLOT`	`Slot(0)` TBD

Fork to Light-client patch

Fork trigger

TBD. Social consensus, along with state conditions such as epoch boundary, finality, deposits, active validator count, etc. may be part of the decision process to trigger the fork. For now we assume the condition will be triggered at slot LIGHTCLIENT_PATCH_FORK_SLOT, where LIGHTCLIENT_PATCH_FORK_SLOT % SLOTS_PER_EPOCH == 0.

Upgrading the state

After process_slots of Phase 0 finishes, if state.slot == LIGHTCLIENT_PATCH_FORK_SLOT, an irregular state change is made to upgrade to light-client patch.

def upgrade_to_lightclient_patch(pre: phase0.BeaconState) -> BeaconState:
    epoch = get_current_epoch(pre)
    post = BeaconState(
        genesis_time=pre.genesis_time,
        slot=pre.slot,
        fork=Fork(
            previous_version=pre.fork.current_version,
            current_version=LIGHTCLIENT_PATCH_FORK_VERSION,
            epoch=epoch,
        ),
        # History
        latest_block_header=pre.latest_block_header,
        block_roots=pre.block_roots,
        state_roots=pre.state_roots,
        historical_roots=pre.historical_roots,
        # Eth1
        eth1_data=pre.eth1_data,
        eth1_data_votes=pre.eth1_data_votes,
        eth1_deposit_index=pre.eth1_deposit_index,
        # Registry
        validators=pre.validators,
        balances=pre.balances,
        # Randomness
        randao_mixes=pre.randao_mixes,
        # Slashings
        slashings=pre.slashings,
        # Attestations
        # previous_epoch_attestations is cleared on upgrade. 
        previous_epoch_attestations=List[PendingAttestation, MAX_ATTESTATIONS * SLOTS_PER_EPOCH](),
        # empty in pre state, since the upgrade is performed just after an epoch boundary.
        current_epoch_attestations=List[PendingAttestation, MAX_ATTESTATIONS * SLOTS_PER_EPOCH](),
        # Finality
        justification_bits=pre.justification_bits,
        previous_justified_checkpoint=pre.previous_justified_checkpoint,
        current_justified_checkpoint=pre.current_justified_checkpoint,
        finalized_checkpoint=pre.finalized_checkpoint,
    )
    # Fill in sync committees
    post.current_sync_committee = get_sync_committee(post, get_current_epoch(post))
    post.next_sync_committee = get_sync_committee(post, get_current_epoch(post) + EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
    return post

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