eth2.0-specs/specs/phase1/fork-choice.md

Ethereum 2.0 Phase 1 -- Beacon Chain Fork Choice

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

Table of contents

• Introduction
• Fork choice
  • Helpers
    • Extended LatestMessage
    • Updated update_latest_messages
  • Handlers

Introduction

This document is the beacon chain fork choice spec for part of Ethereum 2.0 Phase 1.

Fork choice

Due to the changes in the structure of IndexedAttestation in Phase 1, on_attestation must be re-specified to handle this. The bulk of on_attestation has been moved out into a few helpers to reduce code duplication where possible.

The rest of the fork choice remains stable.

Helpers

Extended LatestMessage

@dataclass(eq=True, frozen=True)
class LatestMessage(object):
    epoch: Epoch
    root: Root
    shard: Shard
    shard_root: Root

Updated update_latest_messages

def update_latest_messages(store: Store, attesting_indices: Sequence[ValidatorIndex], attestation: Attestation) -> None:
    target = attestation.data.target
    beacon_block_root = attestation.data.beacon_block_root
    # TODO: separate shard chain vote
    shard = attestation.data.shard
    for i in attesting_indices:
        if i not in store.latest_messages or target.epoch > store.latest_messages[i].epoch:
            store.latest_messages[i] = LatestMessage(
                epoch=target.epoch, root=beacon_block_root, shard=shard, shard_root=attestation.data.shard_head_root
            )

Handlers

def on_attestation(store: Store, attestation: Attestation) -> None:
    """
    Run ``on_attestation`` upon receiving a new ``attestation`` from either within a block or directly on the wire.

    An ``attestation`` that is asserted as invalid may be valid at a later time,
    consider scheduling it for later processing in such case.
    """
    validate_on_attestation(store, attestation)
    store_target_checkpoint_state(store, attestation.data.target)

    # Get state at the `target` to fully validate attestation
    target_state = store.checkpoint_states[attestation.data.target]
    indexed_attestation = get_indexed_attestation(target_state, attestation)
    assert is_valid_indexed_attestation(target_state, indexed_attestation)

    # Update latest messages for attesting indices
    attesting_indices = [
        index for i, index in enumerate(indexed_attestation.committee)
        if attestation.aggregation_bits[i]
    ]
    update_latest_messages(store, attesting_indices, attestation)