6.6 KiB
6.6 KiB
Ethereum 2.0 Phase 1 -- Beacon Chain + Shard Chain Fork Choice
Notice: This document is a work-in-progress for researchers and implementers.
Table of contents
Introduction
This document is the shard chain fork choice spec for part of Ethereum 2.0 Phase 1. It assumes the beacon chain fork choice spec.
Fork choice
Helpers
get_forkchoice_shard_store
def get_forkchoice_shard_store(anchor_state: BeaconState, shard: Shard) -> ShardStore:
return ShardStore(
shard=shard,
signed_blocks={
anchor_state.shard_states[shard].latest_block_root: SignedShardBlock(
message=ShardBlock(slot=compute_previous_slot(anchor_state.slot), shard=shard)
)
},
block_states={anchor_state.shard_states[shard].latest_block_root: anchor_state.copy().shard_states[shard]},
)
get_shard_latest_attesting_balance
def get_shard_latest_attesting_balance(store: Store, shard: Shard, root: Root) -> Gwei:
shard_store = store.shard_stores[shard]
state = store.checkpoint_states[store.justified_checkpoint]
active_indices = get_active_validator_indices(state, get_current_epoch(state))
return Gwei(sum(
state.validators[i].effective_balance for i in active_indices
if (
i in shard_store.latest_messages
# TODO: check the latest message logic: currently, validator's previous vote of another shard
# would be ignored once their newer vote is accepted. Check if it makes sense.
and get_shard_ancestor(
store,
shard,
shard_store.latest_messages[i].root,
shard_store.signed_blocks[root].message.slot,
) == root
)
))
get_shard_head
def get_shard_head(store: Store, shard: Shard) -> Root:
# Execute the LMD-GHOST fork choice
"""
Execute the LMD-GHOST fork choice.
"""
shard_store = store.shard_stores[shard]
beacon_head_root = get_head(store)
shard_head_state = store.block_states[beacon_head_root].shard_states[shard]
shard_head_root = shard_head_state.latest_block_root
shard_blocks = {
root: signed_shard_block.message for root, signed_shard_block in shard_store.signed_blocks.items()
if signed_shard_block.message.slot > shard_head_state.slot
}
while True:
# Find the valid child block roots
children = [
root for root, shard_block in shard_blocks.items()
if shard_block.shard_parent_root == shard_head_root
]
if len(children) == 0:
return shard_head_root
# Sort by latest attesting balance with ties broken lexicographically
shard_head_root = max(
children, key=lambda root: (get_shard_latest_attesting_balance(store, shard, root), root)
)
get_shard_ancestor
def get_shard_ancestor(store: Store, shard: Shard, root: Root, slot: Slot) -> Root:
shard_store = store.shard_stores[shard]
block = shard_store.signed_blocks[root].message
if block.slot > slot:
return get_shard_ancestor(store, shard, block.shard_parent_root, slot)
elif block.slot == slot:
return root
else:
# root is older than queried slot, thus a skip slot. Return most recent root prior to slot
return root
get_pending_shard_blocks
def get_pending_shard_blocks(store: Store, shard: Shard) -> Sequence[SignedShardBlock]:
"""
Return the canonical shard block branch that has not yet been crosslinked.
"""
shard_store = store.shard_stores[shard]
beacon_head_root = get_head(store)
beacon_head_state = store.block_states[beacon_head_root]
latest_shard_block_root = beacon_head_state.shard_states[shard].latest_block_root
shard_head_root = get_shard_head(store, shard)
root = shard_head_root
signed_shard_blocks = []
while root != latest_shard_block_root:
signed_shard_block = shard_store.signed_blocks[root]
signed_shard_blocks.append(signed_shard_block)
root = signed_shard_block.message.shard_parent_root
signed_shard_blocks.reverse()
return signed_shard_blocks
Handlers
on_shard_block
def on_shard_block(store: Store, signed_shard_block: SignedShardBlock) -> None:
shard_block = signed_shard_block.message
shard = shard_block.shard
shard_store = store.shard_stores[shard]
# Check shard parent exists
assert shard_block.shard_parent_root in shard_store.block_states
shard_parent_state = shard_store.block_states[shard_block.shard_parent_root]
# Check beacon parent exists
assert shard_block.beacon_parent_root in store.block_states
beacon_parent_state = store.block_states[shard_block.beacon_parent_root]
# Check that block is later than the finalized shard state slot (optimization to reduce calls to get_ancestor)
finalized_beacon_state = store.block_states[store.finalized_checkpoint.root]
finalized_shard_state = finalized_beacon_state.shard_states[shard]
assert shard_block.slot > finalized_shard_state.slot
# Check block is a descendant of the finalized block at the checkpoint finalized slot
finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
assert (
get_ancestor(store, shard_block.beacon_parent_root, finalized_slot) == store.finalized_checkpoint.root
)
# Check the block is valid and compute the post-state
shard_state = shard_parent_state.copy()
shard_state_transition(shard_state, signed_shard_block, beacon_parent_state, validate_result=True)
# Add new block to the store
# Note: storing `SignedShardBlock` format for computing `ShardTransition.proposer_signature_aggregate`
shard_store.signed_blocks[hash_tree_root(shard_block)] = signed_shard_block
# Add new state for this block to the store
shard_store.block_states[hash_tree_root(shard_block)] = shard_state