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nimbus-eth2/beacon_chain/spec/beaconstate.nim

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# beacon_chain
# Copyright (c) 2018-2019 Status Research & Development GmbH
# Licensed and distributed under either of
# * MIT license (license terms in the root directory or at http://opensource.org/licenses/MIT).
# * Apache v2 license (license terms in the root directory or at http://www.apache.org/licenses/LICENSE-2.0).
# at your option. This file may not be copied, modified, or distributed except according to those terms.
import
chronicles, math, options, sequtils,
../extras, ../ssz,
./bitfield, ./crypto, ./datatypes, ./digest, ./helpers, ./validator
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#get_effective_balance
func get_effective_balance*(state: BeaconState, index: ValidatorIndex): Gwei =
## Return the effective balance (also known as "balance at stake") for a
## validator with the given ``index``.
min(state.validator_balances[index], MAX_DEPOSIT_AMOUNT)
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#verify_merkle_branch
func verify_merkle_branch(leaf: Eth2Digest, proof: openarray[Eth2Digest], depth: uint64, index: uint64, root: Eth2Digest): bool =
## Verify that the given ``leaf`` is on the merkle branch ``proof``
## starting with the given ``root``.
var
value = leaf
buf: array[64, byte]
for i in 0 ..< depth.int:
if (index div (1'u64 shl i)) mod 2 != 0:
buf[0..31] = proof[i.int].data
buf[32..63] = value.data
else:
buf[0..31] = value.data
buf[32..63] = proof[i.int].data
value = eth2hash(buf)
value == root
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.1/specs/core/0_beacon-chain.md#process_deposit
func process_deposit*(state: var BeaconState, deposit: Deposit): bool =
## Process a deposit from Ethereum 1.0.
## Note that this function mutates ``state``.
let deposit_input = deposit.deposit_data.deposit_input
## Should equal 8 bytes for deposit_data.amount +
## 8 bytes for deposit_data.timestamp +
## 176 bytes for deposit_data.deposit_input
## It should match the deposit_data in the eth1.0 deposit contract
## TODO actual serialize func useful after all
var serialized_deposit_data: array[8 + 8 + 176, byte]
serialized_deposit_data[0..7] = deposit.deposit_data.amount.int_to_bytes8()
serialized_deposit_data[8..15] =
deposit.deposit_data.timestamp.int_to_bytes8()
serialized_deposit_data[16..63] = deposit_input.pubkey.getBytes()
serialized_deposit_data[64..95] = deposit_input.withdrawal_credentials.data
serialized_deposit_data[96..191] =
deposit_input.proof_of_possession.getBytes()
# Verify the Merkle branch
let merkle_branch_is_valid = verify_merkle_branch(
eth2hash(serialized_deposit_data),
deposit.proof,
DEPOSIT_CONTRACT_TREE_DEPTH,
deposit.index,
state.latest_eth1_data.deposit_root)
## TODO enable this check, after using merkle_root (not in spec anymore, but
## useful to construct proofs) to build proofs (i.e. the other child in each
## pair of children at each level of the merkle tree), and injecting a proof
## sequence corresponding to their hash values, into the `Deposits`, in that
## tests/testutil.nim area of code. Currently it's checking against garbage,
## either when creating genesis states or in the block processing of deposit
## lists from Eth1Data.
# doAssert merkle_branch_is_valid
## Increment the next deposit index we are expecting. Note that this
## needs to be done here because while the deposit contract will never
## create an invalid Merkle branch, it may admit an invalid deposit
## object, and we need to be able to skip over it
state.deposit_index += 1
## if not validate_proof_of_possession(
## state, pubkey, proof_of_possession, withdrawal_credentials):
## return
## TODO re-enable (but it wasn't running to begin with, and
## PoP isn't really a phase 0 concern, so this isn't meaningful
## regardless.
let
validator_pubkeys = state.validator_registry.mapIt(it.pubkey)
pubkey = deposit_input.pubkey
amount = deposit.deposit_data.amount
withdrawal_credentials = deposit_input.withdrawal_credentials
if pubkey notin validator_pubkeys:
# Add new validator
let validator = Validator(
pubkey: pubkey,
withdrawal_credentials: withdrawal_credentials,
activation_epoch: FAR_FUTURE_EPOCH,
exit_epoch: FAR_FUTURE_EPOCH,
withdrawable_epoch: FAR_FUTURE_EPOCH,
initiated_exit: false,
slashed: false,
)
## Note: In phase 2 registry indices that have been withdrawn for a long
## time will be recycled.
state.validator_registry.add(validator)
state.validator_balances.add(amount)
else:
# Increase balance by deposit amount
let index = validator_pubkeys.find(pubkey)
let validator = addr state.validator_registry[index]
doAssert state.validator_registry[index].withdrawal_credentials ==
withdrawal_credentials
state.validator_balances[index] += amount
true
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#get_delayed_activation_exit_epoch
func get_delayed_activation_exit_epoch*(epoch: Epoch): Epoch =
## Return the epoch at which an activation or exit triggered in ``epoch``
## takes effect.
epoch + 1 + ACTIVATION_EXIT_DELAY
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#activate_validator
func activate_validator(state: var BeaconState,
index: ValidatorIndex,
is_genesis: bool) =
## Activate the validator with the given ``index``.
## Note that this function mutates ``state``.
let validator = addr state.validator_registry[index]
validator.activation_epoch =
if is_genesis:
GENESIS_EPOCH
else:
get_delayed_activation_exit_epoch(get_current_epoch(state))
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#initiate_validator_exit
func initiate_validator_exit*(state: var BeaconState,
index: ValidatorIndex) =
## Initiate exit for the validator with the given ``index``.
## Note that this function mutates ``state``.
var validator = addr state.validator_registry[index]
validator.initiated_exit = true
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#exit_validator
func exit_validator*(state: var BeaconState,
index: ValidatorIndex) =
## Exit the validator with the given ``index``.
## Note that this function mutates ``state``.
let
validator = addr state.validator_registry[index]
delayed_activation_exit_epoch =
get_delayed_activation_exit_epoch(get_current_epoch(state))
# The following updates only occur if not previous exited
if validator.exit_epoch <= delayed_activation_exit_epoch:
return
validator.exit_epoch = delayed_activation_exit_epoch
func reduce_balance*(balance: var uint64, amount: uint64) =
# Not in spec, but useful to avoid underflow.
balance -= min(amount, balance)
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#slash_validator
func slash_validator*(state: var BeaconState, index: ValidatorIndex) =
## Slash the validator with index ``index``.
## Note that this function mutates ``state``.
let validator = addr state.validator_registry[index]
doAssert state.slot < get_epoch_start_slot(validator.withdrawable_epoch) ##\
## [TO BE REMOVED IN PHASE 2]
exit_validator(state, index)
state.latest_slashed_balances[
(get_current_epoch(state) mod LATEST_SLASHED_EXIT_LENGTH).int
] += get_effective_balance(state, index)
let
whistleblower_index = get_beacon_proposer_index(state, state.slot)
whistleblower_reward = get_effective_balance(state, index) div
WHISTLEBLOWER_REWARD_QUOTIENT
## TODO here and elsewhere, if reduce_balance can't reduce balance by full
## whistleblower_reward (to prevent underflow) should increase be full? It
## seems wrong for the amounts to differ.
state.validator_balances[whistleblower_index] += whistleblower_reward
reduce_balance(state.validator_balances[index], whistleblower_reward)
validator.slashed = true
validator.withdrawable_epoch =
get_current_epoch(state) + LATEST_SLASHED_EXIT_LENGTH
func update_shuffling_cache*(state: var BeaconState) =
let
list_size = state.validator_registry.len.uint64
shuffling_seq = mapIt(
get_shuffled_seq(state.current_shuffling_seed, list_size),
# No intrinsic reason for this conversion; SSZ requirement artifact.
it.int)
doAssert state.shuffling_cache.index in [0, 1]
# Do a dance to keep everything JSON-encodable.
state.shuffling_cache.seeds[state.shuffling_cache.index] =
state.current_shuffling_seed
state.shuffling_cache.list_sizes[state.shuffling_cache.index] = list_size
if state.shuffling_cache.index == 0:
state.shuffling_cache.shuffling_0 = shuffling_seq
else:
state.shuffling_cache.shuffling_1 = shuffling_seq
state.shuffling_cache.index = 1 - state.shuffling_cache.index
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.1/specs/core/0_beacon-chain.md#get_temporary_block_header
func get_temporary_block_header*(blck: BeaconBlock): BeaconBlockHeader =
## Return the block header corresponding to a block with ``state_root`` set
## to ``ZERO_HASH``.
BeaconBlockHeader(
slot: blck.slot,
previous_block_root: blck.previous_block_root,
state_root: ZERO_HASH,
block_body_root: hash_tree_root(blck.body),
# signed_root(block) is used for block id purposes so signature is a stub
signature: EMPTY_SIGNATURE,
)
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.1/specs/core/0_beacon-chain.md#on-genesis
func get_empty_block*(): BeaconBlock =
# Nim default values fill this in mostly correctly.
BeaconBlock(slot: GENESIS_SLOT)
func get_genesis_beacon_state*(
genesis_validator_deposits: openArray[Deposit],
genesis_time: uint64,
genesis_eth1_data: Eth1Data,
flags: UpdateFlags = {}): BeaconState =
## Get the genesis ``BeaconState``.
##
## Before the beacon chain starts, validators will register in the Eth1 chain
## and deposit ETH. When enough many validators have registered, a
## `ChainStart` log will be emitted and the beacon chain can start beaconing.
##
## Because the state root hash is part of the genesis block, the beacon state
## must be calculated before creating the genesis block.
# Induct validators
# Not in spec: the system doesn't work unless there are at least SLOTS_PER_EPOCH
# validators - there needs to be at least one member in each committee -
# good to know for testing, though arguably the system is not that useful at
# at that point :)
doAssert genesis_validator_deposits.len >= SLOTS_PER_EPOCH
var state = BeaconState(
# Misc
slot: GENESIS_SLOT,
genesis_time: genesis_time,
fork: Fork(
previous_version: GENESIS_FORK_VERSION,
current_version: GENESIS_FORK_VERSION,
epoch: GENESIS_EPOCH,
),
validator_registry_update_epoch: GENESIS_EPOCH,
# validator_registry and validator_balances automatically initalized
# Randomness and committees
# latest_randao_mixes automatically initialized
previous_shuffling_start_shard: GENESIS_START_SHARD,
current_shuffling_start_shard: GENESIS_START_SHARD,
previous_shuffling_epoch: GENESIS_EPOCH,
current_shuffling_epoch: GENESIS_EPOCH,
previous_shuffling_seed: ZERO_HASH,
current_shuffling_seed: ZERO_HASH,
# Finality
# previous_epoch_attestations and current_epoch_attestations automatically
# initialized
previous_justified_epoch: GENESIS_EPOCH,
current_justified_epoch: GENESIS_EPOCH,
justification_bitfield: 0,
finalized_epoch: GENESIS_EPOCH,
finalized_root: ZERO_HASH,
# Recent state
# latest_block_roots, latest_state_roots, latest_active_index_roots,
# latest_slashed_balances, and latest_slashed_balances automatically
# initialized
latest_block_header: get_temporary_block_header(get_empty_block()),
# Ethereum 1.0 chain data
# eth1_data_votes automatically initialized
latest_eth1_data: genesis_eth1_data,
deposit_index: 0,
)
for i in 0 ..< SHARD_COUNT:
state.latest_crosslinks[i] = Crosslink(
epoch: GENESIS_EPOCH, crosslink_data_root: ZERO_HASH)
# Process genesis deposits
for deposit in genesis_validator_deposits:
discard process_deposit(state, deposit)
# Process genesis activations
for validator_index in 0 ..< state.validator_registry.len:
let vi = validator_index.ValidatorIndex
if get_effective_balance(state, vi) >= MAX_DEPOSIT_AMOUNT:
activate_validator(state, vi, true)
let genesis_active_index_root = hash_tree_root(
get_active_validator_indices(state.validator_registry, GENESIS_EPOCH))
for index in 0 ..< LATEST_ACTIVE_INDEX_ROOTS_LENGTH:
state.latest_active_index_roots[index] = genesis_active_index_root
state.current_shuffling_seed = generate_seed(state, GENESIS_EPOCH)
# Not in spec.
update_shuffling_cache(state)
state
# TODO candidate for spec?
# https://github.com/ethereum/eth2.0-specs/blob/0.5.1/specs/core/0_beacon-chain.md#on-genesis
func get_initial_beacon_block*(state: BeaconState): BeaconBlock =
BeaconBlock(
slot: GENESIS_SLOT,
state_root: hash_tree_root(state)
# parent_root, randao_reveal, eth1_data, signature, and body automatically
# initialized to default values.
)
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#get_block_root
func get_block_root*(state: BeaconState,
slot: Slot): Eth2Digest =
# Return the block root at a recent ``slot``.
doAssert state.slot <= slot + SLOTS_PER_HISTORICAL_ROOT
doAssert slot < state.slot
state.latest_block_roots[slot mod SLOTS_PER_HISTORICAL_ROOT]
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.1/specs/core/0_beacon-chain.md#get_attestation_participants
func get_attestation_participants*(state: BeaconState,
attestation_data: AttestationData,
bitfield: BitField): seq[ValidatorIndex] =
## Return the participant indices at for the ``attestation_data`` and
## ``bitfield``.
## Attestation participants in the attestation data are called out in a
## bit field that corresponds to the committee of the shard at the time;
## this function converts it to list of indices in to BeaconState.validators
##
## Returns empty list if the shard is not found
## Return the participant indices at for the ``attestation_data`` and ``bitfield``.
##
# TODO Linear search through shard list? borderline ok, it's a small list
# TODO iterator candidate
# Find the committee in the list with the desired shard
let crosslink_committees = get_crosslink_committees_at_slot(
state, attestation_data.slot)
doAssert anyIt(
crosslink_committees,
it[1] == attestation_data.shard)
let crosslink_committee = mapIt(
filterIt(crosslink_committees, it.shard == attestation_data.shard),
it.committee)[0]
# TODO this and other attestation-based fields need validation so we don't
# crash on a malicious attestation!
doAssert verify_bitfield(bitfield, len(crosslink_committee))
# Find the participating attesters in the committee
result = @[]
for i, validator_index in crosslink_committee:
let aggregation_bit = get_bitfield_bit(bitfield, i)
if aggregation_bit:
result.add(validator_index)
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#ejections
func process_ejections*(state: var BeaconState) =
## Iterate through the validator registry and eject active validators with
## balance below ``EJECTION_BALANCE``
for index in get_active_validator_indices(
state.validator_registry, get_current_epoch(state)):
if state.validator_balances[index] < EJECTION_BALANCE:
exit_validator(state, index)
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#get_total_balance
func get_total_balance*(state: BeaconState, validators: auto): Gwei =
# Return the combined effective balance of an array of validators.
foldl(validators, a + get_effective_balance(state, b), 0'u64)
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#validator-registry-and-shuffling-seed-data
func should_update_validator_registry*(state: BeaconState): bool =
# Must have finalized a new block
if state.finalized_epoch <= state.validator_registry_update_epoch:
return false
# Must have processed new crosslinks on all shards of the current epoch
allIt(0 ..< get_current_epoch_committee_count(state).int,
not (state.latest_crosslinks[
((state.current_shuffling_start_shard + it.uint64) mod
SHARD_COUNT).int].epoch <= state.validator_registry_update_epoch))
func update_validator_registry*(state: var BeaconState) =
## Update validator registry.
## Note that this function mutates ``state``.
let
current_epoch = get_current_epoch(state)
# The active validators
active_validator_indices =
get_active_validator_indices(state.validator_registry, current_epoch)
# The total effective balance of active validators
total_balance = get_total_balance(state, active_validator_indices)
# The maximum balance churn in Gwei (for deposits and exits separately)
max_balance_churn = max(
MAX_DEPOSIT_AMOUNT,
total_balance div (2 * MAX_BALANCE_CHURN_QUOTIENT)
)
# Activate validators within the allowable balance churn
var balance_churn = 0'u64
for index, validator in state.validator_registry:
if validator.activation_epoch == FAR_FUTURE_EPOCH and
state.validator_balances[index] >= MAX_DEPOSIT_AMOUNT:
# Check the balance churn would be within the allowance
balance_churn += get_effective_balance(state, index.ValidatorIndex)
if balance_churn > max_balance_churn:
break
# Activate validator
activate_validator(state, index.ValidatorIndex, false)
# Exit validators within the allowable balance churn
balance_churn = 0
for index, validator in state.validator_registry:
if validator.activation_epoch == FAR_FUTURE_EPOCH and
validator.initiated_exit:
# Check the balance churn would be within the allowance
balance_churn += get_effective_balance(state, index.ValidatorIndex)
if balance_churn > max_balance_churn:
break
# Exit validator
exit_validator(state, index.ValidatorIndex)
state.validator_registry_update_epoch = current_epoch
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.1/specs/core/0_beacon-chain.md#attestations
proc checkAttestation*(
state: var BeaconState, attestation: Attestation, flags: UpdateFlags): bool =
## Check that an attestation follows the rules of being included in the state
## at the current slot. When acting as a proposer, the same rules need to
## be followed!
# Can't submit attestations that are too far in history (or in prehistory)
if not (attestation.data.slot >= GENESIS_SLOT):
warn("Attestation predates genesis slot",
attestation_slot = attestation.data.slot,
state_slot = humaneSlotNum(state.slot))
return
if not (state.slot <= attestation.data.slot + SLOTS_PER_EPOCH):
warn("Attestation too old",
attestation_slot = humaneSlotNum(attestation.data.slot),
state_slot = humaneSlotNum(state.slot))
return
# Can't submit attestations too quickly
if not (
attestation.data.slot + MIN_ATTESTATION_INCLUSION_DELAY <= state.slot):
warn("Can't submit attestations too quickly",
attestation_slot = humaneSlotNum(attestation.data.slot),
state_slot = humaneSlotNum(state.slot))
return
# # Verify that the justified epoch and root is correct
if slot_to_epoch(attestation.data.slot) >= get_current_epoch(state):
# Case 1: current epoch attestations
if not (attestation.data.source_epoch == state.current_justified_epoch):
warn("Source epoch is not current justified epoch",
attestation_slot = humaneSlotNum(attestation.data.slot),
state_slot = humaneSlotNum(state.slot))
return
if not (attestation.data.source_root == state.current_justified_root):
warn("Source root is not current justified root",
attestation_slot = humaneSlotNum(attestation.data.slot),
state_slot = humaneSlotNum(state.slot))
return
else:
# Case 2: previous epoch attestations
if not (attestation.data.source_epoch == state.previous_justified_epoch):
warn("Source epoch is not previous justified epoch",
attestation_slot = humaneSlotNum(attestation.data.slot),
state_slot = humaneSlotNum(state.slot))
return
if not (attestation.data.source_root == state.previous_justified_root):
warn("Source root is not previous justified root",
attestation_slot = humaneSlotNum(attestation.data.slot),
state_slot = humaneSlotNum(state.slot))
return
# Check that the crosslink data is valid
let acceptable_crosslink_data = @[
# Case 1: Latest crosslink matches the one in the state
attestation.data.previous_crosslink,
# Case 2: State has already been updated, state's latest crosslink matches
# the crosslink the attestation is trying to create
Crosslink(
crosslink_data_root: attestation.data.crosslink_data_root,
epoch: slot_to_epoch(attestation.data.slot)
)
]
if not (state.latest_crosslinks[attestation.data.shard] in
acceptable_crosslink_data):
warn("Unexpected crosslink shard",
state_latest_crosslinks_attestation_data_shard =
state.latest_crosslinks[attestation.data.shard],
attestation_data_previous_crosslink = attestation.data.previous_crosslink,
epoch = humaneEpochNum(slot_to_epoch(attestation.data.slot)),
actual_epoch = slot_to_epoch(attestation.data.slot),
crosslink_data_root = attestation.data.crosslink_data_root,
acceptable_crosslink_data = acceptable_crosslink_data)
return
# Attestation must be nonempty!
doAssert anyIt(attestation.aggregation_bitfield.bits, it != 0)
# Custody must be empty (to be removed in phase 1)
doAssert allIt(attestation.custody_bitfield.bits, it == 0)
# Get the committee for the specific shard that this attestation is for
let crosslink_committee = mapIt(
filterIt(get_crosslink_committees_at_slot(state, attestation.data.slot),
it.shard == attestation.data.shard),
it.committee)[0]
# Custody bitfield must be a subset of the attestation bitfield
doAssert allIt(0 ..< len(crosslink_committee),
if not get_bitfield_bit(attestation.aggregation_bitfield, it):
not get_bitfield_bit(attestation.custody_bitfield, it)
else:
true)
# Verify aggregate signature
let
participants = get_attestation_participants(
state, attestation.data, attestation.aggregation_bitfield)
## TODO when the custody_bitfield assertion-to-emptiness disappears do this
## and fix the custody_bit_0_participants check to depend on it.
# custody_bit_1_participants = {nothing, always, because assertion above}
custody_bit_1_participants: seq[ValidatorIndex] = @[]
custody_bit_0_participants = participants
if skipValidation notin flags:
# Verify that aggregate_signature verifies using the group pubkey.
doAssert bls_verify_multiple(
@[
bls_aggregate_pubkeys(mapIt(custody_bit_0_participants,
state.validator_registry[it].pubkey)),
bls_aggregate_pubkeys(mapIt(custody_bit_1_participants,
state.validator_registry[it].pubkey)),
],
@[
hash_tree_root(AttestationDataAndCustodyBit(
data: attestation.data, custody_bit: false)),
hash_tree_root(AttestationDataAndCustodyBit(
data: attestation.data, custody_bit: true)),
],
attestation.aggregate_signature,
get_domain(state.fork, slot_to_epoch(attestation.data.slot),
DOMAIN_ATTESTATION),
)
# Crosslink data root is zero (to be removed in phase 1)
if attestation.data.crosslink_data_root != ZERO_HASH:
warn("Invalid crosslink data root")
return
true
# https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#prepare_validator_for_withdrawal
func prepare_validator_for_withdrawal*(state: var BeaconState, index: ValidatorIndex) =
## Set the validator with the given ``index`` as withdrawable
## ``MIN_VALIDATOR_WITHDRAWABILITY_DELAY`` after the current epoch.
## Note that this function mutates ``state``.
var validator = addr state.validator_registry[index]
validator.withdrawable_epoch = get_current_epoch(state) +
MIN_VALIDATOR_WITHDRAWABILITY_DELAY
proc makeAttestationData*(
state: BeaconState, shard: uint64,
beacon_block_root: Eth2Digest): AttestationData =
## Fine points:
## Head must be the head state during the slot that validator is
## part of committee - notably, it can't be a newer or older state (!)
# TODO update when https://github.com/ethereum/eth2.0-specs/issues/742
let
epoch_start_slot = get_epoch_start_slot(slot_to_epoch(state.slot))
epoch_boundary_root =
if epoch_start_slot == state.slot: beacon_block_root
else: get_block_root(state, epoch_start_slot)
justified_slot = get_epoch_start_slot(state.current_justified_epoch)
justified_block_root = get_block_root(state, justified_slot)
AttestationData(
slot: state.slot,
shard: shard,
beacon_block_root: beacon_block_root,
crosslink_data_root: Eth2Digest(), # Stub in phase0
previous_crosslink: state.latest_crosslinks[shard],
source_epoch: state.current_justified_epoch,
)
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