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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.
# State transition - epoch processing, as described in
# https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
#
# The purpose of this code right is primarily educational, to help piece
# together the mechanics of the beacon state and to discover potential problem
# areas.
#
# The entry point is `process_epoch` which is at the bottom of this file.
#
# General notes about the code (TODO):
# * It's inefficient - we quadratically copy, allocate and iterate when there
# are faster options
# * Weird styling - the sections taken from the spec use python styling while
# the others use NEP-1 - helps grepping identifiers in spec
# * We mix procedural and functional styles for no good reason, except that the
# spec does so also.
# * There are likely lots of bugs.
# * For indices, we get a mix of uint64, ValidatorIndex and int - this is currently
# swept under the rug with casts
# * The spec uses uint64 for data types, but functions in the spec often assume
# signed bigint semantics - under- and overflows ensue
# * Sane error handling is missing in most cases (yay, we'll get the chance to
# debate exceptions again!)
# When updating the code, add TODO sections to mark where there are clear
# improvements to be made - other than that, keep things similar to spec for
# now.
import # TODO - cleanup imports
algorithm, math, options, sequtils, tables,
stew/[bitseqs, bitops2], chronicles, json_serialization/std/sets,
../extras, ../ssz, ../beacon_node_types,
beaconstate, crypto, datatypes, digest, helpers, validator
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#get_total_active_balance
func get_total_active_balance*(state: BeaconState): Gwei =
return get_total_balance(
state,
get_active_validator_indices(state, get_current_epoch(state)))
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#helper-functions-1
func get_matching_source_attestations(state: BeaconState, epoch: Epoch):
seq[PendingAttestation] =
doAssert epoch in [get_current_epoch(state), get_previous_epoch(state)]
if epoch == get_current_epoch(state):
state.current_epoch_attestations
else:
state.previous_epoch_attestations
func get_matching_target_attestations(state: BeaconState, epoch: Epoch):
seq[PendingAttestation] =
filterIt(
get_matching_source_attestations(state, epoch),
it.data.target.root == get_block_root(state, epoch)
)
func get_matching_head_attestations(state: BeaconState, epoch: Epoch):
seq[PendingAttestation] =
filterIt(
get_matching_source_attestations(state, epoch),
it.data.beacon_block_root ==
get_block_root_at_slot(state, get_attestation_data_slot(state, it.data))
)
func get_attesting_indices(
state: BeaconState, attestations: openarray[PendingAttestation],
stateCache: var StateCache): HashSet[ValidatorIndex] =
result = initSet[ValidatorIndex]()
for a in attestations:
result = result.union(get_attesting_indices(
state, a.data, a.aggregation_bits, stateCache))
func get_unslashed_attesting_indices*(
state: BeaconState, attestations: openarray[PendingAttestation],
stateCache: var StateCache): HashSet[ValidatorIndex] =
result = get_attesting_indices(state, attestations, stateCache)
for index in result:
if state.validators[index].slashed:
result.excl index
func get_attesting_balance(
state: BeaconState, attestations: seq[PendingAttestation],
stateCache: var StateCache): Gwei =
get_total_balance(state, get_unslashed_attesting_indices(
state, attestations, stateCache))
# Not exactly in spec, but for get_winning_crosslink_and_attesting_indices
func lowerThan(candidate, current: Eth2Digest): bool =
# return true iff candidate is "lower" than current, per spec rule:
# "ties broken in favor of lexicographically higher hash
for i, v in current.data:
if v > candidate.data[i]: return true
false
func get_winning_crosslink_and_attesting_indices(
state: BeaconState, epoch: Epoch, shard: Shard,
stateCache: var StateCache): tuple[a: Crosslink, b: HashSet[ValidatorIndex]] =
let
attestations =
filterIt(
get_matching_source_attestations(state, epoch),
it.data.crosslink.shard == shard)
root_current_shard_crosslink =
hash_tree_root(state.current_crosslinks[shard])
crosslinks =
filterIt(
mapIt(attestations, it.data.crosslink),
root_current_shard_crosslink == it.parent_root or
root_current_shard_crosslink == hash_tree_root(it))
# default=Crosslink()
if len(crosslinks) == 0:
return (Crosslink(), initSet[ValidatorIndex]())
## Not from spec. Don't repeatedly search/filter attestations in an O(n^2)
## way, but create lookup table in O(n) time with O(1) lookup by crosslink
## to cut out expensive inner loop.
##
## Could also sort attestations by .data.crosslink first, and rely on that
## ordering, among other approaches which don't change this function sig.
var attesting_indices = initTable[Eth2Digest, HashSet[ValidatorIndex]]()
for attestation in attestations:
let
crosslink = attestation.data.crosslink
crosslink_key = crosslink.data_root
var crosslink_attestation_indices =
if crosslink_key in attesting_indices:
attesting_indices[crosslink_key]
else:
initSet[ValidatorIndex]()
## See also how get_attesting_balance(...) works. This inverts the loop
## nesting order. Also, this ensures no duplicate indices, though it is
## not supposed to happen, regardless, if validators are only attesting
## on their assigned shards. Still, the right response there is slashed
## balances, not crashing clients.
crosslink_attestation_indices.incl(
get_unslashed_attesting_indices(state, [attestation], stateCache))
attesting_indices[crosslink_key] = crosslink_attestation_indices
## Winning crosslink has the crosslink data root with the most balance voting
## for it (ties broken lexicographically)
var
winning_crosslink: Crosslink
winning_crosslink_balance = 0.Gwei
for candidate_crosslink in crosslinks:
## TODO when confident this exactly reproduces the spec version,
## remove the when false'd scaffolding.
when false:
let crosslink_balance_uncached =
get_attesting_balance(
state,
filterIt(attestations, it.data.crosslink == candidate_crosslink),
stateCache)
# TODO verify if one can assume this cached balance always exists here, by
# doAsserting candidate_crosslink_key in attesting_indices
let
candidate_crosslink_key = candidate_crosslink.data_root
crosslink_balance =
if candidate_crosslink_key in attesting_indices:
get_total_balance(state, attesting_indices[candidate_crosslink_key])
else:
## See `get_total_balance(...)`
## But see above, this branch might never happen.
1.Gwei
## TODO factor out precalculation mechanism; consider adding compilation
## flag to enable long calculation & consistency/assumption checking.
when false:
doAssert crosslink_balance == crosslink_balance_uncached
if (crosslink_balance > winning_crosslink_balance or
(winning_crosslink_balance == crosslink_balance and
lowerThan(winning_crosslink.data_root,
candidate_crosslink.data_root))):
winning_crosslink = candidate_crosslink
winning_crosslink_balance = crosslink_balance
let winning_attestations =
filterIt(attestations, it.data.crosslink == winning_crosslink)
(winning_crosslink,
get_unslashed_attesting_indices(state, winning_attestations, stateCache))
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#justification-and-finalization
proc process_justification_and_finalization*(
state: var BeaconState, stateCache: var StateCache) =
if get_current_epoch(state) <= GENESIS_EPOCH + 1:
return
let
previous_epoch = get_previous_epoch(state)
current_epoch = get_current_epoch(state)
old_previous_justified_checkpoint = state.previous_justified_checkpoint
old_current_justified_checkpoint = state.current_justified_checkpoint
# Process justifications
state.previous_justified_checkpoint = state.current_justified_checkpoint
## Spec:
## state.justification_bits[1:] = state.justification_bits[:-1]
## state.justification_bits[0] = 0b0
state.justification_bits = state.justification_bits shl 1
# This is a somewhat expensive approach
let active_validator_indices =
toSet(mapIt(
get_active_validator_indices(state, get_current_epoch(state)), it.int))
let matching_target_attestations_previous =
get_matching_target_attestations(state, previous_epoch) # Previous epoch
## This epoch processing is the last time these previous attestations can
## matter -- in the next epoch, they'll be 2 epochs old, when BeaconState
## tracks current_epoch_attestations and previous_epoch_attestations only
## per
## https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#attestations
## and `get_matching_source_attestations(...)` via
## https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#helper-functions-1
## and
## https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#final-updates
## after which the state.previous_epoch_attestations is replaced.
trace "Non-attesting indices in previous epoch",
missing_all_validators=
difference(active_validator_indices,
toSet(mapIt(get_attesting_indices(state,
matching_target_attestations_previous, stateCache), it.int))),
missing_unslashed_validators=
difference(active_validator_indices,
toSet(mapIt(get_unslashed_attesting_indices(state,
matching_target_attestations_previous, stateCache), it.int))),
prev_attestations_len=len(state.previous_epoch_attestations),
cur_attestations_len=len(state.current_epoch_attestations),
num_active_validators=len(active_validator_indices),
required_balance = get_total_active_balance(state) * 2,
attesting_balance_prev = get_attesting_balance(state, matching_target_attestations_previous, stateCache)
if get_attesting_balance(state, matching_target_attestations_previous,
stateCache) * 3 >= get_total_active_balance(state) * 2:
state.current_justified_checkpoint =
Checkpoint(epoch: previous_epoch,
root: get_block_root(state, previous_epoch))
state.justification_bits.raiseBit 1
let matching_target_attestations_current =
get_matching_target_attestations(state, current_epoch) # Current epoch
if get_attesting_balance(state, matching_target_attestations_current,
stateCache) * 3 >= get_total_active_balance(state) * 2:
state.current_justified_checkpoint =
Checkpoint(epoch: current_epoch,
root: get_block_root(state, current_epoch))
state.justification_bits.raiseBit 0
# Process finalizations
let bitfield = state.justification_bits
## The 2nd/3rd/4th most recent epochs are justified, the 2nd using the 4th
## as source
if (bitfield and 0b1110) == 0b1110 and
old_previous_justified_checkpoint.epoch + 3 == current_epoch:
state.finalized_checkpoint = old_previous_justified_checkpoint
## The 2nd/3rd most recent epochs are justified, the 2nd using the 3rd as
## source
if (bitfield and 0b110) == 0b110 and
old_previous_justified_checkpoint.epoch + 2 == current_epoch:
state.finalized_checkpoint = old_previous_justified_checkpoint
## The 1st/2nd/3rd most recent epochs are justified, the 1st using the 3rd as
## source
if (bitfield and 0b111) == 0b111 and
old_current_justified_checkpoint.epoch + 2 == current_epoch:
state.finalized_checkpoint = old_current_justified_checkpoint
## The 1st/2nd most recent epochs are justified, the 1st using the 2nd as
## source
if (bitfield and 0b11) == 0b11 and
old_current_justified_checkpoint.epoch + 1 == current_epoch:
state.finalized_checkpoint = old_current_justified_checkpoint
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#crosslinks
func process_crosslinks*(state: var BeaconState, stateCache: var StateCache) =
state.previous_crosslinks = state.current_crosslinks
for epoch in @[get_previous_epoch(state), get_current_epoch(state)]:
for offset in 0'u64 ..< get_committee_count(state, epoch):
let
shard = (get_start_shard(state, epoch) + offset) mod SHARD_COUNT
crosslink_committee =
toSet(get_crosslink_committee(state, epoch, shard, stateCache))
# In general, it'll loop over the same shards twice, and
# get_winning_root_and_participants is defined to return
# the same results from the previous epoch as current.
# TODO cache like before, in 0.5 version of this function
(winning_crosslink, attesting_indices) =
get_winning_crosslink_and_attesting_indices(
state, epoch, shard, stateCache)
if 3'u64 * get_total_balance(state, attesting_indices) >=
2'u64 * get_total_balance(state, crosslink_committee):
state.current_crosslinks[shard] = winning_crosslink
# https://github.com/ethereum/eth2.0-specs/blob/v0.7.1/specs/core/0_beacon-chain.md#rewards-and-penalties-1
func get_base_reward(state: BeaconState, index: ValidatorIndex): Gwei =
let
total_balance = get_total_active_balance(state)
effective_balance = state.validators[index].effective_balance
effective_balance * BASE_REWARD_FACTOR div
integer_squareroot(total_balance) div BASE_REWARDS_PER_EPOCH
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#rewards-and-penalties-1
func get_attestation_deltas(state: BeaconState, stateCache: var StateCache):
tuple[a: seq[Gwei], b: seq[Gwei]] =
let
previous_epoch = get_previous_epoch(state)
total_balance = get_total_active_balance(state)
var
rewards = repeat(0'u64, len(state.validators))
penalties = repeat(0'u64, len(state.validators))
eligible_validator_indices : seq[ValidatorIndex] = @[]
for index, v in state.validators:
if is_active_validator(v, previous_epoch) or
(v.slashed and previous_epoch + 1 < v.withdrawable_epoch):
eligible_validator_indices.add index.ValidatorIndex
# Micro-incentives for matching FFG source, FFG target, and head
let
matching_source_attestations =
get_matching_source_attestations(state, previous_epoch)
matching_target_attestations =
get_matching_target_attestations(state, previous_epoch)
matching_head_attestations =
get_matching_head_attestations(state, previous_epoch)
for attestations in
[matching_source_attestations, matching_target_attestations,
matching_head_attestations]:
let
unslashed_attesting_indices =
get_unslashed_attesting_indices(state, attestations, stateCache)
attesting_balance = get_total_balance(state, unslashed_attesting_indices)
for index in eligible_validator_indices:
if index in unslashed_attesting_indices:
rewards[index] +=
get_base_reward(state, index) * attesting_balance div total_balance
else:
penalties[index] += get_base_reward(state, index)
# Early-out not explicitly in spec
if matching_source_attestations.len == 0:
return (rewards, penalties)
# Proposer and inclusion delay micro-rewards
## This depends on matching_source_attestations being an indexable seq, not a
## set, hash table, etc.
let source_attestation_attesting_indices =
mapIt(
matching_source_attestations,
get_attesting_indices(state, it.data, it.aggregation_bits, stateCache))
## TODO if this is still a profiling issue, do higher-level semantic
## translation
for index in get_unslashed_attesting_indices(
state, matching_source_attestations, stateCache):
# Translation of attestation = min([...])
doAssert matching_source_attestations.len > 0
var attestation = matching_source_attestations[0]
for source_attestation_index, a in matching_source_attestations:
if index notin
source_attestation_attesting_indices[source_attestation_index]:
continue
if a.inclusion_delay < attestation.inclusion_delay:
attestation = a
let proposer_reward =
(get_base_reward(state, index) div PROPOSER_REWARD_QUOTIENT).Gwei
rewards[attestation.proposer_index.int] += proposer_reward
let max_attester_reward = get_base_reward(state, index) - proposer_reward
rewards[index] +=
(max_attester_reward *
((SLOTS_PER_EPOCH + MIN_ATTESTATION_INCLUSION_DELAY).uint64 -
attestation.inclusion_delay) div SLOTS_PER_EPOCH).Gwei
# Inactivity penalty
let finality_delay = previous_epoch - state.finalized_checkpoint.epoch
if finality_delay > MIN_EPOCHS_TO_INACTIVITY_PENALTY:
let matching_target_attesting_indices =
get_unslashed_attesting_indices(
state, matching_target_attestations, stateCache)
for index in eligible_validator_indices:
penalties[index] +=
BASE_REWARDS_PER_EPOCH.uint64 * get_base_reward(state, index)
if index notin matching_target_attesting_indices:
penalties[index] +=
state.validators[index].effective_balance *
finality_delay div INACTIVITY_PENALTY_QUOTIENT
(rewards, penalties)
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#rewards-and-penalties-1
func get_crosslink_deltas*(state: BeaconState, cache: var StateCache):
tuple[a: seq[Gwei], b: seq[Gwei]] =
var
rewards = repeat(0'u64, len(state.validators))
penalties = repeat(0'u64, len(state.validators))
let epoch = get_previous_epoch(state)
for offset in 0'u64 ..< get_committee_count(state, epoch):
let
shard = (get_start_shard(state, epoch) + offset) mod SHARD_COUNT
crosslink_committee =
toSet(get_crosslink_committee(state, epoch, shard, cache))
(winning_crosslink, attesting_indices) =
get_winning_crosslink_and_attesting_indices(
state, epoch, shard, cache)
attesting_balance = get_total_balance(state, attesting_indices)
committee_balance = get_total_balance(state, crosslink_committee)
for index in crosslink_committee:
let base_reward = get_base_reward(state, index)
if index in attesting_indices:
rewards[index] +=
base_reward * attesting_balance div committee_balance
else:
penalties[index] += base_reward
(rewards, penalties)
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#rewards-and-penalties-1
func process_rewards_and_penalties(
state: var BeaconState, cache: var StateCache) =
if get_current_epoch(state) == GENESIS_EPOCH:
return
let
(rewards1, penalties1) = get_attestation_deltas(state, cache)
(rewards2, penalties2) = get_crosslink_deltas(state, cache)
for i in 0 ..< len(state.validators):
increase_balance(state, i.ValidatorIndex, rewards1[i] + rewards2[i])
decrease_balance(state, i.ValidatorIndex, penalties1[i] + penalties2[i])
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#slashings
func process_slashings*(state: var BeaconState) =
let
epoch = get_current_epoch(state)
total_balance = get_total_active_balance(state)
for index, validator in state.validators:
if validator.slashed and epoch + EPOCHS_PER_SLASHINGS_VECTOR div 2 ==
validator.withdrawable_epoch:
let increment = EFFECTIVE_BALANCE_INCREMENT # Factored out from penalty
# numerator to avoid uint64 overflow
let penalty_numerator =
validator.effective_balance div increment *
min(sum(state.slashings) * 3, total_balance)
let penalty = penalty_numerator div total_balance * increment
decrease_balance(state, index.ValidatorIndex, penalty)
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#final-updates
proc process_final_updates*(state: var BeaconState) =
let
current_epoch = get_current_epoch(state)
next_epoch = current_epoch + 1
# Reset eth1 data votes
if (state.slot + 1) mod SLOTS_PER_ETH1_VOTING_PERIOD == 0:
state.eth1_data_votes = @[]
# Update effective balances with hysteresis
for index, validator in state.validators:
let balance = state.balances[index]
const HALF_INCREMENT = EFFECTIVE_BALANCE_INCREMENT div 2
if balance < validator.effective_balance or
validator.effective_balance + 3'u64 * HALF_INCREMENT < balance:
state.validators[index].effective_balance =
min(
balance - balance mod EFFECTIVE_BALANCE_INCREMENT,
MAX_EFFECTIVE_BALANCE)
# Set active index root
let
index_epoch = next_epoch + ACTIVATION_EXIT_DELAY
index_root_position = index_epoch mod EPOCHS_PER_HISTORICAL_VECTOR
indices_list = sszList(get_active_validator_indices(state, index_epoch), VALIDATOR_REGISTRY_LIMIT)
state.active_index_roots[index_root_position] = hash_tree_root(indices_list)
# Set committees root
let committee_root_position = next_epoch mod EPOCHS_PER_HISTORICAL_VECTOR
state.compact_committees_roots[committee_root_position] =
get_compact_committees_root(state, next_epoch)
# Reset slashings
state.slashings[next_epoch mod EPOCHS_PER_SLASHINGS_VECTOR] = 0.Gwei
# Set randao mix
state.randao_mixes[next_epoch mod EPOCHS_PER_HISTORICAL_VECTOR] =
get_randao_mix(state, current_epoch)
# Set historical root accumulator
if next_epoch mod (SLOTS_PER_HISTORICAL_ROOT div SLOTS_PER_EPOCH).uint64 == 0:
let historical_batch = HistoricalBatch(
block_roots: state.block_roots,
state_roots: state.state_roots,
)
state.historical_roots.add (hash_tree_root(historical_batch))
# Update start shard
state.start_shard =
(state.start_shard + get_shard_delta(state, current_epoch)) mod
SHARD_COUNT
# Rotate current/previous epoch attestations
state.previous_epoch_attestations = state.current_epoch_attestations
state.current_epoch_attestations = @[]
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#per-epoch-processing
proc process_epoch*(state: var BeaconState) =
# @proc are placeholders
trace "process_epoch",
current_epoch = get_current_epoch(state)
var per_epoch_cache = get_empty_per_epoch_cache()
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#justification-and-finalization
process_justification_and_finalization(state, per_epoch_cache)
trace "ran process_justification_and_finalization",
current_epoch = get_current_epoch(state)
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#crosslinks
process_crosslinks(state, per_epoch_cache)
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#rewards-and-penalties-1
process_rewards_and_penalties(state, per_epoch_cache)
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#registry-updates
# Don't rely on caching here.
process_registry_updates(state)
## Caching here for get_crosslink_committee(...) can break otherwise, since
## get_active_validator_indices(...) usually changes.
clear(per_epoch_cache.crosslink_committee_cache)
# @process_reveal_deadlines
# @process_challenge_deadlines
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#slashings
process_slashings(state)
# https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#final-updates
process_final_updates(state)
# @after_process_final_updates
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