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nimbus-eth2/beacon_chain/spec/state_transition_epoch.nim

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# beacon_chain
# Copyright (c) 2018-2020 Status Research & Development GmbH
# Licensed and distributed under either of
# * MIT license (license terms in the root directory or at https://opensource.org/licenses/MIT).
# * Apache v2 license (license terms in the root directory or at https://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.
{.push raises: [Defect].}
import
math, sequtils, tables,
stew/[bitops2], chronicles, json_serialization/std/sets,
metrics, ../extras, ../ssz/merkleization,
beaconstate, crypto, datatypes, digest, helpers, validator,
state_transition_helpers,
../../nbench/bench_lab
# Logging utilities
# --------------------------------------------------------
logScope: topics = "consens"
# https://github.com/ethereum/eth2.0-metrics/blob/master/metrics.md#interop-metrics
declareGauge beacon_finalized_epoch, "Current finalized epoch" # On epoch transition
declareGauge beacon_finalized_root, "Current finalized root" # On epoch transition
declareGauge beacon_current_justified_epoch, "Current justified epoch" # On epoch transition
declareGauge beacon_current_justified_root, "Current justified root" # On epoch transition
declareGauge beacon_previous_justified_epoch, "Current previously justified epoch" # On epoch transition
declareGauge beacon_previous_justified_root, "Current previously justified root" # On epoch transition
# Non-spec
declareGauge epoch_transition_justification_and_finalization, "Epoch transition justification and finalization time"
declareGauge epoch_transition_times_rewards_and_penalties, "Epoch transition reward and penalty time"
declareGauge epoch_transition_registry_updates, "Epoch transition registry updates time"
declareGauge epoch_transition_slashings, "Epoch transition slashings time"
declareGauge epoch_transition_final_updates, "Epoch transition final updates time"
declareGauge beacon_current_epoch, "Current epoch"
# Spec
# --------------------------------------------------------
# https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#get_total_active_balance
func get_total_active_balance*(state: BeaconState, cache: var StateCache): Gwei =
# Return the combined effective balance of the active validators.
# Note: ``get_total_balance`` returns ``EFFECTIVE_BALANCE_INCREMENT`` Gwei
# minimum to avoid divisions by zero.
# TODO refactor get_epoch_per_epoch_cache() not to be, well, empty, so can
# avoid this ever refilling, and raiseAssert, and get rid of var
let
epoch = state.slot.compute_epoch_at_slot
try:
if epoch notin cache.shuffled_active_validator_indices:
cache.shuffled_active_validator_indices[epoch] =
get_shuffled_active_validator_indices(state, epoch)
get_total_balance(state, cache.shuffled_active_validator_indices[epoch])
except KeyError:
raiseAssert("get_total_active_balance(): cache always filled before usage")
# https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/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.asSeq
else:
state.previous_epoch_attestations.asSeq
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_target_attestations(state, epoch),
it.data.beacon_block_root ==
get_block_root_at_slot(state, it.data.slot)
)
func get_attesting_balance(
state: BeaconState, attestations: seq[PendingAttestation],
stateCache: var StateCache): Gwei =
# Return the combined effective balance of the set of unslashed validators
# participating in ``attestations``.
# Note: ``get_total_balance`` returns ``EFFECTIVE_BALANCE_INCREMENT`` Gwei
# minimum to avoid divisions by zero.
get_total_balance(state, get_unslashed_attesting_indices(
state, attestations, stateCache))
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.4/specs/core/0_beacon-chain.md#justification-and-finalization
proc process_justification_and_finalization*(state: var BeaconState,
stateCache: var StateCache, updateFlags: UpdateFlags = {}) {.nbench.} =
logScope: pcs = "process_justification_and_finalization"
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
# TODO JUSTIFICATION_BITS_LENGTH is a constant in spec, move there or fix
# BitVector serialization in SSZ layer
const JUSTIFICATION_BITS_LENGTH = 4
state.justification_bits = (state.justification_bits shl 1) and
cast[uint8]((2^JUSTIFICATION_BITS_LENGTH) - 1)
# This is a somewhat expensive approach
let active_validator_indices {.used.} =
toHashSet(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/v0.11.3/specs/phase0/beacon-chain.md#attestations
## and `get_matching_source_attestations(...)` via
## https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#helper-functions-1
## and
## https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#final-updates
## after which the state.previous_epoch_attestations is replaced.
let total_active_balance = get_total_active_balance(state, stateCache)
trace "Non-attesting indices in previous epoch",
missing_all_validators=
difference(active_validator_indices,
toHashSet(mapIt(get_attesting_indices(state,
matching_target_attestations_previous, stateCache), it.int))),
missing_unslashed_validators=
difference(active_validator_indices,
toHashSet(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 = total_active_balance * 2,
attesting_balance_prev = get_attesting_balance(state, matching_target_attestations_previous, stateCache)
if get_attesting_balance(state, matching_target_attestations_previous,
stateCache) * 3 >= total_active_balance * 2:
state.current_justified_checkpoint =
Checkpoint(epoch: previous_epoch,
root: get_block_root(state, previous_epoch))
state.justification_bits.setBit 1
trace "Justified with previous epoch",
current_epoch = current_epoch,
checkpoint = shortLog(state.current_justified_checkpoint),
cat = "justification"
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 >= total_active_balance * 2:
state.current_justified_checkpoint =
Checkpoint(epoch: current_epoch,
root: get_block_root(state, current_epoch))
state.justification_bits.setBit 0
trace "Justified with current epoch",
current_epoch = current_epoch,
checkpoint = shortLog(state.current_justified_checkpoint),
cat = "justification"
# 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
trace "Finalized with rule 234",
current_epoch = current_epoch,
checkpoint = shortLog(state.finalized_checkpoint),
cat = "finalization"
## 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
trace "Finalized with rule 23",
current_epoch = current_epoch,
checkpoint = shortLog(state.finalized_checkpoint),
cat = "finalization"
## 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
trace "Finalized with rule 123",
current_epoch = current_epoch,
checkpoint = shortLog(state.finalized_checkpoint),
cat = "finalization"
## 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
trace "Finalized with rule 12",
current_epoch = current_epoch,
checkpoint = shortLog(state.finalized_checkpoint),
cat = "finalization"
# https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#rewards-and-penalties-1
func get_base_reward(state: BeaconState, index: ValidatorIndex,
total_balance: auto): Gwei =
# Spec function recalculates total_balance every time, which creates an
# O(n^2) situation.
let 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/v0.11.3/specs/phase0/beacon-chain.md#rewards-and-penalties-1
func get_attestation_deltas(state: BeaconState, stateCache: var StateCache):
tuple[a: seq[Gwei], b: seq[Gwei]] {.nbench.}=
let
previous_epoch = get_previous_epoch(state)
total_balance = get_total_active_balance(state, stateCache)
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:
# Factored out from balance totals to avoid uint64 overflow
const increment = EFFECTIVE_BALANCE_INCREMENT
let reward_numerator = get_base_reward(state, index, total_balance) *
(attesting_balance div increment)
rewards[index] += reward_numerator div (total_balance div increment)
else:
penalties[index] += get_base_reward(state, index, total_balance)
# 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))
for index in get_unslashed_attesting_indices(
state, matching_source_attestations, stateCache):
# Translation of attestation = min([...])
doAssert matching_source_attestations.len > 0
# Start by filtering the right attestations
var filtered_matching_source_attestations: seq[PendingAttestation]
for source_attestation_index, a in matching_source_attestations:
if index notin
source_attestation_attesting_indices[source_attestation_index]:
continue
filtered_matching_source_attestations.add a
# The first filtered attestation serves as min until we find something
# better
var attestation = filtered_matching_source_attestations[0]
for source_attestation_index, a in filtered_matching_source_attestations:
if a.inclusion_delay < attestation.inclusion_delay:
attestation = a
let
base_reward = get_base_reward(state, index, total_balance)
proposer_reward = (base_reward div PROPOSER_REWARD_QUOTIENT).Gwei
rewards[attestation.proposer_index.int] += proposer_reward
let max_attester_reward = base_reward - proposer_reward
rewards[index] += max_attester_reward div attestation.inclusion_delay
# 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, total_balance)
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/v0.11.3/specs/phase0/beacon-chain.md#rewards-and-penalties-1
func process_rewards_and_penalties(
state: var BeaconState, cache: var StateCache) {.nbench.}=
if get_current_epoch(state) == GENESIS_EPOCH:
return
let (rewards, penalties) = get_attestation_deltas(state, cache)
for i in 0 ..< len(state.validators):
increase_balance(state, i.ValidatorIndex, rewards[i])
decrease_balance(state, i.ValidatorIndex, penalties[i])
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.4/specs/core/0_beacon-chain.md#slashings
func process_slashings*(state: var BeaconState, cache: var StateCache) {.nbench.}=
let
epoch = get_current_epoch(state)
total_balance = get_total_active_balance(state, cache)
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/v0.11.3/specs/phase0/beacon-chain.md#final-updates
func process_final_updates*(state: var BeaconState) {.nbench.}=
let
current_epoch = get_current_epoch(state)
next_epoch = current_epoch + 1
# Reset eth1 data votes
if next_epoch mod EPOCHS_PER_ETH1_VOTING_PERIOD == 0:
state.eth1_data_votes = default(type state.eth1_data_votes)
# Update effective balances with hysteresis
for index, validator in state.validators:
let balance = state.balances[index]
const
HYSTERESIS_INCREMENT =
EFFECTIVE_BALANCE_INCREMENT div HYSTERESIS_QUOTIENT
DOWNWARD_THRESHOLD =
HYSTERESIS_INCREMENT * HYSTERESIS_DOWNWARD_MULTIPLIER
UPWARD_THRESHOLD = HYSTERESIS_INCREMENT * HYSTERESIS_UPWARD_MULTIPLIER
if balance + DOWNWARD_THRESHOLD < validator.effective_balance or
validator.effective_balance + UPWARD_THRESHOLD < balance:
state.validators[index].effective_balance =
min(
balance - balance mod EFFECTIVE_BALANCE_INCREMENT,
MAX_EFFECTIVE_BALANCE)
# 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:
# Equivalent to hash_tree_root(foo: HistoricalBatch), but without using
# significant additional stack or heap.
# https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#historicalbatch
# In response to https://github.com/status-im/nim-beacon-chain/issues/921
state.historical_roots.add hash_tree_root(
[hash_tree_root(state.block_roots), hash_tree_root(state.state_roots)])
# Rotate current/previous epoch attestations
state.previous_epoch_attestations = state.current_epoch_attestations
state.current_epoch_attestations = default(type state.current_epoch_attestations)
# https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#epoch-processing
proc process_epoch*(state: var BeaconState, updateFlags: UpdateFlags,
per_epoch_cache: var StateCache) {.nbench.} =
let currentEpoch = get_current_epoch(state)
trace "process_epoch",
current_epoch = currentEpoch
# https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#justification-and-finalization
process_justification_and_finalization(state, per_epoch_cache, updateFlags)
# state.slot hasn't been incremented yet.
if verifyFinalization in updateFlags and currentEpoch >= 3:
# Rule 2/3/4 finalization results in the most pessimal case. The other
# three finalization rules finalize more quickly as long as the any of
# the finalization rules triggered.
doAssert state.finalized_checkpoint.epoch + 3 >= currentEpoch
# https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#rewards-and-penalties-1
process_rewards_and_penalties(state, per_epoch_cache)
# https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#registry-updates
process_registry_updates(state, per_epoch_cache)
# https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#slashings
process_slashings(state, per_epoch_cache)
# https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#final-updates
process_final_updates(state)
# Once per epoch metrics
beacon_current_epoch.set(currentEpoch.int64)
beacon_finalized_epoch.set(state.finalized_checkpoint.epoch.int64)
beacon_finalized_root.set(state.finalized_checkpoint.root.toGaugeValue)
beacon_current_justified_epoch.set(
state.current_justified_checkpoint.epoch.int64)
beacon_current_justified_root.set(
state.current_justified_checkpoint.root.toGaugeValue)
beacon_previous_justified_epoch.set(
state.previous_justified_checkpoint.epoch.int64)
beacon_previous_justified_root.set(
state.previous_justified_checkpoint.root.toGaugeValue)
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