nimbus-eth2/beacon_chain/state_transition.nim

271 lines
12 KiB
Nim

# 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, 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 `state_transition` which is at the bottom of the 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
chronicles,
./extras, ./ssz, metrics,
./spec/[datatypes, crypto, digest, helpers, validator],
./spec/[state_transition_block, state_transition_epoch],
../nbench/bench_lab
# https://github.com/ethereum/eth2.0-metrics/blob/master/metrics.md#additional-metrics
declareGauge beacon_current_validators, """Number of status="pending|active|exited|withdrawable" validators in current epoch""" # On epoch transition
declareGauge beacon_previous_validators, """Number of status="pending|active|exited|withdrawable" validators in previous epoch""" # On epoch transition
# Canonical state transition functions
# ---------------------------------------------------------------
# https://github.com/ethereum/eth2.0-specs/blob/v0.10.1/specs/phase0/beacon-chain.md#beacon-chain-state-transition-function
func process_slot*(state: var BeaconState) {.nbench.}=
# Cache state root
let previous_state_root = hash_tree_root(state)
state.state_roots[state.slot mod SLOTS_PER_HISTORICAL_ROOT] =
previous_state_root
# Cache latest block header state root
if state.latest_block_header.state_root == ZERO_HASH:
state.latest_block_header.state_root = previous_state_root
# Cache block root
state.block_roots[state.slot mod SLOTS_PER_HISTORICAL_ROOT] =
hash_tree_root(state.latest_block_header)
func get_epoch_validator_count(state: BeaconState): int64 =
# https://github.com/ethereum/eth2.0-metrics/blob/master/metrics.md#additional-metrics
#
# This O(n) loop doesn't add to the algorithmic complexity of the epoch
# transition -- registry update already does this. It is not great, but
# isn't new, either. If profiling shows issues some of this can be loop
# fusion'ed.
for index, validator in state.validators:
# These work primarily for the `beacon_current_validators` metric defined
# as 'Number of status="pending|active|exited|withdrawable" validators in
# current epoch'. This is, in principle, equivalent to checking whether a
# validator's either at less than MAX_EFFECTIVE_BALANCE, or has withdrawn
# already because withdrawable_epoch has passed, which more precisely has
# intuitive meaning of all-the-current-relevant-validators. So, check for
# not-(either (not-even-pending) or withdrawn). That is validators change
# from not-even-pending to pending to active to exited to withdrawable to
# withdrawn, and this avoids bugs on potential edge cases and off-by-1's.
if (validator.activation_epoch != FAR_FUTURE_EPOCH or
validator.effective_balance > MAX_EFFECTIVE_BALANCE) and
validator.withdrawable_epoch > get_current_epoch(state):
result += 1
# https://github.com/ethereum/eth2.0-specs/blob/v0.10.1/specs/phase0/beacon-chain.md#beacon-chain-state-transition-function
proc process_slots*(state: var BeaconState, slot: Slot) {.nbench.}=
if not (state.slot <= slot):
warn("Trying to apply old block",
state_slot = state.slot,
slot = slot)
return
# Catch up to the target slot
while state.slot < slot:
process_slot(state)
let is_epoch_transition = (state.slot + 1) mod SLOTS_PER_EPOCH == 0
if is_epoch_transition:
# Note: Genesis epoch = 0, no need to test if before Genesis
beacon_previous_validators.set(get_epoch_validator_count(state))
process_epoch(state)
state.slot += 1
if is_epoch_transition:
beacon_current_validators.set(get_epoch_validator_count(state))
# https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/beacon-chain.md#verify_block_signature
proc verify_block_signature*(
state: BeaconState, signedBlock: SignedBeaconBlock): bool {.nbench.} =
if signedBlock.message.proposer_index >= state.validators.len.uint64:
notice "Invalid proposer index in block",
blck = shortLog(signedBlock.message)
return false
let
proposer = state.validators[signedBlock.message.proposer_index]
domain = get_domain(
state, DOMAIN_BEACON_PROPOSER,
compute_epoch_at_slot(signedBlock.message.slot))
signing_root = compute_signing_root(signedBlock.message, domain)
if not bls_verify(proposer.pubKey, signing_root.data, signedBlock.signature):
notice "Block: signature verification failed",
blck = shortLog(signedBlock),
signingRoot = shortLog(signing_root)
return false
true
# https://github.com/ethereum/eth2.0-specs/blob/v0.9.4/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
proc verifyStateRoot(state: BeaconState, blck: BeaconBlock): bool =
# This is inlined in state_transition(...) in spec.
let state_root = hash_tree_root(state)
if state_root != blck.state_root:
notice "Block: root verification failed",
block_state_root = blck.state_root, state_root
false
else:
true
proc state_transition*(
state: var BeaconState, signedBlock: SignedBeaconBlock, flags: UpdateFlags): bool {.nbench.}=
## Time in the beacon chain moves by slots. Every time (haha.) that happens,
## we will update the beacon state. Normally, the state updates will be driven
## by the contents of a new block, but it may happen that the block goes
## missing - the state updates happen regardless.
##
## Each call to this function will advance the state by one slot - new_block,
## must match that slot. If the update fails, the state will remain unchanged.
##
## The flags are used to specify that certain validations should be skipped
## for the new block. This is done during block proposal, to create a state
## whose hash can be included in the new block.
#
# TODO this function can be written with a loop inside to handle all empty
# slots up to the slot of the new_block - but then again, why not eagerly
# update the state as time passes? Something to ponder...
# One reason to keep it this way is that you need to look ahead if you're
# the block proposer, though in reality we only need a partial update for
# that ===> Implemented as process_slots
# TODO There's a discussion about what this function should do, and when:
# https://github.com/ethereum/eth2.0-specs/issues/284
# TODO check to which extent this copy can be avoided (considering forks etc),
# for now, it serves as a reminder that we need to handle invalid blocks
# somewhere..
# many functions will mutate `state` partially without rolling back
# the changes in case of failure (look out for `var BeaconState` and
# bool return values...)
## TODO, of cacheState/processEpoch/processSlot/processBloc, only the last
## might fail, so should this bother capturing here, or?
var old_state = state
# These should never fail.
process_slots(state, signedBlock.message.slot)
# Block updates - these happen when there's a new block being suggested
# by the block proposer. Every actor in the network will update its state
# according to the contents of this block - but first they will validate
# that the block is sane.
# TODO what should happen if block processing fails?
# https://github.com/ethereum/eth2.0-specs/issues/293
if skipBLSValidation in flags or
verify_block_signature(state, signedBlock):
var per_epoch_cache = get_empty_per_epoch_cache()
if processBlock(state, signedBlock.message, flags, per_epoch_cache):
# This is a bit awkward - at the end of processing we verify that the
# state we arrive at is what the block producer thought it would be -
# meaning that potentially, it could fail verification
if skipStateRootValidation in flags or verifyStateRoot(state, signedBlock.message):
# State root is what it should be - we're done!
return true
# Block processing failed, roll back changes
state = old_state
false
# Hashed-state transition functions
# ---------------------------------------------------------------
# https://github.com/ethereum/eth2.0-specs/blob/v0.10.1/specs/phase0/beacon-chain.md#beacon-chain-state-transition-function
func process_slot(state: var HashedBeaconState) =
# Cache state root
let previous_slot_state_root = state.root
state.data.state_roots[state.data.slot mod SLOTS_PER_HISTORICAL_ROOT] =
previous_slot_state_root
# Cache latest block header state root
if state.data.latest_block_header.state_root == ZERO_HASH:
state.data.latest_block_header.state_root = previous_slot_state_root
# Cache block root
state.data.block_roots[state.data.slot mod SLOTS_PER_HISTORICAL_ROOT] =
hash_tree_root(state.data.latest_block_header)
# https://github.com/ethereum/eth2.0-specs/blob/v0.10.1/specs/phase0/beacon-chain.md#beacon-chain-state-transition-function
proc process_slots*(state: var HashedBeaconState, slot: Slot) =
# TODO: Eth specs strongly assert that state.data.slot <= slot
# This prevents receiving attestation in any order
# (see tests/test_attestation_pool)
# but it maybe an artifact of the test case
# as this was not triggered in the testnet1
# after a hour
if state.data.slot > slot:
notice(
"Unusual request for a slot in the past",
state_root = shortLog(state.root),
current_slot = state.data.slot,
target_slot = slot
)
# Catch up to the target slot
while state.data.slot < slot:
process_slot(state)
let is_epoch_transition = (state.data.slot + 1) mod SLOTS_PER_EPOCH == 0
if is_epoch_transition:
# Note: Genesis epoch = 0, no need to test if before Genesis
beacon_previous_validators.set(get_epoch_validator_count(state.data))
process_epoch(state.data)
state.data.slot += 1
if is_epoch_transition:
beacon_current_validators.set(get_epoch_validator_count(state.data))
state.root = hash_tree_root(state.data)
proc state_transition*(
state: var HashedBeaconState, signedBlock: SignedBeaconBlock, flags: UpdateFlags): bool =
# Save for rollback
var old_state = state
process_slots(state, signedBlock.message.slot)
if skipBLSValidation in flags or
verify_block_signature(state.data, signedBlock):
var per_epoch_cache = get_empty_per_epoch_cache()
if processBlock(state.data, signedBlock.message, flags, per_epoch_cache):
if skipStateRootValidation in flags or verifyStateRoot(state.data, signedBlock.message):
# State root is what it should be - we're done!
# TODO when creating a new block, state_root is not yet set.. comparing
# with zero hash here is a bit fragile however, but this whole thing
# should go away with proper hash caching
state.root =
if signedBlock.message.state_root == Eth2Digest(): hash_tree_root(state.data)
else: signedBlock.message.state_root
return true
# Block processing failed, roll back changes
state = old_state
false