nimbus-eth2/beacon_chain/spec/state_transition.nim

297 lines
13 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):
# * 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.
# * For indices, we get a mix of uint64, ValidatorIndex and int - this is currently
# swept under the rug with casts
# * 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
tables,
chronicles,
stew/results,
../extras, ../ssz/merkleization, metrics,
datatypes, crypto, digest, helpers, signatures, validator,
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
func get_epoch_validator_count(state: BeaconState): int64 {.nbench.} =
# 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/v0.12.2/specs/phase0/beacon-chain.md#beacon-chain-state-transition-function
proc verify_block_signature*(
state: BeaconState, signed_block: SomeSignedBeaconBlock): bool {.nbench.} =
let
proposer_index = signed_block.message.proposer_index
if proposer_index >= state.validators.lenu64:
notice "Invalid proposer index in block",
blck = shortLog(signed_block.message)
return false
if not verify_block_signature(
state.fork, state.genesis_validators_root, signed_block.message.slot,
signed_block.message, state.validators[proposer_index].pubkey,
signed_block.signature):
notice "Block: signature verification failed",
blck = shortLog(signedBlock)
return false
true
# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/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 = shortLog(blck.state_root), state_root = shortLog(state_root)
false
else:
true
proc verifyStateRoot(state: BeaconState, blck: TrustedBeaconBlock): bool =
# This is inlined in state_transition(...) in spec.
true
type
RollbackProc* = proc(v: var BeaconState) {.gcsafe, raises: [Defect].}
proc noRollback*(state: var BeaconState) =
trace "Skipping rollback of broken state"
type
RollbackHashedProc* = proc(state: var HashedBeaconState) {.gcsafe.}
# Hashed-state transition functions
# ---------------------------------------------------------------
# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/beacon-chain.md#beacon-chain-state-transition-function
func process_slot*(state: var HashedBeaconState) {.nbench.} =
# 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)
func clear_epoch_from_cache(cache: var StateCache, epoch: Epoch) =
cache.shuffled_active_validator_indices.del epoch
let
start_slot = epoch.compute_start_slot_at_epoch
end_slot = (epoch + 1).compute_start_slot_at_epoch
for i in start_slot ..< end_slot:
cache.beacon_proposer_indices.del i
# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/beacon-chain.md#beacon-chain-state-transition-function
proc advance_slot*(
state: var HashedBeaconState, updateFlags: UpdateFlags,
epochCache: var StateCache) {.nbench.} =
# Special case version of process_slots that moves one slot at a time - can
# run faster if the state root is known already (for example when replaying
# existing slots)
process_slot(state)
let is_epoch_transition = (state.data.slot + 1).isEpoch
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, updateFlags, epochCache)
clear_epoch_from_cache(
epochCache, (state.data.slot + 1).compute_epoch_at_slot)
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)
# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/beacon-chain.md#beacon-chain-state-transition-function
proc process_slots*(state: var HashedBeaconState, slot: Slot,
cache: var StateCache, updateFlags: UpdateFlags = {}): bool {.nbench.} =
# TODO this function is not _really_ necessary: when replaying states, we
# advance slots one by one before calling `state_transition` - this way,
# we avoid the state root calculation - as such, instead of advancing
# slots "automatically" in `state_transition`, perhaps it would be better
# to keep a pre-condition that state must be at the right slot already?
if not (state.data.slot < slot):
if slotProcessed notin updateFlags or 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
)
return false
# Catch up to the target slot
while state.data.slot < slot:
advance_slot(state, updateFlags, cache)
true
proc noRollback*(state: var HashedBeaconState) =
trace "Skipping rollback of broken state"
proc state_transition*(
preset: RuntimePreset,
state: var HashedBeaconState, signedBlock: SomeSignedBeaconBlock,
# TODO this is ... okay, but not perfect; align with EpochRef
stateCache: var StateCache,
flags: UpdateFlags, rollback: RollbackHashedProc): 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.
##
## 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.
##
## `rollback` is called if the transition fails and the given state has been
## partially changed. If a temporary state was given to `state_transition`,
## it is safe to use `noRollback` and leave it broken, else the state
## object should be rolled back to a consistent state. If the transition fails
## before the state has been updated, `rollback` will not be called.
doAssert not rollback.isNil, "use noRollback if it's ok to mess up state"
# This only fails if it hasn't changed stateCache, so it can't create a false
# not-followed future history in stateCache.
if not process_slots(state, signedBlock.message.slot, stateCache, flags):
rollback(state)
return false
# 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.
if skipBLSValidation in flags or
verify_block_signature(state.data, signedBlock):
# TODO after checking scaffolding, remove this
trace "state_transition: processing block, signature passed",
signature = shortLog(signedBlock.signature),
blockRoot = shortLog(signedBlock.root)
if process_block(preset, state.data, signedBlock.message, flags, stateCache):
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
# TODO shouldn't ever have to recalculate; verifyStateRoot() does it
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
rollback(state)
false
proc state_transition*(
preset: RuntimePreset,
state: var HashedBeaconState, signedBlock: SomeSignedBeaconBlock,
flags: UpdateFlags, rollback: RollbackHashedProc): bool {.nbench.} =
# TODO consider moving this to testutils or similar, since non-testing
# and fuzzing code should always be coming from block pool which should
# always be providing cache or equivalent
var cache = StateCache()
state_transition(preset, state, signedBlock, cache, flags, rollback)
# https://github.com/ethereum/eth2.0-specs/blob/v0.12.2/specs/phase0/validator.md#preparing-for-a-beaconblock
# TODO There's more to do here - the spec has helpers that deal set up some of
# the fields in here!
proc makeBeaconBlock*(
preset: RuntimePreset,
state: var HashedBeaconState,
proposer_index: ValidatorIndex,
parent_root: Eth2Digest,
randao_reveal: ValidatorSig,
eth1_data: Eth1Data,
graffiti: GraffitiBytes,
attestations: seq[Attestation],
deposits: seq[Deposit],
rollback: RollbackHashedProc,
cache: var StateCache): Option[BeaconBlock] =
## Create a block for the given state. The last block applied to it must be
## the one identified by parent_root and process_slots must be called up to
## the slot for which a block is to be created.
# To create a block, we'll first apply a partial block to the state, skipping
# some validations.
var blck = BeaconBlock(
slot: state.data.slot,
proposer_index: proposer_index.uint64,
parent_root: parent_root,
body: BeaconBlockBody(
randao_reveal: randao_reveal,
eth1_data: eth1data,
graffiti: graffiti,
attestations: List[Attestation, Limit MAX_ATTESTATIONS](attestations),
deposits: List[Deposit, Limit MAX_DEPOSITS](deposits)))
let ok = process_block(preset, state.data, blck, {skipBlsValidation}, cache)
if not ok:
warn "Unable to apply new block to state", blck = shortLog(blck)
rollback(state)
return
state.root = hash_tree_root(state.data)
blck.state_root = state.root
some(blck)