5.0 KiB
Altair -- Fork Logic
Table of contents
Introduction
This document describes the process of the first upgrade of the beacon chain: the Altair hard fork, introducing light client support and other improvements.
Configuration
Warning: this configuration is not definitive.
| Name | Value |
|---|---|
ALTAIR_FORK_VERSION |
Version('0x01000000') |
ALTAIR_FORK_EPOCH |
Epoch(74240) (Oct 27, 2021, 10:56:23am UTC) |
Helper functions
Misc
compute_fork_version
def compute_fork_version(epoch: Epoch) -> Version:
"""
Return the fork version at the given ``epoch``.
"""
if epoch >= ALTAIR_FORK_EPOCH:
return ALTAIR_FORK_VERSION
return GENESIS_FORK_VERSION
Fork to Altair
Fork trigger
The fork is triggered at epoch ALTAIR_FORK_EPOCH.
Note that for the pure Altair networks, we don't apply upgrade_to_altair since it starts with Altair version logic.
Upgrading the state
If state.slot % SLOTS_PER_EPOCH == 0 and compute_epoch_at_slot(state.slot) == ALTAIR_FORK_EPOCH, an irregular state change is made to upgrade to Altair.
The upgrade occurs after the completion of the inner loop of process_slots that sets state.slot equal to ALTAIR_FORK_EPOCH * SLOTS_PER_EPOCH.
Care must be taken when transitioning through the fork boundary as implementations will need a modified state transition function that deviates from the Phase 0 document.
In particular, the outer state_transition function defined in the Phase 0 document will not expose the precise fork slot to execute the upgrade in the presence of skipped slots at the fork boundary. Instead the logic must be within process_slots.
def translate_participation(state: BeaconState, pending_attestations: Sequence[phase0.PendingAttestation]) -> None:
for attestation in pending_attestations:
data = attestation.data
inclusion_delay = attestation.inclusion_delay
# Translate attestation inclusion info to flag indices
participation_flag_indices = get_attestation_participation_flag_indices(state, data, inclusion_delay)
# Apply flags to all attesting validators
epoch_participation = state.previous_epoch_participation
for index in get_attesting_indices(state, data, attestation.aggregation_bits):
for flag_index in participation_flag_indices:
epoch_participation[index] = add_flag(epoch_participation[index], flag_index)
def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
epoch = phase0.get_current_epoch(pre)
post = BeaconState(
# Versioning
genesis_time=pre.genesis_time,
genesis_validators_root=pre.genesis_validators_root,
slot=pre.slot,
fork=Fork(
previous_version=pre.fork.current_version,
current_version=ALTAIR_FORK_VERSION,
epoch=epoch,
),
# History
latest_block_header=pre.latest_block_header,
block_roots=pre.block_roots,
state_roots=pre.state_roots,
historical_roots=pre.historical_roots,
# Eth1
eth1_data=pre.eth1_data,
eth1_data_votes=pre.eth1_data_votes,
eth1_deposit_index=pre.eth1_deposit_index,
# Registry
validators=pre.validators,
balances=pre.balances,
# Randomness
randao_mixes=pre.randao_mixes,
# Slashings
slashings=pre.slashings,
# Participation
previous_epoch_participation=[ParticipationFlags(0b0000_0000) for _ in range(len(pre.validators))],
current_epoch_participation=[ParticipationFlags(0b0000_0000) for _ in range(len(pre.validators))],
# Finality
justification_bits=pre.justification_bits,
previous_justified_checkpoint=pre.previous_justified_checkpoint,
current_justified_checkpoint=pre.current_justified_checkpoint,
finalized_checkpoint=pre.finalized_checkpoint,
# Inactivity
inactivity_scores=[uint64(0) for _ in range(len(pre.validators))],
)
# Fill in previous epoch participation from the pre state's pending attestations
translate_participation(post, pre.previous_epoch_attestations)
# Fill in sync committees
# Note: A duplicate committee is assigned for the current and next committee at the fork boundary
post.current_sync_committee = get_next_sync_committee(post)
post.next_sync_committee = get_next_sync_committee(post)
return post