Merge pull request #520 from ethereum/next-epoch-shuffling
helpers and notes for shuffling lookahead
This commit is contained in:
Danny Ryan
GitHub
commit
e400c28372
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@ -64,6 +64,7 @@
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- [`get_shuffling`](#get_shuffling)
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- [`get_previous_epoch_committee_count`](#get_previous_epoch_committee_count)
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- [`get_current_epoch_committee_count`](#get_current_epoch_committee_count)
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- [`get_next_epoch_committee_count`](#get_next_epoch_committee_count)
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- [`get_crosslink_committees_at_slot`](#get_crosslink_committees_at_slot)
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- [`get_block_root`](#get_block_root)
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- [`get_randao_mix`](#get_randao_mix)
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@ -72,6 +73,7 @@
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- [`get_beacon_proposer_index`](#get_beacon_proposer_index)
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- [`merkle_root`](#merkle_root)
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- [`get_attestation_participants`](#get_attestation_participants)
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- [`is_power_of_two`](#is_power_of_two)
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- [`int_to_bytes1`, `int_to_bytes2`, ...](#int_to_bytes1-int_to_bytes2-)
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- [`get_effective_balance`](#get_effective_balance)
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- [`get_fork_version`](#get_fork_version)
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@ -815,31 +817,61 @@ def get_current_epoch_committee_count(state: BeaconState) -> int:
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return get_epoch_committee_count(len(current_active_validators))
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```
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### `get_next_epoch_committee_count`
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```python
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def get_next_epoch_committee_count(state: BeaconState) -> int:
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next_active_validators = get_active_validator_indices(
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state.validator_registry,
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get_current_epoch(state) + 1,
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)
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return get_epoch_committee_count(len(next_active_validators))
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```
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### `get_crosslink_committees_at_slot`
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```python
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def get_crosslink_committees_at_slot(state: BeaconState,
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slot: SlotNumber) -> List[Tuple[List[ValidatorIndex], ShardNumber]]:
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slot: SlotNumber,
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registry_change=False: bool) -> List[Tuple[List[ValidatorIndex], ShardNumber]]:
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"""
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Return the list of ``(committee, shard)`` tuples for the ``slot``.
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Note: There are two possible shufflings for crosslink committees for a
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``slot`` in the next epoch -- with and without a `registry_change`
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"""
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epoch = slot_to_epoch(slot)
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current_epoch = get_current_epoch(state)
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previous_epoch = current_epoch - 1 if current_epoch > GENESIS_EPOCH else current_epoch
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next_epoch = current_epoch + 1
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assert previous_epoch <= epoch < next_epoch
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assert previous_epoch <= epoch <= next_epoch
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if epoch < current_epoch:
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if epoch == previous_epoch:
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committees_per_epoch = get_previous_epoch_committee_count(state)
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seed = state.previous_epoch_seed
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shuffling_epoch = state.previous_calculation_epoch
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shuffling_start_shard = state.previous_epoch_start_shard
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else:
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elif epoch == current_epoch:
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committees_per_epoch = get_current_epoch_committee_count(state)
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seed = state.current_epoch_seed
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shuffling_epoch = state.current_calculation_epoch
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shuffling_start_shard = state.current_epoch_start_shard
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elif epoch == next_epoch:
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current_committees_per_epoch = get_current_epoch_committee_count(state)
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committees_per_epoch = get_next_epoch_committee_count(state)
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shuffling_epoch = next_epoch
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epochs_since_last_registry_update = current_epoch - state.validator_registry_update_epoch
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if registry_change:
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seed = generate_seed(state, next_epoch)
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shuffling_start_shard = (state.current_epoch_start_shard + current_committees_per_epoch) % SHARD_COUNT
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elif epochs_since_last_registry_update > 1 and is_power_of_two(epochs_since_last_registry_update):
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seed = generate_seed(state, next_epoch)
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shuffling_start_shard = state.current_epoch_start_shard
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else:
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seed = state.current_epoch_seed
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shuffling_start_shard = state.current_epoch_start_shard
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shuffling = get_shuffling(
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seed,
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@ -896,7 +928,7 @@ def get_active_index_root(state: BeaconState,
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"""
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Return the index root at a recent ``epoch``.
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"""
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assert get_current_epoch(state) - LATEST_INDEX_ROOTS_LENGTH < epoch <= get_current_epoch(state)
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assert get_current_epoch(state) - LATEST_INDEX_ROOTS_LENGTH + ENTRY_EXIT_DELAY < epoch <= get_current_epoch(state) + ENTRY_EXIT_DELAY
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return state.latest_index_roots[epoch % LATEST_INDEX_ROOTS_LENGTH]
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```
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@ -966,6 +998,19 @@ def get_attestation_participants(state: BeaconState,
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return participants
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```
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### `is_power_of_two`
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```
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def is_power_of_two(value: int) -> bool:
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"""
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Check if ``value`` is a power of two integer.
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"""
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if value == 0:
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return False
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else:
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return 2**int(math.log2(value)) == value
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```
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### `int_to_bytes1`, `int_to_bytes2`, ...
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`int_to_bytes1(x): return x.to_bytes(1, 'big')`, `int_to_bytes2(x): return x.to_bytes(2, 'big')`, and so on for all integers, particularly 1, 2, 3, 4, 8, 32, 48, 96.
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@ -1507,7 +1552,9 @@ def get_initial_beacon_state(initial_validator_deposits: List[Deposit],
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if get_effective_balance(state, validator_index) >= MAX_DEPOSIT_AMOUNT:
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activate_validator(state, validator_index, is_genesis=True)
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state.latest_index_roots[GENESIS_EPOCH % LATEST_INDEX_ROOTS_LENGTH] = hash_tree_root(get_active_validator_indices(state.validator_registry, GENESIS_EPOCH))
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genesis_active_index_root = hash_tree_root(get_active_validator_indices(state, GENESIS_EPOCH))
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for index in range(LATEST_INDEX_ROOTS_LENGTH):
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state.latest_index_roots[index] = genesis_active_index_root
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state.current_epoch_seed = generate_seed(state, GENESIS_EPOCH)
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return state
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@ -1933,7 +1980,6 @@ First, update the following:
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* Set `state.previous_calculation_epoch = state.current_calculation_epoch`.
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* Set `state.previous_epoch_start_shard = state.current_epoch_start_shard`.
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* Set `state.previous_epoch_seed = state.current_epoch_seed`.
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* Set `state.latest_index_roots[next_epoch % LATEST_INDEX_ROOTS_LENGTH] = hash_tree_root(get_active_validator_indices(state, next_epoch))`.
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If the following are satisfied:
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@ -1996,7 +2042,7 @@ and perform the following updates:
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If a validator registry update does _not_ happen do the following:
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* Let `epochs_since_last_registry_update = current_epoch - state.validator_registry_update_epoch`.
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* If `epochs_since_last_registry_update` is an exact power of 2:
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* If `epochs_since_last_registry_update > 1` and `is_power_of_two(epochs_since_last_registry_update)`:
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* Set `state.current_calculation_epoch = next_epoch`.
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* Set `state.current_epoch_seed = generate_seed(state, state.current_calculation_epoch)`
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* _Note_ that `state.current_epoch_start_shard` is left unchanged.
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@ -2048,6 +2094,7 @@ def process_penalties_and_exits(state: BeaconState) -> None:
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#### Final updates
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* Set `state.latest_index_roots[(next_epoch + ENTRY_EXIT_DELAY) % LATEST_INDEX_ROOTS_LENGTH] = hash_tree_root(get_active_validator_indices(state, next_epoch + ENTRY_EXIT_DELAY))`.
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* Set `state.latest_penalized_balances[(next_epoch) % LATEST_PENALIZED_EXIT_LENGTH] = state.latest_penalized_balances[current_epoch % LATEST_PENALIZED_EXIT_LENGTH]`.
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* Set `state.latest_randao_mixes[next_epoch % LATEST_RANDAO_MIXES_LENGTH] = get_randao_mix(state, current_epoch)`.
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* Remove any `attestation` in `state.latest_attestations` such that `slot_to_epoch(attestation.data.slot) < current_epoch`.
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@ -50,6 +50,7 @@ __NOTICE__: This document is a work-in-progress for researchers and implementers
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- [Aggregation bitfield](#aggregation-bitfield)
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- [Custody bitfield](#custody-bitfield)
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- [Aggregate signature](#aggregate-signature)
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- [Responsibility lookahead](#responsibility-lookahead)
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- [How to avoid slashing](#how-to-avoid-slashing)
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- [Proposer slashing](#proposer-slashing)
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- [Attester slashing](#attester-slashing)
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@ -329,6 +330,39 @@ signed_attestation_data = bls_sign(
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)
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```
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## Responsibility lookahead
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The beacon chain shufflings are designed to provide a minimum of 1 epoch lookahead on the validator's upcoming responsibilities of proposing and attesting dictated by the shuffling and slot.
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There are three possibilities for the shuffling at the next epoch:
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1. The shuffling changes due to a "validator registry change".
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2. The shuffling changes due to `epochs_since_last_registry_update` being an exact power of 2 greater than 1.
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3. The shuffling remains the same (i.e. the validator is in the same shard committee).
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Either (2) or (3) occurs if (1) fails. The choice between (2) and (3) is deterministic based upon `epochs_since_last_registry_update`.
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`get_crosslink_committees_at_slot` is designed to be able to query slots in the next epoch. When querying slots in the next epoch there are two options -- with and without a `registry_change` -- which is the optional third parameter of the function. The following helper can be used to get the potential crosslink committees in the next epoch for a given `validator_index`. This function returns a list of 2 shard committee tuples.
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```python
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def get_next_epoch_crosslink_committees(state: BeaconState,
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validator_index: ValidatorIndex) -> List[Tuple[ValidatorIndex], ShardNumber]:
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current_epoch = get_current_epoch(state)
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next_epoch = current_epoch + 1
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next_epoch_start_slot = get_epoch_start_slot(next_epoch)
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potential_committees = []
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for validator_registry in [False, True]:
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for slot in range(next_epoch_start_slot, next_epoch_start_slot + EPOCH_LENGTH):
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shard_committees = get_crosslink_committees_at_slot(state, slot, validator_registry)
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selected_committees = [committee for committee in shard_committees if validator_index in committee[0]]
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if len(selected_committees) > 0:
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potential_assignments.append(selected_committees)
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break
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return potential_assignments
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```
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`get_next_epoch_crosslink_committees` should be called at the beginning of each epoch to plan for the next epoch. A validator should always plan for both values of `registry_change` as a possibility unless the validator can concretely eliminate one of the options. Planning for a future shuffling involves noting at which slot one might have to attest and propose and also which shard one should begin syncing (in phase 1+).
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## How to avoid slashing
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"Slashing" is the burning of some amount of validator funds and immediate ejection from the active validator set. In Phase 0, there are two ways in which funds can be slashed -- [proposer slashing](#proposer-slashing) and [attester slashing](#attester-slashing). Although being slashed has serious repercussions, it is simple enough to avoid being slashed all together by remaining _consistent_ with respect to the messages a validator has previously signed.
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