Merge pull request #253 from ethereum/JustinDrake-patch-8

Bug fixes and cleanups around exit_validator

specs/core/0_beacon-chain.md

@@ -57,6 +57,7 @@
         - [Beacon chain fork choice rule](#beacon-chain-fork-choice-rule)
     - [Beacon chain state transition function](#beacon-chain-state-transition-function)
         - [Helper functions](#helper-functions)
+            - [`is_active_validator`](#is_active_validator)
             - [`get_active_validator_indices`](#get_active_validator_indices)
             - [`shuffle`](#shuffle)
             - [`split`](#split)
@@ -86,6 +87,7 @@
             - [Attestations](#attestations-1)
             - [Deposits](#deposits-1)
             - [Exits](#exits-1)
+        - [Ejections](#ejections)
     - [Per-epoch processing](#per-epoch-processing)
         - [Helpers](#helpers)
         - [Receipt roots](#receipt-roots)
@@ -143,7 +145,7 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
 | - | - | :-: |
 | `SHARD_COUNT` | `2**10` (= 1,024) | shards |
 | `TARGET_COMMITTEE_SIZE` | `2**8` (= 256) | [validators](#dfn-validator) |
-| `MIN_BALANCE` | `2**4` (= 16) | ETH |
+| `EJECTION_BALANCE` | `2**4` (= 16) | ETH |
 | `MAX_BALANCE_CHURN_QUOTIENT` | `2**5` (= 32) | - |
 | `GWEI_PER_ETH` | `10**9` | Gwei/ETH |
 | `BEACON_CHAIN_SHARD_NUMBER` | `2**64 - 1` | - |
@@ -701,6 +703,15 @@ The per-slot transitions generally focus on verifying aggregate signatures and s
 
 Note: The definitions below are for specification purposes and are not necessarily optimal implementations.
 
+#### `is_active_validator`
+ ```python
+def is_active_validator(validator: ValidatorRecord) -> bool:
+    """
+    Checks if ``validator`` is active.
+    """
+    return validator.status in [ACTIVE, ACTIVE_PENDING_EXIT]
+```
+
 #### `get_active_validator_indices`
 
 ```python
@@ -708,7 +719,7 @@ def get_active_validator_indices(validators: [ValidatorRecord]) -> List[int]:
     """
     Gets indices of active validators from ``validators``.
     """
-    return [i for i, v in enumerate(validators) if v.status in [ACTIVE, ACTIVE_PENDING_EXIT]]
+    return [i for i, v in enumerate(validators) if is_active_validator(v)]
 ```
 
 #### `shuffle`
@@ -895,7 +906,7 @@ def get_updated_ancestor_hashes(latest_block: BeaconBlock,
 #### `get_attestation_participants`
 
 ```python
-def get_attestation_participants(state: State,
+def get_attestation_participants(state: BeaconState,
                                  attestation_data: AttestationData,
                                  participation_bitfield: bytes) -> List[int]:
     """
@@ -963,7 +974,7 @@ def get_domain(fork_data: ForkData,
 #### `verify_casper_votes`
 
 ```python
-def verify_casper_votes(state: State, votes: SlashableVoteData) -> bool:
+def verify_casper_votes(state: BeaconState, votes: SlashableVoteData) -> bool:
     if len(votes.aggregate_signature_poc_0_indices) + len(votes.aggregate_signature_poc_1_indices) > MAX_CASPER_VOTES:
         return False
 
@@ -1032,8 +1043,8 @@ def on_startup(initial_validator_entries: List[Any],
             proof_of_possession=proof_of_possession,
             withdrawal_credentials=withdrawal_credentials,
             randao_commitment=randao_commitment,
-            current_slot=INITIAL_SLOT_NUMBER,
             status=ACTIVE,
+            current_slot=INITIAL_SLOT_NUMBER,
         )
 
     # Setup state
@@ -1166,8 +1177,7 @@ def process_deposit(state: BeaconState,
                     proof_of_possession: bytes,
                     withdrawal_credentials: Hash32,
                     randao_commitment: Hash32,
-                    status: int,
+                    status: int) -> int:
-                    current_slot: int) -> int:
     """
     Process a deposit from Ethereum 1.0.
     Note that this function mutates ``state``.
@@ -1185,7 +1195,7 @@ def process_deposit(state: BeaconState,
         withdrawal_credentials=withdrawal_credentials,
         randao_commitment=randao_commitment,
         status=status,
-        current_slot=current_slot,
+        current_slot=state.slot,
     )
 
     return index
@@ -1196,8 +1206,7 @@ def process_deposit(state: BeaconState,
 ```python
 def exit_validator(index: int,
                    state: BeaconState,
-                   penalize: bool,
+                   new_status: bool) -> None:
-                   current_slot: int) -> None:
     """
     Exit the validator with the given ``index``.
     Note that this function mutates ``state``.
@@ -1205,7 +1214,8 @@ def exit_validator(index: int,
     state.validator_registry_exit_count += 1
 
     validator = state.validator_registry[index]
-    validator.latest_status_change_slot = current_slot
+    validator.status = new_status
+    validator.latest_status_change_slot = state.slot
     validator.exit_count = state.validator_registry_exit_count
 
     # Remove validator from persistent committees
@@ -1215,16 +1225,13 @@ def exit_validator(index: int,
                 committee.pop(i)
                 break
 
-    if penalize:
+    if new_status == EXITED_WITH_PENALTY:
-        validator.status = EXITED_WITH_PENALTY
+        state.latest_penalized_exit_balances[state.slot // COLLECTIVE_PENALTY_CALCULATION_PERIOD] += get_effective_balance(validator)
-        state.latest_penalized_exit_balances[current_slot // COLLECTIVE_PENALTY_CALCULATION_PERIOD] += get_effective_balance(validator)
         
-        whistleblower = state.validator_registry[get_beacon_proposer_index(state, current_slot)]
+        whistleblower = state.validator_registry[get_beacon_proposer_index(state, state.slot)]
         whistleblower_reward = validator.balance // WHISTLEBLOWER_REWARD_QUOTIENT
         whistleblower.balance += whistleblower_reward
         validator.balance -= whistleblower_reward
-    else:
-        validator.status = ACTIVE_PENDING_EXIT
 
     state.validator_registry_delta_chain_tip = get_new_validator_registry_delta_chain_tip(
         validator_registry_delta_chain_tip=state.validator_registry_delta_chain_tip,
@@ -1244,11 +1251,13 @@ Below are the processing steps that happen at every slot.
 * Set `state.latest_block_hashes = state.latest_block_hashes + [latest_hash]`. (The output of `get_block_hash` should not change, except that it will no longer throw for `state.slot - 1`).
 
 If there is a block from the proposer for `state.slot`, we process that incoming block:
+
 * Let `block` be that associated incoming block.
 * Verify that `block.slot == state.slot`
 * Verify that `block.ancestor_hashes` equals `get_updated_ancestor_hashes(latest_block, latest_hash)`.
 
 If there is no block from the proposer at state.slot:
+
 * Set `state.validator_registry[get_beacon_proposer_index(state, state.slot)].randao_skips += 1`.
 * Skip all other per-slot processing. Move directly to [per-epoch processing](#per-epoch-processing).
 
@@ -1287,7 +1296,7 @@ For each `proposer_slashing` in `block.body.proposer_slashings`:
 * Verify that `proposer_slashing.proposal_data_1.shard == proposer_slashing.proposal_data_2.shard`.
 * Verify that `proposer_slashing.proposal_data_1.block_hash != proposer_slashing.proposal_data_2.block_hash`.
 * Verify that `proposer.status != EXITED_WITH_PENALTY`.
-* Run `exit_validator(proposer_slashing.proposer_index, state, penalize=True, current_slot=state.slot)`.
+* Run `exit_validator(proposer_slashing.proposer_index, state, new_status=EXITED_WITH_PENALTY)`.
 
 #### Casper slashings
 
@@ -1302,7 +1311,7 @@ For each `casper_slashing` in `block.body.casper_slashings`:
 * Let `intersection = [x for x in indices(casper_slashing.votes_1) if x in indices(casper_slashing.votes_2)]`.
 * Verify that `len(intersection) >= 1`.
 * Verify that `casper_slashing.votes_1.data.justified_slot + 1 < casper_slashing.votes_2.data.justified_slot + 1 == casper_slashing.votes_2.data.slot < casper_slashing.votes_1.data.slot` or `casper_slashing.votes_1.data.slot == casper_slashing.votes_2.data.slot`.
-* For each [validator](#dfn-validator) index `i` in `intersection`, if `state.validator_registry[i].status` does not equal `EXITED_WITH_PENALTY`, then run `exit_validator(i, state, penalize=True, current_slot=state.slot)`
+* For each [validator](#dfn-validator) index `i` in `intersection`, if `state.validator_registry[i].status` does not equal `EXITED_WITH_PENALTY`, then run `exit_validator(i, state, new_status=EXITED_WITH_PENALTY)`
 
 #### Attestations
 
@@ -1353,8 +1362,7 @@ process_deposit(
     proof_of_possession=deposit.deposit_data.deposit_parameters.proof_of_possession,
     withdrawal_credentials=deposit.deposit_data.deposit_parameters.withdrawal_credentials,
     randao_commitment=deposit.deposit_data.deposit_parameters.randao_commitment,
-    status=PENDING_ACTIVATION,
+    status=PENDING_ACTIVATION
-    current_slot=state.slot
 )
 ```
 
@@ -1369,7 +1377,22 @@ For each `exit` in `block.body.exits`:
 * Verify that `validator.status == ACTIVE`.
 * Verify that `state.slot >= exit.slot`.
 * Verify that `state.slot >= validator.latest_status_change_slot + SHARD_PERSISTENT_COMMITTEE_CHANGE_PERIOD`.
-* Run `exit_validator(validator_index, state, penalize=False, current_slot=state.slot)`.
+* Run `exit_validator(validator_index, state, new_status=ACTIVE_PENDING_EXIT)`.
+
+### Ejections
+
+* Run `process_ejections(state)`.
+
+ ```python
+def process_ejections(state: BeaconState) -> None:
+    """
+    Iterate through the validator registry
+    and eject active validators with balance below ``EJECTION_BALANCE``.
+    """
+    for i, v in enumerate(state.validator_registry):
+        if is_active_validator(v) and v.balance < EJECTION_BALANCE:
+            exit_validator(i, state, new_status=EXITED_WITHOUT_PENALTY)
+```
 
 ## Per-epoch processing
 
@@ -1437,6 +1460,7 @@ If `state.slot % POW_RECEIPT_ROOT_VOTING_PERIOD == 0`:
 ### Finalization
 
 Set `state.finalized_slot = state.previous_justified_slot` if any of the following are true:
+
 * `state.previous_justified_slot == state.slot - 2 * EPOCH_LENGTH and state.justification_bitfield % 4 == 3`
 * `state.previous_justified_slot == state.slot - 3 * EPOCH_LENGTH and state.justification_bitfield % 8 == 7`
 * `state.previous_justified_slot == state.slot - 4 * EPOCH_LENGTH and state.justification_bitfield % 16 in (15, 14)`
@@ -1620,7 +1644,6 @@ while len(state.persistent_committee_reassignments) > 0 and state.persistent_com
 ### Final updates
 
 * Remove any `attestation` in `state.latest_attestations` such that `attestation.data.slot < state.slot - EPOCH_LENGTH`.
-* Run `exit_validator(i, state, penalize=False, current_slot=state.slot)` for indices `i` such that `state.validator_registry[i].status == ACTIVE and state.validator_registry[i].balance < MIN_BALANCE`.
 * Set `state.latest_block_hashes = state.latest_block_hashes[EPOCH_LENGTH:]`.
 
 ## State root processing