Implement #459

Contents: * Peg entries and exits to epoch boundaries * Add a store of historical active index roots * Mix it into the randomness * Remove the delta hash chain Note that the actual light client implementation is beyond the scope of the spec. [Note to reviewers: verify that the invariant added in the PR is correct] Question: * Do we want to also only store epoch-boundary randao values? I don't think we use the epoch-intermediate ones anywhere.....
ethereum · Jan 19, 2019 · 9e75a76 · 9e75a76
1 parent 8d5c75c
commit 9e75a76
Showing 1 changed file with 35 additions and 56 deletions.
diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
@@ -18,7 +18,6 @@
         - [Reward and penalty quotients](#reward-and-penalty-quotients)
         - [Status flags](#status-flags)
         - [Max operations per block](#max-operations-per-block)
-        - [Validator registry delta flags](#validator-registry-delta-flags)
         - [Signature domains](#signature-domains)
     - [Data structures](#data-structures)
         - [Beacon chain operations](#beacon-chain-operations)
@@ -47,7 +46,6 @@
             - [`Crosslink`](#crosslink)
             - [`PendingAttestation`](#pendingattestation)
             - [`Fork`](#fork)
-            - [`ValidatorRegistryDeltaBlock`](#validatorregistrydeltablock)
             - [`Eth1Data`](#eth1data)
             - [`Eth1DataVote`](#eth1datavote)
     - [Ethereum 1.0 deposit contract](#ethereum-10-deposit-contract)
@@ -73,6 +71,7 @@
             - [`get_crosslink_committees_at_slot`](#get_crosslink_committees_at_slot)
             - [`get_block_root`](#get_block_root)
             - [`get_randao_mix`](#get_randao_mix)
+            - [`get_active_index_root`](#get_active_index_root)
             - [`get_beacon_proposer_index`](#get_beacon_proposer_index)
             - [`merkle_root`](#merkle_root)
             - [`get_attestation_participants`](#get_attestation_participants)
@@ -168,6 +167,7 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
 | `MAX_CASPER_VOTES` | `2**10` (= 1,024) | votes |
 | `LATEST_BLOCK_ROOTS_LENGTH` | `2**13` (= 8,192) | block roots |
 | `LATEST_RANDAO_MIXES_LENGTH` | `2**13` (= 8,192) | randao mixes |
+| `LATEST_INDEX_ROOTS_LENGTH` | `2**13` (= 8,192) | index roots |
 | `LATEST_PENALIZED_EXIT_LENGTH` | `2**13` (= 8,192) | epochs | ~36 days |
 | `MAX_WITHDRAWALS_PER_EPOCH` | `2**2` (= 4) | withdrawals |
 
@@ -235,13 +235,6 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
 | `MAX_DEPOSITS` | `2**4` (= 16) |
 | `MAX_EXITS` | `2**4` (= 16) |
 
-### Validator registry delta flags
-
-| Name | Value |
-| - | - |
-| `ACTIVATION` | `0` |
-| `EXIT` | `1` |
-
 ### Signature domains
 
 | Name | Value |
@@ -478,7 +471,6 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
     'validator_balances': ['uint64'],
     'validator_registry_update_slot': 'uint64',
     'validator_registry_exit_count': 'uint64',
-    'validator_registry_delta_chain_tip': 'hash32',  # For light clients to track deltas
 
     # Randomness and committees
     'latest_randao_mixes': ['hash32'],
@@ -487,8 +479,8 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
     'current_epoch_start_shard': 'uint64',
     'previous_epoch_calculation_slot': 'uint64',
     'current_epoch_calculation_slot': 'uint64',
-    'previous_epoch_randao_mix': 'hash32',
-    'current_epoch_randao_mix': 'hash32',
+    'previous_epoch_seed': 'hash32',
+    'current_epoch_seed': 'hash32',
 
     # Custody challenges
     'custody_challenges': [CustodyChallenge],
@@ -502,6 +494,7 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
     # Recent state
     'latest_crosslinks': [Crosslink],
     'latest_block_roots': ['hash32'],  # Needed to process attestations, older to newer
+    'latest_index_roots': ['hash32'],
     'latest_penalized_balances': ['uint64'],  # Balances penalized at every withdrawal period
     'latest_attestations': [PendingAttestation],
     'batched_block_roots': ['hash32'],
@@ -584,18 +577,6 @@ Unless otherwise indicated, code appearing in `this style` is to be interpreted
 }
 ```
 
-#### `ValidatorRegistryDeltaBlock`
-
-```python
-{
-    'latest_registry_delta_root': 'hash32',
-    'validator_index': 'uint24',
-    'pubkey': 'uint384',
-    'slot': 'uint64',
-    'flag': 'uint64',
-}
-```
-
 #### `Eth1Data`
 
 ```python
@@ -954,15 +935,15 @@ def get_crosslink_committees_at_slot(state: BeaconState,
     if slot < state_epoch_slot:
         committees_per_slot = get_previous_epoch_committee_count_per_slot(state)
         shuffling = get_shuffling(
-            state.previous_epoch_randao_mix,
+            state.previous_epoch_seed,
             state.validator_registry,
             state.previous_epoch_calculation_slot,
         )
         slot_start_shard = (state.previous_epoch_start_shard + committees_per_slot * offset) % SHARD_COUNT
     else:
         committees_per_slot = get_current_epoch_committee_count_per_slot(state)
         shuffling = get_shuffling(
-            state.current_epoch_randao_mix,
+            state.current_epoch_seed,
             state.validator_registry,
             state.current_epoch_calculation_slot,
         )
@@ -1007,6 +988,19 @@ def get_randao_mix(state: BeaconState,
     return state.latest_randao_mixes[slot % LATEST_RANDAO_MIXES_LENGTH]
 ```
 
+#### `get_active_index_root`
+
+```python
+def get_active_index_root(state: BeaconState,
+                          slot: int) -> Hash32:
+    """
+    Returns the randao mix at a recent ``slot``.
+    """
+    assert state.slot // EPOCH_LENGTH < slot // EPOCH_LENGTH + LATEST_INDEX_ROOTS_LENGTH
+    assert slot // EPOCH_LENGTH <= state.slot // EPOCH_LENGTH
+    return state.latest_index_roots[(slot // EPOCH_LENGTH) % LATEST_INDEX_ROOTS_LENGTH]
+```
+
 #### `get_beacon_proposer_index`
 
 ```python
@@ -1235,7 +1229,6 @@ def get_initial_beacon_state(initial_validator_deposits: List[Deposit],
         validator_balances=[],
         validator_registry_update_slot=GENESIS_SLOT,
         validator_registry_exit_count=0,
-        validator_registry_delta_chain_tip=ZERO_HASH,
 
         # Randomness and committees
         latest_randao_mixes=[ZERO_HASH for _ in range(LATEST_RANDAO_MIXES_LENGTH)],
@@ -1244,8 +1237,8 @@ def get_initial_beacon_state(initial_validator_deposits: List[Deposit],
         current_epoch_start_shard=GENESIS_START_SHARD,
         previous_epoch_calculation_slot=GENESIS_SLOT,
         current_epoch_calculation_slot=GENESIS_SLOT,
-        previous_epoch_randao_mix=ZERO_HASH,
-        current_epoch_randao_mix=ZERO_HASH,
+        previous_epoch_seed=ZERO_HASH,
+        current_epoch_seed=ZERO_HASH,
 
         # Custody challenges
         custody_challenges=[],
@@ -1259,6 +1252,7 @@ def get_initial_beacon_state(initial_validator_deposits: List[Deposit],
         # Recent state
         latest_crosslinks=[Crosslink(slot=GENESIS_SLOT, shard_block_root=ZERO_HASH) for _ in range(SHARD_COUNT)],
         latest_block_roots=[ZERO_HASH for _ in range(LATEST_BLOCK_ROOTS_LENGTH)],
+        latest_index_roots=[ZERO_HASH for _ in range(LATEST_INDEX_ROOTS_LENGTH)],
         latest_penalized_balances=[0 for _ in range(LATEST_PENALIZED_EXIT_LENGTH)],
         latest_attestations=[],
         batched_block_roots=[],
@@ -1383,16 +1377,7 @@ Note: All functions in this section mutate `state`.
 def activate_validator(state: BeaconState, index: int, genesis: bool) -> None:
     validator = state.validator_registry[index]
 
-    validator.activation_slot = GENESIS_SLOT if genesis else (state.slot + ENTRY_EXIT_DELAY)
-    state.validator_registry_delta_chain_tip = hash_tree_root(
-        ValidatorRegistryDeltaBlock(
-            latest_registry_delta_root=state.validator_registry_delta_chain_tip,
-            validator_index=index,
-            pubkey=validator.pubkey,
-            slot=validator.activation_slot,
-            flag=ACTIVATION,
-        )
-    )
+    validator.activation_slot = GENESIS_SLOT if genesis else (state.slot - state.slot % EPOCH_LENGTH + ENTRY_EXIT_DELAY)
 ```
 
 ```python
@@ -1406,22 +1391,13 @@ def exit_validator(state: BeaconState, index: int) -> None:
     validator = state.validator_registry[index]
 
     # The following updates only occur if not previous exited
-    if validator.exit_slot <= state.slot + ENTRY_EXIT_DELAY:
+    if validator.exit_slot <= state.slot - state.slot % EPOCH_LENGTH + ENTRY_EXIT_DELAY:
         return
 
-    validator.exit_slot = state.slot + ENTRY_EXIT_DELAY
+    validator.exit_slot = state.slot - state.slot % EPOCH_LENGTH + ENTRY_EXIT_DELAY
 
     state.validator_registry_exit_count += 1
     validator.exit_count = state.validator_registry_exit_count
-    state.validator_registry_delta_chain_tip = hash_tree_root(
-        ValidatorRegistryDeltaBlock(
-            latest_registry_delta_root=state.validator_registry_delta_chain_tip,
-            validator_index=index,
-            pubkey=validator.pubkey,
-            slot=validator.exit_slot,
-            flag=EXIT,
-        )
-    )
 ```
 
 ```python
@@ -1583,7 +1559,7 @@ Verify that `len(block.body.exits) <= MAX_EXITS`.
 For each `exit` in `block.body.exits`:
 
 * Let `validator = state.validator_registry[exit.validator_index]`.
-* Verify that `validator.exit_slot > state.slot + ENTRY_EXIT_DELAY`.
+* Verify that `validator.exit_slot > state.slot - state.slot % EPOCH_LENGTH + ENTRY_EXIT_DELAY`.
 * Verify that `state.slot >= exit.slot`.
 * Let `exit_message = hash_tree_root(Exit(slot=exit.slot, validator_index=exit.validator_index, signature=EMPTY_SIGNATURE))`.
 * Verify that `bls_verify(pubkey=validator.pubkey, message=exit_message, signature=exit.signature, domain=get_domain(state.fork, exit.slot, DOMAIN_EXIT))`.
@@ -1763,7 +1739,7 @@ def update_validator_registry(state: BeaconState) -> None:
     # Activate validators within the allowable balance churn
     balance_churn = 0
     for index, validator in enumerate(state.validator_registry):
-        if validator.activation_slot > state.slot + ENTRY_EXIT_DELAY and state.validator_balances[index] >= MAX_DEPOSIT_AMOUNT:
+        if validator.activation_slot > state.slot - state.slot % EPOCH_LENGTH + ENTRY_EXIT_DELAY and state.validator_balances[index] >= MAX_DEPOSIT_AMOUNT:
             # Check the balance churn would be within the allowance
             balance_churn += get_effective_balance(state, index)
             if balance_churn > max_balance_churn:
@@ -1775,7 +1751,7 @@ def update_validator_registry(state: BeaconState) -> None:
     # Exit validators within the allowable balance churn
     balance_churn = 0
     for index, validator in enumerate(state.validator_registry):
-        if validator.exit_slot > state.slot + ENTRY_EXIT_DELAY and validator.status_flags & INITIATED_EXIT:
+        if validator.exit_slot > state.slot - state.slot % EPOCH_LENGTH + ENTRY_EXIT_DELAY and validator.status_flags & INITIATED_EXIT:
             # Check the balance churn would be within the allowance
             balance_churn += get_effective_balance(state, index)
             if balance_churn > max_balance_churn:
@@ -1791,17 +1767,19 @@ and perform the following updates:
 
 * Set `state.previous_epoch_calculation_slot = state.current_epoch_calculation_slot`
 * Set `state.previous_epoch_start_shard = state.current_epoch_start_shard`
-* Set `state.previous_epoch_randao_mix = state.current_epoch_randao_mix`
+* Set `state.previous_epoch_seed = state.current_epoch_seed`
 * Set `state.current_epoch_calculation_slot = state.slot`
 * Set `state.current_epoch_start_shard = (state.current_epoch_start_shard + get_current_epoch_committee_count_per_slot(state) * EPOCH_LENGTH) % SHARD_COUNT`
-* Set `state.current_epoch_randao_mix = get_randao_mix(state, state.current_epoch_calculation_slot - SEED_LOOKAHEAD)`
+* Set `state.current_epoch_seed = hash(get_randao_mix(state, state.current_epoch_calculation_slot - SEED_LOOKAHEAD) + get_active_index_root(state, state.current_epoch_calculation_slot))`
 
 If a validator registry update does _not_ happen do the following:
 
 * Set `state.previous_epoch_calculation_slot = state.current_epoch_calculation_slot`
 * Set `state.previous_epoch_start_shard = state.current_epoch_start_shard`
 * Let `epochs_since_last_registry_change = (state.slot - state.validator_registry_update_slot) // EPOCH_LENGTH`.
-* If `epochs_since_last_registry_change` is an exact power of 2, set `state.current_epoch_calculation_slot = state.slot` and `state.current_epoch_randao_mix = state.latest_randao_mixes[(state.current_epoch_calculation_slot - SEED_LOOKAHEAD) % LATEST_RANDAO_MIXES_LENGTH]`. Note that `state.current_epoch_start_shard` is left unchanged.
+* If `epochs_since_last_registry_change` is an exact power of 2, set `state.current_epoch_calculation_slot = state.slot` and `state.current_epoch_seed = hash(get_randao_mix(state, state.current_epoch_calculation_slot - SEED_LOOKAHEAD) + get_active_index_root(state, state.current_epoch_calculation_slot))`. Note that `state.current_epoch_start_shard` is left unchanged.
+
+**Invariant**: the active index root that is hashed into the shuffling seed actually is the `hash_tree_root` of the validator set that is used for that epoch.
 
 Regardless of whether or not a validator set change happens, run the following:
 
@@ -1844,6 +1822,7 @@ def process_penalties_and_exits(state: BeaconState) -> None:
 
 * Let `e = state.slot // EPOCH_LENGTH`. Set `state.latest_penalized_balances[(e+1) % LATEST_PENALIZED_EXIT_LENGTH] = state.latest_penalized_balances[e % LATEST_PENALIZED_EXIT_LENGTH]`
 * Remove any `attestation` in `state.latest_attestations` such that `attestation.data.slot < state.slot - EPOCH_LENGTH`.
+* Let `epoch = state.slot // EPOCH_LENGTH`. Set `state.latest_index_roots[epoch % LATEST_INDEX_ROOTS_LENGTH] = hash_tree_root(get_active_validator_indices(state, state.slot))`
 
 ## State root processing