Merge pull request #2746 from ethereum/vbuterin-patch-12

Simplify sync protocol and update to calculate optimistic heads
ethereum · Dec 15, 2021 · 2fa396f · 2fa396f
2 parents 4cd2334 + de89238
commit 2fa396f
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Showing 5 changed files with 194 additions and 120 deletions.
diff --git a/presets/mainnet/altair.yaml b/presets/mainnet/altair.yaml
@@ -22,3 +22,5 @@ EPOCHS_PER_SYNC_COMMITTEE_PERIOD: 256
 # ---------------------------------------------------------------
 # 1
 MIN_SYNC_COMMITTEE_PARTICIPANTS: 1
+# SLOTS_PER_EPOCH * EPOCHS_PER_SYNC_COMMITTEE_PERIOD (= 32 * 256)
+UPDATE_TIMEOUT: 8192
diff --git a/presets/minimal/altair.yaml b/presets/minimal/altair.yaml
@@ -22,3 +22,5 @@ EPOCHS_PER_SYNC_COMMITTEE_PERIOD: 8
 # ---------------------------------------------------------------
 # 1
 MIN_SYNC_COMMITTEE_PARTICIPANTS: 1
+# SLOTS_PER_EPOCH * EPOCHS_PER_SYNC_COMMITTEE_PERIOD (= 8 * 8)
+UPDATE_TIMEOUT: 64
diff --git a/setup.py b/setup.py
@@ -683,6 +683,7 @@ def combine_dicts(old_dict: Dict[str, T], new_dict: Dict[str, T]) -> Dict[str, T
     'uint8', 'uint16', 'uint32', 'uint64', 'uint128', 'uint256',
     'bytes', 'byte', 'ByteList', 'ByteVector',
     'Dict', 'dict', 'field', 'ceillog2', 'floorlog2', 'Set',
+    'Optional',
 ]
 
 

diff --git a/specs/altair/sync-protocol.md b/specs/altair/sync-protocol.md
@@ -13,12 +13,14 @@
 - [Preset](#preset)
   - [Misc](#misc)
 - [Containers](#containers)
-  - [`LightClientSnapshot`](#lightclientsnapshot)
   - [`LightClientUpdate`](#lightclientupdate)
   - [`LightClientStore`](#lightclientstore)
 - [Helper functions](#helper-functions)
   - [`get_subtree_index`](#get_subtree_index)
+  - [`get_active_header`](#get_active_header)
+  - [`get_safety_threshold`](#get_safety_threshold)
 - [Light client state updates](#light-client-state-updates)
+    - [`process_slot_for_light_client_store`](#process_slot_for_light_client_store)
     - [`validate_light_client_update`](#validate_light_client_update)
     - [`apply_light_client_update`](#apply_light_client_update)
     - [`process_light_client_update`](#process_light_client_update)
@@ -47,38 +49,27 @@ uses sync committees introduced in [this beacon chain extension](./beacon-chain.
 
 ### Misc
 
-| Name | Value |
-| - | - |
-| `MIN_SYNC_COMMITTEE_PARTICIPANTS` | `1` |
+| Name | Value | Notes |
+| - | - | - |
+| `MIN_SYNC_COMMITTEE_PARTICIPANTS` | `1` | |
+| `UPDATE_TIMEOUT` | `SLOTS_PER_EPOCH * EPOCHS_PER_SYNC_COMMITTEE_PERIOD` | ~27.3 hours |
 
 ## Containers
 
-### `LightClientSnapshot`
-
-```python
-class LightClientSnapshot(Container):
-    # Beacon block header
-    header: BeaconBlockHeader
-    # Sync committees corresponding to the header
-    current_sync_committee: SyncCommittee
-    next_sync_committee: SyncCommittee
-```
-
 ### `LightClientUpdate`
 
 ```python
 class LightClientUpdate(Container):
-    # Update beacon block header
-    header: BeaconBlockHeader
-    # Next sync committee corresponding to the header
+    # The beacon block header that is attested to by the sync committee
+    attested_header: BeaconBlockHeader
+    # Next sync committee corresponding to the active header
     next_sync_committee: SyncCommittee
     next_sync_committee_branch: Vector[Bytes32, floorlog2(NEXT_SYNC_COMMITTEE_INDEX)]
-    # Finality proof for the update header
-    finality_header: BeaconBlockHeader
+    # The finalized beacon block header attested to by Merkle branch
+    finalized_header: BeaconBlockHeader
     finality_branch: Vector[Bytes32, floorlog2(FINALIZED_ROOT_INDEX)]
     # Sync committee aggregate signature
-    sync_committee_bits: Bitvector[SYNC_COMMITTEE_SIZE]
-    sync_committee_signature: BLSSignature
+    sync_committee_aggregate: SyncAggregate
     # Fork version for the aggregate signature
     fork_version: Version
 ```
@@ -88,8 +79,18 @@ class LightClientUpdate(Container):
 ```python
 @dataclass
 class LightClientStore(object):
-    snapshot: LightClientSnapshot
-    valid_updates: Set[LightClientUpdate]
+    # Beacon block header that is finalized
+    finalized_header: BeaconBlockHeader
+    # Sync committees corresponding to the header
+    current_sync_committee: SyncCommittee
+    next_sync_committee: SyncCommittee
+    # Best available header to switch finalized head to if we see nothing else
+    best_valid_update: Optional[LightClientUpdate]
+    # Most recent available reasonably-safe header
+    optimistic_header: BeaconBlockHeader
+    # Max number of active participants in a sync committee (used to calculate safety threshold)
+    previous_max_active_participants: uint64
+    current_max_active_participants: uint64
 ```
 
 ## Helper functions
@@ -101,95 +102,157 @@ def get_subtree_index(generalized_index: GeneralizedIndex) -> uint64:
     return uint64(generalized_index % 2**(floorlog2(generalized_index)))
 ```
 
+### `get_active_header`
+
+```python
+def get_active_header(update: LightClientUpdate) -> BeaconBlockHeader:
+    # The "active header" is the header that the update is trying to convince us
+    # to accept. If a finalized header is present, it's the finalized header,
+    # otherwise it's the attested header
+    if update.finalized_header != BeaconBlockHeader():
+        return update.finalized_header
+    else:
+        return update.attested_header
+```
+
+### `get_safety_threshold`
+
+```python
+def get_safety_threshold(store: LightClientStore) -> uint64:
+    return max(
+        store.previous_max_active_participants,
+        store.current_max_active_participants,
+    ) // 2
+```
+
 ## Light client state updates
 
-A light client maintains its state in a `store` object of type `LightClientStore` and receives `update` objects of type `LightClientUpdate`. Every `update` triggers `process_light_client_update(store, update, current_slot)` where `current_slot` is the current slot based on some local clock.
+A light client maintains its state in a `store` object of type `LightClientStore` and receives `update` objects of type `LightClientUpdate`. Every `update` triggers `process_light_client_update(store, update, current_slot)` where `current_slot` is the current slot based on some local clock. `process_slot_for_light_client_store` is processed every time the current slot increments.
+
+#### `process_slot_for_light_client_store`
+
+```python
+def process_slot_for_light_client_store(store: LightClientStore, current_slot: Slot) -> None:
+    if current_slot % UPDATE_TIMEOUT == 0:
+        store.previous_max_active_participants = store.current_max_active_participants
+        store.current_max_active_participants = 0
+    if (
+        current_slot > store.finalized_header.slot + UPDATE_TIMEOUT
+        and store.best_valid_update is not None
+    ):
+        # Forced best update when the update timeout has elapsed
+        apply_light_client_update(store, store.best_valid_update)
+        store.best_valid_update = None
+```
 
 #### `validate_light_client_update`
 
 ```python
-def validate_light_client_update(snapshot: LightClientSnapshot,
+def validate_light_client_update(store: LightClientStore,
                                  update: LightClientUpdate,
+                                 current_slot: Slot,
                                  genesis_validators_root: Root) -> None:
-    # Verify update slot is larger than snapshot slot
-    assert update.header.slot > snapshot.header.slot
+    # Verify update slot is larger than slot of current best finalized header
+    active_header = get_active_header(update)
+    assert current_slot >= active_header.slot > store.finalized_header.slot
 
     # Verify update does not skip a sync committee period
-    snapshot_period = compute_epoch_at_slot(snapshot.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
-    update_period = compute_epoch_at_slot(update.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
-    assert update_period in (snapshot_period, snapshot_period + 1)
+    finalized_period = compute_epoch_at_slot(store.finalized_header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
+    update_period = compute_epoch_at_slot(active_header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
+    assert update_period in (finalized_period, finalized_period + 1)
 
-    # Verify update header root is the finalized root of the finality header, if specified
-    if update.finality_header == BeaconBlockHeader():
-        signed_header = update.header
+    # Verify that the `finalized_header`, if present, actually is the finalized header saved in the
+    # state of the `attested header`
+    if update.finalized_header == BeaconBlockHeader():
         assert update.finality_branch == [Bytes32() for _ in range(floorlog2(FINALIZED_ROOT_INDEX))]
     else:
-        signed_header = update.finality_header
         assert is_valid_merkle_branch(
-            leaf=hash_tree_root(update.header),
+            leaf=hash_tree_root(update.finalized_header),
             branch=update.finality_branch,
             depth=floorlog2(FINALIZED_ROOT_INDEX),
             index=get_subtree_index(FINALIZED_ROOT_INDEX),
-            root=update.finality_header.state_root,
+            root=update.attested_header.state_root,
         )
 
     # Verify update next sync committee if the update period incremented
-    if update_period == snapshot_period:
-        sync_committee = snapshot.current_sync_committee
+    if update_period == finalized_period:
+        sync_committee = store.current_sync_committee
         assert update.next_sync_committee_branch == [Bytes32() for _ in range(floorlog2(NEXT_SYNC_COMMITTEE_INDEX))]
     else:
-        sync_committee = snapshot.next_sync_committee
+        sync_committee = store.next_sync_committee
         assert is_valid_merkle_branch(
             leaf=hash_tree_root(update.next_sync_committee),
             branch=update.next_sync_committee_branch,
             depth=floorlog2(NEXT_SYNC_COMMITTEE_INDEX),
             index=get_subtree_index(NEXT_SYNC_COMMITTEE_INDEX),
-            root=update.header.state_root,
+            root=active_header.state_root,
         )
+
+    sync_aggregate = update.sync_committee_aggregate
 
     # Verify sync committee has sufficient participants
-    assert sum(update.sync_committee_bits) >= MIN_SYNC_COMMITTEE_PARTICIPANTS
+    assert sum(sync_aggregate.sync_committee_bits) >= MIN_SYNC_COMMITTEE_PARTICIPANTS
 
     # Verify sync committee aggregate signature
-    participant_pubkeys = [pubkey for (bit, pubkey) in zip(update.sync_committee_bits, sync_committee.pubkeys) if bit]
+    participant_pubkeys = [
+        pubkey for (bit, pubkey) in zip(sync_aggregate.sync_committee_bits, sync_committee.pubkeys)
+        if bit
+    ]
     domain = compute_domain(DOMAIN_SYNC_COMMITTEE, update.fork_version, genesis_validators_root)
-    signing_root = compute_signing_root(signed_header, domain)
-    assert bls.FastAggregateVerify(participant_pubkeys, signing_root, update.sync_committee_signature)
+    signing_root = compute_signing_root(update.attested_header, domain)
+    assert bls.FastAggregateVerify(participant_pubkeys, signing_root, sync_aggregate.sync_committee_signature)
 ```
 
 #### `apply_light_client_update`
 
 ```python
-def apply_light_client_update(snapshot: LightClientSnapshot, update: LightClientUpdate) -> None:
-    snapshot_period = compute_epoch_at_slot(snapshot.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
-    update_period = compute_epoch_at_slot(update.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
-    if update_period == snapshot_period + 1:
-        snapshot.current_sync_committee = snapshot.next_sync_committee
-        snapshot.next_sync_committee = update.next_sync_committee
-    snapshot.header = update.header
+def apply_light_client_update(store: LightClientStore, update: LightClientUpdate) -> None:
+    active_header = get_active_header(update)
+    finalized_period = compute_epoch_at_slot(store.finalized_header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
+    update_period = compute_epoch_at_slot(active_header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
+    if update_period == finalized_period + 1:
+        store.current_sync_committee = store.next_sync_committee
+        store.next_sync_committee = update.next_sync_committee
+    store.finalized_header = active_header
 ```
 
 #### `process_light_client_update`
 
 ```python
-def process_light_client_update(store: LightClientStore, update: LightClientUpdate, current_slot: Slot,
+def process_light_client_update(store: LightClientStore,
+                                update: LightClientUpdate,
+                                current_slot: Slot,
                                 genesis_validators_root: Root) -> None:
-    validate_light_client_update(store.snapshot, update, genesis_validators_root)
-    store.valid_updates.add(update)
+    validate_light_client_update(store, update, current_slot, genesis_validators_root)
 
-    update_timeout = SLOTS_PER_EPOCH * EPOCHS_PER_SYNC_COMMITTEE_PERIOD
+    sync_committee_bits = update.sync_committee_aggregate.sync_committee_bits
+
+    # Update the best update in case we have to force-update to it if the timeout elapses
     if (
-        sum(update.sync_committee_bits) * 3 >= len(update.sync_committee_bits) * 2
-        and update.finality_header != BeaconBlockHeader()
+        store.best_valid_update is None
+        or sum(sync_committee_bits) > sum(store.best_valid_update.sync_committee_aggregate.sync_committee_bits)
     ):
-        # Apply update if (1) 2/3 quorum is reached and (2) we have a finality proof.
-        # Note that (2) means that the current light client design needs finality.
-        # It may be changed to re-organizable light client design. See the on-going issue consensus-specs#2182.
-        apply_light_client_update(store.snapshot, update)
-        store.valid_updates = set()
-    elif current_slot > store.snapshot.header.slot + update_timeout:
-        # Forced best update when the update timeout has elapsed
-        apply_light_client_update(store.snapshot,
-                                  max(store.valid_updates, key=lambda update: sum(update.sync_committee_bits)))
-        store.valid_updates = set()
+        store.best_valid_update = update
+
+    # Track the maximum number of active participants in the committee signatures
+    store.current_max_active_participants = max(
+        store.current_max_active_participants,
+        sum(sync_committee_bits),
+    )
+
+    # Update the optimistic header
+    if (
+        sum(sync_committee_bits) > get_safety_threshold(store)
+        and update.attested_header.slot > store.optimistic_header.slot
+    ):
+        store.optimistic_header = update.attested_header
+
+    # Update finalized header
+    if (
+        sum(sync_committee_bits) * 3 >= len(sync_committee_bits) * 2
+        and update.finalized_header != BeaconBlockHeader()
+    ):
+        # Normal update through 2/3 threshold
+        apply_light_client_update(store, update)
+        store.best_valid_update = None
 ```