Add LightClient(Finality|Optimistic)Update

Introduces reduced `LightClientUpdate` structures to allow keeping track
of the latest `finalized_header` and `optimistic_header`. This may also
help in scheduling the next query for a full `LightClientUpdate` once
sync committee finality has been reached.

specs/altair/light-client/full-node.md

@@ -14,6 +14,8 @@
 - [Deriving light client data](#deriving-light-client-data)
   - [`create_light_client_bootstrap`](#create_light_client_bootstrap)
   - [`create_light_client_update`](#create_light_client_update)
+  - [`create_light_client_finality_update`](#create_light_client_finality_update)
+  - [`create_light_client_optimistic_update`](#create_light_client_optimistic_update)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 <!-- /TOC -->
@@ -133,3 +135,31 @@ Full nodes SHOULD provide the best derivable `LightClientUpdate` (according to `
 - `LightClientUpdate` are assigned to sync committee periods based on their `attested_header.slot`
 - `LightClientUpdate` are only considered if `compute_sync_committee_period(compute_epoch_at_slot(update.attested_header.slot)) == compute_sync_committee_period(compute_epoch_at_slot(update.signature_slot))`
 - Only `LightClientUpdate` with `next_sync_committee` as selected by fork choice are provided, regardless of ranking by `is_better_update`. To uniquely identify a non-finalized sync committee fork, all of `period`, `current_sync_committee` and `next_sync_committee` need to be incorporated, as sync committees may reappear over time.
+
+### `create_light_client_finality_update`
+
+```python
+def create_light_client_finality_update(update: LightClientUpdate) -> LightClientFinalityUpdate:
+    return LightClientFinalityUpdate(
+        attested_header=update.attested_header,
+        finalized_header=update.finalized_header,
+        finality_branch=update.finality_branch,
+        sync_aggregate=update.sync_aggregate,
+        signature_slot=update.signature_slot,
+    )
+```
+
+Full nodes SHOULD provide the `LightClientFinalityUpdate` with the highest `attested_header.slot` (if multiple, highest `signature_slot`) as selected by fork choice, and SHOULD support a push mechanism to deliver new `LightClientFinalityUpdate` whenever `finalized_header` changes.
+
+### `create_light_client_optimistic_update`
+
+```python
+def create_light_client_optimistic_update(update: LightClientUpdate) -> LightClientOptimisticUpdate:
+    return LightClientOptimisticUpdate(
+        attested_header=update.attested_header,
+        sync_aggregate=update.sync_aggregate,
+        signature_slot=update.signature_slot,
+    )
+```
+
+Full nodes SHOULD provide the `LightClientOptimisticUpdate` with the highest `attested_header.slot` (if multiple, highest `signature_slot`) as selected by fork choice, and SHOULD support a push mechanism to deliver new `LightClientOptimisticUpdate` whenever `attested_header` changes.

specs/altair/light-client/sync-protocol.md

@@ -15,6 +15,8 @@
 - [Containers](#containers)
   - [`LightClientBootstrap`](#lightclientbootstrap)
   - [`LightClientUpdate`](#lightclientupdate)
+  - [`LightClientFinalityUpdate`](#lightclientfinalityupdate)
+  - [`LightClientOptimisticUpdate`](#lightclientoptimisticupdate)
   - [`LightClientStore`](#lightclientstore)
 - [Helper functions](#helper-functions)
   - [`is_sync_committee_update`](#is_sync_committee_update)
@@ -31,6 +33,8 @@
   - [`validate_light_client_update`](#validate_light_client_update)
   - [`apply_light_client_update`](#apply_light_client_update)
   - [`process_light_client_update`](#process_light_client_update)
+  - [`process_light_client_finality_update`](#process_light_client_finality_update)
+  - [`process_light_client_optimistic_update`](#process_light_client_optimistic_update)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 <!-- /TOC -->
@@ -96,6 +100,33 @@ class LightClientUpdate(Container):
     signature_slot: Slot
 ```
 
+### `LightClientFinalityUpdate`
+
+```python
+class LightClientFinalityUpdate(Container):
+    # The beacon block header that is attested to by the sync committee
+    attested_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_aggregate: SyncAggregate
+    # Slot at which the aggregate signature was created (untrusted)
+    signature_slot: Slot
+```
+
+### `LightClientOptimisticUpdate`
+
+```python
+class LightClientOptimisticUpdate(Container):
+    # The beacon block header that is attested to by the sync committee
+    attested_header: BeaconBlockHeader
+    # Sync committee aggregate signature
+    sync_aggregate: SyncAggregate
+    # Slot at which the aggregate signature was created (untrusted)
+    signature_slot: Slot
+```
+
 ### `LightClientStore`
 
 ```python
@@ -250,7 +281,7 @@ def initialize_light_client_store(trusted_block_root: Root,
 
 ## Light client state updates
 
-A light client receives `update` objects of type `LightClientUpdate`. Every `update` triggers `process_light_client_update(store, update, current_slot, genesis_validators_root)` where `current_slot` is the current slot based on a local clock. `process_slot_for_light_client_store` is triggered every time the current slot increments.
+A light client receives `update`, `finality_update` and `optimistic_update` objects of type `LightClientUpdate`, `LightClientFinalityUpdate` and `LightClientOptimisticUpdate`. Every `update` triggers `process_light_client_update(store, update, current_slot, genesis_validators_root)` where `current_slot` is the current slot based on a local clock. Likewise, every `finality_update` triggers `process_light_client_finality_update`, and every `optimistic_update` triggers `process_light_client_optimistic_update`. `process_slot_for_light_client_store` is triggered every time the current slot increments.
 
 ### `process_slot_for_light_client_store`
 
@@ -420,3 +451,41 @@ def process_light_client_update(store: LightClientStore,
         apply_light_client_update(store, update)
         store.best_valid_update = None
 ```
+
+### `process_light_client_finality_update`
+
+```python
+def process_light_client_finality_update(store: LightClientStore,
+                                         finality_update: LightClientFinalityUpdate,
+                                         current_slot: Slot,
+                                         genesis_validators_root: Root) -> None:
+    update = LightClientUpdate(
+        attested_header=finality_update.attested_header,
+        next_sync_committee=SyncCommittee(),
+        next_sync_committee_branch=[Bytes32() for _ in range(floorlog2(NEXT_SYNC_COMMITTEE_INDEX))],
+        finalized_header=finality_update.finalized_header,
+        finality_branch=finality_update.finality_branch,
+        sync_aggregate=finality_update.sync_aggregate,
+        signature_slot=finality_update.signature_slot,
+    )
+    process_light_client_update(store, update, current_slot, genesis_validators_root)
+```
+
+### `process_light_client_optimistic_update`
+
+```python
+def process_light_client_optimistic_update(store: LightClientStore,
+                                           optimistic_update: LightClientOptimisticUpdate,
+                                           current_slot: Slot,
+                                           genesis_validators_root: Root) -> None:
+    update = LightClientUpdate(
+        attested_header=optimistic_update.attested_header,
+        next_sync_committee=SyncCommittee(),
+        next_sync_committee_branch=[Bytes32() for _ in range(floorlog2(NEXT_SYNC_COMMITTEE_INDEX))],
+        finalized_header=BeaconBlockHeader(),
+        finality_branch=[Bytes32() for _ in range(floorlog2(FINALIZED_ROOT_INDEX))],
+        sync_aggregate=optimistic_update.sync_aggregate,
+        signature_slot=optimistic_update.signature_slot,
+    )
+    process_light_client_update(store, update, current_slot, genesis_validators_root)
+```