send LC finality update on event stream on supermajority (#5602)

When new finality is reached without supermajority sync committee
support, trigger another event push on beacon-API and libp2p once
the finality gains supermajority support.

- https://github.com/ethereum/consensus-specs/pull/3549

beacon_chain/consensus_object_pools/blockchain_dag_light_client.nim

@@ -563,6 +563,11 @@ proc createLightClientUpdates(
     var finalized_slot = attested_data.finalized_slot
     if finalized_slot == forkyLatest.finalized_header.beacon.slot:
       forkyLatest.finality_branch = attested_data.finality_branch
+      let old_num_active_participants =
+        forkyLatest.sync_aggregate.num_active_participants.uint64
+      if not hasSupermajoritySyncParticipation(old_num_active_participants) and
+          hasSupermajoritySyncParticipation(num_active_participants):
+        newFinality = true
     elif finalized_slot < dag.tail.slot or
         not load_finalized_bid(finalized_slot):
       forkyLatest.finalized_header.reset()

beacon_chain/consensus_object_pools/light_client_pool.nim

@@ -19,6 +19,10 @@ type
       ## Latest finality update that was forwarded on libp2p gossip.
       ## Tracks `finality_update.finalized_header.beacon.slot`.
 
+    latestForwardedFinalityHasSupermajority*: bool
+      ## Whether or not the latest finality update that was forwarded on
+      ## libp2p gossip had supermajority (> 2/3) sync committee participation.
+
     latestForwardedOptimisticSlot*: Slot
       ## Latest optimistic update that was forwarded on libp2p gossip.
       ## Tracks `optimistic_update.attested_header.beacon.slot`.

beacon_chain/gossip_processing/gossip_validation.nim

@@ -1371,15 +1371,28 @@ proc validateLightClientFinalityUpdate*(
     pool: var LightClientPool, dag: ChainDAGRef,
     finality_update: ForkedLightClientFinalityUpdate,
     wallTime: BeaconTime): Result[void, ValidationError] =
+  # [IGNORE] The `finalized_header.beacon.slot` is greater than that of all
+  # previously forwarded `finality_update`s, or it matches the highest
+  # previously forwarded slot and also has a `sync_aggregate` indicating
+  # supermajority (> 2/3) sync committee participation while the previously
+  # forwarded `finality_update` for that slot did not indicate supermajority
   let finalized_slot = withForkyFinalityUpdate(finality_update):
     when lcDataFork > LightClientDataFork.None:
       forkyFinalityUpdate.finalized_header.beacon.slot
     else:
       GENESIS_SLOT
-  if finalized_slot <= pool.latestForwardedFinalitySlot:
-    # [IGNORE] The `finalized_header.beacon.slot` is greater than that of all
-    # previously forwarded `finality_update`s
+  if finalized_slot < pool.latestForwardedFinalitySlot:
     return errIgnore("LightClientFinalityUpdate: slot already forwarded")
+  let has_supermajority = withForkyFinalityUpdate(finality_update):
+    when lcDataFork > LightClientDataFork.None:
+      forkyFinalityUpdate.sync_aggregate.hasSupermajoritySyncParticipation
+    else:
+      false
+  if finalized_slot == pool.latestForwardedFinalitySlot:
+    if pool.latestForwardedFinalityHasSupermajority:
+      return errIgnore("LightClientFinalityUpdate: already have supermajority")
+    if not has_supermajority:
+      return errIgnore("LightClientFinalityUpdate: no new supermajority")
 
   let
     signature_slot = withForkyFinalityUpdate(finality_update):
@@ -1400,6 +1413,7 @@ proc validateLightClientFinalityUpdate*(
     return errIgnore("LightClientFinalityUpdate: not matching local")
 
   pool.latestForwardedFinalitySlot = finalized_slot
+  pool.latestForwardedFinalityHasSupermajority = has_supermajority
   ok()
 
 # https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.4/specs/altair/light-client/p2p-interface.md#light_client_optimistic_update

beacon_chain/spec/datatypes/altair.nim

@@ -638,6 +638,14 @@ func init*(T: type SyncAggregate): SyncAggregate =
 func num_active_participants*(v: SomeSyncAggregate): int =
   countOnes(v.sync_committee_bits)
 
+func hasSupermajoritySyncParticipation*(
+    num_active_participants: uint64): bool =
+  const max_active_participants = SYNC_COMMITTEE_SIZE.uint64
+  num_active_participants * 3 >= static(max_active_participants * 2)
+
+func hasSupermajoritySyncParticipation*(v: SomeSyncAggregate): bool =
+  hasSupermajoritySyncParticipation(v.num_active_participants.uint64)
+
 func shortLog*(v: SyncAggregate): auto =
   $(v.sync_committee_bits)
 

beacon_chain/spec/helpers.nim

@@ -303,12 +303,11 @@ template toMeta*(update: ForkedLightClientUpdate): LightClientUpdateMetadata =
 
 func is_better_data*(new_meta, old_meta: LightClientUpdateMetadata): bool =
   # Compare supermajority (> 2/3) sync committee participation
-  const max_active_participants = SYNC_COMMITTEE_SIZE.uint64
   let
     new_has_supermajority =
-      new_meta.num_active_participants * 3 >= max_active_participants * 2
+      hasSupermajoritySyncParticipation(new_meta.num_active_participants)
     old_has_supermajority =
-      old_meta.num_active_participants * 3 >= max_active_participants * 2
+      hasSupermajoritySyncParticipation(old_meta.num_active_participants)
   if new_has_supermajority != old_has_supermajority:
     return new_has_supermajority > old_has_supermajority
   if not new_has_supermajority:

beacon_chain/validators/beacon_validators.nim

@@ -261,6 +261,8 @@ proc isSynced*(node: BeaconNode, head: BlockRef): bool =
     head.slot + node.config.syncHorizon >= wallSlot.slot
 
 proc handleLightClientUpdates*(node: BeaconNode, slot: Slot) {.async.} =
+  template pool: untyped = node.lightClientPool[]
+
   static: doAssert lightClientFinalityUpdateSlotOffset ==
     lightClientOptimisticUpdateSlotOffset
   let sendTime = node.beaconClock.fromNow(
@@ -280,14 +282,28 @@ proc handleLightClientUpdates*(node: BeaconNode, slot: Slot) {.async.} =
       if num_active_participants < MIN_SYNC_COMMITTEE_PARTICIPANTS:
         return
 
-      let finalized_slot = forkyFinalityUpdate.finalized_header.beacon.slot
-      if finalized_slot > node.lightClientPool[].latestForwardedFinalitySlot:
+      let
+        finalized_slot =
+          forkyFinalityUpdate.finalized_header.beacon.slot
+        has_supermajority =
+          hasSupermajoritySyncParticipation(num_active_participants.uint64)
+        newFinality =
+          if finalized_slot > pool.latestForwardedFinalitySlot:
+            true
+          elif finalized_slot < pool.latestForwardedFinalitySlot:
+            false
+          elif pool.latestForwardedFinalityHasSupermajority:
+            false
+          else:
+            has_supermajority
+      if newFinality:
         template msg(): auto = forkyFinalityUpdate
         let sendResult =
           await node.network.broadcastLightClientFinalityUpdate(msg)
 
         # Optimization for message with ephemeral validity, whether sent or not
-        node.lightClientPool[].latestForwardedFinalitySlot = finalized_slot
+        pool.latestForwardedFinalitySlot = finalized_slot
+        pool.latestForwardedFinalityHasSupermajority = has_supermajority
 
         if sendResult.isOk:
           beacon_light_client_finality_updates_sent.inc()
@@ -297,13 +313,13 @@ proc handleLightClientUpdates*(node: BeaconNode, slot: Slot) {.async.} =
             error = sendResult.error()
 
       let attested_slot = forkyFinalityUpdate.attested_header.beacon.slot
-      if attested_slot > node.lightClientPool[].latestForwardedOptimisticSlot:
+      if attested_slot > pool.latestForwardedOptimisticSlot:
         let msg = forkyFinalityUpdate.toOptimistic
         let sendResult =
           await node.network.broadcastLightClientOptimisticUpdate(msg)
 
         # Optimization for message with ephemeral validity, whether sent or not
-        node.lightClientPool[].latestForwardedOptimisticSlot = attested_slot
+        pool.latestForwardedOptimisticSlot = attested_slot
 
         if sendResult.isOk:
           beacon_light_client_optimistic_updates_sent.inc()