feat: implement is_better_update for light client

beacon-chain/blockchain/lightclient.go

@@ -16,7 +16,7 @@ import (
 )
 
 const (
-	finalityBranchNumOfLeaves = 6
+	FinalityBranchNumOfLeaves = 6
 )
 
 // CreateLightClientFinalityUpdate - implements https://github.com/ethereum/consensus-specs/blob/3d235740e5f1e641d3b160c8688f26e7dc5a1894/specs/altair/light-client/full-node.md#create_light_client_finality_update
@@ -215,8 +215,8 @@ func NewLightClientFinalityUpdateFromBeaconState(
 			BodyRoot:      make([]byte, 32),
 		}
 
-		finalityBranch = make([][]byte, finalityBranchNumOfLeaves)
-		for i := 0; i < finalityBranchNumOfLeaves; i++ {
+		finalityBranch = make([][]byte, FinalityBranchNumOfLeaves)
+		for i := 0; i < FinalityBranchNumOfLeaves; i++ {
 			finalityBranch[i] = make([]byte, 32)
 		}
 	}

beacon-chain/blockchain/lightclient_test.go

@@ -153,7 +153,7 @@ func TestLightClient_NewLightClientFinalityUpdateFromBeaconState(t *testing.T) {
 	require.DeepSSZEqual(t, zeroHash, update.FinalizedHeader.ParentRoot, "Finalized header parent root is not zero")
 	require.DeepSSZEqual(t, zeroHash, update.FinalizedHeader.StateRoot, "Finalized header state root is not zero")
 	require.DeepSSZEqual(t, zeroHash, update.FinalizedHeader.BodyRoot, "Finalized header body root is not zero")
-	require.Equal(t, finalityBranchNumOfLeaves, len(update.FinalityBranch), "Invalid finality branch leaves")
+	require.Equal(t, FinalityBranchNumOfLeaves, len(update.FinalityBranch), "Invalid finality branch leaves")
 	for _, leaf := range update.FinalityBranch {
 		require.DeepSSZEqual(t, zeroHash, leaf, "Leaf is not zero")
 	}

beacon-chain/rpc/eth/light-client/BUILD.bazel

@@ -34,20 +34,28 @@ go_library(
 
 go_test(
     name = "go_default_test",
-    srcs = ["handlers_test.go"],
+    srcs = [
+        "handlers_test.go",
+        "helpers_test.go",
+    ],
     embed = [":go_default_library"],
     deps = [
         "//api/server/structs:go_default_library",
+        "//beacon-chain/blockchain:go_default_library",
         "//beacon-chain/blockchain/testing:go_default_library",
         "//beacon-chain/core/helpers:go_default_library",
         "//beacon-chain/rpc/testutil:go_default_library",
         "//beacon-chain/state:go_default_library",
+        "//config/fieldparams:go_default_library",
         "//config/params:go_default_library",
         "//consensus-types/blocks:go_default_library",
         "//consensus-types/interfaces:go_default_library",
         "//consensus-types/primitives:go_default_library",
         "//encoding/bytesutil:go_default_library",
+        "//proto/eth/v1:go_default_library",
+        "//proto/eth/v2:go_default_library",
         "//proto/prysm/v1alpha1:go_default_library",
+        "//testing/assert:go_default_library",
         "//testing/require:go_default_library",
         "//testing/util:go_default_library",
         "@com_github_ethereum_go_ethereum//common/hexutil:go_default_library",

beacon-chain/rpc/eth/light-client/helpers.go

@@ -3,6 +3,7 @@ package lightclient
 import (
 	"context"
 	"fmt"
+	"reflect"
 	"strconv"
 
 	"github.com/ethereum/go-ethereum/common/hexutil"
@@ -311,3 +312,64 @@ func newLightClientUpdateToJSON(input *v2.LightClientUpdate) *structs.LightClien
 		SignatureSlot:           strconv.FormatUint(uint64(input.SignatureSlot), 10),
 	}
 }
+
+func IsSyncCommitteeUpdate(update *v2.LightClientUpdate) bool {
+	nextSyncCommitteeBranch := make([][]byte, fieldparams.NextSyncCommitteeBranchDepth)
+	return !reflect.DeepEqual(update.NextSyncCommitteeBranch, nextSyncCommitteeBranch)
+}
+
+func IsFinalityUpdate(update *v2.LightClientUpdate) bool {
+	finalityBranch := make([][]byte, blockchain.FinalityBranchNumOfLeaves)
+	return !reflect.DeepEqual(update.FinalityBranch, finalityBranch)
+}
+
+func IsBetterUpdate(newUpdate, oldUpdate *v2.LightClientUpdate) bool {
+	maxActiveParticipants := newUpdate.SyncAggregate.SyncCommitteeBits.Len()
+	newNumActiveParticipants := newUpdate.SyncAggregate.SyncCommitteeBits.Count()
+	oldNumActiveParticipants := oldUpdate.SyncAggregate.SyncCommitteeBits.Count()
+	newHasSupermajority := newNumActiveParticipants*3 >= maxActiveParticipants*2
+	oldHasSupermajority := oldNumActiveParticipants*3 >= maxActiveParticipants*2
+
+	if newHasSupermajority != oldHasSupermajority {
+		return newHasSupermajority
+	}
+	if !newHasSupermajority && newNumActiveParticipants != oldNumActiveParticipants {
+		return newNumActiveParticipants > oldNumActiveParticipants
+	}
+
+	// Compare presence of relevant sync committee
+	newHasRelevantSyncCommittee := IsSyncCommitteeUpdate(newUpdate) && (slots.SyncCommitteePeriod(slots.ToEpoch(newUpdate.AttestedHeader.Slot)) == slots.SyncCommitteePeriod(slots.ToEpoch(newUpdate.SignatureSlot)))
+	oldHasRelevantSyncCommittee := IsSyncCommitteeUpdate(oldUpdate) && (slots.SyncCommitteePeriod(slots.ToEpoch(oldUpdate.AttestedHeader.Slot)) == slots.SyncCommitteePeriod(slots.ToEpoch(oldUpdate.SignatureSlot)))
+
+	if newHasRelevantSyncCommittee != oldHasRelevantSyncCommittee {
+		return newHasRelevantSyncCommittee
+	}
+
+	// Compare indication of any finality
+	newHasFinality := IsFinalityUpdate(newUpdate)
+	oldHasFinality := IsFinalityUpdate(oldUpdate)
+	if newHasFinality != oldHasFinality {
+		return newHasFinality
+	}
+
+	// Compare sync committee finality
+	if newHasFinality {
+		newHasSyncCommitteeFinality := slots.SyncCommitteePeriod(slots.ToEpoch(newUpdate.FinalizedHeader.Slot)) == slots.SyncCommitteePeriod(slots.ToEpoch(newUpdate.AttestedHeader.Slot))
+		oldHasSyncCommitteeFinality := slots.SyncCommitteePeriod(slots.ToEpoch(oldUpdate.FinalizedHeader.Slot)) == slots.SyncCommitteePeriod(slots.ToEpoch(oldUpdate.AttestedHeader.Slot))
+
+		if newHasSyncCommitteeFinality != oldHasSyncCommitteeFinality {
+			return newHasSyncCommitteeFinality
+		}
+	}
+
+	// Tiebreaker 1: Sync committee participation beyond supermajority
+	if newNumActiveParticipants != oldNumActiveParticipants {
+		return newNumActiveParticipants > oldNumActiveParticipants
+	}
+
+	// Tiebreaker 2: Prefer older data (fewer changes to best)
+	if newUpdate.AttestedHeader.Slot != oldUpdate.AttestedHeader.Slot {
+		return newUpdate.AttestedHeader.Slot < oldUpdate.AttestedHeader.Slot
+	}
+	return newUpdate.SignatureSlot < oldUpdate.SignatureSlot
+}