From 364ad3fbdad2fcab5f1f6ce9fcde53c1f601899a Mon Sep 17 00:00:00 2001
From: terencechain <terence@prysmaticlabs.com>
Date: Sun, 29 May 2022 12:32:42 -0700
Subject: [PATCH] Reinsert reorg atts (#10767)

* Add common ancestor root for protoarray

* More efficient algo

* Tests

* Fix linting

* Fix linting

* Fix linting

* Fix linting

* Fix linting

* Fix linting

* Apply suggestions from code review

Co-authored-by: Potuz <potuz@prysmaticlabs.com>

* Feedbacks

* Revert saveHead changes

* Revert "Revert saveHead changes"

This reverts commit a15fddc2e68b2e370290b1783eb1ee626af17503.

* Fix rest of the tests

* Update beacon-chain/blockchain/head.go

Co-authored-by: Potuz <potuz@prysmaticlabs.com>

Co-authored-by: Potuz <potuz@prysmaticlabs.com>
Co-authored-by: prylabs-bulldozer[bot] <58059840+prylabs-bulldozer[bot]@users.noreply.github.com>
---
 beacon-chain/blockchain/head.go               |  87 +++--
 beacon-chain/blockchain/head_test.go          | 366 ++++++++++++++++--
 .../blockchain/receive_attestation_test.go    |   2 +
 beacon-chain/forkchoice/BUILD.bazel           |   2 +
 .../forkchoice/doubly-linked-tree/BUILD.bazel |   2 +
 .../doubly-linked-tree/forkchoice.go          |  46 +++
 .../doubly-linked-tree/forkchoice_test.go     | 156 ++++++++
 beacon-chain/forkchoice/error.go              |   5 +
 beacon-chain/forkchoice/interfaces.go         |   1 +
 .../forkchoice/protoarray/BUILD.bazel         |   2 +
 beacon-chain/forkchoice/protoarray/store.go   |  47 +++
 .../forkchoice/protoarray/store_test.go       | 154 ++++++++
 12 files changed, 808 insertions(+), 62 deletions(-)
 create mode 100644 beacon-chain/forkchoice/error.go

diff --git a/beacon-chain/blockchain/head.go b/beacon-chain/blockchain/head.go
index 4feaccfe71..00b4b5b85d 100644
--- a/beacon-chain/blockchain/head.go
+++ b/beacon-chain/blockchain/head.go
@@ -9,6 +9,7 @@ import (
 	"github.com/prysmaticlabs/prysm/beacon-chain/core/feed"
 	statefeed "github.com/prysmaticlabs/prysm/beacon-chain/core/feed/state"
 	"github.com/prysmaticlabs/prysm/beacon-chain/core/helpers"
+	"github.com/prysmaticlabs/prysm/beacon-chain/forkchoice"
 	doublylinkedtree "github.com/prysmaticlabs/prysm/beacon-chain/forkchoice/doubly-linked-tree"
 	"github.com/prysmaticlabs/prysm/beacon-chain/forkchoice/protoarray"
 	"github.com/prysmaticlabs/prysm/beacon-chain/state"
@@ -107,16 +108,16 @@ func (s *Service) updateHead(ctx context.Context, balances []uint64) ([32]byte,
 
 // This saves head info to the local service cache, it also saves the
 // new head root to the DB.
-func (s *Service) saveHead(ctx context.Context, headRoot [32]byte, headBlock interfaces.SignedBeaconBlock, headState state.BeaconState) error {
+func (s *Service) saveHead(ctx context.Context, newHeadRoot [32]byte, headBlock interfaces.SignedBeaconBlock, headState state.BeaconState) error {
 	ctx, span := trace.StartSpan(ctx, "blockChain.saveHead")
 	defer span.End()
 
 	// Do nothing if head hasn't changed.
-	r, err := s.HeadRoot(ctx)
+	oldHeadroot, err := s.HeadRoot(ctx)
 	if err != nil {
 		return err
 	}
-	if headRoot == bytesutil.ToBytes32(r) {
+	if newHeadRoot == bytesutil.ToBytes32(oldHeadroot) {
 		return nil
 	}
 	if err := wrapper.BeaconBlockIsNil(headBlock); err != nil {
@@ -128,7 +129,7 @@ func (s *Service) saveHead(ctx context.Context, headRoot [32]byte, headBlock int
 
 	// If the head state is not available, just return nil.
 	// There's nothing to cache
-	if !s.cfg.BeaconDB.HasStateSummary(ctx, headRoot) {
+	if !s.cfg.BeaconDB.HasStateSummary(ctx, newHeadRoot) {
 		return nil
 	}
 
@@ -140,7 +141,7 @@ func (s *Service) saveHead(ctx context.Context, headRoot [32]byte, headBlock int
 	headSlot := s.HeadSlot()
 	newHeadSlot := headBlock.Block().Slot()
 	newStateRoot := headBlock.Block().StateRoot()
-	if bytesutil.ToBytes32(headBlock.Block().ParentRoot()) != bytesutil.ToBytes32(r) {
+	if bytesutil.ToBytes32(headBlock.Block().ParentRoot()) != bytesutil.ToBytes32(oldHeadroot) {
 		log.WithFields(logrus.Fields{
 			"newSlot": fmt.Sprintf("%d", newHeadSlot),
 			"oldSlot": fmt.Sprintf("%d", headSlot),
@@ -156,7 +157,7 @@ func (s *Service) saveHead(ctx context.Context, headRoot [32]byte, headBlock int
 				Slot:                newHeadSlot,
 				Depth:               absoluteSlotDifference,
 				OldHeadBlock:        oldHeadRoot[:],
-				NewHeadBlock:        headRoot[:],
+				NewHeadBlock:        newHeadRoot[:],
 				OldHeadState:        oldStateRoot,
 				NewHeadState:        newStateRoot,
 				Epoch:               slots.ToEpoch(newHeadSlot),
@@ -164,25 +165,24 @@ func (s *Service) saveHead(ctx context.Context, headRoot [32]byte, headBlock int
 			},
 		})
 
-		if err := s.saveOrphanedAtts(ctx, bytesutil.ToBytes32(r)); err != nil {
+		if err := s.saveOrphanedAtts(ctx, bytesutil.ToBytes32(oldHeadroot), newHeadRoot); err != nil {
 			return err
 		}
-
 		reorgCount.Inc()
 	}
 
 	// Cache the new head info.
-	s.setHead(headRoot, headBlock, headState)
+	s.setHead(newHeadRoot, headBlock, headState)
 
 	// Save the new head root to DB.
-	if err := s.cfg.BeaconDB.SaveHeadBlockRoot(ctx, headRoot); err != nil {
+	if err := s.cfg.BeaconDB.SaveHeadBlockRoot(ctx, newHeadRoot); err != nil {
 		return errors.Wrap(err, "could not save head root in DB")
 	}
 
 	// Forward an event capturing a new chain head over a common event feed
 	// done in a goroutine to avoid blocking the critical runtime main routine.
 	go func() {
-		if err := s.notifyNewHeadEvent(ctx, newHeadSlot, headState, newStateRoot, headRoot[:]); err != nil {
+		if err := s.notifyNewHeadEvent(ctx, newHeadSlot, headState, newStateRoot, newHeadRoot[:]); err != nil {
 			log.WithError(err).Error("Could not notify event feed of new chain head")
 		}
 	}()
@@ -355,35 +355,48 @@ func (s *Service) notifyNewHeadEvent(
 	return nil
 }
 
-// This saves the attestations inside the beacon block with respect to root `orphanedRoot` back into the
-// attestation pool. It also filters out the attestations that is one epoch older as a
-// defense so invalid attestations don't flow into the attestation pool.
-func (s *Service) saveOrphanedAtts(ctx context.Context, orphanedRoot [32]byte) error {
-	orphanedBlk, err := s.getBlock(ctx, orphanedRoot)
-	if err != nil {
+// This saves the attestations between `orphanedRoot` and the common ancestor root that is derived using `newHeadRoot`.
+// It also filters out the attestations that is one epoch older as a defense so invalid attestations don't flow into the attestation pool.
+func (s *Service) saveOrphanedAtts(ctx context.Context, orphanedRoot [32]byte, newHeadRoot [32]byte) error {
+	commonAncestorRoot, err := s.ForkChoicer().CommonAncestorRoot(ctx, newHeadRoot, orphanedRoot)
+	switch {
+	// Exit early if there's no common ancestor as there would be nothing to save.
+	case errors.Is(err, forkchoice.ErrUnknownCommonAncestor):
+		return nil
+	case err != nil:
 		return err
 	}
-
-	if orphanedBlk == nil || orphanedBlk.IsNil() {
-		return errors.New("orphaned block can't be nil")
-	}
-
-	for _, a := range orphanedBlk.Block().Body().Attestations() {
-		// Is the attestation one epoch older.
-		if a.Data.Slot+params.BeaconConfig().SlotsPerEpoch < s.CurrentSlot() {
-			continue
+	for orphanedRoot != commonAncestorRoot {
+		if ctx.Err() != nil {
+			return ctx.Err()
 		}
-		if helpers.IsAggregated(a) {
-			if err := s.cfg.AttPool.SaveAggregatedAttestation(a); err != nil {
-				return err
-			}
-		} else {
-			if err := s.cfg.AttPool.SaveUnaggregatedAttestation(a); err != nil {
-				return err
-			}
-		}
-		saveOrphanedAttCount.Inc()
-	}
 
+		orphanedBlk, err := s.getBlock(ctx, orphanedRoot)
+		if err != nil {
+			return err
+		}
+		// If the block is an epoch older, break out of the loop since we can't include atts anyway.
+		// This prevents stuck within this for loop longer than necessary.
+		if orphanedBlk.Block().Slot()+params.BeaconConfig().SlotsPerEpoch <= s.CurrentSlot() {
+			break
+		}
+		for _, a := range orphanedBlk.Block().Body().Attestations() {
+			// if the attestation is one epoch older, it wouldn't been useful to save it.
+			if a.Data.Slot+params.BeaconConfig().SlotsPerEpoch < s.CurrentSlot() {
+				continue
+			}
+			if helpers.IsAggregated(a) {
+				if err := s.cfg.AttPool.SaveAggregatedAttestation(a); err != nil {
+					return err
+				}
+			} else {
+				if err := s.cfg.AttPool.SaveUnaggregatedAttestation(a); err != nil {
+					return err
+				}
+			}
+			saveOrphanedAttCount.Inc()
+		}
+		orphanedRoot = bytesutil.ToBytes32(orphanedBlk.Block().ParentRoot())
+	}
 	return nil
 }
diff --git a/beacon-chain/blockchain/head_test.go b/beacon-chain/blockchain/head_test.go
index 10ff994175..7447fa352b 100644
--- a/beacon-chain/blockchain/head_test.go
+++ b/beacon-chain/blockchain/head_test.go
@@ -3,6 +3,7 @@ package blockchain
 import (
 	"bytes"
 	"context"
+	"sort"
 	"testing"
 	"time"
 
@@ -10,9 +11,11 @@ import (
 	testDB "github.com/prysmaticlabs/prysm/beacon-chain/db/testing"
 	doublylinkedtree "github.com/prysmaticlabs/prysm/beacon-chain/forkchoice/doubly-linked-tree"
 	"github.com/prysmaticlabs/prysm/beacon-chain/state/stategen"
+	"github.com/prysmaticlabs/prysm/config/features"
 	"github.com/prysmaticlabs/prysm/config/params"
 	types "github.com/prysmaticlabs/prysm/consensus-types/primitives"
 	"github.com/prysmaticlabs/prysm/consensus-types/wrapper"
+	"github.com/prysmaticlabs/prysm/encoding/bytesutil"
 	ethpbv1 "github.com/prysmaticlabs/prysm/proto/eth/v1"
 	ethpb "github.com/prysmaticlabs/prysm/proto/prysm/v1alpha1"
 	"github.com/prysmaticlabs/prysm/testing/assert"
@@ -49,6 +52,8 @@ func TestSaveHead_Different(t *testing.T) {
 	require.NoError(t, service.cfg.BeaconDB.SaveBlock(context.Background(), oldBlock))
 	oldRoot, err := oldBlock.Block().HashTreeRoot()
 	require.NoError(t, err)
+	require.NoError(t, service.cfg.ForkChoiceStore.InsertOptimisticBlock(
+		ctx, oldBlock.Block().Slot(), oldRoot, bytesutil.ToBytes32(oldBlock.Block().ParentRoot()), [32]byte{}, 0, 0))
 	service.head = &head{
 		slot:  0,
 		root:  oldRoot,
@@ -64,6 +69,8 @@ func TestSaveHead_Different(t *testing.T) {
 	require.NoError(t, service.cfg.BeaconDB.SaveBlock(context.Background(), wsb))
 	newRoot, err := newHeadBlock.HashTreeRoot()
 	require.NoError(t, err)
+	require.NoError(t, service.cfg.ForkChoiceStore.InsertOptimisticBlock(
+		ctx, wsb.Block().Slot(), newRoot, bytesutil.ToBytes32(wsb.Block().ParentRoot()), [32]byte{}, 0, 0))
 	headState, err := util.NewBeaconState()
 	require.NoError(t, err)
 	require.NoError(t, headState.SetSlot(1))
@@ -93,6 +100,8 @@ func TestSaveHead_Different_Reorg(t *testing.T) {
 	require.NoError(t, service.cfg.BeaconDB.SaveBlock(context.Background(), oldBlock))
 	oldRoot, err := oldBlock.Block().HashTreeRoot()
 	require.NoError(t, err)
+	require.NoError(t, service.cfg.ForkChoiceStore.InsertOptimisticBlock(
+		ctx, oldBlock.Block().Slot(), oldRoot, bytesutil.ToBytes32(oldBlock.Block().ParentRoot()), [32]byte{}, 0, 0))
 	service.head = &head{
 		slot:  0,
 		root:  oldRoot,
@@ -110,6 +119,8 @@ func TestSaveHead_Different_Reorg(t *testing.T) {
 	require.NoError(t, service.cfg.BeaconDB.SaveBlock(context.Background(), wsb))
 	newRoot, err := newHeadBlock.HashTreeRoot()
 	require.NoError(t, err)
+	require.NoError(t, service.cfg.ForkChoiceStore.InsertOptimisticBlock(
+		ctx, wsb.Block().Slot(), newRoot, bytesutil.ToBytes32(wsb.Block().ParentRoot()), [32]byte{}, 0, 0))
 	headState, err := util.NewBeaconState()
 	require.NoError(t, err)
 	require.NoError(t, headState.SetSlot(1))
@@ -229,50 +240,355 @@ func Test_notifyNewHeadEvent(t *testing.T) {
 	})
 }
 
-func TestSaveOrphanedAtts(t *testing.T) {
-	genesis, keys := util.DeterministicGenesisState(t, 64)
-	b, err := util.GenerateFullBlock(genesis, keys, util.DefaultBlockGenConfig(), 1)
-	assert.NoError(t, err)
-	r, err := b.Block.HashTreeRoot()
-	require.NoError(t, err)
-
+func TestSaveOrphanedAtts_NoCommonAncestor(t *testing.T) {
 	ctx := context.Background()
 	beaconDB := testDB.SetupDB(t)
 	service := setupBeaconChain(t, beaconDB)
-	service.genesisTime = time.Now()
+	service.genesisTime = time.Now().Add(time.Duration(-10*int64(1)*int64(params.BeaconConfig().SecondsPerSlot)) * time.Second)
 
-	wsb, err := wrapper.WrappedSignedBeaconBlock(b)
+	// Chain setup
+	// 0 -- 1 -- 2 -- 3
+	//    -4
+	st, keys := util.DeterministicGenesisState(t, 64)
+	blkG, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 0)
+	assert.NoError(t, err)
+	b, err := wrapper.WrappedSignedBeaconBlock(blkG)
+	assert.NoError(t, err)
+	require.NoError(t, service.cfg.BeaconDB.SaveBlock(ctx, b))
+	rG, err := blkG.Block.HashTreeRoot()
 	require.NoError(t, err)
-	require.NoError(t, service.cfg.BeaconDB.SaveBlock(ctx, wsb))
-	require.NoError(t, service.saveOrphanedAtts(ctx, r))
 
-	require.Equal(t, len(b.Block.Body.Attestations), service.cfg.AttPool.AggregatedAttestationCount())
-	savedAtts := service.cfg.AttPool.AggregatedAttestations()
-	atts := b.Block.Body.Attestations
-	require.DeepSSZEqual(t, atts, savedAtts)
+	blk1, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 1)
+	assert.NoError(t, err)
+	blk1.Block.ParentRoot = rG[:]
+	r1, err := blk1.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk2, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 2)
+	assert.NoError(t, err)
+	blk2.Block.ParentRoot = r1[:]
+	r2, err := blk2.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk3, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 3)
+	assert.NoError(t, err)
+	blk3.Block.ParentRoot = r2[:]
+	r3, err := blk3.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk4 := util.NewBeaconBlock()
+	blk4.Block.Slot = 4
+	r4, err := blk4.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	for _, blk := range []*ethpb.SignedBeaconBlock{blkG, blk1, blk2, blk3, blk4} {
+		r, err := blk.Block.HashTreeRoot()
+		require.NoError(t, err)
+		require.NoError(t, service.ForkChoicer().InsertOptimisticBlock(ctx, blk.Block.Slot, r, bytesutil.ToBytes32(blk.Block.ParentRoot), [32]byte{}, 0, 0))
+		b, err := wrapper.WrappedSignedBeaconBlock(blk)
+		require.NoError(t, err)
+		require.NoError(t, beaconDB.SaveBlock(ctx, b))
+	}
+
+	require.NoError(t, service.saveOrphanedAtts(ctx, r3, r4))
+	require.Equal(t, 0, service.cfg.AttPool.AggregatedAttestationCount())
+}
+
+func TestSaveOrphanedAtts(t *testing.T) {
+	ctx := context.Background()
+	beaconDB := testDB.SetupDB(t)
+	service := setupBeaconChain(t, beaconDB)
+	service.genesisTime = time.Now().Add(time.Duration(-10*int64(1)*int64(params.BeaconConfig().SecondsPerSlot)) * time.Second)
+
+	// Chain setup
+	// 0 -- 1 -- 2 -- 3
+	//  \-4
+	st, keys := util.DeterministicGenesisState(t, 64)
+	blkG, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 0)
+	assert.NoError(t, err)
+	b, err := wrapper.WrappedSignedBeaconBlock(blkG)
+	assert.NoError(t, err)
+	require.NoError(t, service.cfg.BeaconDB.SaveBlock(ctx, b))
+	rG, err := blkG.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk1, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 1)
+	assert.NoError(t, err)
+	blk1.Block.ParentRoot = rG[:]
+	r1, err := blk1.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk2, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 2)
+	assert.NoError(t, err)
+	blk2.Block.ParentRoot = r1[:]
+	r2, err := blk2.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk3, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 3)
+	assert.NoError(t, err)
+	blk3.Block.ParentRoot = r2[:]
+	r3, err := blk3.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk4 := util.NewBeaconBlock()
+	blk4.Block.Slot = 4
+	blk4.Block.ParentRoot = rG[:]
+	r4, err := blk4.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	for _, blk := range []*ethpb.SignedBeaconBlock{blkG, blk1, blk2, blk3, blk4} {
+		r, err := blk.Block.HashTreeRoot()
+		require.NoError(t, err)
+		require.NoError(t, service.ForkChoicer().InsertOptimisticBlock(ctx, blk.Block.Slot, r, bytesutil.ToBytes32(blk.Block.ParentRoot), [32]byte{}, 0, 0))
+		b, err := wrapper.WrappedSignedBeaconBlock(blk)
+		require.NoError(t, err)
+		require.NoError(t, beaconDB.SaveBlock(ctx, b))
+	}
+
+	require.NoError(t, service.saveOrphanedAtts(ctx, r3, r4))
+	require.Equal(t, 3, service.cfg.AttPool.AggregatedAttestationCount())
+	wantAtts := []*ethpb.Attestation{
+		blk3.Block.Body.Attestations[0],
+		blk2.Block.Body.Attestations[0],
+		blk1.Block.Body.Attestations[0],
+	}
+	atts := service.cfg.AttPool.AggregatedAttestations()
+	sort.Slice(atts, func(i, j int) bool {
+		return atts[i].Data.Slot > atts[j].Data.Slot
+	})
+	require.DeepEqual(t, wantAtts, atts)
 }
 
 func TestSaveOrphanedAtts_CanFilter(t *testing.T) {
-	genesis, keys := util.DeterministicGenesisState(t, 64)
-	b, err := util.GenerateFullBlock(genesis, keys, util.DefaultBlockGenConfig(), 1)
+	ctx := context.Background()
+	beaconDB := testDB.SetupDB(t)
+	service := setupBeaconChain(t, beaconDB)
+	service.genesisTime = time.Now().Add(time.Duration(-1*int64(params.BeaconConfig().SlotsPerEpoch+2)*int64(params.BeaconConfig().SecondsPerSlot)) * time.Second)
+
+	// Chain setup
+	// 0 -- 1 -- 2
+	//  \-4
+	st, keys := util.DeterministicGenesisState(t, 64)
+	blkG, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 0)
 	assert.NoError(t, err)
-	r, err := b.Block.HashTreeRoot()
+	b, err := wrapper.WrappedSignedBeaconBlock(blkG)
+	assert.NoError(t, err)
+	require.NoError(t, service.cfg.BeaconDB.SaveBlock(ctx, b))
+	rG, err := blkG.Block.HashTreeRoot()
 	require.NoError(t, err)
 
+	blk1, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 1)
+	assert.NoError(t, err)
+	blk1.Block.ParentRoot = rG[:]
+	r1, err := blk1.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk2, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 2)
+	assert.NoError(t, err)
+	blk2.Block.ParentRoot = r1[:]
+	r2, err := blk2.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk4 := util.NewBeaconBlock()
+	blk4.Block.Slot = 4
+	blk4.Block.ParentRoot = rG[:]
+	r4, err := blk4.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	for _, blk := range []*ethpb.SignedBeaconBlock{blkG, blk1, blk2, blk4} {
+		r, err := blk.Block.HashTreeRoot()
+		require.NoError(t, err)
+		require.NoError(t, service.ForkChoicer().InsertOptimisticBlock(ctx, blk.Block.Slot, r, bytesutil.ToBytes32(blk.Block.ParentRoot), [32]byte{}, 0, 0))
+		b, err := wrapper.WrappedSignedBeaconBlock(blk)
+		require.NoError(t, err)
+		require.NoError(t, beaconDB.SaveBlock(ctx, b))
+	}
+
+	require.NoError(t, service.saveOrphanedAtts(ctx, r2, r4))
+	require.Equal(t, 0, service.cfg.AttPool.AggregatedAttestationCount())
+}
+
+func TestSaveOrphanedAtts_NoCommonAncestor_DoublyLinkedTrie(t *testing.T) {
+	resetCfg := features.InitWithReset(&features.Flags{
+		EnableForkChoiceDoublyLinkedTree: true,
+	})
+	defer resetCfg()
+
 	ctx := context.Background()
 	beaconDB := testDB.SetupDB(t)
 	service := setupBeaconChain(t, beaconDB)
-	service.genesisTime = time.Now().Add(time.Duration(-1*int64(params.BeaconConfig().SlotsPerEpoch+1)*int64(params.BeaconConfig().SecondsPerSlot)) * time.Second)
+	service.genesisTime = time.Now().Add(time.Duration(-10*int64(1)*int64(params.BeaconConfig().SecondsPerSlot)) * time.Second)
 
-	wsb, err := wrapper.WrappedSignedBeaconBlock(b)
+	// Chain setup
+	// 0 -- 1 -- 2 -- 3
+	//    -4
+	st, keys := util.DeterministicGenesisState(t, 64)
+	blkG, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 0)
+	assert.NoError(t, err)
+	b, err := wrapper.WrappedSignedBeaconBlock(blkG)
+	assert.NoError(t, err)
+	require.NoError(t, service.cfg.BeaconDB.SaveBlock(ctx, b))
+	rG, err := blkG.Block.HashTreeRoot()
 	require.NoError(t, err)
-	require.NoError(t, service.cfg.BeaconDB.SaveBlock(ctx, wsb))
-	require.NoError(t, service.saveOrphanedAtts(ctx, r))
 
+	blk1, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 1)
+	assert.NoError(t, err)
+	blk1.Block.ParentRoot = rG[:]
+	r1, err := blk1.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk2, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 2)
+	assert.NoError(t, err)
+	blk2.Block.ParentRoot = r1[:]
+	r2, err := blk2.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk3, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 3)
+	assert.NoError(t, err)
+	blk3.Block.ParentRoot = r2[:]
+	r3, err := blk3.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk4 := util.NewBeaconBlock()
+	blk4.Block.Slot = 4
+	r4, err := blk4.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	for _, blk := range []*ethpb.SignedBeaconBlock{blkG, blk1, blk2, blk3, blk4} {
+		r, err := blk.Block.HashTreeRoot()
+		require.NoError(t, err)
+		require.NoError(t, service.ForkChoicer().InsertOptimisticBlock(ctx, blk.Block.Slot, r, bytesutil.ToBytes32(blk.Block.ParentRoot), [32]byte{}, 0, 0))
+		b, err := wrapper.WrappedSignedBeaconBlock(blk)
+		require.NoError(t, err)
+		require.NoError(t, beaconDB.SaveBlock(ctx, b))
+	}
+
+	require.NoError(t, service.saveOrphanedAtts(ctx, r3, r4))
+	require.Equal(t, 0, service.cfg.AttPool.AggregatedAttestationCount())
+}
+
+func TestSaveOrphanedAtts_DoublyLinkedTrie(t *testing.T) {
+	resetCfg := features.InitWithReset(&features.Flags{
+		EnableForkChoiceDoublyLinkedTree: true,
+	})
+	defer resetCfg()
+
+	ctx := context.Background()
+	beaconDB := testDB.SetupDB(t)
+	service := setupBeaconChain(t, beaconDB)
+	service.genesisTime = time.Now().Add(time.Duration(-10*int64(1)*int64(params.BeaconConfig().SecondsPerSlot)) * time.Second)
+
+	// Chain setup
+	// 0 -- 1 -- 2 -- 3
+	//  \-4
+	st, keys := util.DeterministicGenesisState(t, 64)
+	blkG, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 0)
+	assert.NoError(t, err)
+	b, err := wrapper.WrappedSignedBeaconBlock(blkG)
+	assert.NoError(t, err)
+	require.NoError(t, service.cfg.BeaconDB.SaveBlock(ctx, b))
+	rG, err := blkG.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk1, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 1)
+	assert.NoError(t, err)
+	blk1.Block.ParentRoot = rG[:]
+	r1, err := blk1.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk2, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 2)
+	assert.NoError(t, err)
+	blk2.Block.ParentRoot = r1[:]
+	r2, err := blk2.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk3, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 3)
+	assert.NoError(t, err)
+	blk3.Block.ParentRoot = r2[:]
+	r3, err := blk3.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk4 := util.NewBeaconBlock()
+	blk4.Block.Slot = 4
+	blk4.Block.ParentRoot = rG[:]
+	r4, err := blk4.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	for _, blk := range []*ethpb.SignedBeaconBlock{blkG, blk1, blk2, blk3, blk4} {
+		r, err := blk.Block.HashTreeRoot()
+		require.NoError(t, err)
+		require.NoError(t, service.ForkChoicer().InsertOptimisticBlock(ctx, blk.Block.Slot, r, bytesutil.ToBytes32(blk.Block.ParentRoot), [32]byte{}, 0, 0))
+		b, err := wrapper.WrappedSignedBeaconBlock(blk)
+		require.NoError(t, err)
+		require.NoError(t, beaconDB.SaveBlock(ctx, b))
+	}
+
+	require.NoError(t, service.saveOrphanedAtts(ctx, r3, r4))
+	require.Equal(t, 3, service.cfg.AttPool.AggregatedAttestationCount())
+	wantAtts := []*ethpb.Attestation{
+		blk3.Block.Body.Attestations[0],
+		blk2.Block.Body.Attestations[0],
+		blk1.Block.Body.Attestations[0],
+	}
+	atts := service.cfg.AttPool.AggregatedAttestations()
+	sort.Slice(atts, func(i, j int) bool {
+		return atts[i].Data.Slot > atts[j].Data.Slot
+	})
+	require.DeepEqual(t, wantAtts, atts)
+}
+
+func TestSaveOrphanedAtts_CanFilter_DoublyLinkedTrie(t *testing.T) {
+	resetCfg := features.InitWithReset(&features.Flags{
+		EnableForkChoiceDoublyLinkedTree: true,
+	})
+	defer resetCfg()
+
+	ctx := context.Background()
+	beaconDB := testDB.SetupDB(t)
+	service := setupBeaconChain(t, beaconDB)
+	service.genesisTime = time.Now().Add(time.Duration(-1*int64(params.BeaconConfig().SlotsPerEpoch+2)*int64(params.BeaconConfig().SecondsPerSlot)) * time.Second)
+
+	// Chain setup
+	// 0 -- 1 -- 2
+	//  \-4
+	st, keys := util.DeterministicGenesisState(t, 64)
+	blkG, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 0)
+	assert.NoError(t, err)
+	b, err := wrapper.WrappedSignedBeaconBlock(blkG)
+	assert.NoError(t, err)
+	require.NoError(t, service.cfg.BeaconDB.SaveBlock(ctx, b))
+	rG, err := blkG.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk1, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 1)
+	assert.NoError(t, err)
+	blk1.Block.ParentRoot = rG[:]
+	r1, err := blk1.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk2, err := util.GenerateFullBlock(st, keys, util.DefaultBlockGenConfig(), 2)
+	assert.NoError(t, err)
+	blk2.Block.ParentRoot = r1[:]
+	r2, err := blk2.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	blk4 := util.NewBeaconBlock()
+	blk4.Block.Slot = 4
+	blk4.Block.ParentRoot = rG[:]
+	r4, err := blk4.Block.HashTreeRoot()
+	require.NoError(t, err)
+
+	for _, blk := range []*ethpb.SignedBeaconBlock{blkG, blk1, blk2, blk4} {
+		r, err := blk.Block.HashTreeRoot()
+		require.NoError(t, err)
+		require.NoError(t, service.ForkChoicer().InsertOptimisticBlock(ctx, blk.Block.Slot, r, bytesutil.ToBytes32(blk.Block.ParentRoot), [32]byte{}, 0, 0))
+		b, err := wrapper.WrappedSignedBeaconBlock(blk)
+		require.NoError(t, err)
+		require.NoError(t, beaconDB.SaveBlock(ctx, b))
+	}
+
+	require.NoError(t, service.saveOrphanedAtts(ctx, r2, r4))
 	require.Equal(t, 0, service.cfg.AttPool.AggregatedAttestationCount())
-	savedAtts := service.cfg.AttPool.AggregatedAttestations()
-	atts := b.Block.Body.Attestations
-	require.DeepNotSSZEqual(t, atts, savedAtts)
 }
 
 func TestUpdateHead_noSavedChanges(t *testing.T) {
diff --git a/beacon-chain/blockchain/receive_attestation_test.go b/beacon-chain/blockchain/receive_attestation_test.go
index e3551367e0..521b112496 100644
--- a/beacon-chain/blockchain/receive_attestation_test.go
+++ b/beacon-chain/blockchain/receive_attestation_test.go
@@ -233,6 +233,8 @@ func TestService_ProcessAttestationsAndUpdateHead(t *testing.T) {
 	r, err := b.Block.HashTreeRoot()
 	require.NoError(t, err)
 	require.NoError(t, service.cfg.BeaconDB.SaveBlock(ctx, wb))
+	require.NoError(t, service.cfg.ForkChoiceStore.InsertOptimisticBlock(
+		ctx, wb.Block().Slot(), r, bytesutil.ToBytes32(wb.Block().ParentRoot()), [32]byte{}, 0, 0))
 	service.head.root = r // Old head
 	require.Equal(t, 1, len(service.cfg.AttPool.ForkchoiceAttestations()))
 	require.NoError(t, err, service.UpdateHead(ctx))
diff --git a/beacon-chain/forkchoice/BUILD.bazel b/beacon-chain/forkchoice/BUILD.bazel
index 2a785ec490..57ce97f928 100644
--- a/beacon-chain/forkchoice/BUILD.bazel
+++ b/beacon-chain/forkchoice/BUILD.bazel
@@ -4,6 +4,7 @@ go_library(
     name = "go_default_library",
     srcs = [
         "doc.go",
+        "error.go",
         "interfaces.go",
     ],
     importpath = "github.com/prysmaticlabs/prysm/beacon-chain/forkchoice",
@@ -16,5 +17,6 @@ go_library(
         "//config/fieldparams:go_default_library",
         "//consensus-types/primitives:go_default_library",
         "//proto/prysm/v1alpha1:go_default_library",
+        "@com_github_pkg_errors//:go_default_library",
     ],
 )
diff --git a/beacon-chain/forkchoice/doubly-linked-tree/BUILD.bazel b/beacon-chain/forkchoice/doubly-linked-tree/BUILD.bazel
index 47c1170188..c3c91b6a5f 100644
--- a/beacon-chain/forkchoice/doubly-linked-tree/BUILD.bazel
+++ b/beacon-chain/forkchoice/doubly-linked-tree/BUILD.bazel
@@ -20,6 +20,7 @@ go_library(
         "//testing/spectest:__subpackages__",
     ],
     deps = [
+        "//beacon-chain/forkchoice:go_default_library",
         "//beacon-chain/forkchoice/types:go_default_library",
         "//config/fieldparams:go_default_library",
         "//config/params:go_default_library",
@@ -49,6 +50,7 @@ go_test(
     ],
     embed = [":go_default_library"],
     deps = [
+        "//beacon-chain/forkchoice:go_default_library",
         "//beacon-chain/forkchoice/types:go_default_library",
         "//config/params:go_default_library",
         "//consensus-types/primitives:go_default_library",
diff --git a/beacon-chain/forkchoice/doubly-linked-tree/forkchoice.go b/beacon-chain/forkchoice/doubly-linked-tree/forkchoice.go
index e4067dc7f1..24011ab063 100644
--- a/beacon-chain/forkchoice/doubly-linked-tree/forkchoice.go
+++ b/beacon-chain/forkchoice/doubly-linked-tree/forkchoice.go
@@ -5,6 +5,7 @@ import (
 	"fmt"
 
 	"github.com/pkg/errors"
+	"github.com/prysmaticlabs/prysm/beacon-chain/forkchoice"
 	fieldparams "github.com/prysmaticlabs/prysm/config/fieldparams"
 	"github.com/prysmaticlabs/prysm/config/params"
 	types "github.com/prysmaticlabs/prysm/consensus-types/primitives"
@@ -379,3 +380,48 @@ func (f *ForkChoice) UpdateFinalizedCheckpoint(fc *pbrpc.Checkpoint) error {
 	f.store.finalizedEpoch = fc.Epoch
 	return nil
 }
+
+// CommonAncestorRoot returns the common ancestor root between the two block roots r1 and r2.
+func (f *ForkChoice) CommonAncestorRoot(ctx context.Context, r1 [32]byte, r2 [32]byte) ([32]byte, error) {
+	ctx, span := trace.StartSpan(ctx, "doublelinkedtree.CommonAncestorRoot")
+	defer span.End()
+
+	// Do nothing if the input roots are the same.
+	if r1 == r2 {
+		return r1, nil
+	}
+
+	f.store.nodesLock.RLock()
+	defer f.store.nodesLock.RUnlock()
+
+	n1, ok := f.store.nodeByRoot[r1]
+	if !ok || n1 == nil {
+		return [32]byte{}, ErrNilNode
+	}
+	n2, ok := f.store.nodeByRoot[r2]
+	if !ok || n2 == nil {
+		return [32]byte{}, ErrNilNode
+	}
+
+	for {
+		if ctx.Err() != nil {
+			return [32]byte{}, ctx.Err()
+		}
+		if n1.slot > n2.slot {
+			n1 = n1.parent
+			// Reaches the end of the tree and unable to find common ancestor.
+			if n1 == nil {
+				return [32]byte{}, forkchoice.ErrUnknownCommonAncestor
+			}
+		} else {
+			n2 = n2.parent
+			// Reaches the end of the tree and unable to find common ancestor.
+			if n2 == nil {
+				return [32]byte{}, forkchoice.ErrUnknownCommonAncestor
+			}
+		}
+		if n1 == n2 {
+			return n1.root, nil
+		}
+	}
+}
diff --git a/beacon-chain/forkchoice/doubly-linked-tree/forkchoice_test.go b/beacon-chain/forkchoice/doubly-linked-tree/forkchoice_test.go
index 7f30f9243c..d01a0e8e4c 100644
--- a/beacon-chain/forkchoice/doubly-linked-tree/forkchoice_test.go
+++ b/beacon-chain/forkchoice/doubly-linked-tree/forkchoice_test.go
@@ -5,6 +5,7 @@ import (
 	"encoding/binary"
 	"testing"
 
+	"github.com/prysmaticlabs/prysm/beacon-chain/forkchoice"
 	"github.com/prysmaticlabs/prysm/config/params"
 	types "github.com/prysmaticlabs/prysm/consensus-types/primitives"
 	"github.com/prysmaticlabs/prysm/crypto/hash"
@@ -251,3 +252,158 @@ func TestStore_UpdateCheckpoints(t *testing.T) {
 	require.Equal(t, f.store.justifiedEpoch, jc.Epoch)
 	require.Equal(t, f.store.finalizedEpoch, fc.Epoch)
 }
+
+func TestStore_CommonAncestor(t *testing.T) {
+	{
+		ctx := context.Background()
+		f := setup(0, 0)
+
+		//  /-- b -- d -- e
+		// a
+		//  \-- c -- f
+		//        \-- g
+		//        \ -- h -- i -- j
+		require.NoError(t, f.InsertOptimisticBlock(ctx, 0, [32]byte{'a'}, params.BeaconConfig().ZeroHash, [32]byte{'A'}, 1, 1))
+		require.NoError(t, f.InsertOptimisticBlock(ctx, 1, [32]byte{'b'}, [32]byte{'a'}, [32]byte{'B'}, 1, 1))
+		require.NoError(t, f.InsertOptimisticBlock(ctx, 2, [32]byte{'c'}, [32]byte{'a'}, [32]byte{'C'}, 1, 1))
+		require.NoError(t, f.InsertOptimisticBlock(ctx, 3, [32]byte{'d'}, [32]byte{'b'}, [32]byte{}, 1, 1))
+		require.NoError(t, f.InsertOptimisticBlock(ctx, 4, [32]byte{'e'}, [32]byte{'d'}, [32]byte{}, 1, 1))
+		require.NoError(t, f.InsertOptimisticBlock(ctx, 5, [32]byte{'f'}, [32]byte{'c'}, [32]byte{}, 1, 1))
+		require.NoError(t, f.InsertOptimisticBlock(ctx, 6, [32]byte{'g'}, [32]byte{'c'}, [32]byte{}, 1, 1))
+		require.NoError(t, f.InsertOptimisticBlock(ctx, 7, [32]byte{'h'}, [32]byte{'c'}, [32]byte{}, 1, 1))
+		require.NoError(t, f.InsertOptimisticBlock(ctx, 8, [32]byte{'i'}, [32]byte{'h'}, [32]byte{}, 1, 1))
+		require.NoError(t, f.InsertOptimisticBlock(ctx, 9, [32]byte{'j'}, [32]byte{'i'}, [32]byte{}, 1, 1))
+
+		tests := []struct {
+			name     string
+			r1       [32]byte
+			r2       [32]byte
+			wantRoot [32]byte
+		}{
+			{
+				name:     "Common ancestor between c and b is a",
+				r1:       [32]byte{'c'},
+				r2:       [32]byte{'b'},
+				wantRoot: [32]byte{'a'},
+			},
+			{
+				name:     "Common ancestor between c and d is a",
+				r1:       [32]byte{'c'},
+				r2:       [32]byte{'d'},
+				wantRoot: [32]byte{'a'},
+			},
+			{
+				name:     "Common ancestor between c and e is a",
+				r1:       [32]byte{'c'},
+				r2:       [32]byte{'e'},
+				wantRoot: [32]byte{'a'},
+			},
+			{
+				name:     "Common ancestor between g and f is c",
+				r1:       [32]byte{'g'},
+				r2:       [32]byte{'f'},
+				wantRoot: [32]byte{'c'},
+			},
+			{
+				name:     "Common ancestor between f and h is c",
+				r1:       [32]byte{'f'},
+				r2:       [32]byte{'h'},
+				wantRoot: [32]byte{'c'},
+			},
+			{
+				name:     "Common ancestor between g and h is c",
+				r1:       [32]byte{'g'},
+				r2:       [32]byte{'h'},
+				wantRoot: [32]byte{'c'},
+			},
+			{
+				name:     "Common ancestor between b and h is a",
+				r1:       [32]byte{'b'},
+				r2:       [32]byte{'h'},
+				wantRoot: [32]byte{'a'},
+			},
+			{
+				name:     "Common ancestor between e and h is a",
+				r1:       [32]byte{'e'},
+				r2:       [32]byte{'h'},
+				wantRoot: [32]byte{'a'},
+			},
+			{
+				name:     "Common ancestor between i and f is c",
+				r1:       [32]byte{'i'},
+				r2:       [32]byte{'f'},
+				wantRoot: [32]byte{'c'},
+			},
+			{
+				name:     "Common ancestor between e and h is a",
+				r1:       [32]byte{'j'},
+				r2:       [32]byte{'g'},
+				wantRoot: [32]byte{'c'},
+			},
+		}
+		for _, tc := range tests {
+			t.Run(tc.name, func(t *testing.T) {
+				gotRoot, err := f.CommonAncestorRoot(ctx, tc.r1, tc.r2)
+				require.NoError(t, err)
+				require.Equal(t, tc.wantRoot, gotRoot)
+			})
+		}
+
+		// a -- b -- c -- d
+		f = setup(0, 0)
+		require.NoError(t, f.InsertOptimisticBlock(ctx, 0, [32]byte{'a'}, params.BeaconConfig().ZeroHash, [32]byte{'A'}, 1, 1))
+		require.NoError(t, f.InsertOptimisticBlock(ctx, 1, [32]byte{'b'}, [32]byte{'a'}, [32]byte{'B'}, 1, 1))
+		require.NoError(t, f.InsertOptimisticBlock(ctx, 2, [32]byte{'c'}, [32]byte{'b'}, [32]byte{'C'}, 1, 1))
+		require.NoError(t, f.InsertOptimisticBlock(ctx, 3, [32]byte{'d'}, [32]byte{'c'}, [32]byte{}, 1, 1))
+		tests = []struct {
+			name     string
+			r1       [32]byte
+			r2       [32]byte
+			wantRoot [32]byte
+		}{
+			{
+				name:     "Common ancestor between a and b is a",
+				r1:       [32]byte{'a'},
+				r2:       [32]byte{'b'},
+				wantRoot: [32]byte{'a'},
+			},
+			{
+				name:     "Common ancestor between b and d is b",
+				r1:       [32]byte{'d'},
+				r2:       [32]byte{'b'},
+				wantRoot: [32]byte{'b'},
+			},
+			{
+				name:     "Common ancestor between d and a is a",
+				r1:       [32]byte{'d'},
+				r2:       [32]byte{'a'},
+				wantRoot: [32]byte{'a'},
+			},
+		}
+		for _, tc := range tests {
+			t.Run(tc.name, func(t *testing.T) {
+				gotRoot, err := f.CommonAncestorRoot(ctx, tc.r1, tc.r2)
+				require.NoError(t, err)
+				require.Equal(t, tc.wantRoot, gotRoot)
+			})
+		}
+
+		// Equal inputs should return the same root.
+		r, err := f.CommonAncestorRoot(ctx, [32]byte{'b'}, [32]byte{'b'})
+		require.NoError(t, err)
+		require.Equal(t, [32]byte{'b'}, r)
+		// Requesting finalized root (last node) should return the same root.
+		r, err = f.CommonAncestorRoot(ctx, [32]byte{'a'}, [32]byte{'a'})
+		require.NoError(t, err)
+		require.Equal(t, [32]byte{'a'}, r)
+		// Requesting unknown root
+		_, err = f.CommonAncestorRoot(ctx, [32]byte{'a'}, [32]byte{'z'})
+		require.ErrorIs(t, err, ErrNilNode)
+		_, err = f.CommonAncestorRoot(ctx, [32]byte{'z'}, [32]byte{'a'})
+		require.ErrorIs(t, err, ErrNilNode)
+		require.NoError(t, f.InsertOptimisticBlock(ctx, 100, [32]byte{'y'}, [32]byte{'z'}, [32]byte{}, 1, 1))
+		// broken link
+		_, err = f.CommonAncestorRoot(ctx, [32]byte{'y'}, [32]byte{'a'})
+		require.ErrorIs(t, err, forkchoice.ErrUnknownCommonAncestor)
+	}
+}
diff --git a/beacon-chain/forkchoice/error.go b/beacon-chain/forkchoice/error.go
new file mode 100644
index 0000000000..7341d9d89d
--- /dev/null
+++ b/beacon-chain/forkchoice/error.go
@@ -0,0 +1,5 @@
+package forkchoice
+
+import "github.com/pkg/errors"
+
+var ErrUnknownCommonAncestor = errors.New("unknown common ancestor")
diff --git a/beacon-chain/forkchoice/interfaces.go b/beacon-chain/forkchoice/interfaces.go
index db764c21d4..76738bee18 100644
--- a/beacon-chain/forkchoice/interfaces.go
+++ b/beacon-chain/forkchoice/interfaces.go
@@ -62,6 +62,7 @@ type Getter interface {
 	ProposerBoost() [fieldparams.RootLength]byte
 	HasParent(root [32]byte) bool
 	AncestorRoot(ctx context.Context, root [32]byte, slot types.Slot) ([]byte, error)
+	CommonAncestorRoot(ctx context.Context, root1 [32]byte, root2 [32]byte) ([32]byte, error)
 	IsCanonical(root [32]byte) bool
 	FinalizedEpoch() types.Epoch
 	JustifiedEpoch() types.Epoch
diff --git a/beacon-chain/forkchoice/protoarray/BUILD.bazel b/beacon-chain/forkchoice/protoarray/BUILD.bazel
index 1fa101ac3e..6621bceea6 100644
--- a/beacon-chain/forkchoice/protoarray/BUILD.bazel
+++ b/beacon-chain/forkchoice/protoarray/BUILD.bazel
@@ -20,6 +20,7 @@ go_library(
         "//testing/spectest:__subpackages__",
     ],
     deps = [
+        "//beacon-chain/forkchoice:go_default_library",
         "//beacon-chain/forkchoice/types:go_default_library",
         "//config/fieldparams:go_default_library",
         "//config/params:go_default_library",
@@ -50,6 +51,7 @@ go_test(
     ],
     embed = [":go_default_library"],
     deps = [
+        "//beacon-chain/forkchoice:go_default_library",
         "//beacon-chain/forkchoice/types:go_default_library",
         "//config/params:go_default_library",
         "//consensus-types/primitives:go_default_library",
diff --git a/beacon-chain/forkchoice/protoarray/store.go b/beacon-chain/forkchoice/protoarray/store.go
index f1518f6353..cf9ed50f2a 100644
--- a/beacon-chain/forkchoice/protoarray/store.go
+++ b/beacon-chain/forkchoice/protoarray/store.go
@@ -6,6 +6,7 @@ import (
 	"fmt"
 
 	"github.com/pkg/errors"
+	"github.com/prysmaticlabs/prysm/beacon-chain/forkchoice"
 	fieldparams "github.com/prysmaticlabs/prysm/config/fieldparams"
 	"github.com/prysmaticlabs/prysm/config/params"
 	types "github.com/prysmaticlabs/prysm/consensus-types/primitives"
@@ -195,6 +196,52 @@ func (f *ForkChoice) AncestorRoot(ctx context.Context, root [32]byte, slot types
 	return f.store.nodes[i].root[:], nil
 }
 
+// CommonAncestorRoot returns the common ancestor root between the two block roots r1 and r2.
+func (f *ForkChoice) CommonAncestorRoot(ctx context.Context, r1 [32]byte, r2 [32]byte) ([32]byte, error) {
+	ctx, span := trace.StartSpan(ctx, "protoArray.CommonAncestorRoot")
+	defer span.End()
+
+	// Do nothing if the two input roots are the same.
+	if r1 == r2 {
+		return r1, nil
+	}
+
+	i1, ok := f.store.nodesIndices[r1]
+	if !ok || i1 >= uint64(len(f.store.nodes)) {
+		return [32]byte{}, errInvalidNodeIndex
+	}
+
+	i2, ok := f.store.nodesIndices[r2]
+	if !ok || i2 >= uint64(len(f.store.nodes)) {
+		return [32]byte{}, errInvalidNodeIndex
+	}
+
+	for {
+		if ctx.Err() != nil {
+			return [32]byte{}, ctx.Err()
+		}
+		if i1 > i2 {
+			n1 := f.store.nodes[i1]
+			i1 = n1.parent
+			// Reaches the end of the tree and unable to find common ancestor.
+			if i1 >= uint64(len(f.store.nodes)) {
+				return [32]byte{}, forkchoice.ErrUnknownCommonAncestor
+			}
+		} else {
+			n2 := f.store.nodes[i2]
+			i2 = n2.parent
+			// Reaches the end of the tree and unable to find common ancestor.
+			if i2 >= uint64(len(f.store.nodes)) {
+				return [32]byte{}, forkchoice.ErrUnknownCommonAncestor
+			}
+		}
+		if i1 == i2 {
+			n1 := f.store.nodes[i1]
+			return n1.root, nil
+		}
+	}
+}
+
 // PruneThreshold of fork choice store.
 func (s *Store) PruneThreshold() uint64 {
 	return s.pruneThreshold
diff --git a/beacon-chain/forkchoice/protoarray/store_test.go b/beacon-chain/forkchoice/protoarray/store_test.go
index b2d3355457..1ab7b7e1cb 100644
--- a/beacon-chain/forkchoice/protoarray/store_test.go
+++ b/beacon-chain/forkchoice/protoarray/store_test.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"testing"
 
+	"github.com/prysmaticlabs/prysm/beacon-chain/forkchoice"
 	"github.com/prysmaticlabs/prysm/config/params"
 	types "github.com/prysmaticlabs/prysm/consensus-types/primitives"
 	"github.com/prysmaticlabs/prysm/encoding/bytesutil"
@@ -560,6 +561,159 @@ func TestStore_PruneBranched(t *testing.T) {
 	}
 }
 
+func TestStore_CommonAncestor(t *testing.T) {
+	ctx := context.Background()
+	f := setup(0, 0)
+
+	//  /-- b -- d -- e
+	// a
+	//  \-- c -- f
+	//        \-- g
+	//        \ -- h -- i -- j
+	require.NoError(t, f.InsertOptimisticBlock(ctx, 0, [32]byte{'a'}, params.BeaconConfig().ZeroHash, [32]byte{'A'}, 1, 1))
+	require.NoError(t, f.InsertOptimisticBlock(ctx, 1, [32]byte{'b'}, [32]byte{'a'}, [32]byte{'B'}, 1, 1))
+	require.NoError(t, f.InsertOptimisticBlock(ctx, 2, [32]byte{'c'}, [32]byte{'a'}, [32]byte{'C'}, 1, 1))
+	require.NoError(t, f.InsertOptimisticBlock(ctx, 3, [32]byte{'d'}, [32]byte{'b'}, [32]byte{}, 1, 1))
+	require.NoError(t, f.InsertOptimisticBlock(ctx, 4, [32]byte{'e'}, [32]byte{'d'}, [32]byte{}, 1, 1))
+	require.NoError(t, f.InsertOptimisticBlock(ctx, 5, [32]byte{'f'}, [32]byte{'c'}, [32]byte{}, 1, 1))
+	require.NoError(t, f.InsertOptimisticBlock(ctx, 6, [32]byte{'g'}, [32]byte{'c'}, [32]byte{}, 1, 1))
+	require.NoError(t, f.InsertOptimisticBlock(ctx, 7, [32]byte{'h'}, [32]byte{'c'}, [32]byte{}, 1, 1))
+	require.NoError(t, f.InsertOptimisticBlock(ctx, 8, [32]byte{'i'}, [32]byte{'h'}, [32]byte{}, 1, 1))
+	require.NoError(t, f.InsertOptimisticBlock(ctx, 9, [32]byte{'j'}, [32]byte{'i'}, [32]byte{}, 1, 1))
+
+	tests := []struct {
+		name     string
+		r1       [32]byte
+		r2       [32]byte
+		wantRoot [32]byte
+	}{
+		{
+			name:     "Common ancestor between c and b is a",
+			r1:       [32]byte{'c'},
+			r2:       [32]byte{'b'},
+			wantRoot: [32]byte{'a'},
+		},
+		{
+			name:     "Common ancestor between c and d is a",
+			r1:       [32]byte{'c'},
+			r2:       [32]byte{'d'},
+			wantRoot: [32]byte{'a'},
+		},
+		{
+			name:     "Common ancestor between c and e is a",
+			r1:       [32]byte{'c'},
+			r2:       [32]byte{'e'},
+			wantRoot: [32]byte{'a'},
+		},
+		{
+			name:     "Common ancestor between g and f is c",
+			r1:       [32]byte{'g'},
+			r2:       [32]byte{'f'},
+			wantRoot: [32]byte{'c'},
+		},
+		{
+			name:     "Common ancestor between f and h is c",
+			r1:       [32]byte{'f'},
+			r2:       [32]byte{'h'},
+			wantRoot: [32]byte{'c'},
+		},
+		{
+			name:     "Common ancestor between g and h is c",
+			r1:       [32]byte{'g'},
+			r2:       [32]byte{'h'},
+			wantRoot: [32]byte{'c'},
+		},
+		{
+			name:     "Common ancestor between b and h is a",
+			r1:       [32]byte{'b'},
+			r2:       [32]byte{'h'},
+			wantRoot: [32]byte{'a'},
+		},
+		{
+			name:     "Common ancestor between e and h is a",
+			r1:       [32]byte{'e'},
+			r2:       [32]byte{'h'},
+			wantRoot: [32]byte{'a'},
+		},
+		{
+			name:     "Common ancestor between i and f is c",
+			r1:       [32]byte{'i'},
+			r2:       [32]byte{'f'},
+			wantRoot: [32]byte{'c'},
+		},
+		{
+			name:     "Common ancestor between e and h is a",
+			r1:       [32]byte{'j'},
+			r2:       [32]byte{'g'},
+			wantRoot: [32]byte{'c'},
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			gotRoot, err := f.CommonAncestorRoot(ctx, tc.r1, tc.r2)
+			require.NoError(t, err)
+			require.Equal(t, tc.wantRoot, gotRoot)
+		})
+	}
+
+	// a -- b -- c -- d
+	f = setup(0, 0)
+	require.NoError(t, f.InsertOptimisticBlock(ctx, 0, [32]byte{'a'}, params.BeaconConfig().ZeroHash, [32]byte{'A'}, 1, 1))
+	require.NoError(t, f.InsertOptimisticBlock(ctx, 1, [32]byte{'b'}, [32]byte{'a'}, [32]byte{'B'}, 1, 1))
+	require.NoError(t, f.InsertOptimisticBlock(ctx, 2, [32]byte{'c'}, [32]byte{'b'}, [32]byte{'C'}, 1, 1))
+	require.NoError(t, f.InsertOptimisticBlock(ctx, 3, [32]byte{'d'}, [32]byte{'c'}, [32]byte{}, 1, 1))
+	tests = []struct {
+		name     string
+		r1       [32]byte
+		r2       [32]byte
+		wantRoot [32]byte
+	}{
+		{
+			name:     "Common ancestor between a and b is a",
+			r1:       [32]byte{'a'},
+			r2:       [32]byte{'b'},
+			wantRoot: [32]byte{'a'},
+		},
+		{
+			name:     "Common ancestor between b and d is b",
+			r1:       [32]byte{'d'},
+			r2:       [32]byte{'b'},
+			wantRoot: [32]byte{'b'},
+		},
+		{
+			name:     "Common ancestor between d and a is a",
+			r1:       [32]byte{'d'},
+			r2:       [32]byte{'a'},
+			wantRoot: [32]byte{'a'},
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			gotRoot, err := f.CommonAncestorRoot(ctx, tc.r1, tc.r2)
+			require.NoError(t, err)
+			require.Equal(t, tc.wantRoot, gotRoot)
+		})
+	}
+
+	// Equal inputs should return the same root.
+	r, err := f.CommonAncestorRoot(ctx, [32]byte{'b'}, [32]byte{'b'})
+	require.NoError(t, err)
+	require.Equal(t, [32]byte{'b'}, r)
+	// Requesting finalized root (last node) should return the same root.
+	r, err = f.CommonAncestorRoot(ctx, [32]byte{'a'}, [32]byte{'a'})
+	require.NoError(t, err)
+	require.Equal(t, [32]byte{'a'}, r)
+	// Requesting unknown root
+	_, err = f.CommonAncestorRoot(ctx, [32]byte{'a'}, [32]byte{'z'})
+	require.ErrorIs(t, err, errInvalidNodeIndex)
+	_, err = f.CommonAncestorRoot(ctx, [32]byte{'z'}, [32]byte{'a'})
+	require.ErrorIs(t, err, errInvalidNodeIndex)
+	require.NoError(t, f.InsertOptimisticBlock(ctx, 100, [32]byte{'y'}, [32]byte{'z'}, [32]byte{}, 1, 1))
+	// broken link
+	_, err = f.CommonAncestorRoot(ctx, [32]byte{'y'}, [32]byte{'a'})
+	require.ErrorIs(t, err, forkchoice.ErrUnknownCommonAncestor)
+}
+
 func TestStore_LeadsToViableHead(t *testing.T) {
 	tests := []struct {
 		n              *Node