Use proposer lookahead for data column verification (#16202)

Replace the proposer indices cache usage in data column sidecar
verification with direct state lookahead access. Since data column
sidecars require the Fulu fork, the state always has a ProposerLookahead
field that provides O(1) proposer index lookups for current and next
epoch.

This simplifies SidecarProposerExpected() by removing:
- Checkpoint-based proposer cache lookup
- Singleflight wrapper (not needed for O(1) access)
- Target root computation for cache keys

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

beacon-chain/verification/blob_test.go

@@ -687,6 +687,12 @@ func sbrNotFound(t *testing.T, expectedRoot [32]byte) *mockStateByRooter {
 	}}
 }
 
+func sbrReturnsState(st state.BeaconState) *mockStateByRooter {
+	return &mockStateByRooter{sbr: func(_ context.Context, _ [32]byte) (state.BeaconState, error) {
+		return st, nil
+	}}
+}
+
 func sbrForValOverride(idx primitives.ValidatorIndex, val *ethpb.Validator) *mockStateByRooter {
 	return sbrForValOverrideWithT(nil, idx, val)
 }

beacon-chain/verification/data_column.go

@@ -11,12 +11,10 @@ import (
 	"github.com/OffchainLabs/prysm/v7/beacon-chain/core/helpers"
 	"github.com/OffchainLabs/prysm/v7/beacon-chain/core/peerdas"
 	"github.com/OffchainLabs/prysm/v7/beacon-chain/core/transition"
-	forkchoicetypes "github.com/OffchainLabs/prysm/v7/beacon-chain/forkchoice/types"
 	"github.com/OffchainLabs/prysm/v7/beacon-chain/state"
 	fieldparams "github.com/OffchainLabs/prysm/v7/config/fieldparams"
 	"github.com/OffchainLabs/prysm/v7/config/params"
 	"github.com/OffchainLabs/prysm/v7/consensus-types/blocks"
-	"github.com/OffchainLabs/prysm/v7/consensus-types/primitives"
 	"github.com/OffchainLabs/prysm/v7/encoding/bytesutil"
 	"github.com/OffchainLabs/prysm/v7/runtime/logging"
 	"github.com/OffchainLabs/prysm/v7/time/slots"
@@ -484,88 +482,19 @@ func (dv *RODataColumnsVerifier) SidecarProposerExpected(ctx context.Context) (e
 
 	defer dv.recordResult(RequireSidecarProposerExpected, &err)
 
-	type slotParentRoot struct {
-		slot       primitives.Slot
-		parentRoot [fieldparams.RootLength]byte
-	}
-
-	targetRootBySlotParentRoot := make(map[slotParentRoot][fieldparams.RootLength]byte)
-
-	var targetRootFromCache = func(slot primitives.Slot, parentRoot [fieldparams.RootLength]byte) ([fieldparams.RootLength]byte, error) {
-		// Use cached values if available.
-		slotParentRoot := slotParentRoot{slot: slot, parentRoot: parentRoot}
-		if root, ok := targetRootBySlotParentRoot[slotParentRoot]; ok {
-			return root, nil
-		}
-
-		// Compute the epoch of the data column slot.
-		dataColumnEpoch := slots.ToEpoch(slot)
-		if dataColumnEpoch > 0 {
-			dataColumnEpoch = dataColumnEpoch - 1
-		}
-
-		// Compute the target root for the epoch.
-		targetRoot, err := dv.fc.TargetRootForEpoch(parentRoot, dataColumnEpoch)
-		if err != nil {
-			return [fieldparams.RootLength]byte{}, columnErrBuilder(errors.Wrap(err, "target root from epoch"))
-		}
-
-		// Store the target root in the cache.
-		targetRootBySlotParentRoot[slotParentRoot] = targetRoot
-
-		return targetRoot, nil
-	}
-
 	for _, dataColumn := range dv.dataColumns {
-		// Extract the slot of the data column.
 		dataColumnSlot := dataColumn.Slot()
 
-		// Extract the root of the parent block corresponding to the data column.
+		// Get the verifying state, it is guaranteed to have the correct proposer in the lookahead.
-		parentRoot := dataColumn.ParentRoot()
-
-		// Compute the target root for the data column.
-		targetRoot, err := targetRootFromCache(dataColumnSlot, parentRoot)
-		if err != nil {
-			return columnErrBuilder(errors.Wrap(err, "target root"))
-		}
-
-		// Compute the epoch of the data column slot.
-		dataColumnEpoch := slots.ToEpoch(dataColumnSlot)
-		if dataColumnEpoch > 0 {
-			dataColumnEpoch = dataColumnEpoch - 1
-		}
-
-		// Create a checkpoint for the target root.
-		checkpoint := &forkchoicetypes.Checkpoint{Root: targetRoot, Epoch: dataColumnEpoch}
-
-		// Try to extract the proposer index from the data column in the cache.
-		idx, cached := dv.pc.Proposer(checkpoint, dataColumnSlot)
-
-		if !cached {
-			parentRoot := dataColumn.ParentRoot()
-			// Ensure the expensive index computation is only performed once for
-			// concurrent requests for the same signature data.
-			idxAny, err, _ := dv.sg.Do(concatRootSlot(parentRoot, dataColumnSlot), func() (any, error) {
 		verifyingState, err := dv.getVerifyingState(ctx, dataColumn)
 		if err != nil {
-					return nil, columnErrBuilder(errors.Wrap(err, "verifying state"))
+			return columnErrBuilder(errors.Wrap(err, "verifying state"))
 		}
 
-				idx, err = helpers.BeaconProposerIndexAtSlot(ctx, verifyingState, dataColumnSlot)
+		// Use proposer lookahead directly
+		idx, err := helpers.BeaconProposerIndexAtSlot(ctx, verifyingState, dataColumnSlot)
 		if err != nil {
-					return nil, columnErrBuilder(errors.Wrap(err, "compute proposer"))
+			return columnErrBuilder(errors.Wrap(err, "proposer from lookahead"))
-				}
-
-				return idx, nil
-			})
-			if err != nil {
-				return err
-			}
-
-			var ok bool
-			if idx, ok = idxAny.(primitives.ValidatorIndex); !ok {
-				return columnErrBuilder(errors.New("type assertion to ValidatorIndex failed"))
-			}
 		}
 
 		if idx != dataColumn.ProposerIndex() {
@@ -626,7 +555,3 @@ func inclusionProofKey(c blocks.RODataColumn) ([32]byte, error) {
 
 	return sha256.Sum256(unhashedKey), nil
 }
-
-func concatRootSlot(root [fieldparams.RootLength]byte, slot primitives.Slot) string {
-	return string(root[:]) + fmt.Sprintf("%d", slot)
-}

beacon-chain/verification/data_column_test.go

@@ -2,7 +2,6 @@ package verification
 
 import (
 	"reflect"
-	"sync"
 	"testing"
 	"time"
 
@@ -795,87 +794,90 @@ func TestDataColumnsSidecarProposerExpected(t *testing.T) {
 		blobCount  = 1
 	)
 
-	parentRoot := [fieldparams.RootLength]byte{}
-	columns := GenerateTestDataColumns(t, parentRoot, columnSlot, blobCount)
-	firstColumn := columns[0]
 	ctx := t.Context()
-	testCases := []struct {
+	parentRoot := [fieldparams.RootLength]byte{}
-		name          string
-		stateByRooter StateByRooter
-		proposerCache proposerCache
-		columns       []blocks.RODataColumn
-		error         string
-	}{
-		{
-			name:          "Cached, matches",
-			stateByRooter: nil,
-			proposerCache: &mockProposerCache{
-				ProposerCB: pcReturnsIdx(firstColumn.ProposerIndex()),
-			},
-			columns: columns,
-		},
-		{
-			name:          "Cached, does not match",
-			stateByRooter: nil,
-			proposerCache: &mockProposerCache{
-				ProposerCB: pcReturnsIdx(firstColumn.ProposerIndex() + 1),
-			},
-			columns: columns,
-			error:   errSidecarUnexpectedProposer.Error(),
-		},
-		{
-			name:          "Not cached, state lookup failure",
-			stateByRooter: sbrNotFound(t, firstColumn.ParentRoot()),
-			proposerCache: &mockProposerCache{
-				ProposerCB: pcReturnsNotFound(),
-			},
-			columns: columns,
-			error:   "verifying state",
-		},
-	}
 
-	for _, tc := range testCases {
+	// Create a Fulu state to get the expected proposer from the lookahead.
-		t.Run(tc.name, func(t *testing.T) {
+	fuluState, _ := util.DeterministicGenesisStateFulu(t, 32)
+	expectedProposer, err := fuluState.ProposerLookahead()
+	require.NoError(t, err)
+	expectedProposerIdx := primitives.ValidatorIndex(expectedProposer[columnSlot])
+
+	// Generate data columns with the expected proposer index.
+	matchingColumns := generateTestDataColumnsWithProposer(t, parentRoot, columnSlot, blobCount, expectedProposerIdx)
+	// Generate data columns with wrong proposer index.
+	wrongColumns := generateTestDataColumnsWithProposer(t, parentRoot, columnSlot, blobCount, expectedProposerIdx+1)
+
+	t.Run("Proposer matches", func(t *testing.T) {
 		initializer := Initializer{
 			shared: &sharedResources{
-					sr:  tc.stateByRooter,
+				sr: sbrReturnsState(fuluState),
-					pc:  tc.proposerCache,
+				hsp: &mockHeadStateProvider{
-					hsp: &mockHeadStateProvider{},
+					headRoot:          parentRoot[:],
-					fc: &mockForkchoicer{
+					headSlot:          columnSlot, // Same epoch so HeadStateReadOnly is used
-						TargetRootForEpochCB: fcReturnsTargetRoot([fieldparams.RootLength]byte{}),
+					headStateReadOnly: fuluState,
 				},
+				fc: &mockForkchoicer{},
 			},
 		}
 
-			verifier := initializer.NewDataColumnsVerifier(tc.columns, GossipDataColumnSidecarRequirements)
+		verifier := initializer.NewDataColumnsVerifier(matchingColumns, GossipDataColumnSidecarRequirements)
-			var wg sync.WaitGroup
-
-			var err1, err2 error
-			wg.Go(func() {
-				err1 = verifier.SidecarProposerExpected(ctx)
-			})
-			wg.Go(func() {
-				err2 = verifier.SidecarProposerExpected(ctx)
-			})
-			wg.Wait()
-
-			require.Equal(t, true, verifier.results.executed(RequireSidecarProposerExpected))
-
-			if len(tc.error) > 0 {
-				require.ErrorContains(t, tc.error, err1)
-				require.ErrorContains(t, tc.error, err2)
-				require.NotNil(t, verifier.results.result(RequireSidecarProposerExpected))
-				return
-			}
-
-			require.NoError(t, err1)
-			require.NoError(t, err2)
-			require.NoError(t, verifier.results.result(RequireSidecarProposerExpected))
-
 		err := verifier.SidecarProposerExpected(ctx)
 		require.NoError(t, err)
+		require.Equal(t, true, verifier.results.executed(RequireSidecarProposerExpected))
+		require.NoError(t, verifier.results.result(RequireSidecarProposerExpected))
 	})
+
+	t.Run("Proposer does not match", func(t *testing.T) {
+		initializer := Initializer{
+			shared: &sharedResources{
+				sr: sbrReturnsState(fuluState),
+				hsp: &mockHeadStateProvider{
+					headRoot:          parentRoot[:],
+					headSlot:          columnSlot, // Same epoch so HeadStateReadOnly is used
+					headStateReadOnly: fuluState,
+				},
+				fc: &mockForkchoicer{},
+			},
 		}
+
+		verifier := initializer.NewDataColumnsVerifier(wrongColumns, GossipDataColumnSidecarRequirements)
+		err := verifier.SidecarProposerExpected(ctx)
+		require.ErrorContains(t, errSidecarUnexpectedProposer.Error(), err)
+		require.Equal(t, true, verifier.results.executed(RequireSidecarProposerExpected))
+		require.NotNil(t, verifier.results.result(RequireSidecarProposerExpected))
+	})
+
+	t.Run("State lookup failure", func(t *testing.T) {
+		columns := GenerateTestDataColumns(t, parentRoot, columnSlot, blobCount)
+		initializer := Initializer{
+			shared: &sharedResources{
+				sr:  sbrNotFound(t, columns[0].ParentRoot()),
+				hsp: &mockHeadStateProvider{},
+				fc:  &mockForkchoicer{},
+			},
+		}
+
+		verifier := initializer.NewDataColumnsVerifier(columns, GossipDataColumnSidecarRequirements)
+		err := verifier.SidecarProposerExpected(ctx)
+		require.ErrorContains(t, "verifying state", err)
+		require.Equal(t, true, verifier.results.executed(RequireSidecarProposerExpected))
+		require.NotNil(t, verifier.results.result(RequireSidecarProposerExpected))
+	})
+}
+
+func generateTestDataColumnsWithProposer(t *testing.T, parent [fieldparams.RootLength]byte, slot primitives.Slot, blobCount int, proposer primitives.ValidatorIndex) []blocks.RODataColumn {
+	roBlock, roBlobs := util.GenerateTestDenebBlockWithSidecar(t, parent, slot, blobCount, util.WithProposer(proposer))
+	blobs := make([]kzg.Blob, 0, len(roBlobs))
+	for i := range roBlobs {
+		blobs = append(blobs, kzg.Blob(roBlobs[i].Blob))
+	}
+
+	cellsPerBlob, proofsPerBlob := util.GenerateCellsAndProofs(t, blobs)
+	roDataColumnSidecars, err := peerdas.DataColumnSidecars(cellsPerBlob, proofsPerBlob, peerdas.PopulateFromBlock(roBlock))
+	require.NoError(t, err)
+
+	return roDataColumnSidecars
 }
 
 func TestColumnRequirementSatisfaction(t *testing.T) {
@@ -922,12 +924,3 @@ func TestColumnRequirementSatisfaction(t *testing.T) {
 	require.NoError(t, err)
 }
 
-func TestConcatRootSlot(t *testing.T) {
-	root := [fieldparams.RootLength]byte{1, 2, 3}
-	const slot = primitives.Slot(3210)
-
-	const expected = "\x01\x02\x03\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x003210"
-
-	actual := concatRootSlot(root, slot)
-	require.Equal(t, expected, actual)
-}

changelog/potuz_dcs_pc_removal.md

@@ -0,0 +1,2 @@
+### Changed
+- Use lookahead to validate data column sidecar proposer index.

testing/util/deneb.go

@@ -44,6 +44,13 @@ func WithProposerSigning(idx primitives.ValidatorIndex, sk bls.SecretKey, valRoo
 	}
 }
 
+// WithProposer sets the proposer index for the generated block without signing.
+func WithProposer(idx primitives.ValidatorIndex) DenebBlockGeneratorOption {
+	return func(g *denebBlockGenerator) {
+		g.proposer = idx
+	}
+}
+
 func WithPayloadSetter(p *enginev1.ExecutionPayloadDeneb) DenebBlockGeneratorOption {
 	return func(g *denebBlockGenerator) {
 		g.payload = p