From 3def16caaa6063eb6b026effdb6a9a976aeb666e Mon Sep 17 00:00:00 2001
From: Potuz <potuz@prysmaticlabs.com>
Date: Wed, 16 Jul 2025 00:16:11 -0300
Subject: [PATCH] Fix InitializeProposerLookahead (#15450)

* Fix InitializeProposerLookahead

Get the right Active validator indices for each epoch after the fork
transition.

Co-Authored-By: Claude <noreply@anthropic.com>

* Add test

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: james-prysm <90280386+james-prysm@users.noreply.github.com>
---
 beacon-chain/core/helpers/beacon_committee.go |  8 ++--
 .../core/helpers/beacon_committee_test.go     | 43 +++++++++++++++++++
 changelog/potuz_fix_initialize_lookahead.md   |  2 +
 3 files changed, 49 insertions(+), 4 deletions(-)
 create mode 100644 changelog/potuz_fix_initialize_lookahead.md

diff --git a/beacon-chain/core/helpers/beacon_committee.go b/beacon-chain/core/helpers/beacon_committee.go
index a7f282811d..16f734bbd8 100644
--- a/beacon-chain/core/helpers/beacon_committee.go
+++ b/beacon-chain/core/helpers/beacon_committee.go
@@ -669,11 +669,11 @@ func ComputeCommittee(
 // InitializeProposerLookahead computes the list of the proposer indices for the next MIN_SEED_LOOKAHEAD + 1 epochs.
 func InitializeProposerLookahead(ctx context.Context, state state.ReadOnlyBeaconState, epoch primitives.Epoch) ([]uint64, error) {
 	lookAhead := make([]uint64, 0, uint64(params.BeaconConfig().MinSeedLookahead+1)*uint64(params.BeaconConfig().SlotsPerEpoch))
-	indices, err := ActiveValidatorIndices(ctx, state, epoch)
-	if err != nil {
-		return nil, errors.Wrap(err, "could not get active indices")
-	}
 	for i := range params.BeaconConfig().MinSeedLookahead + 1 {
+		indices, err := ActiveValidatorIndices(ctx, state, epoch+i)
+		if err != nil {
+			return nil, errors.Wrap(err, "could not get active indices")
+		}
 		proposerIndices, err := PrecomputeProposerIndices(state, indices, epoch+i)
 		if err != nil {
 			return nil, errors.Wrap(err, "could not compute proposer indices")
diff --git a/beacon-chain/core/helpers/beacon_committee_test.go b/beacon-chain/core/helpers/beacon_committee_test.go
index 86ede9fcb0..d168e8e8ab 100644
--- a/beacon-chain/core/helpers/beacon_committee_test.go
+++ b/beacon-chain/core/helpers/beacon_committee_test.go
@@ -916,3 +916,46 @@ func TestAssignmentForValidator(t *testing.T) {
 		require.DeepEqual(t, &helpers.LiteAssignment{}, got)
 	})
 }
+
+// Regression for #15450
+func TestInitializeProposerLookahead_RegressionTest(t *testing.T) {
+	ctx := t.Context()
+
+	state, _ := util.DeterministicGenesisState(t, 128)
+	// Set some validators to activate in epoch 3 instead of 0
+	validators := state.Validators()
+	for i := 64; i < 128; i++ {
+		validators[i].ActivationEpoch = 3
+	}
+	require.NoError(t, state.SetValidators(validators))
+	require.NoError(t, state.SetSlot(64)) // epoch 2
+	epoch := slots.ToEpoch(state.Slot())
+
+	proposerLookahead, err := helpers.InitializeProposerLookahead(ctx, state, epoch)
+	require.NoError(t, err)
+	slotsPerEpoch := int(params.BeaconConfig().SlotsPerEpoch)
+	for epochOffset := primitives.Epoch(0); epochOffset < 2; epochOffset++ {
+		targetEpoch := epoch + epochOffset
+
+		activeIndices, err := helpers.ActiveValidatorIndices(ctx, state, targetEpoch)
+		require.NoError(t, err)
+
+		expectedProposers, err := helpers.PrecomputeProposerIndices(state, activeIndices, targetEpoch)
+		require.NoError(t, err)
+
+		startIdx := int(epochOffset) * slotsPerEpoch
+		endIdx := startIdx + slotsPerEpoch
+		actualProposers := proposerLookahead[startIdx:endIdx]
+
+		expectedUint64 := make([]uint64, len(expectedProposers))
+		for i, proposer := range expectedProposers {
+			expectedUint64[i] = uint64(proposer)
+		}
+
+		// This assertion would fail with the original bug:
+		for i, expected := range expectedUint64 {
+			require.Equal(t, expected, actualProposers[i],
+				"Proposer index mismatch at slot %d in epoch %d", i, targetEpoch)
+		}
+	}
+}
diff --git a/changelog/potuz_fix_initialize_lookahead.md b/changelog/potuz_fix_initialize_lookahead.md
new file mode 100644
index 0000000000..17e67b7ada
--- /dev/null
+++ b/changelog/potuz_fix_initialize_lookahead.md
@@ -0,0 +1,2 @@
+### Fixed
+- Fixed lookahead initialization at the fulu fork.