Use head to validate atts for previous epoch (#16109)

In the event that the target checkpoint of an attestation is for the
previous epoch, and the head state has the same dependent root at that
epoch. The reason being that this guarantees that both seed and active
validator indices are guaranteed to be the same at the checkpoint's
epoch, from the point of view of the attester (even on a different
branch) and the head view.

beacon-chain/blockchain/process_attestation_helpers.go

@@ -22,7 +22,7 @@ import (
 // The caller of this function must have a lock on forkchoice.
 func (s *Service) getRecentPreState(ctx context.Context, c *ethpb.Checkpoint) state.ReadOnlyBeaconState {
 	headEpoch := slots.ToEpoch(s.HeadSlot())
-	if c.Epoch < headEpoch || c.Epoch == 0 {
+	if c.Epoch+1 < headEpoch || c.Epoch == 0 {
 		return nil
 	}
 	// Only use head state if the head state is compatible with the target checkpoint.
@@ -30,11 +30,13 @@ func (s *Service) getRecentPreState(ctx context.Context, c *ethpb.Checkpoint) st
 	if err != nil {
 		return nil
 	}
-	headDependent, err := s.cfg.ForkChoiceStore.DependentRootForEpoch([32]byte(headRoot), c.Epoch-1)
+	// headEpoch - 1 equals c.Epoch if c is from the previous epoch and equals c.Epoch - 1 if c is from the current epoch.
+	// We don't use the smaller c.Epoch - 1 because forkchoice would not have the data to answer that.
+	headDependent, err := s.cfg.ForkChoiceStore.DependentRootForEpoch([32]byte(headRoot), headEpoch-1)
 	if err != nil {
 		return nil
 	}
-	targetDependent, err := s.cfg.ForkChoiceStore.DependentRootForEpoch([32]byte(c.Root), c.Epoch-1)
+	targetDependent, err := s.cfg.ForkChoiceStore.DependentRootForEpoch([32]byte(c.Root), headEpoch-1)
 	if err != nil {
 		return nil
 	}
@@ -43,7 +45,7 @@ func (s *Service) getRecentPreState(ctx context.Context, c *ethpb.Checkpoint) st
 	}
 
 	// If the head state alone is enough, we can return it directly read only.
-	if c.Epoch == headEpoch {
+	if c.Epoch <= headEpoch {
 		st, err := s.HeadStateReadOnly(ctx)
 		if err != nil {
 			return nil

beacon-chain/blockchain/process_attestation_test.go

@@ -170,12 +170,13 @@ func TestService_GetRecentPreState(t *testing.T) {
 	err = s.SetFinalizedCheckpoint(cp0)
 	require.NoError(t, err)
 
-	st, root, err := prepareForkchoiceState(ctx, 31, [32]byte(ckRoot), [32]byte{}, [32]byte{'R'}, cp0, cp0)
+	st, blk, err := prepareForkchoiceState(ctx, 31, [32]byte(ckRoot), [32]byte{}, [32]byte{'R'}, cp0, cp0)
 	require.NoError(t, err)
-	require.NoError(t, service.cfg.ForkChoiceStore.InsertNode(ctx, st, root))
+	require.NoError(t, service.cfg.ForkChoiceStore.InsertNode(ctx, st, blk))
 	service.head = &head{
 		root:  [32]byte(ckRoot),
 		state: s,
+		block: blk,
 		slot:  31,
 	}
 	require.NotNil(t, service.getRecentPreState(ctx, &ethpb.Checkpoint{Epoch: 1, Root: ckRoot}))
@@ -197,12 +198,13 @@ func TestService_GetRecentPreState_Old_Checkpoint(t *testing.T) {
 	err = s.SetFinalizedCheckpoint(cp0)
 	require.NoError(t, err)
 
-	st, root, err := prepareForkchoiceState(ctx, 33, [32]byte(ckRoot), [32]byte{}, [32]byte{'R'}, cp0, cp0)
+	st, blk, err := prepareForkchoiceState(ctx, 33, [32]byte(ckRoot), [32]byte{}, [32]byte{'R'}, cp0, cp0)
 	require.NoError(t, err)
-	require.NoError(t, service.cfg.ForkChoiceStore.InsertNode(ctx, st, root))
+	require.NoError(t, service.cfg.ForkChoiceStore.InsertNode(ctx, st, blk))
 	service.head = &head{
 		root:  [32]byte(ckRoot),
 		state: s,
+		block: blk,
 		slot:  33,
 	}
 	require.IsNil(t, service.getRecentPreState(ctx, &ethpb.Checkpoint{}))
@@ -227,6 +229,7 @@ func TestService_GetRecentPreState_Same_DependentRoots(t *testing.T) {
 	require.NoError(t, service.cfg.ForkChoiceStore.InsertNode(ctx, st, blk))
 	st, blk, err = prepareForkchoiceState(ctx, 64, [32]byte{'T'}, blk.Root(), [32]byte{}, cp0, cp0)
 	require.NoError(t, err)
+	headBlock := blk
 	require.NoError(t, service.cfg.ForkChoiceStore.InsertNode(ctx, st, blk))
 	st, blk, err = prepareForkchoiceState(ctx, 33, [32]byte{'U'}, [32]byte(ckRoot), [32]byte{}, cp0, cp0)
 	require.NoError(t, err)
@@ -235,8 +238,9 @@ func TestService_GetRecentPreState_Same_DependentRoots(t *testing.T) {
 
 	service.head = &head{
 		root:  [32]byte{'T'},
-		state: s,
+		block: headBlock,
 		slot:  64,
+		state: s,
 	}
 	require.NotNil(t, service.getRecentPreState(ctx, &ethpb.Checkpoint{Epoch: 2, Root: cpRoot[:]}))
 }
@@ -263,6 +267,7 @@ func TestService_GetRecentPreState_Different_DependentRoots(t *testing.T) {
 	require.NoError(t, service.cfg.ForkChoiceStore.InsertNode(ctx, st, blk))
 	st, blk, err = prepareForkchoiceState(ctx, 64, [32]byte{'U'}, blk.Root(), [32]byte{}, cp0, cp0)
 	require.NoError(t, err)
+	headBlock := blk
 	require.NoError(t, service.cfg.ForkChoiceStore.InsertNode(ctx, st, blk))
 	st, blk, err = prepareForkchoiceState(ctx, 33, [32]byte{'V'}, [32]byte(ckRoot), [32]byte{}, cp0, cp0)
 	require.NoError(t, err)
@@ -270,7 +275,8 @@ func TestService_GetRecentPreState_Different_DependentRoots(t *testing.T) {
 	cpRoot := blk.Root()
 
 	service.head = &head{
-		root:  [32]byte{'T'},
+		root:  [32]byte{'U'},
+		block: headBlock,
 		state: s,
 		slot:  64,
 	}
@@ -287,12 +293,13 @@ func TestService_GetRecentPreState_Different(t *testing.T) {
 	err = s.SetFinalizedCheckpoint(cp0)
 	require.NoError(t, err)
 
-	st, root, err := prepareForkchoiceState(ctx, 33, [32]byte(ckRoot), [32]byte{}, [32]byte{'R'}, cp0, cp0)
+	st, blk, err := prepareForkchoiceState(ctx, 33, [32]byte(ckRoot), [32]byte{}, [32]byte{'R'}, cp0, cp0)
 	require.NoError(t, err)
-	require.NoError(t, service.cfg.ForkChoiceStore.InsertNode(ctx, st, root))
+	require.NoError(t, service.cfg.ForkChoiceStore.InsertNode(ctx, st, blk))
 	service.head = &head{
 		root:  [32]byte(ckRoot),
 		state: s,
+		block: blk,
 		slot:  33,
 	}
 	require.IsNil(t, service.getRecentPreState(ctx, &ethpb.Checkpoint{}))

changelog/potuz_use_head_previous_epoch.md

@@ -0,0 +1,3 @@
+### Added
+
+- Use the head state to validate attestations for the previous epoch if head is compatible with the target checkpoint.