Return parent root from forkchoice (#14293)

beacon-chain/blockchain/chain_info_forkchoice.go

@@ -99,3 +99,10 @@ func (s *Service) FinalizedBlockHash() [32]byte {
 	defer s.cfg.ForkChoiceStore.RUnlock()
 	return s.cfg.ForkChoiceStore.FinalizedPayloadBlockHash()
 }
+
+// ParentRoot wraps a call to the corresponding method in forkchoice
+func (s *Service) ParentRoot(root [32]byte) ([32]byte, error) {
+	s.cfg.ForkChoiceStore.RLock()
+	defer s.cfg.ForkChoiceStore.RUnlock()
+	return s.cfg.ForkChoiceStore.ParentRoot(root)
+}

beacon-chain/forkchoice/doubly-linked-tree/forkchoice.go

@@ -676,3 +676,18 @@ func (f *ForkChoice) TargetRootForEpoch(root [32]byte, epoch primitives.Epoch) (
 	}
 	return f.TargetRootForEpoch(targetNode.root, epoch)
 }
+
+// ParentRoot returns the block root of the parent node if it is in forkchoice.
+// The exception is for the finalized checkpoint root which we return the zero
+// hash.
+func (f *ForkChoice) ParentRoot(root [32]byte) ([32]byte, error) {
+	n, ok := f.store.nodeByRoot[root]
+	if !ok || n == nil {
+		return [32]byte{}, ErrNilNode
+	}
+	// Return the zero hash for the tree root
+	if n.parent == nil {
+		return [32]byte{}, nil
+	}
+	return n.parent.root, nil
+}

beacon-chain/forkchoice/doubly-linked-tree/forkchoice_test.go

@@ -861,3 +861,29 @@ func TestForkChoiceSlot(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, primitives.Slot(3), slot)
 }
+
+func TestForkchoiceParentRoot(t *testing.T) {
+	f := setup(0, 0)
+	ctx := context.Background()
+	root1 := [32]byte{'a'}
+	st, root, err := prepareForkchoiceState(ctx, 3, root1, params.BeaconConfig().ZeroHash, [32]byte{'A'}, 0, 0)
+	require.NoError(t, err)
+	require.NoError(t, f.InsertNode(ctx, st, root))
+
+	root2 := [32]byte{'b'}
+	st, root, err = prepareForkchoiceState(ctx, 3, root2, root1, [32]byte{'A'}, 0, 0)
+	require.NoError(t, err)
+	require.NoError(t, f.InsertNode(ctx, st, root))
+
+	root, err = f.ParentRoot(root2)
+	require.NoError(t, err)
+	require.Equal(t, root1, root)
+
+	_, err = f.ParentRoot([32]byte{'c'})
+	require.ErrorIs(t, err, ErrNilNode)
+
+	zeroHash := [32]byte{}
+	root, err = f.ParentRoot(zeroHash)
+	require.NoError(t, err)
+	require.Equal(t, zeroHash, root)
+}

beacon-chain/forkchoice/interfaces.go

@@ -80,6 +80,7 @@ type FastGetter interface {
 	TargetRootForEpoch([32]byte, primitives.Epoch) ([32]byte, error)
 	UnrealizedJustifiedPayloadBlockHash() [32]byte
 	Weight(root [32]byte) (uint64, error)
+	ParentRoot(root [32]byte) ([32]byte, error)
 }
 
 // Setter allows to set forkchoice information

beacon-chain/forkchoice/ro.go

@@ -169,3 +169,10 @@ func (ro *ROForkChoice) TargetRootForEpoch(root [32]byte, epoch primitives.Epoch
 	defer ro.l.RUnlock()
 	return ro.getter.TargetRootForEpoch(root, epoch)
 }
+
+// ParentRoot delegates to the underlying forkchoice call, under a lock.
+func (ro *ROForkChoice) ParentRoot(root [32]byte) ([32]byte, error) {
+	ro.l.RLock()
+	defer ro.l.RUnlock()
+	return ro.getter.ParentRoot(root)
+}

beacon-chain/forkchoice/ro_test.go

@@ -37,6 +37,7 @@ const (
 	slotCalled
 	lastRootCalled
 	targetRootForEpochCalled
+	parentRootCalled
 )
 
 func _discard(t *testing.T, e error) {
@@ -291,3 +292,8 @@ func (ro *mockROForkchoice) TargetRootForEpoch(_ [32]byte, _ primitives.Epoch) (
 	ro.calls = append(ro.calls, targetRootForEpochCalled)
 	return [32]byte{}, nil
 }
+
+func (ro *mockROForkchoice) ParentRoot(_ [32]byte) ([32]byte, error) {
+	ro.calls = append(ro.calls, parentRootCalled)
+	return [32]byte{}, nil
+}