fix(trie): empty value encoding for leaves

internal/trie/node/decode.go

@@ -138,6 +138,10 @@ func decodeLeaf(reader io.Reader, partialKeyLength uint16) (node *Node, err erro
 		return nil, fmt.Errorf("%w: %s", ErrDecodeValue, err)
 	}
 
+	// The scale decoding above can encounter either io.EOF,
+	// leaving the value byte slice as `[]byte(nil)`, or
+	// `[]byte{0}` which decodes to `[]byte{}`.
+	// This implementation forces empty node values to `[]byte(nil)`.
 	if len(value) > 0 {
 		node.SubValue = value
 	}

internal/trie/node/encode.go

@@ -27,8 +27,7 @@ func (n *Node) Encode(buffer Buffer) (err error) {
 		return fmt.Errorf("cannot write LE key to buffer: %w", err)
 	}
 
-	kind := n.Kind()
-	nodeIsBranch := kind == Branch
+	nodeIsBranch := n.Kind() == Branch
 	if nodeIsBranch {
 		childrenBitmap := common.Uint16ToBytes(n.ChildrenBitmap())
 		_, err = buffer.Write(childrenBitmap)
@@ -37,9 +36,10 @@ func (n *Node) Encode(buffer Buffer) (err error) {
 		}
 	}
 
-	// Only encode node value if the node is a leaf or
-	// the node is a branch with a non empty value.
-	if !nodeIsBranch || (nodeIsBranch && n.SubValue != nil) {
+	// Only encode node value if it's not empty.
+	// See https://spec.polkadot.network/#defn-node-subvalue
+	// See https://github.com/paritytech/substrate/blob/a7ba55d3c54b9957c242f659e02f5b5a0f47b3ff/primitives/trie/src/node_codec.rs#L123
+	if n.SubValue != nil {
 		encoder := scale.NewEncoder(buffer)
 		err = encoder.Encode(n.SubValue)
 		if err != nil {

internal/trie/node/encode_test.go

@@ -4,9 +4,11 @@
 package node
 
 import (
+	"bytes"
 	"errors"
 	"testing"
 
+	"github.com/ChainSafe/gossamer/pkg/scale"
 	"github.com/golang/mock/gomock"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -102,8 +104,6 @@ func Test_Node_Encode(t *testing.T) {
 			writes: []writeCall{
 				{written: []byte{leafVariant.bits | 3}}, // partial key length 3
 				{written: []byte{0x01, 0x23}},           // partial key
-				{written: []byte{0}},                    // node value encoded length
-				{written: nil},                          // node value
 			},
 			expectedEncoding: []byte{1, 2, 3},
 		},
@@ -310,3 +310,20 @@ func Test_Node_Encode(t *testing.T) {
 		})
 	}
 }
+
+func Test_scaleEncodeEmptyByteSlices(t *testing.T) {
+	t.Parallel()
+
+	buffer := bytes.NewBuffer(nil)
+	encoder := scale.NewEncoder(buffer)
+	err := encoder.Encode([]byte(nil))
+	require.NoError(t, err)
+	assert.Equal(t, []byte{0}, buffer.Bytes())
+
+	buffer.Reset()
+
+	encoder = scale.NewEncoder(buffer)
+	err = encoder.Encode([]byte{})
+	require.NoError(t, err)
+	assert.Equal(t, []byte{0}, buffer.Bytes())
+}