Add option to disallow unknown fields

nbt/decode.go

@@ -370,12 +370,12 @@ func (d *Decoder) unmarshal(val reflect.Value, tagType byte) error {
 					if err != nil {
 						return fmt.Errorf("fail to decode tag %q: %w", tn, err)
 					}
-				} else {
+				} else if d.disallowUnknownFields {
-					if err := d.rawRead(tt); err != nil {
+					return fmt.Errorf("unknown field %q", tn)
+				} else if err := d.rawRead(tt); err != nil {
 					return err
 				}
 			}
-			}
 		case reflect.Map:
 			if val.Type().Key().Kind() != reflect.String {
 				return errors.New("cannot parse TagCompound as " + val.Type().String())

nbt/decode_test.go

@@ -3,10 +3,12 @@ package nbt
 import (
 	"bytes"
 	"compress/gzip"
+	"fmt"
 	"io"
 	"math"
 	"reflect"
 	"strconv"
+	"strings"
 	"testing"
 )
 
@@ -428,3 +430,21 @@ func TestDecoder_Decode_textUnmarshaler(t *testing.T) {
 		t.Errorf("b should be true")
 	}
 }
+
+func TestDecoder_Decode_ErrorUnknownField(t *testing.T) {
+	data := []byte{
+		TagCompound, 0, 1, 'S',
+		TagByte, 0, 1, 'A', 1,
+		TagByte, 0, 1, 'B', 2,
+		TagEnd,
+	}
+	var v struct {
+		A byte `nbt:"a"`
+	}
+	d := NewDecoder(bytes.NewReader(data))
+	d.DisallowUnknownFields()
+	if _, err := d.Decode(&v); err == nil || !strings.Contains(err.Error(), "unknown field") {
+		t.Errorf("should return an error unmarshalling unknown field")
+		fmt.Println(err)
+	}
+}

nbt/nbt.go

@@ -34,6 +34,7 @@ type DecoderReader = interface {
 }
 type Decoder struct {
 	r                     DecoderReader
+	disallowUnknownFields bool
 }
 
 func NewDecoder(r io.Reader) *Decoder {
@@ -46,6 +47,12 @@ func NewDecoder(r io.Reader) *Decoder {
 	return d
 }
 
+// DisallowUnknownFields makes the decoder return an error when unmarshalling a compound
+// tag item that has a tag name not present in the destination struct.
+func (d *Decoder) DisallowUnknownFields() {
+	d.disallowUnknownFields = true
+}
+
 type reader struct {
 	io.Reader
 }