From e717b9c6dba9243d58beb0ef9bc6241fd9e023e8 Mon Sep 17 00:00:00 2001
From: Jack Lee <jacklee1792@gmail.com>
Date: Tue, 21 Feb 2023 02:02:04 -0500
Subject: [PATCH] Add option to disallow unknown fields

---
 nbt/decode.go      |  8 ++++----
 nbt/decode_test.go | 20 ++++++++++++++++++++
 nbt/nbt.go         |  9 ++++++++-
 3 files changed, 32 insertions(+), 5 deletions(-)

diff --git a/nbt/decode.go b/nbt/decode.go
index 4d76f6a..b63858e 100644
--- a/nbt/decode.go
+++ b/nbt/decode.go
@@ -370,10 +370,10 @@ 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 {
-					if err := d.rawRead(tt); err != nil {
-						return err
-					}
+				} else if d.disallowUnknownFields {
+					return fmt.Errorf("unknown field %q", tn)
+				} else if err := d.rawRead(tt); err != nil {
+					return err
 				}
 			}
 		case reflect.Map:
diff --git a/nbt/decode_test.go b/nbt/decode_test.go
index e8ecf45..5aac4db 100644
--- a/nbt/decode_test.go
+++ b/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)
+	}
+}
diff --git a/nbt/nbt.go b/nbt/nbt.go
index 983b088..7fc6786 100644
--- a/nbt/nbt.go
+++ b/nbt/nbt.go
@@ -33,7 +33,8 @@ type DecoderReader = interface {
 	io.Reader
 }
 type Decoder struct {
-	r DecoderReader
+	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
 }