Fix unmarshal error in nbt

nbt/decode.go

@@ -51,13 +51,10 @@ func (d *Decoder) checkCompressed(head byte) (compress string) {
 var ErrEND = errors.New("unexpected TAG_End")
 
 func (d *Decoder) unmarshal(val reflect.Value, tagType byte, tagName string) error {
-	if val.CanInterface() {
-		if i, ok := val.Interface().(Unmarshaler); ok {
-			return i.Unmarshal(tagType, tagName, d.r)
+	u, val := indirect(val, tagType == TagEnd)
+	if u != nil {
+		return u.Unmarshal(tagType, tagName, d.r)
 	}
-	}
-	// TODO: use function like json.indirect() to handle pointer better
-	val = val.Elem()
 
 	switch tagType {
 	default:
@@ -358,6 +355,70 @@ func (d *Decoder) unmarshal(val reflect.Value, tagType byte, tagName string) err
 	return nil
 }
 
+// indirect walks down v allocating pointers as needed,
+// until it gets to a non-pointer.
+// If it encounters an Unmarshaler, indirect stops and returns that.
+// If decodingNull is true, indirect stops at the first settable pointer so it
+// can be set to nil.
+//
+// This function is copied and modified from encoding/json
+func indirect(v reflect.Value, decodingNull bool) (Unmarshaler, reflect.Value) {
+	v0 := v
+	haveAddr := false
+
+	// If v is a named type and is addressable,
+	// start with its address, so that if the type has pointer methods,
+	// we find them.
+	if v.Kind() != reflect.Ptr && v.Type().Name() != "" && v.CanAddr() {
+		haveAddr = true
+		v = v.Addr()
+	}
+	for {
+		// Load value from interface, but only if the result will be
+		// usefully addressable.
+		if v.Kind() == reflect.Interface && !v.IsNil() {
+			e := v.Elem()
+			if e.Kind() == reflect.Ptr && !e.IsNil() && (!decodingNull || e.Elem().Kind() == reflect.Ptr) {
+				haveAddr = false
+				v = e
+				continue
+			}
+		}
+
+		if v.Kind() != reflect.Ptr {
+			break
+		}
+
+		if decodingNull && v.CanSet() {
+			break
+		}
+
+		// Prevent infinite loop if v is an interface pointing to its own address:
+		//     var v interface{}
+		//     v = &v
+		if v.Elem().Kind() == reflect.Interface && v.Elem().Elem() == v {
+			v = v.Elem()
+			break
+		}
+		if v.IsNil() {
+			v.Set(reflect.New(v.Type().Elem()))
+		}
+		if v.Type().NumMethod() > 0 && v.CanInterface() {
+			if u, ok := v.Interface().(Unmarshaler); ok {
+				return u, reflect.Value{}
+			}
+		}
+
+		if haveAddr {
+			v = v0 // restore original value after round-trip Value.Addr().Elem()
+			haveAddr = false
+		} else {
+			v = v.Elem()
+		}
+	}
+	return nil, v
+}
+
 // rawRead read and discard a value
 func (d *Decoder) rawRead(tagType byte) error {
 	var buf [8]byte