Allow LexiconTypeDecoder to handle unknown types (at least from JSON)

lex/util/decoder.go

@@ -48,7 +48,7 @@ func JsonDecodeValue(b []byte) (any, error) {
 
 	t, ok := lexTypesMap[tstr]
 	if !ok {
-		return nil, fmt.Errorf("%w: %q", ErrUnrecognizedType, tstr)
+		t = reflect.TypeOf(UnknownType{})
 	}
 
 	val := reflect.New(t)
@@ -76,6 +76,10 @@ func CborDecodeValue(b []byte) (CBOR, error) {
 
 	t, ok := lexTypesMap[tstr]
 	if !ok {
+		// It is possible to gracefully handle this case, like in JsonDecodeValue,
+		// but since cbg.CBORUnmarshaller takes io.Reader as an input,
+		// that requires a function that would correctly advance the reader
+		// to the end of a value (and return the bytes).
 		return nil, fmt.Errorf("%w: %q", ErrUnrecognizedType, tstr)
 	}
 
@@ -112,6 +116,12 @@ func (ltd *LexiconTypeDecoder) MarshalJSON() ([]byte, error) {
 	if ltd == nil || ltd.Val == nil {
 		return nil, fmt.Errorf("LexiconTypeDecoder MarshalJSON called on a nil")
 	}
+
+	if v, ok := ltd.Val.(*UnknownType); ok {
+		// Shortcut for passing through unrecognized values
+		return v.MarshalJSON()
+	}
+
 	v := reflect.ValueOf(ltd.Val)
 	t := v.Type()
 	sf, ok := t.Elem().FieldByName("LexiconTypeID")

lex/util/decoder_test.go

@@ -1,6 +1,7 @@
 package util
 
 import (
+	"encoding/json"
 	"testing"
 )
 
@@ -37,3 +38,14 @@ func TestNewFromType(t *testing.T) {
 		t.Fatal("expect bogus generation to fail")
 	}
 }
+
+func TestMissingTypeField(t *testing.T) {
+	var out struct {
+		Field *LexiconTypeDecoder
+	}
+	const input = `{"Field":{"Some_stuff":"but $type is missing"}}`
+
+	if err := json.Unmarshal([]byte(input), &out); err != nil {
+		t.Fatalf("failed to unmarshal: %s", err)
+	}
+}

lex/util/util.go

@@ -44,3 +44,19 @@ func CborTypeExtractReader(r io.Reader) (string, []byte, error) {
 
 	return tcheck.Type, buf.Bytes(), nil
 }
+
+type UnknownType struct {
+	JSON json.RawMessage
+}
+
+func (t *UnknownType) MarshalJSON() ([]byte, error) {
+	return t.JSON.MarshalJSON()
+}
+
+func (t *UnknownType) UnmarshalJSON(b []byte) error {
+	return t.JSON.UnmarshalJSON(b)
+}
+
+func (t *UnknownType) MarshalCBOR(w io.Writer) error {
+	return fmt.Errorf("converting unrecognized types from JSON to CBOR is not implemented")
+}