rlp: add support for optional struct fields (#22832)

This adds support for a new struct tag "optional". Using this tag, structs used
for RLP encoding/decoding can be extended in a backwards-compatible way,
by adding new fields at the end.

rlp/decode.go

@@ -229,7 +229,7 @@ func decodeBigInt(s *Stream, val reflect.Value) error {
 		i = new(big.Int)
 		val.Set(reflect.ValueOf(i))
 	}
-	// Reject leading zero bytes
+	// Reject leading zero bytes.
 	if len(b) > 0 && b[0] == 0 {
 		return wrapStreamError(ErrCanonInt, val.Type())
 	}
@@ -394,9 +394,16 @@ func makeStructDecoder(typ reflect.Type) (decoder, error) {
 		if _, err := s.List(); err != nil {
 			return wrapStreamError(err, typ)
 		}
-		for _, f := range fields {
+		for i, f := range fields {
 			err := f.info.decoder(s, val.Field(f.index))
 			if err == EOL {
+				if f.optional {
+					// The field is optional, so reaching the end of the list before
+					// reaching the last field is acceptable. All remaining undecoded
+					// fields are zeroed.
+					zeroFields(val, fields[i:])
+					break
+				}
 				return &decodeError{msg: "too few elements", typ: typ}
 			} else if err != nil {
 				return addErrorContext(err, "."+typ.Field(f.index).Name)
@@ -407,6 +414,13 @@ func makeStructDecoder(typ reflect.Type) (decoder, error) {
 	return dec, nil
 }
 
+func zeroFields(structval reflect.Value, fields []field) {
+	for _, f := range fields {
+		fv := structval.Field(f.index)
+		fv.Set(reflect.Zero(fv.Type()))
+	}
+}
+
 // makePtrDecoder creates a decoder that decodes into the pointer's element type.
 func makePtrDecoder(typ reflect.Type, tag tags) (decoder, error) {
 	etype := typ.Elem()