go.mod: upgrade gencodec

[fjl]

This updates the gencodec tool version to latest. The newer version can generate conversions between slices and arrays. This feature is not used yet, we plan to start using it for blsync.

[fjl]

Example:

//go:generate go run github.com/fjl/gencodec@latest -type SyncAggregate -field-override syncAggregateMarshaling -out gen_syncaggregate_json.go

type SyncAggregate struct {
	Signers   [params.SyncCommitteeBitmaskSize]byte `gencodec:"required" json:"sync_committee_bits"`
	Signature [params.BLSSignatureSize]byte         `gencodec:"required" json:"sync_committee_signature"`
}

type syncAggregateMarshaling struct {
	Signers   hexutil.Bytes
	Signature hexutil.Bytes
}

gen_syncaggregate.go

// MarshalJSON marshals as JSON.
func (s SyncAggregate) MarshalJSON() ([]byte, error) {
	type SyncAggregate struct {
		Signers   hexutil.Bytes `gencodec:"required" json:"sync_committee_bits"`
		Signature hexutil.Bytes `gencodec:"required" json:"sync_committee_signature"`
	}
	var enc SyncAggregate
	enc.Signers = s.Signers[:]
	enc.Signature = s.Signature[:]
	return json.Marshal(&enc)
}

// UnmarshalJSON unmarshals from JSON.
func (s *SyncAggregate) UnmarshalJSON(input []byte) error {
	type SyncAggregate struct {
		Signers   *hexutil.Bytes `gencodec:"required" json:"sync_committee_bits"`
		Signature *hexutil.Bytes `gencodec:"required" json:"sync_committee_signature"`
	}
	var dec SyncAggregate
	if err := json.Unmarshal(input, &dec); err != nil {
		return err
	}
	if dec.Signers == nil {
		return errors.New("missing required field 'sync_committee_bits' for SyncAggregate")
	}
	if len(*dec.Signers) != len(s.Signers) {
		return errors.New("field 'sync_committee_bits' has wrong length, need 64 items")
	}
	copy(s.Signers[:], *dec.Signers)
	if dec.Signature == nil {
		return errors.New("missing required field 'sync_committee_signature' for SyncAggregate")
	}
	if len(*dec.Signature) != len(s.Signature) {
		return errors.New("field 'sync_committee_signature' has wrong length, need 96 items")
	}
	copy(s.Signature[:], *dec.Signature)
	return nil
}

[holiman]

.... that's great I suppose.. :)

EDIT: Have reviewed the code example above now, looks good to me. A bit strange that you use if len(*dec.Signature) != len(s.Signature), but return errors.New("field 'sync_committee_signature' has wrong length, need 96 items").

Instead of, say, consistently use 96, or consistently use len(s.Signature).

a:

if len(*dec.Signature) != 96 {
		return errors.New("field 'sync_committee_signature' has wrong length, need 96 items")
	}

b:

if len(*dec.Signature) != len(s.Signature) {
		return fmt.Errorf("field 'sync_committee_signature' has wrong length, need %d items", len(s.Signature))
	}

Doesn't matter though, looks good to me