[#263] Define generic protobuf message

Define `Message` interface of the generic protobuf message. In the initial
implementation, the message should be convertible to and from related gRPC
message.

Implement encoding functions over the `Message` that can:

 * unmarshal the `Message` via related gRPC message;
 * decode the `Message` from JSON format via related gRPC message;
 * encode the `Message` to JSON.

Implement nested `test` package for testing the implementation.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

rpc/message/encoding.go

@@ -0,0 +1,48 @@
+package message
+
+import (
+	"github.com/nspcc-dev/neofs-api-go/rpc/grpc"
+	"google.golang.org/protobuf/encoding/protojson"
+	"google.golang.org/protobuf/proto"
+)
+
+// GRPCConvertedMessage is an interface
+// of the gRPC message that is used
+// for Message encoding/decoding.
+type GRPCConvertedMessage interface {
+	grpc.Message
+	proto.Message
+}
+
+// Unmarshal decodes m from its Protobuf binary representation
+// via related gRPC message.
+//
+// gm should be tof the same type as the m.ToGRPCMessage() return.
+func Unmarshal(m Message, data []byte, gm GRPCConvertedMessage) error {
+	if err := proto.Unmarshal(data, gm); err != nil {
+		return err
+	}
+
+	return m.FromGRPCMessage(gm)
+}
+
+// MarshalJSON encodes m to Protobuf JSON representation.
+func MarshalJSON(m Message) ([]byte, error) {
+	return protojson.MarshalOptions{
+		EmitUnpopulated: true,
+	}.Marshal(
+		m.ToGRPCMessage().(proto.Message),
+	)
+}
+
+// UnmarshalJSON decodes m from its Protobuf JSON representation
+// via related gRPC message.
+//
+// gm should be tof the same type as the m.ToGRPCMessage() return.
+func UnmarshalJSON(m Message, data []byte, gm GRPCConvertedMessage) error {
+	if err := protojson.Unmarshal(data, gm); err != nil {
+		return err
+	}
+
+	return m.FromGRPCMessage(gm)
+}

rpc/message/message.go

@@ -0,0 +1,43 @@
+package message
+
+import (
+	"fmt"
+
+	"github.com/nspcc-dev/neofs-api-go/rpc/grpc"
+)
+
+// Message represents raw Protobuf message
+// that can be transmitted via several
+// transport protocols.
+type Message interface {
+	// Must return gRPC message that can
+	// be used for gRPC protocol transmission.
+	ToGRPCMessage() grpc.Message
+
+	// Must restore the message from related
+	// gRPC message.
+	//
+	// If gRPC message is not a related one,
+	// ErrUnexpectedMessageType can be returned
+	// to indicate this.
+	FromGRPCMessage(grpc.Message) error
+}
+
+// ErrUnexpectedMessageType is an error that
+// is used to indicate message mismatch.
+type ErrUnexpectedMessageType struct {
+	exp, act interface{}
+}
+
+// NewUnexpectedMessageType initializes an error about message mismatch
+// between act and exp.
+func NewUnexpectedMessageType(act, exp interface{}) ErrUnexpectedMessageType {
+	return ErrUnexpectedMessageType{
+		exp: exp,
+		act: act,
+	}
+}
+
+func (e ErrUnexpectedMessageType) Error() string {
+	return fmt.Sprintf("unexpected message type %T: expected %T", e.act, e.exp)
+}

rpc/message/test/message.go

@@ -0,0 +1,68 @@
+package messagetest
+
+import (
+	"encoding/json"
+	"fmt"
+	"testing"
+
+	"github.com/nspcc-dev/neofs-api-go/rpc/message"
+	"github.com/pkg/errors"
+	"github.com/stretchr/testify/require"
+)
+
+type jsonMessage interface {
+	json.Marshaler
+	json.Unmarshaler
+}
+
+type binaryMessage interface {
+	StableMarshal([]byte) ([]byte, error)
+	Unmarshal([]byte) error
+}
+
+func TestRPCMessage(t *testing.T, msgGens ...func(empty bool) message.Message) {
+	for _, msgGen := range msgGens {
+		msg := msgGen(false)
+
+		t.Run(fmt.Sprintf("convert_%T", msg), func(t *testing.T) {
+			msg := msgGen(false)
+
+			err := msg.FromGRPCMessage(100)
+
+			require.True(t, errors.As(err, new(message.ErrUnexpectedMessageType)))
+
+			msg2 := msgGen(true)
+
+			err = msg2.FromGRPCMessage(msg.ToGRPCMessage())
+			require.NoError(t, err)
+
+			require.Equal(t, msg, msg2)
+		})
+
+		t.Run("encoding", func(t *testing.T) {
+			if jm, ok := msg.(jsonMessage); ok {
+				t.Run(fmt.Sprintf("JSON_%T", msg), func(t *testing.T) {
+					data, err := jm.MarshalJSON()
+					require.NoError(t, err)
+
+					jm2 := msgGen(true).(jsonMessage)
+					require.NoError(t, jm2.UnmarshalJSON(data))
+
+					require.Equal(t, jm, jm2)
+				})
+			}
+
+			if bm, ok := msg.(binaryMessage); ok {
+				t.Run(fmt.Sprintf("Binary_%T", msg), func(t *testing.T) {
+					data, err := bm.StableMarshal(nil)
+					require.NoError(t, err)
+
+					bm2 := msgGen(true).(binaryMessage)
+					require.NoError(t, bm2.Unmarshal(data))
+
+					require.Equal(t, bm, bm2)
+				})
+			}
+		})
+	}
+}