Refactor server handler

- rename protocols.Handler to ServerHandler
- rename ServerHandler methods to better express their purpose
- pass command and response as parameters
- simplify state machine and handle errors in serial/seriallink.go
- implement command read timeout
- remove currentCommand and currentResponse fields from MsgPackHandler

cmd/clientsim/main.go

@@ -20,15 +20,14 @@ limitations under the License.
 package main
 
 import (
-	"bytes"
 	"context"
+	"crypto/rand"
 	"fmt"
 	"io"
 	"os"
 	"sync"
 	"time"
 
-	"github.com/justincpresley/go-cobs"
 	"github.com/shamaton/msgpackgen/msgpack"
 	"github.com/sirupsen/logrus"
 
@@ -37,8 +36,6 @@ import (
 	"git.cacert.org/cacert-gosigner/pkg/messages"
 )
 
-var cobsConfig = cobs.Config{SpecialByte: protocol.CobsDelimiter, Delimiter: true, EndingSave: true}
-
 type protocolState int8
 
 const (
@@ -160,16 +157,27 @@ func (g *TestCommandGenerator) HandleResponse(frame []byte) error {
 }
 
 func (g *TestCommandGenerator) GenerateCommands(ctx context.Context) error {
-	const (
-		healthInterval = 5 * time.Second
-		startPause     = 3 * time.Second
-	)
-
 	var (
 		announce *messages.CommandAnnounce
 		err      error
 	)
 
+	// write some leading garbage to test signer robustness
+	_, _ = io.CopyN(os.Stdout, rand.Reader, 50) //nolint:gomnd
+
+	announce, err = messages.BuildCommandAnnounce(messages.CmdHealth)
+	if err != nil {
+		return fmt.Errorf("build command announce failed: %w", err)
+	}
+
+	g.commands <- &protocol.Command{Announce: announce, Command: &messages.HealthCommand{}}
+
+	const (
+		healthInterval = 5 * time.Second
+		crlInterval    = 15 * time.Minute
+		startPause     = 3 * time.Second
+	)
+
 	g.logger.Info("start generating commands")
 
 	time.Sleep(startPause)
@@ -184,17 +192,18 @@ func (g *TestCommandGenerator) GenerateCommands(ctx context.Context) error {
 		Command:  &messages.FetchCRLCommand{IssuerID: "sub-ecc_person_2022"},
 	}
 
-	timer := time.NewTimer(healthInterval)
+	healthTimer := time.NewTimer(healthInterval)
+	crlTimer := time.NewTimer(crlInterval)
 
 	for {
 		select {
 		case <-ctx.Done():
-			_ = timer.Stop()
+			_ = healthTimer.Stop()
 
 			g.logger.Info("stopping health check loop")
 
 			return nil
-		case <-timer.C:
+		case <-healthTimer.C:
 			announce, err = messages.BuildCommandAnnounce(messages.CmdHealth)
 			if err != nil {
 				return fmt.Errorf("build command announce failed: %w", err)
@@ -204,9 +213,14 @@ func (g *TestCommandGenerator) GenerateCommands(ctx context.Context) error {
 				Announce: announce,
 				Command:  &messages.HealthCommand{},
 			}
-		}
 
-		timer.Reset(healthInterval)
+			healthTimer.Reset(healthInterval)
+		case <-crlTimer.C:
+			g.commands <- &protocol.Command{
+				Announce: announce,
+				Command:  &messages.FetchCRLCommand{IssuerID: "sub-ecc_person_2022"},
+			}
+		}
 	}
 }
 
@@ -215,106 +229,11 @@ type clientSimulator struct {
 	logger           *logrus.Logger
 	lock             sync.Mutex
 	framesIn         chan []byte
+	framesOut        chan []byte
+	framer           protocol.Framer
 	commandGenerator *TestCommandGenerator
 }
 
-func (c *clientSimulator) readFrames() error {
-	const readInterval = 50 * time.Millisecond
-
-	var frame []byte
-
-	buffer := &bytes.Buffer{}
-
-	delimiter := []byte{cobsConfig.SpecialByte}
-
-	for {
-		readBytes, err := c.readFromStdin()
-		if err != nil {
-			c.logger.WithError(err).Error("stdin read error")
-
-			close(c.framesIn)
-
-			return err
-		}
-
-		if len(readBytes) == 0 {
-			time.Sleep(readInterval)
-
-			continue
-		}
-
-		c.logger.Tracef("read %d bytes", len(readBytes))
-
-		buffer.Write(readBytes)
-
-		c.logger.Tracef("read buffer is now %d bytes long", buffer.Len())
-
-		rest := buffer.Bytes()
-
-		if !bytes.Contains(rest, delimiter) {
-			c.logger.Tracef("read data does not contain the delimiter %x", delimiter)
-
-			continue
-		}
-
-		for bytes.Contains(rest, delimiter) {
-			parts := bytes.SplitAfterN(rest, delimiter, 2)
-			frame, rest = parts[0], parts[1]
-
-			c.logger.Tracef("frame of length %d", len(frame))
-
-			if len(frame) == 0 {
-				continue
-			}
-
-			err = cobs.Verify(frame, cobsConfig)
-			if err != nil {
-				return fmt.Errorf("frame verification failed: %w", err)
-			}
-
-			decoded := cobs.Decode(frame, cobsConfig)
-
-			c.logger.Tracef("frame decoded to length %d", len(decoded))
-
-			c.framesIn <- decoded
-
-			c.logger.Tracef("%d bytes remaining", len(rest))
-		}
-
-		buffer.Truncate(0)
-		buffer.Write(rest)
-
-		c.logger.Tracef("read buffer is now %d bytes long", buffer.Len())
-	}
-}
-
-func (c *clientSimulator) writeFrame(frame []byte) error {
-	encoded := cobs.Encode(frame, cobsConfig)
-
-	if _, err := io.Copy(os.Stdout, bytes.NewBuffer(encoded)); err != nil {
-		return fmt.Errorf("could not write data: %w", err)
-	}
-
-	return nil
-}
-
-func (c *clientSimulator) readFromStdin() ([]byte, error) {
-	const bufferSize = 1024
-
-	buf := make([]byte, bufferSize)
-
-	c.logger.Trace("waiting for input")
-
-	count, err := os.Stdin.Read(buf)
-	if err != nil {
-		return nil, fmt.Errorf("reading input failed: %w", err)
-	}
-
-	c.logger.Tracef("read %d bytes from stdin", count)
-
-	return buf[:count], nil
-}
-
 func (c *clientSimulator) writeCmdAnnouncement() error {
 	frame, err := c.commandGenerator.CmdAnnouncement()
 	if err != nil {
@@ -323,9 +242,7 @@ func (c *clientSimulator) writeCmdAnnouncement() error {
 
 	c.logger.Trace("writing command announcement")
 
-	if err := c.writeFrame(frame); err != nil {
-		return err
-	}
+	c.framesOut <- frame
 
 	if err := c.nextState(); err != nil {
 		return err
@@ -342,9 +259,7 @@ func (c *clientSimulator) writeCommand() error {
 
 	c.logger.Trace("writing command data")
 
-	if err := c.writeFrame(frame); err != nil {
-		return err
-	}
+	c.framesOut <- frame
 
 	if err := c.nextState(); err != nil {
 		return err
@@ -353,44 +268,61 @@ func (c *clientSimulator) writeCommand() error {
 	return nil
 }
 
+const responseAnnounceTimeout = 30 * time.Second
+const responseDataTimeout = 2 * time.Second
+
 func (c *clientSimulator) handleResponseAnnounce() error {
 	c.logger.Trace("waiting for response announce")
 
-	frame := <-c.framesIn
+	select {
+	case frame := <-c.framesIn:
+		if frame == nil {
+			return nil
+		}
+
+		if err := c.commandGenerator.HandleResponseAnnounce(frame); err != nil {
+			return fmt.Errorf("response announce handling failed: %w", err)
+		}
+
+		if err := c.nextState(); err != nil {
+			return err
+		}
+	case <-time.After(responseAnnounceTimeout):
+		c.logger.Warn("response announce timeout expired")
+
+		c.protocolState = cmdAnnounce
 
-	if frame == nil {
 		return nil
 	}
 
-	if err := c.commandGenerator.HandleResponseAnnounce(frame); err != nil {
-		return fmt.Errorf("response announce handling failed: %w", err)
-	}
-
-	if err := c.nextState(); err != nil {
-		return err
-	}
-
 	return nil
 }
 
 func (c *clientSimulator) handleResponseData() error {
 	c.logger.Trace("waiting for response data")
 
-	frame := <-c.framesIn
+	select {
+	case frame := <-c.framesIn:
+		if frame == nil {
+			return nil
+		}
+
+		if err := c.commandGenerator.HandleResponse(frame); err != nil {
+			return fmt.Errorf("response handler failed: %w", err)
+		}
+
+		if err := c.nextState(); err != nil {
+			return err
+		}
+
+		return nil
+	case <-time.After(responseDataTimeout):
+		c.logger.Warn("response data timeout expired")
+
+		c.protocolState = cmdAnnounce
 
-	if frame == nil {
 		return nil
 	}
-
-	if err := c.commandGenerator.HandleResponse(frame); err != nil {
-		return fmt.Errorf("response handler failed: %w", err)
-	}
-
-	if err := c.nextState(); err != nil {
-		return err
-	}
-
-	return nil
 }
 
 func (c *clientSimulator) Run(ctx context.Context) error {
@@ -398,7 +330,13 @@ func (c *clientSimulator) Run(ctx context.Context) error {
 	errors := make(chan error)
 
 	go func() {
-		err := c.readFrames()
+		err := c.framer.ReadFrames(os.Stdin, c.framesIn)
+
+		errors <- err
+	}()
+
+	go func() {
+		err := c.framer.WriteFrames(os.Stdout, c.framesOut)
 
 		errors <- err
 	}()
@@ -480,8 +418,10 @@ func main() {
 			logger:   logger,
 			commands: make(chan *protocol.Command),
 		},
-		logger:   logger,
-		framesIn: make(chan []byte),
+		logger:    logger,
+		framesIn:  make(chan []byte),
+		framesOut: make(chan []byte),
+		framer:    protocol.NewCOBSFramer(logger),
 	}
 
 	err := sim.Run(context.Background())

internal/handler/msgpack.go

@@ -21,6 +21,7 @@ import (
 	"errors"
 	"fmt"
 	"sync"
+	"time"
 
 	"github.com/shamaton/msgpackgen/msgpack"
 	"github.com/sirupsen/logrus"
@@ -32,185 +33,186 @@ import (
 	"git.cacert.org/cacert-gosigner/pkg/messages"
 )
 
-// MsgPackHandler is a Handler implementation for the msgpack serialization format.
+const readCommandTimeOut = 5 * time.Second
+
+var errReadCommandTimeout = errors.New("read command timeout expired")
+
+// MsgPackHandler is a ServerHandler implementation for the msgpack serialization format.
 type MsgPackHandler struct {
 	logger          *logrus.Logger
 	healthHandler   *health.Handler
 	fetchCRLHandler *revoking.FetchCRLHandler
-	currentCommand  *protocol.Command
-	currentResponse *protocol.Response
 	lock            sync.Mutex
 }
 
-func (m *MsgPackHandler) HandleCommandAnnounce(frame []byte) error {
+func (m *MsgPackHandler) CommandAnnounce(frames chan []byte) (*protocol.Command, error) {
 	m.lock.Lock()
 	defer m.lock.Unlock()
 
+	frame := <-frames
+
 	var ann messages.CommandAnnounce
 
 	if err := msgpack.Unmarshal(frame, &ann); err != nil {
-		return fmt.Errorf("could not unmarshal command announcement: %w", err)
+		return nil, fmt.Errorf("could not unmarshal command announcement: %w", err)
 	}
 
 	m.logger.WithField("announcement", &ann).Info("received command announcement")
 
-	m.currentCommand = &protocol.Command{Announce: &ann}
-
-	return nil
+	return &protocol.Command{Announce: &ann}, nil
 }
 
-func (m *MsgPackHandler) HandleCommand(frame []byte) error {
+func (m *MsgPackHandler) CommandData(frames chan []byte, command *protocol.Command) error {
 	m.lock.Lock()
 	defer m.lock.Unlock()
 
-	err := m.parseCommand(frame)
-	if err != nil {
-		m.currentResponse = m.buildErrorResponse(err.Error())
-
-		m.logCommandResponse()
+	select {
+	case frame := <-frames:
+		err := m.parseCommand(frame, command)
+		if err != nil {
+			return err
+		}
 
 		return nil
+	case <-time.After(readCommandTimeOut):
+		return errReadCommandTimeout
 	}
+}
 
-	err = m.handleCommand()
+func (m *MsgPackHandler) HandleCommand(command *protocol.Command) (*protocol.Response, error) {
+	m.lock.Lock()
+	defer m.lock.Unlock()
+
+	var (
+		response *protocol.Response
+		err      error
+	)
+
+	response, err = m.handleCommand(command)
 	if err != nil {
 		m.logger.WithError(err).Error("command handling failed")
 
-		return err
+		response = m.buildErrorResponse(command.Announce.ID, "command handling failed")
 	}
 
-	m.logCommandResponse()
+	m.logCommandResponse(command, response)
 
-	m.currentCommand = nil
+	return response, nil
+}
+
+func (m *MsgPackHandler) logCommandResponse(command *protocol.Command, response *protocol.Response) {
+	m.logger.WithField("command", command.Announce).Info("handled command")
+	m.logger.WithField("command", command).WithField("response", response).Debug("command and response")
+}
+
+func (m *MsgPackHandler) Respond(response *protocol.Response, out chan []byte) error {
+	m.lock.Lock()
+	defer m.lock.Unlock()
+
+	announce, err := msgpack.Marshal(response)
+	if err != nil {
+		return fmt.Errorf("could not marshal response announcement: %w", err)
+	}
+
+	m.logger.WithField("length", len(announce)).Debug("write response announcement")
+
+	out <- announce
+
+	data, err := msgpack.Marshal(response.Response)
+	if err != nil {
+		return fmt.Errorf("could not marshal response: %w", err)
+	}
+
+	m.logger.WithField("length", len(data)).Debug("write response")
+
+	out <- announce
 
 	return nil
 }
 
-func (m *MsgPackHandler) logCommandResponse() {
-	m.logger.WithField("command", m.currentCommand.Announce).Info("handled command")
-	m.logger.WithField(
-		"command",
-		m.currentCommand,
-	).WithField(
-		"response",
-		m.currentResponse,
-	).Debug("command and response")
-}
-
-func (m *MsgPackHandler) ResponseAnnounce() ([]byte, error) {
-	m.lock.Lock()
-	defer m.lock.Unlock()
-
-	announceData, err := msgpack.Marshal(m.currentResponse.Announce)
-	if err != nil {
-		return nil, fmt.Errorf("could not marshal response announcement: %w", err)
-	}
-
-	m.logger.WithField("announcement", m.currentResponse.Announce).Debug("write response announcement")
-
-	return announceData, nil
-}
-
-func (m *MsgPackHandler) ResponseData() ([]byte, error) {
-	m.lock.Lock()
-	defer m.lock.Unlock()
-
-	responseData, err := msgpack.Marshal(m.currentResponse.Response)
-	if err != nil {
-		return nil, fmt.Errorf("could not marshal response: %w", err)
-	}
-
-	m.logger.WithField("response", m.currentResponse.Response).Debug("write response")
-
-	return responseData, nil
-}
-
-func (m *MsgPackHandler) parseHealthCommand(frame []byte) error {
+func (m *MsgPackHandler) parseHealthCommand(frame []byte) (*messages.HealthCommand, error) {
 	var command messages.HealthCommand
 
 	if err := msgpack.Unmarshal(frame, &command); err != nil {
 		m.logger.WithError(err).Error("unmarshal failed")
 
-		return errors.New("could not unmarshal health command")
+		return nil, errors.New("could not unmarshal health command")
 	}
 
-	m.currentCommand.Command = &command
-
-	return nil
+	return &command, nil
 }
 
-func (m *MsgPackHandler) parseFetchCRLCommand(frame []byte) error {
+func (m *MsgPackHandler) parseFetchCRLCommand(frame []byte) (*messages.FetchCRLCommand, error) {
 	var command messages.FetchCRLCommand
 
 	if err := msgpack.Unmarshal(frame, &command); err != nil {
 		m.logger.WithError(err).Error("unmarshal failed")
 
-		return errors.New("could not unmarshal fetch crl command")
+		return nil, errors.New("could not unmarshal fetch crl command")
 	}
 
-	m.currentCommand.Command = &command
-
-	return nil
+	return &command, nil
 }
 
-func (m *MsgPackHandler) currentID() string {
-	return m.currentCommand.Announce.ID
-}
-
-func (m *MsgPackHandler) handleCommand() error {
+func (m *MsgPackHandler) handleCommand(command *protocol.Command) (*protocol.Response, error) {
 	var (
-		err          error
-		responseData interface{}
 		responseCode messages.ResponseCode
+		responseData interface{}
 	)
 
-	switch m.currentCommand.Command.(type) {
+	switch cmd := command.Command.(type) {
 	case *messages.HealthCommand:
 		response, err := m.handleHealthCommand()
 		if err != nil {
-			return err
+			return nil, err
 		}
 
 		responseCode, responseData = messages.RespHealth, response
 	case *messages.FetchCRLCommand:
-		response, err := m.handleFetchCRLCommand()
+		response, err := m.handleFetchCRLCommand(cmd)
 		if err != nil {
-			return err
+			return nil, err
 		}
 
 		responseCode, responseData = messages.RespFetchCRL, response
 	default:
-		return fmt.Errorf("unhandled command %s", m.currentCommand.Announce)
+		return nil, fmt.Errorf("unhandled command %s", command.Announce)
 	}
 
-	if err != nil {
-		return fmt.Errorf("error from command handler: %w", err)
-	}
-
-	m.currentResponse = &protocol.Response{
-		Announce: messages.BuildResponseAnnounce(responseCode, m.currentID()),
+	return &protocol.Response{
+		Announce: messages.BuildResponseAnnounce(responseCode, command.Announce.ID),
 		Response: responseData,
-	}
-
-	return nil
+	}, nil
 }
 
-func (m *MsgPackHandler) buildErrorResponse(errMsg string) *protocol.Response {
+func (m *MsgPackHandler) buildErrorResponse(commandID string, errMsg string) *protocol.Response {
 	return &protocol.Response{
-		Announce: messages.BuildResponseAnnounce(messages.RespError, m.currentID()),
+		Announce: messages.BuildResponseAnnounce(messages.RespError, commandID),
 		Response: &messages.ErrorResponse{Message: errMsg},
 	}
 }
 
-func (m *MsgPackHandler) parseCommand(frame []byte) error {
-	switch m.currentCommand.Announce.Code {
+func (m *MsgPackHandler) parseCommand(frame []byte, command *protocol.Command) error {
+	switch command.Announce.Code {
 	case messages.CmdHealth:
-		return m.parseHealthCommand(frame)
+		healthCommand, err := m.parseHealthCommand(frame)
+		if err != nil {
+			return err
+		}
+
+		command.Command = healthCommand
 	case messages.CmdFetchCRL:
-		return m.parseFetchCRLCommand(frame)
+		fetchCRLCommand, err := m.parseFetchCRLCommand(frame)
+		if err != nil {
+			return err
+		}
+
+		command.Command = fetchCRLCommand
 	default:
-		return fmt.Errorf("unhandled command code %s", m.currentCommand.Announce.Code)
+		return fmt.Errorf("unhandled command code %s", command.Announce.Code)
 	}
+
+	return nil
 }
 
 func (m *MsgPackHandler) handleHealthCommand() (*messages.HealthResponse, error) {
@@ -235,27 +237,20 @@ func (m *MsgPackHandler) handleHealthCommand() (*messages.HealthResponse, error)
 	return response, nil
 }
 
-func (m *MsgPackHandler) handleFetchCRLCommand() (*messages.FetchCRLResponse, error) {
-	fetchCRLPayload, ok := m.currentCommand.Command.(*messages.FetchCRLCommand)
-	if !ok {
-		return nil, fmt.Errorf("could not use payload as FetchCRLPayload")
-	}
-
-	res, err := m.fetchCRLHandler.FetchCRL(fetchCRLPayload.IssuerID)
+func (m *MsgPackHandler) handleFetchCRLCommand(command *messages.FetchCRLCommand) (*messages.FetchCRLResponse, error) {
+	res, err := m.fetchCRLHandler.FetchCRL(command.IssuerID)
 	if err != nil {
 		return nil, fmt.Errorf("could not fetch CRL: %w", err)
 	}
 
-	response := &messages.FetchCRLResponse{
+	return &messages.FetchCRLResponse{
 		IsDelta:   false,
 		CRLNumber: res.Number,
 		CRLData:   res.CRLData,
-	}
-
-	return response, nil
+	}, nil
 }
 
-func New(logger *logrus.Logger, handlers ...RegisterHandler) (protocol.Handler, error) {
+func New(logger *logrus.Logger, handlers ...RegisterHandler) (protocol.ServerHandler, error) {
 	messages.RegisterGeneratedResolver()
 
 	h := &MsgPackHandler{

internal/serial/seriallink.go

@@ -15,18 +15,14 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-// Package seriallink provides a handler for the serial connection of the signer machine.
+// Package serial provides a handler for the serial connection of the signer machine.
 package serial
 
 import (
-	"bytes"
 	"context"
+	"errors"
 	"fmt"
-	"io"
-	"sync"
-	"time"
 
-	"github.com/justincpresley/go-cobs"
 	"github.com/sirupsen/logrus"
 	"github.com/tarm/serial"
 
@@ -39,22 +35,15 @@ type protocolState int8
 const (
 	cmdAnnounce protocolState = iota
 	cmdData
-	respAnnounce
-	respData
+	handleCommand
+	respond
 )
 
-var validTransitions = map[protocolState]protocolState{
-	cmdAnnounce:  cmdData,
-	cmdData:      respAnnounce,
-	respAnnounce: respData,
-	respData:     cmdAnnounce,
-}
-
 var protocolStateNames = map[protocolState]string{
-	cmdAnnounce:  "CMD ANNOUNCE",
-	cmdData:      "CMD DATA",
-	respAnnounce: "RESP ANNOUNCE",
-	respData:     "RESP DATA",
+	cmdAnnounce:   "CMD ANNOUNCE",
+	cmdData:       "CMD DATA",
+	handleCommand: "RESP ANNOUNCE",
+	respond:       "RESP DATA",
 }
 
 func (p protocolState) String() string {
@@ -66,13 +55,14 @@ func (p protocolState) String() string {
 }
 
 type Handler struct {
-	protocolHandler protocol.Handler
+	protocolHandler protocol.ServerHandler
 	protocolState   protocolState
+	framer          protocol.Framer
 	config          *serial.Config
 	port            *serial.Port
 	logger          *logrus.Logger
-	lock            sync.Mutex
 	framesIn        chan []byte
+	framesOut       chan []byte
 }
 
 func (h *Handler) setupConnection() error {
@@ -95,256 +85,137 @@ func (h *Handler) Close() error {
 	return nil
 }
 
-var cobsConfig = cobs.Config{SpecialByte: protocol.CobsDelimiter, Delimiter: true, EndingSave: true}
-
 func (h *Handler) Run(ctx context.Context) error {
 	h.protocolState = cmdAnnounce
-	errors := make(chan error)
+	protocolErrors, framerErrors := make(chan error), make(chan error)
 
 	go func() {
-		err := h.readFrames()
+		err := h.framer.ReadFrames(h.port, h.framesIn)
 
-		errors <- err
+		framerErrors <- err
+	}()
+
+	go func() {
+		err := h.framer.WriteFrames(h.port, h.framesOut)
+
+		framerErrors <- err
+	}()
+
+	go func() {
+		err := h.handleProtocolState()
+
+		protocolErrors <- err
 	}()
 
 	for {
 		select {
 		case <-ctx.Done():
 			return nil
-
-		case err := <-errors:
+		case err := <-framerErrors:
 			if err != nil {
-				return fmt.Errorf("error from handler loop: %w", err)
+				return fmt.Errorf("error from framer: %w", err)
 			}
 
 			return nil
-
-		default:
-			if err := h.handleProtocolState(); err != nil {
-				return err
+		case err := <-protocolErrors:
+			if err != nil {
+				return fmt.Errorf("error from protocol handler: %w", err)
 			}
+
+			return nil
 		}
 	}
 }
 
-func (h *Handler) readFrames() error {
-	const (
-		readInterval = 50 * time.Millisecond
-	)
-
-	var frame []byte
-
-	buffer := &bytes.Buffer{}
-
-	delimiter := []byte{cobsConfig.SpecialByte}
-
-	for {
-		readBytes, err := h.readFromPort()
-		if err != nil {
-			close(h.framesIn)
-
-			return err
-		}
-
-		if len(readBytes) == 0 {
-			time.Sleep(readInterval)
-
-			continue
-		}
-
-		h.logger.Tracef("read %d bytes", len(readBytes))
-
-		buffer.Write(readBytes)
-
-		h.logger.Tracef("read buffer is now %d bytes long", buffer.Len())
-
-		rest := buffer.Bytes()
-
-		if !bytes.Contains(rest, delimiter) {
-			continue
-		}
-
-		for bytes.Contains(rest, delimiter) {
-			parts := bytes.SplitAfterN(rest, delimiter, 2)
-			frame, rest = parts[0], parts[1]
-
-			h.logger.Tracef("frame of length %d", len(frame))
-
-			if len(frame) == 0 {
-				continue
-			}
-
-			if err := cobs.Verify(frame, cobsConfig); err != nil {
-				close(h.framesIn)
-
-				return fmt.Errorf("could not verify COBS frame: %w", err)
-			}
-
-			decoded := cobs.Decode(frame, cobsConfig)
-
-			h.logger.Tracef("frame decoded to length %d", len(decoded))
-
-			h.framesIn <- decoded
-		}
-
-		buffer.Truncate(0)
-		buffer.Write(rest)
-
-		h.logger.Tracef("read buffer is now %d bytes long", buffer.Len())
-	}
-}
-
-func (h *Handler) writeFrame(frame []byte) error {
-	encoded := cobs.Encode(frame, cobsConfig)
-
-	return h.writeToPort(encoded)
-}
-
-func (h *Handler) nextState() error {
-	next, ok := validTransitions[h.protocolState]
-	if !ok {
-		return fmt.Errorf("illegal protocol state %s", h.protocolState)
-	}
-
-	h.protocolState = next
-
-	return nil
-}
+var errCommandExpected = errors.New("command must not be nil")
+var errResponseExpected = errors.New("response must not be nil")
 
 func (h *Handler) handleProtocolState() error {
-	h.logger.Tracef("handling protocol state %s", h.protocolState)
+	var (
+		command  *protocol.Command
+		response *protocol.Response
+		err      error
+	)
 
-	h.lock.Lock()
-	defer h.lock.Unlock()
+	for {
+		h.logger.Debugf("handling protocol state %s", h.protocolState)
 
-	switch h.protocolState {
-	case cmdAnnounce:
-		if err := h.handleCmdAnnounce(); err != nil {
-			return err
+		switch h.protocolState {
+		case cmdAnnounce:
+			command, err = h.protocolHandler.CommandAnnounce(h.framesIn)
+			if err != nil {
+				h.logger.WithError(err).Error("could not handle command announce")
+
+				break
+			}
+
+			h.protocolState = cmdData
+		case cmdData:
+			if command == nil {
+				return errCommandExpected
+			}
+
+			err = h.protocolHandler.CommandData(h.framesIn, command)
+			if err != nil {
+				h.logger.WithError(err).Error("could not handle command data")
+
+				h.protocolState = cmdAnnounce
+
+				break
+			}
+
+			h.protocolState = handleCommand
+		case handleCommand:
+			if command == nil {
+				return errCommandExpected
+			}
+
+			response, err = h.protocolHandler.HandleCommand(command)
+			if err != nil {
+				h.logger.WithError(err).Error("could not handle command")
+
+				h.protocolState = cmdAnnounce
+
+				break
+			}
+
+			command = nil
+
+			h.protocolState = respond
+		case respond:
+			if response == nil {
+				return errResponseExpected
+			}
+
+			err = h.protocolHandler.Respond(response, h.framesOut)
+			if err != nil {
+				h.logger.WithError(err).Error("could not respond")
+
+				h.protocolState = cmdAnnounce
+
+				break
+			}
+
+			response = nil
+
+			h.protocolState = cmdAnnounce
+		default:
+			return fmt.Errorf("unknown protocol state %s", h.protocolState)
 		}
-	case cmdData:
-		if err := h.handleCmdData(); err != nil {
-			return err
-		}
-	case respAnnounce:
-		if err := h.handleRespAnnounce(); err != nil {
-			return err
-		}
-	case respData:
-		if err := h.handleRespData(); err != nil {
-			return err
-		}
-	default:
-		return fmt.Errorf("unknown protocol state %s", h.protocolState)
 	}
-
-	return nil
 }
 
-func (h *Handler) writeToPort(data []byte) error {
-	reader := bytes.NewReader(data)
-
-	n, err := io.Copy(h.port, reader)
-	if err != nil {
-		return fmt.Errorf("could not write data: %w", err)
-	}
-
-	h.logger.Tracef("wrote %d bytes", n)
-
-	return nil
-}
-
-func (h *Handler) readFromPort() ([]byte, error) {
-	const bufferSize = 1024
-
-	buf := make([]byte, bufferSize)
-
-	count, err := h.port.Read(buf)
-	if err != nil {
-		return nil, fmt.Errorf("could not read from serial port: %w", err)
-	}
-
-	return buf[:count], nil
-}
-
-func (h *Handler) handleCmdAnnounce() error {
-	h.logger.Trace("waiting for command announce")
-
-	frame := <-h.framesIn
-	if frame == nil {
-		return nil
-	}
-
-	if err := h.protocolHandler.HandleCommandAnnounce(frame); err != nil {
-		return fmt.Errorf("command announce handling failed: %w", err)
-	}
-
-	if err := h.nextState(); err != nil {
-		return err
-	}
-
-	return nil
-}
-
-func (h *Handler) handleCmdData() error {
-	h.logger.Trace("waiting for command data")
-
-	frame := <-h.framesIn
-
-	if frame == nil {
-		return nil
-	}
-
-	if err := h.protocolHandler.HandleCommand(frame); err != nil {
-		return fmt.Errorf("command handler failed: %w", err)
-	}
-
-	if err := h.nextState(); err != nil {
-		return err
-	}
-
-	return nil
-}
-
-func (h *Handler) handleRespAnnounce() error {
-	frame, err := h.protocolHandler.ResponseAnnounce()
-	if err != nil {
-		return fmt.Errorf("could not get response announcement: %w", err)
-	}
-
-	if err := h.writeFrame(frame); err != nil {
-		return err
-	}
-
-	if err := h.nextState(); err != nil {
-		return err
-	}
-
-	return nil
-}
-
-func (h *Handler) handleRespData() error {
-	frame, err := h.protocolHandler.ResponseData()
-	if err != nil {
-		return fmt.Errorf("could not get response data: %w", err)
-	}
-
-	if err := h.writeFrame(frame); err != nil {
-		return err
-	}
-
-	if err := h.nextState(); err != nil {
-		return err
-	}
-
-	return nil
-}
-
-func New(cfg *config.Serial, logger *logrus.Logger, protocolHandler protocol.Handler) (*Handler, error) {
+func New(
+	cfg *config.Serial,
+	logger *logrus.Logger,
+	protocolHandler protocol.ServerHandler,
+) (*Handler, error) {
 	h := &Handler{
 		protocolHandler: protocolHandler,
 		logger:          logger,
 		framesIn:        make(chan []byte),
+		framesOut:       make(chan []byte),
+		framer:          protocol.NewCOBSFramer(logger),
 	}
 	h.config = &serial.Config{Name: cfg.Device, Baud: cfg.Baud, ReadTimeout: cfg.Timeout}
 

pkg/messages/messages.go

@@ -43,7 +43,7 @@ const (
 
 var commandNames = map[CommandCode]string{
 	CmdHealth:   "HEALTH",
-	CmdFetchCRL: "FETCH URL",
+	CmdFetchCRL: "FETCH CRL",
 }
 
 func (c CommandCode) String() string {

pkg/protocol/protocol.go

@@ -19,7 +19,14 @@ limitations under the License.
 package protocol
 
 import (
+	"bytes"
+	"errors"
 	"fmt"
+	"io"
+	"time"
+
+	"github.com/justincpresley/go-cobs"
+	"github.com/sirupsen/logrus"
 
 	"git.cacert.org/cacert-gosigner/pkg/messages"
 )
@@ -44,14 +51,130 @@ func (r *Response) String() string {
 	return fmt.Sprintf("Rsp[announce={%s}, data={%s}]", r.Announce, r.Response)
 }
 
-// Handler is responsible for parsing incoming frames and calling commands
-type Handler interface {
-	// HandleCommandAnnounce handles the initial announcement of a command.
-	HandleCommandAnnounce([]byte) error
-	// HandleCommand handles the command data.
-	HandleCommand([]byte) error
-	// ResponseAnnounce generates the announcement for a response.
-	ResponseAnnounce() ([]byte, error)
-	// ResponseData generates the response data.
-	ResponseData() ([]byte, error)
+// ServerHandler is responsible for parsing incoming frames and calling commands
+type ServerHandler interface {
+	// CommandAnnounce handles the initial announcement of a command.
+	CommandAnnounce(chan []byte) (*Command, error)
+	// CommandData handles the command data.
+	CommandData(chan []byte, *Command) error
+	// HandleCommand executes the command, generating a response.
+	HandleCommand(*Command) (*Response, error)
+	// Respond generates the response for a command.
+	Respond(*Response, chan []byte) error
+}
+
+// Framer handles bytes on the wire by adding or removing framing information.
+type Framer interface {
+	// ReadFrames reads data frames and publishes unframed data to the channel.
+	ReadFrames(io.Reader, chan []byte) error
+	// WriteFrames takes data from the channel and writes framed data to the writer.
+	WriteFrames(io.Writer, chan []byte) error
+}
+
+const bufferSize = 1024
+const readInterval = 50 * time.Millisecond
+
+type COBSFramer struct {
+	config cobs.Config
+	logger *logrus.Logger
+}
+
+func NewCOBSFramer(logger *logrus.Logger) *COBSFramer {
+	return &COBSFramer{
+		config: cobs.Config{SpecialByte: CobsDelimiter, Delimiter: true, EndingSave: true},
+		logger: logger,
+	}
+}
+
+func (c *COBSFramer) ReadFrames(reader io.Reader, frameChan chan []byte) error {
+	var (
+		err              error
+		raw, data, frame []byte
+	)
+
+	buffer := &bytes.Buffer{}
+
+	for {
+		raw, err = c.readRaw(reader)
+		if err != nil {
+			close(frameChan)
+
+			return err
+		}
+
+		if len(raw) == 0 {
+			time.Sleep(readInterval)
+
+			continue
+		}
+
+		c.logger.Tracef("read %d raw bytes", len(raw))
+
+		buffer.Write(raw)
+
+		for {
+			data, err = buffer.ReadBytes(c.config.SpecialByte)
+			if err != nil {
+				if errors.Is(err, io.EOF) {
+					buffer.Write(data)
+
+					break
+				}
+
+				return fmt.Errorf("could not read from buffer: %w", err)
+			}
+
+			if err = cobs.Verify(data, c.config); err != nil {
+				c.logger.WithError(err).Warnf("skipping invalid frame of %d bytes", len(data))
+
+				break
+			}
+
+			frame = cobs.Decode(data, c.config)
+
+			c.logger.Tracef("frame decoded to length %d", len(frame))
+
+			frameChan <- frame
+		}
+
+		c.logger.Tracef("read buffer is now %d bytes long", buffer.Len())
+	}
+}
+
+func (c *COBSFramer) readRaw(reader io.Reader) ([]byte, error) {
+	buf := make([]byte, bufferSize)
+
+	count, err := reader.Read(buf)
+	if err != nil {
+		if errors.Is(err, io.EOF) {
+			return []byte{}, nil
+		}
+
+		return nil, fmt.Errorf("could not read data: %w", err)
+	}
+
+	raw := buf[:count]
+
+	return raw, nil
+}
+
+func (c *COBSFramer) WriteFrames(writer io.Writer, frameChan chan []byte) error {
+	for {
+		frame := <-frameChan
+
+		if frame == nil {
+			c.logger.Debug("channel closed")
+
+			return nil
+		}
+
+		encoded := cobs.Encode(frame, c.config)
+
+		n, err := io.Copy(writer, bytes.NewReader(encoded))
+		if err != nil {
+			return fmt.Errorf("cold not write data: %w", err)
+		}
+
+		c.logger.Tracef("wrote %d bytes", n)
+	}
 }