daemon-socket-go-example/main.go

package main

import (
	"encoding/binary"
	"encoding/json"
	"errors"
	"github.com/sevlyar/go-daemon"
	"io"
	"log"
	"log/slog"
	"net"
	"os"
	"time"
)

type Message interface {
	ID() uint32
}

const (
	EchoRequestID = iota
)

type EchoRequest struct {
	EchoByte byte
}

type MessageHandler func(m Message) error

var Handlers = map[uint32]MessageHandler{
	EchoRequestID: handleEchoRequest,
}

func (EchoRequest) ID() uint32 {
	return EchoRequestID
}

func unixSocketListen() error {
	listener, err := net.Listen("unix", "/tmp/liberum.sock")
	if err != nil {
		return err
	}

	for conn, err := listener.Accept(); err == nil; {
		err = handleConnection(conn)
		if err != nil {
			slog.Error("Error handling connection:", err)
		}
	}

	return nil
}

func handleConnection(conn net.Conn) error {
	msgChan := readSocketMessages(conn)

	for msg := range msgChan {
		err := handleMessage(msg)
		if err != nil {
			return err
		}
	}

	return nil
}

func readSocketMessages(conn net.Conn) chan Message {
	msgChan := make(chan Message, 64)

	go func() {
		for {
			msgBytes, err := readMessage(conn)
			if err != nil {
				slog.Error("Error reading message", "error", err)
			}

			msg, err := decodeMessage(msgBytes)
			if err != nil {
				slog.Error("Error parsing message", "error", err)
			}

			msgChan <- msg
		}
	}()

	return msgChan
}

func decodeMessage(msgBytes []byte) (Message, error) {
	if len(msgBytes) < 4 {
		return nil, errors.New("message too short to have type ID")
	}

	msgID := binary.LittleEndian.Uint32(msgBytes[0:4])
	msgRest := msgBytes[4:]
	var err error
	var msg Message

	switch msgID {
	case EchoRequestID:
		var echoReq EchoRequest
		err = json.Unmarshal(msgRest, &echoReq)
		msg = echoReq
	default:
		err = errors.New("unknown message type ID")
	}

	if err != nil {
		return nil, err
	}

	return msg, nil
}

func readMessage(conn net.Conn) ([]byte, error) {
	msgLenBuf := make([]byte, 4)
	n, err := io.ReadFull(conn, msgLenBuf)
	if err != nil {
		return nil, err
	}

	if n != 4 {
		return nil, errors.New("could not read message length")
	}

	msgLen := binary.LittleEndian.Uint32(msgLenBuf)
	msgContent := make([]byte, msgLen)
	n, err = io.ReadFull(conn, msgContent)
	if err != nil {
		return nil, err
	}

	if uint32(n) != msgLen {
		return nil, errors.New("could not read full message")
	}

	return msgContent, nil
}

func handleMessage(msg Message) error {
	handler, ok := Handlers[msg.ID()]
	if !ok {
		return errors.New("message handler not defined for a given ID")
	}

	err := handler(msg)
	if err != nil {
		return err
	}

	return nil
}

func handleEchoRequest(echoRequestMsg Message) error {
	echoRequest := echoRequestMsg.(EchoRequest)
	slog.Info("Got echo request", "EchoByte", echoRequest.EchoByte)

	return nil
}

func writeMessage(conn net.Conn, msg Message) error {
	msgBytes, err := encodeMessage(msg)
	if err != nil {
		return err
	}

	n, err := conn.Write(msgBytes)
	if err != nil {
		return err
	}

	if n != len(msgBytes) {
		return errors.New("could not write full message")
	}

	return nil
}

func encodeMessage(msg Message) ([]byte, error) {
	var msgBytes []byte
	var msgJsonBytes []byte
	var err error

	msgJsonBytes, err = json.Marshal(msg)
	if err != nil {
		return nil, err
	}

	// +4 for type field length
	msgLen := len(msgJsonBytes) + 4
	msgLenBytes := make([]byte, 4)
	binary.LittleEndian.PutUint32(msgLenBytes, uint32(msgLen))
	msgBytes = append(msgBytes, msgLenBytes...)

	msgTypeBytes := make([]byte, 4)
	binary.LittleEndian.PutUint32(msgTypeBytes, msg.ID())
	msgBytes = append(msgBytes, msgTypeBytes...)

	msgBytes = append(msgBytes, msgJsonBytes...)

	return msgBytes, nil
}

func writeSocketMessages(conn net.Conn, msgChan <-chan Message) error {
	for msg := range msgChan {
		err := writeMessage(conn, msg)
		if err != nil {
			return err
		}
	}

	return nil
}

func unixSocketConnect() (net.Conn, error) {
	conn, err := net.Dial("unix", "/tmp/liberum.sock")
	if err != nil {
		return nil, err
	}

	return conn, nil
}

func main() {
	ctx := &daemon.Context{
		PidFileName: "liberum-daemon.pid",
		PidFilePerm: 0644,
		LogFileName: "liberum-daemon.log",
		LogFilePerm: 0640,
		WorkDir:     "./",
		Umask:       027,
	}

	d, err := ctx.Reborn()
	if err != nil {
		panic(err)
	}

	if d != nil {
		return
	}

	defer func(ctx *daemon.Context) {
		slog.Info("Stopping daemon")
		_ = ctx.Release()
	}(ctx)
	log.Println("Daemon started")

	_ = os.Remove("/tmp/liberum.sock")

	go func() {
		err = unixSocketListen()
		if err != nil {
			slog.Error("Error listening unix socket", "error", err)
		}
	}()

	time.Sleep(1 * time.Second)
	conn, err := unixSocketConnect()
	if err != nil {
		panic(err)
	}

	msgWriteChan := make(chan Message, 64)

	go func() {
		err = writeSocketMessages(conn, msgWriteChan)
		if err != nil {
			panic(err)
		}
	}()

	msgWriteChan <- EchoRequest{123}
	time.Sleep(time.Second * 5)
}