backend_syslog.go

// Copyright (c) 2022 Exograd SAS.
//
// Permission to use, copy, modify, and/or distribute this software for
// any purpose with or without fee is hereby granted, provided that the
// above copyright notice and this permission notice appear in all
// copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
// WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
// WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
// AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
// DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
// PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
// TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
// PERFORMANCE OF THIS SOFTWARE.

package log

import (
	"bytes"
	"fmt"
	"net"
	"os"
	"strconv"
	"strings"
	"sync"
	"time"
)

const (
	BOM          = string('\uFEFF')
	FacilityCode = 16 // local use 0
)

type SyslogBackendCfg struct {
	Addr            string `json:"addr"`
	ApplicationName string `json:"application_name"`
}

type SyslogBackend struct {
	Cfg SyslogBackendCfg

	mut  sync.Mutex
	conn net.Conn
}

func NewSyslogBackend(cfg SyslogBackendCfg) (*SyslogBackend, error) {
	b := &SyslogBackend{
		Cfg: cfg,
	}

	if err := b.connect(); err != nil {
		err2 := fmt.Errorf("cannot initialize syslog backend: %w", err)
		return nil, err2
	}

	return b, nil
}

// The function is unsafe and MUST be called with b.mut held.
func (b *SyslogBackend) connect() error {
	if b.conn != nil {
		return nil
	}

	conn, err := net.Dial("tcp", b.Cfg.Addr)
	if err != nil {
		b.conn = nil
		err2 := fmt.Errorf("cannot connect to the syslog daemon: %w", err)
		return err2
	}

	b.conn = conn
	return nil
}

func (b *SyslogBackend) writeAndRetry(msg bytes.Buffer) error {
	b.mut.Lock()
	defer b.mut.Unlock()

	var buf bytes.Buffer
	buf.WriteString(strconv.Itoa(msg.Len()) + " ")
	buf.Write(msg.Bytes())

	if err := b.connect(); err != nil {
		return fmt.Errorf("cannot write log message: %w", err)
	}

	if _, err := b.conn.Write(buf.Bytes()); err != nil {
		_ = b.conn.Close()
		if err := b.connect(); err != nil {
			return err
		}
		if _, err := b.conn.Write(buf.Bytes()); err != nil {
			_ = b.conn.Close()
			b.conn = nil
			return fmt.Errorf("cannot write log message: %w", err)
		}
	}

	return nil
}

func (b *SyslogBackend) Log(msg Message) {
	var buf bytes.Buffer

	// https://datatracker.ietf.org/doc/html/rfc5424#section-6.2.1
	pri := FacilityCode*8 + getSeverityCode(msg.Level)

	// https://datatracker.ietf.org/doc/html/rfc5424#section-6.2.2
	version := 1

	// https://datatracker.ietf.org/doc/html/rfc5424#section-6.2.3
	datetime := msg.Time.Format(time.RFC3339Nano)

	// https://datatracker.ietf.org/doc/html/rfc5424#section-6.2.4
	hostname, err := os.Hostname()
	if err != nil {
		hostname = "-"
	}

	// https://datatracker.ietf.org/doc/html/rfc5424#section-6.2.5
	appname := "-"
	if b.Cfg.ApplicationName != "" {
		appname = b.Cfg.ApplicationName
	}

	// https://datatracker.ietf.org/doc/html/rfc5424#section-6.2.6
	procid := os.Getpid()

	// https://datatracker.ietf.org/doc/html/rfc5424#section-6.2.7
	msgid := "-"

	// https://datatracker.ietf.org/doc/html/rfc5424#section-6.3.1
	sdElementId := "go-log@32473"
	sdElementParameters := []string{}

	for key, value := range msg.Data {
		escapedValue := escapeSdElementValue(formatDatum2(value))
		sdElementParameters =
			append(sdElementParameters,
				fmt.Sprintf("%s=\"%s\"", key, escapedValue))
	}

	// https://datatracker.ietf.org/doc/html/rfc5424#section-6.4
	message := BOM + msg.Message

	var format string
	var arguments []interface{}

	// https://datatracker.ietf.org/doc/html/rfc5424#section-6
	if len(sdElementParameters) == 0 {
		format = "<%d>%d %s %s %s %d %s [%s] %s"
		arguments = []interface{}{pri, version, datetime,
			hostname, appname, procid, msgid, sdElementId,
			message}
	} else {
		format = "<%d>%d %s %s %s %d %s [%s %s] %s"
		arguments = []interface{}{pri, version, datetime,
			hostname, appname, procid, msgid, sdElementId,
			strings.Join(sdElementParameters, " "),
			message}
	}

	fmt.Fprintf(&buf, format, arguments...)

	if err := b.writeAndRetry(buf); err != nil {
		fmt.Fprintf(os.Stderr, "%v\n", err)
	}
}

func getSeverityCode(l Level) int {
	var code int

	switch l {
	case LevelDebug:
		code = 7
	case LevelInfo:
		code = 6
	case LevelError:
		code = 3
	}

	return code
}

func escapeSdElementValue(src string) string {
	var dest bytes.Buffer

	for _, rune := range src {
		switch rune {
		case '\\':
			dest.WriteString("\\\\")
		case '"':
			dest.WriteString("\\\"")
		case ']':
			dest.WriteString("\\]")
		default:
			dest.WriteRune(rune)
		}
	}

	return dest.String()
}

func formatDatum2(datum Datum) string {
	switch v := datum.(type) {
	case fmt.Stringer:
		return formatDatum2(v.String())
	case string:
		return v
	default:
		return fmt.Sprintf("%v", v)
	}
}