prepare switch to log/slog

This commit prepares the switch from log to log/slog, which was
introduced in Go 1.21.
slog provides some useful facilities to add metadata to log entries,
which should be helpful for debugging problems.

This commit also adds a small transaction ID generator. It provides a
common identifier between log messages, so that multiple errors can be
viewed together in their order.

1 files changed, 52 insertions, 1 deletions

diff --git a/cmd/monfront/main.go b/cmd/monfront/main.go
index a318e75..19be6e0 100644
--- a/cmd/monfront/main.go
+++ b/cmd/monfront/main.go
@@ -6,8 +6,10 @@ import (
 	"flag"
 	"fmt"
 	"html/template"
+	"io"
 	"io/ioutil"
 	"log"
+	"log/slog"
 	"net"
 	"net/http"
 	"os"
@@ -49,6 +51,11 @@ type (
 			Mode string   `toml:"mode"`
 			List []string `toml:"list"`
 		}
+		Log struct {
+			Format string `toml:"format"`
+			Level  string `toml:"level"`
+			Output string `toml:"output"`
+		}
 	}
 
 	MapEntry struct {
@@ -99,6 +106,8 @@ func main() {
 		log.Fatalf("could not parse config: %s", err)
 	}
 
+	logger := parseLogger(config)
+
 	db, err := sql.Open("postgres", config.DB)
 	if err != nil {
 		log.Fatalf("could not open database connection: %s", err)
@@ -168,7 +177,7 @@ func main() {
 		l = tls.NewListener(l, tlsConf)
 	}
 
-	s := newServer(l, db, tmpl, auth, autho)
+	s := newServer(l, db, logger, tmpl, auth, autho)
 	s.Handle("/", showChecks)
 	s.Handle("/create", showCreate)
 	s.Handle("/check", showCheck)
@@ -179,6 +188,48 @@ func main() {
 	log.Fatalf("http server stopped: %s", s.ListenAndServe())
 }
 
+func parseLogger(config Config) *slog.Logger {
+	var output io.Writer
+	switch config.Log.Output {
+	case "", "stderr":
+		output = os.Stderr
+	case "stdout":
+		output = os.Stdout
+	default:
+		var err error
+		output, err = os.OpenFile(config.Log.Output, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0640)
+		if err != nil {
+			log.Fatalf("could not open log file handler: %s", err)
+		}
+	}
+
+	var level slog.Level
+	switch config.Log.Level {
+	case "debug":
+		level = slog.LevelDebug
+	case "", "info":
+		level = slog.LevelInfo
+	case "warn":
+		level = slog.LevelWarn
+	case "error":
+		level = slog.LevelError
+	default:
+		log.Fatalf("unknown log level '%s', only 'debug', 'info', 'warn' and 'error' are supported", config.Log.Level)
+	}
+
+	var handler slog.Handler
+	switch config.Log.Format {
+	case "", "text":
+		handler = slog.NewTextHandler(output, &slog.HandlerOptions{Level: level})
+	case "json":
+		handler = slog.NewJSONHandler(output, &slog.HandlerOptions{Level: level})
+	default:
+		log.Fatalf("unknown log format '%s', only 'text' and 'json' are supported", config.Log.Format)
+	}
+
+	return slog.New(handler)
+}
+
 func checkAction(con *Context) {
 	if con.r.Method != "POST" {
 		con.w.WriteHeader(http.StatusMethodNotAllowed)