Support logging to file

config.scfg

@@ -19,6 +19,9 @@ log-level info
 # Options: text, json
 # Default: text
 log-format text
+# Write logs to this file. If unset, the logs will be written to stderr.
+# Default: ""
+log-file /var/log/ntfy-alertmanager.log
 # When multiple alerts are grouped together by Alertmanager, they can either be sent
 # each on their own (single mode) or be kept together (multi mode)
 # Options: single, multi

config/config.go

@@ -25,6 +25,7 @@ type Config struct {
 	HTTPAddress string
 	LogLevel    string
 	LogFormat   string
+	LogFile     string
 	AlertMode   AlertMode
 	User        string
 	Password    string
@@ -97,6 +98,13 @@ func parseBlock(block scfg.Block, config *Config) error {
 		}
 	}
 
+	d = block.Get("log-file")
+	if d != nil {
+		if err := d.ParseParams(&config.LogFile); err != nil {
+			return err
+		}
+	}
+
 	d = block.Get("http-address")
 	if d != nil {
 		if err := d.ParseParams(&config.HTTPAddress); err != nil {

config/config_test.go

@@ -14,6 +14,7 @@ base-url https://ntfy-alertmanager.example.com
 http-address :8080
 log-level info
 log-format json
+log-file /var/log/ntfy-alertmanager.log
 alert-mode multi
 user webhookUser
 password webhookPass
@@ -70,6 +71,7 @@ cache {
 		HTTPAddress: ":8080",
 		LogLevel:    "info",
 		LogFormat:   "json",
+		LogFile:     "/var/log/ntfy-alertmanager.log",
 		AlertMode:   Multi,
 		User:        "webhookUser",
 		Password:    "webhookPass",

main.go

@@ -14,6 +14,7 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"io"
 	"log/slog"
 	"net/http"
 	"os"
@@ -505,7 +506,21 @@ func main() {
 		os.Exit(1)
 	}
 
-	logger, err := logging.New(cfg.LogLevel, cfg.LogFormat, os.Stderr)
+	var logOutput io.Writer
+	if cfg.LogFile != "" {
+		f, err := os.OpenFile(cfg.LogFile, os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0600)
+		if err != nil {
+			slog.Error("Failed to open logfile",
+				slog.String("error", err.Error()))
+			os.Exit(1)
+		}
+		defer f.Close()
+		logOutput = f
+	} else {
+		logOutput = os.Stderr
+	}
+
+	logger, err := logging.New(cfg.LogLevel, cfg.LogFormat, logOutput)
 	if err != nil {
 		slog.Error("Failed to create logger",
 			slog.String("error", err.Error()))