Allow to configure log level, don't try to parse output of nft exit…

…ed with non-zero status (#29)

README.md

@@ -23,10 +23,14 @@ nftables_exporter:
   url_path: "/metrics"
   nft_location: /sbin/nft
   fake_nft_json: /path/to/nft.json
+  log_level: warn
 ```
 
 `fake_nft_json` used for debugging. I create this file with the command `nft -j list ruleset > /path/to/nft.json`. For normal exporter usage, this option is not needed.
 
+`log_level` can be one of the following: `debug`, `info`, `warn`, `error`.
+Default: `warn`.
+
 ## Example metrics
 
 ```config

main.go

@@ -1,7 +1,7 @@
 package main
 
 import (
-	"log"
+	"errors"
 	"log/slog"
 	"net/http"
 	"os"
@@ -32,7 +32,7 @@ func (i nftablesManagerCollector) Describe(ch chan<- *prometheus.Desc) {
 func (i nftablesManagerCollector) Collect(ch chan<- prometheus.Metric) {
 	json, err := readData(i.opts)
 	if err != nil {
-		log.Printf("failed parsing nftables data: %s", err)
+		slog.Error("failed to parse nftables data", "error", err)
 		ch <- prometheus.MustNewConstMetric(upDesc, prometheus.GaugeValue, 0)
 	} else {
 		ch <- prometheus.MustNewConstMetric(upDesc, prometheus.GaugeValue, 1)
@@ -42,8 +42,6 @@ func (i nftablesManagerCollector) Collect(ch chan<- prometheus.Metric) {
 }
 
 func main() {
-	// set json logger as default for all log statements
-	slog.SetDefault(slog.New(slog.NewJSONHandler(os.Stdout, nil)))
 	opts := loadOptions()
 	reg := prometheus.NewPedanticRegistry()
 	reg.MustRegister(
@@ -53,11 +51,17 @@ func main() {
 
 	prometheus.WrapRegistererWithPrefix("", reg).MustRegister(nftablesManagerCollector{opts: opts})
 
-	log.Printf("starting on %s%s", opts.Nft.BindTo, opts.Nft.URLPath)
+	slog.Info("starting http server", "bind_to", opts.Nft.BindTo, "url_path", opts.Nft.URLPath)
 	http.Handle(opts.Nft.URLPath, promhttp.HandlerFor(reg, promhttp.HandlerOpts{}))
 	server := http.Server{
 		Addr:              opts.Nft.BindTo,
 		ReadHeaderTimeout: 1 * time.Minute,
 	}
-	log.Fatal(server.ListenAndServe())
+
+	err := server.ListenAndServe()
+	// ListenAndServe always returns non-nil error
+	if !errors.Is(err, http.ErrServerClosed) {
+		slog.Error("http server exited", "error", err)
+		os.Exit(1)
+	}
 }

nftables.go

@@ -2,7 +2,7 @@ package main
 
 import (
 	"fmt"
-	"log"
+	"log/slog"
 	"strings"
 
 	"github.com/prometheus/client_golang/prometheus"
@@ -17,7 +17,7 @@ type nftables struct {
 
 // newNFTables is NFTables constructor
 func newNFTables(json gjson.Result, ch chan<- prometheus.Metric) nftables {
-	log.Print("collecting metrics")
+	slog.Debug("collecting metrics")
 	return nftables{
 		ch:   ch,
 		json: json,

options.go

@@ -3,6 +3,7 @@ package main
 import (
 	"flag"
 	"log"
+	"log/slog"
 	"os"
 
 	"github.com/metal-stack/v"
@@ -20,6 +21,7 @@ type nftOptions struct {
 	URLPath     string `yaml:"url_path"`
 	FakeNftJSON string `yaml:"fake_nft_json"`
 	NFTLocation string `yaml:"nft_location"`
+	LogLevel    string `yaml:"log_level"`
 }
 
 // Parse options from yaml config file
@@ -33,10 +35,15 @@ func loadOptions() options {
 		os.Exit(0)
 	}
 
-	log.Printf("read options from %s\n", *configFile)
+	logLevel := new(slog.LevelVar)
+	slog.SetDefault(slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
+		Level: logLevel,
+	})))
+	slog.Info("reading options from config file", "path", *configFile)
 	yamlFile, err := os.ReadFile(*configFile)
 	if err != nil {
-		log.Fatalf("failed read %s: %s", *configFile, err)
+		slog.Error("failed to read config file", "path", *configFile, "error", err)
+		os.Exit(1)
 	}
 
 	opts := options{
@@ -45,12 +52,22 @@ func loadOptions() options {
 			URLPath:     "/metrics",
 			FakeNftJSON: "",
 			NFTLocation: "/sbin/nft",
+			LogLevel:    "warn",
 		},
 	}
 
 	if yaml.Unmarshal(yamlFile, &opts) != nil {
-		log.Fatalf("failed parse %s: %s", *configFile, err)
+		slog.Error("failed to parse config file", "path", *configFile, "error", err)
+		os.Exit(1)
 	}
-	log.Printf("parsed options: %#v", opts)
+
+	err = logLevel.UnmarshalText([]byte(opts.Nft.LogLevel))
+	if err != nil {
+		slog.Error("cannot parse log level", "error", err)
+		os.Exit(1)
+	}
+
+	slog.Info("parsed options", "opts", opts)
+
 	return opts
 }

readjson.go

@@ -2,7 +2,8 @@ package main
 
 import (
 	"errors"
-	"log"
+	"fmt"
+	"log/slog"
 	"os"
 	"os/exec"
 
@@ -12,28 +13,28 @@ import (
 // Parse json to gjson object
 func parseJSON(data string) (gjson.Result, error) {
 	if !gjson.Valid(data) {
-		return gjson.Parse("{}"), errors.New("invalid JSON")
+		return gjson.Result{}, errors.New("invalid JSON")
 	}
 	return gjson.Get(data, "nftables"), nil
 }
 
 // Reading fake nftables json
 func readFakeNFTables(opts options) (gjson.Result, error) {
-	log.Printf("read fake nftables data from json: %s", opts.Nft.FakeNftJSON)
+	slog.Debug("read fake nftables data from json", "path", opts.Nft.FakeNftJSON)
 	jsonFile, err := os.ReadFile(opts.Nft.FakeNftJSON)
 	if err != nil {
-		log.Printf("fake nftables data reading error: %s", err)
+		return gjson.Result{}, fmt.Errorf("fake nftables data reading error: %w", err)
 	}
 	return parseJSON(string(jsonFile))
 }
 
 // Get json from nftables and parse it
 func readNFTables(opts options) (gjson.Result, error) {
-	log.Print("collecting nftables counters...")
+	slog.Debug("collecting nftables counters...")
 	nft := opts.Nft.NFTLocation
 	out, err := exec.Command(nft, "-j", "list", "ruleset").Output()
 	if err != nil {
-		log.Printf("nftables reading error: %s", err)
+		return gjson.Result{}, fmt.Errorf("nftables reading error: %w", err)
 	}
 	return parseJSON(string(out))
 }