Introduce common Kubernetes config CLI flags

cmd/icinga-kubernetes/main.go

@@ -21,6 +21,7 @@ import (
 	"github.com/pkg/errors"
 	promapi "github.com/prometheus/client_golang/api"
 	promv1 "github.com/prometheus/client_golang/api/prometheus/v1"
+	"github.com/spf13/pflag"
 	"golang.org/x/sync/errgroup"
 	"k8s.io/apimachinery/pkg/util/runtime"
 	"k8s.io/client-go/informers"
@@ -35,24 +36,41 @@ import (
 func main() {
 	runtime.ReallyCrash = true
 
-	kconfig, err := kclientcmd.NewNonInteractiveDeferredLoadingClientConfig(
-		kclientcmd.NewDefaultClientConfigLoadingRules(), &kclientcmd.ConfigOverrides{}).ClientConfig()
-	if err != nil {
-		klog.Fatal(err)
-	}
-
 	var configLocation string
+	var showVersion bool
 
 	klog.InitFlags(nil)
+	pflag.CommandLine.AddGoFlagSet(flag.CommandLine)
+
+	pflag.BoolVar(&showVersion, "version", false, "print version and exit")
+	pflag.StringVar(&configLocation, "config", "./config.yml", "path to the config file")
+
+	loadingRules := kclientcmd.NewDefaultClientConfigLoadingRules()
+	loadingRules.DefaultClientConfig = &kclientcmd.DefaultClientConfig
+	pflag.StringVar(&loadingRules.ExplicitPath, "kubeconfig", "", "Path to a kube config. Only required if out-of-cluster")
+
+	overrides := kclientcmd.ConfigOverrides{}
+	kflags := kclientcmd.RecommendedConfigOverrideFlags("")
+	kflags.ContextOverrideFlags.Namespace = kclientcmd.FlagInfo{}
+	kclientcmd.BindOverrideFlags(&overrides, pflag.CommandLine, kflags)
 
-	flag.BoolFunc("version", "print version and exit", func(_ string) error {
+	pflag.Parse()
+
+	if showVersion {
 		internal.Version.Print()
 		os.Exit(0)
+	}
 
-		return nil
-	})
-	flag.StringVar(&configLocation, "config", "./config.yml", "path to the config file")
-	flag.Parse()
+	kconfig, err := kclientcmd.NewNonInteractiveDeferredLoadingClientConfig(loadingRules, &overrides).ClientConfig()
+	if err != nil {
+		if kclientcmd.IsEmptyConfig(err) {
+			klog.Fatal(
+				"no configuration provided: set KUBECONFIG environment variable or --kubeconfig CLI flag to" +
+					" a kubeconfig file with cluster access configured")
+		}
+
+		klog.Fatal(errors.Wrap(err, "can't configure Kubernetes client"))
+	}
 
 	clientset, err := kubernetes.NewForConfig(kconfig)
 	if err != nil {

doc/02-Installation.md

@@ -79,15 +79,19 @@ All available settings can be found under [Configuration](03-Configuration.md).
 
 ## Running Icinga for Kubernetes
 
-With locally accessible kubeconfig and `config.yml` files, `icinga-kubernetes` can be executed by running:
+With locally accessible
+[kubeconfig](https://kubernetes.io/docs/concepts/configuration/organize-cluster-access-kubeconfig/)
+and `config.yml` files, `icinga-kubernetes` can be executed by running:
 
 ```bash
 icinga-kubernetes -config /path/to/config.yml
 ```
 
 ## Using a Container
 
-With locally accessible kubeconfig and `config.yml` files,
+With locally accessible
+[kubeconfig](https://kubernetes.io/docs/concepts/configuration/organize-cluster-access-kubeconfig/)
+and `config.yml` files,
 run the `icinga/icinga-kubernetes` image using a container runtime of you choice, e.g. Docker:
 
 ```bash

go.mod

@@ -15,6 +15,7 @@ require (
 	github.com/pkg/errors v0.9.1
 	github.com/prometheus/client_golang v1.19.1
 	github.com/prometheus/common v0.53.0
+	github.com/spf13/pflag v1.0.5
 	go.uber.org/zap v1.27.0
 	golang.org/x/exp v0.0.0-20240506185415-9bf2ced13842
 	golang.org/x/sync v0.7.0
@@ -52,7 +53,6 @@ require (
 	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/prometheus/client_model v0.6.0 // indirect
 	github.com/robfig/cron/v3 v3.0.1 // indirect
-	github.com/spf13/pflag v1.0.5 // indirect
 	github.com/ssgreg/journald v1.0.0 // indirect
 	go.uber.org/atomic v1.9.0 // indirect
 	go.uber.org/multierr v1.10.0 // indirect