the pod namespace is now passed using the env var POD_NAMESPACE

Before, the namespace used to be guessed by looking up the service
account's `namespace` file at

  /var/run/secrets/kubernetes.io/serviceaccount/namespace

Although this way is "OK" since the agent will always have a service
account token mounted to the pod, we decided that passing the namespace
to the pod using an explicit env var would be better.

pkg/agent/config.go

@@ -23,10 +23,6 @@ import (
 	"github.com/jetstack/preflight/pkg/version"
 )
 
-const (
-	inClusterNamespacePath = "/var/run/secrets/kubernetes.io/serviceaccount/namespace"
-)
-
 // Config wraps the options for a run of the agent.
 type Config struct {
 	// Deprecated: Schedule doesn't do anything. Use `period` instead.
@@ -154,9 +150,8 @@ type AgentCmdFlags struct {
 	// InstallNS (--install-namespace) is the namespace in which the agent is
 	// running in. Only needed when running the agent outside of Kubernetes.
 	//
-	// May be left empty when running in Kubernetes. In this case, the namespace
-	// is read from the file
-	// /var/run/secrets/kubernetes.io/serviceaccount/namespace.
+	// May be left empty when running in Kubernetes. In Kubernetes, the
+	// namespace is read from the environment variable `POD_NAMESPACE`.
 	InstallNS string
 
 	// Profiling (--enable-pprof) enables the pprof server.
@@ -726,21 +721,12 @@ func createCredentialClient(log *log.Logger, credentials client.Credentials, cfg
 
 // Inspired by the controller-runtime project.
 func getInClusterNamespace() (string, error) {
-	// Check whether the namespace file exists.
-	// If not, we are not running in cluster so can't guess the namespace.
-	_, err := os.Stat(inClusterNamespacePath)
-	if os.IsNotExist(err) {
-		return "", fmt.Errorf("not running in cluster, please use --install-namespace to specify the namespace in which the agent is running")
-	}
-	if err != nil {
-		return "", fmt.Errorf("error checking namespace file: %w", err)
+	ns := os.Getenv("POD_NAMESPACE")
+	if ns != "" {
+		return ns, nil
 	}
 
-	namespace, err := os.ReadFile(inClusterNamespacePath)
-	if err != nil {
-		return "", fmt.Errorf("error reading namespace file: %w", err)
-	}
-	return string(namespace), nil
+	return "", fmt.Errorf("POD_NAMESPACE env var not set, meaning that you are probably not running in cluster. Please use --install-namespace or POD_NAMESPACE to specify the namespace in which the agent is running.")
 }
 
 func reMarshal(rawConfig interface{}, config datagatherer.Config) error {