cmd/k8s-operator: require namespace config (#14648)

Most users should not run into this because it's set in the helm chart
and the deploy manifest, but if namespace is not set we get confusing
authz errors because the kube client tries to fetch some namespaced resources
as though they're cluster-scoped and reports permission denied. Try to
detect namespace from the default projected volume, and otherwise fatal.

Fixes #cleanup

Change-Id: I64b34191e440b61204b9ad30bbfa117abbbe09c3

Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>

cmd/k8s-operator/operator.go

@@ -88,6 +88,15 @@ func main() {
 	zlog := kzap.NewRaw(opts...).Sugar()
 	logf.SetLogger(zapr.NewLogger(zlog.Desugar()))
 
+	if tsNamespace == "" {
+		const namespaceFile = "/var/run/secrets/kubernetes.io/serviceaccount/namespace"
+		b, err := os.ReadFile(namespaceFile)
+		if err != nil {
+			zlog.Fatalf("Could not get operator namespace from OPERATOR_NAMESPACE environment variable or default projected volume: %v", err)
+		}
+		tsNamespace = strings.TrimSpace(string(b))
+	}
+
 	// The operator can run either as a plain operator or it can
 	// additionally act as api-server proxy
 	// https://tailscale.com/kb/1236/kubernetes-operator/?q=kubernetes#accessing-the-kubernetes-control-plane-using-an-api-server-proxy.