cmd/k8s-operator: allow setting same host value for tls and ingress rules

We were too strict and required the user not specify the host field at all
in the ingress rules, but that degrades compatibility with existing helm charts.

Relax the constraint so that rule.Host can either be empty, or match the tls.Host[0]
value exactly.

Fixes #9548

Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/k8s-operator/ingress.go

@@ -192,8 +192,15 @@ func (a *IngressReconciler) maybeProvision(ctx context.Context, logger *zap.Suga
 		}
 	}
 	addIngressBackend(ing.Spec.DefaultBackend, "/")
+
+	var tlsHost string // hostname or FQDN or empty
+	if ing.Spec.TLS != nil && len(ing.Spec.TLS) > 0 && len(ing.Spec.TLS[0].Hosts) > 0 {
+		tlsHost = ing.Spec.TLS[0].Hosts[0]
+	}
 	for _, rule := range ing.Spec.Rules {
-		if rule.Host != "" {
+		// Host is optional, but if it's present it must match the TLS host
+		// otherwise we ignore the rule.
+		if rule.Host != "" && rule.Host != tlsHost {
 			a.recorder.Eventf(ing, corev1.EventTypeWarning, "InvalidIngressBackend", "rule with host %q ignored, unsupported", rule.Host)
 			continue
 		}
@@ -208,8 +215,8 @@ func (a *IngressReconciler) maybeProvision(ctx context.Context, logger *zap.Suga
 		tags = strings.Split(tstr, ",")
 	}
 	hostname := ing.Namespace + "-" + ing.Name + "-ingress"
-	if ing.Spec.TLS != nil && len(ing.Spec.TLS) > 0 && len(ing.Spec.TLS[0].Hosts) > 0 {
-		hostname, _, _ = strings.Cut(ing.Spec.TLS[0].Hosts[0], ".")
+	if tlsHost != "" {
+		hostname, _, _ = strings.Cut(tlsHost, ".")
 	}
 
 	sts := &tailscaleSTSConfig{