Allow single MinIO tenant in a namespace (#181)

* Allow single MinIO tenant in a namespace

This PR blocks attempts to create a new MinIO tenant in a namespace
that already has another MinIO tenant.

* Update pkg/controller/cluster/main-controller.go

Co-authored-by: Ritesh H Shukla <kerneltime@gmail.com>

Co-authored-by: Ritesh H Shukla <kerneltime@gmail.com>
Co-authored-by: Daniel Valdivia <hola@danielvaldivia.com>

pkg/controller/cluster/main-controller.go

@@ -32,6 +32,7 @@ import (
 	"k8s.io/apimachinery/pkg/api/errors"
 	apierrors "k8s.io/apimachinery/pkg/api/errors"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/labels"
 	"k8s.io/apimachinery/pkg/types"
 	"k8s.io/apimachinery/pkg/util/runtime"
 	"k8s.io/apimachinery/pkg/util/wait"
@@ -91,6 +92,7 @@ const (
 	statusUpdatingContainerArguments = "Updating Container Arguments"
 	statusUpdatingConsoleVersion     = "Updating Console Version"
 	statusNotOwned                   = "Statefulset not controlled by operator"
+	statusFailedAlreadyExists        = "Failed: Another MinIO Tenant already exists in the namespace"
 )
 
 // Controller struct watches the Kubernetes API for changes to Tenant resources
@@ -436,6 +438,19 @@ func (c *Controller) syncHandler(key string) error {
 		}
 	}
 
+	// List all MinIO instances in this namespace.
+	li, err := c.tenantsLister.Tenants(mi.Namespace).List(labels.NewSelector())
+	if err != nil {
+		return err
+	}
+	// Only 1 minio tenant per namespace allowed.
+	if len(li) > 1 {
+		if _, err = c.updateTenantStatus(ctx, mi, statusFailedAlreadyExists, 0); err != nil {
+			return err
+		}
+		return fmt.Errorf("Failed creating MinIO Tenant '%s' because another MinIO Tenant '%s' already exists in the namespace '%s'", mi.Name, li[0].Name, mi.Namespace)
+	}
+
 	// For each zone check it's stateful set
 	var totalReplicas int32
 	for _, zone := range mi.Spec.Zones {