fix: ensure that Gateway managed DataPlanes are reduced

controllers/gateway/controller.go

@@ -36,6 +36,7 @@ import (
 	gatewayutils "github.com/kong/gateway-operator/pkg/utils/gateway"
 	k8sutils "github.com/kong/gateway-operator/pkg/utils/kubernetes"
 	"github.com/kong/gateway-operator/pkg/utils/kubernetes/compare"
+	k8sreduce "github.com/kong/gateway-operator/pkg/utils/kubernetes/reduce"
 	k8sresources "github.com/kong/gateway-operator/pkg/utils/kubernetes/resources"
 	"github.com/kong/gateway-operator/pkg/vars"
 )
@@ -382,11 +383,16 @@ func (r *Reconciler) provisionDataPlane(
 
 	count := len(dataplanes)
 	if count > 1 {
-		err = fmt.Errorf("data plane deployments found: %d, expected: 1", count)
+		err = fmt.Errorf("data planes found: %d, expected: 1", count)
 		k8sutils.SetCondition(
 			createDataPlaneCondition(metav1.ConditionFalse, k8sutils.UnableToProvisionReason, err.Error(), gateway.Generation),
 			gatewayConditionsAndListenersAware(gateway),
 		)
+		log.Debug(logger, "reducing dataplanes", gateway, "count", count)
+		rErr := k8sreduce.ReduceDataPlanes(ctx, r.Client, dataplanes)
+		if rErr != nil {
+			return nil, fmt.Errorf("failed reducing data planes: %w", rErr)
+		}
 		return nil, err
 	}
 	if count == 0 {

pkg/utils/kubernetes/reduce/filters.go

@@ -10,6 +10,7 @@ import (
 	networkingv1 "k8s.io/api/networking/v1"
 	rbacv1 "k8s.io/api/rbac/v1"
 
+	operatorv1beta1 "github.com/kong/gateway-operator/apis/v1beta1"
 	"github.com/kong/gateway-operator/pkg/consts"
 )
 
@@ -330,3 +331,24 @@ func filterValidatingWebhookConfigurations(webhookConfigurations []admregv1.Vali
 
 	return append(webhookConfigurations[:best], webhookConfigurations[best+1:]...)
 }
+
+// -----------------------------------------------------------------------------
+// Filter functions - DataPlanes
+// -----------------------------------------------------------------------------
+
+// filterDataPlanes filters out the DataPlanes to be kept and returns all the DataPlanes
+// to be deleted. The oldest DataPlane is kept.
+func filterDataPlanes(dataplanes []operatorv1beta1.DataPlane) []operatorv1beta1.DataPlane {
+	if len(dataplanes) < 2 {
+		return []operatorv1beta1.DataPlane{}
+	}
+
+	best := 0
+	for i, dataplane := range dataplanes {
+		if dataplane.CreationTimestamp.Before(&dataplanes[best].CreationTimestamp) {
+			best = i
+		}
+	}
+
+	return append(dataplanes[:best], dataplanes[best+1:]...)
+}

pkg/utils/kubernetes/reduce/reduce.go

@@ -12,6 +12,8 @@ import (
 	networkingv1 "k8s.io/api/networking/v1"
 	rbacv1 "k8s.io/api/rbac/v1"
 	"sigs.k8s.io/controller-runtime/pkg/client"
+
+	operatorv1beta1 "github.com/kong/gateway-operator/apis/v1beta1"
 )
 
 // PreDeleteHook is a function that can be executed before deleting an object.
@@ -176,3 +178,17 @@ func ReduceValidatingWebhookConfigurations(ctx context.Context, k8sClient client
 	}
 	return nil
 }
+
+// +kubebuilder:rbac:groups=gateway-operator.konghq.com,resources=dataplanes,verbs=get;list;watch;update;patch
+
+// ReduceDataPlanes detects the best DataPlane in the set and deletes all the others.
+func ReduceDataPlanes(ctx context.Context, k8sClient client.Client, dataplanes []operatorv1beta1.DataPlane) error {
+	filteredDataPlanes := filterDataPlanes(dataplanes)
+	for _, dataplane := range filteredDataPlanes {
+		dataplane := dataplane
+		if err := k8sClient.Delete(ctx, &dataplane); err != nil {
+			return err
+		}
+	}
+	return nil
+}