Add structure for feature flags

Documentation/operator.md

@@ -49,6 +49,8 @@ Usage of ./operator:
     	Namespaces not to scope the interaction of the Prometheus Operator (deny list). This is mutually exclusive with --namespaces.
   -enable-config-reloader-probes
     	Enable liveness and readiness for the config-reloader container. Default: false
+  -feature-gates value
+    	Feature gates are a set of key=value pairs that describe Prometheus-Operator features. At the moment there are no feature gates available.
   -key-file string
     	- NOT RECOMMENDED FOR PRODUCTION - Path to private TLS certificate file.
   -kubelet-node-address-priority value

cmd/operator/main.go

@@ -41,6 +41,8 @@ import (
 	"k8s.io/apimachinery/pkg/runtime/schema"
 	"k8s.io/client-go/kubernetes"
 	"k8s.io/client-go/rest"
+	k8sflag "k8s.io/component-base/cli/flag"
+	"k8s.io/utils/ptr"
 
 	logging "github.com/prometheus-operator/prometheus-operator/internal/log"
 	"github.com/prometheus-operator/prometheus-operator/pkg/admission"
@@ -114,6 +116,8 @@ var (
 	kubeletObject       string
 	kubeletSelector     operator.LabelSelector
 	nodeAddressPriority operator.NodeAddressPriority
+
+	featureGates *k8sflag.MapStringBool
 )
 
 func parseFlags(fs *flag.FlagSet) {
@@ -166,6 +170,11 @@ func parseFlags(fs *flag.FlagSet) {
 	fs.Var(&cfg.ThanosRulerSelector, "thanos-ruler-instance-selector", "Label selector to filter ThanosRuler Custom Resources to watch.")
 	fs.Var(&cfg.SecretListWatchSelector, "secret-field-selector", "Field selector to filter Secrets to watch")
 
+	featureGates = k8sflag.NewMapStringBool(ptr.To(make(map[string]bool)))
+	fs.Var(featureGates, "feature-gates", "Feature gates are a set of key=value pairs that describe Prometheus-Operator features. At the moment there are no feature gates available.")
+	// Once the first feature gate is added, the line below should be uncommented and the line above deleted.
+	//fs.Var(featureGates, "feature-gates", fmt.Sprintf("Feature gates are a set of key=value pairs that describe Prometheus-Operator features. Available features: %q.", operator.AvailableFeatureGates()))
+
 	logging.RegisterFlags(fs, &logConfig)
 	versionutil.RegisterFlags(fs)
 
@@ -193,8 +202,17 @@ func run(fs *flag.FlagSet) int {
 		level.Warn(logger).Log("msg", "Failed to set GOMAXPROCS automatically", "err", err)
 	}
 
+	gates, err := operator.ValidateFeatureGates(featureGates)
+	if err != nil {
+		level.Error(logger).Log(
+			"msg", "error validating feature gates",
+			"error", err)
+		return 1
+	}
+
 	level.Info(logger).Log("msg", "Starting Prometheus Operator", "version", version.Info())
 	level.Info(logger).Log("build_context", version.BuildContext())
+	level.Info(logger).Log("feature_gates", gates)
 
 	if len(cfg.Namespaces.AllowList) > 0 && len(cfg.Namespaces.DenyList) > 0 {
 		level.Error(logger).Log(

pkg/operator/feature_gates.go

@@ -0,0 +1,63 @@
+// Copyright 2024 The prometheus-operator Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package operator
+
+import (
+	"fmt"
+	"slices"
+	"strings"
+
+	k8sflag "k8s.io/component-base/cli/flag"
+)
+
+// At the moment, the are no feature gates available.
+var defaultFeatureGates = map[string]bool{}
+
+// ValidateFeatureGates merges the feature gate default values with
+// the values provided by the user.
+func ValidateFeatureGates(flags *k8sflag.MapStringBool) (string, error) {
+	gates := defaultFeatureGates
+	if flags.Empty() {
+		return mapToString(gates), nil
+	}
+
+	imgs := *flags.Map
+	for k, v := range imgs {
+		if _, ok := gates[k]; !ok {
+			return "", fmt.Errorf("feature gate %v is unknown", k)
+		}
+		gates[k] = v
+	}
+	return mapToString(gates), nil
+}
+
+func AvailableFeatureGates() []string {
+	i := 0
+	gates := make([]string, len(defaultFeatureGates))
+	for k := range defaultFeatureGates {
+		gates[i] = k
+		i++
+	}
+	slices.Sort(gates)
+	return gates
+}
+
+func mapToString(m map[string]bool) string {
+	var s []string
+	for k, v := range m {
+		s = append(s, fmt.Sprintf("%s=%t", k, v))
+	}
+	return strings.Join(s, ",")
+}