client-go/features: warn when ordering initialization issue

ReplaceFeatureGates logs a warning when the default env var
implementation has been already used.
Such a situation indicates a potential ordering issue and usually is unwanted.

staging/src/k8s.io/client-go/features/envvar.go

@@ -77,6 +77,10 @@ type envVarFeatureGates struct {
 	// enabled holds a map[Feature]bool
 	// with values explicitly set via env var
 	enabled atomic.Value
+
+	// readEnvVars holds the boolean value which
+	// indicates whether readEnvVarsOnce has been called.
+	readEnvVars atomic.Bool
 }
 
 // Enabled returns true if the key is enabled.  If the key is not known, this call will panic.
@@ -116,6 +120,7 @@ func (f *envVarFeatureGates) getEnabledMapFromEnvVar() map[Feature]bool {
 			}
 		}
 		f.enabled.Store(featureGatesState)
+		f.readEnvVars.Store(true)
 
 		for feature, featureSpec := range f.known {
 			if featureState, ok := featureGatesState[feature]; ok {
@@ -127,3 +132,7 @@ func (f *envVarFeatureGates) getEnabledMapFromEnvVar() map[Feature]bool {
 	})
 	return f.enabled.Load().(map[Feature]bool)
 }
+
+func (f *envVarFeatureGates) hasAlreadyReadEnvVar() bool {
+	return f.readEnvVars.Load()
+}

staging/src/k8s.io/client-go/features/envvar_test.go

@@ -146,3 +146,11 @@ func TestEnvVarFeatureGatesEnabledPanic(t *testing.T) {
 	target := newEnvVarFeatureGates(nil)
 	require.PanicsWithError(t, fmt.Errorf("feature %q is not registered in FeatureGates %q", "UnknownFeature", target.callSiteName).Error(), func() { target.Enabled("UnknownFeature") })
 }
+
+func TestHasAlreadyReadEnvVar(t *testing.T) {
+	target := newEnvVarFeatureGates(nil)
+	require.False(t, target.hasAlreadyReadEnvVar())
+
+	_ = target.getEnabledMapFromEnvVar()
+	require.True(t, target.hasAlreadyReadEnvVar())
+}

staging/src/k8s.io/client-go/features/features.go

@@ -17,6 +17,9 @@ limitations under the License.
 package features
 
 import (
+	"errors"
+
+	utilruntime "k8s.io/apimachinery/pkg/util/runtime"
 	"sync/atomic"
 )
 
@@ -99,8 +102,23 @@ func AddFeaturesToExistingFeatureGates(registry Registry) error {
 //	// then replace client-go's feature gates implementation with your implementation
 //	clientgofeaturegate.ReplaceFeatureGates(utilfeature.DefaultMutableFeatureGate)
 func ReplaceFeatureGates(newFeatureGates Gates) {
+	if replaceFeatureGatesWithWarningIndicator(newFeatureGates) {
+		utilruntime.HandleError(errors.New("the default feature gates implementation has already been used and now it's being overwritten. This might lead to unexpected behaviour. Check your initialization order"))
+	}
+}
+
+func replaceFeatureGatesWithWarningIndicator(newFeatureGates Gates) bool {
+	shouldProduceWarning := false
+
+	if defaultFeatureGates, ok := FeatureGates().(*envVarFeatureGates); ok {
+		if defaultFeatureGates.hasAlreadyReadEnvVar() {
+			shouldProduceWarning = true
+		}
+	}
 	wrappedFeatureGates := &featureGatesWrapper{newFeatureGates}
 	featureGates.Store(wrappedFeatureGates)
+
+	return shouldProduceWarning
 }
 
 func init() {