client-go/features: introduce feature gates

This PR add a feature gates mechanisim to client-go
as described in https://docs.google.com/document/d/1g9BGCRw-7ucUxO6OtCWbb3lfzUGA_uU9178wLdXAIfs

In particular:
  - Adds a default feature gate implementation based on environment variables.
  - Adds a set of methods for reading, overwriting the default implementation, and adding features to an external registry.

Co-authored-by: deads2k <deads@redhat.com>
Co-authored-by: Ben Luddy <bluddy@redhat.com>

Kubernetes-commit: d74c57d4f592d20a992afb54b1ee64f56215210e

features/testing/features_init_test.go

@@ -0,0 +1,79 @@
+/*
+Copyright 2024 The Kubernetes 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 testing
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"k8s.io/client-go/features"
+)
+
+func TestDriveInitDefaultFeatureGates(t *testing.T) {
+	featureGates := features.FeatureGates()
+	assertFunctionPanicsWithMessage(t, func() { featureGates.Enabled("FakeFeatureGate") }, "features.FeatureGates().Enabled", fmt.Sprintf("feature %q is not registered in FeatureGate", "FakeFeatureGate"))
+
+	fakeFeatureGates := &alwaysEnabledFakeGates{}
+	require.True(t, fakeFeatureGates.Enabled("FakeFeatureGate"))
+
+	features.ReplaceFeatureGates(fakeFeatureGates)
+	featureGates = features.FeatureGates()
+
+	assertFeatureGatesType(t, featureGates)
+	require.True(t, featureGates.Enabled("FakeFeatureGate"))
+}
+
+type alwaysEnabledFakeGates struct{}
+
+func (f *alwaysEnabledFakeGates) Enabled(features.Feature) bool {
+	return true
+}
+
+func assertFeatureGatesType(t *testing.T, fg features.Gates) {
+	_, ok := fg.(*alwaysEnabledFakeGates)
+	if !ok {
+		t.Fatalf("passed features.FeatureGates() is NOT of type *alwaysEnabledFakeGates, it is of type = %T", fg)
+	}
+}
+
+func assertFunctionPanicsWithMessage(t *testing.T, f func(), fName, errMessage string) {
+	didPanic, panicMessage := didFunctionPanic(f)
+	if !didPanic {
+		t.Fatalf("function %q did not panicked", fName)
+	}
+
+	panicError, ok := panicMessage.(error)
+	if !ok || !strings.Contains(panicError.Error(), errMessage) {
+		t.Fatalf("func %q should panic with error message:\t%#v\n\tPanic value:\t%#v\n", fName, errMessage, panicMessage)
+	}
+}
+
+func didFunctionPanic(f func()) (didPanic bool, panicMessage interface{}) {
+	didPanic = true
+
+	defer func() {
+		panicMessage = recover()
+	}()
+
+	f()
+	didPanic = false
+
+	return
+}