Support overrides for registered feature defaults.

This is to support the goal of enabling a feature by default for a single component only when the
feature in question is consumed by multiple components.

Overriden defaults are reflected in KnownFeatures and registered flag text.

staging/src/k8s.io/component-base/featuregate/feature_gate.go

@@ -115,6 +115,17 @@ type MutableFeatureGate interface {
 	GetAll() map[Feature]FeatureSpec
 	// AddMetrics adds feature enablement metrics
 	AddMetrics()
+	// OverrideDefault sets a local override for the registered default value of a named
+	// feature. If the feature has not been previously registered (e.g. by a call to Add), has a
+	// locked default, or if the gate has already registered itself with a FlagSet, a non-nil
+	// error is returned.
+	//
+	// When two or more components consume a common feature, one component can override its
+	// default at runtime in order to adopt new defaults before or after the other
+	// components. For example, a new feature can be evaluated with a limited blast radius by
+	// overriding its default to true for a limited number of components without simultaneously
+	// changing its default for all consuming components.
+	OverrideDefault(name Feature, override bool) error
 }
 
 // featureGate implements FeatureGate as well as pflag.Value for flag parsing.
@@ -296,6 +307,29 @@ func (f *featureGate) Add(features map[Feature]FeatureSpec) error {
 	return nil
 }
 
+func (f *featureGate) OverrideDefault(name Feature, override bool) error {
+	f.lock.Lock()
+	defer f.lock.Unlock()
+
+	known := map[Feature]FeatureSpec{}
+	for name, spec := range f.known.Load().(map[Feature]FeatureSpec) {
+		known[name] = spec
+	}
+
+	if spec, ok := known[name]; !ok {
+		return fmt.Errorf("cannot override default: feature %q is not registered", name)
+	} else if spec.LockToDefault && override != spec.Default {
+		return fmt.Errorf("cannot override default: feature %q default is locked to %t", name, spec.Default)
+	} else {
+		spec.Default = override
+		known[name] = spec
+	}
+
+	f.known.Store(known)
+
+	return nil
+}
+
 // GetAll returns a copy of the map of known feature names to feature specs.
 func (f *featureGate) GetAll() map[Feature]FeatureSpec {
 	retval := map[Feature]FeatureSpec{}

staging/src/k8s.io/component-base/featuregate/feature_gate_test.go

@@ -508,3 +508,131 @@ func TestFeatureGateString(t *testing.T) {
 		})
 	}
 }
+
+func TestFeatureGateOverrideDefault(t *testing.T) {
+	t.Run("overrides take effect", func(t *testing.T) {
+		f := NewFeatureGate()
+		if err := f.Add(map[Feature]FeatureSpec{
+			"TestFeature1": {Default: true},
+			"TestFeature2": {Default: false},
+		}); err != nil {
+			t.Fatal(err)
+		}
+		if err := f.OverrideDefault("TestFeature1", false); err != nil {
+			t.Fatal(err)
+		}
+		if err := f.OverrideDefault("TestFeature2", true); err != nil {
+			t.Fatal(err)
+		}
+		if f.Enabled("TestFeature1") {
+			t.Error("expected TestFeature1 to have effective default of false")
+		}
+		if !f.Enabled("TestFeature2") {
+			t.Error("expected TestFeature2 to have effective default of true")
+		}
+	})
+
+	t.Run("overrides are preserved across deep copies", func(t *testing.T) {
+		f := NewFeatureGate()
+		if err := f.Add(map[Feature]FeatureSpec{"TestFeature": {Default: false}}); err != nil {
+			t.Fatal(err)
+		}
+		if err := f.OverrideDefault("TestFeature", true); err != nil {
+			t.Fatal(err)
+		}
+		fcopy := f.DeepCopy()
+		if !fcopy.Enabled("TestFeature") {
+			t.Error("default override was not preserved by deep copy")
+		}
+	})
+
+	t.Run("reflected in known features", func(t *testing.T) {
+		f := NewFeatureGate()
+		if err := f.Add(map[Feature]FeatureSpec{"TestFeature": {
+			Default:    false,
+			PreRelease: Alpha,
+		}}); err != nil {
+			t.Fatal(err)
+		}
+		if err := f.OverrideDefault("TestFeature", true); err != nil {
+			t.Fatal(err)
+		}
+		var found bool
+		for _, s := range f.KnownFeatures() {
+			if !strings.Contains(s, "TestFeature") {
+				continue
+			}
+			found = true
+			if !strings.Contains(s, "default=true") {
+				t.Errorf("expected override of default to be reflected in known feature description %q", s)
+			}
+		}
+		if !found {
+			t.Error("found no entry for TestFeature in known features")
+		}
+	})
+
+	t.Run("may not change default for specs with locked defaults", func(t *testing.T) {
+		f := NewFeatureGate()
+		if err := f.Add(map[Feature]FeatureSpec{
+			"LockedFeature": {
+				Default:       true,
+				LockToDefault: true,
+			},
+		}); err != nil {
+			t.Fatal(err)
+		}
+		if err := f.OverrideDefault("LockedFeature", true); err != nil {
+			t.Errorf("expected no error from attempt to override default to the registered default")
+		}
+		if f.OverrideDefault("LockedFeature", false) == nil {
+			t.Error("expected error when attempting to override the default for a feature with a locked default")
+		}
+	})
+
+	t.Run("does not supersede explicitly-set value", func(t *testing.T) {
+		f := NewFeatureGate()
+		if err := f.Add(map[Feature]FeatureSpec{"TestFeature": {Default: true}}); err != nil {
+			t.Fatal(err)
+		}
+		if err := f.OverrideDefault("TestFeature", false); err != nil {
+			t.Fatal(err)
+		}
+		if err := f.SetFromMap(map[string]bool{"TestFeature": true}); err != nil {
+			t.Fatal(err)
+		}
+		if !f.Enabled("TestFeature") {
+			t.Error("expected feature to be effectively enabled despite default override")
+		}
+	})
+
+	t.Run("prevents re-registration of feature spec after overriding default", func(t *testing.T) {
+		f := NewFeatureGate()
+		if err := f.Add(map[Feature]FeatureSpec{
+			"TestFeature": {
+				Default:    true,
+				PreRelease: Alpha,
+			},
+		}); err != nil {
+			t.Fatal(err)
+		}
+		if err := f.OverrideDefault("TestFeature", false); err != nil {
+			t.Fatal(err)
+		}
+		if err := f.Add(map[Feature]FeatureSpec{
+			"TestFeature": {
+				Default:    true,
+				PreRelease: Alpha,
+			},
+		}); err == nil {
+			t.Error("expected re-registration to return a non-nil error after overriding its default")
+		}
+	})
+
+	t.Run("does not allow override for an unknown feature", func(t *testing.T) {
+		f := NewFeatureGate()
+		if err := f.OverrideDefault("TestFeature", true); err == nil {
+			t.Error("expected an error to be returned in attempt to override default for unregistered feature")
+		}
+	})
+}