feature_gate.go

/*
Copyright 2016 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 feature

import (
	"fmt"
	"sort"
	"strconv"
	"strings"
	"sync"
	"sync/atomic"

	"github.com/golang/glog"
	"github.com/spf13/pflag"
)

type Feature string

const (
	flagName = "feature-gates"

	// allAlphaGate is a global toggle for alpha features. Per-feature key
	// values override the default set by allAlphaGate. Examples:
	//   AllAlpha=false,NewFeature=true  will result in newFeature=true
	//   AllAlpha=true,NewFeature=false  will result in newFeature=false
	allAlphaGate Feature = "AllAlpha"
)

var (
	// The generic features.
	defaultFeatures = map[Feature]FeatureSpec{
		allAlphaGate: {Default: false, PreRelease: Alpha},
	}

	// Special handling for a few gates.
	specialFeatures = map[Feature]func(known map[Feature]FeatureSpec, enabled map[Feature]bool, val bool){
		allAlphaGate: setUnsetAlphaGates,
	}

	// DefaultFeatureGate is a shared global FeatureGate.
	DefaultFeatureGate FeatureGate = NewFeatureGate()
)

type FeatureSpec struct {
	Default    bool
	PreRelease prerelease
}

type prerelease string

const (
	// Values for PreRelease.
	Alpha = prerelease("ALPHA")
	Beta  = prerelease("BETA")
	GA    = prerelease("")

	// Deprecated
	Deprecated = prerelease("DEPRECATED")
)

// FeatureGate parses and stores flag gates for known features from
// a string like feature1=true,feature2=false,...
type FeatureGate interface {
	// AddFlag adds a flag for setting global feature gates to the specified FlagSet.
	AddFlag(fs *pflag.FlagSet)
	// Set parses and stores flag gates for known features
	// from a string like feature1=true,feature2=false,...
	Set(value string) error
	// SetFromMap stores flag gates for known features from a map[string]bool or returns an error
	SetFromMap(m map[string]bool) error
	// Enabled returns true if the key is enabled.
	Enabled(key Feature) bool
	// Add adds features to the featureGate.
	Add(features map[Feature]FeatureSpec) error
	// KnownFeatures returns a slice of strings describing the FeatureGate's known features.
	KnownFeatures() []string
	// DeepCopy returns a deep copy of the FeatureGate object, such that gates can be
	// set on the copy without mutating the original. This is useful for validating
	// config against potential feature gate changes before committing those changes.
	DeepCopy() FeatureGate
}

// featureGate implements FeatureGate as well as pflag.Value for flag parsing.
type featureGate struct {
	special map[Feature]func(map[Feature]FeatureSpec, map[Feature]bool, bool)

	// lock guards writes to known, enabled, and reads/writes of closed
	lock sync.Mutex
	// known holds a map[Feature]FeatureSpec
	known *atomic.Value
	// enabled holds a map[Feature]bool
	enabled *atomic.Value
	// closed is set to true when AddFlag is called, and prevents subsequent calls to Add
	closed bool
}

func setUnsetAlphaGates(known map[Feature]FeatureSpec, enabled map[Feature]bool, val bool) {
	for k, v := range known {
		if v.PreRelease == Alpha {
			if _, found := enabled[k]; !found {
				enabled[k] = val
			}
		}
	}
}

// Set, String, and Type implement pflag.Value
var _ pflag.Value = &featureGate{}

func NewFeatureGate() *featureGate {
	known := map[Feature]FeatureSpec{}
	for k, v := range defaultFeatures {
		known[k] = v
	}

	knownValue := &atomic.Value{}
	knownValue.Store(known)

	enabled := map[Feature]bool{}
	enabledValue := &atomic.Value{}
	enabledValue.Store(enabled)

	f := &featureGate{
		known:   knownValue,
		special: specialFeatures,
		enabled: enabledValue,
	}
	return f
}

// Set parses a string of the form "key1=value1,key2=value2,..." into a
// map[string]bool of known keys or returns an error.
func (f *featureGate) Set(value string) error {
	m := make(map[string]bool)
	for _, s := range strings.Split(value, ",") {
		if len(s) == 0 {
			continue
		}
		arr := strings.SplitN(s, "=", 2)
		k := strings.TrimSpace(arr[0])
		if len(arr) != 2 {
			return fmt.Errorf("missing bool value for %s", k)
		}
		v := strings.TrimSpace(arr[1])
		boolValue, err := strconv.ParseBool(v)
		if err != nil {
			return fmt.Errorf("invalid value of %s=%s, err: %v", k, v, err)
		}
		m[k] = boolValue
	}
	return f.SetFromMap(m)
}

// SetFromMap stores flag gates for known features from a map[string]bool or returns an error
func (f *featureGate) SetFromMap(m map[string]bool) error {
	f.lock.Lock()
	defer f.lock.Unlock()

	// Copy existing state
	known := map[Feature]FeatureSpec{}
	for k, v := range f.known.Load().(map[Feature]FeatureSpec) {
		known[k] = v
	}
	enabled := map[Feature]bool{}
	for k, v := range f.enabled.Load().(map[Feature]bool) {
		enabled[k] = v
	}

	for k, v := range m {
		k := Feature(k)
		featureSpec, ok := known[k]
		if !ok {
			return fmt.Errorf("unrecognized feature gate: %s", k)
		}
		enabled[k] = v
		// Handle "special" features like "all alpha gates"
		if fn, found := f.special[k]; found {
			fn(known, enabled, v)
		}

		if featureSpec.PreRelease == Deprecated {
			glog.Warningf("Setting deprecated feature gate %s=%t. It will be removed in a future release.", k, v)
		} else if featureSpec.PreRelease == GA {
			glog.Warningf("Setting GA feature gate %s=%t. It will be removed in a future release.", k, v)
		}
	}

	// Persist changes
	f.known.Store(known)
	f.enabled.Store(enabled)

	glog.V(1).Infof("feature gates: %v", f.enabled)
	return nil
}

// String returns a string containing all enabled feature gates, formatted as "key1=value1,key2=value2,...".
func (f *featureGate) String() string {
	pairs := []string{}
	for k, v := range f.enabled.Load().(map[Feature]bool) {
		pairs = append(pairs, fmt.Sprintf("%s=%t", k, v))
	}
	sort.Strings(pairs)
	return strings.Join(pairs, ",")
}

func (f *featureGate) Type() string {
	return "mapStringBool"
}

// Add adds features to the featureGate.
func (f *featureGate) Add(features map[Feature]FeatureSpec) error {
	f.lock.Lock()
	defer f.lock.Unlock()

	if f.closed {
		return fmt.Errorf("cannot add a feature gate after adding it to the flag set")
	}

	// Copy existing state
	known := map[Feature]FeatureSpec{}
	for k, v := range f.known.Load().(map[Feature]FeatureSpec) {
		known[k] = v
	}

	for name, spec := range features {
		if existingSpec, found := known[name]; found {
			if existingSpec == spec {
				continue
			}
			return fmt.Errorf("feature gate %q with different spec already exists: %v", name, existingSpec)
		}

		known[name] = spec
	}

	// Persist updated state
	f.known.Store(known)

	return nil
}

// Enabled returns true if the key is enabled.
func (f *featureGate) Enabled(key Feature) bool {
	if v, ok := f.enabled.Load().(map[Feature]bool)[key]; ok {
		return v
	}
	return f.known.Load().(map[Feature]FeatureSpec)[key].Default
}

// AddFlag adds a flag for setting global feature gates to the specified FlagSet.
func (f *featureGate) AddFlag(fs *pflag.FlagSet) {
	f.lock.Lock()
	// TODO(mtaufen): Shouldn't we just close it on the first Set/SetFromMap instead?
	// Not all components expose a feature gates flag using this AddFlag method, and
	// in the future, all components will completely stop exposing a feature gates flag,
	// in favor of componentconfig.
	f.closed = true
	f.lock.Unlock()

	known := f.KnownFeatures()
	fs.Var(f, flagName, ""+
		"A set of key=value pairs that describe feature gates for alpha/experimental features. "+
		"Options are:\n"+strings.Join(known, "\n"))
}

// KnownFeatures returns a slice of strings describing the FeatureGate's known features.
// Deprecated and GA features are hidden from the list.
func (f *featureGate) KnownFeatures() []string {
	var known []string
	for k, v := range f.known.Load().(map[Feature]FeatureSpec) {
		if v.PreRelease == GA || v.PreRelease == Deprecated {
			continue
		}
		known = append(known, fmt.Sprintf("%s=true|false (%s - default=%t)", k, v.PreRelease, v.Default))
	}
	sort.Strings(known)
	return known
}

// DeepCopy returns a deep copy of the FeatureGate object, such that gates can be
// set on the copy without mutating the original. This is useful for validating
// config against potential feature gate changes before committing those changes.
func (f *featureGate) DeepCopy() FeatureGate {
	// Copy existing state.
	known := map[Feature]FeatureSpec{}
	for k, v := range f.known.Load().(map[Feature]FeatureSpec) {
		known[k] = v
	}
	enabled := map[Feature]bool{}
	for k, v := range f.enabled.Load().(map[Feature]bool) {
		enabled[k] = v
	}

	// Store copied state in new atomics.
	knownValue := &atomic.Value{}
	knownValue.Store(known)
	enabledValue := &atomic.Value{}
	enabledValue.Store(enabled)

	// Construct a new featureGate around the copied state.
	// Note that specialFeatures is treated as immutable by convention,
	// and we maintain the value of f.closed across the copy.
	return &featureGate{
		special: specialFeatures,
		known:   knownValue,
		enabled: enabledValue,
		closed:  f.closed,
	}
}