Merge "Change type of Once keys to OnceKey"

// Once computes a value the first time it is called with a given key per OncePer, and returns the

// value without recomputing when called with the same key.  key must be hashable.

func (once *OncePer) Once(key interface{}, value func() interface{}) interface{} {

func (once *OncePer) Once(key OnceKey, value func() interface{}) interface{} {

	// Fast path: check if the key is already in the map

	if v, ok := once.values.Load(key); ok {

		return v

// Get returns the value previously computed with Once for a given key.  If Once has not been called for the given

// key Get will panic.

func (once *OncePer) Get(key interface{}) interface{} {

func (once *OncePer) Get(key OnceKey) interface{} {

	v, ok := once.values.Load(key)

	if !ok {

		panic(fmt.Errorf("Get() called before Once()"))

}

// OnceStringSlice is the same as Once, but returns the value cast to a []string

func (once *OncePer) OnceStringSlice(key interface{}, value func() []string) []string {

func (once *OncePer) OnceStringSlice(key OnceKey, value func() []string) []string {

	return once.Once(key, func() interface{} { return value() }).([]string)

}

// OnceStringSlice is the same as Once, but returns two values cast to []string

func (once *OncePer) Once2StringSlice(key interface{}, value func() ([]string, []string)) ([]string, []string) {

func (once *OncePer) Once2StringSlice(key OnceKey, value func() ([]string, []string)) ([]string, []string) {

	type twoStringSlice [2][]string

	s := once.Once(key, func() interface{} {

		var s twoStringSlice