Prepare for a type-safe OnceKey

Add an opaque OnceKey type and use it for all calls to Once in
build/soong.  A future patch will convert the arguments to
Once* to OnceKey once users outside build/soong have been updated.

Test: onceper_test.go
Change-Id: Ifcb338e6e603e804e507203c9508d30ffb2df966

android/onceper.go

@@ -24,8 +24,6 @@ type OncePer struct {
 	valuesLock sync.Mutex
 }
 
-type valueMap map[interface{}]interface{}
-
 // 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{} {
@@ -50,6 +48,8 @@ func (once *OncePer) Once(key interface{}, value func() interface{}) interface{}
 	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{} {
 	v, ok := once.values.Load(key)
 	if !ok {
@@ -59,10 +59,12 @@ func (once *OncePer) Get(key interface{}) interface{} {
 	return v
 }
 
+// OnceStringSlice is the same as Once, but returns the value cast to a []string
 func (once *OncePer) OnceStringSlice(key interface{}, 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) {
 	type twoStringSlice [2][]string
 	s := once.Once(key, func() interface{} {
@@ -72,3 +74,21 @@ func (once *OncePer) Once2StringSlice(key interface{}, value func() ([]string, [
 	}).(twoStringSlice)
 	return s[0], s[1]
 }
+
+// OnceKey is an opaque type to be used as the key in calls to Once.
+type OnceKey struct {
+	key interface{}
+}
+
+// NewOnceKey returns an opaque OnceKey object for the provided key.  Two calls to NewOnceKey with the same key string
+// DO NOT produce the same OnceKey object.
+func NewOnceKey(key string) OnceKey {
+	return OnceKey{&key}
+}
+
+// NewCustomOnceKey returns an opaque OnceKey object for the provided key.  The key can be any type that is valid as the
+// key in a map, i.e. comparable.  Two calls to NewCustomOnceKey with key values that compare equal will return OnceKey
+// objects that access the same value stored with Once.
+func NewCustomOnceKey(key interface{}) OnceKey {
+	return OnceKey{key}
+}