Change ReadSubKeyNames signature to iterator

In order to add users and groups iterators for os/user,
ReadSubKeyNames must be changed. Instead of returning
a full slice of all subkey names, it can be changed to
accept callback function, which receives sequentially
iterated subkey names. As such, ReadSubKeyNames becomes
an iterator and will use less memory for larger sets of
data.

Author: Mantelijo

src/internal/syscall/windows/registry/key.go

@@ -87,9 +87,9 @@ func OpenKey(k Key, path string, access uint32) (Key, error) {
 	return Key(subkey), nil
 }
 
-// ReadSubKeyNames returns the names of subkeys of key k.
-func (k Key) ReadSubKeyNames() ([]string, error) {
-	names := make([]string, 0)
+// ReadSubKeyNames iterates over the names of subkeys of key k. Callback function fn receives each iterated subkey name.
+// If fn returns non-nil error, iteration is terminated and that error is returned.
+func (k Key) ReadSubKeyNames(fn func(string) error) error {
 	// Registry key size limit is 255 bytes and described there:
 	// https://msdn.microsoft.com/library/windows/desktop/ms724872.aspx
 	buf := make([]uint16, 256) //plus extra room for terminating zero byte
@@ -110,11 +110,15 @@ loopItems:
 			if err == _ERROR_NO_MORE_ITEMS {
 				break loopItems
 			}
-			return names, err
+			return err
+		}
+
+		// Callback with key name string
+		if err := fn(syscall.UTF16ToString(buf[:l])); err != nil{
+			return err
 		}
-		names = append(names, syscall.UTF16ToString(buf[:l]))
 	}
-	return names, nil
+	return nil
 }
 
 // CreateKey creates a key named path under open key k.

src/internal/syscall/windows/registry/registry_test.go

@@ -35,7 +35,11 @@ func TestReadSubKeyNames(t *testing.T) {
 	}
 	defer k.Close()
 
-	names, err := k.ReadSubKeyNames()
+	var names []string
+	err = k.ReadSubKeyNames(func(s string) error {
+		names = append(names, s)
+		return nil
+	})
 	if err != nil {
 		t.Fatal(err)
 	}

src/mime/type_windows.go

@@ -13,10 +13,15 @@ func init() {
 }
 
 func initMimeWindows() {
-	names, err := registry.CLASSES_ROOT.ReadSubKeyNames()
+	var names []string
+	err := registry.CLASSES_ROOT.ReadSubKeyNames(func(s string) error {
+		names = append(names, s)
+		return nil
+	})
 	if err != nil {
 		return
 	}
+
 	for _, name := range names {
 		if len(name) < 2 || name[0] != '.' { // looking for extensions only
 			continue

src/time/zoneinfo_windows.go

@@ -67,7 +67,11 @@ func toEnglishName(stdname, dstname string) (string, error) {
 	}
 	defer k.Close()
 
-	names, err := k.ReadSubKeyNames()
+	var names []string
+	err = k.ReadSubKeyNames(func(s string) error {
+		names = append(names, s)
+		return nil
+	})
 	if err != nil {
 		return "", err
 	}