windows/registry: improve ReadSubKeyNames permissions

The existing implementation requires QUERY_VALUE and ENUMERATE_SUB_KEYS permissions to enumerate
subkeys, so, using registry key name limits, Stat function could be excluded
from methods body and improved method requires only ENUMERATE_SUB_KEYS permission

Registry elements size limits described there:
https://msdn.microsoft.com/library/windows/desktop/ms724872.aspx

Fixes golang/go#23869

Change-Id: Id96beb9b0b294f01cc6eb1bb53bee5f50d02ea7e
Reviewed-on: https://go-review.googlesource.com/95655
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

windows/registry/key.go

@@ -113,12 +113,10 @@ func OpenRemoteKey(pcname string, k Key) (Key, error) {
 // The parameter n controls the number of returned names,
 // analogous to the way os.File.Readdirnames works.
 func (k Key) ReadSubKeyNames(n int) ([]string, error) {
-	ki, err := k.Stat()
-	if err != nil {
-		return nil, err
-	}
-	names := make([]string, 0, ki.SubKeyCount)
-	buf := make([]uint16, ki.MaxSubKeyLen+1) // extra room for terminating zero byte
+	names := make([]string, 0)
+	// 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
 loopItems:
 	for i := uint32(0); ; i++ {
 		if n > 0 {

windows/registry/registry_test.go

@@ -29,7 +29,7 @@ func randKeyName(prefix string) string {
 }
 
 func TestReadSubKeyNames(t *testing.T) {
-	k, err := registry.OpenKey(registry.CLASSES_ROOT, "TypeLib", registry.ENUMERATE_SUB_KEYS|registry.QUERY_VALUE)
+	k, err := registry.OpenKey(registry.CLASSES_ROOT, "TypeLib", registry.ENUMERATE_SUB_KEYS)
 	if err != nil {
 		t.Fatal(err)
 	}