registry: Explain how GetMUIStringValue works and where it falls short

GetMUIStringValue tries as a convenience to resolve string values even for pathless
resource DLLs by searching the system directory (one of several paths used
by the system's standard DLL search order algorithm). This would not be
needed if regLoadMUIString searched for pathless DLLs itself, but it
doesn't, instead it needs an absolute path, otherwise it will fail.

This approach works fine for solving issue #12015 (handle localized time
zone names; for which GetMUIStringValue was created) since tzres.dll that
is used to resolve localized time zone names has no path in the registry
but is located under the system directory.

However, this approach will fail if a pathless DLL is located somewhere
else than the system directory.

Because of this limitation GetMUIStringValue may have to be revised in the
future to allow for custom paths, possibly through another version of the
function.

See also:
https://msdn.microsoft.com/en-us/library/windows/desktop/ms724890%28v=vs.85%29.aspx
https://msdn.microsoft.com/en-us/library/windows/desktop/ms682586%28v=vs.85%29.aspx

Change-Id: Ida66a0ef1928e0461ce248c795827902d785e6cd
Reviewed-on: https://go-review.googlesource.com/13929
Reviewed-by: Rob Pike <r@golang.org>

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

@@ -131,6 +131,14 @@ func (k Key) GetMUIStringValue(name string) (string, error) {
 
 	err = regLoadMUIString(syscall.Handle(k), pname, &buf[0], uint32(len(buf)), &buflen, 0, pdir)
 	if err == syscall.ERROR_FILE_NOT_FOUND { // Try fallback path
+
+		// Try to resolve the string value using the system directory as
+		// a DLL search path; this assumes the string value is of the form
+		// @[path]\dllname,-strID but with no path given, e.g. @tzres.dll,-320.
+
+		// This approach works with tzres.dll but may have to be revised
+		// in the future to allow callers to provide custom search paths.
+
 		var s string
 		s, err = ExpandString("%SystemRoot%\\system32\\")
 		if err != nil {