runtime: make mksyscall_windows.go flags do what they say they do

The -systemdll and -xsys flags generate broken code in some situations
(see issue for details). Fix all that.

This CL only fixes bugs in existing code, but I have more changes comming:

golang.org/x/sys/windows is not the only package that uses mksyscall_windows.go.
golang.org/x/exp/shiny and github.com/derekparker/delve do too. I also have
few personal packages that use mksyscall_windows.go. None of those packages
are aware of new -xsys flag. I would like to change mksyscall_windows.go, so
external packages do not need to use -xsys flag. I would love to get rid of
-xsys flag altogether, but I don't see how it is possible. So I will, probably,
replace -xsys with a flag that means opposite to -xsys, and use new flag
everywhere in standard libraries. Flag name suggestions are welcome.

-systemdll flag makes users code more "secure". I would like to make -systemdll
behaviour a default for all mksyscall_windows.go users. We use that already in
standard library. If we think "secure" is important, we should encourage it in
all users code. If mksyscall_windows.go user insist on using old code, provide
-use_old_loaddll (need good name here) flag for that. So -systemdll flag will
be replaced with -use_old_loaddll.

Fixes #15167

Change-Id: I516369507867358ba1b66aabe00a17a7b477016e
Reviewed-on: https://go-review.googlesource.com/21645Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/mksyscall_windows.go

@@ -617,9 +617,6 @@ func ParseFiles(fs []string) (*Source, error) {
 			"unsafe",
 		},
 	}
-	if *systemDLL {
-		src.Import("internal/syscall/windows/sysdll")
-	}
 	for _, file := range fs {
 		if err := src.ParseFile(file); err != nil {
 			return nil, err
@@ -691,8 +688,29 @@ func (src *Source) ParseFile(path string) error {
 
 // Generate output source file from a source set src.
 func (src *Source) Generate(w io.Writer) error {
-	if *sysRepo && packageName != "windows" {
-		src.Import("golang.org/x/sys/windows")
+	const (
+		pkgStd         = iota // any package in std library
+		pkgXSysWindows        // x/sys/windows package
+		pkgOther
+	)
+	var pkgtype int
+	switch {
+	case !*sysRepo:
+		pkgtype = pkgStd
+	case packageName == "windows":
+		// TODO: this needs better logic than just using package name
+		pkgtype = pkgXSysWindows
+	default:
+		pkgtype = pkgOther
+	}
+	if *systemDLL {
+		switch pkgtype {
+		case pkgStd:
+			src.Import("internal/syscall/windows/sysdll")
+		case pkgXSysWindows:
+		default:
+			src.Import("golang.org/x/sys/windows")
+		}
 	}
 	if packageName != "syscall" {
 		src.Import("syscall")
@@ -702,18 +720,17 @@ func (src *Source) Generate(w io.Writer) error {
 		"syscalldot":  syscalldot,
 		"newlazydll": func(dll string) string {
 			arg := "\"" + dll + ".dll\""
-			if *systemDLL {
-				arg = "sysdll.Add(" + arg + ")"
-			}
-			if *sysRepo {
-				if packageName == "windows" {
-					return "NewLazySystemDLL(" + arg + ")"
-				} else {
-					return "windows.NewLazySystemDLL(" + arg + ")"
-				}
-			} else {
+			if !*systemDLL {
 				return syscalldot() + "NewLazyDLL(" + arg + ")"
 			}
+			switch pkgtype {
+			case pkgStd:
+				return syscalldot() + "NewLazyDLL(sysdll.Add(" + arg + "))"
+			case pkgXSysWindows:
+				return "NewLazySystemDLL(" + arg + ")"
+			default:
+				return "windows.NewLazySystemDLL(" + arg + ")"
+			}
 		},
 	}
 	t := template.Must(template.New("main").Funcs(funcMap).Parse(srcTemplate))