os: TempDir uses GetTempPath2 on Windows if available

This generates GetTempPath2. Go now tries to determine if the windows
it runs on has GetTempPath2 by finding it only once at the loading time.
If GetTempPath2 exists, it sets the flag so that any calls to tempDir
will use it. If it doesn't exist, Go then uses GetTempPath.
GetTempPath2 was generated into internal/syscall/windows since
syscall is locked down.

Fixes golang#56899

src/internal/syscall/windows/syscall_windows.go

@@ -151,6 +151,7 @@ const (
 //sys	GetModuleFileName(module syscall.Handle, fn *uint16, len uint32) (n uint32, err error) = kernel32.GetModuleFileNameW
 //sys	SetFileInformationByHandle(handle syscall.Handle, fileInformationClass uint32, buf uintptr, bufsize uint32) (err error) = kernel32.SetFileInformationByHandle
 //sys	VirtualQuery(address uintptr, buffer *MemoryBasicInformation, length uintptr) (err error) = kernel32.VirtualQuery
+//sys	GetTempPath2(buflen uint32, buf *uint16) (n uint32, err error) = GetTempPath2W
 
 const (
 	// flags for CreateToolhelp32Snapshot
@@ -363,6 +364,10 @@ func LoadGetFinalPathNameByHandle() error {
 	return procGetFinalPathNameByHandleW.Find()
 }
 
+func ErrorLoadingGetTempPath2() error {
+	return procGetTempPath2W.Find()
+}
+
 //sys	CreateEnvironmentBlock(block **uint16, token syscall.Token, inheritExisting bool) (err error) = userenv.CreateEnvironmentBlock
 //sys	DestroyEnvironmentBlock(block *uint16) (err error) = userenv.DestroyEnvironmentBlock
 

src/os/file_windows.go

@@ -9,6 +9,7 @@ import (
 	"internal/poll"
 	"internal/syscall/windows"
 	"runtime"
+	"sync"
 	"syscall"
 	"unicode/utf16"
 	"unsafe"
@@ -299,11 +300,23 @@ func Pipe() (r *File, w *File, err error) {
 	return newFile(p[0], "|0", "pipe"), newFile(p[1], "|1", "pipe"), nil
 }
 
+var (
+	useGetTempPath2Once sync.Once
+	useGetTempPath2     bool
+)
+
 func tempDir() string {
+	useGetTempPath2Once.Do(func() {
+		useGetTempPath2 = (windows.ErrorLoadingGetTempPath2() == nil)
+	})
+	getTempPath := syscall.GetTempPath
+	if useGetTempPath2 {
+		getTempPath = windows.GetTempPath2
+	}
 	n := uint32(syscall.MAX_PATH)
 	for {
 		b := make([]uint16, n)
-		n, _ = syscall.GetTempPath(uint32(len(b)), &b[0])
+		n, _ = getTempPath(uint32(len(b)), &b[0])
 		if n > uint32(len(b)) {
 			continue
 		}