os: reimplement windows os.Stat

Currently windows Stat uses combination of Lstat and Readlink to
walk symlinks until it reaches file or directory. Windows Readlink
is implemented via Windows DeviceIoControl(FSCTL_GET_REPARSE_POINT, ...)
call, but that call does not work on network shares or inside of
Docker container (see issues #18555 ad #19922 for details).

But Raymond Chen suggests different approach:
https://blogs.msdn.microsoft.com/oldnewthing/20100212-00/?p=14963/
- he suggests to use Windows I/O manager to dereferences the
symbolic link.

This appears to work for all normal symlinks, but also for network
shares and inside of Docker container.

This CL implements described procedure.

I also had to adjust TestStatSymlinkLoop, because the test is
expecting Stat to return syscall.ELOOP for symlink with a loop.
But new Stat returns Windows error of ERROR_CANT_RESOLVE_FILENAME
= 1921 instead. I could map ERROR_CANT_RESOLVE_FILENAME into
syscall.ELOOP, but I suspect the former is broader than later.
And ERROR_CANT_RESOLVE_FILENAME message text of "The name of
the file cannot be resolved by the system." sounds fine to me.

Fixes #10935
Fixes #18555
Fixes #19922

Change-Id: I979636064cdbdb9c7c840cf8ae73fe2c24499879
Reviewed-on: https://go-review.googlesource.com/41834
Reviewed-by: Harshavardhana <hrshvardhana@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/os_windows_test.go

@@ -667,8 +667,8 @@ func TestStatSymlinkLoop(t *testing.T) {
 	defer os.Remove("x")
 
 	_, err = os.Stat("x")
-	if perr, ok := err.(*os.PathError); !ok || perr.Err != syscall.ELOOP {
-		t.Errorf("expected *PathError with ELOOP, got %T: %v\n", err, err)
+	if _, ok := err.(*os.PathError); !ok {
+		t.Errorf("expected *PathError, got %T: %v\n", err, err)
 	}
 }
 

src/os/stat_windows.go

@@ -61,32 +61,85 @@ func (file *File) Stat() (FileInfo, error) {
 // Stat returns a FileInfo structure describing the named file.
 // If there is an error, it will be of type *PathError.
 func Stat(name string) (FileInfo, error) {
-	var fi FileInfo
-	var err error
-	link := name
-	for i := 0; i < 255; i++ {
-		fi, err = Lstat(link)
+	if len(name) == 0 {
+		return nil, &PathError{"Stat", name, syscall.Errno(syscall.ERROR_PATH_NOT_FOUND)}
+	}
+	if name == DevNull {
+		return &devNullStat, nil
+	}
+	namep, err := syscall.UTF16PtrFromString(fixLongPath(name))
+	if err != nil {
+		return nil, &PathError{"Stat", name, err}
+	}
+
+	// Use Windows I/O manager to dereferences the symbolic link, as per
+	// https://blogs.msdn.microsoft.com/oldnewthing/20100212-00/?p=14963/
+	h, err := syscall.CreateFile(namep, 0, 0, nil,
+		syscall.OPEN_EXISTING, syscall.FILE_FLAG_BACKUP_SEMANTICS, 0)
+	if err != nil {
+		if err == windows.ERROR_SHARING_VIOLATION {
+			// try FindFirstFile now that CreateFile failed
+			return statWithFindFirstFile(name, namep)
+		}
+		return nil, &PathError{"CreateFile", name, err}
+	}
+	defer syscall.CloseHandle(h)
+
+	var d syscall.ByHandleFileInformation
+	err = syscall.GetFileInformationByHandle(h, &d)
+	if err != nil {
+		return nil, &PathError{"GetFileInformationByHandle", name, err}
+	}
+	return &fileStat{
+		name: basename(name),
+		sys: syscall.Win32FileAttributeData{
+			FileAttributes: d.FileAttributes,
+			CreationTime:   d.CreationTime,
+			LastAccessTime: d.LastAccessTime,
+			LastWriteTime:  d.LastWriteTime,
+			FileSizeHigh:   d.FileSizeHigh,
+			FileSizeLow:    d.FileSizeLow,
+		},
+		vol:   d.VolumeSerialNumber,
+		idxhi: d.FileIndexHigh,
+		idxlo: d.FileIndexLow,
+		// fileStat.path is used by os.SameFile to decide, if it needs
+		// to fetch vol, idxhi and idxlo. But these are already set,
+		// so set fileStat.path to "" to prevent os.SameFile doing it again.
+		// Also do not set fileStat.filetype, because it is only used for
+		// console and stdin/stdout. But you cannot call os.Stat for these.
+	}, nil
+}
+
+// statWithFindFirstFile is used by Stat to handle special case of stating
+// c:\pagefile.sys. We might discovered other files need similar treatment.
+func statWithFindFirstFile(name string, namep *uint16) (FileInfo, error) {
+	var fd syscall.Win32finddata
+	h, err := syscall.FindFirstFile(namep, &fd)
+	if err != nil {
+		return nil, &PathError{"FindFirstFile", name, err}
+	}
+	syscall.FindClose(h)
+
+	fullpath := name
+	if !isAbs(fullpath) {
+		fullpath, err = syscall.FullPath(fullpath)
 		if err != nil {
-			return nil, err
-		}
-		if fi.Mode()&ModeSymlink == 0 {
-			fi.(*fileStat).name = basename(name)
-			return fi, nil
-		}
-		newlink, err := Readlink(link)
-		if err != nil {
-			return nil, err
-		}
-		switch {
-		case isAbs(newlink):
-			link = newlink
-		case len(newlink) > 0 && IsPathSeparator(newlink[0]):
-			link = volumeName(link) + newlink
-		default:
-			link = dirname(link) + `\` + newlink
+			return nil, &PathError{"FullPath", name, err}
 		}
 	}
-	return nil, &PathError{"Stat", name, syscall.ELOOP}
+	return &fileStat{
+		name: basename(name),
+		path: fullpath,
+		sys: syscall.Win32FileAttributeData{
+			FileAttributes: fd.FileAttributes,
+			CreationTime:   fd.CreationTime,
+			LastAccessTime: fd.LastAccessTime,
+			LastWriteTime:  fd.LastWriteTime,
+			FileSizeHigh:   fd.FileSizeHigh,
+			FileSizeLow:    fd.FileSizeLow,
+		},
+	}, nil
 }
 
 // Lstat returns the FileInfo structure describing the named file.
@@ -111,19 +164,7 @@ func Lstat(name string) (FileInfo, error) {
 			return nil, &PathError{"GetFileAttributesEx", name, e}
 		}
 		// try FindFirstFile now that GetFileAttributesEx failed
-		var fd syscall.Win32finddata
-		h, e2 := syscall.FindFirstFile(namep, &fd)
-		if e2 != nil {
-			return nil, &PathError{"FindFirstFile", name, e}
-		}
-		syscall.FindClose(h)
-
-		fs.sys.FileAttributes = fd.FileAttributes
-		fs.sys.CreationTime = fd.CreationTime
-		fs.sys.LastAccessTime = fd.LastAccessTime
-		fs.sys.LastWriteTime = fd.LastWriteTime
-		fs.sys.FileSizeHigh = fd.FileSizeHigh
-		fs.sys.FileSizeLow = fd.FileSizeLow
+		return statWithFindFirstFile(name, namep)
 	}
 	fs.path = name
 	if !isAbs(fs.path) {