Implement proper renameFile on Windows

Symlinks on Windows are implemented using NTFS reparse points. Reparse
points are also used when creating volume mounts on Windows. As far as
golang is concerned, mountpoints and symlinks look the same. This change
makes the distinction between the two and allows safely renaming
symlinks, while erring out for anything else that looks like a symlink.

Signed-off-by: Gabriel Adrian Samfira <[email protected]>

diskwriter.go

@@ -205,7 +205,8 @@ func (dw *DiskWriter) HandleChange(kind ChangeKind, p string, fi os.FileInfo, er
 				return errors.Wrapf(err, "failed to remove %s", destPath)
 			}
 		}
-		if err := os.Rename(newPath, destPath); err != nil {
+
+		if err := renameFile(newPath, destPath); err != nil {
 			return errors.Wrapf(err, "failed to rename %s to %s", newPath, destPath)
 		}
 	}

diskwriter_unix.go

@@ -51,3 +51,10 @@ func handleTarTypeBlockCharFifo(path string, stat *types.Stat) error {
 	}
 	return nil
 }
+
+func renameFile(src, dst string) error {
+	if err := os.Rename(src, dst); err != nil {
+		return errors.Wrapf(err, "failed to rename %s to %s", src, dst)
+	}
+	return nil
+}

diskwriter_windows.go

@@ -1,8 +1,15 @@
+//go:build windows
 // +build windows
 
 package fsutil
 
 import (
+	"fmt"
+	ioFS "io/fs"
+	"os"
+	"syscall"
+
+	"github.com/Microsoft/go-winio"
 	"github.com/pkg/errors"
 	"github.com/tonistiigi/fsutil/types"
 )
@@ -16,3 +23,75 @@ func rewriteMetadata(p string, stat *types.Stat) error {
 func handleTarTypeBlockCharFifo(path string, stat *types.Stat) error {
 	return errors.New("Not implemented on windows")
 }
+
+func getFileHandle(path string, info ioFS.FileInfo) (syscall.Handle, error) {
+	p, err := syscall.UTF16PtrFromString(path)
+	if err != nil {
+		return 0, errors.Wrap(err, "converting string to UTF-16")
+	}
+	attrs := uint32(syscall.FILE_FLAG_BACKUP_SEMANTICS)
+	if info.Mode()&os.ModeSymlink != 0 {
+		// Use FILE_FLAG_OPEN_REPARSE_POINT, otherwise CreateFile will follow symlink.
+		// See https://docs.microsoft.com/en-us/windows/desktop/FileIO/symbolic-link-effects-on-file-systems-functions#createfile-and-createfiletransacted
+		attrs |= syscall.FILE_FLAG_OPEN_REPARSE_POINT
+	}
+	h, err := syscall.CreateFile(p, 0, 0, nil, syscall.OPEN_EXISTING, attrs, 0)
+	if err != nil {
+		return 0, errors.Wrap(err, "getting file handle")
+	}
+	return h, nil
+}
+
+func readlink(path string, info ioFS.FileInfo) ([]byte, error) {
+	h, err := getFileHandle(path, info)
+	if err != nil {
+		return nil, errors.Wrap(err, "getting file handle")
+	}
+	defer syscall.CloseHandle(h)
+
+	rdbbuf := make([]byte, syscall.MAXIMUM_REPARSE_DATA_BUFFER_SIZE)
+	var bytesReturned uint32
+	err = syscall.DeviceIoControl(h, syscall.FSCTL_GET_REPARSE_POINT, nil, 0, &rdbbuf[0], uint32(len(rdbbuf)), &bytesReturned, nil)
+	if err != nil {
+		return nil, errors.Wrap(err, "sending I/O control command")
+	}
+	return rdbbuf[:bytesReturned], nil
+}
+
+func getReparsePoint(path string, info ioFS.FileInfo) (*winio.ReparsePoint, error) {
+	target, err := readlink(path, info)
+	if err != nil {
+		return nil, errors.Wrap(err, "fetching link")
+	}
+	rp, err := winio.DecodeReparsePoint(target)
+	if err != nil {
+		return nil, errors.Wrap(err, "decoding reparse point")
+	}
+	return rp, nil
+}
+
+func renameFile(src, dst string) error {
+	info, err := os.Lstat(dst)
+	if err != nil {
+		if !os.IsNotExist(err) {
+			return errors.Wrap(err, "getting file info")
+		}
+	}
+
+	if info != nil && info.Mode()&os.ModeSymlink != 0 {
+		dstInfoRp, err := getReparsePoint(dst, info)
+		if err != nil {
+			return errors.Wrap(err, "getting reparse point")
+		}
+		if dstInfoRp.IsMountPoint {
+			return fmt.Errorf("%s is a mount point", dst)
+		}
+		if err := os.Remove(dst); err != nil {
+			return errors.Wrapf(err, "removing %s", dst)
+		}
+	}
+	if err := os.Rename(src, dst); err != nil {
+		return errors.Wrapf(err, "failed to rename %s to %s", src, dst)
+	}
+	return nil
+}