Pass disk handle for FormatWritableLayerVhd on RS5

On RS5 the HcsFormatWritableLayerVhd call expects to receive a disk handle.
On 19h1+ you can pass a vhd handle and internally they will do the same
work we're doing in this change to grab a disk handle to perform the format.

Signed-off-by: Daniel Canter <dcanter@microsoft.com>

computestorage/format.go

@@ -2,13 +2,41 @@ package computestorage
 
 import (
 	"context"
+	"os"
+	"syscall"
 
+	"github.com/Microsoft/go-winio/vhd"
 	"github.com/Microsoft/hcsshim/internal/oc"
+	"github.com/Microsoft/hcsshim/osversion"
 	"github.com/pkg/errors"
 	"go.opencensus.io/trace"
 	"golang.org/x/sys/windows"
 )
 
+func openDisk(path string) (windows.Handle, error) {
+	u16, err := windows.UTF16PtrFromString(path)
+	if err != nil {
+		return 0, err
+	}
+	h, err := windows.CreateFile(
+		u16,
+		windows.GENERIC_READ|windows.GENERIC_WRITE,
+		windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE,
+		nil,
+		windows.OPEN_EXISTING,
+		windows.FILE_ATTRIBUTE_NORMAL|windows.FILE_FLAG_NO_BUFFERING,
+		0,
+	)
+	if err != nil {
+		return 0, &os.PathError{
+			Op:   "CreateFile",
+			Path: path,
+			Err:  err,
+		}
+	}
+	return h, nil
+}
+
 // FormatWritableLayerVhd formats a virtual disk for use as a writable container layer.
 //
 // If the VHD is not mounted it will be temporarily mounted.
@@ -18,9 +46,31 @@ func FormatWritableLayerVhd(ctx context.Context, vhdHandle windows.Handle) (err
 	defer span.End()
 	defer func() { oc.SetSpanStatus(span, err) }()
 
-	err = hcsFormatWritableLayerVhd(vhdHandle)
+	h := vhdHandle
+	// On RS5 HcsFormatWritableLayerVhd expects to receive a disk handle instead of a vhd handle.
+	if osversion.Build() < osversion.V19H1 {
+		if err := vhd.AttachVirtualDisk(syscall.Handle(vhdHandle), vhd.AttachVirtualDiskFlagNone, &vhd.AttachVirtualDiskParameters{Version: 1}); err != nil {
+			return err
+		}
+		defer func() {
+			if detachErr := vhd.DetachVirtualDisk(syscall.Handle(vhdHandle)); err != nil {
+				err = detachErr
+			}
+		}()
+		diskPath, err := vhd.GetVirtualDiskPhysicalPath(syscall.Handle(vhdHandle))
+		if err != nil {
+			return err
+		}
+		diskHandle, err := openDisk(diskPath)
+		if err != nil {
+			return err
+		}
+		defer windows.CloseHandle(diskHandle) // nolint: errcheck
+		h = diskHandle
+	}
+	err = hcsFormatWritableLayerVhd(h)
 	if err != nil {
 		return errors.Wrap(err, "failed to format writable layer vhd")
 	}
-	return nil
+	return
 }