Remove dependency on tar2vhd for LCOW differ

This change no longer requires the use of a UtlityVM on Windows to convert the
layer tar to an ext4 vhd for LCOW. This has a significant performance boost
that makes linux/amd64 layer extraction comparable to native Linux performance.

Signed-off-by: Justin Terry (VM) <juterry@microsoft.com>

diff/lcow/lcow.go

@@ -21,10 +21,11 @@ package lcow
 import (
 	"context"
 	"io"
-	"os/exec"
+	"os"
 	"path"
 	"time"
 
+	"github.com/Microsoft/hcsshim/ext4/tar2ext4"
 	"github.com/containerd/containerd/archive/compression"
 	"github.com/containerd/containerd/content"
 	"github.com/containerd/containerd/diff"
@@ -40,6 +41,11 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
+const (
+	// maxLcowVhdSizeGB is the max size in GB of any layer
+	maxLcowVhdSizeGB = 128 * 1024 * 1024 * 1024
+)
+
 func init() {
 	plugin.Register(&plugin.Registration{
 		Type: plugin.DiffPlugin,
@@ -131,15 +137,22 @@ func (s windowsLcowDiff) Apply(ctx context.Context, desc ocispec.Descriptor, mou
 		r: io.TeeReader(rdr, digester.Hash()),
 	}
 
-	cmd := exec.Command(
-		"runhcs.exe",
-		"tar2vhd",
-		"--scratchpath", path.Join(layer, "sandbox.vhdx"), // TODO: JTERRY75 when the snapshotter changes this to be scratch.vhdx update it here too.
-		"--destpath", path.Join(layer, "layer.vhd"))
+	layerPath := path.Join(layer, "layer.vhd")
+	outFile, err := os.Create(layerPath)
+	if err != nil {
+		return emptyDesc, err
+	}
+	defer outFile.Close()
+	defer func() {
+		if err != nil {
+			outFile.Close()
+			os.Remove(layerPath)
+		}
+	}()
 
-	cmd.Stdin = rc
-	if bytes, err := cmd.CombinedOutput(); err != nil {
-		return emptyDesc, errors.Wrapf(err, "failed to exec runhcs.exe tar2vhd: %s", string(bytes))
+	err = tar2ext4.Convert(rc, outFile, tar2ext4.ConvertWhiteout, tar2ext4.AppendVhdFooter, tar2ext4.MaximumDiskSize(maxLcowVhdSizeGB))
+	if err != nil {
+		return emptyDesc, errors.Wrapf(err, "failed to convert tar to ext4 vhd")
 	}
 
 	return ocispec.Descriptor{