Don't create container scratch per base layer (#2002)

For WCIFS based layers, a container scratch base VHD (and a differencing VHD) both are created per unique base
layer. However, with UnionFS we
don't add any reparse points and the VHD is empty, so we don't need to create a VHD per unique base layer. Now
the CimFS snapshotter will handle container scratch VHD creation and the LayerWriter will only create the VHD
for the UtilityVM. (The UtilityVM VHD still needs to be created per unique base layer since the BCD of that
layer is configured to boot from the UtilityVM VHD and the BCD is unique per image)

Signed-off-by: Amit Barve <ambarve@microsoft.com>

internal/wclayer/cim/process.go

@@ -22,65 +22,6 @@ import (
 
 const defaultVHDXBlockSizeInMB = 1
 
-func createContainerBaseLayerVHDs(ctx context.Context, layerPath string) (err error) {
-	baseVhdPath := filepath.Join(layerPath, wclayer.ContainerBaseVhd)
-	diffVhdPath := filepath.Join(layerPath, wclayer.ContainerScratchVhd)
-	defaultVhdSize := uint64(20)
-
-	if _, err := os.Stat(baseVhdPath); err == nil {
-		if err := os.RemoveAll(baseVhdPath); err != nil {
-			return fmt.Errorf("failed to remove base vhdx path:  %w", err)
-		}
-	}
-	if _, err := os.Stat(diffVhdPath); err == nil {
-		if err := os.RemoveAll(diffVhdPath); err != nil {
-			return fmt.Errorf("failed to remove differencing vhdx: %w", err)
-		}
-	}
-
-	createParams := &vhd.CreateVirtualDiskParameters{
-		Version: 2,
-		Version2: vhd.CreateVersion2{
-			MaximumSize:      defaultVhdSize * memory.GiB,
-			BlockSizeInBytes: defaultVHDXBlockSizeInMB * memory.MiB,
-		},
-	}
-	handle, err := vhd.CreateVirtualDisk(baseVhdPath, vhd.VirtualDiskAccessNone, vhd.CreateVirtualDiskFlagNone, createParams)
-	if err != nil {
-		return fmt.Errorf("failed to create vhdx: %w", err)
-	}
-
-	defer func() {
-		if err != nil {
-			os.RemoveAll(baseVhdPath)
-			os.RemoveAll(diffVhdPath)
-		}
-	}()
-
-	err = computestorage.FormatWritableLayerVhd(ctx, windows.Handle(handle))
-	// we always wanna close the handle whether format succeeds for not.
-	closeErr := syscall.CloseHandle(handle)
-	if err != nil {
-		return err
-	} else if closeErr != nil {
-		return fmt.Errorf("failed to close vhdx handle: %w", closeErr)
-	}
-
-	// Create the differencing disk that will be what's copied for the final rw layer
-	// for a container.
-	if err = vhd.CreateDiffVhd(diffVhdPath, baseVhdPath, defaultVHDXBlockSizeInMB); err != nil {
-		return fmt.Errorf("failed to create differencing disk: %w", err)
-	}
-
-	if err = security.GrantVmGroupAccess(baseVhdPath); err != nil {
-		return fmt.Errorf("failed to grant vm group access to %s: %w", baseVhdPath, err)
-	}
-	if err = security.GrantVmGroupAccess(diffVhdPath); err != nil {
-		return fmt.Errorf("failed to grant vm group access to %s: %w", diffVhdPath, err)
-	}
-	return nil
-}
-
 // processUtilityVMLayer is similar to createContainerBaseLayerVHDs but along with the scratch creation it
 // also does some BCD modifications to allow the UVM to boot from the CIM. It expects that the UVM BCD file is
 // present at layerPath/`wclayer.BcdFilePath` and a UVM SYSTEM hive is present at
@@ -228,10 +169,6 @@ func processLayoutFile(layerPath string) ([]pendingCimOp, error) {
 // inside the cim, some registry file links must be updated. This function takes care of all those
 // steps. This function opens the cim file for writing and updates it.
 func (cw *CimLayerWriter) processBaseLayer(ctx context.Context, processUtilityVM bool) (err error) {
-	if err = createContainerBaseLayerVHDs(ctx, cw.path); err != nil {
-		return fmt.Errorf("failed to create container base VHDs: %w", err)
-	}
-
 	if processUtilityVM {
 		if err = processUtilityVMLayer(ctx, cw.path); err != nil {
 			return fmt.Errorf("process utilityVM layer: %w", err)