Implement windowsDiff.Compare via hcsshim/pkg/ociwclayer

This parallels the implementation of windowsDiff.Apply, including
bouncing very briefly though archive.WriteDiff and then straight back
out into Windows-specific code.

It's mostly pulling existing mechanisms from non-Windows Compare or
Windows Apply, and highlights that there's probably a lot of scope for
refactoring on top of this.

Now the export-related integration tests pass CI on Windows.

Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>

archive/tar.go

@@ -63,13 +63,34 @@ func Diff(ctx context.Context, a, b string) io.ReadCloser {
 }
 
 // WriteDiff writes a tar stream of the computed difference between the
-// provided directories.
+// provided paths.
 //
 // Produces a tar using OCI style file markers for deletions. Deleted
 // files will be prepended with the prefix ".wh.". This style is
 // based off AUFS whiteouts.
 // See https://github.com/opencontainers/image-spec/blob/master/layer.md
-func WriteDiff(ctx context.Context, w io.Writer, a, b string) error {
+func WriteDiff(ctx context.Context, w io.Writer, a, b string, opts ...WriteDiffOpt) error {
+	var options WriteDiffOptions
+	for _, opt := range opts {
+		if err := opt(&options); err != nil {
+			return errors.Wrap(err, "failed to apply option")
+		}
+	}
+	if options.writeDiffFunc == nil {
+		options.writeDiffFunc = writeDiffNaive
+	}
+
+	return options.writeDiffFunc(ctx, w, a, b, options)
+}
+
+// writeDiffNaive writes a tar stream of the computed difference between the
+// provided directories on disk.
+//
+// Produces a tar using OCI style file markers for deletions. Deleted
+// files will be prepended with the prefix ".wh.". This style is
+// based off AUFS whiteouts.
+// See https://github.com/opencontainers/image-spec/blob/master/layer.md
+func writeDiffNaive(ctx context.Context, w io.Writer, a, b string, _ WriteDiffOptions) error {
 	cw := newChangeWriter(w, b)
 	err := fs.Changes(ctx, a, b, cw.HandleChange)
 	if err != nil {

archive/tar_linux_test.go

@@ -70,7 +70,7 @@ func TestOverlayApplyNoParents(t *testing.T) {
 	}
 	fstest.FSSuite(t, overlayDiffApplier{
 		tmp: base,
-		diff: func(ctx context.Context, w io.Writer, a, b string) error {
+		diff: func(ctx context.Context, w io.Writer, a, b string, _ ...WriteDiffOpt) error {
 			cw := newChangeWriter(w, b)
 			cw.addedDirs = nil
 			err := fs.Changes(ctx, a, b, cw.HandleChange)
@@ -85,7 +85,7 @@ func TestOverlayApplyNoParents(t *testing.T) {
 
 type overlayDiffApplier struct {
 	tmp  string
-	diff func(context.Context, io.Writer, string, string) error
+	diff func(context.Context, io.Writer, string, string, ...WriteDiffOpt) error
 	t    *testing.T
 }
 

archive/tar_opts.go

@@ -73,3 +73,13 @@ func WithParents(p []string) ApplyOpt {
 		return nil
 	}
 }
+
+// WriteDiffOptions provides additional options for a WriteDiff operation
+type WriteDiffOptions struct {
+	ParentLayers []string // Windows needs the full list of parent layers
+
+	writeDiffFunc func(context.Context, io.Writer, string, string, WriteDiffOptions) error
+}
+
+// WriteDiffOpt allows setting mutable archive write properties on creation
+type WriteDiffOpt func(options *WriteDiffOptions) error

archive/tar_opts_windows.go

@@ -40,3 +40,33 @@ func AsWindowsContainerLayer() ApplyOpt {
 		return nil
 	}
 }
+
+// writeDiffWindowsLayers writes a tar stream of the computed difference between the
+// provided Windows layers
+//
+// Produces a tar using OCI style file markers for deletions. Deleted
+// files will be prepended with the prefix ".wh.". This style is
+// based off AUFS whiteouts.
+// See https://github.com/opencontainers/image-spec/blob/master/layer.md
+func writeDiffWindowsLayers(ctx context.Context, w io.Writer, _, layer string, options WriteDiffOptions) error {
+	return ociwclayer.ExportLayerToTar(ctx, w, layer, options.ParentLayers)
+}
+
+// AsWindowsContainerLayerPair indicates that the paths to diff are a pair of
+// Windows Container Layers. The caller must be holding SeBackupPrivilege.
+func AsWindowsContainerLayerPair() WriteDiffOpt {
+	return func(options *WriteDiffOptions) error {
+		options.writeDiffFunc = writeDiffWindowsLayers
+		return nil
+	}
+}
+
+// WithParentLayers provides the Windows Container Layers that are the parents
+// of the target (right-hand, "upper") layer, if any. The source (left-hand, "lower")
+// layer passed to WriteDiff should be "" in this case.
+func WithParentLayers(p []string) WriteDiffOpt {
+	return func(options *WriteDiffOptions) error {
+		options.ParentLayers = p
+		return nil
+	}
+}