add WithFilePrefix

This adds an option to prefix every file path with a given string.

rootfs/options.go

@@ -0,0 +1,20 @@
+package rootfs
+
+type options struct {
+	filePrefix string
+}
+
+type Option interface {
+	apply(*options)
+}
+
+type filePrefixOption string
+
+func (p filePrefixOption) apply(opts *options) {
+	opts.filePrefix = string(p)
+}
+
+// WithFilePrefixOption adds a prefix to all files in the output archive.
+func WithFilePrefix(p string) Option {
+	return filePrefixOption(p)
+}

rootfs/rootfs.go

@@ -35,7 +35,14 @@ type (
 // Flatten flattens a docker image to a tarball. The underlying io.Writer
 // should be an open file handle, which the caller is responsible for closing
 // themselves
-func Flatten(rd io.ReadSeeker, w io.Writer) (_err error) {
+func Flatten(rd io.ReadSeeker, w io.Writer, opts ...Option) (_err error) {
+	options := options{
+		filePrefix: "",
+	}
+	for _, o := range opts {
+		o.apply(&options)
+	}
+
 	tr := tar.NewReader(rd)
 	var closer func() error
 	var err error
@@ -177,7 +184,8 @@ func Flatten(rd io.ReadSeeker, w io.Writer) (_err error) {
 			if hdr.Typeflag != tar.TypeDir && file2layer[hdr.Name] != i {
 				continue
 			}
-			if err := writeFile(tr, tw, hdr); err != nil {
+			prefix := options.filePrefix
+			if err := writeFile(tr, tw, hdr, prefix); err != nil {
 				return err
 			}
 		}
@@ -188,10 +196,10 @@ func Flatten(rd io.ReadSeeker, w io.Writer) (_err error) {
 	return nil
 }
 
-func writeFile(tr *tar.Reader, tw *tar.Writer, hdr *tar.Header) error {
+func writeFile(tr *tar.Reader, tw *tar.Writer, hdr *tar.Header, prefix string) error {
 	hdrOut := &tar.Header{
 		Typeflag: hdr.Typeflag,
-		Name:     hdr.Name,
+		Name:     prefix + hdr.Name,
 		Linkname: hdr.Linkname,
 		Size:     hdr.Size,
 		Mode:     int64(hdr.Mode & 0777),

rootfs/rootfs_test.go

@@ -34,6 +34,7 @@ func TestRootFS(t *testing.T) {
 
 	tests := []struct {
 		name    string
+		opts    []Option
 		image   tarball
 		want    []extractable
 		wantErr string
@@ -54,7 +55,7 @@ func TestRootFS(t *testing.T) {
 			wantErr: "layer0/layer.tar defined in manifest, missing in tarball",
 		},
 		{
-			name: "basic file overwrite, layer order mixed",
+			name: "ok: basic file overwrite, layer order mixed",
 			image: tarball{
 				file{Name: "layer1/layer.tar", Contents: layer1.Buffer()},
 				file{Name: "layer0/layer.tar", Contents: layer0.Buffer()},
@@ -172,7 +173,7 @@ func TestRootFS(t *testing.T) {
 			},
 		},
 		{
-			name: "archived layer",
+			name: "compressed layer",
 			image: tarball{
 				file{Name: "layer1/layer.tar", Contents: layer1.Gzip()},
 				file{Name: "layer0/layer.tar", Contents: layer0.Gzip()},
@@ -183,6 +184,19 @@ func TestRootFS(t *testing.T) {
 				file{Name: "/file", UID: 1, Contents: bytes.NewBufferString("from 1")},
 			},
 		},
+		{
+			name: "ok: add a file prefix",
+			opts: []Option{WithFilePrefix("/opt")},
+			image: tarball{
+				file{Name: "layer1/layer.tar", Contents: layer1.Buffer()},
+				file{Name: "layer0/layer.tar", Contents: layer0.Buffer()},
+				manifest{"layer0/layer.tar", "layer1/layer.tar"},
+			},
+			want: []extractable{
+				dir{Name: "/opt/", UID: 0},
+				file{Name: "/opt/file", UID: 1, Contents: bytes.NewBufferString("from 1")},
+			},
+		},
 	}
 
 	for _, tt := range tests {
@@ -190,7 +204,7 @@ func TestRootFS(t *testing.T) {
 			in := bytes.NewReader(tt.image.Buffer().Bytes())
 			out := bytes.Buffer{}
 
-			err := Flatten(in, &out)
+			err := Flatten(in, &out, tt.opts...)
 			if tt.wantErr != "" {
 				if err == nil {
 					t.Fatal("expected error, got nil")