add WithFilePrefix

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

.gitignore

@@ -1,3 +1,4 @@
+/undocker
 /undocker-*
 coverage.html
 sha256sum.txt*

main.go

@@ -2,6 +2,7 @@
 package main
 
 import (
+	"flag"
 	"fmt"
 	"io"
 	"os"
@@ -15,33 +16,56 @@ var Version = "unknown"
 var VersionHash = "unknown"
 
 const _usage = `Usage:
-  %s <infile> <outfile>
+  %s [OPTION]... <infile> <outfile>
 
 Flatten a Docker container image to a root file system.
 
 Arguments:
-  <infile>:  Input Docker container. Tarball.
-  <outfile>: Output tarball, the root file system. '-' is stdout.
+  <infile>   Input Docker container. Tarball.
+  <outfile>  Output tarball, the root file system. '-' is stdout.
+
+Options:
+  --prefix=<prefix>  prefix all destination files with a given string.
 
 undocker %s (%s)
 Built with %s
 `
 
-func main() {
-	runtime.GOMAXPROCS(1) // no need to create that many threads
-
-	if len(os.Args) != 3 {
-		fmt.Fprintf(os.Stderr, _usage,
+func usage(pre string, out io.Writer) {
+	fmt.Fprintf(out, pre+_usage,
 		filepath.Base(os.Args[0]),
 		Version,
 		VersionHash,
 		runtime.Version(),
 	)
-		os.Exit(1)
+}
+
+func usageErr(pre string) {
+	usage(pre, os.Stderr)
+	os.Exit(2)
+}
+
+func main() {
+	runtime.GOMAXPROCS(1) // no need to create that many threads
+
+	var filePrefix string
+	fs := flag.NewFlagSet("undocker", flag.ExitOnError)
+	fs.Usage = func() { usageErr("") }
+	fs.StringVar(&filePrefix, "prefix", "", "prefix files in the tarball")
+
+	if len(os.Args) == 1 {
+		usageErr("")
+	}
+
+	_ = fs.Parse(os.Args[1:]) // ExitOnError captures it
+
+	args := fs.Args()
+	if len(args) != 2 {
+		usageErr("invalid number of arguments\n")
 	}
 
 	c := &command{flattener: rootfs.Flatten, Stdout: os.Stdout}
-	if err := c.execute(os.Args[1], os.Args[2]); err != nil {
+	if err := c.execute(args[0], args[1], filePrefix); err != nil {
 		fmt.Printf("Error: %v\n", err)
 		os.Exit(1)
 	}
@@ -49,11 +73,11 @@ func main() {
 }
 
 type command struct {
-	flattener func(io.ReadSeeker, io.Writer) error
+	flattener func(io.ReadSeeker, io.Writer, ...rootfs.Option) error
 	Stdout    io.Writer
 }
 
-func (c *command) execute(infile string, outfile string) (_err error) {
+func (c *command) execute(infile, outfile, filePrefix string) (_err error) {
 	rd, err := os.Open(infile)
 	if err != nil {
 		return err
@@ -84,5 +108,5 @@ func (c *command) execute(infile string, outfile string) (_err error) {
 		out = outf
 	}
 
-	return c.flattener(rd, out)
+	return c.flattener(rd, out, rootfs.WithFilePrefix(filePrefix))
 }

main_test.go

@@ -9,6 +9,8 @@ import (
 	"path/filepath"
 	"regexp"
 	"testing"
+
+	"git.sr.ht/~motiejus/undocker/rootfs"
 )
 
 func TestExecute(t *testing.T) {
@@ -17,7 +19,7 @@ func TestExecute(t *testing.T) {
 	tests := []struct {
 		name      string
 		fixture   func(*testing.T, string)
-		flattener func(io.ReadSeeker, io.Writer) error
+		flattener func(io.ReadSeeker, io.Writer, ...rootfs.Option) error
 		infile    string
 		outfile   string
 		wantErr   string
@@ -99,7 +101,7 @@ func TestExecute(t *testing.T) {
 			inf := filepath.Join(dir, tt.infile)
 
 			c := &command{Stdout: &stdout, flattener: tt.flattener}
-			err := c.execute(inf, tt.outfile)
+			err := c.execute(inf, tt.outfile, "")
 
 			if tt.assertion != nil {
 				tt.assertion(t, dir)
@@ -135,11 +137,11 @@ func TestExecute(t *testing.T) {
 	}
 }
 
-func flattenPassthrough(r io.ReadSeeker, w io.Writer) error {
+func flattenPassthrough(r io.ReadSeeker, w io.Writer, _ ...rootfs.Option) error {
 	_, err := io.Copy(w, r)
 	return err
 }
 
-func flattenBad(_ io.ReadSeeker, _ io.Writer) error {
+func flattenBad(_ io.ReadSeeker, _ io.Writer, _ ...rootfs.Option) error {
 	return errors.New("some error")
 }

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")