add some tests

rootfs/BUILD

@@ -8,4 +8,8 @@ go_library(
     ],
     importpath = "github.com/motiejus/code/undocker/rootfs",
     visibility = ["//visibility:public"],
+    deps = [
+        "@com_github_stretchr_testify//assert",
+        "@com_github_stretchr_testify//require",
+    ],
 )

rootfs/rootfs.go

@@ -3,7 +3,6 @@ package rootfs
 import (
 	"archive/tar"
 	"encoding/json"
-	"fmt"
 	"io"
 	"strings"
 )
@@ -50,12 +49,12 @@ func RootFS(in io.ReadSeeker, out io.Writer) error {
 		case hdr.Name == _manifestJSON:
 			dec := json.NewDecoder(tr)
 			if err := dec.Decode(&manifest); err != nil {
-				return fmt.Errorf("decode manifest.json: %w", err)
+				return err
 			}
 		case strings.HasSuffix(hdr.Name, "/layer.tar"):
 			here, err := in.Seek(0, io.SeekCurrent)
 			if err != nil {
-				return fmt.Errorf("seek: %w", err)
+				return err
 			}
 			layerOffsets[hdr.Name] = here
 		}
@@ -74,7 +73,7 @@ func RootFS(in io.ReadSeeker, out io.Writer) error {
 	// for all kinds of files.
 	for i, offset := range layers {
 		if _, err := in.Seek(offset, io.SeekStart); err != nil {
-			fmt.Errorf("seek: %w", err)
+			return err
 		}
 		tr = tar.NewReader(in)
 
@@ -83,6 +82,9 @@ func RootFS(in io.ReadSeeker, out io.Writer) error {
 			if err == io.EOF {
 				break
 			}
+			if err != nil {
+				return err
+			}
 			file2layer[hdr.Name] = i
 		}
 	}
@@ -90,7 +92,7 @@ func RootFS(in io.ReadSeeker, out io.Writer) error {
 	// phase 3: iterate through all layers and write files.
 	for i, offset := range layers {
 		if _, err := in.Seek(offset, io.SeekStart); err != nil {
-			fmt.Errorf("seek: %w", err)
+			return err
 		}
 		tr = tar.NewReader(in)
 
@@ -99,6 +101,9 @@ func RootFS(in io.ReadSeeker, out io.Writer) error {
 			if err == io.EOF {
 				break
 			}
+			if err != nil {
+				return err
+			}
 			if file2layer[hdr.Name] != i {
 				continue
 			}

rootfs/rootfs_tests.go

@@ -4,7 +4,11 @@ import (
 	"archive/tar"
 	"bytes"
 	"encoding/json"
+	"io"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 type layers []string
@@ -24,8 +28,8 @@ type tarrable interface {
 
 type file struct {
 	name     string
-	contents []byte
 	uid      int
+	contents []byte
 }
 
 func (f file) tar(tw *tar.Writer) {
@@ -56,6 +60,27 @@ func (t tarball) bytes() []byte {
 	return buf.Bytes()
 }
 
+func extract(t *testing.T, tarball io.Reader) []file {
+	t.Helper()
+	var ret []file
+	tr := tar.NewReader(tarball)
+	for {
+		hdr, err := tr.Next()
+		if err == io.EOF {
+			break
+		}
+		require.NoError(t, err)
+
+		elem := file{name: hdr.Name, uid: hdr.Uid}
+		if hdr.Typeflag == tar.TypeReg {
+			buf := bytes.Buffer{}
+			io.Copy(&buf, tr)
+			elem.contents = buf.Bytes()
+		}
+	}
+	return ret
+}
+
 var (
 	_layer0 = tarball{
 		dir{name: "/", uid: 0},
@@ -77,6 +102,35 @@ var (
 	}
 )
 
-func RootFSTest(t *testing.T) {
+func TestRootFS(t *testing.T) {
+	tests := []struct {
+		name  string
+		image tarball
+		want  []file
+	}{
+		{
+			name:  "empty",
+			image: tarball{},
+			want:  []file{},
+		},
+		{
+			name:  "basic overwrite, 2 layers",
+			image: _image,
+			want: []file{
+				{name: "/", uid: 1},
+				{name: "file", uid: 0, contents: []byte("from 1")},
+			},
+		},
+	}
 
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			in := bytes.NewReader(tt.image.bytes())
+			out := bytes.Buffer{}
+
+			require.NoError(t, RootFS(in, &out))
+			got := extract(t, &out)
+			assert.Equal(t, tt.want, got)
+		})
+	}
 }