Add the ability to skip unpacking the initial file system (#2234)

- Adds a new option, InitialFSUnpacked
- When opts.InitialFSUnpacked is true, the first stage builder will
  skip unpacking the file system; later stages are unaffected

Signed-off-by: Natalie Arellano <narellano@vmware.com>

Signed-off-by: Natalie Arellano <narellano@vmware.com>

pkg/config/options.go

@@ -82,6 +82,7 @@ type KanikoOptions struct {
 	CacheCopyLayers          bool
 	CacheRunLayers           bool
 	ForceBuildMetadata       bool
+	InitialFSUnpacked        bool
 }
 
 type KanikoGitOptions struct {

pkg/executor/build.go

@@ -55,6 +55,7 @@ const emptyTarSize = 1024
 // for testing
 var (
 	initializeConfig = initConfig
+	getFSFromImage   = util.GetFSFromImage
 )
 
 type cachePusher func(*config.KanikoOptions, string, string, string) error
@@ -322,12 +323,15 @@ func (s *stageBuilder) build() error {
 	if len(s.crossStageDeps[s.stage.Index]) > 0 {
 		shouldUnpack = true
 	}
+	if s.stage.Index == 0 && s.opts.InitialFSUnpacked {
+		shouldUnpack = false
+	}
 
 	if shouldUnpack {
 		t := timing.Start("FS Unpacking")
 
 		retryFunc := func() error {
-			_, err := util.GetFSFromImage(config.RootDir, s.image, util.ExtractFile)
+			_, err := getFSFromImage(config.RootDir, s.image, util.ExtractFile)
 			return err
 		}
 
@@ -622,7 +626,6 @@ func DoBuild(opts *config.KanikoOptions) (v1.Image, error) {
 	var args *dockerfile.BuildArgs
 
 	for index, stage := range kanikoStages {
-
 		sb, err := newStageBuilder(
 			args, opts, stage,
 			crossStageDependencies,

pkg/executor/build_test.go

@@ -727,17 +727,20 @@ func Test_stageBuilder_populateCompositeKey(t *testing.T) {
 
 func Test_stageBuilder_build(t *testing.T) {
 	type testcase struct {
-		description       string
-		opts              *config.KanikoOptions
-		args              map[string]string
-		layerCache        *fakeLayerCache
-		expectedCacheKeys []string
-		pushedCacheKeys   []string
-		commands          []commands.DockerCommand
-		fileName          string
-		rootDir           string
-		image             v1.Image
-		config            *v1.ConfigFile
+		description        string
+		opts               *config.KanikoOptions
+		args               map[string]string
+		layerCache         *fakeLayerCache
+		expectedCacheKeys  []string
+		pushedCacheKeys    []string
+		commands           []commands.DockerCommand
+		fileName           string
+		rootDir            string
+		image              v1.Image
+		config             *v1.ConfigFile
+		stage              config.KanikoStage
+		crossStageDeps     map[int][]string
+		mockGetFSFromImage func(root string, img v1.Image, extract util.ExtractFunction) ([]string, error)
 	}
 
 	testCases := []testcase{
@@ -1238,6 +1241,15 @@ RUN foobar
 				rootDir:           dir,
 			}
 		}(),
+		{
+			description:    "fs unpacked",
+			opts:           &config.KanikoOptions{InitialFSUnpacked: true},
+			stage:          config.KanikoStage{Index: 0},
+			crossStageDeps: map[int][]string{0: {"some-dep"}},
+			mockGetFSFromImage: func(root string, img v1.Image, extract util.ExtractFunction) ([]string, error) {
+				return nil, fmt.Errorf("getFSFromImage shouldn't be called if fs is already unpacked")
+			},
+		},
 	}
 	for _, tc := range testCases {
 		t.Run(tc.description, func(t *testing.T) {
@@ -1294,6 +1306,13 @@ RUN foobar
 			if tc.rootDir != "" {
 				config.RootDir = tc.rootDir
 			}
+			sb.stage = tc.stage
+			sb.crossStageDeps = tc.crossStageDeps
+			if tc.mockGetFSFromImage != nil {
+				original := getFSFromImage
+				defer func() { getFSFromImage = original }()
+				getFSFromImage = tc.mockGetFSFromImage
+			}
 			err := sb.build()
 			if err != nil {
 				t.Errorf("Expected error to be nil but was %v", err)