linuxkit/build: implement --modified-only option

src/cmd/linuxkit/build.go

@@ -56,6 +56,7 @@ func buildCmd() *cobra.Command {
 		noSbom             bool
 		sbomOutputFilename string
 		inputTar           string
+		modifiedOnly       bool
 		sbomCurrentTime    bool
 		dryRun             bool
 	)
@@ -117,6 +118,9 @@ The generated image can be in one of multiple formats which can be run on variou
 			if inputTar != "" && pull {
 				return fmt.Errorf("cannot use --input-tar and --pull together")
 			}
+			if inputTar == "" && modifiedOnly {
+				return fmt.Errorf("--modified-only can be used together with the --input-tar")
+			}
 
 			var outfile *os.File
 			if outputFile != "" {
@@ -235,7 +239,7 @@ The generated image can be in one of multiple formats which can be run on variou
 					return fmt.Errorf("error creating sbom generator: %v", err)
 				}
 			}
-			err = moby.Build(m, w, moby.BuildOpts{Pull: pull, BuilderType: tp, DecompressKernel: decompressKernel, CacheDir: cacheDir.String(), DockerCache: docker, Arch: arch, SbomGenerator: sbomGenerator, InputTar: inputTar})
+			err = moby.Build(m, w, moby.BuildOpts{Pull: pull, BuilderType: tp, DecompressKernel: decompressKernel, CacheDir: cacheDir.String(), DockerCache: docker, Arch: arch, SbomGenerator: sbomGenerator, InputTar: inputTar, ModifiedOnly: modifiedOnly})
 			if err != nil {
 				return fmt.Errorf("%v", err)
 			}
@@ -266,6 +270,7 @@ The generated image can be in one of multiple formats which can be run on variou
 	cmd.Flags().StringVar(&arch, "arch", runtime.GOARCH, "target architecture for which to build")
 	cmd.Flags().VarP(&buildFormats, "format", "f", "Formats to create [ "+strings.Join(outputTypes, " ")+" ]")
 	cmd.Flags().StringVar(&inputTar, "input-tar", "", "path to tar from previous linuxkit build to use as input; if provided, will take files from images from this tar, using OCI images only to replace or update files. Always copies to a temporary working directory to avoid overwriting. Only works if input-tar file has the linuxkit.yaml used to build it in the exact same location. Incompatible with --pull")
+	cmd.Flags().BoolVar(&modifiedOnly, "modified-only", false, "Output tarball contains modified files only. Works only with the --input-tar option.")
 	cacheDir = flagOverEnvVarOverDefaultString{def: defaultLinuxkitCache(), envVar: envVarCacheDir}
 	cmd.Flags().Var(&cacheDir, "cache", fmt.Sprintf("Directory for caching and finding cached image, overrides env var %s", envVarCacheDir))
 	cmd.Flags().BoolVar(&noSbom, "no-sbom", false, "suppress consolidation of sboms on input container images to a single sbom and saving in the output filesystem")

src/cmd/linuxkit/moby/build.go

@@ -206,8 +206,10 @@ func Build(m Moby, w io.Writer, opts BuildOpts) error {
 		// first check if the existing one had it
 		//if config != nil && len(oldConfig.initRefs) > index+1 && oldConfig.initRefs[index].String() == image {
 		if oldConfig != nil && oldConfig.Kernel.ref != nil && oldConfig.Kernel.ref.String() == m.Kernel.ref.String() {
-			if err := extractPackageFilesFromTar(in, iw, m.Kernel.ref.String(), "kernel"); err != nil {
-				return err
+			if !opts.ModifiedOnly {
+				if err := extractPackageFilesFromTar(in, iw, m.Kernel.ref.String(), "kernel"); err != nil {
+					return err
+				}
 			}
 		} else {
 			// get kernel and initrd tarball and ucode cpio archive from container
@@ -231,8 +233,10 @@ func Build(m Moby, w io.Writer, opts BuildOpts) error {
 	apkTar := newAPKTarWriter(iw, "init")
 	for i, ii := range m.initRefs {
 		if oldConfig != nil && len(oldConfig.initRefs) > i && oldConfig.initRefs[i].String() == ii.String() {
-			if err := extractPackageFilesFromTar(in, apkTar, ii.String(), fmt.Sprintf("init[%d]", i)); err != nil {
-				return err
+			if !opts.ModifiedOnly {
+				if err := extractPackageFilesFromTar(in, apkTar, ii.String(), fmt.Sprintf("init[%d]", i)); err != nil {
+					return err
+				}
 			}
 		} else {
 			log.Infof("Process init image: %s", ii)
@@ -251,8 +255,10 @@ func Build(m Moby, w io.Writer, opts BuildOpts) error {
 	}
 	for i, image := range m.Onboot {
 		if oldConfig != nil && len(oldConfig.Onboot) > i && oldConfig.Onboot[i].Equal(image) {
-			if err := extractPackageFilesFromTar(in, iw, image.Image, fmt.Sprintf("onboot[%d]", i)); err != nil {
-				return err
+			if !opts.ModifiedOnly {
+				if err := extractPackageFilesFromTar(in, iw, image.Image, fmt.Sprintf("onboot[%d]", i)); err != nil {
+					return err
+				}
 			}
 		} else {
 			so := fmt.Sprintf("%03d", i)
@@ -267,8 +273,10 @@ func Build(m Moby, w io.Writer, opts BuildOpts) error {
 	}
 	for i, image := range m.Onshutdown {
 		if oldConfig != nil && len(oldConfig.Onshutdown) > i && oldConfig.Onshutdown[i].Equal(image) {
-			if err := extractPackageFilesFromTar(in, iw, image.Image, fmt.Sprintf("onshutdown[%d]", i)); err != nil {
-				return err
+			if !opts.ModifiedOnly {
+				if err := extractPackageFilesFromTar(in, iw, image.Image, fmt.Sprintf("onshutdown[%d]", i)); err != nil {
+					return err
+				}
 			}
 		} else {
 			so := fmt.Sprintf("%03d", i)
@@ -283,8 +291,10 @@ func Build(m Moby, w io.Writer, opts BuildOpts) error {
 	}
 	for i, image := range m.Services {
 		if oldConfig != nil && len(oldConfig.Services) > i && oldConfig.Services[i].Equal(image) {
-			if err := extractPackageFilesFromTar(in, iw, image.Image, fmt.Sprintf("services[%d]", i)); err != nil {
-				return err
+			if !opts.ModifiedOnly {
+				if err := extractPackageFilesFromTar(in, iw, image.Image, fmt.Sprintf("services[%d]", i)); err != nil {
+					return err
+				}
 			}
 		} else {
 			if err := outputImage(image, "services", i, "", m, idMap, dupMap, iw, opts); err != nil {

src/cmd/linuxkit/moby/opts.go

@@ -10,4 +10,5 @@ type BuildOpts struct {
 	Arch             string
 	SbomGenerator    *SbomGenerator
 	InputTar         string
+	ModifiedOnly     bool
 }