Add support for cache export

Signed-off-by: Avi Deitcher <avi@deitcher.net>

src/cmd/linuxkit/cache.go

@@ -15,6 +15,7 @@ func cacheUsage() {
 	fmt.Printf("Supported commands are\n")
 	// Please keep these in alphabetical order
 	fmt.Printf("  clean\n")
+	fmt.Printf("  export\n")
 	fmt.Printf("  ls\n")
 	fmt.Printf("\n")
 	fmt.Printf("'options' are the backend specific options.\n")
@@ -33,6 +34,8 @@ func cache(args []string) {
 		cacheClean(args[1:])
 	case "ls":
 		cacheList(args[1:])
+	case "export":
+		cacheExport(args[1:])
 	case "help", "-h", "-help", "--help":
 		cacheUsage()
 		os.Exit(0)

src/cmd/linuxkit/cache_export.go

@@ -0,0 +1,71 @@
+package main
+
+import (
+	"flag"
+	"io"
+	"os"
+	"runtime"
+
+	"github.com/containerd/containerd/reference"
+	cachepkg "github.com/linuxkit/linuxkit/src/cmd/linuxkit/cache"
+	"github.com/linuxkit/linuxkit/src/cmd/linuxkit/util"
+	log "github.com/sirupsen/logrus"
+)
+
+func cacheExport(args []string) {
+	fs := flag.NewFlagSet("export", flag.ExitOnError)
+
+	cacheDir := fs.String("cache", defaultLinuxkitCache(), "Directory for caching and finding cached image")
+	arch := fs.String("arch", runtime.GOARCH, "Architecture to resolve an index to an image, if the provided image name is an index")
+	outfile := fs.String("outfile", "", "Path to file to save output, '-' for stdout")
+
+	if err := fs.Parse(args); err != nil {
+		log.Fatal("Unable to parse args")
+	}
+
+	// get the requested images
+	if fs.NArg() < 1 {
+		log.Fatal("At least one image name is required")
+	}
+	names := fs.Args()
+	name := names[0]
+	fullname := util.ReferenceExpand(name)
+
+	p, err := cachepkg.NewProvider(*cacheDir)
+	if err != nil {
+		log.Fatalf("unable to read a local cache: %v", err)
+	}
+	desc, err := p.FindDescriptor(fullname)
+	if err != nil {
+		log.Fatalf("unable to find image named %s: %v", name, err)
+	}
+	ref, err := reference.Parse(fullname)
+	if err != nil {
+		log.Fatalf("invalid image name %s: %v", name, err)
+	}
+
+	src := p.NewSource(&ref, *arch, desc)
+	reader, err := src.V1TarReader()
+	if err != nil {
+		log.Fatalf("error getting reader for image %s: %v", name, err)
+	}
+	defer reader.Close()
+
+	// try to write the output file
+	var w io.Writer
+	switch {
+	case outfile == nil, *outfile == "":
+		log.Fatal("'outfile' flag is required")
+	case *outfile == "-":
+		w = os.Stdout
+	default:
+		f, err := os.OpenFile(*outfile, os.O_CREATE|os.O_RDWR, 0644)
+		if err != nil {
+			log.Fatalf("unable to open %s: %v", *outfile, err)
+		}
+		defer f.Close()
+		w = f
+	}
+
+	io.Copy(w, reader)
+}