func downloadUnpackGadget(sto Store, model *asserts.Model, opts *Options, local *localInfos) error {
	if err := os.MkdirAll(opts.GadgetUnpackDir, 0755); err != nil {
		return fmt.Errorf("cannot create gadget unpack dir %q: %s", opts.GadgetUnpackDir, err)
	}

	dlOpts := &DownloadOptions{
		TargetDir: opts.GadgetUnpackDir,
		Channel:   opts.Channel,
	}
	snapFn, _, err := acquireSnap(sto, model.Gadget(), dlOpts, local)
	if err != nil {
		return err
	}
	// FIXME: jumping through layers here, we need to make
	//        unpack part of the container interface (again)
	snap := squashfs.New(snapFn)
	return snap.Unpack("*", opts.GadgetUnpackDir)
}

func makeStore(model *asserts.Model) Store {
	cfg := store.DefaultConfig()
	cfg.Architecture = model.Architecture()
	cfg.Series = model.Series()
	cfg.StoreID = model.Store()
	return store.New(cfg, nil)
}

func bootstrapToRootDir(sto Store, model *asserts.Model, opts *Options, local *localInfos) error {
	// FIXME: try to avoid doing this
	if opts.RootDir != "" {
		dirs.SetRootDir(opts.RootDir)
		defer dirs.SetRootDir("/")
	}

	// sanity check target
	if osutil.FileExists(dirs.SnapStateFile) {
		return fmt.Errorf("cannot bootstrap over existing system")
	}

	// TODO: developer database in home or use snapd (but need
	// a bit more API there, potential issues when crossing stores/series)
	db, err := asserts.OpenDatabase(&asserts.DatabaseConfig{
		Backstore: asserts.NewMemoryBackstore(),
		Trusted:   sysdb.Trusted(),
	})
	if err != nil {
		return err
	}
	f := makeFetcher(sto, &DownloadOptions{}, db)

	if err := f.Save(model); err != nil {
		if !osutil.GetenvBool("UBUNTU_IMAGE_SKIP_COPY_UNVERIFIED_MODEL") {
			return fmt.Errorf("cannot fetch and check prerequisites for the model assertion: %v", err)
		} else {
			logger.Noticef("Cannot fetch and check prerequisites for the model assertion, it will not be copied into the image: %v", err)
			f.addedRefs = nil
		}
	}

	// put snaps in place
	if err := os.MkdirAll(dirs.SnapBlobDir, 0755); err != nil {
		return err
	}

	snapSeedDir := filepath.Join(dirs.SnapSeedDir, "snaps")
	assertSeedDir := filepath.Join(dirs.SnapSeedDir, "assertions")
	dlOpts := &DownloadOptions{
		TargetDir: snapSeedDir,
		Channel:   opts.Channel,
		DevMode:   false, // XXX: should this be true?
	}

	for _, d := range []string{snapSeedDir, assertSeedDir} {
		if err := os.MkdirAll(d, 0755); err != nil {
			return err
		}
	}

	snaps := []string{}
	// core,kernel,gadget first
	snaps = append(snaps, local.PreferLocal(defaultCore))
	snaps = append(snaps, local.PreferLocal(model.Kernel()))
	snaps = append(snaps, local.PreferLocal(model.Gadget()))
	// then required and the user requested stuff
	for _, snapName := range model.RequiredSnaps() {
		snaps = append(snaps, local.PreferLocal(snapName))
	}
	snaps = append(snaps, opts.Snaps...)

	seen := make(map[string]bool)
	downloadedSnapsInfo := map[string]*snap.Info{}
	var seedYaml snap.Seed
	for _, snapName := range snaps {
		name := local.Name(snapName)
		if seen[name] {
			fmt.Fprintf(Stdout, "%s already prepared, skipping\n", name)
			continue
		}

		if name != snapName {
			fmt.Fprintf(Stdout, "Copying %q (%s)\n", snapName, name)
		} else {
			fmt.Fprintf(Stdout, "Fetching %s\n", snapName)
		}

		fn, info, err := acquireSnap(sto, name, dlOpts, local)
		if err != nil {
			return err
		}

		seen[name] = true

		// if it comes from the store fetch the snap assertions too
		// TODO: support somehow including available assertions
		// also for local snaps
		if info.SnapID != "" {
			err = FetchAndCheckSnapAssertions(fn, info, f, db)
			if err != nil {
				return err
			}
		}

		typ := info.Type
		// kernel/os are required for booting
		if typ == snap.TypeKernel || typ == snap.TypeOS {
			dst := filepath.Join(dirs.SnapBlobDir, filepath.Base(fn))
			if err := osutil.CopyFile(fn, dst, 0); err != nil {
//.........这里部分代码省略.........