buildkit/util/dockerexporter/dockerexporter.go

package dockerexporter

import (
	"archive/tar"
	"context"
	"encoding/json"
	"io"
	"path"
	"sort"

	"github.com/containerd/containerd/content"
	"github.com/containerd/containerd/images"
	"github.com/containerd/containerd/platforms"
	ocispecs "github.com/opencontainers/image-spec/specs-go"
	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
	"github.com/pkg/errors"
)

// DockerExporter implements containerd/images.Exporter to
// Docker Combined Image JSON + Filesystem Changeset Format v1.1
// https://github.com/moby/moby/blob/master/image/spec/v1.1.md#combined-image-json--filesystem-changeset-format
// The outputed tarball is also compatible wih OCI Image Format Specification
type DockerExporter struct {
	Name string
}

var _ images.Exporter = &DockerExporter{}

// Export exports tarball into writer.
func (de *DockerExporter) Export(ctx context.Context, store content.Provider, desc ocispec.Descriptor, writer io.Writer) error {
	tw := tar.NewWriter(writer)
	defer tw.Close()

	dockerManifest, err := dockerManifestRecord(ctx, store, desc, de.Name)
	if err != nil {
		return err
	}

	records := []tarRecord{
		ociLayoutFile(""),
		ociIndexRecord(desc),
		*dockerManifest,
	}

	algorithms := map[string]struct{}{}
	exportHandler := func(ctx context.Context, desc ocispec.Descriptor) ([]ocispec.Descriptor, error) {
		records = append(records, blobRecord(store, desc))
		algorithms[desc.Digest.Algorithm().String()] = struct{}{}
		return nil, nil
	}

	// Get all the children for a descriptor
	childrenHandler := images.ChildrenHandler(store)
	// Filter the childen by the platform
	childrenHandler = images.FilterPlatforms(childrenHandler, platforms.Default())

	handlers := images.Handlers(
		childrenHandler,
		images.HandlerFunc(exportHandler),
	)

	// Walk sequentially since the number of fetchs is likely one and doing in
	// parallel requires locking the export handler
	if err := images.Walk(ctx, handlers, desc); err != nil {
		return err
	}

	if len(algorithms) > 0 {
		records = append(records, directoryRecord("blobs/", 0755))
		for alg := range algorithms {
			records = append(records, directoryRecord("blobs/"+alg+"/", 0755))
		}
	}

	return writeTar(ctx, tw, records)
}

type tarRecord struct {
	Header *tar.Header
	CopyTo func(context.Context, io.Writer) (int64, error)
}

func dockerManifestRecord(ctx context.Context, provider content.Provider, desc ocispec.Descriptor, name string) (*tarRecord, error) {
	switch desc.MediaType {
	case images.MediaTypeDockerSchema2Manifest, ocispec.MediaTypeImageManifest:
		p, err := content.ReadBlob(ctx, provider, desc.Digest)
		if err != nil {
			return nil, err
		}
		var manifest ocispec.Manifest
		if err := json.Unmarshal(p, &manifest); err != nil {
			return nil, err
		}
		type mfstItem struct {
			Config   string
			RepoTags []string
			Layers   []string
		}
		item := mfstItem{
			Config: path.Join("blobs", manifest.Config.Digest.Algorithm().String(), manifest.Config.Digest.Hex()),
		}

		for _, l := range manifest.Layers {
			item.Layers = append(item.Layers, path.Join("blobs", l.Digest.Algorithm().String(), l.Digest.Hex()))
		}

		if name != "" {
			item.RepoTags = append(item.RepoTags, name)
		}

		dt, err := json.Marshal([]mfstItem{item})
		if err != nil {
			return nil, err
		}

		return &tarRecord{
			Header: &tar.Header{
				Name:     "manifest.json",
				Mode:     0444,
				Size:     int64(len(dt)),
				Typeflag: tar.TypeReg,
			},
			CopyTo: func(ctx context.Context, w io.Writer) (int64, error) {
				n, err := w.Write(dt)
				return int64(n), err
			},
		}, nil
	default:
		return nil, errors.Errorf("%v not supported for Docker exporter", desc.MediaType)
	}

}

func blobRecord(cs content.Provider, desc ocispec.Descriptor) tarRecord {
	path := "blobs/" + desc.Digest.Algorithm().String() + "/" + desc.Digest.Hex()
	return tarRecord{
		Header: &tar.Header{
			Name:     path,
			Mode:     0444,
			Size:     desc.Size,
			Typeflag: tar.TypeReg,
		},
		CopyTo: func(ctx context.Context, w io.Writer) (int64, error) {
			r, err := cs.ReaderAt(ctx, desc.Digest)
			if err != nil {
				return 0, err
			}
			defer r.Close()

			// Verify digest
			dgstr := desc.Digest.Algorithm().Digester()

			n, err := io.Copy(io.MultiWriter(w, dgstr.Hash()), content.NewReader(r))
			if err != nil {
				return 0, err
			}
			if dgstr.Digest() != desc.Digest {
				return 0, errors.Errorf("unexpected digest %s copied", dgstr.Digest())
			}
			return n, nil
		},
	}
}

func directoryRecord(name string, mode int64) tarRecord {
	return tarRecord{
		Header: &tar.Header{
			Name:     name,
			Mode:     mode,
			Typeflag: tar.TypeDir,
		},
	}
}

func ociLayoutFile(version string) tarRecord {
	if version == "" {
		version = ocispec.ImageLayoutVersion
	}
	layout := ocispec.ImageLayout{
		Version: version,
	}

	b, err := json.Marshal(layout)
	if err != nil {
		panic(err)
	}

	return tarRecord{
		Header: &tar.Header{
			Name:     ocispec.ImageLayoutFile,
			Mode:     0444,
			Size:     int64(len(b)),
			Typeflag: tar.TypeReg,
		},
		CopyTo: func(ctx context.Context, w io.Writer) (int64, error) {
			n, err := w.Write(b)
			return int64(n), err
		},
	}

}

func ociIndexRecord(manifests ...ocispec.Descriptor) tarRecord {
	index := ocispec.Index{
		Versioned: ocispecs.Versioned{
			SchemaVersion: 2,
		},
		Manifests: manifests,
	}

	b, err := json.Marshal(index)
	if err != nil {
		panic(err)
	}

	return tarRecord{
		Header: &tar.Header{
			Name:     "index.json",
			Mode:     0644,
			Size:     int64(len(b)),
			Typeflag: tar.TypeReg,
		},
		CopyTo: func(ctx context.Context, w io.Writer) (int64, error) {
			n, err := w.Write(b)
			return int64(n), err
		},
	}
}

func writeTar(ctx context.Context, tw *tar.Writer, records []tarRecord) error {
	sort.Slice(records, func(i, j int) bool {
		return records[i].Header.Name < records[j].Header.Name
	})

	for _, record := range records {
		if err := tw.WriteHeader(record.Header); err != nil {
			return err
		}
		if record.CopyTo != nil {
			n, err := record.CopyTo(ctx, tw)
			if err != nil {
				return err
			}
			if n != record.Header.Size {
				return errors.Errorf("unexpected copy size for %s", record.Header.Name)
			}
		} else if record.Header.Size > 0 {
			return errors.Errorf("no content to write to record with non-zero size for %s", record.Header.Name)
		}
	}
	return nil
}