vendor/github.com/moby/buildkit/util/compression/compression.go

package compression

import (
	"bytes"
	"context"
	"io"

	"github.com/containerd/containerd/content"
	"github.com/containerd/containerd/images"
	digest "github.com/opencontainers/go-digest"
	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
	"github.com/pkg/errors"
	"github.com/sirupsen/logrus"
)

// Type represents compression type for blob data.
type Type int

const (
	// Uncompressed indicates no compression.
	Uncompressed Type = iota

	// Gzip is used for blob data.
	Gzip

	// UnknownCompression means not supported yet.
	UnknownCompression Type = -1
)

var Default = Gzip

func (ct Type) String() string {
	switch ct {
	case Uncompressed:
		return "uncompressed"
	case Gzip:
		return "gzip"
	default:
		return "unknown"
	}
}

// DetectLayerMediaType returns media type from existing blob data.
func DetectLayerMediaType(ctx context.Context, cs content.Store, id digest.Digest, oci bool) (string, error) {
	ra, err := cs.ReaderAt(ctx, ocispec.Descriptor{Digest: id})
	if err != nil {
		return "", err
	}
	defer ra.Close()

	ct, err := detectCompressionType(content.NewReader(ra))
	if err != nil {
		return "", err
	}

	switch ct {
	case Uncompressed:
		if oci {
			return ocispec.MediaTypeImageLayer, nil
		}
		return images.MediaTypeDockerSchema2Layer, nil
	case Gzip:
		if oci {
			return ocispec.MediaTypeImageLayerGzip, nil
		}
		return images.MediaTypeDockerSchema2LayerGzip, nil
	default:
		return "", errors.Errorf("failed to detect layer %v compression type", id)
	}
}

// detectCompressionType detects compression type from real blob data.
func detectCompressionType(cr io.Reader) (Type, error) {
	var buf [10]byte
	var n int
	var err error

	if n, err = cr.Read(buf[:]); err != nil && err != io.EOF {
		// Note: we'll ignore any io.EOF error because there are some
		// odd cases where the layer.tar file will be empty (zero bytes)
		// and we'll just treat it as a non-compressed stream and that
		// means just create an empty layer.
		//
		// See issue docker/docker#18170
		return UnknownCompression, err
	}

	for c, m := range map[Type][]byte{
		Gzip: {0x1F, 0x8B, 0x08},
	} {
		if n < len(m) {
			continue
		}
		if bytes.Equal(m, buf[:len(m)]) {
			return c, nil
		}
	}
	return Uncompressed, nil
}

var toDockerLayerType = map[string]string{
	ocispec.MediaTypeImageLayer:            images.MediaTypeDockerSchema2Layer,
	images.MediaTypeDockerSchema2Layer:     images.MediaTypeDockerSchema2Layer,
	ocispec.MediaTypeImageLayerGzip:        images.MediaTypeDockerSchema2LayerGzip,
	images.MediaTypeDockerSchema2LayerGzip: images.MediaTypeDockerSchema2LayerGzip,
}

var toOCILayerType = map[string]string{
	ocispec.MediaTypeImageLayer:            ocispec.MediaTypeImageLayer,
	images.MediaTypeDockerSchema2Layer:     ocispec.MediaTypeImageLayer,
	ocispec.MediaTypeImageLayerGzip:        ocispec.MediaTypeImageLayerGzip,
	images.MediaTypeDockerSchema2LayerGzip: ocispec.MediaTypeImageLayerGzip,
}

func convertLayerMediaType(mediaType string, oci bool) string {
	var converted string
	if oci {
		converted = toOCILayerType[mediaType]
	} else {
		converted = toDockerLayerType[mediaType]
	}
	if converted == "" {
		logrus.Warnf("unhandled conversion for mediatype %q", mediaType)
		return mediaType
	}
	return converted
}

func ConvertAllLayerMediaTypes(oci bool, descs ...ocispec.Descriptor) []ocispec.Descriptor {
	var converted []ocispec.Descriptor
	for _, desc := range descs {
		desc.MediaType = convertLayerMediaType(desc.MediaType, oci)
		converted = append(converted, desc)
	}
	return converted
}