docker/tarfile/src.go
func (s *Source) GetManifest(ctx context.Context, instanceDigest *digest.Digest) ([]byte, string, error) {
	if instanceDigest != nil {
		// How did we even get here? GetManifest(ctx, nil) has returned a manifest.DockerV2Schema2MediaType.
		return nil, "", errors.Errorf(`Manifest lists are not supported by "docker-daemon:"`)
	}
	if s.generatedManifest == nil {
		if err := s.ensureCachedDataIsPresent(); err != nil {
			return nil, "", err
		}
		m := manifest.Schema2{
			SchemaVersion: 2,
			MediaType:     manifest.DockerV2Schema2MediaType,
			ConfigDescriptor: manifest.Schema2Descriptor{
				MediaType: manifest.DockerV2Schema2ConfigMediaType,
				Size:      int64(len(s.configBytes)),
				Digest:    s.configDigest,
			},
			LayersDescriptors: []manifest.Schema2Descriptor{},
		}
		for _, diffID := range s.orderedDiffIDList {
			li, ok := s.knownLayers[diffID]
			if !ok {
				return nil, "", errors.Errorf("Internal inconsistency: Information about layer %s missing", diffID)
			}
			m.LayersDescriptors = append(m.LayersDescriptors, manifest.Schema2Descriptor{
				Digest:    diffID, // diffID is a digest of the uncompressed tarball
				MediaType: manifest.DockerV2Schema2LayerMediaType,
				Size:      li.size,
			})
		}
		manifestBytes, err := json.Marshal(&m)
		if err != nil {
			return nil, "", err
		}
		s.generatedManifest = manifestBytes
	}
	return s.generatedManifest, manifest.DockerV2Schema2MediaType, nil
}
func (s *Source) GetManifest(ctx context.Context, instanceDigest *digest.Digest) ([]byte, string, error) {
	if instanceDigest != nil {
		// How did we even get here? GetManifest(ctx, nil) has returned a manifest.DockerV2Schema2MediaType.
		return nil, "", errors.Errorf(`Manifest lists are not supported by "docker-daemon:"`)
	}
	if s.generatedManifest == nil {
		if err := s.ensureCachedDataIsPresent(); err != nil {
			return nil, "", err
		}
		m := manifest.Schema2{
			SchemaVersion: 2,
			MediaType:     manifest.DockerV2Schema2MediaType,
			ConfigDescriptor: manifest.Schema2Descriptor{
				MediaType: manifest.DockerV2Schema2ConfigMediaType,
				Size:      int64(len(s.configBytes)),
				Digest:    s.configDigest,
			},
			LayersDescriptors: []manifest.Schema2Descriptor{},
		}
		for _, diffID := range s.orderedDiffIDList {
			li, ok := s.knownLayers[diffID]
			if !ok {
				return nil, "", errors.Errorf("Internal inconsistency: Information about layer %s missing", diffID)
			}
			m.LayersDescriptors = append(m.LayersDescriptors, manifest.Schema2Descriptor{
				Digest:    diffID, // diffID is a digest of the uncompressed tarball
				MediaType: manifest.DockerV2Schema2LayerMediaType,
				Size:      li.size,
			})
		}
		manifestBytes, err := json.Marshal(&m)
		if err != nil {
			return nil, "", err
		}
		s.generatedManifest = manifestBytes
	}
	return s.generatedManifest, manifest.DockerV2Schema2MediaType, nil
}
// GetManifest returns the image's manifest along with its MIME type (which may be empty when it can't be determined but the manifest is available).
// It may use a remote (= slow) service.
// If instanceDigest is not nil, it contains a digest of the specific manifest instance to retrieve (when the primary manifest is a manifest list);
// this never happens if the primary manifest is not a manifest list (e.g. if the source never returns manifest lists).